Indian Patents. 263110:"IMAGE FORMING APPARATUS AND IMAGE FORMING METHOD"

Title of Invention	"IMAGE FORMING APPARATUS AND IMAGE FORMING METHOD"
Abstract	The present invention discloses an image forming apparatus for forming an image on a medium by jetting one or more droplets on the medium. The image forming apparatus includes a dot brightness changing part for changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part.

Full Text	DESCRIPTION IMAGE FORMING APPARATUS, IMAGE FORMING METHOD, RECORDING MEDIUM, AND PROGRAM TECHNICAL FIELD The present invention relates to an image forming apparatus, an image forming method, a recording medium, and a program. BACKGROUND ART Typically, an image forming apparatus (multi-function machine) , which has one or more of the functions of a printer, a facsimile machine, or a copier, forms images (image forming) by conveying a medium (hereinafter also referred to as "sheet" or "paper") and jetting a liquid (hereinafter also referred to as "recording liquid" or "i nk") onto the conveyed paper by using a liquid jetting apparatus having a recording head (including one or more liquid jet heads) from which liquid (ink) droplets are ejected (jetted) . It is to be noted that, although the medium is hereinafter referred to as "sheet" or "paper", the material of the medium is not to be limited only to those produced by manufacturing paper. The medium may include, for example, a paper material, a textile material, a fiber material, a fabric material, a leather material, a metal material, a plastic material, a glass material, a wood material, or a ceramic material. The medium may be, for example, a recording medium (recording paper) or a transfer material (transfer paper) . It is to be noted that "image forming" has substantially the same meaning as "recording" or "printing". For example, "image forming" includes forming images having meaning (e.g. characters, figures, symbols) and also image having no particular meaning (e.g. patterns) . Furthermore, the liquid used for an image forming apparatus is not limited to a recording liquid or ink and may be other kinds of liquid as long as images can be formed with the liquid. The image forming apparatus may include, for example, a serial type image forming apparatus which forms images by scanning a liquid jet head mounted on a carriage or a line type image forming apparatus having a line type liquid jet head. The serial type image forming apparatus has a printing speed which is determined according to conditions such as image resoluticn nozzle density , dot formation driving frequency and sub-scanning speed. Among such conditions, nozzle density is constrained by the precision in manufacturing the nozzles,- the liquid chamber, the flow path, and the actuator of the liquid jet head of the image forming apparatus. Particularly, in a case where a liquid jet head uses a piezoelectric element, the only ways to separately form channels corresponding to the nozzles of the liquid jet head are to use a mechanical method (e.g., dicing) or form a thin-film PZT by printing. Accordingly, the nozzle density is relatively low compared to a thermal type liquid jet head fabricated by a semiconductor process. The upper limit of nozzle density of the piezoelectric type liquid jet head is currently approximately 360 dpi. Meanwhile, in order to improve printing speed, it is preferable to form a printing area by scanning the liquid jet head in the main scanning direction in a single time. For example, in a case of forming an image with a resolution of 300 dpi in the sub-scanning direction by using a liquid jet head having a nozzle density of 300 dpi, it is possible to form the image by moving the liquid jet head in the main scanning direction in a single time. In a case of forming an image with a resolution of 600 dpi, the image is to be formed by using an interlace method (in this case, scanning two times in the main scanning direction and one time in the sub-scanning direction (sheet conveying direction) ) . Thus, it is apparent that the non-interlace method which forms an immage with a single scan has greater printing speed than the interlace method. Furthermore, as for methods of forming a single line in the main scanning direction, there are a single pass printing method that forms an image(s) by a single pass in the main scanning direction and a multi-pass printing method that forms an image (s) by plural passes in the main scanning direction. It is apparent that the single pass printing method has greater printing speed than the multi-pass printing method. However, in a case where an image is formed by using a single pass/non-interlace method, the resolution of the image is inevitably low. In order to improve image quality in a case where resolution of the image is low, it is effective to use multiple values (multi-values) for a single pixel. As for methods of using multi-values, there is, for example, a method of changing the size of a single dot, a method of forming a single pixel by jetting plural small dots, or a method of changing the density of ink. Although the method of acquiring high quality images by using multi-values is effective for images of pictures and photographs, no significant effects can be attained for images of graphics and characters (letters) . This is due to the graphics and characters requiring a dot size large enough to fill the background areas of an image, that is, characters or graphic images become low density when small size dots are used. Therefore, binary images such as character images/graphic images face inherent problems of low resolution Part icularly, in a case where characters (letters) are expressed with binary images, the quality of the characters deteriorates such that the characters become illegible. Besides a method of improving the nozzle density of a recording head for improving the resolution of the recording head, there is a method of assembling nozzles in a misaligned manner so that the apparent resolution will be improved. Both methods, however, cannot avoid the increase of manufacturing costs of the recording head. Furthermore, the improvement of resolution increases the workload for processing the image data and complicates the control system of the image forming apparatus, to thereby lead to an increase in the overall cost of the image forming apparatus. These problems become particularly noticeable for a recording head provided with an extended length for increasing printing speed. Furthermore, the problem of low resolution becomes more serious for a line printer since the line printer, unlike the serial printer, is unable to use the interlace method and the multi-pass method such that the printing resolution becomes fixed according to the nozzle resolution of the recording head. Accordingly, in order to obtain satisfactory images while maintaining low manufacturing costs and high speed printing, it is important to attain a satisfactory image quality within a limited recording head resolution. Next, the inherent low RESOLUTION problem is described in further detail. The images recorded by using a liquid jetting method are expressed by using dots formed in a matrix manner in a recording head scanning direction and in a sheet conveying direction that perpendicularly intersects the recording head scanning direction. In a case where a character (letter) is printed as a dot image, the quality of the character significantly differs depending on the resolution of the printed image. For example, in a case where a character of the same size is printed with 300 dpi and 600 dpi, the ratio in the number of dots comprising the character is approximately four times. Therefore, finer details can be expressed and character quality is higher by printing with 600 dpi. Particularly, in a case of expressing a diagonal part (slanted part) of a character, the number of dots increases/decreases in a stepwise manner according to resolution. Therefore, indentation (jaggy) becomes more noticeable when printing with 300 dpi. As a method of reducing the jaggy appearing at the outline upon low resolution printing, there is a smoothing method referred to as anti-aliasing. The anti-aliasing method can perform smoothing with high precision since an outline can be expressed by using dots of many tones (scales). However, the anti-aliasing method is very complicated and requires a large amount of processing time. Therefore, the anti-aliasing method is not suitable for recent image forming apparatuses (e.g., ink jet printer) that are expected to provide high throughput Accordingly, Japanese Registered Patent No. 2886192 (hereinafter referred to as "Patent Document 1") discloses a method of comparing a bit pattern of a sample window in a character bitmap image and a predetermined bit pattern and correcting the main pixels in the sample window into small dots when the bit patterns match. Furthermore, Japanese Registered Patent No. 3029533 (hereinafter referred to as "Patent Document 2") discloses a method of determining an outline part of an image from black dot data and reducing the size of printed dots other than edge dots and black dots. Furthermore, Japanese Laid-Open Patent Application No. 2003-334938 (hereinafter referred to as "Patent Document 3") discloses a method of reducing jaggy at an outline part by changing the size of dots of the outline part according to the inclination of the outline and forming dots at a blank part surrounding the outline part. Furthermore, Japanese Laid-Open Patent Application No. 2004-114303 (hereinafter referred to as "Patent Document 4") discloses a method of expressing a stepwise surrounding part of the dots forming an outline part of an image by using dots smaller than the dots forming the image. Furthermore, Japanese Laid-Open Patent Application No. 2004-017552 (hereinafter referred to as "Patent Document 5") diauloses a method of performing a smoothing process using small dots when image data of a character or graphics is black and refraining from performing the smoothing process when the image data is not black. Furthermore, Japanese Laid-Open Patent Application No. 2004-017546 (hereinafter referred to as "Patent Document 6") discloses a method of converting data so that dots of a stepwise surrounding part of the dots forming an outline part of a character or graphics are expressed as dots smaller than the dots other than those of the stepwise surrounding part and changing the data converting method according to the inclination of the outline part. .Furthermore, Japanese Laid-Open Patent Application No. 2005-193384 (hereinafter referred to as "Patent Document 7") discloses a method including a step of detecting a stepwise changing part of dots forming an outline part of a character and/or graphics on which a half tone process is performed and a step of converting dot data of the dots surrounding the stepwise changing part detected in the detection step into dot data of dots having a size no greater than those of the stepwise changing part, wherein the converting method of the data conversion step is different depending on the inclination of the outline part. Furthermore, Japanese Laid-Open Patent Application No. 2002-166603 (hereinafter referred to as "Patent Document 8") discloses a method of obtaining the difference of density between designated pixels forming a target printing character and pixels surrounding the designated pixels, determining that the designated pixels are pixels of the outline part of the character when there are predetermined number of pixels having greater density with respect to the predetermined pixels, and coloring the determined designated pixels with a black color member comprising a mixture of a cyan, magenta, and yellow color members. Furthermore, Japanese Registered Patent No. 3244411 (hereinafter referred to as "Patent Document 9" discloses a method including a binary character image-data generation step for generating binary character image data by setting the output concentration in a pixel used as the inside of a profile configuration as a 1st concentration when the profile configuration concerned is drawn based on the outline data corresponding to the profile configuration of the character which should be output on the pixel coordinates that specify the pixel of an output unit and setting the output concentration in the pixel used as the outside of the profile configuration as a 2nd concentration different from said 1st concentration, an auxiliary line data extraction step for extracting the partial borderline which fulfills predetermined conditions as auxiliary line data from one or more partial border lines which constitute the outline data; a gradation image-data generation step for generating gradation image data by setting the output concentration of the pixel having a positional relationship fulfilling a predetermined criteria with respect to the drawn partial borderline as the 3rd concentration existing in the middle of the 1st and 2nd concentration when the partial borderline corresponding to the auxiliary line data is drawn on the pixel coordinate, and a compositing step for generating a character image data by compositing binary character image data and gradation image data. The methods disclosed in Patent Documents 1 and 2 are effective for LED printers and laser printers . Since the particle diameter of the toner used by the LED printers and laser printers is no greater than 10 μm, the LED printers and laser printers hardly exhibit ink spreading on plain paper and are able to form small dots as designated. Furthermore, the LED printers and laser printers can form dots of designated sizes at optimum locations by slightly adjusting the laser irradiation position and the laser irradiation length. However, a liquid jet type image forming apparatus exhibits greater ink spreading compared to a laser printer. Furthermore, since the liquid jet type image forming apparatus, which changes dot size by modifying the number or length of the drive pulses in a drive period, requires more time to form dots compared to LED printers and laser printers, it is difficult for the liquid jet type image forming apparatus to change the dot size into many different types. That is, the liquid jet type image forming apparatus can change dots to only a few different sizes. Due to a similar reason, the dots of the liquid jet type image forming apparatus arc formed in a substantially fixed location (position) inside a single pixel. That is, unlike the LED printer or the laser printer, it is difficult for the liquid jet type image forming apparatus to freely change the dot formation location (position) inside a single pixel. As described above, although there are various methods for performing outline correction by using dots of different sizes, the sizes of the dots are limited to more or less than three types even for a multi-value printer capable of forming dots of different sizes'. Furthermore, the types of dot size that can be used for correction could be further limited depending on, for example, character correction, covered area of the medium (paper), or gradation property. Furthermore, since dots are formed at the center of (the address of) each pixel determined by printing resolution, correction might not be satisfactorily achieved. Moreover, in a case where small dots are used for reducing brightness, spaces are easily formed between the character framework and the correcting dot. Such space or deviation may degrade the quality of a character. Furthermore, although outline correction can be performed by changing density with use of composite black, the amount of ink adhered on a single pixel increases due to composite black being created by blending cyan (C) , magenta (M), and yellow (Y) ink. This causes bleeding of ink, particularly, when plain paper is used and results to degradation of image quality. Furthermore, the increase in the amount of ink also increases the time required for drying (reduction of productivity) and also raises ink cost. Furthermore, undesired color areas may be formed in a case where a target impact position for one color ink deviates from a target impact position of another color ink. This also becomes a cause for degrading the quality of a character. Since the kinds of ink that can be mounted in a typical inkjet recording apparatus are approximately four to eight colors, the color that can express black is generally limited to K (K pigment, K dye, or light black) and a CMY composite. Therefore, little variation can be made in the density of ink. In addition, correction of color characters is difficult (similar color ink of different density is required). From another aspect, although the above-described Patent Document 3 discloses an inkjet recording method capable of reducing jaggy by changing dot size, the changing of dot sizes may cause dots to be formed at undesired positions on a medium. This problem is described in detail below. A liquid jet head used in a liquid jet type image forming apparatus includes a pressure generating part for applying pressure to a liquid (ink) inside a liquid chamber. For example, the pressure generating part may be a thermal type head using a thermal type head which uses a heating resistor for generating bubbles or a piezoelectric type head which uses a piezoelectric element (electromechanical transducer) for changing the wall of the "liquid chamber. In oder to chage the dot diameter, it is common to use a method of changing the energy applied to the pressure generating part. For example, this method may be changing the driving voltage of the pressure generating part, changing the pulse width of the drive pulse, or changing the pulse number of the drive pulse. Among these methods, the method of changing the drive voltage requires different signals corresponding to different drive voltages and plural switching parts for selectively switching for each channel corresponding to different drive voltages. Thereby, the driving element (driver IC) becomes larger in correspondence with the increase in the types of drive voltages. In a case of controlling (changing) pulse width or number of pulses, the pulse width and number of pulses can be changed by controlling the switching part based on time. In this case, a single switching part for each channel would be needed. Such pulse width modulation method or pulse number modulation method are used particularly by an image forming apparatus using a piezoelectric head. However, the amount of ink differs depending on the pulse width modulation method or the pulse number modulation method. That is, in forming ink droplets having different dot diameter, since the length of the drive pulse is different, the timing for jetting ink droplets (jet upon end of drive pulse) in accordance with the drive pulse is different even if the timing of the start of the rise of the meniscus is the same upon input of the drive pulse. Accordingly, the time for the droplets to reach the medium surface is different- As a result, the dot formation area on the medium differs depending on the dot size. Thus, even in a case of attempting to improve image quality by replacing (correcting) the dots at the outline part with small dots, it is difficult to provide satisfactory images since the small dots cannot be formed at desired areas of the medium. Furthermore, the deviation of dot formation position is also caused by viscosity resistance of ink due to ambient conditions (e.g., temperature, humidity). Furthermore, the deviation of dot formation location differs depending on image formation conditions (e.g., type of paper, resolution). DISCLOSURE OF INVENTION It is a general object of the present invention to provide an image forming apparatus, an image forming method, a recording medium, and a program that substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art. Features and advantages of the present invention are set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention can be realized and attained by an image forming apparatus, an image forming method, a recording medium, and a program particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention. To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an embodiment of the present invention provides an image forming apparatus for forming an image on a medium by jetting one or more droplets on the medium, the image forming apparatus including: a dot brightness changing part for changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part. Furthermore, another embodiment of the present invention provides a program for causing a computer to execute an image forming process for forming an image on a medium by jetting one or more droplets on the medium, the program including a step of: changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part. Furthermore, another embodiment of the present invention provides a computer-readable recording medium for causing a computer to execute an image forming process for forming an image on a medium by jetting one or more droplets on the medium, the computer-readable recording medium including: the program according to the embodiment of the present invention. Furthermore, another embodiment of the present invention provides an image forming method for forming an image on a medium by jetting one or more droplets on the medium, the image forming method including a step of: changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part. Furthermore, another embodiment of the present invention provides an image forming apparatus for forming an image on a medium by jetting one or more droplets on the medium, the image forming apparatus including: a dot brightness changing part for changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part; a dot size changing part for changing a dot size of at least the target dot forming the outline part of the image to a dot size different from the dot size of the other dots forming the outline part; wherein at least one of the dot brightness and the dot size of the target dot is changed according to the color of the image by the dot brightness changing part and the dot size changing part. Furthermore, another embodiment of the present invention provides an image forming apparatus for forming an image including one or more dots on a medium by jetting one or more droplets on the medium, the image forming apparatus including: an outline correction part, for correcting at least one target dot forming an outline part of the image by replacing the target dot with a replacement dot having a dot size different from the dot size of the target dot; and a replacement dot changing part for changing the method of replacing the target dot according to a factor causing deviation of dot formation position or deviation amount of the dot formation position. Furthermore, another embodiment of the present invention provides a program for causing a computer of an image forming apparatus to execute an image forming process for forming an image including one or more dots on a medium by jetting one or more droplets on the medium, the program including the steps of: correcting at least one target dot forming an outline part of the image by replacing the target dot with a replacement dot having a dot size different from the dot size of the target dot; and changing the method of replacing the target dot according to a factor causing deviation of dot formation position or deviation amount of the dot formation position. Furthermore, another embodiment of the present invention provides a computer-readable recording medium for causing a computer to execute an image forming process for forming an image including one or more dots on a medium by jetting one or more droplets on the medium, the computer-readable recording medium including: the program according to an embodiment of the present invention. Furthermore, another embodiment of the present invention provides an image forming method for forming an image including one or more dots on a medium by jetting one or more droplets on the medium, the image forming method including the steps of: correcting at least one target dot forming an outline part of the image by replacing the target dot with a replacement dot having a dot size different from the dot size of the target dot; and changing the method of replacing the target dot according to a factor causing deviation of dot formation position or deviation amount of the dot formation position. BRIEF DESCRIPTION OF DRAWINGS Fig.l is a side view for describing an overall configuration of an image forming apparatus including a program for performing an image forming method according to an embodiment of the present invention; Fig.2 is a plan view of an image forming apparatus according to an embodiment of the present invention; Fig. 3 is a cross-sectional view along a longitudinal direction of a liquid chamber of a recording head according to an embodiment of the present invention; Fig.4 is a cross-sectional view along a lateral direction of the liquid chamber of the recording head according to an embodiment of the present invention; Fig.5 is a block diagram showing a control part of an image forming apparatus according to an embodiment of the present invention; Fig.6 is a block diagram showing an example of a printing control part of an image forming apparatus according to an embodiment of the present invention: Fig. 7 is a schematic diagram for describing an example of a drive waveform generated and output by a drive waveform generating part of a printing control part according to an embodiment of the present invention; Fig.8 is a schematic diagram for describing drive signals for a small droplet, a medium droplet, a large droplet, a fine drive selected according to a drive waveform according to an embodiment of the present invention; Fig.9 is a schematic diagram for describing differences of output characters due to different resolution; Fig.10 is a schematic diagram for describing an example of performing jaggy correction (outline correction) according to an embodiment of the present invention; Fig.11 is a flowchart for describing an outline correction process (method) according to an embodiment of the present invention; Fig.12 is an exemplary table used for assigning correction patterns according to printing mode/resolution according to an embodiment of the present invention; Fig.13 is a schematic diagram for describing an outline correction process (method) using a correction pattern according to an embodiment of the present invention; Fig.14 is a schematic diagram for describing jaggy correction (outline correction) where dot formation position is changed according to an embodiment of the present invention; Fig.15 is a schematic diagram for describing an example of performing jaggy correction (outline correction) according to an embodiment of the present invention; Fig. 16 is a schematic diagram for describing a state before outline correction, the result of outline correction when there is no deviation of dot formation position, and the result of outline correction when there is deviation of dot formation position; Fig.17 is a schematic diagram for describing an outline corrected character in a case where there is no deviation of dot formation position and a case where there is deviation of dot formation position; Fig. 18 is a schematic diagram for describing a case of applying a correction pattern according to an embodiment of the present invention; Fig.19 is a flowchart for describing a process (method) for controlling an outline correction process (method) according to an embodiment of the present invention; and Fig.20 is an exemplary table used for selection of correction patterns according to an embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail based on the embodiments illustrated in the drawings. First, an exemplary image forming apparatus 1000 is below with reference to Figs. 1 and 2, where Fig. 1 is a side view and Fig. 2 is a plan view of the image forming apparatus 1000. The illustrated image forming apparatus 1000 has guide members including a guide rod 1 and a guide rail 2. The guide rod 1 and the guide rail 2 are mounted in traversed positions between left and right side boards (not shown) of the image forming apparatus. The guide rod 1 and the guide rail 2 hold a carriage 3 so that the carriage 3 can slide in the main scanning direction. Amain scanning motor 4 drives the sliding movement of the carriage 3 via a timing belt 5 stretched between a driving pulley 6A and a driven pulley 6B. Thereby, the carriage 3 is able to travel (scan) in the arrow directions shown in Fig.2 (main scanning direction). The carriage 3 has a recording head (liquid jetting head) 7 including, for example, four recording head parts 7y, 7c, 7m, and 7k for jetting ink droplets of yellow (Y) , cyan (C) , magenta (M), and black (K) , respectively. The recording head 7 having plural ink jetting holes aligned in a direction perpendicular to the main scanning direction is attached to the carriage 3 so that ink droplets can be jetted downward therefrom. The recording head 7 may include a pressure generating part that generates pressure used for jetting ink droplets from the recording head 7 . For example, the pressure generating part may be a thermal actuator which utilizes the pressure changes of ink boiled by an electric heat converting element (e.g. heating resistor), a shape-memory alloy actuator which utilizes the changes of shape of an alloy in accordance with temperature, or an electrostatic actuator utilizing static electricity. Furthermore, the recording head 7 is not limited to having plural recording head parts corresponding to each color. For example, the recording head 7 may have plural ink jetting nozzles for jetting ink of plural colors. The carriage 3 also has a sub-tank 8 for supplying ink of each color to the recording head 7. The sub-tank 8 is supplied with ink from a main tank (i.e. ink cartridge, not shown) via an ink supplying tube(s) 9. The image forming apparatus also includes a sheet feeding portion for feeding sheets of paper 12 stacked on a sheet stacking part 11 of a sheet feed cassette 10. The sheet feeding portion includes a separating pad 14 having a friction coefficient sufficient for separating sheets of paper 12 from the sheet stacking part and a sheet feeding roller 13 (in this example, a half moon shaped roller) for conveying the sheets of paper 12 one at a time from the sheet stacking part 11. The separating pad 14 is configured to urge the sheets in the direction toward the sheet feeding roller 13. The paper 12 conveyed from the sheet feeding part is conveyed to an area below the recording head 7. In order to convey the paper 12 to the area below the recording head 7, the image forming apparatus is provided with a conveyor belt 21 that conveys the paper 12 by attracting the paper 12 with electrostatic force; a counter roller 22 and the conveyor belt 21 having the paper 12 delivered inbetween after receiving the paper 12 conveyed from the sheet feeding part via a guide 15; a conveyor belt guide 23 for placing the paper 12 flat on the conveyor belt 21 by changing the orientation of the paper 12 conveyed in a substantially upright (perpendicular) position by an angle of approximately 90 degrees; and a pressing member 24 for pressing a pressing roller 25 against the conveyor belt 21. Furthermore, the image forming apparatus includes a charging roller (charging part) 26 for charging the surface of the conveyor belt 21. In this example, the conveyor belt 21 is an endless belt stretched between a conveyor roller 27 and a tension roller 28. A sub-scanning motor 31 rotates the conveyor roller 27 via a timing belt 21 and a timing roller 33 so that the conveyor belt 21 is rotated in the belt conveying direction shown in Fig.2 (sub-scanning direction). It is to be noted that a guide member 29 is positioned at the backside of the conveyor belt 21 in correspondence with a target image forming area of the recording head 7. Furthermore,- the charging roller 26 is positioned contacting the top surface of the conveyor belt 21 so that the charging roller 26 rotates in accordance with the rotation of the conveyor belt 21. As shown in Fig. 2, the image forming apparatus also includes a rotary encoder 36. The rotary encoder 36 includes a slit disk 34 attached to a rotary shaft of the conveyor roller 27 and a sensor 35 for detecting a slit(s) formed in the slit disk 34. The image forming apparatus also includes a sheet discharging portion for discharging the sheet of paper 12 onto which data are recorded by the recording head 7. The sheet discharging portion includes a separating claw 51 for separating the paper 12 from the conveyor belt 21, a first sheet discharging roller 53, a second sheet discharging roller 53, and a sheet discharge tray 54 for stacking the paper(s) 12 thereon. Furthermore, a double-side sheet feeding unit (not shown) may be detachably attached to a rear portion of the image forming apparatus. By rotating the conveyor belt in the reverse direction, the paper 12 is delivered to the double-side sheet feeding unit so as to have the paper 12 flipped upside down. Then, the flipped paper 12 is conveyed back to the part between the counter roller 22 and the conveyor belt 21. Furthermore, as shown in Fig.2, a nozzle recovery mechanism 56 for maintaining/restoring the operating status of the nozzle (s) may be provided at a non-printing area toward one side (in this example, toward the back side) of the main scanning direction of the carriage 3. The nozzle recovery mechanism 56 includes, for example, plural caps 57 for covering the surface of each of the nozzles of the recording head 7, a wiper blade 58 for wiping off residual ink from the surface of the nozzles, and an ink receptacle 59 for receiving accumulated ink that is jetted in a process of disposing of undesired ink. Accordingly, with the image forming apparatus having the above-described configuration, sheets of paper 12 are separated and conveyed sheet by sheet from the sheet feeding part, then the separated conveyed paper 12 is guided to the part between the conveyor belt 21 and the counter roller 22 in an upright manner by the guide 15, and then the orientation of the conveyed paper is changed approximately 90 degrees by guiding the tip part of the paper with the conveyor guide 23 and pressing the paper 12 against the conveyor belt 21 with the pressing roller 25. In this conveying operation, an AC bias supplying part of a control part (not shown) of the image forming apparatus alternately applies negative and positive alternate voltages to the charging roller 26 in accordance with an alternate charging pattern. Thereby, the conveyor belt 21 is alternately charged with negative and positive voltages at intervals of a predetermined width in accordance with the alternate charging pattern. When the paper 12 is conveyed onto the charged conveyor belt 21, the paper 12 is attracted to the conveyor belt 21 by electrostatic force. Thus held, the paper 12 is conveyed in the sub-scanning direction by the rotation of the conveyor belt 21. Then, the recording head 7 jets ink droplets onto the paper 12 while the paper 12 is being moved in correspondence with the forward and backward movement of the carriage 3. After the recording head 7 records (prints) a single row by jetting ink in accordance with image signals, the paper 12 is further conveyed a predetermined distance for recording the next row. The recording operation of the recording head 7 is completed when a signal is received indicative of the completion of the recording operation or indicative of the rear end of the paper 12 reaching the edge of the recording area. After the completion of the recording operation, the paper is discharged to the discharge tray 54. In a case of conducting double side printing, the paper 12 is flipped upside down after the recording of the front side (the side which is printed first) of the paper 12 is completed. The paper 12 is flipped so that the back side of the paper is the printing surface by rotating the conveyor belt 21 in reverse and delivering the paper 12 to the double side sheet feeding unit (not shown) . Then, the flipped paper 12 is conveyed to the part between the counter roller 22 and the conveyor belt 21. After the paper 12 is placed on the conveyor belt 21, the recording head 7 conducts the above-described recording operation on the back side of the paper 12. After the recording operation is completed, the paper 12 is discharged to the discharge tray 54. In a case where the image forming apparatus is standing by to conduct a printing (recording) operation, the carriage 3 is moved toward the recovery mechanism 56. The cap 57 covers the nozzle side of the recording head 7 to keep the nozzles moist. This prevents poor jetting performance caused by dried ink. Furthermore, where the cap covers the nozzle side of the recording head 7, a recovery operation may be performed by suctioning accumulated viscous ink (recording liquid) from the nozzles and ejecting the ink and bubbles. Then, the wiper blade 58 wipes off the ink that has adhered to the nozzle side of the recording head 7 during the recovery operation. Furthermore, an empty jetting (idling) operation that is irrelevant to a printing operation may be performed in which ink is jetted, for example, prior to a recording operation or during the recording operation. Next, an example of a recording head part included in the recording head 7 is described with reference to Figs. 3 and 4 . Fig. 3 is a cross-sectional view along a longitudinal direction of a liquid chamber of the recording head 7. Fig.4 is a cross-sectional view along a lateral direction of the liquid chamber of the recording head 7. The recording head 7 includes a layered structure formed by bonding together a flow plate 101 (for example, formed by performing anisotropic etching on a single crystal silicon substrate), a vibration plate 102 (for example, formed by performing electroforming on a nickel plate) provided on a lower surface of the flow plate 101, and a nozzle communication path 103 provided on an upper surface of the flow plate 101. This layered structure is formed with, for example, a nozzle communication path 105 in flow communication with the nozzle (s) 104 of the recording head 7, a liquid chamber 106 serving as a pressure generating chamber, a common liquid chamber 108 for supplying ink to the liquid chamber 106 via a fluid resistance part (supply path) 107, and an ink supply port 109 in flow communication with the common liquid chamber 108. Furthermore, the recording head 7 includes two rows (although only one row is illustrated in Fig.3) of layered structure type piezoelectric elements (also referred to as "pressure generating part" or "actuator part") 121 for applying pressure to the ink inside the liquid chamber 106 by deforming the vibration plate 102, and a base substrate 122 affixed to the piezoelectric elements 121. It is to be noted that plural pillar parts 123 are formed in-between the piezoelectric elements 121. Although the pillar parts 123 are formed at the same time of forming the piezoelectric elements 121 when cutting a base material of the piezoelectric element 121, the pillar parts 123 simply become normal pillars since no drive voltage is applied thereto. Furthermore, the piezoelectric element 121 is connected to an FPC cable 12 6 on which a driving circuit (driving IC, not shown) is mounted. The peripheral portions of the vibration plate 102A are bonded to a frame member 130. The frame member 130 is fabricated to form a void portion 131 for installing an actuator unit (including, for example, the piezoelectric element 121, the base substrate 122) therein, a concave part including the common liquid chamber 108, and an ink supply hole 132 for supplying ink from the outside to the common liquid chamber 108. The frame member 130 is fabricated by injection molding with use of, for example, a thermal setting resin (e.g. epoxy type resin) or polyphenylene sulfate. The flow plate 101 is fabricated to form various concave parts and hole parts including the nozzle communication path 105 and the liquid chamber 106. The flow plate 101 is fabricated, for example, by using an anisotropic etching method in which an alkali type etching liquid (e.g. potassium hydroxide, KOH) is applied to a single crystal silicon substrate having a crystal plane orientation of (110). It is however to be noted that other materials may be used for fabricating the flow substrate 101 besides a single crystal silicon substrate. For example, a stainless steel substrate or a photosensitive resin may also be used. The vibration plate 102 is fabrication, from example by performing an electroforming method on a metal plate formed of nickel. It is however to be noted that other metal plates or a bonded member formed by bonding together a metal plate and a resin plate may also be used. The piezoelectric elements 121 and the pillar parts 123, and the frame member 130 are bonded to the vibration plate 102 by using an adhesive agent. The nozzle plate 103 is formed with nozzles 104 having diameters ranging from 10 urn - 30 μm in correspondence with the sizes of respective liquid chambers 106. The nozzle plate 103 is bonded to the flow plate 101 by using an adhesive agent. The nozzle plate 103 includes, for example, a metal material member having a water repellent layer formed on its outermost surface. The piezoelectric element (in this example, PZT) 121 has a layered structure in which piezoelectric material 151 and internal electrodes 152 are alternately layered on top of one another. The internal electrodes 152, which are alternately extended to the side edge planes of the piezoelectric element 121, are connected to an independent electrode 153 and a common electrode 154. In this example, the pressure is applied to the ink in the liquid chamber 106 by using a piezoelectric constant d33 material for the piezoelectric material 151. It is however to be noted that pressure may also be applied to the ink in the liquid chamber 106 by using a piezoelectric constant d31 material for the piezoelectric material 151. Furthermore, a single row of piezoelectric elements 121 may be provided in correspondence with a single base substrate 121. Accordingly, in a case of jetting ink (recording liquid) from the nozzles 104 of the above-described recording head 7, the piezoelectric element 121 is contracted by lowering the voltage applied to the piezoelectric element 121 to a voltage below a reference electric potential. Thereby, the volume of the liquid chamber 106 increases as the vibration plate 102 is lowered in correspondence with the contraction of the piezoelectric element 121. Then, ink flows into the liquid chamber 106. Then, the voltage applied to the piezoelectric element is raised so that the piezoelectric element 121 expands in the layered direction of the piezoelectric element 121. Thereby, the volume of the liquid chamber 106 decreases as the vibration plate 102 deforms in a manner protruding toward the nozzle 104 in correspondence with the expansion of the piezoelectric element 121. As a result, pressure is applied to the ink inside the liquid chamber 106, thereby jetting ink out from the nozzle 104. Then, the position of the vibration plate 102 returns to its original position by lowering the voltage applied to the piezoelectric element 121 to the reference electric potential. As the vibration plate 102 returns to the original position, the liquid chamber 106 expands to create a negative pressure in the liquid chamber 106. The negative pressure in the liquid chamber 106 allows ink to be supplied into the liquid chamber 106 from the common liquid chamber 108. The recording operation of the recording head 7 moves on to the next ink jetting process after the vibration of the meniscus face of the nozzle 104 attenuates and becomes stable. It is to be noted that the method of driving the recording head 7 is not limited to the above-described example (pull/push method) . For example, a pull method or a push method may be employed by controlling the drive waveform applied to the recording head 7. Next, an example of a control part 200 of the image forming apparatus is described with reference to Fig.5. The control part 200 of FIG. 5 includes, for example, a CPU 201 for overall control (including control of correction of contour parts (jaggy correction)) of the image forming apparatus, a ROM 202 for storing programs and data installed via a computer-readable recording medium 500 for execution by the CPU 201, a RAM for temporarily storing image data and the like, a rewritable non-volatile memory 204 for maintaining data when the power of the image forming apparatus is turned off, and an ASIC 205 for processing various signals corresponding to image data, input/output signals for performing image processing (e.g., image sorting), and controlling various parts of the image forming apparatus. The control part 200 further includes, for example, an I/F 206 for exchanging data and signals with the host, a printing control part 207 including a data transfer part and a drive waveform generating part for controlling the recording head 7, a head driver (driver IC) 208 for driving the recording head 7 provided on the carriage 3, a motor driving part 210 for driving the main scanning motor 4 and the sub-scanning motor 31, an AC bias supply part 212 for supplying AC bias to the charge roller 34, and an I/O 213 for receiving various detection signals from the encoder sensors 43, 35, the temperature sensor 215, and other sensors. The control part 200 is connected to a control panel 214 for inputting data to the image forming apparatus and displaying data. The control part 200 receives data such as image data from the host side at the I/F 206 via a cable or a network (e.g., the Internet) . The host side is connected to, for example, an information processing apparatus (e.g., a personal computer (PC)) 600, an image reading apparatus (e.g., an image scanner) and/or a photographing apparatus (e.g., a digital camera). The CPU 201 of the control part 200 reads out and analyzes the image data (printing data) stored in a reception buffer of the I/F 206. Then, the ASIC 205 performs various processes on the image data such as image processing and data rearrangement. Then, the processed image data are transferred from the printing control part (head drive control part) 207 to the head driver 208. It is to be noted that the generation of dot patterns for outputting images is conducted in the printer driver of the host side (described below). The printing control part 207 transfers image data in the form of serial data to the head driver 208. In addition, the printing control part 207 outputs transfer clocks (required for transferring the image data), latch signals, and droplet control signals (mask signals) to the head driver 208. The printing control part 207 has a drive waveform generating part including a D/A converter for performing D/A conversion on pattern data of drive signals stored in the ROM 202 and a drive waveform selecting part for selecting the waveform to be output to the head driver 208. Accordingly, the printing control part 207 generates drive waveforms including one or more drive pulses (drive signals) and outputs the drive waveforms to the head driver 208. The head driver 208 applies drive signals included in the waveforms output from the printing control part 207 to a driving element (e.g. the above-described piezoelectric element 121) . The driving element generates energy for enabling ink droplets to be selectively jetted from the recording head 7. The head driver 208 applies the drive signals based on serially input image data corresponding to a single line of the recording head 7. By selecting the drive pulse included in the drive waveform, ink droplets of different sizes including large droplets (large dots), medium droplets (medium dots), and small droplets (small dots) can be jetted from the recording head 7. The CPU 201 calculates the drive output value (control value) for controlling the main scanning motor 4 and drives the main scanning motor 4 via the motor driving part 210 in accordance with the calculated value. The calculation of the CPU 201 is based on the detected speed value and the detected position value obtained by sampling the detection pulses of the encoder sensor 43 (i.e. linear encoder) and the target speed value and the target position value stored beforehand in a speed/position profile. In the same manner, the CPU 201 calculates the drive output value (control value) for controlling the sub-scanning motor 31 and drives the sub-scanning motor 31 via the motor driving part 210 in accordance with the calculated value. The calculation of the CPU 201 is based on the detected speed value and the detected position value obtained by sampling the detection pulses of the encoder sensor 35 (i.e. rotary encoder) and the target speed value and the target position value stored beforehand in a speed/position profile. Next, examples of the printing control part 207 and the head driver 208 are described with reference to Fig.6. As described above, the printing control part 207 has a drive waveform generating part 301 for generating a drive waveform including plural drive pulses (drive signals) and outputting the drive waveform in a single printing cycle and a data transfer part for outputting two bit image data corresponding to the output (print) image (gradation signal 0, 1) , latch signals (LAT), and droplet control signals M0-M3. The droplet control signal is a two bit signal for instructing opening and closing of an analog switch (switching part) of the head driver 208 with respect to each droplet. In correspondence with the printing period of a common drive waveform, the droplet control signal makes a state-transition to an H level (ON) with respect to a selected waveform and makes a state-transition to a L level (OFF) with respect to a non-selected waveform. The head driver 208 includes a shift register 311 for inputting transfer clocks (shift clocks) and serial image data (gradation data: two bit/CH) from the data transfer part 302, a latch circuit 312 for latching each resistance value of the shift register 311 with latch signals, a decoder 313 for decoding the gradation data and the droplet control signals M0-M3 and outputting the decoding results, a level shifter 314 for converting the level of the logic level voltage signal of the decoder 313 into a level operable for an analog switch 315, and the analog switch 315 for switching on and off (open/close) according to the output of the decoder 313 via the level shifter 314. The analog switch 315 is connected to a selection electrode (independent electrode) 153 of each piezoelectric element 121 for receiving a common drive waveform from the drive waveform generating part 301. Therefore, in accordance with the results of decoding the serially transferred image data (gradation data) and the droplet control signals MN0-MN3 by the decoder 313, the analog switch 315 is switched on, to thereby allow a predetermined drive signal of the common drive waveform to pass through (to be selected) and applied to the piezoelectric element 121. Next, examples of a drive waveform are described with reference to Figs.7 and 8. As shown in Fig. 7, the drive waveform generating part 301 generates a drive signal (drive waveform) including eight drive pulses P1-P8 in a single printing period (one drive period) . The drive pulses P1-P8 include, for example, a waveform element dropping from a reference electric potential Ve and a waveform element rising from the dropped state. The drive pulse to be used is selected according to the droplet control signals M0-M3 from the data transfer part 302. The waveform element of a drive pulse having a potential V dropping from the reference electric potential Ve corresponds to a pulling waveform element for causing the piezoelectric element 121 to contract and increase the volume of the liquid chamber 106. The waveform element of a drive pulse rising from a dropped state corresponds to a pushing waveform element for causing the piezoelectric element 121 to expand and reduce the volume of the liquid chamber 106. Furthermore, in accordance with the droplet control signals M0-M3 from the data transfer part 302, the drive pulse P1 is selected in a case of forming a small-sized droplet (small dot) (See (a) of Fig.8), the drive pulses P4-P6 are selected in a case of forming medium-sized droplets (medium dots) (See (b) of Fig. 8) , the drive pulses P2-P8 are selected in a case of forming large-sized droplets (large dots) (See (c) of Fig.8), and drive pulse P2 is selected in a case of performing fine driving (vibration of meniscus with no jetting of droplets) . Accordingly, the selected drive pulses are applied to the piezoelectric element 121 of the recording head 7. In a case of forming a medium droplet (medium dot) , a first droplet is jetted at the drive pulse P4, a second droplet is jetted at the drive pulse P5, and a third droplet is jetted at the drive pulse P6. The jetted first, second, and third droplets are combined during flight and make impact on the medium as a single droplet. In this case, when the inherent vibration period of the pressure chamber (liquid chamber 106) is represented as "Tc", the interval of the jetting timing between the drive pulse P4 and P5 is preferably " 2Tc ± 0.5 μs. Since the drive pulses P4 and P5 have a simple pulling waveform element, the ink droplet speed of the third droplet will become too fast if the drive pulse P6 is also provided with a simple pulling waveform element. This could cause the impact location of the droplet to deviate from the impact location of the other droplets. Therefore, by reducing the pulling voltage (reducing drop of electric potential) for the drive pulse P6. the pulling of meniscus can be reduced, to thereby restrain the ink droplet speed of third droplet. However, the rise voltage is not to be reduced for saving necessary ink droplet volume. In other words, by relatively reducing the pull voltage for the last drive pulse in a set of drive pulses, the jetting speed corresponding to the last drive pulse can be relatively reduced. Accordingly, the impact location of the last droplet can be matched with the impact locations of the other droplets. The fine drive pulse P2 has a drive waveform for vibrating the meniscus of a nozzle without jetting ink droplets so that the meniscus of the nozzle can be prevented from drying. The fine drive pulse P2 is applied to the recording head 7 for a non-printing area of a medium. By using the drive pulse P2 having a fine waveform as one of the drive pulses for forming a large droplet, the drive period can be shortened (accelerated). By setting the interval of the jetting timing between the fine drive pulse P2 and the drive pulse P3 within a range of ± 0.5 μs with respect to the inherent vibration period "Tc", the volume of ink droplet jetted by the drive pulse P3 can be gained (increased). That is, by overlapping (supplementing) the expansion of the pressure chamber 6 from the drive pulse P3 with the pressure vibration of the vibration period in the pressure chamber from the drive pulse P2, the volume of the liquid droplet jetted at the drive pulse P3 is greater compared to the volume of the liquid droplet jetted by the drive pulse P3 alone. Next., an exemplary operation of performing outline correction of an image with the above-described image forming apparatus is described. [First Example] A liquid jet type image forming apparatus 10 00 exhibits jagged areas at a diagonal part of an outline of an image since the dots formed in the diagonal part by the image forming apparatus 1000 are arranged in a stepwise manner. As shown in (a) of Fig. 9, jagged areas are hardly recognizable in a case where characters are printed in high resolution. However, as shown in (b) of Fig. 9, significant jagged areas (jaggy) clearly appear and character quality is poor in a case where characters are printed in an insufficiently low resolution. As described above, exemplary methods for reducing (correcting) the jagged areas include using dots smaller than other dots to form the outline part or adding new dots to empty spaces in the outline part. However, the method of using dots smaller than other dots at the outline part cannot be used as the correcting method in a case of forming an image with binary values, in other words, forming an image by printing or not printing a dot(s) (one type of dot size). Meanwhile, in a case of using recording liquid (ink) of four colors (black (K), cyan (C), magenta (M), yellow (Y)), a composite black dot of C, M, Y can be formed by jetting ink droplets of C, M, and Y onto the same impact location. It is known that a dot printed with the composite black ink has a greater brightness than a dot printed with black (K) ink alone. Furthermore, another composite black dot of C, M, Y, and K having lesser brightness can be printed by jetting ink droplets of C, M, Y, and K onto the same impact location. Accordingly, by compositing (combining) ink of different colors, black dots having different brightnesses can be used to reduce jagged areas at an outline part of, for example, a black character. Furthermore, in a case where a color ink besides those of the aforementioned four colors (K, C, M, Y) is used (e.g. , black ink with greater brightness such as light black, gray), a dot itself can have different brightness by compositing the colors such as black, light black, cyan, magenta, and yellow. In a case of forming an image by using dots of different sizes, an outline part of an image can also be corrected by changing the sizes of dots at the outline part into sizes smaller than other dots or by adding dots smaller than other dots to a part surrounding the outline part. From a broad (macro) perspective, the part printed with smaller dots will appear to have greater brightness since the area covered by ink is smaller compared to other parts printed with larger dots. Accordingly, an outline correcting effect can be attained. It is also regarded that the different dot sizes also serve to physically reduce the jagged areas. Furthermore, with a configuration capable of forming more dot sizes, a more suitable outline correction can be achieved owing to more choices of dot sizes and more variations of dot correction patterns. In addition, such configuration can be used in forming character other than black. Therefore, an image forming apparatus capable of forming dots of various sizes can use both the above-described outline correcting method using different dot sizes (e.g., using dots smaller than other dots as the dots of an outline part or adding dots no larger than the other dots to a part surrounding an outline part) and the above-described outline correcting method using different dot brightnesses (e.g., in a case of black dots, using different kinds of ink to form different dot brightnesses) . Accordingly, in a case of forming black characters or thin lines, a combination of the outline correcting method using different dot sizes and the outline correcting method using different dot brightnesses can be used whereas in a case of forming characters or thin lines of a color besides black, the outline correcting method using different dot sizes can be used. In other words, in an image forming apparatus capable of forming dots of different sizes, either one or both of the following method (part) can be used according to the image to be corrected (target image), in which one method (part) is for changing the brightness of at least a single dot situated in an outline part (e.g., outline part of a character or a thin line) or the brightness of at least a single dot added to a part surrounding the outline part to a brightness relatively greater than the brightness of other dots situated in the outline part and the other method (part) is for changing the dot size of at least a single dot situated in an outline part (e.g. , outline part of a character or a thin line) or the dot size of at least a single dot added to a part surrounding the outline part. As described below, it is to be noted that the two methods (parts) can be combined as a single method (part) for performing a brightness changing process and a dot size changing process at the same time by using a correction pattern. Accordingly, by changing dot size and dot brightness, correction can be performed at a higher level and with more variations. In addition, improving character quality by changing dot size and changing dot brightness can reduce the tradeoffs between preventing drying of the nozzle and preventing increase of ink cost. For example, in a case of printing a document mainly of black, although jetting failure due to drying of a color nozzle tends to occur, a process of cleaning the nozzle would reduce printing speed. Meanwhile, in a case of compositing black, ink costs tend to increase (particularly, cost increase unrelated to improvement of image quality). Thus, performing outline correction only by changing dot brightness causes increase of ink cost and ink bleeding. Therefore, by enabling both dot size change and dot brightness change, nozzle drying and ink cost increase can both be prevented. Next, exemplary cases in performing the above-described outline correction are described with reference to Fig.10. Fig.10(a) shows a case where no outline correction is performed. In Fig. 10 (a), stepwise jagged parts are noticeable at the outline part. Meanwhile, Fig.10(b) shows a case where outline correction is performed by having empty (blank) dots of the dots surrounding the outline part changed into image (printed) dots Ds, that is, changing the empty (blank) dots into dots having a size smaller (or equal) than that of the dots of the outline part. In addition to or as an alternative of the outline correction of Fig.10(b), the dots of the outline part can be changed into dots having a relatively small size. Accordingly, by adding small dots to the outline part, the jagged parts at the outline part become smooth. In a case of an image forming apparatus capable of using dots size, outline correction can be performed in a greater variety of ways. Thus, the image forming apparatus can perform more suitable outline correction. It is to be noted that the number of multi-value or dot formation location, number of dots, sizes of dots are not limited to those described in the embodiments of the present invention, such as in Fig.10(b). Furthermore, this outline correction can be performed not only for black characters but also for color characters. Fig.10(c) shows a case where outline correction is performed by changing the dots of the outline part into dots Dp having relatively greater brightness than that of the dots of the outline part. The changing of brightness may be performed on the dots to be added to a part surrounding the outline part or on the dots which originally form, in this example, a black character. By using a different brightness for the dots forming a character, the jagged parts at the outline part become less noticeable. Thereby, an outline correction effect can be attained. Furthermore, by the use of dots with different brightnesses, outline correction can be performed not only with an image forming apparatus capable of forming dots of different sizes but even with an image forming apparatus capable of forming dots of a single dot size. It is to be noted that the number of multi-value or dot formation location, number of dots, sizes of dots, brightness of dots, or the kind of ink used for forming the dots are not limited to those described in the embodiments of the present invention, such as in Fig.lO(c). Fig.10(d) shows a case where outline correction is performed by changing the dots of the outline part into dots Dp having relatively greater brightness than that of the dots of the outline part and also changing (adding) the dots surrounding the outline parts into dots Dps having relatively greater brightness as well as smaller dot size than the other dots of the outline part. In other words, Fig.10(d) shows a case of combining the dot size changing process and dot brightness changing process. Thereby, a more efficient outline correction effect can be attained. The dot brightness changing process may be performed on the dots originally forming, in this example, a black character, the dots to be added to the outline part, or the dots changed into a smaller size. It is to be noted that the number of multi-value or dot formation location, number of dots, sizes of dots, brightness of dots, or the kind of ink used for forming the dots are not limited to those described in the embodiments of the present invention, such as in Fig.10(d). It is to be noted that the liquid jet head capable of jetting dots of different sizes is not limited to a piezoelectric type head and an electrostatic type head. For example, the liquid jet head may be a thermal type head including nozzles of different diameters or a heater having a non-linear characteristic for controlling heater resistance. The above-described outline correction effect can be attained with an image forming apparatus having any one of the heads. Accordingly, jagged areas (jaggy) can be hardly noticeable and image quality can be improved by changing the brightness of at least one dot in the dots forming an outline part (for example, an outline part of a character or a linear image) or one dot to be added to a part surrounding the outline part to a bri ghtness relatively greater than that of the other dots forming the outline part. From another aspect, black characters, in general, are preferred to be formed darkly and clearly for better visibility. However, in a case where there is a significant contrast between the paper and the image (character), the edges at the outline part of the image become emphasized and jagged areas become noticeable. Therefore, in a case of performing outline correction by using dots having different brightnesses, the jagged areas can be reduced without deterioration of visibility by using dots of low brightness for the framework of the character while using dots of higher brightness for the outline part of the character. For example, in a case of forming a black character with recording liquids of four colors (K, C, M, Y) , the density of the black character can be increased by using a composite black ink including the four colors (four color composite black ink) . However, in this case, jagged areas can be reduced without deterioration of visibility by using the four color composite black ink for the framework of the black character while using a single black color (K) ink or a three color composite black ink for the outline part of the black character. Furthermore, the amount of ink consumed can be less compared to using the four color composite black ink. Furthermore, by detecting the angle of an outline part and changing the dot size changing process and/or the dot brightness changing process (e.g., switching a correction pattern (described in detail below) ) according to the detected angle, an optimum outline correction can be realized according to the angle of the outline part. For example, in detecting the angle of the outline part, first, the dots forming the outline part are detected. Then, an outline formed by connecting the detected dots is obtained. Then, the angle of the outline part is calculated by comparing the outline and a predetermined direction (e.g., main scanning direction or sub-scanning direction) . Accordingly, a correction pattern corresponding to the calculated angle of the outline part is selected. Exemplary cases of performing outline correction using correction patterns according to various conditions and factors (e.g., angle of the outline part, etc.) are described below. In the case of performing outline correction according to the angle of the outline part, a correction pattern includes one or more outline pattern data and correction process data corresponding to the outline pattern data. Thus, the outline correction is performed by matching a detected outline pattern and a correction pattern. Whether jagged areas (jaggy) in the outline part are noticeable may also depend on printing conditions (e.g., resolution, type of paper). Since dot formation location can be specified in detail in a case of high resolution printing, jaggy in the outline part is less noticeable. Therefore, in a case where sufficient resolution can be obtained., outline correction miaht not be required. Furthermore, with respect to types of paper, dots tend to blur when plain paper is used. Meanwhile, when glossy paper is used, the shapes of dots clearly appear and the density of the paper surface is uniform. Therefore, the edges tend to appear and jaggy is noticeable. Therefore, it is determined whether it is necessary to perform the outline detection and outline correction depends on printing conditions (e.g., resolution, type of paper) and performing outline correction; when it is determined to be necessary, with a correction pattern based on the printing conditions, an optimum outline correction can be performed without unnecessary data processing. Since the outline correction method according to an embodiment of the present invention is applicable to characters and thin line images, the calculation workload performed for the outline correction method can be reduced by obtaining object data of the image to be printed (e.g., text data, photograph data, graphic data, thin line data) and performing the outline detection process and the outline correction process on character data and thin line data only. Furthermore, as for other printing conditions (factors) of an liquid jet type image forming apparatus, jetting failure (e.g., broken (missing) line patterns, or bent line patterns) tend to occur as ink becomes dry and the viscosity of the ink increases. Particularly, in a case where the image forming apparatus is used under a low temperature and/or low humidity environment or a case where the image forming apparatus has not been used for a long period, such problems are liable to occur. Furthermore, as for other printing conditions, when printing image data (e.g., printing an image consisting of mostly black characters but with a part expressed with color characters) in a case where a certain ink (e.g., black) is frequently used while other color inks are not often used, the low usage rate of the other color inks may cause jetting failure and may skip (miss) printing the color character part. In order to prevent the above-described jetting failures, the nozzle face is cleaned and blank (empty) ink jetting is performed. However, these processes reduce printing throughput and unnecessarily consume ink. Furthermore, although color inks may be used by printing a black character part with composite black ink for preventing the jetting failure, this leads to an increase in the amount of ink consumption and the appearance of jagged areas due to an increase of density of the characters. More specifically, in a case where a composite black of four colors (K, C, M, Y) is used, jagged areas will clearly appear due to increased density of the characters. In a case where a composite black of three colors (C, M, Y) is used, visibility of the characters is reduced due to reduced density of the characters. By referring to object data of image data to be printed (target image data) or color component data calculated from the target image data, an optimum outline correction can be performed from the aspect of steady nozzle jetting performance and suitable ink consumption based on the object data and the color component data. That is, a correction pattern can be selected from the aspect of attaining a balanced usage of ink, for example, by selecting a correction pattern using a large amount of color ink in a case of printing an image consisting mostly of characters/thin lines where the color ink usage rate is low or by switching to a correction pattern that controls the consumption of each ink to a predetermined amount based on color component data. In a case of performing an outline correction using the dot size changing process where the target image data includes characters of a color other than black, suitable outline correction can be performed by changing the correction patterns based on color component data. For example, since jagged areas are more noticeable when using colors as black, cyan, and magenta at an outline part compared to using a color such as yellow, the correction pattern can be changed based on color component data so that optimum outline correction can be achieved. Furthermore, correction patterns may also be switched according to status data of the image forming apparatus (e.g., ambient temperature or humidity of the image forming apparatus, lapsed time from previous use of the image forming apparatus) . For example, under conditions where jetting failure tends to occur (e.g., low temperature, low humidity, long unused period), it is preferable to select a correction pattern that can use various inks in a balanced manner for attaining steady jetting performance. For example, under conditions where it is difficult for ink to dry (e.g., high temperature, high humidity) it is preferable to select a correction pattern that can reduce ink consumption for preventing problems such as cockling and bleeding. Furthermore, in a case where the image forming apparatus is capable of forming dots of multiple sizes (multi-value dot size), ambient conditions may cause jetting failure to occur for a particular size droplet (e.g., small droplet), to thereby degrade image quality. Therefore, by switching the correction pattern and changing the combination of dot sizes, degrading of image quality due to jetting failure can be prevented. Next, an exemplary outline correction operation (method) is described with reference to Fig.11. This outline correction operation is executed by having a CPU 201 execute a program stored in a ROM 202. In the outline correction operation, first, printing data are obtained. For example, the printing data include printing mode (resolution, type of paper) , image data (e.g., color component data, object data) , and status data of the image forming apparatus (e.g., ambient data). Then, the CPU 201 determines whether outline correction is necessary based on the obtained printing data. In a case where outline correction is determined unnecessary (e.g., sufficient high resolution, no character/thin line to be printed), outline correction is not performed. In a case where outline correction is determined necessary, the CPU 201 selects a correction pattern to be used based on a table as shown in Fig. 12. It is to be noted that the table shown in Fig.12 is merely an example. Thus, the choices of patterns or the number of patterns is not limited to those shown in Fig.12. Since the optimum correction process differs depending on, for example, resolution and type of paper, the correction pattern is different according to resolution and type of paper. Furthermore, even if the resolution or the type of paper is the same, it may be desirable to switch the correction pattern according to the ambient conditions of the image forming apparatus or data configuration of the image data. For example, a correction pattern that increases the usage rate of C, M, Y may be selected for preventing nozzle failure. In another example, a correction pattern that controls ink consumption may be selected for reducing ink cost. Furthermore, in a case where the image forming apparatus is capable of forming dots of multiple sizes, a combination of the dot size changing process and the dot brightness changing process may be used when correcting black characters and the dot size changing process may be used when correcting color characters. Thus, different correction patterns are assigned to various correction levels which are determined (defined) by calculating various printing data. As shown in Fig. 13, each correction pattern includes plural outline patterns and dot arrangement data (e.g., dot formation location, number of dots, dot size, type of ink) corresponding to the outline patterns. The CPU 201 performs a pattern matching process on the shape (pattern) of a detected outline part and the outline patterns included in the correction pattern. The CPU 201 performs outline correction on the detected outline part by using a matching outline pattern in the correction pattern. More specifically, in performing the outline correction process based on the printing data according to an embodiment of the present invention, the CPU 201 selects a correction pattern from the table shown in Fig.12 and performs outline correction on the detected outline part by using a corresponding outline pattern of the selected correction pattern. For example, an outline pattern having an angle corresponding to that of the detected outline part may be used in the outline correction process. In an exemplary table shown in Fig. 12, the correction levels (correction rank) α, β, γ are defined (categorized) according to ambient conditions of the image forming apparatus and/or configuration (constitution) of the image data. Each correction level is assigned with correction patterns A through G according to printing mode. For example, in a case where the correction levels α, β, γ are categorized according to ambient temperature (α indicating low temperature, β indicating medium temperature, γ indicating high temperature), a correction pattern A that can use various inks in a balanced manner can be assigned for correction level α (i.e. using C, M, and Y inks for increasing brightness rather than using K ink only in a low temperature condition). Meanwhile, a correction pattern C that that can reduce ink consumption can be assigned for correction level γ (i. e. using one type of ink rather than using C, M, Y inks) . For example, in a case where the correction levels α, β, γ are categorized according to the data configuration of the image data (α indicating small proportion of black color data, β indicating medium proportion of black color data, γ indicating large proportion of black color data), a correction pattern A having high C, M, Y usage rates can be assigned for correction level α (i.e. using C, M, and Y inks for increasing brightness instead of using K ink only when the proportion of black color data in the image data is small) . Meanwhile, a correction pattern C having low C, M, Y usage rates can be assigned for correction level γ . It is to be noted that the categorization of the correction level or the number of correction levels is not to be limited to those described above. Furthermore, in a case of forming black characters or black thin lines, the timing for jetting liquid droplets of one or more colors can be different with respect to those of other colors. For example, in a case of performing outline correction with a configuration capable of jetting four colors of K, C, M, Y, the timing for jetting K color ink can be different from the timing for jetting C, M, Y color inks. This allows outline correcting dots to be formed at areas where a satisfactory outline correction effect can be attained. It is to be noted that various head configurations can be used (e.g., a configuration having a head corresponding to each color, a configuration having one head for K ink and another head for C, M, Y ink, and a configuration having a head with plural nozzle arrays). Fig.14 shows exemplary cases where droplets are jetted at different timings. Fig.14(a) shows a case where no outline correction is performed. In Fig. 14 (a) , jagged areas are noticeable at the outline part. Fig.14(b) shows a case where outline correction is performed by adding small dots Ds to the outline part. However, since the kinds of dot sizes are limited and dot formation locations are defined by resolution, the outline correction effect is still insufficient. In Fig.14 (c) , the dot formation locations of a portion of the dots D are shifted (deviated) a half pitch of the resolution with respect to the dot formation locations of the other dots (in this example, the dots are shifted in a main scanning direction which perpendicularly intersects the nozzle array direction) . Accordingly, the outline correcting dots can be formed in areas where greater outline correction effect can be attained. In addition to the outline correction effect (smoothing effect) using a different brightness at the outline part, the dots also serve to physically cover the jagged areas. The example shown in Fig. 14 (c) can be executed not only by an image forming apparatus capable of forming multi-value dot patterns but also by an image forming apparatus capable of forming binary dot patterns. Thus, depending on the selected recording head, the correction process of using different dot brightnesses can be used in combination. As shown in Fig.l4(d), an image forming apparatus capable of forming multi-value dot patterns can perform a combination of changing dot size and changing dot formation location. Thus, depending on the selected recording head, the correction process of using different dot brightnesses can be used in combination. It is to be noted that the outline correction processes described with Fig. 14 may be performed with a recording head, dot formation location, number of dots, dot size, and ink other than those described above. In changing the timing for jetting the ink, a recording head having drive waveforms of different rise timings may be used for selectively applying the drive waveform to the recording head. Although the above-described embodiments of the present invention are described with a serial type image forming apparatus, the present invention may also be effectively applied to a line type image forming apparatus. In the line type image forming apparatus, nozzle are arranged substantially across the entire paper in the paper width direction. Furthermore, the line type image forming apparatus performs recording (printing) by conveying the paper instead of scanning in the width direction. Therefore, the line type image forming apparatus is only capable of using the interlace method and the multi-pass method. This makes it difficult for the line type image forming apparatus to increase image forming (printing) resolution. Accordingly, the problem of jaggy may be more serious for the line type image forming apparatus compared to the serial type image forming apparatus. Furthermore, since the line type image forming apparatus performs recording by conveying paper with respect to an affixed recording head, it is difficult to perform a head cleaning process and a blank jetting process with the line type image forming apparatus. Furthermore, since the line type image forming apparatus has a longer recording head than that of the serial image forming apparatus, a greater amount of ink is used in the cleaning process and the blank jetting process. Therefore, the difficulties of outline correction, steady jetting performance, and ink consumption also apply to the line type image forming apparatus. Therefore, the present invention is also effective for the line type image forming apparatus. The above-described program according to an embodiment of the present invention may be installed in a computer-readable recording medium 500 for enabling an image forming apparatus including a CPU (computer) 201 to perform the outline correction operation (method) , to thereby eliminate jaggy and improve image quality. Although the above-described image forming apparatus is explained having a configuration of a printer, the present invention may also be applied to an image forming apparatus having a configuration of a facsimile machine, a plotting apparatus, a copier, or a multifunction machine having the functions of a printer, a facsimile, and a copier, for example. [Second Example] Next, another exemplary operation of performing outline correction of an image with the above-described image forming apparatus is described. As described above, in a case of jetting liquid droplets of different dot diameters by applying drive pulses of different widths or different numbers of drive pulses to the recording head 7, even if the timing of the start of the rise of the meniscus is the same upon input of the drive pulse, the timing for jetting liquid droplets upon the end of the drive pulse is different. Therefore, the time for the droplets to reach the medium surface is different. As a result, the dot formation position on the paper is different depending on dot size. Furthermore, since the viscosity resistance of ink changes according to ambient temperature and ambient humidity, the meniscus may not be appropriately controlled even if a drive pulse is applied to the recording head 7. This results in deviation of dot impact position. The ambient conditions also make it difficult to maintain suitable dot size and dot shape. This is also results to deviation of dot impact position. Furthermore, in a case where the image forming apparatus 1000 is not used for a long period of time, dried ink may accumulate surrounding of the nozzle. Moreover, in a case where the image forming apparatus 1000 has been used for a long period of time since the manufacture of the image forming apparatus 1000, changes in the characteristics of its recording head 7 may affect liquid jet precision and lead to deviation of dot formation position. Although an example of a piezoelectric type recording head is used for describing the problem of dot formation position, this problem may also occur in a case of using a thermal type recording head or an electrostatic type recording head (a head using electrostatic force between a vibration plate and an electrode facing the vibration plate for applying pressure to a liquid chamber via the vibration plate) as the recording head 7. For example, the present invention may be applied to a case of using a thermal type recording head which forms multi-value dot patterns by controlling the resistance value of its thermoelectric transducer. Next, exemplary cases in performing the above-described outline correction are described with reference to Fig.15. Fig.15(a) shows a case where no outline correction is performed. As shown in Fig.15 (a), satisfactory image quality cannot be attained and stepwise jagged parts are noticeable at the outline part in a case where resolution is not high enough. Accordingly, by replacing the dots at the parts surrounding the outline part with other dots, the jagged areas can be reduced. For example, Fig. 15 (b) shows a case where one blank dot situated in the stepwise part of the outline part is replaced with a small size image dot (addition of dots) . Fig. 15 (c) shows a case where two blank dots situated in the stepwise part of the outline part are replaced with one small size image dot and another medium size image dot (addition of dots) . Fig. 15 (d) shows a case where two more dots situated in the stepwise part of the outline part are replaced with another small size image dot and another medium size image dot in addition to those of Fig.15 (c). However, as described above, in a case where there is a deviation of dot formation position, the above-described outline correction process may adversely affect image quality. For example, in a case of performing the outline correction by adding dot Dh (see Fig. 16 (b) ) for reducing jaggy in the outline part shown in Fig.16 (a) , deviation of dot formation position (see Fig.16 (c) ) may cause undesired overlapped or blurred areas, to thereby degrade image quality. More specifically, as shown in Figs. 17 (a) and 17 (b) , the roughness at the outline part of the character in Fig.17(b) is significantly noticeable compared to that of Fig.17(a). Accordingly, the present invention selectively changes the method (pattern) for replacing a target dot with another dot having, for example, different dot size and/or brightness with respect to the target dot by referring to factors (e.g., ambient temperature, ambient humidity, lapsed time from manufacture, lapsed time from previous image forming process) that cause deviation of dot formation position (dot impact position) or by referring to the actual amount (degree) by which the dots deviate from a desired dot formation position on the paper (medium) , so as not to use a deviated dot(s) that would adversely affect image quality. For example, the method (pattern) of replacing dots can be changed by selectively changing a correction pattern for maintaining image quality of an output image (described in detail below). The same as Fig. 15 (a), Fig. 18 (a) shows a case where no outline correction is performed. Likewise, jagged areas are noticeable at the outline part. Accordingly, as shown in Fig.18 (b), an outline correction process using different size droplets (medium droplet and small droplet : medium dot and small dot) is performed. In this case, when deviation of a droplet impact position occurs (e.g., deviation of small droplet impact position) , the dot corresponding to the deviated droplet deviates from its target dot formation position (small dot Ds deviating one dot from target dot formation position in Fig. 18 (c) ) . Thus, in addition to not being able to attain a satisfactory outline correction effect, such deviation causes degrading of image quality such as creation of overlapped lines, mist, and blurring. In a case where there is a significant deviation of a small droplet impact location, a correction pattern which uses only medium droplets (no small droplets) can be selected as the correction pattern for performing the outline correction process (See Fig. 18 (d) ) . Likewise, in a case where there is a significant deviation of a medium droplet impact location, a correction pattern which uses only small droplets (no medium droplets) can be selected as the correction pattern for performing the outline correction process (See Fig. 18 (e) ) . Thus, an optimum correction pattern can be used in the outline correction process (dot replacement process) according to the deviation status (or the cause of deviation) . It is to be noted that the types of droplets, droplet sizes (large, medium, small), or correction pattern are not limited to those described with Fig.18. Next, factors causing deviation of dot formation position, how the factors are detected, and how the factors are input are described. As described above, the factors causing deviation of dot formation position may be, for example, ambient temperature, ambient humidity, elapsed time from the time of manufacture of the image forming apparatus, and elapsed time from the previous time of performing an image forming process. Accordingly, the factors can be detected by providing the image forming apparatus 1000 with a part for detecting the ambient temperature, ambient humidity (e.g., temperature sensor 215) and a part for counting the time elapsed from the time of manufacturing the image forming apparatus or the previous time of performing an image forming process and updating the factors by storing them in a non-volatile memory (RAM) 204 of the control part 200. Furthermore, the actual amount (degree) of a dot deviating from a target dot formation position on a medium may also be detected. For example, in detecting the actual amount of dot formation position deviation, first, a dot pattern enabling detection of the amount of deviation from the dot formation position during an actual image forming process (e.g., a ruled pattern formed with different size dots) is printed (output) on a medium. Then, dot formation data (impact precision data), density data, and brightness data are obtained by reading the printed dot pattern with, for example, a photo-sensor or a scanning unit. Then, the CPU 201 calculates how much a dot (s) is deviated from a target dot formation position (deviation amount) based on the obtained data. Alternatively, in a case where the configuration for detecting the actual deviation amount becomes too complicated, the user may input data related to the deviation amount. Likewise, a dot pattern enabling detection of the amount of deviation from the dot formation position during an actual image forming process is printed (output) on a medium. Then, the user determines the deviation amount by examining the output dot pattern and inputs data related to the determined deviation amount. For example, the user has the deviation amount divided into multiple levels (first level to n level) beforehand. Then, the user determines the level to which the output dot pattern belongs. Then, the user inputs data (e.g., numeral) corresponding to the determined level via the control panel 214. Next, a relationship between dot formation position deviation and resolution and a relationship between medium type (paper type) and image quality are described. As described above, in general, correction of the outline part (particularly, the stepwise part of the outline part) of an image is required in a case where resolution is low. Meanwhile, in a case of high resolution, correction of the outline part of an image may not be required since a substantially satisfactory image quality can be attained. Furthermore, jagged areas become more noticeable at the stepwise part as resolution becomes lower. Therefore, it is preferable to change the method of correcting the outline part (method of dot replacement) according to the resolution of the image to be formed. Furthermore, deviation of dot formation position becomes more noticeable (thereby, regarded as having poor image quality) in a case of a type of medium having less liquid bleeding (blurring) characteristics. For example, even if the deviation amount of the dot formation position is the same, impact deviation has a direct influence on image quality in a case of using glossy paper. This is because glossy paper has high contrast and exhibits little blurring of impact droplets. Therefore, it is preferable to change the method of correcting the outline part (method of dot replacement) according to the type to medium. Next, in view of the above, an exemplary outline correction operation (method) is described with reference to the flowchart of Fig.19. This outline correction operation is executed by having a CPU 201 execute a program stored in a ROM 202. In this outline correction operation (method), a correction pattern is prepared beforehand for performing pattern matching with respect to an outline part of an image. Then, in a case where there is a match between the outline part of an image and the correction pattern as a result of the pattern matching process, outline correction is performed by replacing a dot(s) surrounding a stepwise part of the outline with a dot(s) having a size designated by the matching correction pattern. It is to be noted that the outline correction process performed by dot replacement includes replacing a blank dot with an image dot and modifying the size of the image dot. More specifically, in the outline correction process, first, the CPU 201 determines whether the resolution of the printing mode is no greater than a predetermined resolution for determining whether correction of the outline part is necessary. As described above, in a case where printing resolution is high enough that jagged areas are hardly noticeable, the CPU 201 does not need to execute the outline correction process. As an alternative of or in addition to the step of determining the resolution, a step of determining the type of medium can be performed for determining whether the outline correction process is necessary. In a case where the outline correction process is determined necessary, the factors that cause dot formation position deviation (hereinafter also referred to as "dot deviation data") are obtained. As described above, the dot deviation data may include ambient temperature, ambient humidity, elapsed time from time of manufacturing the image forming apparatus, elapsed time from previous time of executing an image forming process, actual deviation amount based on an output dot pattern, or data related to the deviation amount input by the user. Then, the obtained dot deviation data are compared with a selection condition (s) for selecting a correction pattern including predetermined resolution data (or medium type data) . Then, a correction pattern is selected based on the comparison result. Then, an outline correction process is executed by using the selected correction pattern. It is to be noted that, the outline correction process (dot replacement process) may not be executed in a case where there is no correction pattern that satisfies the selection condition. For example, as exemplarily shown in the table of Fig. 20, a correction pattern can be selected based on a combination of resolution and ambient temperature. More specifically, in this example, outline correction is not performed regardless of the ambient temperature when the resolution is equal to or greater than 600 dpi X 600 dpi. Furthermore, pattern A or B is selected according to resolution when the temperature T is less than a predetermined temperature Tl. Furthermore, pattern C or D is selected according to resolution when the temperature T is no less than the predetermined temperature Tl but less than a predetermined temperature 12. Furthermore, pattern E or F is selected according to resolution when the temperature T is no less than the predetermined temperature T2. Generally, in terms of ambient temperature, ink viscosity tends to increase and jetting failure of small droplets tends to occur when the ambient temperature of the image forming apparatus 1000 is low. Therefore, it is preferable to select a correction pattern that does not use small droplets or a correction pattern having a reduced small droplet usage rate (Pattern A, B) . In a case of normal temperature, jetting failure hardly occurs. Therefore, a correction pattern using every type (size) of dot can be selected (Pattern C, D) . In a case of high temperature, problems such as decrease of ink viscosity, increase of ink consumption, increase of jetting speed, scattering of ink in a mist-like manner upon impact, or trailing dots upon impact tend to occur. Therefore, a correction pattern using droplets which do not cause such problems or a correction pattern using less ink can be selected Pattern E, F) . In addition, a further preferable correction pattern can be selected according to printing conditions (e.g., resolution, type of medium). It is to be noted that a correction pattern can be selected based on the deviation amount of dot formation position and/or factors that cause deviation of dot formation position. Initially, correction patterns that cause little deviation according to various printing conditions (e.g., resolution, type of paper) are prepared first hand. However, in a case where a dot of a certain size may cause unsatisfactory correction results, a correction pattern using dots of the certain size is avoided and an alternative correction pattern is selected (a correction pattern having the most effective correction performance without using dots of the certain size) . In a case where an effective correction effect is unlikely to be attained by selecting any one of the correction patterns, a correction pattern which performs no out line correction may be selected. Although one example of changing the method of correction is described as changing the correction pattern by preparing multiple correction patterns corresponding to. factors that cause dot formation position deviation and selecting a correction pattern based on the detection results of the factors, other examples of changing the correction method may be used. For example, instead of changing the correction pattern, the size of the dots for performing dot replacement can be changed by comparing the factors with a predetermined condition(s) according to a pattern matching result. Hence, by providing a part for changing the method of replacing the dots forming an outline part of an image based on factors that affect dot formation position or the amount of deviation from a dot formation position on a medium (detection data or data input from outside) , the method of the dot replacement (correction method) can be changed when dot formation position deviation occurs so that outline correction can be performed without being adversely affected by the dot formation position deviation. Thereby, image quality can be prevented from degrading. Furthermore, by providing a program enabling a computer to execute a process for changing the method of replacing the dots forming an outline part of an image based on factors that affect dot formation position or the amount of deviation from a dot formation position on a medium (detection data or data input from outside), the method of the dot replacement (correction method) can be changed when dot formation position deviation occurs so that outline correction can be performed without being adversely affected by the dot formation position deviation. Thereby, image quality can be prevented from degrading. Furthermore, by installing the program on a computer-readable recording medium 500, a program enabling the method of the dot replacement (correction method) to be changed when dot formation position deviation occurs can be provided so that outline correction can be performed without being adversely affected by the dot formation position deviation. Thereby, image quality can be prevented from degrading. Furthermore, by providing an image forming method for changing the method of replacing the dots forming an outline part of an image based on factors that affect dot formation position or the amount of deviation from a dot formation position on a medium (detection data or data input from outside), the method of the dot replacement (correction method) can be changed when dot formation position deviation occurs so that outline correction can be performed without being adversely affected by the dot formation position deviation. Thereby, image quality can be prevented from degrading. It is to be noted that the image forming apparatus 1000 according to an embodiment of the present invention may include a function to display the status of the image formina apparatus 1000 or a method of improving the image forming apparatus 1000 to the user or administrator of the image forming apparatus 1000 via an apparatus (e.g., display apparatus, host computer (data processing apparatus)) connected to the image forming apparatus 1000 based on various data such as temperature, humidity, and dot impact data (data related to deviation of dot formation position). For example, in a case where an effective outline correction performance (e.g., jaggy reduction effect) cannot be realized even by applying the correction patterns stored beforehand in the image forming apparatus 1000 due to temperature, humidity, dot impact data (data related to deviation of dot formation position) , or other factors that cause deviation of dot formation position, the image forming apparatus 1000 may be informed with data related to the inability to realize an effective correction performance via the control panel 214 of the image forming apparatus 1000 or a printer driver of a data processing apparatus on the host side. For example, the data related to the inability to realize an effective correction performance may include data notifying that the image forming apparatus 1000 is in such state, data indicating ambient/printing conditions (ambient temperature, ambient humidity, resolution, deviation amount) , data indicating the status of the image forming apparatus 1000, data including information for improving the performance of the image forming apparatus 1000 such as messages advising change of ambient/printing conditions, maintenance information (e.g., head cleaning, head adjustment), and help/support service information. With the above-described display function, the user can be informed (persuaded) to adjust various settings and ambient conditions and perform other adjustment (e.g., calibration) for improving the performance of the image forming apparatus 1000 including elimination of dot formation position deviation according to various conditions. The image forming apparatus 1000 may also include a function to report the above-described data related to the inability to realize an effective correction performance to a support service of the image forming apparatus 1000. With the reporting function, the support service can promptly and automatically detect failure of the image forming apparatus 1000. Thus, owing to the above-described functions, not only can the image forming apparatus 1000 achieve optimum outline correction according to various conditions but can also receive various support for maintaining satisfactory performance. Although the above-described image forming apparatus is explained having a configuration of a printer, the present invention may also be applied to an image forming apparatus having a configuration of a facsimile machine, a plottina apparatus, a copier, or a multifunction machine having the functions of a printer,- a facsimile, and a copier, for example. In the above-described embodiments of the present invention, a dot (pixel) of an image is not limited to a dot (pixel) that is formed by a single type of recording liquid (ink) . That is, a dot of an image may be formed by forming plural types of recording liquid (ink) to a single position. Therefore, a dot (pixel) can be formed by jetting one or more droplets to a single position on a medium. Furthermore, in the above-described embodiments of the present invention, unless described as otherwise, brightness (dot brightness) refers to the brightness that is defined by a dot itself. That'is, brightness does not include brightness of a covered area when viewed from a broad (macro) perspective unless described as otherwise. Furthermore, in the above-described embodiments of the present invention, a character may be a letter, a symbol, or a thin line. For example, a black character may include a thin black line. Furthermore, in the above-described embodiments of the present invention, the term "change" may include not only the meaning "change" but also a meaning "replace", "correct", or "correct". Furthermore, in the above-described embodiments of the present invention, among the dots that form an outline part, the dots which are not corrected (e.g., corrected by change of dot brightness) may be described as other dots forming an outline part, other dots of an outline part, or other dots situated in an outline part. Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention. The present application is based on Japanese Priority Application Nos.2006-252046 and 2006-252053 both filed on September 19, 2006, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference. CLAIMS 1. An image forming apparatus for forming an image on a medium by jetting one or more droplets on the medium, the image forming apparatus comprising: a dot brightness changing part for changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part. 2. The image forming apparatus as claimed in claim 1, wherein the target dot is formed with a size smaller than the size of other dots forming the outline part. 3. The image forming apparatus as claimed in claim 1, wherein when the image is a black image consisting of only black color, the target dot is formed with a size smaller than the size of other dots forming the outline part. 4. The image forming apparatus as claimed in claim 1, wherein when the image is a black image consisting of only black color, the dots of the image are formed by a plurality of black recording liquids having different brightness. 5. The image forming apparatus as claimed in claim 1, wherein when the image is a black image consisting of only black color, the dots of the image are formed by a black recording liquid and a recording liquid having a color other than black. 6. The image forming apparatus as claimed in claim 1, wherein the dot brightness of the target dot is changed by changing a correction pattern. 7. The image forming apparatus as claimed in claim 1, wherein the target dot is formed in a position deviating from a target dot formation position in a nozzle array direction or a direction perpendicularly intersecting the nozzle array direction. 8. A program for causing a computer to execute an image forming process for forming an image on a medium by jetting one or more droplets on the medium, the program comprising a step of: changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part. 9. The program as claimed in claim 8, wherein the target dot is formed with a size smaller than the size of other dots forming the outline part. 10. The program as claimed in claim 8, wherein when the image is a black image consisting of only black color, the target dot is formed with a size smaller than the size of other dots forming the outline part. 11. The program as claimed in claim 8, wherein when the image is a black image consisting of only black color, the dots of the image are formed by a plurality of black recording liquids having different brightness. 12. The program as claimed in claim 8, wherein when the image is a black image consisting of only black color, the dots of the image are formed by a black recording liquid and a recording liquid having a color other than black. 13. The program as claimed in claim 8, wherein the dot brightness of the target dot is changed by changing a correction pattern. 14. The program as claimed in claim 8, wherein the target dot is formed in a position deviating from a target dot formation position in a nozzle array direction or a direction perpendicularly intersecting the nozzle array direction. 15. A computer-readable recording medium for causing a computer to execute an image forming process for forming an image on a medium by jetting one or more droplets on the medium, the computer-readable recording medium comprising: the program as claimed in claim 8. 16. An image forming method for forming an image on a medium by jetting one or more droplets on the medium, the image forming method comprising a step of: changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part. 17. An image forming apparatus for forming an image on a medium by jetting one or more droplets on the medium, the image forming apparatus comprising: a dot brightness changing part for changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part; a dot size changing part for changing a dot size of at least the target dot forming the outline part of the image to a dot size different from the dot size of the other dots forming the outline part; wherein at least one of the dot brightness and the dot size of the target dot is changed according to the color of the image by the dot brightness changing part and the dot size changing part. 18. An image forming apparatus for forming an image including one or more dots on a medium by jetting one or more droplets on the medium, the image forming apparatus comprising: an outline correction part for correcting at least one target dot forming an outline part of the image by replacing the target dot with a replacement dot having a dot size different from the dot size of the target dot; and a replacement dot changing part for changing the method of replacing the target dot according to a factor causing deviation of dot formation position or deviation amount of the dot formation position. 19. The image forming apparatus as claimed in claim 18, wherein the factor causing deviation of dot formation position include at least one of ambient temperature, ambient humidity, lapsed time from manufacture of the image forming apparatus, and lapsed time from previous an image forming process. 20. The image forming apparatus as claimed in claim 18, further comprising: a deviation amount detecting part for detecting the deviation amount of the dot formation position. 21. The image forming apparatus as claimed in claim 18, wherein data related to the deviation amount of the dot formation position are input from outside of the image forming apparatus. 22. The image forming apparatus as claimed in claim 18, wherein the method of replacing the target dot is changed according to printing condition of the image forming apparatus. 23. The image forming apparatus as claimed in claim 22, wherein the printing condition includes at least one of resolution and type of medium. 24. The image forming apparatus as claimed in claim 18, wherein the replacement dot changing part is configured to form the replacement dot by using a droplet relatively difficult to cause deviation of dot formation position or by using less droplets that tend to cause deviation of dot formation position. 25. The image forming apparatus as claimed in claim 18, wherein the changing of the method of replacing the target dot includes not replacing the target dot. 26. A program for causing a computer of an image forming apparatus to execute an image forming process for forming an image including one or more dots on a medium by jetting one or more droplets on the medium, the program comprising the steps of: correcting at least one target dot forming an outline part of the image by replacing the target dot with a replacement dot having a dot size different from the dot size of the target dot; and changing the method of replacing the target dot according to a factor causing deviation of dot formation position or deviation amount of the dot formation position. 27. The program as claimed in claim 26, wherein the factor causing deviation of dot formation position include at least one of ambient temperature, ambient humidity, lapsed time from manufacture of the image forming apparatus, and lapsed time from previous an image forming process. 28. The program as claimed in claim 26, further comprising a step of: detecting the deviation amount of the dot formation position. 29. The program as claimed in claim 26, wherein data related to the deviation amount of the dot formation position are input from outside of the image forming apparatus. 30. The program as claimed in claim 26, wherein the method of replacing the target dot is changed according to printing condition of the image forming apparatus. 31. The program as claimed in claim 30, wherein the printing condition includes at least one of resolution and type of medium. 32. The program as claimed in claim 26, wherein the replacement dot changing step is for forming the replacement dot by using a droplet relatively difficult to cause deviation of dot formation position or by using less droplets that tend to cause deviation of dot formation position. 33. The program as claimed in claim 26, wherein the changing of the method of replacing the target dot includes not replacing the target dot. 34. A computer-readable recording medium for causing a computer to execute an image forming process for forming an image including one or more dots on a medium by jetting one or more droplets on the medium, the computer-readable recording medium comprising: the program as claimed in claim 26. 35. An image forming method for forming an image including one or more dots on a medium by jetting one or more droplets on the medium, the image forming method comprising the steps of: correcting at least one target dot forming an outline part of the image by replacing the target dot with a replacement dot having a dot size different from the dot size of the target dot; and changing the method of replacing the target dot according to a factor causing deviation of dot formation position or deviation amount of the dot formation position. The present invention discloses an image forming apparatus for forming an image on a medium by jetting one or more droplets on the medium. The image forming apparatus includes a dot brightness changing part for changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part.

Full Text

DESCRIPTION
IMAGE FORMING APPARATUS, IMAGE FORMING METHOD, RECORDING MEDIUM,
AND PROGRAM
TECHNICAL FIELD
The present invention relates to an image forming apparatus,
an image forming method, a recording medium, and a program.
BACKGROUND ART
Typically, an image forming apparatus
(multi-function machine) , which has one or more of the functions
of a printer, a facsimile machine, or a copier, forms images (image
forming) by conveying a medium (hereinafter also referred to as
"sheet" or "paper") and jetting a liquid (hereinafter also
referred to as "recording liquid" or "i nk") onto the conveyed paper
by using a liquid jetting apparatus having a recording head
(including one or more liquid jet heads) from which liquid (ink)
droplets are ejected (jetted) . It is to be noted that, although
the medium is hereinafter referred to as "sheet" or "paper", the
material of the medium is not to be limited only to those produced
by manufacturing paper. The medium may include, for example, a
paper material, a textile material, a fiber material, a fabric
material, a leather material, a metal material, a plastic material,
a glass material, a wood material, or a ceramic material. The
medium may be, for example, a recording medium (recording paper)

or a transfer material (transfer paper) . It is to be noted that
"image forming" has substantially the same meaning as "recording"
or "printing". For example, "image forming" includes forming
images having meaning (e.g. characters, figures, symbols) and also
image having no particular meaning (e.g. patterns) . Furthermore,
the liquid used for an image forming apparatus is not limited to
a recording liquid or ink and may be other kinds of liquid as long
as images can be formed with the liquid. The image forming
apparatus may include, for example, a serial type image forming
apparatus which forms images by scanning a liquid jet head mounted
on a carriage or a line type image forming apparatus having a line
type liquid jet head.
The serial type image forming apparatus has a printing
speed which is determined according to conditions such as image
resoluticn nozzle density , dot formation driving frequency and
sub-scanning speed. Among such conditions, nozzle density is
constrained by the precision in manufacturing the nozzles,- the
liquid chamber, the flow path, and the actuator of the liquid jet
head of the image forming apparatus. Particularly, in a case
where a liquid jet head uses a piezoelectric element, the only
ways to separately form channels corresponding to the nozzles of
the liquid jet head are to use a mechanical method (e.g., dicing)
or form a thin-film PZT by printing. Accordingly, the nozzle
density is relatively low compared to a thermal type liquid jet
head fabricated by a semiconductor process. The upper limit of

nozzle density of the piezoelectric type liquid jet head is
currently approximately 360 dpi.
Meanwhile, in order to improve printing speed, it is
preferable to form a printing area by scanning the liquid jet head
in the main scanning direction in a single time. For example,
in a case of forming an image with a resolution of 300 dpi in the
sub-scanning direction by using a liquid jet head having a nozzle
density of 300 dpi, it is possible to form the image by moving
the liquid jet head in the main scanning direction in a single
time. In a case of forming an image with a resolution of 600 dpi,
the image is to be formed by using an interlace method (in this
case, scanning two times in the main scanning direction and one
time in the sub-scanning direction (sheet conveying direction) ) .
Thus, it is apparent that the non-interlace method which forms
an immage with a single scan has greater printing speed than the
interlace method. Furthermore, as for methods of forming a single
line in the main scanning direction, there are a single pass
printing method that forms an image(s) by a single pass in the
main scanning direction and a multi-pass printing method that
forms an image (s) by plural passes in the main scanning direction.
It is apparent that the single pass printing method has greater
printing speed than the multi-pass printing method.
However, in a case where an image is formed by using
a single pass/non-interlace method, the resolution of the image
is inevitably low. In order to improve image quality in a case

where resolution of the image is low, it is effective to use
multiple values (multi-values) for a single pixel. As for methods
of using multi-values, there is, for example, a method of changing
the size of a single dot, a method of forming a single pixel by
jetting plural small dots, or a method of changing the density
of ink.
Although the method of acquiring high quality images
by using multi-values is effective for images of pictures and
photographs, no significant effects can be attained for images
of graphics and characters (letters) . This is due to the graphics
and characters requiring a dot size large enough to fill the
background areas of an image, that is, characters or graphic images
become low density when small size dots are used. Therefore,
binary images such as character images/graphic images face
inherent problems of low resolution Part icularly, in a case
where characters (letters) are expressed with binary images, the
quality of the characters deteriorates such that the characters
become illegible.
Besides a method of improving the nozzle density of
a recording head for improving the resolution of the recording
head, there is a method of assembling nozzles in a misaligned
manner so that the apparent resolution will be improved. Both
methods, however, cannot avoid the increase of manufacturing costs
of the recording head. Furthermore, the improvement of
resolution increases the workload for processing the image data

and complicates the control system of the image forming apparatus,
to thereby lead to an increase in the overall cost of the image
forming apparatus. These problems become particularly
noticeable for a recording head provided with an extended length
for increasing printing speed. Furthermore, the problem of low
resolution becomes more serious for a line printer since the line
printer, unlike the serial printer, is unable to use the interlace
method and the multi-pass method such that the printing resolution
becomes fixed according to the nozzle resolution of the recording
head.
Accordingly, in order to obtain satisfactory images
while maintaining low manufacturing costs and high speed printing,
it is important to attain a satisfactory image quality within a
limited recording head resolution.
Next, the inherent low RESOLUTION problem is
described in further detail. The images recorded by using a
liquid jetting method are expressed by using dots formed in a
matrix manner in a recording head scanning direction and in a sheet
conveying direction that perpendicularly intersects the
recording head scanning direction. In a case where a character
(letter) is printed as a dot image, the quality of the character
significantly differs depending on the resolution of the printed
image. For example, in a case where a character of the same size
is printed with 300 dpi and 600 dpi, the ratio in the number of
dots comprising the character is approximately four times.

Therefore, finer details can be expressed and character quality
is higher by printing with 600 dpi. Particularly, in a case of
expressing a diagonal part (slanted part) of a character, the
number of dots increases/decreases in a stepwise manner according
to resolution. Therefore, indentation (jaggy) becomes more
noticeable when printing with 300 dpi.
As a method of reducing the jaggy appearing at the
outline upon low resolution printing, there is a smoothing method
referred to as anti-aliasing. The anti-aliasing method can
perform smoothing with high precision since an outline can be
expressed by using dots of many tones (scales). However, the
anti-aliasing method is very complicated and requires a large
amount of processing time. Therefore, the anti-aliasing method
is not suitable for recent image forming apparatuses (e.g., ink jet
printer) that are expected to provide high throughput
Accordingly, Japanese Registered Patent No. 2886192
(hereinafter referred to as "Patent Document 1") discloses a
method of comparing a bit pattern of a sample window in a character
bitmap image and a predetermined bit pattern and correcting the
main pixels in the sample window into small dots when the bit
patterns match.
Furthermore, Japanese Registered Patent No. 3029533
(hereinafter referred to as "Patent Document 2") discloses a
method of determining an outline part of an image from black dot
data and reducing the size of printed dots other than edge dots

and black dots.
Furthermore, Japanese Laid-Open Patent Application
No. 2003-334938 (hereinafter referred to as "Patent Document 3")
discloses a method of reducing jaggy at an outline part by changing
the size of dots of the outline part according to the inclination
of the outline and forming dots at a blank part surrounding the
outline part.
Furthermore, Japanese Laid-Open Patent Application
No. 2004-114303 (hereinafter referred to as "Patent Document 4")
discloses a method of expressing a stepwise surrounding part of
the dots forming an outline part of an image by using dots smaller
than the dots forming the image.
Furthermore, Japanese Laid-Open Patent Application
No. 2004-017552 (hereinafter referred to as "Patent Document 5")
diauloses a method of performing a smoothing process using small
dots when image data of a character or graphics is black and
refraining from performing the smoothing process when the image
data is not black.
Furthermore, Japanese Laid-Open Patent Application
No. 2004-017546 (hereinafter referred to as "Patent Document 6")
discloses a method of converting data so that dots of a stepwise
surrounding part of the dots forming an outline part of a character
or graphics are expressed as dots smaller than the dots other than
those of the stepwise surrounding part and changing the data
converting method according to the inclination of the outline

part.
.Furthermore, Japanese Laid-Open Patent Application
No. 2005-193384 (hereinafter referred to as "Patent Document 7")
discloses a method including a step of detecting a stepwise
changing part of dots forming an outline part of a character and/or
graphics on which a half tone process is performed and a step of
converting dot data of the dots surrounding the stepwise changing
part detected in the detection step into dot data of dots having
a size no greater than those of the stepwise changing part, wherein
the converting method of the data conversion step is different
depending on the inclination of the outline part.
Furthermore, Japanese Laid-Open Patent Application
No. 2002-166603 (hereinafter referred to as "Patent Document 8")
discloses a method of obtaining the difference of density between
designated pixels forming a target printing character and pixels
surrounding the designated pixels, determining that the
designated pixels are pixels of the outline part of the character
when there are predetermined number of pixels having greater
density with respect to the predetermined pixels, and coloring
the determined designated pixels with a black color member
comprising a mixture of a cyan, magenta, and yellow color members.
Furthermore, Japanese Registered Patent No. 3244411
(hereinafter referred to as "Patent Document 9" discloses a method
including a binary character image-data generation step for
generating binary character image data by setting the output

concentration in a pixel used as the inside of a profile
configuration as a 1st concentration when the profile
configuration concerned is drawn based on the outline data
corresponding to the profile configuration of the character which
should be output on the pixel coordinates that specify the pixel
of an output unit and setting the output concentration in the pixel
used as the outside of the profile configuration as a 2nd
concentration different from said 1st concentration, an auxiliary
line data extraction step for extracting the partial borderline
which fulfills predetermined conditions as auxiliary line data
from one or more partial border lines which constitute the outline
data; a gradation image-data generation step for generating
gradation image data by setting the output concentration of the
pixel having a positional relationship fulfilling a predetermined
criteria with respect to the drawn partial borderline as the 3rd
concentration existing in the middle of the 1st and 2nd
concentration when the partial borderline corresponding to the
auxiliary line data is drawn on the pixel coordinate, and a
compositing step for generating a character image data by
compositing binary character image data and gradation image data.
The methods disclosed in Patent Documents 1 and 2 are
effective for LED printers and laser printers . Since the particle
diameter of the toner used by the LED printers and laser printers
is no greater than 10 μm, the LED printers and laser printers
hardly exhibit ink spreading on plain paper and are able to form

small dots as designated. Furthermore, the LED printers and laser
printers can form dots of designated sizes at optimum locations
by slightly adjusting the laser irradiation position and the laser
irradiation length.
However, a liquid jet type image forming apparatus
exhibits greater ink spreading compared to a laser printer.
Furthermore, since the liquid jet type image forming apparatus,
which changes dot size by modifying the number or length of the
drive pulses in a drive period, requires more time to form dots
compared to LED printers and laser printers, it is difficult for
the liquid jet type image forming apparatus to change the dot size
into many different types. That is, the liquid jet type image
forming apparatus can change dots to only a few different sizes.
Due to a similar reason, the dots of the liquid jet type image
forming apparatus arc formed in a substantially fixed location
(position) inside a single pixel. That is, unlike the LED printer
or the laser printer, it is difficult for the liquid jet type image
forming apparatus to freely change the dot formation location
(position) inside a single pixel.
As described above, although there are various
methods for performing outline correction by using dots of
different sizes, the sizes of the dots are limited to more or less
than three types even for a multi-value printer capable of forming
dots of different sizes'. Furthermore, the types of dot size that
can be used for correction could be further limited depending on,

for example, character correction, covered area of the medium
(paper), or gradation property. Furthermore, since dots are
formed at the center of (the address of) each pixel determined
by printing resolution, correction might not be satisfactorily
achieved. Moreover, in a case where small dots are used for
reducing brightness, spaces are easily formed between the
character framework and the correcting dot. Such space or
deviation may degrade the quality of a character.
Furthermore, although outline correction can be
performed by changing density with use of composite black, the
amount of ink adhered on a single pixel increases due to composite
black being created by blending cyan (C) , magenta (M), and yellow
(Y) ink. This causes bleeding of ink, particularly, when plain
paper is used and results to degradation of image quality.
Furthermore, the increase in the amount of ink also increases the
time required for drying (reduction of productivity) and also
raises ink cost. Furthermore, undesired color areas may be formed
in a case where a target impact position for one color ink deviates
from a target impact position of another color ink. This also
becomes a cause for degrading the quality of a character. Since
the kinds of ink that can be mounted in a typical inkjet recording
apparatus are approximately four to eight colors, the color that
can express black is generally limited to K (K pigment, K dye,
or light black) and a CMY composite. Therefore, little variation
can be made in the density of ink. In addition, correction of

color characters is difficult (similar color ink of different
density is required).
From another aspect, although the above-described
Patent Document 3 discloses an inkjet recording method capable
of reducing jaggy by changing dot size, the changing of dot sizes
may cause dots to be formed at undesired positions on a medium.
This problem is described in detail below.
A liquid jet head used in a liquid jet type image
forming apparatus includes a pressure generating part for applying
pressure to a liquid (ink) inside a liquid chamber. For example,
the pressure generating part may be a thermal type head using a
thermal type head which uses a heating resistor for generating
bubbles or a piezoelectric type head which uses a piezoelectric
element (electromechanical transducer) for changing the wall of
the "liquid chamber. In oder to chage the dot diameter, it is
common to use a method of changing the energy applied to the
pressure generating part. For example, this method may be
changing the driving voltage of the pressure generating part,
changing the pulse width of the drive pulse, or changing the pulse
number of the drive pulse.
Among these methods, the method of changing the drive
voltage requires different signals corresponding to different
drive voltages and plural switching parts for selectively
switching for each channel corresponding to different drive
voltages. Thereby, the driving element (driver IC) becomes

larger in correspondence with the increase in the types of drive
voltages. In a case of controlling (changing) pulse width or
number of pulses, the pulse width and number of pulses can be
changed by controlling the switching part based on time. In this
case, a single switching part for each channel would be needed.
Such pulse width modulation method or pulse number modulation
method are used particularly by an image forming apparatus using
a piezoelectric head.
However, the amount of ink differs depending on the
pulse width modulation method or the pulse number modulation
method. That is, in forming ink droplets having different dot
diameter, since the length of the drive pulse is different, the
timing for jetting ink droplets (jet upon end of drive pulse) in
accordance with the drive pulse is different even if the timing
of the start of the rise of the meniscus is the same upon input
of the drive pulse. Accordingly, the time for the droplets to
reach the medium surface is different- As a result, the dot
formation area on the medium differs depending on the dot size.
Thus, even in a case of attempting to improve image quality by
replacing (correcting) the dots at the outline part with small
dots, it is difficult to provide satisfactory images since the
small dots cannot be formed at desired areas of the medium.
Furthermore, the deviation of dot formation position is also
caused by viscosity resistance of ink due to ambient conditions
(e.g., temperature, humidity). Furthermore, the deviation of dot

formation location differs depending on image formation
conditions (e.g., type of paper, resolution).
DISCLOSURE OF INVENTION
It is a general object of the present invention to
provide an image forming apparatus, an image forming method, a
recording medium, and a program that substantially obviate one
or more of the problems caused by the limitations and disadvantages
of the related art.
Features and advantages of the present invention are
set forth in the description which follows, and in part will become
apparent from the description and the accompanying drawings, or
may be learned by practice of the invention according to the
teachings provided in the description. Objects as well as other
features and advantages of the present invention can be realized
and attained by an image forming apparatus, an image forming method,
a recording medium, and a program particularly pointed out in the
specification in such full, clear, concise, and exact terms as
to enable a person having ordinary skill in the art to practice
the invention.
To achieve these and other advantages and in
accordance with the purpose of the invention, as embodied and
broadly described herein, an embodiment of the present invention
provides an image forming apparatus for forming an image on a
medium by jetting one or more droplets on the medium, the image

forming apparatus including: a dot brightness changing part for
changing a dot brightness of at least one target dot forming an
outline part of the image to a brightness relatively greater than
the dot brightness of other dots forming the outline part.
Furthermore, another embodiment of the present
invention provides a program for causing a computer to execute
an image forming process for forming an image on a medium by jetting
one or more droplets on the medium, the program including a step
of: changing a dot brightness of at least one target dot forming
an outline part of the image to a brightness relatively greater
than the dot brightness of other dots forming the outline part.
Furthermore, another embodiment of the present
invention provides a computer-readable recording medium for
causing a computer to execute an image forming process for forming
an image on a medium by jetting one or more droplets on the medium,
the computer-readable recording medium including: the program
according to the embodiment of the present invention.
Furthermore, another embodiment of the present
invention provides an image forming method for forming an image
on a medium by jetting one or more droplets on the medium, the
image forming method including a step of: changing a dot brightness
of at least one target dot forming an outline part of the image
to a brightness relatively greater than the dot brightness of other
dots forming the outline part.
Furthermore, another embodiment of the present

invention provides an image forming apparatus for forming an image
on a medium by jetting one or more droplets on the medium, the
image forming apparatus including: a dot brightness changing part
for changing a dot brightness of at least one target dot forming
an outline part of the image to a brightness relatively greater
than the dot brightness of other dots forming the outline part;
a dot size changing part for changing a dot size of at least the
target dot forming the outline part of the image to a dot size
different from the dot size of the other dots forming the outline
part; wherein at least one of the dot brightness and the dot size
of the target dot is changed according to the color of the image
by the dot brightness changing part and the dot size changing part.
Furthermore, another embodiment of the present
invention provides an image forming apparatus for forming an image
including one or more dots on a medium by jetting one or more
droplets on the medium, the image forming apparatus including:
an outline correction part, for correcting at least one target dot
forming an outline part of the image by replacing the target dot
with a replacement dot having a dot size different from the dot
size of the target dot; and a replacement dot changing part for
changing the method of replacing the target dot according to a
factor causing deviation of dot formation position or deviation
amount of the dot formation position.
Furthermore, another embodiment of the present
invention provides a program for causing a computer of an image

forming apparatus to execute an image forming process for forming
an image including one or more dots on a medium by jetting one
or more droplets on the medium, the program including the steps
of: correcting at least one target dot forming an outline part
of the image by replacing the target dot with a replacement dot
having a dot size different from the dot size of the target dot;
and changing the method of replacing the target dot according to
a factor causing deviation of dot formation position or deviation
amount of the dot formation position.
Furthermore, another embodiment of the present
invention provides a computer-readable recording medium for
causing a computer to execute an image forming process for forming
an image including one or more dots on a medium by jetting one
or more droplets on the medium, the computer-readable recording
medium including: the program according to an embodiment of the
present invention.
Furthermore, another embodiment of the present
invention provides an image forming method for forming an image
including one or more dots on a medium by jetting one or more
droplets on the medium, the image forming method including the
steps of: correcting at least one target dot forming an outline
part of the image by replacing the target dot with a replacement
dot having a dot size different from the dot size of the target
dot; and changing the method of replacing the target dot according
to a factor causing deviation of dot formation position or

deviation amount of the dot formation position.
BRIEF DESCRIPTION OF DRAWINGS
Fig.l is a side view for describing an overall
configuration of an image forming apparatus including a program
for performing an image forming method according to an embodiment
of the present invention;
Fig.2 is a plan view of an image forming apparatus
according to an embodiment of the present invention;
Fig. 3 is a cross-sectional view along a longitudinal
direction of a liquid chamber of a recording head according to
an embodiment of the present invention;
Fig.4 is a cross-sectional view along a lateral
direction of the liquid chamber of the recording head according
to an embodiment of the present invention;
Fig.5 is a block diagram showing a control part of
an image forming apparatus according to an embodiment of the
present invention;
Fig.6 is a block diagram showing an example of a
printing control part of an image forming apparatus according to
an embodiment of the present invention:
Fig. 7 is a schematic diagram for describing an example
of a drive waveform generated and output by a drive waveform
generating part of a printing control part according to an
embodiment of the present invention;

Fig.8 is a schematic diagram for describing drive
signals for a small droplet, a medium droplet, a large droplet,
a fine drive selected according to a drive waveform according to
an embodiment of the present invention;
Fig.9 is a schematic diagram for describing
differences of output characters due to different resolution;
Fig.10 is a schematic diagram for describing an
example of performing jaggy correction (outline correction)
according to an embodiment of the present invention;
Fig.11 is a flowchart for describing an outline
correction process (method) according to an embodiment of the
present invention;
Fig.12 is an exemplary table used for assigning
correction patterns according to printing mode/resolution
according to an embodiment of the present invention;
Fig.13 is a schematic diagram for describing an
outline correction process (method) using a correction pattern
according to an embodiment of the present invention;
Fig.14 is a schematic diagram for describing jaggy
correction (outline correction) where dot formation position is
changed according to an embodiment of the present invention;
Fig.15 is a schematic diagram for describing an
example of performing jaggy correction (outline correction)
according to an embodiment of the present invention;
Fig. 16 is a schematic diagram for describing a state

before outline correction, the result of outline correction when
there is no deviation of dot formation position, and the result
of outline correction when there is deviation of dot formation
position;
Fig.17 is a schematic diagram for describing an
outline corrected character in a case where there is no deviation
of dot formation position and a case where there is deviation of
dot formation position;
Fig. 18 is a schematic diagram for describing a case
of applying a correction pattern according to an embodiment of
the present invention;
Fig.19 is a flowchart for describing a process
(method) for controlling an outline correction process (method)
according to an embodiment of the present invention; and
Fig.20 is an exemplary table used for selection of
correction patterns according to an embodiment of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is described in detail based
on the embodiments illustrated in the drawings.
First, an exemplary image forming apparatus 1000 is
below with reference to Figs. 1 and 2, where Fig. 1 is a side view
and Fig. 2 is a plan view of the image forming apparatus 1000.
The illustrated image forming apparatus 1000 has

guide members including a guide rod 1 and a guide rail 2. The
guide rod 1 and the guide rail 2 are mounted in traversed positions
between left and right side boards (not shown) of the image forming
apparatus. The guide rod 1 and the guide rail 2 hold a carriage
3 so that the carriage 3 can slide in the main scanning direction.
Amain scanning motor 4 drives the sliding movement of the carriage
3 via a timing belt 5 stretched between a driving pulley 6A and
a driven pulley 6B. Thereby, the carriage 3 is able to travel
(scan) in the arrow directions shown in Fig.2 (main scanning
direction).
The carriage 3 has a recording head (liquid jetting
head) 7 including, for example, four recording head parts 7y, 7c,
7m, and 7k for jetting ink droplets of yellow (Y) , cyan (C) , magenta
(M), and black (K) , respectively. The recording head 7 having
plural ink jetting holes aligned in a direction perpendicular to
the main scanning direction is attached to the carriage 3 so that
ink droplets can be jetted downward therefrom.
The recording head 7 may include a pressure generating
part that generates pressure used for jetting ink droplets from
the recording head 7 . For example, the pressure generating part
may be a thermal actuator which utilizes the pressure changes of
ink boiled by an electric heat converting element (e.g. heating
resistor), a shape-memory alloy actuator which utilizes the
changes of shape of an alloy in accordance with temperature, or
an electrostatic actuator utilizing static electricity.

Furthermore, the recording head 7 is not limited to
having plural recording head parts corresponding to each color.
For example, the recording head 7 may have plural ink jetting
nozzles for jetting ink of plural colors.
The carriage 3 also has a sub-tank 8 for supplying
ink of each color to the recording head 7. The sub-tank 8 is
supplied with ink from a main tank (i.e. ink cartridge, not shown)
via an ink supplying tube(s) 9.
The image forming apparatus also includes a sheet
feeding portion for feeding sheets of paper 12 stacked on a sheet
stacking part 11 of a sheet feed cassette 10. The sheet feeding
portion includes a separating pad 14 having a friction coefficient
sufficient for separating sheets of paper 12 from the sheet
stacking part and a sheet feeding roller 13 (in this example, a
half moon shaped roller) for conveying the sheets of paper 12 one
at a time from the sheet stacking part 11. The separating pad
14 is configured to urge the sheets in the direction toward the
sheet feeding roller 13.
The paper 12 conveyed from the sheet feeding part is
conveyed to an area below the recording head 7. In order to convey
the paper 12 to the area below the recording head 7, the image
forming apparatus is provided with a conveyor belt 21 that conveys
the paper 12 by attracting the paper 12 with electrostatic force;
a counter roller 22 and the conveyor belt 21 having the paper 12
delivered inbetween after receiving the paper 12 conveyed from

the sheet feeding part via a guide 15; a conveyor belt guide 23
for placing the paper 12 flat on the conveyor belt 21 by changing
the orientation of the paper 12 conveyed in a substantially upright
(perpendicular) position by an angle of approximately 90 degrees;
and a pressing member 24 for pressing a pressing roller 25 against
the conveyor belt 21. Furthermore, the image forming apparatus
includes a charging roller (charging part) 26 for charging the
surface of the conveyor belt 21.
In this example, the conveyor belt 21 is an endless
belt stretched between a conveyor roller 27 and a tension roller
28. A sub-scanning motor 31 rotates the conveyor roller 27 via
a timing belt 21 and a timing roller 33 so that the conveyor belt
21 is rotated in the belt conveying direction shown in Fig.2
(sub-scanning direction). It is to be noted that a guide member
29 is positioned at the backside of the conveyor belt 21 in
correspondence with a target image forming area of the recording
head 7. Furthermore,- the charging roller 26 is positioned
contacting the top surface of the conveyor belt 21 so that the
charging roller 26 rotates in accordance with the rotation of the
conveyor belt 21.
As shown in Fig. 2, the image forming apparatus also
includes a rotary encoder 36. The rotary encoder 36 includes a
slit disk 34 attached to a rotary shaft of the conveyor roller
27 and a sensor 35 for detecting a slit(s) formed in the slit disk
34.

The image forming apparatus also includes a sheet
discharging portion for discharging the sheet of paper 12 onto
which data are recorded by the recording head 7. The sheet
discharging portion includes a separating claw 51 for separating
the paper 12 from the conveyor belt 21, a first sheet discharging
roller 53, a second sheet discharging roller 53, and a sheet
discharge tray 54 for stacking the paper(s) 12 thereon.
Furthermore, a double-side sheet feeding unit (not
shown) may be detachably attached to a rear portion of the image
forming apparatus. By rotating the conveyor belt in the reverse
direction, the paper 12 is delivered to the double-side sheet
feeding unit so as to have the paper 12 flipped upside down. Then,
the flipped paper 12 is conveyed back to the part between the
counter roller 22 and the conveyor belt 21.
Furthermore, as shown in Fig.2, a nozzle recovery
mechanism 56 for maintaining/restoring the operating status of
the nozzle (s) may be provided at a non-printing area toward one
side (in this example, toward the back side) of the main scanning
direction of the carriage 3.
The nozzle recovery mechanism 56 includes, for
example, plural caps 57 for covering the surface of each of the
nozzles of the recording head 7, a wiper blade 58 for wiping off
residual ink from the surface of the nozzles, and an ink receptacle
59 for receiving accumulated ink that is jetted in a process of
disposing of undesired ink.

Accordingly, with the image forming apparatus having
the above-described configuration, sheets of paper 12 are
separated and conveyed sheet by sheet from the sheet feeding part,
then the separated conveyed paper 12 is guided to the part between
the conveyor belt 21 and the counter roller 22 in an upright manner
by the guide 15, and then the orientation of the conveyed paper
is changed approximately 90 degrees by guiding the tip part of
the paper with the conveyor guide 23 and pressing the paper 12
against the conveyor belt 21 with the pressing roller 25.
In this conveying operation, an AC bias supplying part
of a control part (not shown) of the image forming apparatus
alternately applies negative and positive alternate voltages to
the charging roller 26 in accordance with an alternate charging
pattern. Thereby, the conveyor belt 21 is alternately charged
with negative and positive voltages at intervals of a
predetermined width in accordance with the alternate charging
pattern. When the paper 12 is conveyed onto the charged conveyor
belt 21, the paper 12 is attracted to the conveyor belt 21 by
electrostatic force. Thus held, the paper 12 is conveyed in the
sub-scanning direction by the rotation of the conveyor belt 21.
Then, the recording head 7 jets ink droplets onto the
paper 12 while the paper 12 is being moved in correspondence with
the forward and backward movement of the carriage 3. After the
recording head 7 records (prints) a single row by jetting ink in
accordance with image signals, the paper 12 is further conveyed

a predetermined distance for recording the next row. The
recording operation of the recording head 7 is completed when a
signal is received indicative of the completion of the recording
operation or indicative of the rear end of the paper 12 reaching
the edge of the recording area. After the completion of the
recording operation, the paper is discharged to the discharge tray
54.
In a case of conducting double side printing, the
paper 12 is flipped upside down after the recording of the front
side (the side which is printed first) of the paper 12 is completed.
The paper 12 is flipped so that the back side of the paper is the
printing surface by rotating the conveyor belt 21 in reverse and
delivering the paper 12 to the double side sheet feeding unit (not
shown) . Then, the flipped paper 12 is conveyed to the part between
the counter roller 22 and the conveyor belt 21. After the paper
12 is placed on the conveyor belt 21, the recording head 7 conducts
the above-described recording operation on the back side of the
paper 12. After the recording operation is completed, the paper
12 is discharged to the discharge tray 54.
In a case where the image forming apparatus is
standing by to conduct a printing (recording) operation, the
carriage 3 is moved toward the recovery mechanism 56. The cap
57 covers the nozzle side of the recording head 7 to keep the
nozzles moist. This prevents poor jetting performance caused by
dried ink. Furthermore, where the cap covers the nozzle side of

the recording head 7, a recovery operation may be performed by
suctioning accumulated viscous ink (recording liquid) from the
nozzles and ejecting the ink and bubbles. Then, the wiper blade
58 wipes off the ink that has adhered to the nozzle side of the
recording head 7 during the recovery operation. Furthermore, an
empty jetting (idling) operation that is irrelevant to a printing
operation may be performed in which ink is jetted, for example,
prior to a recording operation or during the recording operation.
Next, an example of a recording head part included
in the recording head 7 is described with reference to Figs. 3 and
4 . Fig. 3 is a cross-sectional view along a longitudinal direction
of a liquid chamber of the recording head 7. Fig.4 is a
cross-sectional view along a lateral direction of the liquid
chamber of the recording head 7.
The recording head 7 includes a layered structure
formed by bonding together a flow plate 101 (for example, formed
by performing anisotropic etching on a single crystal silicon
substrate), a vibration plate 102 (for example, formed by
performing electroforming on a nickel plate) provided on a lower
surface of the flow plate 101, and a nozzle communication path
103 provided on an upper surface of the flow plate 101. This
layered structure is formed with, for example, a nozzle
communication path 105 in flow communication with the nozzle (s)
104 of the recording head 7, a liquid chamber 106 serving as a
pressure generating chamber, a common liquid chamber 108 for

supplying ink to the liquid chamber 106 via a fluid resistance
part (supply path) 107, and an ink supply port 109 in flow
communication with the common liquid chamber 108.
Furthermore, the recording head 7 includes two rows
(although only one row is illustrated in Fig.3) of layered
structure type piezoelectric elements (also referred to as
"pressure generating part" or "actuator part") 121 for applying
pressure to the ink inside the liquid chamber 106 by deforming
the vibration plate 102, and a base substrate 122 affixed to the
piezoelectric elements 121. It is to be noted that plural pillar
parts 123 are formed in-between the piezoelectric elements 121.
Although the pillar parts 123 are formed at the same time of forming
the piezoelectric elements 121 when cutting a base material of
the piezoelectric element 121, the pillar parts 123 simply become
normal pillars since no drive voltage is applied thereto.
Furthermore, the piezoelectric element 121 is
connected to an FPC cable 12 6 on which a driving circuit (driving
IC, not shown) is mounted.
The peripheral portions of the vibration plate 102A
are bonded to a frame member 130. The frame member 130 is
fabricated to form a void portion 131 for installing an actuator
unit (including, for example, the piezoelectric element 121, the
base substrate 122) therein, a concave part including the common
liquid chamber 108, and an ink supply hole 132 for supplying ink
from the outside to the common liquid chamber 108. The frame

member 130 is fabricated by injection molding with use of, for
example, a thermal setting resin (e.g. epoxy type resin) or
polyphenylene sulfate.
The flow plate 101 is fabricated to form various
concave parts and hole parts including the nozzle communication
path 105 and the liquid chamber 106. The flow plate 101 is
fabricated, for example, by using an anisotropic etching method
in which an alkali type etching liquid (e.g. potassium hydroxide,
KOH) is applied to a single crystal silicon substrate having a
crystal plane orientation of (110). It is however to be noted
that other materials may be used for fabricating the flow substrate
101 besides a single crystal silicon substrate. For example, a
stainless steel substrate or a photosensitive resin may also be
used.
The vibration plate 102 is fabrication, from example
by performing an electroforming method on a metal plate formed
of nickel. It is however to be noted that other metal plates or
a bonded member formed by bonding together a metal plate and a
resin plate may also be used. The piezoelectric elements 121 and
the pillar parts 123, and the frame member 130 are bonded to the
vibration plate 102 by using an adhesive agent.
The nozzle plate 103 is formed with nozzles 104 having
diameters ranging from 10 urn - 30 μm in correspondence with the
sizes of respective liquid chambers 106. The nozzle plate 103
is bonded to the flow plate 101 by using an adhesive agent. The

nozzle plate 103 includes, for example, a metal material member
having a water repellent layer formed on its outermost surface.
The piezoelectric element (in this example, PZT) 121
has a layered structure in which piezoelectric material 151 and
internal electrodes 152 are alternately layered on top of one
another. The internal electrodes 152, which are alternately
extended to the side edge planes of the piezoelectric element 121,
are connected to an independent electrode 153 and a common
electrode 154. In this example, the pressure is applied to the
ink in the liquid chamber 106 by using a piezoelectric constant
d33 material for the piezoelectric material 151. It is however
to be noted that pressure may also be applied to the ink in the
liquid chamber 106 by using a piezoelectric constant d31 material
for the piezoelectric material 151. Furthermore, a single row
of piezoelectric elements 121 may be provided in correspondence
with a single base substrate 121.
Accordingly, in a case of jetting ink (recording
liquid) from the nozzles 104 of the above-described recording head
7, the piezoelectric element 121 is contracted by lowering the
voltage applied to the piezoelectric element 121 to a voltage below
a reference electric potential. Thereby, the volume of the liquid
chamber 106 increases as the vibration plate 102 is lowered in
correspondence with the contraction of the piezoelectric element
121. Then, ink flows into the liquid chamber 106. Then, the
voltage applied to the piezoelectric element is raised so that

the piezoelectric element 121 expands in the layered direction
of the piezoelectric element 121. Thereby, the volume of the
liquid chamber 106 decreases as the vibration plate 102 deforms
in a manner protruding toward the nozzle 104 in correspondence
with the expansion of the piezoelectric element 121. As a result,
pressure is applied to the ink inside the liquid chamber 106,
thereby jetting ink out from the nozzle 104.
Then, the position of the vibration plate 102 returns
to its original position by lowering the voltage applied to the
piezoelectric element 121 to the reference electric potential.
As the vibration plate 102 returns to the original position, the
liquid chamber 106 expands to create a negative pressure in the
liquid chamber 106. The negative pressure in the liquid chamber
106 allows ink to be supplied into the liquid chamber 106 from
the common liquid chamber 108. The recording operation of the
recording head 7 moves on to the next ink jetting process after
the vibration of the meniscus face of the nozzle 104 attenuates
and becomes stable.
It is to be noted that the method of driving the
recording head 7 is not limited to the above-described example
(pull/push method) . For example, a pull method or a push method
may be employed by controlling the drive waveform applied to the
recording head 7.
Next, an example of a control part 200 of the image
forming apparatus is described with reference to Fig.5.

The control part 200 of FIG. 5 includes, for example,
a CPU 201 for overall control (including control of correction
of contour parts (jaggy correction)) of the image forming
apparatus, a ROM 202 for storing programs and data installed via
a computer-readable recording medium 500 for execution by the CPU
201, a RAM for temporarily storing image data and the like, a
rewritable non-volatile memory 204 for maintaining data when the
power of the image forming apparatus is turned off, and an ASIC
205 for processing various signals corresponding to image data,
input/output signals for performing image processing (e.g., image
sorting), and controlling various parts of the image forming
apparatus.
The control part 200 further includes, for example,
an I/F 206 for exchanging data and signals with the host, a printing
control part 207 including a data transfer part and a drive
waveform generating part for controlling the recording head 7,
a head driver (driver IC) 208 for driving the recording head 7
provided on the carriage 3, a motor driving part 210 for driving
the main scanning motor 4 and the sub-scanning motor 31, an AC
bias supply part 212 for supplying AC bias to the charge roller
34, and an I/O 213 for receiving various detection signals from
the encoder sensors 43, 35, the temperature sensor 215, and other
sensors.
The control part 200 is connected to a control panel
214 for inputting data to the image forming apparatus and

displaying data.
The control part 200 receives data such as image data
from the host side at the I/F 206 via a cable or a network (e.g.,
the Internet) . The host side is connected to, for example, an
information processing apparatus (e.g., a personal computer (PC))
600, an image reading apparatus (e.g., an image scanner) and/or
a photographing apparatus (e.g., a digital camera).
The CPU 201 of the control part 200 reads out and
analyzes the image data (printing data) stored in a reception
buffer of the I/F 206. Then, the ASIC 205 performs various
processes on the image data such as image processing and data
rearrangement. Then, the processed image data are transferred
from the printing control part (head drive control part) 207 to
the head driver 208. It is to be noted that the generation of
dot patterns for outputting images is conducted in the printer
driver of the host side (described below).
The printing control part 207 transfers image data
in the form of serial data to the head driver 208. In addition,
the printing control part 207 outputs transfer clocks (required
for transferring the image data), latch signals, and droplet
control signals (mask signals) to the head driver 208. The
printing control part 207 has a drive waveform generating part
including a D/A converter for performing D/A conversion on pattern
data of drive signals stored in the ROM 202 and a drive waveform
selecting part for selecting the waveform to be output to the head

driver 208. Accordingly, the printing control part 207 generates
drive waveforms including one or more drive pulses (drive signals)
and outputs the drive waveforms to the head driver 208.
The head driver 208 applies drive signals included
in the waveforms output from the printing control part 207 to a
driving element (e.g. the above-described piezoelectric element
121) . The driving element generates energy for enabling ink
droplets to be selectively jetted from the recording head 7. The
head driver 208 applies the drive signals based on serially input
image data corresponding to a single line of the recording head
7. By selecting the drive pulse included in the drive waveform,
ink droplets of different sizes including large droplets (large
dots), medium droplets (medium dots), and small droplets (small
dots) can be jetted from the recording head 7.
The CPU 201 calculates the drive output value (control
value) for controlling the main scanning motor 4 and drives the
main scanning motor 4 via the motor driving part 210 in accordance
with the calculated value. The calculation of the CPU 201 is based
on the detected speed value and the detected position value
obtained by sampling the detection pulses of the encoder sensor
43 (i.e. linear encoder) and the target speed value and the target
position value stored beforehand in a speed/position profile.
In the same manner, the CPU 201 calculates the drive
output value (control value) for controlling the sub-scanning
motor 31 and drives the sub-scanning motor 31 via the motor driving

part 210 in accordance with the calculated value. The calculation
of the CPU 201 is based on the detected speed value and the detected
position value obtained by sampling the detection pulses of the
encoder sensor 35 (i.e. rotary encoder) and the target speed value
and the target position value stored beforehand in a
speed/position profile.
Next, examples of the printing control part 207 and
the head driver 208 are described with reference to Fig.6.
As described above, the printing control part 207 has
a drive waveform generating part 301 for generating a drive
waveform including plural drive pulses (drive signals) and
outputting the drive waveform in a single printing cycle and a
data transfer part for outputting two bit image data corresponding
to the output (print) image (gradation signal 0, 1) , latch signals
(LAT), and droplet control signals M0-M3.
The droplet control signal is a two bit signal for
instructing opening and closing of an analog switch (switching
part) of the head driver 208 with respect to each droplet. In
correspondence with the printing period of a common drive waveform,
the droplet control signal makes a state-transition to an H level
(ON) with respect to a selected waveform and makes a
state-transition to a L level (OFF) with respect to a non-selected
waveform.
The head driver 208 includes a shift register 311 for
inputting transfer clocks (shift clocks) and serial image data

(gradation data: two bit/CH) from the data transfer part 302, a
latch circuit 312 for latching each resistance value of the shift
register 311 with latch signals, a decoder 313 for decoding the
gradation data and the droplet control signals M0-M3 and
outputting the decoding results, a level shifter 314 for
converting the level of the logic level voltage signal of the
decoder 313 into a level operable for an analog switch 315, and
the analog switch 315 for switching on and off (open/close)
according to the output of the decoder 313 via the level shifter
314.
The analog switch 315 is connected to a selection
electrode (independent electrode) 153 of each piezoelectric
element 121 for receiving a common drive waveform from the drive
waveform generating part 301. Therefore, in accordance with the
results of decoding the serially transferred image data (gradation
data) and the droplet control signals MN0-MN3 by the decoder 313,
the analog switch 315 is switched on, to thereby allow a
predetermined drive signal of the common drive waveform to pass
through (to be selected) and applied to the piezoelectric element
121.
Next, examples of a drive waveform are described with
reference to Figs.7 and 8.
As shown in Fig. 7, the drive waveform generating part
301 generates a drive signal (drive waveform) including eight
drive pulses P1-P8 in a single printing period (one drive period) .

The drive pulses P1-P8 include, for example, a waveform element
dropping from a reference electric potential Ve and a waveform
element rising from the dropped state. The drive pulse to be used
is selected according to the droplet control signals M0-M3 from
the data transfer part 302.
The waveform element of a drive pulse having a
potential V dropping from the reference electric potential Ve
corresponds to a pulling waveform element for causing the
piezoelectric element 121 to contract and increase the volume of
the liquid chamber 106. The waveform element of a drive pulse
rising from a dropped state corresponds to a pushing waveform
element for causing the piezoelectric element 121 to expand and
reduce the volume of the liquid chamber 106.
Furthermore, in accordance with the droplet control
signals M0-M3 from the data transfer part 302, the drive pulse
P1 is selected in a case of forming a small-sized droplet (small
dot) (See (a) of Fig.8), the drive pulses P4-P6 are selected in
a case of forming medium-sized droplets (medium dots) (See (b)
of Fig. 8) , the drive pulses P2-P8 are selected in a case of forming
large-sized droplets (large dots) (See (c) of Fig.8), and drive
pulse P2 is selected in a case of performing fine driving
(vibration of meniscus with no jetting of droplets) . Accordingly,
the selected drive pulses are applied to the piezoelectric element
121 of the recording head 7.
In a case of forming a medium droplet (medium dot) ,

a first droplet is jetted at the drive pulse P4, a second droplet
is jetted at the drive pulse P5, and a third droplet is jetted
at the drive pulse P6. The jetted first, second, and third
droplets are combined during flight and make impact on the medium
as a single droplet. In this case, when the inherent vibration
period of the pressure chamber (liquid chamber 106) is represented
as "Tc", the interval of the jetting timing between the drive pulse
P4 and P5 is preferably " 2Tc ± 0.5 μs. Since the drive pulses
P4 and P5 have a simple pulling waveform element, the ink droplet
speed of the third droplet will become too fast if the drive pulse
P6 is also provided with a simple pulling waveform element. This
could cause the impact location of the droplet to deviate from
the impact location of the other droplets. Therefore, by reducing
the pulling voltage (reducing drop of electric potential) for the
drive pulse P6. the pulling of meniscus can be reduced, to thereby
restrain the ink droplet speed of third droplet. However, the
rise voltage is not to be reduced for saving necessary ink droplet
volume.
In other words, by relatively reducing the pull
voltage for the last drive pulse in a set of drive pulses, the
jetting speed corresponding to the last drive pulse can be
relatively reduced. Accordingly, the impact location of the last
droplet can be matched with the impact locations of the other
droplets.
The fine drive pulse P2 has a drive waveform for

vibrating the meniscus of a nozzle without jetting ink droplets
so that the meniscus of the nozzle can be prevented from drying.
The fine drive pulse P2 is applied to the recording head 7 for
a non-printing area of a medium. By using the drive pulse P2 having
a fine waveform as one of the drive pulses for forming a large
droplet, the drive period can be shortened (accelerated).
By setting the interval of the jetting timing between
the fine drive pulse P2 and the drive pulse P3 within a range of
± 0.5 μs with respect to the inherent vibration period "Tc", the
volume of ink droplet jetted by the drive pulse P3 can be gained
(increased). That is, by overlapping (supplementing) the
expansion of the pressure chamber 6 from the drive pulse P3 with
the pressure vibration of the vibration period in the pressure
chamber from the drive pulse P2, the volume of the liquid droplet
jetted at the drive pulse P3 is greater compared to the volume
of the liquid droplet jetted by the drive pulse P3 alone.
Next., an exemplary operation of performing outline
correction of an image with the above-described image forming
apparatus is described.
[First Example]
A liquid jet type image forming apparatus 10 00
exhibits jagged areas at a diagonal part of an outline of an image
since the dots formed in the diagonal part by the image forming
apparatus 1000 are arranged in a stepwise manner. As shown in
(a) of Fig. 9, jagged areas are hardly recognizable in a case where

characters are printed in high resolution. However, as shown in
(b) of Fig. 9, significant jagged areas (jaggy) clearly appear and
character quality is poor in a case where characters are printed
in an insufficiently low resolution. As described above,
exemplary methods for reducing (correcting) the jagged areas
include using dots smaller than other dots to form the outline
part or adding new dots to empty spaces in the outline part.
However, the method of using dots smaller than other dots at the
outline part cannot be used as the correcting method in a case
of forming an image with binary values, in other words, forming
an image by printing or not printing a dot(s) (one type of dot
size).
Meanwhile, in a case of using recording liquid (ink)
of four colors (black (K), cyan (C), magenta (M), yellow (Y)),
a composite black dot of C, M, Y can be formed by jetting ink
droplets of C, M, and Y onto the same impact location. It is known
that a dot printed with the composite black ink has a greater
brightness than a dot printed with black (K) ink alone.
Furthermore, another composite black dot of C, M, Y, and K having
lesser brightness can be printed by jetting ink droplets of C,
M, Y, and K onto the same impact location.
Accordingly, by compositing (combining) ink of
different colors, black dots having different brightnesses can
be used to reduce jagged areas at an outline part of, for example,
a black character.

Furthermore, in a case where a color ink besides those
of the aforementioned four colors (K, C, M, Y) is used (e.g. , black
ink with greater brightness such as light black, gray), a dot
itself can have different brightness by compositing the colors
such as black, light black, cyan, magenta, and yellow.
In a case of forming an image by using dots of
different sizes, an outline part of an image can also be corrected
by changing the sizes of dots at the outline part into sizes smaller
than other dots or by adding dots smaller than other dots to a
part surrounding the outline part. From a broad (macro)
perspective, the part printed with smaller dots will appear to
have greater brightness since the area covered by ink is smaller
compared to other parts printed with larger dots. Accordingly,
an outline correcting effect can be attained. It is also regarded
that the different dot sizes also serve to physically reduce the
jagged areas.
Furthermore, with a configuration capable of forming
more dot sizes, a more suitable outline correction can be achieved
owing to more choices of dot sizes and more variations of dot
correction patterns. In addition, such configuration can be used
in forming character other than black.
Therefore, an image forming apparatus capable of
forming dots of various sizes can use both the above-described
outline correcting method using different dot sizes (e.g., using
dots smaller than other dots as the dots of an outline part or

adding dots no larger than the other dots to a part surrounding
an outline part) and the above-described outline correcting method
using different dot brightnesses (e.g., in a case of black dots,
using different kinds of ink to form different dot brightnesses) .
Accordingly, in a case of forming black characters
or thin lines, a combination of the outline correcting method using
different dot sizes and the outline correcting method using
different dot brightnesses can be used whereas in a case of forming
characters or thin lines of a color besides black, the outline
correcting method using different dot sizes can be used.
In other words, in an image forming apparatus capable
of forming dots of different sizes, either one or both of the
following method (part) can be used according to the image to be
corrected (target image), in which one method (part) is for
changing the brightness of at least a single dot situated in an
outline part (e.g., outline part of a character or a thin line)
or the brightness of at least a single dot added to a part
surrounding the outline part to a brightness relatively greater
than the brightness of other dots situated in the outline part
and the other method (part) is for changing the dot size of at
least a single dot situated in an outline part (e.g. , outline part
of a character or a thin line) or the dot size of at least a single
dot added to a part surrounding the outline part. As described
below, it is to be noted that the two methods (parts) can be
combined as a single method (part) for performing a brightness

changing process and a dot size changing process at the same time
by using a correction pattern.
Accordingly, by changing dot size and dot brightness,
correction can be performed at a higher level and with more
variations. In addition, improving character quality by changing
dot size and changing dot brightness can reduce the tradeoffs
between preventing drying of the nozzle and preventing increase
of ink cost. For example, in a case of printing a document mainly
of black, although jetting failure due to drying of a color nozzle
tends to occur, a process of cleaning the nozzle would reduce
printing speed. Meanwhile, in a case of compositing black, ink
costs tend to increase (particularly, cost increase unrelated to
improvement of image quality). Thus, performing outline
correction only by changing dot brightness causes increase of ink
cost and ink bleeding. Therefore, by enabling both dot size
change and dot brightness change, nozzle drying and ink cost
increase can both be prevented.
Next, exemplary cases in performing the
above-described outline correction are described with reference
to Fig.10.
Fig.10(a) shows a case where no outline correction
is performed. In Fig. 10 (a), stepwise jagged parts are noticeable
at the outline part. Meanwhile, Fig.10(b) shows a case where
outline correction is performed by having empty (blank) dots of
the dots surrounding the outline part changed into image (printed)

dots Ds, that is, changing the empty (blank) dots into dots having
a size smaller (or equal) than that of the dots of the outline
part. In addition to or as an alternative of the outline
correction of Fig.10(b), the dots of the outline part can be
changed into dots having a relatively small size. Accordingly,
by adding small dots to the outline part, the jagged parts at the
outline part become smooth. In a case of an image forming
apparatus capable of using dots size, outline correction can be
performed in a greater variety of ways. Thus, the image forming
apparatus can perform more suitable outline correction. It is
to be noted that the number of multi-value or dot formation
location, number of dots, sizes of dots are not limited to those
described in the embodiments of the present invention, such as
in Fig.10(b). Furthermore, this outline correction can be
performed not only for black characters but also for color
characters.
Fig.10(c) shows a case where outline correction is
performed by changing the dots of the outline part into dots Dp
having relatively greater brightness than that of the dots of the
outline part. The changing of brightness may be performed on the
dots to be added to a part surrounding the outline part or on the
dots which originally form, in this example, a black character.
By using a different brightness for the dots forming a character,
the jagged parts at the outline part become less noticeable.
Thereby, an outline correction effect can be attained.

Furthermore, by the use of dots with different
brightnesses, outline correction can be performed not only with
an image forming apparatus capable of forming dots of different
sizes but even with an image forming apparatus capable of forming
dots of a single dot size. It is to be noted that the number of
multi-value or dot formation location, number of dots, sizes of
dots, brightness of dots, or the kind of ink used for forming the
dots are not limited to those described in the embodiments of the
present invention, such as in Fig.lO(c).
Fig.10(d) shows a case where outline correction is
performed by changing the dots of the outline part into dots Dp
having relatively greater brightness than that of the dots of the
outline part and also changing (adding) the dots surrounding the
outline parts into dots Dps having relatively greater brightness
as well as smaller dot size than the other dots of the outline
part. In other words, Fig.10(d) shows a case of combining the
dot size changing process and dot brightness changing process.
Thereby, a more efficient outline correction effect can be
attained. The dot brightness changing process may be performed
on the dots originally forming, in this example, a black character,
the dots to be added to the outline part, or the dots changed into
a smaller size. It is to be noted that the number of multi-value
or dot formation location, number of dots, sizes of dots,
brightness of dots, or the kind of ink used for forming the dots
are not limited to those described in the embodiments of the

present invention, such as in Fig.10(d).
It is to be noted that the liquid jet head capable
of jetting dots of different sizes is not limited to a
piezoelectric type head and an electrostatic type head. For
example, the liquid jet head may be a thermal type head including
nozzles of different diameters or a heater having a non-linear
characteristic for controlling heater resistance. The
above-described outline correction effect can be attained with
an image forming apparatus having any one of the heads.
Accordingly, jagged areas (jaggy) can be hardly
noticeable and image quality can be improved by changing the
brightness of at least one dot in the dots forming an outline part
(for example, an outline part of a character or a linear image)
or one dot to be added to a part surrounding the outline part to
a bri ghtness relatively greater than that of the other dots forming
the outline part.
From another aspect, black characters, in general,
are preferred to be formed darkly and clearly for better visibility.
However, in a case where there is a significant contrast between
the paper and the image (character), the edges at the outline part
of the image become emphasized and jagged areas become noticeable.
Therefore, in a case of performing outline correction by using
dots having different brightnesses, the jagged areas can be
reduced without deterioration of visibility by using dots of low
brightness for the framework of the character while using dots

of higher brightness for the outline part of the character.
For example, in a case of forming a black character
with recording liquids of four colors (K, C, M, Y) , the density
of the black character can be increased by using a composite black
ink including the four colors (four color composite black ink) .
However, in this case, jagged areas can be reduced without
deterioration of visibility by using the four color composite
black ink for the framework of the black character while using
a single black color (K) ink or a three color composite black ink
for the outline part of the black character. Furthermore, the
amount of ink consumed can be less compared to using the four color
composite black ink.
Furthermore, by detecting the angle of an outline part
and changing the dot size changing process and/or the dot
brightness changing process (e.g., switching a correction pattern
(described in detail below) ) according to the detected angle, an
optimum outline correction can be realized according to the angle
of the outline part. For example, in detecting the angle of the
outline part, first, the dots forming the outline part are detected.
Then, an outline formed by connecting the detected dots is obtained.
Then, the angle of the outline part is calculated by comparing
the outline and a predetermined direction (e.g., main scanning
direction or sub-scanning direction) . Accordingly, a correction
pattern corresponding to the calculated angle of the outline part
is selected.

Exemplary cases of performing outline correction
using correction patterns according to various conditions and
factors (e.g., angle of the outline part, etc.) are described below.
In the case of performing outline correction according to the angle
of the outline part, a correction pattern includes one or more
outline pattern data and correction process data corresponding
to the outline pattern data. Thus, the outline correction is
performed by matching a detected outline pattern and a correction
pattern.
Whether jagged areas (jaggy) in the outline part are
noticeable may also depend on printing conditions (e.g.,
resolution, type of paper). Since dot formation location can be
specified in detail in a case of high resolution printing, jaggy
in the outline part is less noticeable. Therefore, in a case where
sufficient resolution can be obtained., outline correction miaht
not be required.
Furthermore, with respect to types of paper, dots tend
to blur when plain paper is used. Meanwhile, when glossy paper
is used, the shapes of dots clearly appear and the density of the
paper surface is uniform. Therefore, the edges tend to appear
and jaggy is noticeable.
Therefore, it is determined whether it is necessary
to perform the outline detection and outline correction depends
on printing conditions (e.g., resolution, type of paper) and
performing outline correction; when it is determined to be

necessary, with a correction pattern based on the printing
conditions, an optimum outline correction can be performed without
unnecessary data processing.
Since the outline correction method according to an
embodiment of the present invention is applicable to characters
and thin line images, the calculation workload performed for the
outline correction method can be reduced by obtaining object data
of the image to be printed (e.g., text data, photograph data,
graphic data, thin line data) and performing the outline detection
process and the outline correction process on character data and
thin line data only.
Furthermore, as for other printing conditions
(factors) of an liquid jet type image forming apparatus, jetting
failure (e.g., broken (missing) line patterns, or bent line
patterns) tend to occur as ink becomes dry and the viscosity of
the ink increases. Particularly, in a case where the image
forming apparatus is used under a low temperature and/or low
humidity environment or a case where the image forming apparatus
has not been used for a long period, such problems are liable to
occur.
Furthermore, as for other printing conditions, when
printing image data (e.g., printing an image consisting of mostly
black characters but with a part expressed with color characters)
in a case where a certain ink (e.g., black) is frequently used
while other color inks are not often used, the low usage rate of

the other color inks may cause jetting failure and may skip (miss)
printing the color character part.
In order to prevent the above-described jetting
failures, the nozzle face is cleaned and blank (empty) ink jetting
is performed. However, these processes reduce printing
throughput and unnecessarily consume ink. Furthermore, although
color inks may be used by printing a black character part with
composite black ink for preventing the jetting failure, this leads
to an increase in the amount of ink consumption and the appearance
of jagged areas due to an increase of density of the characters.
More specifically, in a case where a composite black of four colors
(K, C, M, Y) is used, jagged areas will clearly appear due to
increased density of the characters. In a case where a composite
black of three colors (C, M, Y) is used, visibility of the
characters is reduced due to reduced density of the characters.
By referring to object data of image data to be printed
(target image data) or color component data calculated from the
target image data, an optimum outline correction can be performed
from the aspect of steady nozzle jetting performance and suitable
ink consumption based on the object data and the color component
data. That is, a correction pattern can be selected from the
aspect of attaining a balanced usage of ink, for example, by
selecting a correction pattern using a large amount of color ink
in a case of printing an image consisting mostly of characters/thin
lines where the color ink usage rate is low or by switching to

a correction pattern that controls the consumption of each ink
to a predetermined amount based on color component data.
In a case of performing an outline correction using
the dot size changing process where the target image data includes
characters of a color other than black, suitable outline
correction can be performed by changing the correction patterns
based on color component data. For example, since jagged areas
are more noticeable when using colors as black, cyan, and magenta
at an outline part compared to using a color such as yellow, the
correction pattern can be changed based on color component data
so that optimum outline correction can be achieved.
Furthermore, correction patterns may also be
switched according to status data of the image forming apparatus
(e.g., ambient temperature or humidity of the image forming
apparatus, lapsed time from previous use of the image forming
apparatus) . For example, under conditions where jetting failure
tends to occur (e.g., low temperature, low humidity, long unused
period), it is preferable to select a correction pattern that can
use various inks in a balanced manner for attaining steady jetting
performance. For example, under conditions where it is difficult
for ink to dry (e.g., high temperature, high humidity) it is
preferable to select a correction pattern that can reduce ink
consumption for preventing problems such as cockling and bleeding.
Furthermore, in a case where the image forming
apparatus is capable of forming dots of multiple sizes

(multi-value dot size), ambient conditions may cause jetting
failure to occur for a particular size droplet (e.g., small
droplet), to thereby degrade image quality. Therefore, by
switching the correction pattern and changing the combination of
dot sizes, degrading of image quality due to jetting failure can
be prevented.
Next, an exemplary outline correction operation
(method) is described with reference to Fig.11. This outline
correction operation is executed by having a CPU 201 execute a
program stored in a ROM 202.
In the outline correction operation, first, printing
data are obtained. For example, the printing data include
printing mode (resolution, type of paper) , image data (e.g., color
component data, object data) , and status data of the image forming
apparatus (e.g., ambient data). Then, the CPU 201 determines
whether outline correction is necessary based on the obtained
printing data.
In a case where outline correction is determined
unnecessary (e.g., sufficient high resolution, no character/thin
line to be printed), outline correction is not performed.
In a case where outline correction is determined
necessary, the CPU 201 selects a correction pattern to be used
based on a table as shown in Fig. 12. It is to be noted that the
table shown in Fig.12 is merely an example. Thus, the choices
of patterns or the number of patterns is not limited to those shown

in Fig.12.
Since the optimum correction process differs
depending on, for example, resolution and type of paper, the
correction pattern is different according to resolution and type
of paper. Furthermore, even if the resolution or the type of paper
is the same, it may be desirable to switch the correction pattern
according to the ambient conditions of the image forming apparatus
or data configuration of the image data. For example, a
correction pattern that increases the usage rate of C, M, Y may
be selected for preventing nozzle failure. In another example,
a correction pattern that controls ink consumption may be selected
for reducing ink cost. Furthermore, in a case where the image
forming apparatus is capable of forming dots of multiple sizes,
a combination of the dot size changing process and the dot
brightness changing process may be used when correcting black
characters and the dot size changing process may be used when
correcting color characters. Thus, different correction
patterns are assigned to various correction levels which are
determined (defined) by calculating various printing data.
As shown in Fig. 13, each correction pattern includes
plural outline patterns and dot arrangement data (e.g., dot
formation location, number of dots, dot size, type of ink)
corresponding to the outline patterns. The CPU 201 performs a
pattern matching process on the shape (pattern) of a detected
outline part and the outline patterns included in the correction

pattern. The CPU 201 performs outline correction on the detected
outline part by using a matching outline pattern in the correction
pattern.
More specifically, in performing the outline
correction process based on the printing data according to an
embodiment of the present invention, the CPU 201 selects a
correction pattern from the table shown in Fig.12 and performs
outline correction on the detected outline part by using a
corresponding outline pattern of the selected correction pattern.
For example, an outline pattern having an angle corresponding to
that of the detected outline part may be used in the outline
correction process.
In an exemplary table shown in Fig. 12, the correction
levels (correction rank) α, β, γ are defined (categorized)
according to ambient conditions of the image forming apparatus
and/or configuration (constitution) of the image data. Each
correction level is assigned with correction patterns A through
G according to printing mode. For example, in a case where the
correction levels α, β, γ are categorized according to ambient
temperature (α indicating low temperature, β indicating medium
temperature, γ indicating high temperature), a correction
pattern A that can use various inks in a balanced manner can be
assigned for correction level α (i.e. using C, M, and Y inks for
increasing brightness rather than using K ink only in a low
temperature condition). Meanwhile, a correction pattern C that

that can reduce ink consumption can be assigned for correction
level γ (i. e. using one type of ink rather than using C, M, Y inks) .
For example, in a case where the correction levels
α, β, γ are categorized according to the data configuration of
the image data (α indicating small proportion of black color
data, β indicating medium proportion of black color data, γ
indicating large proportion of black color data), a correction
pattern A having high C, M, Y usage rates can be assigned for
correction level α (i.e. using C, M, and Y inks for increasing
brightness instead of using K ink only when the proportion of black
color data in the image data is small) . Meanwhile, a correction
pattern C having low C, M, Y usage rates can be assigned for
correction level γ . It is to be noted that the categorization
of the correction level or the number of correction levels is not
to be limited to those described above.
Furthermore, in a case of forming black characters
or black thin lines, the timing for jetting liquid droplets of
one or more colors can be different with respect to those of other
colors. For example, in a case of performing outline correction
with a configuration capable of jetting four colors of K, C, M,
Y, the timing for jetting K color ink can be different from the
timing for jetting C, M, Y color inks. This allows outline
correcting dots to be formed at areas where a satisfactory outline
correction effect can be attained. It is to be noted that various
head configurations can be used (e.g., a configuration having a

head corresponding to each color, a configuration having one head
for K ink and another head for C, M, Y ink, and a configuration
having a head with plural nozzle arrays).
Fig.14 shows exemplary cases where droplets are
jetted at different timings. Fig.14(a) shows a case where no
outline correction is performed. In Fig. 14 (a) , jagged areas are
noticeable at the outline part. Fig.14(b) shows a case where
outline correction is performed by adding small dots Ds to the
outline part. However, since the kinds of dot sizes are limited
and dot formation locations are defined by resolution, the outline
correction effect is still insufficient. In Fig.14 (c) , the dot
formation locations of a portion of the dots D are shifted
(deviated) a half pitch of the resolution with respect to the dot
formation locations of the other dots (in this example, the dots
are shifted in a main scanning direction which perpendicularly
intersects the nozzle array direction) . Accordingly, the outline
correcting dots can be formed in areas where greater outline
correction effect can be attained. In addition to the outline
correction effect (smoothing effect) using a different brightness
at the outline part, the dots also serve to physically cover the
jagged areas. The example shown in Fig. 14 (c) can be executed not
only by an image forming apparatus capable of forming multi-value
dot patterns but also by an image forming apparatus capable of
forming binary dot patterns. Thus, depending on the selected
recording head, the correction process of using different dot

brightnesses can be used in combination.
As shown in Fig.l4(d), an image forming apparatus
capable of forming multi-value dot patterns can perform a
combination of changing dot size and changing dot formation
location. Thus, depending on the selected recording head, the
correction process of using different dot brightnesses can be used
in combination.
It is to be noted that the outline correction
processes described with Fig. 14 may be performed with a recording
head, dot formation location, number of dots, dot size, and ink
other than those described above. In changing the timing for
jetting the ink, a recording head having drive waveforms of
different rise timings may be used for selectively applying the
drive waveform to the recording head.
Although the above-described embodiments of the
present invention are described with a serial type image forming
apparatus, the present invention may also be effectively applied
to a line type image forming apparatus.
In the line type image forming apparatus, nozzle are
arranged substantially across the entire paper in the paper width
direction. Furthermore, the line type image forming apparatus
performs recording (printing) by conveying the paper instead of
scanning in the width direction. Therefore, the line type image
forming apparatus is only capable of using the interlace method
and the multi-pass method. This makes it difficult for the line

type image forming apparatus to increase image forming (printing)
resolution. Accordingly, the problem of jaggy may be more serious
for the line type image forming apparatus compared to the serial
type image forming apparatus.
Furthermore, since the line type image forming
apparatus performs recording by conveying paper with respect to
an affixed recording head, it is difficult to perform a head
cleaning process and a blank jetting process with the line type
image forming apparatus. Furthermore, since the line type image
forming apparatus has a longer recording head than that of the
serial image forming apparatus, a greater amount of ink is used
in the cleaning process and the blank jetting process.
Therefore, the difficulties of outline correction,
steady jetting performance, and ink consumption also apply to the
line type image forming apparatus. Therefore, the present
invention is also effective for the line type image forming
apparatus.
The above-described program according to an
embodiment of the present invention may be installed in a
computer-readable recording medium 500 for enabling an image
forming apparatus including a CPU (computer) 201 to perform the
outline correction operation (method) , to thereby eliminate jaggy
and improve image quality.
Although the above-described image forming apparatus
is explained having a configuration of a printer, the present

invention may also be applied to an image forming apparatus having
a configuration of a facsimile machine, a plotting apparatus, a
copier, or a multifunction machine having the functions of a
printer, a facsimile, and a copier, for example.
[Second Example]
Next, another exemplary operation of performing
outline correction of an image with the above-described image
forming apparatus is described.
As described above, in a case of jetting liquid
droplets of different dot diameters by applying drive pulses of
different widths or different numbers of drive pulses to the
recording head 7, even if the timing of the start of the rise of
the meniscus is the same upon input of the drive pulse, the timing
for jetting liquid droplets upon the end of the drive pulse is
different. Therefore, the time for the droplets to reach the
medium surface is different. As a result, the dot formation
position on the paper is different depending on dot size.
Furthermore, since the viscosity resistance of ink
changes according to ambient temperature and ambient humidity,
the meniscus may not be appropriately controlled even if a drive
pulse is applied to the recording head 7. This results in
deviation of dot impact position. The ambient conditions also
make it difficult to maintain suitable dot size and dot shape.
This is also results to deviation of dot impact position.
Furthermore, in a case where the image forming apparatus 1000 is

not used for a long period of time, dried ink may accumulate
surrounding of the nozzle. Moreover, in a case where the image
forming apparatus 1000 has been used for a long period of time
since the manufacture of the image forming apparatus 1000, changes
in the characteristics of its recording head 7 may affect liquid
jet precision and lead to deviation of dot formation position.
Although an example of a piezoelectric type recording
head is used for describing the problem of dot formation position,
this problem may also occur in a case of using a thermal type
recording head or an electrostatic type recording head (a head
using electrostatic force between a vibration plate and an
electrode facing the vibration plate for applying pressure to a
liquid chamber via the vibration plate) as the recording head 7.
For example, the present invention may be applied to a case of
using a thermal type recording head which forms multi-value dot
patterns by controlling the resistance value of its thermoelectric
transducer.
Next, exemplary cases in performing the
above-described outline correction are described with reference
to Fig.15.
Fig.15(a) shows a case where no outline correction
is performed. As shown in Fig.15 (a), satisfactory image quality
cannot be attained and stepwise jagged parts are noticeable at
the outline part in a case where resolution is not high enough.
Accordingly, by replacing the dots at the parts surrounding the

outline part with other dots, the jagged areas can be reduced.
For example, Fig. 15 (b) shows a case where one blank
dot situated in the stepwise part of the outline part is replaced
with a small size image dot (addition of dots) . Fig. 15 (c) shows
a case where two blank dots situated in the stepwise part of the
outline part are replaced with one small size image dot and another
medium size image dot (addition of dots) . Fig. 15 (d) shows a case
where two more dots situated in the stepwise part of the outline
part are replaced with another small size image dot and another
medium size image dot in addition to those of Fig.15 (c).
However, as described above, in a case where there
is a deviation of dot formation position, the above-described
outline correction process may adversely affect image quality.
For example, in a case of performing the outline correction by
adding dot Dh (see Fig. 16 (b) ) for reducing jaggy in the outline
part shown in Fig.16 (a) , deviation of dot formation position (see
Fig.16 (c) ) may cause undesired overlapped or blurred areas, to
thereby degrade image quality. More specifically, as shown in
Figs. 17 (a) and 17 (b) , the roughness at the outline part of the
character in Fig.17(b) is significantly noticeable compared to
that of Fig.17(a).
Accordingly, the present invention selectively
changes the method (pattern) for replacing a target dot with
another dot having, for example, different dot size and/or
brightness with respect to the target dot by referring to factors

(e.g., ambient temperature, ambient humidity, lapsed time from
manufacture, lapsed time from previous image forming process) that
cause deviation of dot formation position (dot impact position)
or by referring to the actual amount (degree) by which the dots
deviate from a desired dot formation position on the paper (medium) ,
so as not to use a deviated dot(s) that would adversely affect
image quality. For example, the method (pattern) of replacing
dots can be changed by selectively changing a correction pattern
for maintaining image quality of an output image (described in
detail below).
The same as Fig. 15 (a), Fig. 18 (a) shows a case where
no outline correction is performed. Likewise, jagged areas are
noticeable at the outline part. Accordingly, as shown in
Fig.18 (b), an outline correction process using different size
droplets (medium droplet and small droplet : medium dot and small
dot) is performed. In this case, when deviation of a droplet
impact position occurs (e.g., deviation of small droplet impact
position) , the dot corresponding to the deviated droplet deviates
from its target dot formation position (small dot Ds deviating
one dot from target dot formation position in Fig. 18 (c) ) . Thus,
in addition to not being able to attain a satisfactory outline
correction effect, such deviation causes degrading of image
quality such as creation of overlapped lines, mist, and blurring.
In a case where there is a significant deviation of
a small droplet impact location, a correction pattern which uses

only medium droplets (no small droplets) can be selected as the
correction pattern for performing the outline correction process
(See Fig. 18 (d) ) . Likewise, in a case where there is a significant
deviation of a medium droplet impact location, a correction
pattern which uses only small droplets (no medium droplets) can
be selected as the correction pattern for performing the outline
correction process (See Fig. 18 (e) ) . Thus, an optimum correction
pattern can be used in the outline correction process (dot
replacement process) according to the deviation status (or the
cause of deviation) . It is to be noted that the types of droplets,
droplet sizes (large, medium, small), or correction pattern are
not limited to those described with Fig.18.
Next, factors causing deviation of dot formation
position, how the factors are detected, and how the factors are
input are described.
As described above, the factors causing deviation of
dot formation position may be, for example, ambient temperature,
ambient humidity, elapsed time from the time of manufacture of
the image forming apparatus, and elapsed time from the previous
time of performing an image forming process. Accordingly, the
factors can be detected by providing the image forming apparatus
1000 with a part for detecting the ambient temperature, ambient
humidity (e.g., temperature sensor 215) and a part for counting
the time elapsed from the time of manufacturing the image forming
apparatus or the previous time of performing an image forming

process and updating the factors by storing them in a non-volatile
memory (RAM) 204 of the control part 200.
Furthermore, the actual amount (degree) of a dot
deviating from a target dot formation position on a medium may
also be detected. For example, in detecting the actual amount
of dot formation position deviation, first, a dot pattern enabling
detection of the amount of deviation from the dot formation
position during an actual image forming process (e.g., a ruled
pattern formed with different size dots) is printed (output) on
a medium. Then, dot formation data (impact precision data),
density data, and brightness data are obtained by reading the
printed dot pattern with, for example, a photo-sensor or a scanning
unit. Then, the CPU 201 calculates how much a dot (s) is deviated
from a target dot formation position (deviation amount) based on
the obtained data.
Alternatively, in a case where the configuration for
detecting the actual deviation amount becomes too complicated,
the user may input data related to the deviation amount. Likewise,
a dot pattern enabling detection of the amount of deviation from
the dot formation position during an actual image forming process
is printed (output) on a medium. Then, the user determines the
deviation amount by examining the output dot pattern and inputs
data related to the determined deviation amount. For example,
the user has the deviation amount divided into multiple levels
(first level to n level) beforehand. Then, the user determines

the level to which the output dot pattern belongs. Then, the user
inputs data (e.g., numeral) corresponding to the determined level
via the control panel 214.
Next, a relationship between dot formation position
deviation and resolution and a relationship between medium type
(paper type) and image quality are described.
As described above, in general, correction of the
outline part (particularly, the stepwise part of the outline part)
of an image is required in a case where resolution is low.
Meanwhile, in a case of high resolution, correction of the outline
part of an image may not be required since a substantially
satisfactory image quality can be attained. Furthermore, jagged
areas become more noticeable at the stepwise part as resolution
becomes lower. Therefore, it is preferable to change the method
of correcting the outline part (method of dot replacement)
according to the resolution of the image to be formed.
Furthermore, deviation of dot formation position
becomes more noticeable (thereby, regarded as having poor image
quality) in a case of a type of medium having less liquid bleeding
(blurring) characteristics. For example, even if the deviation
amount of the dot formation position is the same, impact deviation
has a direct influence on image quality in a case of using glossy
paper. This is because glossy paper has high contrast and
exhibits little blurring of impact droplets. Therefore, it is
preferable to change the method of correcting the outline part

(method of dot replacement) according to the type to medium.
Next, in view of the above, an exemplary outline
correction operation (method) is described with reference to the
flowchart of Fig.19. This outline correction operation is
executed by having a CPU 201 execute a program stored in a ROM
202.
In this outline correction operation (method), a
correction pattern is prepared beforehand for performing pattern
matching with respect to an outline part of an image. Then, in
a case where there is a match between the outline part of an image
and the correction pattern as a result of the pattern matching
process, outline correction is performed by replacing a dot(s)
surrounding a stepwise part of the outline with a dot(s) having
a size designated by the matching correction pattern. It is to
be noted that the outline correction process performed by dot
replacement includes replacing a blank dot with an image dot and
modifying the size of the image dot.
More specifically, in the outline correction process,
first, the CPU 201 determines whether the resolution of the
printing mode is no greater than a predetermined resolution for
determining whether correction of the outline part is necessary.
As described above, in a case where printing resolution is high
enough that jagged areas are hardly noticeable, the CPU 201 does
not need to execute the outline correction process. As an
alternative of or in addition to the step of determining the

resolution, a step of determining the type of medium can be
performed for determining whether the outline correction process
is necessary.
In a case where the outline correction process is
determined necessary, the factors that cause dot formation
position deviation (hereinafter also referred to as "dot deviation
data") are obtained. As described above, the dot deviation data
may include ambient temperature, ambient humidity, elapsed time
from time of manufacturing the image forming apparatus, elapsed
time from previous time of executing an image forming process,
actual deviation amount based on an output dot pattern, or data
related to the deviation amount input by the user.
Then, the obtained dot deviation data are compared
with a selection condition (s) for selecting a correction pattern
including predetermined resolution data (or medium type data) .
Then, a correction pattern is selected based on the comparison
result. Then, an outline correction process is executed by using
the selected correction pattern. It is to be noted that, the
outline correction process (dot replacement process) may not be
executed in a case where there is no correction pattern that
satisfies the selection condition.
For example, as exemplarily shown in the table of
Fig. 20, a correction pattern can be selected based on a combination
of resolution and ambient temperature. More specifically, in
this example, outline correction is not performed regardless of

the ambient temperature when the resolution is equal to or greater
than 600 dpi X 600 dpi. Furthermore, pattern A or B is selected
according to resolution when the temperature T is less than a
predetermined temperature Tl. Furthermore, pattern C or D is
selected according to resolution when the temperature T is no less
than the predetermined temperature Tl but less than a
predetermined temperature 12. Furthermore, pattern E or F is
selected according to resolution when the temperature T is no less
than the predetermined temperature T2.
Generally, in terms of ambient temperature, ink
viscosity tends to increase and jetting failure of small droplets
tends to occur when the ambient temperature of the image forming
apparatus 1000 is low. Therefore, it is preferable to select a
correction pattern that does not use small droplets or a correction
pattern having a reduced small droplet usage rate (Pattern A, B) .
In a case of normal temperature, jetting failure hardly occurs.
Therefore, a correction pattern using every type (size) of dot
can be selected (Pattern C, D) . In a case of high temperature,
problems such as decrease of ink viscosity, increase of ink
consumption, increase of jetting speed, scattering of ink in a
mist-like manner upon impact, or trailing dots upon impact tend
to occur. Therefore, a correction pattern using droplets which
do not cause such problems or a correction pattern using less ink
can be selected Pattern E, F) . In addition, a further preferable
correction pattern can be selected according to printing

conditions (e.g., resolution, type of medium).
It is to be noted that a correction pattern can be
selected based on the deviation amount of dot formation position
and/or factors that cause deviation of dot formation position.
Initially, correction patterns that cause little deviation
according to various printing conditions (e.g., resolution, type
of paper) are prepared first hand. However, in a case where a
dot of a certain size may cause unsatisfactory correction results,
a correction pattern using dots of the certain size is avoided
and an alternative correction pattern is selected (a correction
pattern having the most effective correction performance without
using dots of the certain size) . In a case where an effective
correction effect is unlikely to be attained by selecting any one
of the correction patterns, a correction pattern which performs
no out line correction may be selected.
Although one example of changing the method of
correction is described as changing the correction pattern by
preparing multiple correction patterns corresponding to. factors
that cause dot formation position deviation and selecting a
correction pattern based on the detection results of the factors,
other examples of changing the correction method may be used. For
example, instead of changing the correction pattern, the size of
the dots for performing dot replacement can be changed by comparing
the factors with a predetermined condition(s) according to a
pattern matching result.

Hence, by providing a part for changing the method
of replacing the dots forming an outline part of an image based
on factors that affect dot formation position or the amount of
deviation from a dot formation position on a medium (detection
data or data input from outside) , the method of the dot replacement
(correction method) can be changed when dot formation position
deviation occurs so that outline correction can be performed
without being adversely affected by the dot formation position
deviation. Thereby, image quality can be prevented from
degrading.
Furthermore, by providing a program enabling a
computer to execute a process for changing the method of replacing
the dots forming an outline part of an image based on factors that
affect dot formation position or the amount of deviation from a
dot formation position on a medium (detection data or data input
from outside), the method of the dot replacement (correction
method) can be changed when dot formation position deviation
occurs so that outline correction can be performed without being
adversely affected by the dot formation position deviation.
Thereby, image quality can be prevented from degrading.
Furthermore, by installing the program on a
computer-readable recording medium 500, a program enabling the
method of the dot replacement (correction method) to be changed
when dot formation position deviation occurs can be provided so
that outline correction can be performed without being adversely

affected by the dot formation position deviation. Thereby, image
quality can be prevented from degrading.
Furthermore, by providing an image forming method for
changing the method of replacing the dots forming an outline part
of an image based on factors that affect dot formation position
or the amount of deviation from a dot formation position on a medium
(detection data or data input from outside), the method of the
dot replacement (correction method) can be changed when dot
formation position deviation occurs so that outline correction
can be performed without being adversely affected by the dot
formation position deviation. Thereby, image quality can be
prevented from degrading.
It is to be noted that the image forming apparatus
1000 according to an embodiment of the present invention may
include a function to display the status of the image formina
apparatus 1000 or a method of improving the image forming apparatus
1000 to the user or administrator of the image forming apparatus
1000 via an apparatus (e.g., display apparatus, host computer
(data processing apparatus)) connected to the image forming
apparatus 1000 based on various data such as temperature, humidity,
and dot impact data (data related to deviation of dot formation
position).
For example, in a case where an effective outline
correction performance (e.g., jaggy reduction effect) cannot be
realized even by applying the correction patterns stored

beforehand in the image forming apparatus 1000 due to temperature,
humidity, dot impact data (data related to deviation of dot
formation position) , or other factors that cause deviation of dot
formation position, the image forming apparatus 1000 may be
informed with data related to the inability to realize an effective
correction performance via the control panel 214 of the image
forming apparatus 1000 or a printer driver of a data processing
apparatus on the host side.
For example, the data related to the inability to
realize an effective correction performance may include data
notifying that the image forming apparatus 1000 is in such state,
data indicating ambient/printing conditions (ambient temperature,
ambient humidity, resolution, deviation amount) , data indicating
the status of the image forming apparatus 1000, data including
information for improving the performance of the image forming
apparatus 1000 such as messages advising change of
ambient/printing conditions, maintenance information (e.g., head
cleaning, head adjustment), and help/support service
information.
With the above-described display function, the user
can be informed (persuaded) to adjust various settings and ambient
conditions and perform other adjustment (e.g., calibration) for
improving the performance of the image forming apparatus 1000
including elimination of dot formation position deviation
according to various conditions.

The image forming apparatus 1000 may also include a
function to report the above-described data related to the
inability to realize an effective correction performance to a
support service of the image forming apparatus 1000.
With the reporting function, the support service can
promptly and automatically detect failure of the image forming
apparatus 1000.
Thus, owing to the above-described functions, not
only can the image forming apparatus 1000 achieve optimum outline
correction according to various conditions but can also receive
various support for maintaining satisfactory performance.
Although the above-described image forming apparatus
is explained having a configuration of a printer, the present
invention may also be applied to an image forming apparatus having
a configuration of a facsimile machine, a plottina apparatus, a
copier, or a multifunction machine having the functions of a
printer,- a facsimile, and a copier, for example.
In the above-described embodiments of the present
invention, a dot (pixel) of an image is not limited to a dot (pixel)
that is formed by a single type of recording liquid (ink) . That
is, a dot of an image may be formed by forming plural types of
recording liquid (ink) to a single position. Therefore, a dot
(pixel) can be formed by jetting one or more droplets to a single
position on a medium. Furthermore, in the above-described
embodiments of the present invention, unless described as

otherwise, brightness (dot brightness) refers to the brightness
that is defined by a dot itself. That'is, brightness does not
include brightness of a covered area when viewed from a broad
(macro) perspective unless described as otherwise. Furthermore,
in the above-described embodiments of the present invention, a
character may be a letter, a symbol, or a thin line. For example,
a black character may include a thin black line. Furthermore,
in the above-described embodiments of the present invention, the
term "change" may include not only the meaning "change" but also
a meaning "replace", "correct", or "correct". Furthermore, in
the above-described embodiments of the present invention, among
the dots that form an outline part, the dots which are not corrected
(e.g., corrected by change of dot brightness) may be described
as other dots forming an outline part, other dots of an outline
part, or other dots situated in an outline part.
Further, the present invention is not limited to these
embodiments, but variations and modifications may be made without
departing from the scope of the present invention.
The present application is based on Japanese Priority
Application Nos.2006-252046 and 2006-252053 both filed on
September 19, 2006, with the Japanese Patent Office, the entire
contents of which are hereby incorporated by reference.

CLAIMS
1. An image forming apparatus for forming an image
on a medium by jetting one or more droplets on the medium, the
image forming apparatus comprising:
a dot brightness changing part for changing a dot
brightness of at least one target dot forming an outline part of
the image to a brightness relatively greater than the dot
brightness of other dots forming the outline part.
2. The image forming apparatus as claimed in claim
1, wherein the target dot is formed with a size smaller than the
size of other dots forming the outline part.
3. The image forming apparatus as claimed in claim
1, wherein when the image is a black image consisting of only black
color, the target dot is formed with a size smaller than the size
of other dots forming the outline part.
4. The image forming apparatus as claimed in claim
1, wherein when the image is a black image consisting of only black
color, the dots of the image are formed by a plurality of black
recording liquids having different brightness.
5. The image forming apparatus as claimed in claim

1, wherein when the image is a black image consisting of only black
color, the dots of the image are formed by a black recording liquid
and a recording liquid having a color other than black.
6. The image forming apparatus as claimed in claim
1, wherein the dot brightness of the target dot is changed by
changing a correction pattern.
7. The image forming apparatus as claimed in claim
1, wherein the target dot is formed in a position deviating from
a target dot formation position in a nozzle array direction or
a direction perpendicularly intersecting the nozzle array
direction.
8. A program for causing a computer to execute an
image forming process for forming an image on a medium by jetting
one or more droplets on the medium, the program comprising a step
of:
changing a dot brightness of at least one target dot
forming an outline part of the image to a brightness relatively
greater than the dot brightness of other dots forming the outline
part.
9. The program as claimed in claim 8, wherein the
target dot is formed with a size smaller than the size of other

dots forming the outline part.
10. The program as claimed in claim 8, wherein when
the image is a black image consisting of only black color, the
target dot is formed with a size smaller than the size of other
dots forming the outline part.
11. The program as claimed in claim 8, wherein when
the image is a black image consisting of only black color, the
dots of the image are formed by a plurality of black recording
liquids having different brightness.
12. The program as claimed in claim 8, wherein when
the image is a black image consisting of only black color, the
dots of the image are formed by a black recording liquid and a
recording liquid having a color other than black.
13. The program as claimed in claim 8, wherein the
dot brightness of the target dot is changed by changing a
correction pattern.
14. The program as claimed in claim 8, wherein the
target dot is formed in a position deviating from a target dot
formation position in a nozzle array direction or a direction
perpendicularly intersecting the nozzle array direction.

15. A computer-readable recording medium for
causing a computer to execute an image forming process for forming
an image on a medium by jetting one or more droplets on the medium,
the computer-readable recording medium comprising:
the program as claimed in claim 8.
16. An image forming method for forming an image on
a medium by jetting one or more droplets on the medium, the image
forming method comprising a step of:
changing a dot brightness of at least one target dot
forming an outline part of the image to a brightness relatively
greater than the dot brightness of other dots forming the outline
part.
17. An image forming apparatus for forming an image
on a medium by jetting one or more droplets on the medium, the
image forming apparatus comprising:
a dot brightness changing part for changing a dot
brightness of at least one target dot forming an outline part of
the image to a brightness relatively greater than the dot
brightness of other dots forming the outline part;
a dot size changing part for changing a dot size of
at least the target dot forming the outline part of the image to
a dot size different from the dot size of the other dots forming

the outline part;
wherein at least one of the dot brightness and the
dot size of the target dot is changed according to the color of
the image by the dot brightness changing part and the dot size
changing part.
18. An image forming apparatus for forming an image
including one or more dots on a medium by jetting one or more
droplets on the medium, the image forming apparatus comprising:
an outline correction part for correcting at least
one target dot forming an outline part of the image by replacing
the target dot with a replacement dot having a dot size different
from the dot size of the target dot; and
a replacement dot changing part for changing the
method of replacing the target dot according to a factor causing
deviation of dot formation position or deviation amount of the
dot formation position.
19. The image forming apparatus as claimed in claim
18, wherein the factor causing deviation of dot formation position
include at least one of ambient temperature, ambient humidity,
lapsed time from manufacture of the image forming apparatus, and
lapsed time from previous an image forming process.
20. The image forming apparatus as claimed in claim

18, further comprising:
a deviation amount detecting part for detecting the
deviation amount of the dot formation position.
21. The image forming apparatus as claimed in claim
18, wherein data related to the deviation amount of the dot
formation position are input from outside of the image forming
apparatus.
22. The image forming apparatus as claimed in claim
18, wherein the method of replacing the target dot is changed
according to printing condition of the image forming apparatus.
23. The image forming apparatus as claimed in claim
22, wherein the printing condition includes at least one of
resolution and type of medium.
24. The image forming apparatus as claimed in claim
18, wherein the replacement dot changing part is configured to
form the replacement dot by using a droplet relatively difficult
to cause deviation of dot formation position or by using less
droplets that tend to cause deviation of dot formation position.
25. The image forming apparatus as claimed in claim
18, wherein the changing of the method of replacing the target

dot includes not replacing the target dot.
26. A program for causing a computer of an image
forming apparatus to execute an image forming process for forming
an image including one or more dots on a medium by jetting one
or more droplets on the medium, the program comprising the steps
of:
correcting at least one target dot forming an outline
part of the image by replacing the target dot with a replacement
dot having a dot size different from the dot size of the target
dot; and
changing the method of replacing the target dot
according to a factor causing deviation of dot formation position
or deviation amount of the dot formation position.
27. The program as claimed in claim 26, wherein the
factor causing deviation of dot formation position include at
least one of ambient temperature, ambient humidity, lapsed time
from manufacture of the image forming apparatus, and lapsed time
from previous an image forming process.
28. The program as claimed in claim 26, further
comprising a step of:
detecting the deviation amount of the dot formation
position.

29. The program as claimed in claim 26, wherein data
related to the deviation amount of the dot formation position are
input from outside of the image forming apparatus.
30. The program as claimed in claim 26, wherein the
method of replacing the target dot is changed according to printing
condition of the image forming apparatus.
31. The program as claimed in claim 30, wherein the
printing condition includes at least one of resolution and type
of medium.
32. The program as claimed in claim 26, wherein the
replacement dot changing step is for forming the replacement dot
by using a droplet relatively difficult to cause deviation of dot
formation position or by using less droplets that tend to cause
deviation of dot formation position.
33. The program as claimed in claim 26, wherein the
changing of the method of replacing the target dot includes not
replacing the target dot.
34. A computer-readable recording medium for
causing a computer to execute an image forming process for forming

an image including one or more dots on a medium by jetting one
or more droplets on the medium, the computer-readable recording
medium comprising:
the program as claimed in claim 26.
35. An image forming method for forming an image
including one or more dots on a medium by jetting one or more
droplets on the medium, the image forming method comprising the
steps of:
correcting at least one target dot forming an outline
part of the image by replacing the target dot with a replacement
dot having a dot size different from the dot size of the target
dot; and
changing the method of replacing the target dot
according to a factor causing deviation of dot formation position
or deviation amount of the dot formation position.

The present invention discloses an image forming
apparatus for forming an image on a medium by jetting one or more droplets on the medium. The image forming apparatus includes a dot brightness changing part for changing a dot brightness of at least one target dot forming an outline part of the image to a brightness relatively greater than the dot brightness of other dots forming the outline part.

Documents:

01962-kolnp-2008-abstract.pdf

01962-kolnp-2008-claims.pdf

01962-kolnp-2008-correspondence others.pdf

01962-kolnp-2008-description complete.pdf

01962-kolnp-2008-drawings.pdf

01962-kolnp-2008-form 1.pdf

01962-kolnp-2008-form 3.pdf

01962-kolnp-2008-form 5.pdf

01962-kolnp-2008-gpa.pdf

01962-kolnp-2008-international publication.pdf

01962-kolnp-2008-international search report.pdf

01962-kolnp-2008-pct priority document notification.pdf

01962-kolnp-2008-pct request form.pdf

1962-KOLNP-2008-(21-03-2013)-CORRESPONDENCE.pdf

1962-KOLNP-2008-(21-03-2013)-FORM 3.pdf

1962-KOLNP-2008-(23-05-2014)-ABSTRACT.pdf

1962-KOLNP-2008-(23-05-2014)-CLAIMS.pdf

1962-KOLNP-2008-(23-05-2014)-CORRESPONDENCE.pdf

1962-KOLNP-2008-(23-05-2014)-DESCRIPTION (COMPLETE).pdf

1962-KOLNP-2008-(23-05-2014)-DRAWINGS.pdf

1962-KOLNP-2008-(23-05-2014)-FORM-1.pdf

1962-KOLNP-2008-(23-05-2014)-FORM-2.pdf

1962-KOLNP-2008-(23-05-2014)-FORM-3.pdf

1962-KOLNP-2008-(23-05-2014)-FORM-5.pdf

1962-KOLNP-2008-(23-05-2014)-PA.pdf

1962-KOLNP-2008-(23-05-2014)-PETITION UNDER RULE 137.pdf

1962-KOLNP-2008-(26-08-2013)-ANNEXURE TO FORM 3.pdf

1962-KOLNP-2008-(26-08-2013)-CORRESPONDENCE.pdf

1962-KOLNP-2008-(31-03-2014)-CORRESPONDENCE.pdf

1962-KOLNP-2008-(31-03-2014)-FORM-3.pdf

1962-KOLNP-2008-ASSIGNMENT.pdf

1962-KOLNP-2008-CORRESPONDENCE 1.1.pdf

1962-kolnp-2008-form 18.pdf

1962-KOLNP-2008-FORM 3.1.pdf

abstract-1962-kolnp-2008.jpg

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Patent Number

263110

Indian Patent Application Number

1962/KOLNP/2008

PG Journal Number

41/2014

Publication Date

10-Oct-2014

Grant Date

07-Oct-2014

Date of Filing

15-May-2008

Name of Patentee

RICOH COMPANY, LTD.

Applicant Address

3-6, NAKAMAGOME 1-CHOME, OHTA-KU TOKYO

Inventors:

#	Inventor's Name	Inventor's Address
1	TAKAGI YASUNOBU	2-37, NAKAHARA 3-CHOME HIRATSUKA-SHI, KANAGAWA 2540075
2	HOSHINO YOSHIAKI	5-8-107, SAIWAICHO, ATSUGI-SHI, KANAGAWA 2430012
3	ITO TAKAYUKI	9-19-206, IZUMICHO, ATSUGI-SHI, KANAGAWA 2430013
4	KIMURA TAKASHI	17-12-309, TAKAISHI 6-CHOME, ASAO-KU, KAWASAKI-SHI, KANAGAWA 2150003
5	HIRANO MASANORI	1253-7, SHIMOOGINO ATSUGI-SHI, KANAGAWA 2430203

PCT International Classification Number

B41J 2/01

PCT International Application Number

PCT/JP2007/068485

PCT International Filing date

2007-09-14

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2006-252053	2006-09-19	Japan
2	2006-252046	2006-09-19	Japan