JP2009006510A - Printing apparatus, printing method, and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique capable of matching halftone processing to the kind of an image to be printed. <P>SOLUTION: The dot forming state of a first type print area including a character area is determined by a first mode commonly using a reference dither matrix having a predetermined layout to halftone processing of each of N colors. The dot formation state of a second type print area that is the other print area is determined by a second mode using a reference dither matrix differed in layout to halftone processing of each of at least a part or two or more colors of the N-colors. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technique for printing an image by forming dots on a print medium.

  2. Description of the Related Art Printing apparatuses that form dots on a printing medium and print images are widely used as output apparatuses for images created by computers and images taken by digital cameras. In such a printing apparatus, since the gradation value of dots that can be formed with respect to the input gradation value is small, gradation expression is performed by halftone processing. As one of the halftone processes, a systematic dither method using a dither matrix is widely used. Since the systematic dither method greatly affects the image quality depending on the contents of the dither matrix, for example, as disclosed in Patent Document 1, analysis such as simulated annealing or genetic algorithm using an evaluation function taking human vision into consideration The dither matrix has been optimized by techniques. On the other hand, as disclosed in Patent Document 2, a technique for improving the dispersibility of a plurality of types of dots in a printed image formed by combining a plurality of types of dots having different densities and hues has been proposed.

JP-A-7-177351 JP-A-10-157167

  By the way, various types of images such as natural images such as landscape images and human images, and character images representing characters (characters) are printed by the printing apparatus. However, the fact is that no sufficient contrivance has been made in terms of adapting halftone processing to the type of image to be printed.

  The present invention has been made to solve at least a part of the above-described problems, and an object of the present invention is to provide a technique capable of adapting halftone processing to the type of image to be printed.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

[Application Example 1] A printing apparatus that performs printing on a print medium, by performing halftone processing using a dither matrix on image data representing an input gradation value of each pixel constituting an original image, A dot data generation unit that generates dot data representing a dot formation state on each print pixel of a print image to be formed on the print medium, and ejects ink of at least N colors (N is an integer of 2 or more) A print head including a plurality of nozzle rows, wherein each of the plurality of nozzle rows forms a dot group having a different color according to the dot data, and the at least N color dot groups are formed in a common print region. And a printing unit that generates the print image by combining them with each other, and the halftone process includes a standard arrangement with a predetermined arrangement for each halftone process of the N colors. A first mode that uses a matrix in common, and a second mode that uses the reference dither matrix that is arranged differently for each halftone process of at least some of the N colors. The dot data generation unit determines the dot formation state of the first type print area including the character area representing the character by the halftone process in the first mode, and the dots of the second type print area which are other print areas. A printing apparatus that determines a forming state by halftone processing in the second mode.

  According to this configuration, the dot formation state of the first type print area including the character area is determined by the first mode in which the reference dither matrix having a predetermined arrangement is commonly used for each halftone process of N colors. Characters can be printed sharply. The second mode uses a reference dither matrix in which the dot formation state of the second type printing area, which is another printing area, is different from each other in the halftone processing of at least some of the N colors. Therefore, it is possible to suppress the color reproduction range from being narrowed. In this way, halftone processing can be adapted to the type of image to be printed.

Application Example 2 The printing apparatus according to Application Example 1, wherein the second type printing area includes a natural image area representing a natural image.

  According to this configuration, it is possible to suppress the color reproduction range from being narrowed when printing a natural image.

[Application Example 3] In the printing apparatus according to Application Example 1 or Application Example 2, in the halftone processing according to the second mode, each of the N color halftone processes may be arranged differently from each other. Printing device that uses a dither matrix.

  According to this configuration, it is possible to further suppress the narrowing of the color reproduction range.

Application Example 4 In the printing apparatus according to any one of Application Example 1 to Application Example 3, the reference matrix includes the N color dot groups formed by halftone processing in the second mode. A printing apparatus in which a mixed color dot pattern is set to have a predetermined spatial frequency characteristic set in advance.

  According to this configuration, it is possible to easily set a reference matrix suitable for the second mode by paying attention to the mixed-color dot pattern for evaluation.

[Application Example 5] The printing apparatus according to any one of Application Examples 1 to 4, wherein the reference matrix includes the N color dot groups formed by halftone processing in the first mode. A printing apparatus in which a mixed color dot pattern is set to have a predetermined spatial frequency characteristic set in advance.

  According to this configuration, it is possible to easily set a reference matrix suitable for the first mode by paying attention to the mixed-color dot pattern for evaluation.

[Application Example 6] In the printing apparatus according to any one of Application Examples 1 to 5, the printing unit further includes a difference in physical conditions related to the formation of the dots in accordance with the dot data. A print image is generated by mutually combining dot groups formed in each of a plurality of assumed pixel groups in a common print area, and the dot pattern of each of the pixel groups is set in advance in the reference matrix. The printing apparatus is set to have a predetermined spatial frequency characteristic.

  According to this configuration, halftone processing considering each of a plurality of pixel groups is possible.

[Application Example 7] The printing apparatus according to Application Example 6, wherein the printing unit performs the main scan of the print head and converts the dot data into the dot data when the print head moves forward and backward. Accordingly, a dot is formed on each of the print pixels to generate a print image, and the plurality of pixel groups include a group of first pixel positions where dots are formed when the print head moves forward, and the print head And a second group of pixel positions where dots are formed during the backward movement.

  According to this configuration, it is possible to perform halftone processing in consideration of each dot pattern during forward movement and during backward movement.

[Application Example 8] The printing apparatus according to Application Example 6 or Application Example 7, wherein the printing unit repeats the main scanning cycle M times (M is an integer of 2 or more) of the print head. According to the data, dots are formed on each of the print pixels to generate a print image, and the plurality of pixel groups are divided according to the remainder obtained by dividing the numerical value representing the order of the main scan lines in the sub-scanning direction by M. A printing device comprising a group of pixel locations.

  According to this configuration, halftone processing can be performed in consideration of each dot pattern of the plurality of main scanning lines.

Application Example 9 The printing apparatus according to any one of Application Examples 4 to 8, wherein the predetermined spatial frequency characteristic is one of a blue noise characteristic and a green noise characteristic.

  According to this configuration, it is possible to suppress graininess in a region where human visual sensitivity is high.

[Application Example 10] The printing apparatus according to any one of Application Examples 6 to 8, wherein the predetermined spatial frequency characteristic is a dot pattern formed on a print pixel belonging to each of the plurality of pixel groups. A printing apparatus showing that each of the spatial frequency distributions and the spatial frequency distribution of the printed image have a positive correlation coefficient.

  According to this configuration, even if a physical difference occurs, the spatial frequency distribution of the dots to be formed is not greatly affected, so that halftone processing with high robustness against the physical difference can be configured.

Application Example 11 A method of generating a printed matter by forming a print image on a print medium, which is a halftone that uses a dither matrix for image data representing an input tone value of each pixel constituting the original image A dot data generation step for generating dot data representing a dot formation state on each print pixel of a print image to be formed on the print medium by performing processing, and at least N colors (N is an integer of 2 or more) ) And a plurality of nozzle rows that eject ink, each of the plurality of nozzle rows forms a dot group having a different color according to the dot data, and the dot group of at least N colors A printing process for generating the print image by mutually combining them in a common print area, and the halftone process includes each halftone of the N colors In fact, the reference dither matrix having a different arrangement is used for the first mode in which the reference dither matrix having a predetermined arrangement is commonly used and the halftone processing of each of at least some of the N colors. A second mode, wherein the dot data generation step determines a dot formation state of a first type print area including a character area representing a character by halftone processing according to the first mode; Determining the dot formation state of the second type printing area, which is an area, by halftone processing in the second mode.

Application Example 12 A computer program for causing a computer to generate print data to be supplied to a printing unit that performs printing on a print medium, the image data representing the input gradation value of each pixel constituting the original image The computer has a dot data generation function for generating dot data representing the dot formation state on each print pixel of the print image to be formed on the print medium by performing halftone processing using a dither matrix. The printing unit includes a print head including a plurality of nozzle rows that eject ink of at least N colors (N is an integer equal to or greater than 2), and each of the plurality of nozzle rows corresponds to the dot data. Forming a group of dots having different colors and combining the dot groups of at least N colors with each other in a common print region. An image is generated, and the halftone process includes a first mode in which a predetermined arrangement of reference dither matrices is commonly used for each of the N color halftone processes, and each of at least some of the N colors. A second mode that uses the reference dither matrix having different arrangements, and the dot data generation function includes a dot formation state of a first type print area including a character area representing a character. A function of determining the dot formation state of the second type print area, which is another print area, by the halftone process according to the second mode. Computer program.

  The present invention can be realized in various forms. For example, a printing method and apparatus, a computer program for realizing the function of the method or apparatus, a recording medium on which the computer program is recorded, etc. Can be realized.

Next, embodiments of the present invention will be described in the following order based on examples.
A. First embodiment:
B. Second embodiment:
C. Third embodiment:
D. Variations:

A. First embodiment:
FIG. 1 is a block diagram illustrating an example of a configuration of a printing system. This printing system includes a computer 90 as a printing control device and a color printer 20 as a printing unit. The combination of the color printer 20 and the computer 90 can be called a “printing apparatus” in a broad sense.

  In the computer 90, an application program 95 operates under a predetermined operating system. A video driver 91 and a printer driver 96 are incorporated in the operating system, and print data PD to be transferred to the color printer 20 is output from the application program 95 via these drivers. The application program 95 performs desired processing on the image to be processed, and displays the image on the CRT 21 via the video driver 91.

  Inside the printer driver 96, there are a resolution conversion module 97 for converting the resolution of the input image into a print resolution, a color conversion module 98 for converting RGB into CMYK, and a dither matrix M and errors generated in the embodiments described later. A halftone module 99 that reduces the input gradation value to the number of output gradations that can be expressed by dot formation using the diffusion method, and print data to be transmitted to the color printer 20 using the halftone data are generated. The print data generation module 100, a color conversion table LUT used as a reference for color conversion by the color conversion module 98, and a recording rate table DT for determining the recording rate of dots of each size for halftone processing are provided. It has been. The printer driver 96 corresponds to a program for realizing a function for generating print data PD. A program for realizing the function of the printer driver 96 is supplied in a form recorded on a computer-readable recording medium. Examples of such a recording medium include a CD-ROM 126, a flexible disk, a magneto-optical disk, an IC card, a ROM cartridge, a punch card, a printed matter on which a code such as a bar code is printed, an internal storage device of a computer (RAM, ROM, etc. A variety of computer-readable media such as an external storage device and an external storage device.

  FIG. 2 is a schematic configuration diagram of the color printer 20. The color printer 20 includes a sub-scan driving unit that transports the print medium P in the sub-scanning direction by a paper feed motor 22, a head driving mechanism that drives the print head 10A to control ink ejection and dot formation, and these papers. A control circuit 40 that controls the exchange of signals with the feed motor 22, the print head 10A, and the operation panel 32 is provided. The control circuit 40 is connected to the computer 90 via the connector 56. In the color printer 20, the main scanning of the print head 10A is not performed.

  FIG. 3 corresponds to the arrow AA in FIG. 2 and is an explanatory diagram showing the nozzle arrangement on the lower surface of the print head 10A. On the lower surface of the print head 10A, a black ink nozzle row K for discharging black ink, a cyan ink nozzle row C for discharging cyan ink, a magenta ink nozzle row Mz for discharging magenta ink, A yellow ink nozzle Y for discharging yellow ink is formed.

  The plurality of nozzles Nz in each nozzle row are aligned at a constant nozzle pitch k · D along a direction perpendicular to the paper feed direction. Here, k is an integer, and D is a pitch (referred to as “dot pitch”) corresponding to the printing resolution in a direction perpendicular to the paper feed direction. In this specification, it is also referred to as “nozzle pitch is k dots”. The unit [dot] at this time means the dot pitch of the printing resolution. Similarly, the unit of [dot] is used for the paper feed amount.

  In each nozzle row C, Mz, Y, and K included in the print head 10A, the nozzle pitch k is 1. The ink of each color can be ejected without causing the pixels to be missing by the print head 10A.

  FIG. 4 corresponds to the arrow BB in FIG. 2 and is an explanatory diagram showing the lateral surface of the print head 10A. In FIG. 4, only the nozzle row that discharges yellow ink Y is shown for easy understanding. In FIG. 4, the print medium P is fed in the paper feed direction indicated by the arrow.

  FIG. 5 is an explanatory diagram conceptually illustrating a part of the dither matrix. The illustrated matrix includes 128 elements in the horizontal direction (main scanning direction), 64 elements in the vertical direction (sub-scanning direction (paper feeding direction)), a total of 8192 elements, and a range of gradation values from 1 to 255. Uniformly selected threshold values are stored. Note that the size of the dither matrix is not limited to the size illustrated in FIG. 5 and can be various sizes including a matrix having the same number of vertical and horizontal elements.

  FIG. 6 is an explanatory diagram showing the concept of dot formation using a dither matrix. For the sake of illustration, only some elements are shown. In the determination of the presence / absence of dot formation, as shown in FIG. 6, the gradation value of the image data is compared with the threshold value stored at the corresponding position in the dither matrix. If the gradation value of the image data is larger than the threshold value stored in the dither table, a dot is formed, and if the gradation value of the image data is smaller, no dot is formed. In FIG. 6, hatched pixels mean pixels that are dots to be formed. In this way, if the dither matrix is used, it is possible to determine the presence or absence of dot formation for each pixel by a simple process of comparing the gradation value of the image data and the threshold value set in the dither matrix. The gradation number conversion process can be performed quickly. Furthermore, when the gradation value of the image data is determined, whether or not dots are formed in each pixel is determined solely by the threshold value set in the dither matrix. It is possible to positively control the dot generation state by the threshold storage position set in the matrix.

  As described above, since the systematic dither method can positively control the occurrence of dots according to the threshold storage position set in the dither matrix, the dot dispersion can be achieved by adjusting the threshold storage position setting. And other image quality can be controlled. This means that the halftone process can be optimized for various target states by the dither matrix optimization process.

  The halftone module 99 (FIG. 1) performs halftone processing by using such a dither matrix (also called a dither mask). This halftone process includes a process in the first mode and a process in the second mode.

  FIG. 7 is an explanatory diagram of a first mode of halftone processing. In the figure, a dither matrix Mc is shown. The dither matrix Mc is a shared dither matrix that is commonly used for halftone processing of ink colors of cyan, magenta, yellow, and black. In particular, in the first mode, the pixel position (arrangement) of the shared dither matrix Mc is common to each ink color. In other words, the threshold value of a certain pixel position on the print medium P is set to a threshold value associated with the pixel position among a plurality of threshold values of the shared dither matrix Mc. The threshold value is common to each ink color (cyan, magenta, yellow, black).

  FIG. 8 is an explanatory diagram showing a dot formation state in the first mode. In the figure, a first partial enlarged view MI1 of the print image is shown. In this first partial enlarged view MI1, a gray color is represented by a composite of cyan dots Cdot, magenta dots Mdot and yellow dots Ydot. Here, the threshold value at each pixel position is common to each ink color. As a result, each ink color dot is likely to be formed at the same pixel position.

  Also, the first line L1 shown in FIG. 8 indicates the boundary between the area A1g representing dark gray and the area A1w representing the brightest white. The gray area A1g is represented by a composite of three color dots Cdot, Mdot, and Ydot. On the other hand, the white area A1w is represented without ink dots. As described above, in the first mode, there is a high possibility that the dots Cdot, Mdot, and Ydot of each ink color are formed at the same pixel position. As a result, it is possible to suppress the blurring of the boundary as compared with the case where the dots Cdot, Mdot, and Ydot of each ink color are formed at different pixel positions.

  In this embodiment, a common dither matrix is used for four colors (cyan, magenta, yellow, and black). Therefore, blurring is similarly suppressed for the edge of the area represented by the composite of any two of these four colors.

  FIG. 9 is an explanatory diagram of the second mode of halftone processing. In the figure, four dither matrices Mc1 to Mc4 are shown. These matrices Mc1 to Mc4 are used for cyan, magenta, yellow, and black halftones, respectively.

  Each of the matrices Mc1 to Mc4 is the same as the shared dither matrix Mc (FIG. 7). However, the arrangement of the matrices Mc1 to Mc4 is different from each other. The shared dither matrix Mc arranged at a predetermined position (pixel position) is used as a cyan matrix Mc1. On the other hand, the shared dither matrix Mc to which the predetermined shift from the predetermined position is performed is used as a magenta matrix Mc2. Similarly, the shared dither matrix Mc to which another predetermined shift is applied from this predetermined position is used as a matrix Mc3 for yellow, and the shared dither matrix Mc to which another predetermined shift is applied from this predetermined position is Used as a matrix Mc4 for black.

  Note that these predetermined shifts differ for each ink color. As the shift, various movements including translational movement and rotational movement can be employed. Further, when the shift amount is excessively small, dots are likely to overlap. Therefore, it is preferable that the shift amount is large. For example, it is preferable that the ratio of the area where the two matrices Mc1 and Mc2 overlap is about 1/4 to 1/2 of the area of the shared dither matrix Mc. The same applies to the other two matrix combinations.

  FIG. 10 is an explanatory diagram showing a dot formation state in the second mode. In the drawing, a second partial enlarged view MI2 of the print image is shown. In the second partial enlarged view MI2, a gray color is represented by a composite of three color dots Cdot, Mdot, and Ydot. In the second mode, since the arrangement of the matrix of each ink color is different from each other, the threshold value at each pixel position is different for each ink color. As a result, it is unlikely that each ink color dot is formed at the same pixel position. As a result, it is possible to suppress a dot of a certain ink color from being hidden under a dot of another ink color, and thus it is possible to suppress a color reproduction range from being narrowed. Further, since the dots of each ink color are formed in a dispersed manner, it is suppressed that the graininess is conspicuous.

  In this embodiment, dither matrices having different arrangements are used for the four colors (cyan, magenta, yellow, and black). Therefore, the same advantage can be obtained for a region represented by a composite of any two of these four colors.

  FIG. 11 is a flowchart showing the procedure of the printing process. In the first step S900, the printer driver 96 (FIG. 1) reads image data. Then, the resolution conversion module 97 converts the resolution of the read image data. In the next step S902, the color conversion module 98 performs color conversion.

  In the next steps S904, S906a, and S906b, the halftone module 99 (FIG. 1) executes halftone processing. As described above, halftone processing includes two modes. The halftone module 99 uses two modes properly as follows.

  If the print region represented by the pixel of interest is a character region (character region), halftone processing in the first mode is executed for the pixel of interest (step S906a). As a result, when characters such as letters (letters) and numbers are printed, blurring of the edges of the characters is suppressed. Further, even when the printed character is small, the character is prevented from being crushed.

  The character area means an area representing a character. Character means a broad concept including letters (letters), numbers, symbols (signs), marks (marks), and symbols. Such characters represent words, language sounds, predetermined meanings, etc. in a visible form.

  If the print region represented by the pixel of interest is not a character region, halftone processing in the second mode is executed for the pixel of interest (step S906b). As a result, the color reproduction range of the print image can be widened as compared with the case of using the first mode. Moreover, it can also suppress that a graininess is conspicuous. For example, in a print area representing a natural image such as a landscape image or a person image, halftone processing in the second mode is executed. As a result, the quality of the printed natural image can be improved.

  The selection of the mode and the execution of the halftone process in the selected mode are executed for all the print pixels.

  An arbitrary method can be adopted as a method for determining whether or not the print area represented by the pixel of interest is a character area. For example, when the print area is represented by using a character code representing a character, the print area may be determined to be a character area. The halftone module 99 (FIG. 1) can make such a determination by analyzing the printing image data supplied for printing. In the embodiment shown in FIG. 1, the image data for printing is supplied by an application program 95.

  In the next step S908 (FIG. 11), the print data generation module 100 (FIG. 1) generates print data from the halftone data generated by the halftone process. This print data includes halftone data (data representing the dot formation state). The generated print data is transmitted to the color printer 20. The color printer 20 executes printing using the received print data.

  As described above, in the first embodiment, the first mode is adopted for the halftone processing of the character area, and the second mode is adopted for the halftone processing of the other areas. Therefore, the character is printed sharply. At the same time, the image quality of other areas can be improved.

  The mounting state of the first embodiment can be confirmed by the following method, for example. For example, if the dot pattern of the same gray print result is different between the first mode and the second mode, it can be determined that the printing is appropriately implemented.

  Various matrices can be used as the shared dither matrix Mc. For example, a matrix generated by various conventional methods can be employed.

B. Second embodiment:
B1. Device configuration:
In the second embodiment, the dither matrix characteristics and generation will be described. In the following description, another color printer 20b is used instead of the color printer 20 shown in FIG. Further, the printer driver 96 (FIG. 1) executes a printing process in the same manner as in the first embodiment shown in FIG. The following description regarding the characteristics and generation of the dither matrix can also be applied to the first embodiment.

  FIG. 12 is a schematic configuration diagram of the color printer 20b. The color printer 20b includes a sub-scan driving unit that transports the printing paper P in the sub-scanning direction by the paper feed motor 22, and a main motor that reciprocates the carriage 30 in the axial direction (main scanning direction) of the paper feed roller 25 by the carriage motor 24. A scanning drive unit, a head drive mechanism that drives a print head unit 60 (also referred to as “print head assembly”) mounted on the carriage 30 to control ink ejection and dot formation, and these paper feed motors 22, A carriage motor 24, a print head unit 60 including the print head 12, and a control circuit 40 that controls exchange of signals with the operation panel 32 are provided. The control circuit 40 is connected to the computer 90 via the connector 56.

  FIG. 13 is an explanatory diagram showing the nozzle arrangement on the lower surface of the print head 12. A cyan ink nozzle row Cb for discharging cyan ink, a magenta ink nozzle row Mzb for discharging magenta ink, and a yellow ink nozzle Yb for discharging yellow ink are formed on the lower surface of the print head 12. Has been.

  The plurality of nozzles Nz in each nozzle row are aligned at a constant nozzle pitch k · D along the sub-scanning direction. Here, k is an integer, and D is a pitch (referred to as “dot pitch”) corresponding to the printing resolution in the sub-scanning direction. In this specification, it is also referred to as “nozzle pitch is k dots”. The unit [dot] at this time means the dot pitch of the printing resolution. Similarly, the unit of [dot] is used for the sub-scan feed amount.

  Each nozzle Nz is provided with a piezo element (not shown) as a drive element for driving each nozzle Nz to eject ink droplets. During printing, ink droplets are ejected from each nozzle while the print head 12 moves in the main scanning direction.

  The color printer 20b having the above-described hardware configuration transports the printing paper P by the paper feed motor 22 and reciprocates the carriage 30 by the carriage motor 24, and simultaneously drives the piezo elements of the print head 12 so that each color. By ejecting ink droplets and forming ink dots, an image optimized for the visual system and the color printer 20b can be formed on the printing paper P. Specifically, a print image is formed as follows. In the following description, for easy understanding, an example (virtual example) of monochrome printing using only a single nozzle row (for example, cyan ink nozzle row Cb) is shown, and then this is extended to color printing. To do.

  FIG. 14 is an explanatory diagram showing an example of a method for generating a monochrome print image in the second embodiment of the present invention. In this example of the image forming method, a print image is generated by forming ink dots on a print medium while performing main scanning and sub-scanning. The main scanning means an operation of moving the print head 12 relative to the printing medium in the main scanning direction. Sub scanning means an operation of moving the print head 12 relative to the print medium in the sub scanning direction. The print head 12 is configured to eject ink droplets onto a print medium to form ink dots. The print head 12 is equipped with ten nozzles (not shown) at intervals of twice the pixel pitch k.

  The print image is generated as follows while performing main scanning and sub-scanning. In the main scan of pass 1, the pixel position number is 1, 3, 5, out of 10 main scan lines with raster numbers 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 Ink dots are formed on 7 pixels. The main scanning line means a line formed by pixels that are continuous in the main scanning direction. Each circle indicates a dot formation position. The numbers in each circle indicate a pixel group composed of a plurality of pixels on which ink dots are formed simultaneously. In pass 1, dots are formed in the print pixels belonging to the first pixel group.

  When the pass 1 main scan is completed, the sub-scan feed is performed in the sub-scan direction with a movement amount Ls that is five times the pixel pitch. In general, the sub-scan feed is performed by moving the print medium. However, in this embodiment, the print head 12 is moved in the sub-scan direction for easy understanding of the explanation. When the sub-scan feed is completed, the main scan of pass 2 is performed.

  In the pass 2 main scan, among the 10 main scan lines with raster numbers 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, the pixel position numbers are 1, 3, 5, Ink dots are formed on 7 pixels. In this way, in pass 2, dots are formed in the print pixels belonging to the second pixel group. The two main scanning lines with raster numbers 22 and 24 are not shown. When the pass 2 main scan is completed, the sub-scan feed similar to that described above is performed, and then the pass 3 main scan is performed.

  In the main scan of pass 3, ink is applied to pixels having pixel position numbers 2, 4, 6, and 8 out of 10 main scan lines including the main scan lines having raster numbers 11, 13, 15, 17, and 19. Dots are formed. In the pass 4 main scan, ink dots are applied to pixels having pixel position numbers 2, 4, 6, and 8 out of 10 main scan lines including three main scan lines with raster numbers 16, 18, and 20. Is formed. In this way, it can be seen that ink dots can be formed without gaps in the sub-scanning positions with raster numbers of 15 and later. In pass 3 and pass 4, dots are formed on the print pixels belonging to the third and fourth pixel groups, respectively.

  Observing the generation of such a printed image by paying attention to a certain area, it can be seen that the following is performed. For example, if an area having raster numbers 15 to 19 and pixel position numbers 1 to 8 is set as a target area, it can be seen that a print image is formed in the target area as follows.

  In pass 1, it can be seen that the same dot pattern as the ink dots formed at the pixel positions with the raster numbers 1 to 5 and the pixel position numbers 1 to 8 is formed in the region of interest. This dot pattern is formed of dots formed on pixels belonging to the first pixel group. That is, in pass 1, dots are formed in the pixels belonging to the first pixel group in the region of interest.

  In pass 2, dots are formed in the pixels belonging to the second pixel group in the region of interest. In pass 3, dots are formed in the pixels belonging to the third pixel group in the region of interest. In pass 4, dots are formed in the pixels belonging to the fourth pixel group in the region of interest.

  As described above, in the monochrome printing of the present embodiment, it is understood that a print image is formed by combining print pixels belonging to each of the first to fourth pixel groups in a common print region. .

  On the other hand, in the color printing of this embodiment, color printing is performed by ejecting Cb, Mzb, and Yb inks from the print head (FIG. 13) to each of the first to fourth pixel groups. An image is formed. Thus, in color printing, a plurality of colors of ink are ejected almost simultaneously in each main scan.

  FIG. 15 is an explanatory diagram illustrating a state in which a print image is generated on a print medium by combining print pixels belonging to each of a plurality of pixel groups in a common print area in the second embodiment of the present invention. It is. In the example of FIG. 15, the print image is a print image of a predetermined intermediate gradation (monochrome). The dot patterns DP1 and DP1a indicate dot patterns formed on a plurality of pixels belonging to the first pixel group. The dot patterns DP2 and DP2a indicate dot patterns formed on a plurality of pixels belonging to the first and second pixel groups. The dot patterns DP3 and DP3a indicate dot patterns formed on a plurality of pixels belonging to the first to third pixel groups. The dot patterns DP4 and DP4a indicate dot patterns formed on a plurality of pixels belonging to all pixel groups.

  The dot patterns DP1, DP2, DP3, and DP4 are dot patterns when a conventional dither matrix is used. The dot patterns DP1a, DP2a, DP3a, and DP4a are dot patterns when the dither matrix of the present invention is used. As can be seen from FIG. 15, when the dither matrix of the present invention is used, the dot dispersibility is more uniform than in the case of using the dither matrix of the prior art, particularly in the dot patterns DP1a and DP2a where the dot pattern overlap is small. It is.

  Since the conventional dither matrix does not have the concept of a pixel group, optimization is performed by paying attention only to the dispersibility of dots in a finally formed printed image (dot pattern DP4 in the example of FIG. 15). .

  However, the inventor of the present application dared to analyze the image quality of the printed image by paying attention to the dot pattern in the dot formation process. As a result of this analysis, it has been found that image unevenness occurs due to the density of the dot pattern in the dot formation process. The unevenness of this image is due to non-uniformity in the overlapping of multiple color dots formed in the same main scan, so the part where multiple color dots touch and the part where multiple color dots are separated and not blurred However, it has been found by the inventor that color unevenness occurs due to the occurrence of mottle.

  Such color unevenness can occur even when a printed image is formed in one pass. However, even if color unevenness occurs uniformly on the entire printed image, it is difficult for the human eye to perceive it. This is because, since they are uniformly generated, ink bleeding does not occur as uneven “unevenness” including low-frequency components.

  However, in a dot pattern formed in a pixel group in which ink dots are formed almost simultaneously in the same main scan, if unevenness occurs in a low-frequency region that is easily recognized by the human eye due to ink bleeding, this is a significant deterioration in image quality. It will become apparent. In this way, when forming a print image by forming ink dots, it is possible to optimize the dither matrix by focusing on the dot pattern formed in the pixel group in which the ink dots are formed almost simultaneously. It was first discovered by the inventor that it leads to

  However, it has been found that the use of a dither matrix for each of a plurality of ink colors causes problems such as processing of the printing system and an increase in the burden of hardware resources. Furthermore, the preparation of a dither matrix for each of a plurality of ink colors is accompanied by a heavy burden.

B2. Concept of the optimized dither matrix in the second embodiment:
FIG. 16 is an explanatory diagram conceptually illustrating the spatial frequency characteristics of threshold values set for each pixel of a blue noise dither matrix having blue noise characteristics as a simple example of dither matrix adjustment. The spatial frequency characteristic of the blue noise matrix is a characteristic in which the length of one cycle has the largest frequency component in a high frequency region near two pixels. Such spatial frequency characteristics are set in consideration of human visual characteristics. That is, the blue noise dither matrix is a dither matrix in which the threshold storage position is adjusted so that the largest frequency component is generated in the high frequency region in consideration of the human visual characteristic that the sensitivity is low in the high frequency region.

  FIG. 16 further illustrates the spatial frequency characteristics of the green noise matrix as a dashed curve. As shown in the figure, the spatial frequency characteristic of the green noise matrix is a characteristic having the largest frequency component in the intermediate frequency region in which the length of one cycle is from 2 pixels to several tens of pixels. Since the threshold value of the green noise matrix is set so as to have such a spatial frequency characteristic, when it is determined whether or not each pixel has a dot formation while referring to the dither matrix having the green noise characteristic, the threshold value is in units of several dots. While dots are formed adjacent to each other, the dots are formed in a dispersed state as a whole. In printers where it is difficult to stably form fine dots of about one pixel, such as so-called laser printers, it is possible to identify isolated dots by determining the presence or absence of dot formation with reference to such a green noise matrix. Occurrence can be suppressed. As a result, it is possible to quickly output an image with stable image quality. In other words, a dither matrix that is referred to when determining the presence or absence of dot formation by a laser printer or the like is set with a threshold value adjusted to have green noise characteristics.

  FIG. 17 is an explanatory diagram conceptually showing a visual spatial frequency characteristic VTF (Visual Transfer Function) which is a sensitivity characteristic with respect to the visual spatial frequency of a human. By using the visual spatial frequency characteristic VTF, the human visual sensitivity is modeled as a transfer function called the visual spatial frequency characteristic VTF, thereby quantifying the graininess of the dots after the halftone process appealing to the human visual sense. It becomes possible. The value quantified in this way is called the graininess index. Formula F1 shows a typical experimental formula representing the visual spatial frequency characteristic VTF. The variable L in the formula F1 represents the observation distance, and the variable u represents the spatial frequency. Formula F2 is a formula that defines the graininess index. The coefficient K in the formula F2 is a coefficient for matching the obtained value with the human sense.

  Such quantification of the granularity that appeals to human vision enables fine optimization of the dither matrix for the human visual system. Specifically, Fourier transform is performed on the dot pattern assumed when each input gradation value is input to the dither matrix to obtain the power spectrum FS, and all the inputs are obtained after multiplication with the visual spatial frequency characteristic VTF. The graininess index that can be obtained by integrating with the gradation value (formula F2) can be used as an evaluation function of the dither matrix. In this example, the optimization can be achieved by adjusting the threshold storage position so that the evaluation function of the dither matrix becomes small.

  What is common to dither matrices such as blue noise dither matrix and green noise matrix set in consideration of such human visual characteristics is the region of the spatial frequency with the highest human visual sensitivity on the print medium. This is that the average value of the components within a predetermined low frequency range from 0.5 millimeters per millimeter to 2 millimeters per cycle with a center frequency of 1 millimeter per cycle is set to be small. For example, the average value of the components in a predetermined low frequency range is at least 5 cycles per millimeter to 20 cycles per millimeter with a frequency of 10 cycles per millimeter at which human visual sensitivity is almost zero. By having a frequency characteristic that is smaller than the average value, graininess can be suppressed in a region where human visual sensitivity is high, so that effective image quality improvement focusing on human visual sensitivity is performed. The inventor has confirmed that this is possible.

  The halftone process in the embodiment of the present invention is a printing system in which the number of dither matrices used for the halftone is reduced by converting and using a common dither matrix generated by the methods of the following embodiments. The processing load and hardware resource load can be reduced.

B3. Dither matrix generation method in the second embodiment:
FIG. 18 is a flowchart showing the processing routine of the dither matrix generation method in the second embodiment of the present invention. The generation method of the second embodiment is configured so that optimization can be achieved in consideration of the dispersibility of dots formed by successive main scans (passes) in the print image forming process. In this example, a small dither matrix of 8 rows and 8 columns is generated for easy understanding. The granularity index (formula F2, FIG. 17) is used as an evaluation representing the optimality of the dither matrix.

  In step S100, a grouping process is performed. In the present embodiment, the grouping process is a process of dividing the dither matrix for each element corresponding to a plurality of pixel groups in which dots are formed by the same main scanning in the print image forming process (FIG. 14). .

  FIG. 19 is an explanatory diagram showing the dither matrix Mc that has undergone the grouping process in the second embodiment of the present invention. In this grouping process, the pixel group is divided into four pixel groups in FIG. The numbers described in each element of the initial dither matrix M0 indicate the pixel group to which each element belongs. For example, the element of 1 row and 1 column belongs to the first pixel group (FIG. 14), and the element of 2 rows and 1 column belongs to the second pixel group.

  FIG. 20 is an explanatory diagram showing four divided matrices M1 to M4 in the second embodiment of the present invention. The division matrix M1 includes a plurality of elements corresponding to the pixels belonging to the first pixel group among the elements of the initial dither matrix M0, and blank elements that are a plurality of blank elements. The blank element is an element in which dots are not always formed regardless of the input gradation value. Each of the divided matrices M2 to M4 includes a plurality of elements corresponding to pixels belonging to the second to fourth pixel groups among the elements of the dither matrix M, and blank elements.

  In this way, when the grouping process (FIG. 20) in step S100 is completed, the process proceeds to step S150.

  In step S150, matrix shift processing is performed. The matrix shift process is a process in which the dither matrix is shifted and arranged in the same state as that used in the above-described halftone process (FIG. 6).

  FIG. 21 is an explanatory diagram showing the contents of matrix shift processing in the second embodiment of the present invention. In the matrix shift processing of the second embodiment, a single shared dither matrix Mc used for halftone processing of cyan dots, magenta dots, and yellow dots is generated. When the shared dither matrix Mc is arranged at a predetermined position, the shared dither matrix Mc is used as a dither matrix Mc1 for determining the formation state of cyan dots. On the other hand, the common dither matrix Mc is subjected to a predetermined shift from a predetermined position, whereby a dither matrix Mc2 used for determining a magenta dot formation state and a dither matrix Mc3 used for determining a yellow dot formation state. Used for halftone processing. A combination of the dither matrices Mc1, Mc2, and Mc3 corresponds to the dither matrix M in FIG.

  In this embodiment, this shift processing is performed so that the divided matrices M1 to M4 coincide with each other for easy understanding of the description. In the example of this figure, it is shifted by Sx (2 pixels) in the main scanning direction, and is shifted by Sy (2 pixels) in the sub-scanning direction. However, such a shift does not require that the shift amounts in the main scanning direction and the sub-scanning direction match, and the shift may be performed only in one direction. Furthermore, the shift may include not only translational movement but also rotational movement. Note that if the shift is performed so that the divided matrices M1 to M4 coincide with each other as in this embodiment, there is an advantage that the burden of the dither matrix generation process can be reduced. This shift amount corresponds to the “predetermined unit shift amount” in the claims.

  In this way, when the matrix shift process (FIG. 21) in step S150 is completed, the process proceeds to step S200.

  In step S200, a target threshold value determination process is performed. The target threshold value determination process is a process for determining a threshold value to be a storage element determination target. In this embodiment, the threshold value is determined by selecting in order from a threshold value having a relatively small value, that is, a threshold value having a value at which dots are easily formed. Thus, if the dots are selected in order from the threshold at which dots are likely to be formed, the elements stored in order from the threshold for controlling the dot arrangement in the highlight area where the graininess of the dots is conspicuous are fixed. This is because a large degree of design freedom can be given to a highlight region where graininess is conspicuous. In this example, it is assumed that four threshold values have already been determined and the fifth threshold value is determined as described later.

  FIG. 22 is a flowchart showing the processing routine of the dither matrix evaluation process in the second embodiment of the present invention. In step S310, the corresponding dot of the determined threshold is turned on. The determined threshold means a threshold at which the storage element is determined. In this embodiment, since the threshold value is selected in order from the value at which dots are likely to be formed as described above, when a dot is formed as the threshold value of interest, the pixel corresponding to the element storing the determined threshold value is not used. Dots are always formed. Conversely, at the smallest input tone value at which dots are formed at the threshold value of interest, no dots are formed at pixels corresponding to elements other than the element storing the determined threshold value.

  FIG. 23 is an explanatory diagram illustrating a state in which dots are formed in each of three pixels of each color corresponding to an element storing four threshold values in which the first to fourth dots are easily formed in the shared dither matrix Mc. is there. The dot pattern Dpacmy configured in this way is used to determine in which pixel the fifth dot should be formed. The * mark will be described later.

  In the example of FIG. 23, these four threshold values are stored in storage elements of 3 rows and 1 column, 4 rows and 6 columns, 6 rows and 12 columns, and 12 rows and 7 columns. Assuming that the rows and columns of the dither matrix Mc1 (FIG. 22) coincide with the pixel positions, the four cyan dots are 3 rows and 1 column, 4 rows and 6 columns, 6 rows and 12 columns, and 12 rows and 7 rows. It is formed at each pixel position of the column. Four magenta dots are formed at pixel positions shifted from cyan dots by 2 rows and 2 columns, that is, pixel positions of 5 rows and 3 columns, 6 rows and 8 columns, 8 rows and 2 columns, and 2 rows and 9 columns, respectively. Yes. The four yellow dots are formed at pixel positions shifted from cyan dots by 4 rows and 4 columns, that is, pixel positions of 7 rows, 5 columns, 8 rows, 10 columns, 10 rows, 4 columns, and 4 rows, 11 columns, respectively. Yes.

  In step S320 (FIG. 22), a candidate storage element selection process is performed. The storage candidate element selection process is a process of selecting a storage candidate element that becomes a threshold storage element candidate from the elements of the divided matrix M1 selected as the evaluation matrix. In this example, as described above, the 1-row, 1-column storage element marked with * is selected as a storage candidate element.

  For selection of the storage candidate elements, for example, all of the storage elements other than the determined elements that are the four storage elements determined as the threshold storage elements of the shared dither matrix Mc may be sequentially selected. Alternatively, as long as there is an element that is not adjacent to the determined element, this may be preferentially selected.

  In step S330 (FIG. 22), it is assumed that a dot has been turned on for the selected storage candidate element. Specifically, dots are turned on in the storage elements of 1 row, 1 column, 3 rows, 3 columns, and 5 rows and 5 columns. The threshold value stored in the storage candidate element is not only used in the halftone process of cyan dots at the pixel position of 1 row and 1 column, but also the pixel position shifted by 2 pixels each in the main scanning direction and the sub-scanning direction This is because it is also used in halftone processing of magenta dots at the pixel position of 3 rows and 3 columns, and further used in halftone processing of yellow dots at the pixel positions of 5 rows and 5 columns. As a result, the shared dither matrix Mc can be evaluated when the threshold value at which the fifth dot is most likely to be formed is stored in the storage candidate element.

  FIG. 24 is an explanatory diagram showing a dot density matrix Ddacmy corresponding to the rating dot pattern Dpacmy (FIG. 23). In the dot density matrix Ddacmy, the numeral 1 indicates the formation of dots (including the case where it is assumed that dots are formed in the pixels corresponding to the storage candidate elements), and the numeral 0 indicates that dots are not formed. . In this way, based on the determination of one threshold storage element, all the formation states of cyan dots, magenta dots, and yellow dots are determined on the assumption that the shared dither matrix Mc is used in halftone processing. Therefore, the optimality of the threshold storage position for determining the dot formation state of each color can be evaluated at the same time.

  FIG. 25 is a flowchart showing a processing routine of evaluation value determination processing in the second embodiment of the present invention. In step S342, the graininess index is calculated by evaluating all inks for all pixels. Specifically, it is calculated by the equation F2 (FIG. 17) based on the dot density matrix Ddacmy (FIG. 24).

  In step S343, the graininess index is calculated by rating each ink for all pixels. Specifically, a dot density matrix Ddac (FIG. 26) in which only cyan dots are extracted from the dot density matrix Ddacmy (FIG. 26), a dot density matrix Ddam (FIG. 27) in which only magenta dots are extracted from the dot density matrix Ddacmy, and Based on a dot density matrix (not shown) in which only yellow dots are extracted from the dot density matrix Ddacmy, the calculation is similarly performed by the expression F2.

  In step S344, the graininess index is calculated by rating all inks for each pixel group (FIG. 20). Specifically, a dot density matrix Dd1cmy (FIG. 28) obtained by extracting only the first pixel group from the dot density matrix Ddacmy (FIG. 24), and a dot density matrix Dd2cmy obtained by extracting only the second pixel group from the dot density matrix Ddacmy. (FIG. 29), a dot density matrix (not shown) in which only the third pixel group is extracted from the dot density matrix Ddacmy, and a dot density matrix (not shown) in which only the fourth pixel group is extracted from the dot density matrix Ddacmy. ) On the basis of the formula F2.

  In step S345, the graininess index is calculated by rating each ink for each pixel group (FIG. 20). Specifically, it is similarly calculated by the formula F2 based on the dot density matrix obtained by extracting only the dots of each ink of each pixel group from the dot density matrix Ddacmy (FIG. 24). As such a dot density matrix, for example, a dot density matrix Dd1c (FIG. 30) obtained by extracting only the cyan dots of the first pixel group from the dot density matrix Ddacmy (FIG. 24) or only the magenta dots of the first pixel group are extracted. The dot density matrix Dd1m (FIG. 31).

  In step S346, a weighted addition process is performed. The weighted addition process is a process for weighting and adding each calculated granularity index.

  FIG. 32 is an explanatory diagram showing a calculation formula used for the weighted addition process. As can be seen from this calculation formula, the evaluation value E is a value obtained by multiplying the granularity index Gacmy (calculated in step S342) of all inks for all pixels by a weighting coefficient W1 (for example, 12), and each ink for all pixels. Are obtained by multiplying the graininess indices Gac, Gam, and Gay (calculated in step S342) by a weighting coefficient W2 (for example, 4), and four graininess indices G1cmy of all inks for each of the first to fourth pixel groups. , G2cmy, G3cmy, G4cmy (calculated in step S344) multiplied by a weighting coefficient W3 (for example, 3), and 12 graininess indexes G1c of each ink for each of the first to fourth pixel groups It is determined as the sum of the sum of G4y (calculated in step S345) and the weighted coefficient W4 (for example, 1).

  A series of processes (FIG. 22) from the storage candidate element selection process (step S320) to the evaluation value determination process (step S340) is performed for all the storage candidate elements (step S350). In this way, when the respective evaluation values are determined for all the storage candidate elements, the process proceeds to step S400 (FIG. 18).

  In step S400, a storage element determination process is performed. In the storage element determination process, the storage candidate element with the smallest evaluation value is determined as the storage element for the threshold of interest.

  Such processing (step S200 to step S400) is repeated while changing the threshold up to the final threshold (step S500). The final threshold value may be a maximum threshold value at which dots are hardly formed, or may be a maximum threshold value within a predetermined threshold range. This also applies to the threshold value to be evaluated first.

  As described above, in the second embodiment, it is assumed that halftone processing is performed for each dot of cyan, magenta, and yellow by arranging a single shared dither matrix Mc by shifting it twice. In such an assumed halftone process, a method for improving the overall dispersibility of the three color dots has been realized. The shared dither matrix Mc generated in this way has the advantage that the number of dither matrices used for halftone can be reduced to reduce the processing load of the printing system and the burden of hardware resources.

  In addition, the dot pattern used as a rating is not limited to the one described above, and may be configured as follows using, for example, a modified calculation formula (FIG. 33). That is, in the second embodiment, a mixed color pattern of three colors of cyan, magenta, and yellow is targeted for evaluation. For example, a reference is made of a mixed color pattern of two colors of cyan and magenta and a mixed color pattern of two colors of cyan and yellow. It may be configured to optimize the mixed color pattern of two colors including one color (cyan in this example) to be evaluated. In this way, the reference matrix can be set more appropriately. The reason why the reference matrix is set more appropriately is that the mixed color pattern of two colors in the halftone area causes a problem of graininess in the printed image. On the contrary, in the mixed color pattern of three colors, the number of dots becomes excessive and evaluation is difficult.

  In addition, the mixed color pattern of two colors of magenta and yellow is not subject to the evaluation. The mixed color pattern of two colors of cyan and magenta is used as the evaluation target, so that the mixed color pattern of two colors of magenta and yellow is simultaneously used. It is because it will be the object of rating. In this embodiment, as can be seen from FIG. 21, the relationship between the dither matrix Mc1 that determines the cyan dot formation state and the dither matrix Mc2 that determines the magenta dot formation state is the same as the formation of the dither matrix Mc2 and the yellow dots. This is because the relationship is the same as that of the dither matrix Mc3 that determines the state. Such a relative relationship is established because the dither matrices Mc1, Mc2, and Mc3 are configured by repeating the same shift process for a single common dither matrix Mc. It should be noted that “repeat the same shift process” can also be grasped as each of the shift processes of integer multiples (in this example, 1 and 2 times) of a predetermined unit shift amount.

  Note that it is not always necessary to perform such adjustment for all gradation values, but the low frequency component becomes relatively high when it is assumed that dots are evenly arranged on the print medium. It is preferable to perform adjustment with gradation values included in the range of the dot density up to.

  Moreover, as an evaluation value calculation formula, not only the formula shown in FIG. 32 and FIG. 33 but various formulas are employable. For example, a sum of terms arbitrarily selected from a plurality of terms included in the calculation formulas of FIGS. 32 and 33 can be employed. In any case, it is preferable to employ a calculation formula including at least the “graininess index Gacmy of all ink for all pixels” in the calculation formula of FIG. Employing such a calculation formula can improve the image quality of a printed image represented by a composite of all inks.

  Further, when gray halftone is reproduced by using a conventional dither matrix, approximately 20% of all dots of all ink colors are superimposed dots that overlap other dots. On the other hand, when the same gray halftone is reproduced by using the dither matrix generated by the above-described method of the second embodiment, the ratio of superimposed dots can be reduced to about 15%. As a result, it is possible to suppress the color reproduction range from being narrowed. Further, since the dots of each ink color are formed in a dispersed manner, it is possible to suppress the noticeable graininess.

  In the second embodiment, as described with reference to FIGS. 21 and 23, the dot formation state in a state where the shared dither matrix Mc is shifted and arranged for each ink color is evaluated. That is, the dot formation state in the second mode shown in FIG. 9 is evaluated. As a result, the image quality of the image printed in the second mode can be improved.

  Here, in order to further improve the image quality of the image printed in the first mode shown in FIG. 7, the dot formation state when the arrangement of the shared dither matrix Mc is common to each ink color may be evaluated. Specifically, the following evaluation value calculation formula can be employed.

Eb = Em1 * Wm1 + Em2 * Wm2
Eb: evaluation value Em1: first mode evaluation value Wm1: first mode weight coefficient Em2: second mode evaluation value Wm2: second mode weight coefficient

  The first mode evaluation value Em1 is an evaluation value calculated from the dot formation state in the first mode. For the calculation of the first mode evaluation value Em1, the dot formation state obtained by forming dots of each ink color at the same pixel position in step S330 of FIG. 22 is used. The second mode evaluation value Em2 is an evaluation value calculated from the dot formation state in the second mode. For the calculation of the second mode evaluation value Em2, a dot formation state obtained by forming dots of each ink color according to the dither matrix shifted for each ink color is used.

  These evaluation values Em1 and Em2 are calculated from the same calculation formula. However, these evaluation values Em1 and Em2 may be calculated from different calculation formulas. In any case, the above-described various calculation formulas can be adopted as calculation formulas for the evaluation values Em1 and Em2.

  Further, the first mode weighting factor Wm1 and the second mode weighting factor Wm2 are set to the same value (for example, 50). However, when one mode is more important than the other mode, the weight of one mode may be set to a value larger than the weight of the other mode.

  Alternatively, the dither matrix may be generated using only the first mode evaluation value Em1 without using the second mode evaluation value Em2.

  As described above, the case of using inks of three colors of cyan, magenta, and yellow has been described. However, when using a combination of other inks, a dither matrix can be similarly generated. For example, even when four colors of ink are used as in the first embodiment, the shared dither matrix Mc can be generated by the method described in the second embodiment.

C. Third embodiment:
In the third embodiment, another aspect of dither matrix characteristics and generation will be described. In the following description, the same color printer 20b (FIGS. 12 and 13) as in the second embodiment is used. Further, the printer driver 96 (FIG. 1) executes a printing process in the same manner as in the first embodiment shown in FIG. The following description regarding the characteristics and generation of the dither matrix can also be applied to the first embodiment.

C1. Generation of optimal dither matrix assuming bi-directional printing:
Bidirectional printing means printing by forming dots on the print pixels both during the forward movement and during the backward movement of the main scanning feed (referred to simply as main scanning in this specification) of the print head 12 (FIG. 12). Printing that generates an image. The dither matrix optimized for bidirectional printing is generated as follows in order to suppress image quality deterioration caused by bidirectional printing.

C2. Image quality degradation due to bidirectional printing and its suppression mechanism:
FIG. 34 is an explanatory diagram showing a dot pattern formed using a conventional dither matrix. In FIG. 34, three dot patterns Dpall, Dpf, and Dpb are respectively the dot pattern Dpall of the print image, the forward movement dot pattern Dpf formed during the main scanning forward movement of the print head 12, and the main pattern of the print head 12. The dot pattern Dpb at the time of backward movement formed at the time of backward movement of scanning is shown. The dot pattern Dpall of the print image is formed by combining the forward movement dot pattern Dpf and the backward movement dot pattern Dpb in a common print region.

  As can be seen from FIG. 34, the dot pattern Dpall of the printed image shows a relatively uniform dot dispersion, whereas the forward movement dot pattern Dpf and the backward movement dot pattern Dpb are dot density. Has occurred. Such density of dots is recognized by the human eye as significant image quality degradation. Such image quality degradation is caused by the fact that the conventional dither matrix is configured to improve the image quality of the dot pattern Dpall of the printed image. If the dot pattern Dpb can be combined without causing an error in the dot formation position as expected in advance, the dot pattern Dpb is not originally apparent.

  FIG. 35 is an explanatory diagram showing how the image quality of a print image formed using a conventional dither matrix deteriorates due to bidirectional printing. In FIG. 35, four dot patterns Dp11, Dp12, Df1, and Db1 are respectively a dot pattern Dp11 (no dot displacement) of the print image, a dot pattern Dp12 (dot displacement) of the print image, and the print head. 12 shows a forward movement dot pattern Df1 formed at the time of 12 main scanning forward movements, and a backward movement dot pattern Db1 formed at the time of backward movement of the main scanning of the print head 12.

  The dot pattern Dp11 (no dot misalignment) of the print image is the same as the dot pattern Dpall in FIG. The forward movement dot pattern Df1 is the same as the dot pattern Dpf of FIG. The backward movement dot pattern Db1 is the same as the dot pattern Dpb of FIG.

  In the dot pattern Dp12 (with dot misalignment) of the print image, the image quality is significantly degraded due to the relative misalignment between the forward dot pattern Df1 and the backward dot pattern Db1. The relative shift of the dot formation position is caused by the shift of the dot formation position in the main scanning direction by the dot patterns Df1 and Db1 as a whole due to the difference (forward or backward) in the main scanning direction during dot formation. . As described above, the image quality is significantly degraded due to the relative displacement of the dot pattern. As described above, the conventional dither matrix does not cause such displacement, and the dots are formed at accurate positions. This is because of the configuration. In other words, if there is no misalignment, the sparse and dense portions of the dot patterns Df1 and Db1 match each other with high precision, so that uniform dot dispersibility matches. In other words, the density of the dots may be matched with each other, and the density of the dots may be emphasized on the contrary, and the image quality is deteriorated.

  Based on such a hypothesis, the inventor of the present application confirmed that such image quality degradation was caused by bidirectional printing by conducting experiments on various images. Further, the inventor of the present application has come up with a dither matrix that is resistant (robust) to the misalignment of dots based on this hypothesis.

  FIG. 36 is an explanatory diagram showing a state in which image quality deterioration of a printed image formed by bidirectional printing is suppressed by the dither matrix according to the embodiment of the present invention. In FIG. 36, four dot patterns Dp21, Dp22, Df2, and Db2 are respectively a dot pattern Dp21 (no dot displacement) of the print image, a dot pattern Dp22 (dot displacement) of the print image, and the print head 12 shows a forward movement dot pattern Df2 formed at the time of 12 main scanning forward movements, and a backward movement dot pattern Db2 formed at the time of backward movement of the print head 12 by the main scanning.

  The dither matrix according to the embodiment of the present invention is configured so that the dot dispersibility of the forward movement dot pattern Df2 and the backward movement dot pattern Db2 is improved, and the dot patterns Df2 and Db2 are less dense. This is different from the dot patterns Df1 and Db1. In the dot pattern Dp22 (with dot misalignment) of the printed image formed by combining such sparse and small dot patterns Df2 and Db2, sparse parts due to the dot misalignment are inevitably generated. Since the overlap between the portions is also reduced, the density of the dots is reduced and the dispersibility is preferable.

  Thus, the inventor of the present invention does not improve the image quality by improving the accuracy of the dot formation position that has been conventionally performed, but the idea of reversal of a dither matrix configuration that has robustness against errors in the dot formation position. I came up with it. The inventor of the present application has also succeeded in realizing practical generation of a dither matrix having such characteristics.

C3. Generate optimal dither matrix based on granularity index:
FIG. 37 is a flowchart showing the processing routine of the dither matrix generation method in the third embodiment of the present invention. This dither matrix generation method is configured to be able to optimize in consideration of the dispersibility of dots formed during both forward movement and backward movement in the print image formation process. In this example, a small dither matrix of 8 rows and 8 columns is generated for easy understanding.

  In step S1100, a grouping process is performed. In the present embodiment, the grouping process is a dither matrix for each element corresponding to a pixel group in which dots are formed during forward movement and a pixel group in which dots are formed during backward movement in the print image formation process. This is a process of dividing.

  FIG. 38 is an explanatory diagram showing a dither matrix M and two divided matrices M1 and M2 on which grouping processing has been performed in the third embodiment of the present invention. In this grouping process, the pixel group is divided into two pixel groups in FIG. The numbers described in each element of the dither matrix M indicate the pixel group to which each element belongs. In this example, the elements in the odd rows belong to the first pixel group, and the elements in the even rows belong to the second pixel group.

  The division matrix M1 includes a plurality of elements corresponding to the pixels belonging to the first pixel group among the elements of the dither matrix M, and blank elements that are blank elements. On the other hand, the division matrix M2 includes a plurality of elements corresponding to the pixels belonging to the second pixel group among the elements of the dither matrix M, and blank elements that are blank elements. Such a grouping process is set assuming the following printing method.

  FIG. 39 is an explanatory diagram showing dot formation target pixels for each main scanning in bidirectional printing according to the third embodiment of the present invention. In this printing, dots are formed in each pixel by bidirectional printing using the nozzle row 10. The nozzle row 10 is a nozzle row representing the nozzle arrangement Yb, Mzb, Cb on the lower surface of the print head 12 (FIG. 13). The nozzle pitch k · D is 2D. Here, D is a pixel pitch corresponding to the printing resolution in the sub-scanning direction.

  In this bidirectional printing, dots are formed in each pixel as follows. In pass 1, which is the first main scanning, the nozzle row 10 performs main scanning sending in the forward direction, and dots are formed at each pixel position. As a result, dots are formed at the pixel positions in the odd-numbered rows of circles surrounding the numeral “0”. The group of pixels in which dots are formed in this way is the first pixel group. Sub-scan feed is performed after pass 1 is completed, and pass 2 main-scan feed is performed. In pass 2, the nozzle row 10 performs main scanning in the backward direction, and dots are formed at each pixel position. As a result, dots are formed at pixel positions in even-numbered rows of circles surrounding the numeral “2”. The group of pixels in which dots are formed in this way is the second pixel group.

  Thus, when the grouping process (FIG. 37) in step S1100 is completed, the process proceeds to the threshold value determination process (step S1200).

  Step S1200 is the same as step S200 in FIG. In the next step S1300, the dither matrix is evaluated according to the same procedure as in FIG. In step S340 of FIG. 22, the evaluation value is calculated according to the following evaluation value calculation formula.

Ec = Ga * Wa + Gf * Wf + Gb * Wb
Ec: Evaluation value Ga: All pixel index Wa: Weight coefficient of all pixel index Gf: Forward pixel index Wf: Forward pixel index weight coefficient Gb: Reverse pixel index Wb: Weight coefficient of backward pixel index

  The total pixel index Ga is a graininess index calculated from the dot formation state of all pixels regarding all ink colors. The forward pixel index Gf is a graininess index calculated from the dot formation state of pixels formed during the forward movement for all ink colors (the dot on / off distribution in the first divided matrix M1 in FIG. 38 is used). ) The backward pixel index Gb is a granularity index calculated from the dot formation state of pixels formed during backward movement for all ink colors (the dot on / off distribution in the second divided matrix M2 in FIG. 38 is used). )

  Further, the weight Wa of the all-pixel index Ga, the weight Gf of the forward pixel index Gf, and the weight Wb of the backward pixel index Gb are set to the same value (for example, 33). However, these weights Wa, Wf, and Wb may be set to different values.

  The next step S1400 in FIG. 37 is the same as step S400 in FIG. That is, the storage candidate element with the smallest evaluation value is determined as the storage element for the focus threshold value. Step S1500 is the same as step S500 in FIG. That is, a series of processes (steps S1200 to S1400) are repeated while changing the threshold up to the final threshold.

  When such processing is performed for all threshold values from the threshold at which dots are most easily formed to the threshold at which dots are hardly formed, the dither matrix generation processing is completed (step S1500).

  Thus, in the third embodiment, the dither matrix is determined so that the forward pixel index Gf and the backward pixel index Gb become smaller in addition to the total pixel index Ga. That is, the dispersibility of each dot in the forward movement image (FIG. 36: forward movement dot pattern Df2) and the backward movement image (FIG. 36: backward movement dot pattern Db2) is improved. The dither matrix is determined. As a result, it is possible to effectively suppress the graininess that the printed image formed by bidirectional printing appeals to human vision.

  As described above, the case of using inks of three colors of cyan, magenta, and yellow has been described. However, when using a combination of other inks, a dither matrix can be similarly generated. For example, even when four colors of ink are used as in the first embodiment, the shared dither matrix Mc can be generated by the method described in the third embodiment.

  Here, the generation of the dither matrix in the third embodiment is executed in consideration of one of the first mode shown in FIG. 7 and the second mode shown in FIG. 9, or both modes. Is possible. That is, the above-described evaluation value Ec may be calculated based on the dot formation state in the first mode. Instead, the evaluation value Ec may be calculated based on the dot formation state in the second mode. Alternatively, a weighted sum of the evaluation value Ec calculated from the dot formation state in the first mode and the evaluation value Ec calculated from the dot formation state in the second mode may be used. In any case, it is preferable to determine the shared dither matrix Mc so that the evaluation value becomes small.

  The above-described evaluation value Ec is calculated by combining all the ink dots. Similar to steps S343 and S345 in FIG. 25, the evaluation value Ec is calculated using the granularity index for each ink color. May be calculated. Further, as shown in FIG. 14, the evaluation value may be calculated using a granularity index for each pass.

D. Variations:
In addition, elements other than the elements claimed in the independent claims among the constituent elements in each of the above embodiments are additional elements and can be omitted as appropriate. The present invention is not limited to the above-described examples and embodiments, and can be implemented in various modes without departing from the gist thereof. For example, the following modifications are possible.

Modification 1:
In each of the above-described embodiments, various areas including a character area can be adopted as the first type print area to be subjected to the halftone process in the first mode. For example, an area representing a table or a graph may be adopted as the first type printing area. In addition, as the second type print area to be subjected to the halftone process in the second mode, various areas including a natural image area representing a natural image can be adopted. For example, an area representing graphics generated using a computer may be adopted as the second type print area.

  Various methods can be adopted as a method for discriminating between the first type print area and the second type print area. For example, the print area represented by using the character code may be identified as the first type print area, and the other area may be identified as the second type print area. Further, the print area represented by using the character code or the vector image data may be identified as the first type print area, and the other area may be identified as the second type print area. Also, the print area represented by using bitmap image data (for example, JPEG image data) may be identified as the second type print area, and the other areas may be identified as the first type print area. The character area is often represented by using a character code. On the other hand, natural image areas are often represented by using bitmap image data. Therefore, it is possible to make an appropriate determination by adopting these methods. In any case, the halftone module 99 (FIG. 1) can perform such identification (determination) by analyzing the printing image data. Further, the halftone module 99 may acquire information indicating the print area type in each print pixel from another processing unit (for example, another processing unit of the operating system), and identify (determine) according to the acquired information. . The conversion from the character code or vector image data to the input gradation value of each pixel is executed by the resolution conversion module 97 (FIG. 1) according to a known method.

Modification 2:
In the second mode of each embodiment described above, the arrangement of the shared dither matrix Mc is different for every ink color, but a common arrangement may be adopted for some ink colors. In general, it is sufficient that the arrangement is different for at least some of the plurality of ink colors. However, the arrangement is preferably different for all ink colors that can be mixed.

Modification 3:
In the third embodiment described above, the dither matrix is optimized by paying attention to the dot pattern formed in the pixel group. However, without paying attention to such a dot pattern, for example, the following method is used. It is also possible to perform optimization.

  FIG. 40 is an explanatory diagram illustrating an example of an actual printing state in the bidirectional printing method. The characters in the circles indicate in which reciprocal main scanning the dots are formed. FIG. 40A shows a dot pattern when there is no deviation in the main scanning direction. FIG. 40B and FIG. 40C show dot patterns when there is a deviation in the main scanning direction.

  In FIG. 40B, the position of the dot formed in the print pixel belonging to the pixel group in which the dot is formed when the print head moves forward belongs to the pixel group in which the dot is formed when the print head moves backward. The position of the dot formed on the print pixel is shifted by one dot pitch to the right. On the other hand, in FIG. 40C, a pixel group in which dots are formed when the print head is moved backward relative to the positions of dots formed in print pixels belonging to a pixel group in which dots are formed when the print head moves forward. The positions of the dots formed in the print pixels belonging to are shifted leftward by one dot pitch.

  In the third embodiment described above, the spatial frequency distribution of blue noise or green noise is applied to both the dot pattern of the pixel group in which dots are formed during forward movement and the dot pattern of the pixel group in which dots are formed during backward movement. Thus, image quality deterioration due to such a shift is suppressed.

  On the other hand, in this modification, the dot pattern formed in the pixel group formed in the forward movement and the dot pattern formed in the pixel group formed in the backward movement are only one dot pitch in the main scanning direction. The dot pattern synthesized by shifting has a spatial frequency distribution of blue noise or green noise, or has a small granularity index.

  The structure of the dither matrix focusing on the graininess index is, for example, the average value of the graininess index when the shift in the main scanning direction is shifted by one dot pitch on one side, by one dot pitch on the other side, and when there is no shift May be configured to be minimized, or the spatial frequency distribution in these cases may be configured to have a high correlation coefficient. The amount of shift may be 1 dot pitch or less, or 2 dot pitch or more, depending on the printing resolution or other printing environment.

  In this modification, the robustness of the image quality with respect to the deviation of the dot formation position in the forward movement and the backward movement can be increased, so the dot formation positions in the forward movement and the backward movement are collectively. In addition, the image quality deterioration is suppressed even when an unspecified shift occurs between a pixel group in which dots are formed during forward movement and a part of pixel groups in which dots are formed during backward movement. be able to. For example, even when the gap between the print head and the printing paper partially fluctuates between forward movement and backward movement due to periodic deformation caused by the main scanning of the main scanning mechanism of the print head, the image quality deteriorates. Can be suppressed.

Modification 4:
In the above-described embodiment, the granularity index is used as the evaluation measure of the dither matrix. However, for example, RMS granularity as described later may be used. This evaluation scale uses a low-pass filter process for the dot density value using a low-pass filter (FIG. 41) and a predetermined calculation formula (FIG. 42) for the density value subjected to the low-pass filter process. It can be determined by calculating the standard deviation.

Modification 5:
In the above-described embodiments and modifications, the evaluation value is calculated using the granularity index of the dot pattern and the RMS granularity. For example, a threshold is set as an element corresponding to a pixel having a low dot density after the low-pass filter processing. May be used in order. When a low-pass filter is used, not only the above weighting but also the range of the low-pass filter may be adjusted according to the degree of image quality deterioration caused by the influence of color mixing. For example, when it is desired to suppress only the contact and overlap rather than the dot dispersibility, it can be dealt with by reducing the range of the low-pass filter.

  Furthermore, the evaluation value is not limited to the dot pattern granularity index and RMS granularity, and the evaluation scale (Grainess scale: GS value) used by Xerox Dooley et al. Is applied to the dot pattern, and the dither matrix is optimized by the GS value. You may comprise so that property may be evaluated. Here, the GS value is obtained by performing a predetermined process including a two-dimensional Fourier transform on the dot pattern to digitize the dot pattern, and performing integration after performing a filter process that multiplies the visual spatial frequency characteristic VTF. It is a graininess evaluation value that can be Further, as a result, a correlation coefficient with a state (blue noise characteristic or green noise characteristic) where the degree of dot contact is small may be used.

<First characteristic>
Here, it is preferable that the dot pattern formed by printing in each of the above-described embodiments (variants) exhibits the first characteristic described below. This first characteristic is one of a blue noise characteristic and a green noise characteristic. If the dot pattern exhibits such characteristics, it is possible to suppress graininess in a region where human visual sensitivity is high.

<Second characteristic>
Further, in the spatial frequency distribution of the dot pattern, it is preferable that the peak frequency is higher than 4 cycles per millimeter. By doing so, it is possible to suppress graininess in a region where human visual sensitivity is high (FIG. 17).

<Third characteristic>
Moreover, it is also preferable that a dot pattern shows the 3rd characteristic demonstrated below. This third characteristic is that the average intensity value of the component in the range from 0.5 cycle per millimeter to 2 cycle per millimeter is on the print medium for the component in the range from 5 cycle per millimeter to 20 cycle per millimeter. This is a characteristic indicating that it is smaller than the average intensity value. If the dot pattern exhibits the third characteristic, it is possible to suppress graininess in an area where human visual sensitivity is high (FIG. 17). In addition, as an average intensity value of components within a certain frequency range, an average intensity value indicated by the measurement result of the spatial frequency distribution can be adopted. Here, as the average value, an average value calculated using an equal weight independent of the frequency can be employed.

  Note that at least one of the dot pattern according to the first mode shown in FIG. 7 and FIG. 8 and the dot pattern according to the second mode shown in FIG. 9 and FIG. It is preferable to show at least a part of the third characteristic). Moreover, it is preferable that the dot pattern of at least some of the plurality of passes as shown in FIG. 14 exhibits at least some of the various characteristics described above. Further, when using bidirectional printing, it is preferable that at least one of the forward movement dot pattern and the backward movement dot pattern exhibits at least a part of the various characteristics described above.

<4th characteristic>
Also, as in the second and third embodiments described above, a print image is generated by superimposing dot patterns of a plurality of pixel groups that are assumed to have different physical conditions regarding dot formation. There is. For example, in the second embodiment shown in FIGS. 14 and 19, a print image is formed by superimposing four pixel groups divided according to the remainder obtained by dividing the pass number by four. In the third embodiment shown in FIGS. 36 and 38, a print image is formed by superimposing a pixel group formed during forward movement and a pixel group formed during backward movement. In such a case, it is preferable that the spatial frequency characteristics of the dot pattern of each pixel group exhibit a fourth characteristic described below.

  This fourth characteristic indicates that each of the spatial frequency distributions of the dot patterns formed on the printing pixels belonging to each of the plurality of pixel groups and the spatial frequency distribution of the printed image have a positive correlation coefficient. ing. Here, the spatial frequency distribution of the print image indicates the spatial frequency distribution of the dot pattern in a state where all pixel groups are superimposed. If the dot pattern of each pixel group exhibits this fourth characteristic, even if a physical difference occurs, the spatial frequency distribution of the formed dots will not be significantly affected, so that the halftone processing with high robustness against the physical difference will occur. Can be configured. Here, the correlation coefficient is preferably 0.7 or more.

  In this specification, “correlation coefficient” means Pearson's product-moment correlation coefficient generally used as a correlation coefficient. Pearson's product moment correlation coefficient is one of the statistical indicators showing the correlation (degree of similarity) between two data strings. It is said that there is a positive correlation between the two data strings when close to each other, and there is a negative correlation when close to -1. When it is close to 0, the correlation between the two data strings is weak. A correlation coefficient of 0.7 or higher generally means that there is a correlation that is so strong that it cannot occur as a coincidence.

  The Pearson product moment correlation coefficient is obtained by dividing the covariance of two data strings by the product of the standard deviations of the two data strings. In other words, it can be said that the value is normalized from −1 to 1 by dividing the covariance by the product of the standard deviations of the two data strings. In the present invention, the two data strings correspond to two arbitrarily selected from a plurality of data strings obtained by discretizing the spatial frequency distribution of each dot pattern.

  In addition, it can confirm using the test | inspection method in statistical engineering, for example that this invention is mounted in a printing apparatus reliably and has an effect. This test method calculates the probability of occurrence of the null hypothesis (the present invention is not implemented), and when the probability is low to some extent, determines that the null hypothesis is wrong and instead uses the alternative hypothesis (the present invention). Is implemented). Here, the probability (significance level) that serves as a reference for determining that the null hypothesis is wrong is determined as a quality assurance requirement for the design. Thereby, design quality assurance can be realized without checking all the gradations and colors.

Specifically, for example, the following method can be used to assure the quality of the design that the present invention is reliably mounted on the printing apparatus and is effective.
(1) A predetermined number of gray-tone samples are printed for each pixel group using a printing apparatus to be evaluated.
(2) The spatial frequency distribution is measured for each printed pattern.
(3) A correlation coefficient between a plurality of measured spatial frequency distributions is obtained.
(4) Confirm that the correlation coefficient is positive or 0.7 or more.
Here, the probability of occurrence of the null hypothesis (the present invention is not implemented) decreases as the number of gray tone samples increases.

  As described above, the correlation coefficient as the predetermined characteristic does not necessarily have to be provided for all gradations reproduced by the halftone process, and is provided for some gradations. Just do it. Here, the “partial gradation” is desirably a gradation having a relatively low dot density. This is because dots are relatively conspicuous in gradations having a relatively low dot density. In such a case, for example, it is only necessary to confirm the correlation coefficient in continuous gradations where the dot density is relatively low.

<Fifth characteristic>
It is also preferable that the spatial frequency characteristics of the dot pattern of each pixel group exhibit a fifth characteristic described below. The fifth characteristic is that the M is within a predetermined low frequency range of 4 cycles per millimeter or less, which is a spatial frequency region where human visual sensitivity is relatively high on a print medium disposed at an observation distance of 300 mm. The predetermined color mixture pattern composed of color dot groups has such a characteristic that there exists a frequency band that is closest to the predetermined characteristic of the spatial frequency of the dot pattern of the printed image. If the dot pattern of each pixel group exhibits this fifth characteristic, image quality deterioration can be suppressed in a region where human visual sensitivity is high.

  Here, the correlation coefficient between the spatial frequency distribution of the dot pattern of the pixel group and the spatial frequency distribution of the dot pattern of the printed image within a low frequency range of 4 cycles per millimeter or less is higher than that of millimeters per 4 cycles. It is preferably larger than the correlation coefficient in the high frequency range.

  Further, as the predetermined spatial frequency characteristics indicated by the dot pattern, the following characteristics may be employed. For example, the fifth characteristic described above is a graininess index calculated by a calculation process including a Fourier transform process, and the graininess index is obtained by a VTF function determined based on a visual spatial frequency characteristic and a Fourier transform process. It is good also as calculating based on the product with the constant calculated beforehand. The predetermined characteristic may be RMS granularity calculated by a calculation process including a low-pass filter process.

  By the way, in the dither matrix generation method described in the above-described embodiments and modifications, the dither matrix is generated so that the intensity of the spatial frequency distribution of the dot pattern is reduced in a relatively low spatial frequency range ( In particular, as shown in FIG. 17, the intensity decreases in the range of 4 cycles per millimeter or less where the visual sensitivity is relatively high). As a result, by using the dither matrix generated by the above-described generation method, it is possible to easily form a dot pattern that exhibits the various characteristics described above.

  In any case, the above-described various characteristics do not need to be shown over the entire color range reproduced by the halftone process, and may be shown in a part of the color reproduction range. In particular, it is preferable that at least a part of the various characteristics described above is shown when reproducing at least a part of gray. Since the gray area is seen in various printed images, the graininess of the various printed images can be suppressed. Note that gray means an achromatic color whose brightness is set to a value between the maximum and minimum.

Modification 6:
In the above-described embodiment, the evaluation process is performed for each storage element having one threshold value. However, the present invention can also be applied to a case in which a plurality of storage elements having a threshold value are simultaneously determined. . Specifically, for example, in the above-described embodiment, when the storage elements for the sixth threshold are determined and the storage elements for the seventh and eighth thresholds are determined, the seventh threshold storage element is also determined. The storage element may be determined based on the evaluation value when the dot is added to the storage element and the evaluation value when the dot is added to each of the storage elements of the seventh and eighth threshold values, or Only the storage element of the seventh threshold value may be determined.

Modification 7:
In the above-described embodiment, the threshold storage elements are determined in order. However, for example, the dither matrix may be generated by adjusting the dither matrix as an initial state prepared in advance. . For example, a dither matrix is prepared as an initial state for storing in each element a plurality of threshold values for determining the presence or absence of dot formation for each pixel according to the input gradation value, and a plurality of threshold values stored in each element A part of this is replaced with a threshold value stored in another element in a randomly or systematically determined manner, and the dither matrix is adjusted by determining whether or not to replace based on the evaluation values before and after the replacement. May be generated.

Modification 8:
In the above-described embodiment, the presence or absence of dot formation is determined for each pixel by comparing the threshold value set in the dither matrix and the gradation value of the image data for each pixel. The sum of values may be compared with a fixed value to determine the presence or absence of dot formation. Furthermore, the presence / absence of dot formation may be determined according to the data generated in advance based on the threshold value and the gradation value without directly using the threshold value.

  Furthermore, the dither matrix of the present invention uses intermediate data (number data) for specifying the dot formation state as disclosed in, for example, Japanese Patent Application Laid-Open Nos. 2005-236768 and 2005-269527. Such a technique has a broad concept including a conversion table (or correspondence table) generated using a dither matrix. Such a conversion table is not only generated directly from the dither matrix generated by the generation method of the present invention, but may be adjusted or improved. In such a case, the conversion table is also generated by the generation method of the present invention. Corresponds to the use of a dither matrix.

  In addition, the use of a dither matrix in the printing device, printing method, and printed material generation method is based on whether or not dots are formed for each pixel by comparing the threshold value set in the dither matrix and the gradation value of the image data for each pixel. However, for example, the presence or absence of dot formation may be determined by comparing the sum of the threshold value and the gradation value with a fixed value. Furthermore, the presence / absence of dot formation may be determined according to the data generated in advance based on the threshold value and the gradation value without directly using the threshold value. In general, the dither method of the present invention only needs to determine the presence or absence of dot formation according to the gradation value of each pixel and the threshold value set at the corresponding pixel position of the dither matrix.

Modification 9:
In the above-described embodiments, various combinations can be adopted as the combination of the nozzle pitch, the total number of nozzles, and the movement amount in the sub-scanning direction. Various patterns can be adopted as the pixel pattern of each pixel group. In determining the dot formation state of one ink color, a large print area is covered by connecting the shared dither matrix Mc according to a predetermined rule. Here, any rule can be adopted as the connection rule. For example, the shared dither matrix Mc arranged around one shared dither matrix Mc may be shifted in at least one of the main scanning direction and the sub-scanning direction with respect to the central shared dither matrix Mc. In any case, the shared dither matrix Mc (dot pattern) is evaluated by using the dot pattern obtained by connecting the shared dither matrix Mc to the same state as the state where the halftone process is actually executed. It is preferable.

Modification 10:
In each of the above-described embodiments, various pixel groups in which differences in physical conditions related to dot formation are assumed can be adopted as the pixel group. For example, a group of pixels having the same dot formation time in the common print area may be employed. In this case, the dot formation time differs for each pixel group. Further, a group of pixels having the same moving direction of the print head at the time of dot formation may be employed. In this case, the moving direction is different for each pixel group. In addition, when using a plurality of nozzle rows, a group of pixels having the same nozzle row used for dot formation may be employed. In this case, the nozzle row is different for each pixel group. Similarly, when using a plurality of print heads, pixel groups having the same print head used for dot formation may be employed. In any case, it is preferable that at least some of the dot patterns of the plurality of pixel groups exhibit at least some of the various characteristics described above.

Modification 11:
In the first embodiment shown in FIG. 2, a plurality of nozzles may be divided into a plurality of print heads. For example, as shown in FIG. 43, two print heads 14A and 14B may be used. FIG. 43 shows the lower surfaces of the print heads 14A and 14B, as in FIG. The configuration of the first print head 14A is the same as that of the print head 10A shown in FIG. Here, the nozzle pitch is set to “2”. In the example of FIG. 43, the nozzle arrangement in each nozzle row is a staggered arrangement, but the nozzles may be arranged in one row.

  A second print head 14B is disposed on the paper feed direction side of the first print head 14A. The configuration of the second print head 14B is the same as that of the first print head 14A. The second print head 14B is disposed at a position shifted from the first print head 14A by one dot pitch (D) in a direction perpendicular to the paper feed direction. The two print heads 14A and 14B can eject ink of each color to all the print pixels. For example, odd-numbered lines are printed by the first print head 14A, and even-numbered lines are printed by the second print head 14B.

  FIG. 44 shows the lateral surfaces of the print heads 14A and 14B, as in FIG. As shown in the drawing, ink droplets are ejected first by the first print head 14A to the common print region, and then ink droplets are ejected by the second print head 14B. As can be seen from FIG. 43, the first print head 14 </ b> A and the second print head 14 </ b> B are pixels different from each other, but eject ink droplets to pixels adjacent to each other in a common print region.

  Here, a dither matrix considering the evaluation for each pixel group as shown in the second embodiment and the third embodiment may be used. The print pixels on the print medium P are divided into a preceding pixel group in which dots are formed by the first print head 14A and a subsequent pixel group in which dots are formed by the second print head 14B (for example, an odd number) Line group and even line group). Here, as in the evaluation of the forward pixel group and the forward pixel group in the third embodiment shown in FIGS. 38 and 39, the dither matrix is generated by evaluating the preceding pixel group and the subsequent pixel group. It is possible. In this way, it is possible to effectively suppress the graininess caused by the dot formation position error between the two print heads 14A and 14B. Here, a plurality of pixel groups obtained by dividing each of the preceding and succeeding pixel groups for each ink may be evaluated.

  In general, a printer having the following characteristics can be used. That is, for each of at least L types (L is an integer of 1 or more) of a plurality of usable inks, there are M sets of nozzle groups (M is an integer of 2 or more) that eject the same ink. A printed image is formed by combining M groups of dots including subgroups and formed by each of the M nozzle subgroups in a common printing area. Here, as the nozzle subgroup, a nozzle row in which a plurality of nozzles are arranged along a predetermined direction can be employed. In addition, the dot formation time in the common print region is different for each of the M nozzle subgroups, and the nozzle position in the predetermined direction is different for each of the M nozzle subgroups. For example, in the example of FIG. 43, four nozzle rows (nozzle subgroups) are provided for each ink (a staggered arrangement corresponds to two rows). When such a printer is used, M pixel groups each handled by the M nozzle subgroups can be used as evaluation targets. Here, some of the plurality of pixel groups may be collected. For example, when a plurality of print heads are used, the whole of a plurality of pixel groups having the same print head used for dot formation may be used as one pixel group. Here, a plurality of pixel groups having different inks may be collected.

  Here, as in the modification shown in FIG. 40, a synthesized dot pattern obtained when a dot formation position shift that differs for each pixel group may be evaluated. As the synthesized dot pattern, a pattern obtained by synthesizing all the pixel groups is preferably adopted, but a pattern obtained by synthesizing some of the plurality of pixel groups may be adopted.

  The above description is applicable not only to a printer that moves the print medium P in a predetermined direction without moving the print head, but also to a printer that moves both the print head and the print medium P.

Modification 12:
In each of the above embodiments, a part of the configuration realized by hardware may be replaced with software, and conversely, a part of the configuration realized by software may be replaced by hardware. . For example, the function of the halftone module 99 in FIG. 1 may be realized by a hardware circuit having a logic circuit.

  In addition, when part or all of the functions of the present invention are realized by software, the software (computer program) can be provided in a form stored in a computer-readable recording medium. In the present invention, the “computer-readable recording medium” is not limited to a portable recording medium such as a flexible disk or a CD-ROM, but an internal storage device in a computer such as various RAMs and ROMs, a hard disk, and the like. An external storage device fixed to the computer is also included.

1 is a block diagram illustrating an example of a configuration of a printing system. 1 is a schematic configuration diagram of a color printer 20. FIG. Explanatory drawing which shows the nozzle arrangement in the lower surface of 10 A of print heads. Explanatory drawing which shows the horizontal surface of 10 A of print heads. Explanatory drawing which illustrated a part of dither matrix conceptually. Explanatory drawing which shows the idea of the presence or absence of the dot formation using a dither matrix. It is explanatory drawing of the 1st mode of a halftone process. It is explanatory drawing which shows the dot formation state in 1st mode. It is explanatory drawing of the 2nd mode of a halftone process. It is explanatory drawing which shows the dot formation state in a 2nd mode. It is a flowchart which shows the procedure of a printing process. FIG. 2 is a schematic configuration diagram of a color printer 20b. FIG. 3 is an explanatory diagram showing a nozzle arrangement on the lower surface of the print head. Explanatory drawing which shows an example of the production | generation method of a monochrome printed image. FIG. 3 is an explanatory diagram illustrating a state in which a print image is generated on a print medium by combining print pixels belonging to each of a plurality of pixel groups with each other in a common print region. Explanatory drawing which illustrated notionally the spatial frequency characteristic of the threshold value set to each pixel of the blue noise dither matrix which has a blue noise characteristic. Explanatory drawing which showed notionally the visual spatial frequency characteristic VTF (Visual Transfer Function) which is a sensitivity characteristic with respect to the visual spatial frequency which a human has. The flowchart which shows the processing routine of the production | generation method of a dither matrix. Explanatory drawing which shows the dither matrix M in which the grouping process was performed. Explanatory drawing which shows four division | segmentation matrices M1-M4. Explanatory drawing which shows the content of the matrix shift process. The flowchart which shows the process routine of a dither matrix evaluation process. Explanatory drawing which shows a mode that the dot was formed in each of three pixels of each color corresponding to the element in which four threshold values in which the first to fourth dots are easily formed in the shared dither matrix Mc are stored. Explanatory drawing which shows the dot density matrix Ddacmy corresponding to the rating dot pattern Dpacmy. The flowchart which shows the processing routine of evaluation value determination processing. Explanatory drawing which shows the dot density matrix Ddac which extracted only the cyan dot from the dot density matrix Ddacmy of all the ink about all the pixels. Explanatory drawing which shows the dot density matrix Ddam which extracted only the magenta dot from the dot density matrix Ddacmy of all the ink about all the pixels. Explanatory drawing which shows dot density matrix Dd1cmy which extracted only the 1st pixel group from dot density matrix Ddacmy of all the ink about all the pixels. Explanatory drawing which shows the dot density matrix Dd1cmy which extracted only the 2nd pixel group from the dot density matrix Ddacmy of all the ink about all the pixels. Explanatory drawing which shows dot density matrix Dd1c which extracted only the cyan dot of the 1st pixel group from dot density matrix Ddacmy of all the ink about all the pixels. Explanatory drawing which shows the dot density matrix Dd1m which extracted only the magenta dot of the 1st pixel group from the dot density matrix Ddacmy of all the ink about all the pixels. Explanatory drawing which shows the calculation formula used for a weighted addition process. Explanatory drawing which shows the calculation formula used for the weighting addition process of a modification. Explanatory drawing which shows the dot pattern formed using the conventional dither matrix. Explanatory drawing which shows a mode that the image quality of the printing image formed using the conventional dither matrix deteriorates by bidirectional printing. Explanatory drawing which shows a mode that the image quality degradation of the printing image formed by bidirectional | two-way printing is suppressed by the dither matrix of the Example of this invention. The flowchart which shows the processing routine of the production | generation method of a dither matrix. Explanatory drawing which shows the dither matrix M and the two division | segmentation matrices M1 and M2 by which the grouping process was performed. Explanatory drawing which shows the dot formation object pixel of each main scanning in bidirectional | two-way printing. Explanatory drawing which shows an example of the actual printing state in a bidirectional | two-way printing system. Explanatory drawing which shows the low-pass filter used for calculation of RMS granularity. Explanatory drawing showing the formula which defines RMS granularity. Explanatory drawing which shows the nozzle arrangement in the lower surface of print head 14A, 14B. Explanatory drawing which shows the horizontal surface of print head 14A, 14B.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Nozzle row 10A, 12 ... Print head 20, 20b ... Color printer 22 ... Motor 24 ... Carriage motor 25 ... Roller 30 ... Carriage 32 ... Operation panel 40 ... Control circuit 56 ... Connector 60 ... Print head unit 90 ... Computer 91 ... Video driver 95 ... Application program 96 ... Printer driver 97 ... Resolution conversion module 98 .. .Color conversion module 99 ... Halftone module 100 ... Print data generation module M ... Dither matrix P ... Print medium (printing paper)

Claims (12)

A printing device for printing on a print medium,
Dots to each print pixel of the print image to be formed on the print medium by performing halftone processing using a dither matrix on the image data representing the input gradation value of each pixel constituting the original image A dot data generation unit that generates dot data representing the formation state of
A print head including a plurality of nozzle rows that eject ink of at least N colors (N is an integer of 2 or more), and a dot group in which each of the plurality of nozzle rows has a different color according to the dot data. A printing unit that forms and generates the print image by combining the dot groups of at least N colors with each other in a common print region; and
With
The halftone process
A first mode in which a predetermined arrangement of reference dither matrices is commonly used for each halftone process of the N colors;
A second mode that uses the reference dither matrix that is arranged differently for each halftone process of at least some of the N colors;
Including
The dot data generation unit
The dot formation state of the first type print area including the character area representing the character is determined by the halftone process in the first mode,
The dot formation state of the second type printing area which is another printing area is determined by the halftone process in the second mode.
Printing device. The printing apparatus according to claim 1,
The second type printing area includes a natural image area representing a natural image. The printing apparatus according to claim 1 or 2, wherein
In the halftone process according to the second mode, the reference dither matrix having a different arrangement is used for each halftone process of the N colors.
Printing device. The printing apparatus according to any one of claims 1 to 3,
The reference matrix is set so that a mixed-color dot pattern composed of the N color dot groups formed by the halftone processing in the second mode has a predetermined spatial frequency characteristic set in advance. , Printing device. The printing apparatus according to any one of claims 1 to 4,
The reference matrix is set so that a mixed-color dot pattern composed of the N color dot groups formed by the halftone process in the first mode has a predetermined spatial frequency characteristic set in advance. , Printing device. A printing apparatus according to any one of claims 1 to 5,
The printing unit further includes, in accordance with the dot data, dot groups formed in each of a plurality of pixel groups assumed to have different physical conditions related to the dot formation in a common print region. Generate print images by combining them,
The printing apparatus, wherein the reference matrix is set so that each dot pattern of the pixel group has a predetermined spatial frequency characteristic set in advance. The printing apparatus according to claim 6,
The printing unit generates a print image by forming dots in the print pixels according to the dot data at each of the forward movement and the backward movement of the print head while performing main scanning of the print head. ,
The plurality of pixel groups include a group of first pixel positions where dots are formed when the print head moves forward and a group of second pixel positions where dots are formed when the print head moves backward. , Printing device. The printing apparatus according to claim 6 or 7, wherein
The printing unit generates a print image by forming dots in the print pixels according to the dot data while repeating a main scanning cycle of M times (M is an integer of 2 or more) of the print head,
The printing apparatus, wherein the plurality of pixel groups include a group of a plurality of pixel positions divided according to a remainder obtained by dividing a numerical value representing an order of main scanning lines in the sub-scanning direction by the M. A printing apparatus according to any one of claims 4 to 8,
The printing apparatus, wherein the predetermined spatial frequency characteristic is one of a blue noise characteristic and a green noise characteristic. A printing apparatus according to any one of claims 6 to 8,
The predetermined spatial frequency characteristics indicate that each of the spatial frequency distributions of the dot patterns formed on the print pixels belonging to each of the plurality of pixel groups and the spatial frequency distribution of the print image have a positive correlation coefficient with each other. A printing device indicating that the printer has the printer. A method of generating a printed matter by forming a print image on a print medium,
Dots to each print pixel of the print image to be formed on the print medium by performing halftone processing using a dither matrix on the image data representing the input gradation value of each pixel constituting the original image A dot data generation step for generating dot data representing the formation state of
A print head including a plurality of nozzle rows that eject ink of at least N colors (N is an integer of 2 or more), and a dot group in which each of the plurality of nozzle rows has a different color according to the dot data. Forming and generating the print image by combining the dot groups of at least N colors with each other in a common print region; and
With
The halftone process
A first mode in which a predetermined arrangement of reference dither matrices is commonly used for each halftone process of the N colors;
A second mode that uses the reference dither matrix that is arranged differently for each halftone process of at least some of the N colors;
Including
The dot data generation step includes
Determining the dot formation state of the first type print area including the character area representing the character by the halftone process in the first mode;
Determining the dot formation state of the second type print area, which is another print area, by halftone processing in the second mode;
A method for producing printed matter, including: A computer program for causing a computer to generate print data to be supplied to a printing unit that performs printing on a print medium,
Dots to each print pixel of the print image to be formed on the print medium by performing halftone processing using a dither matrix on the image data representing the input gradation value of each pixel constituting the original image Causing the computer to execute a dot data generation function for generating dot data representing the formation state of
The printing unit includes a print head including a plurality of nozzle rows that eject ink of at least N colors (N is an integer equal to or greater than 2), and each of the plurality of nozzle rows has a color according to the dot data. Forming different dot groups and generating the print image by combining the dot groups of at least N colors with each other in a common print area;
The halftone process
A first mode in which a predetermined arrangement of reference dither matrices is commonly used for each halftone process of the N colors;
A second mode that uses the reference dither matrix that is arranged differently for each halftone process of at least some of the N colors;
Including
The dot data generation function
A function of determining the dot formation state of the first type print area including the character area representing the character by the halftone process in the first mode;
A function of determining the dot formation state of the second type print area, which is another print area, by halftone processing in the second mode;
Including computer programs.

Images