JP2009241333A - Printer and printing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the image quality of an image output while increasing the number of colors capable of being discharged from one head in a printer of inkjet type having a line head. <P>SOLUTION: The line head installed on the printer includes two or more nozzles for discharging common color inks arranged in the width direction of the line head with a first nozzle density and two or more nozzles for discharging special feature inks arranged in the width direction of the line head with a second nozzle density lower than the first nozzle density. The printer performs a halftone processing using the first dither mask on the data of the color corresponding to the common color in the image expressed by image data while performing the halftone processing using the second dither mask formed by converting the first dither mask based on the difference between the first nozzle density and the second nozzle density on the data of the color corresponding to the special feature ink. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an ink jet printing apparatus having a line head.

  In recent years, printing devices that perform full-color printing using special inks such as light cyan, light magenta, red, and green, as well as general inks that correspond to the basic colors of cyan, magenta, yellow, and black have become widespread. is doing.

  In such a printing apparatus, the number of nozzles that can be formed in one head is limited. Thus, for example, Patent Document 1 proposes a technique for increasing the number of colors that can be ejected from one head by ejecting different color inks from the same nozzle row in a serial inkjet printer. Yes. Patent Document 2 proposes a technique for reducing the number of nozzles of a specific color that has a slight influence on image quality in order to reduce the size of the head in an inkjet printing apparatus having a serial head.

JP 2003-54016 A JP 2000-141714 A

  However, no specific proposal has been made for increasing the number of colors that can be ejected from one head in a line-type ink jet printer instead of a serial type. Further, in a line-type ink jet printer, there has been no specific proposal for a technique for improving the image quality when the number of colors that can be ejected from one head is increased.

  Considering these problems, the problem to be solved by the present invention is to improve the image quality of an output image while increasing the number of colors that can be ejected from one head in an ink jet printing apparatus having a line head. There are things to do.

  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 including a line head and a transport mechanism that transports a print medium relative to the line head,
In the line head, a plurality of nozzles that eject ink of general colors are arranged in the width direction of the line head according to a first nozzle density, and a plurality of nozzles that eject special color ink are arranged in the first nozzle density. Arranged in the width direction of the line head with a lower second nozzle density,
An input unit for inputting image data;
The color data corresponding to the general color ink in the image represented by the image data is subjected to a halftone process using a first dither mask, and the color data corresponding to the special color ink is set to the first color data. A halftone processing unit that performs a halftone process using a second dither mask generated by converting the first dither mask based on the difference between the nozzle density of 1 and the second nozzle density;
A printing apparatus comprising: a print processing unit that controls the line head and the transport mechanism to print the image subjected to the halftone process.

  With this type of printing apparatus, while reducing the nozzle density of the spot color ink, the spot color ink can be obtained for the spot color ink based on the difference between the nozzle density of the spot color ink and the nozzle density of the spot color ink. Halftone processing is performed using a second dither mask generated by converting one dither mask. As a result, it is possible to improve the image quality of the output image while increasing the number of colors that can be ejected from one head.

Application Example 2 The printing apparatus according to Application Example 1, further including a storage unit in which the first dither mask is stored in advance, and a first storage unit stored in the storage unit prior to the halftone process. A printing apparatus comprising: a generation unit that generates the second dither mask based on one dither mask.

  With such a printing apparatus, since the second dither mask can be generated dynamically from the first dither mask stored in advance in the storage unit, the storage capacity of the storage unit is reduced. be able to. In addition, since the second dither mask is generated prior to the halftone process, it is possible to prevent the halftone process for the spot color ink from being delayed.

[Application Example 3] The printing apparatus according to Application Example 2, wherein the second nozzle density is half of the first nozzle density, and color data corresponding to the special color ink. The upper limit value is half of the upper limit value of the color data corresponding to the general color ink, and the generation unit divides the first dither mask into groups every two columns. In the row direction, the dither values are replaced based on the magnitude relation of the dither values to form a first dither column having a small dither value and a second dither column having a large dither value. A printing apparatus for generating the second dither mask.

  With such a printing apparatus, it is possible to perform halftone processing using the first dither row for spot color ink having a nozzle density that is one half of the nozzle density of general color ink.

Application Example 4 In the printing apparatus according to Application Example 3, in the generation unit, the second dither row includes a dither value that does not reach an upper limit value of color data corresponding to the spot color ink. In this case, the printing apparatus raises the dither value to the upper limit value of the color data corresponding to the special color ink.

  With such a printing apparatus, it is possible to suppress the occurrence of a halftone process result that generates dots in a portion where no nozzle for special color ink exists.

Application Example 5 In the printing apparatus according to Application Example 4, when the generation unit increases the dither value, the upper limit value of the color data corresponding to the spot color ink and the dither value A printing device that subtracts the difference from the value before the pull-up from the adjacent dither values of the same group.

  With such a printing apparatus, the dither value corresponding to the increase in the adjacent dither value can be subtracted from the first dither column applied to the special color ink. It is possible to prevent the ink density from fluctuating.

[Application Example 6] The printing apparatus according to any one of Application Example 3 to Application Example 5, wherein the generation unit has an upper limit value of color data corresponding to the spot color ink in the first dither row. A dither value that is less than the upper limit value of the color data corresponding to the special color ink.

  According to the printing apparatus of the above aspect, when the first dither column to be applied to the spot color ink includes a dither value that is equal to or higher than the upper limit value of the color data corresponding to the spot color ink, the dither value is used as the spot color. It can be lowered to the maximum value that is less than the upper limit value of the color data corresponding to the ink. As a result, it is possible to suitably compare the color corresponding to the special color ink and the first dither row.

  The present invention can be configured as a printing method or a computer program in addition to the configuration as the printing apparatus described above. The computer program may be recorded on a computer-readable recording medium. As the recording medium, for example, various media such as a flexible disk, a CD-ROM, a DVD-ROM, a magneto-optical disk, a memory card, and a hard disk can be used.

Hereinafter, embodiments of the present invention will be described in the following order based on examples.
A. General printer configuration:
B. Line head nozzle arrangement:
C. Printing process:
D. Special color dither mask generation:
E. Halftone processing:
F. Variations:

A. General printer configuration:
FIG. 1 is an explanatory diagram showing a schematic configuration of a printer 10 as an embodiment of the present invention. The printer 10 is a line-type inkjet printer, and includes a control unit 20, an ink cartridge 61, a line head 70, and a transport mechanism 80 as illustrated.

  The ink cartridge 61 contains cyan ink (C), magenta ink (M), yellow ink (Y), and black ink (K) as general color ink (hereinafter referred to as general ink). As ink, red ink (R) and green ink (G) are accommodated. Of course, the type and number of inks are not limited to this. In this embodiment, the general ink is ink obtained by adding black ink to ink corresponding to the three primary colors. On the other hand, the special color ink is an ink that is used to widen the color reproduction region (gamut) of a color that is difficult to reproduce only with general ink. As the special color ink, in addition to R and G, for example, light cyan, light magenta, dark yellow, and blue ink can be used.

  The line head 70 is a head having a width corresponding to the width of the print medium P. On the lower surface of the line head 70, a plurality of nozzles that discharge general ink are arranged in the width direction of the line head according to a first nozzle density, and a plurality of nozzles that discharge special color ink has a first nozzle density. Are arranged in the width direction of the line head with a lower second nozzle density. Details of the nozzle arrangement will be described later.

  Each ink stored in the ink cartridge 61 is supplied to a nozzle provided on the lower surface of the line head 70 through a conductive tube connecting the ink cartridge 61 and each nozzle. Each nozzle ejects ink by a thermal jet method and prints on the print medium P. That is, each nozzle generates a bubble in the ink by heating a part of the wall surface of the nozzle, thereby ejecting the ink onto the print medium P. In this embodiment, ink is ejected by the thermal method, but a piezo method may be used.

  The transport mechanism 80 includes a paper feed roller 82 and a paper feed motor 84. The paper feed motor 84 rotates the paper feed roller 82 to transport the print medium P to the line head 70 in a direction (transport direction) perpendicular to the width direction of the line head 70. In this embodiment, the line head 70 is fixed and the print medium P is conveyed. However, the print medium P may be fixed and the line head 70 may be conveyed.

  The control unit 20 controls the overall operation of the printer 10. The control unit 20 includes a memory card slot 92 for inputting image data recorded on the memory card MC, a USB interface 94 for connecting a device such as a digital camera, and an operation panel 96 for performing various operations related to printing. A liquid crystal display 98 for displaying a user interface is connected.

  The control unit 20 includes a CPU 30, a RAM 40, and a ROM 50. The CPU 30 loads the control program stored in the ROM 50 into the RAM 40 and executes it, thereby operating as a spot color dither generation unit 31, a color conversion unit 32, a halftone processing unit 33, and a print processing unit 34.

  The spot color dither generation unit 31 converts the basic dither mask DM1 recorded in the ROM 50 into a spot color dither mask DM2 based on the difference between the density of nozzles ejecting general ink and the density of nozzles ejecting spot color ink. . Details of this conversion processing will be described later.

  FIG. 2 is a diagram showing a data structure of the basic dither mask DM1 recorded in the ROM 50. As shown in FIG. As shown in the figure, the basic dither mask DM1 of this embodiment has a total of 16 dither values (threshold values), four in the Y direction (horizontal direction) and four in the X direction (vertical direction). For example, when the image is binarized, the dither values are arranged so that the spatial frequency characteristic of the image has a blue noise characteristic. In this embodiment, the range of values that each dither value can take is set to 0 to 99 in relation to the dot occurrence rate described later.

  The color converter 32 converts RGB format image data input from a memory card MC or the like into CMYKRG format image data. A known technique such as a tetrahedral interpolation technique can be applied to the color conversion algorithm.

  The halftone processing unit 33 refers to the basic dither mask DM1 and the spot color dither mask DM2 generated by the spot color dither generation unit 31, and performs halftone processing on the image data subjected to the color conversion process. In this embodiment, halftone processing is performed by a systematic dither method using these dither masks. Details of the halftone process will be described later. Data obtained by the halftone process is hereinafter referred to as print data.

  The print processing unit 34 controls the line head 70 and the transport mechanism 80 based on the print data obtained as a result of the halftone process, and the ink of each color of C, M, Y, K, R, G is applied to the print medium P. Color printing is performed by discharging the ink.

B. Line head nozzle arrangement:
Subsequently, the arrangement of the nozzles provided in the line head 70 will be described with reference to FIG. FIG. 3 is a diagram illustrating the nozzle arrangement of the line head 70. As shown in the drawing, in the line head 70 of the present embodiment, a plurality of nozzles are arranged for each of C, M, Y, K, R, and G in the width direction. For R and G, as shown in the enlarged view at the bottom of the figure, the nozzles are arranged on a straight line at an interval of 800 dpi. However, the R nozzle is shifted to the left by 1600 dpi with respect to the G nozzle. On the other hand, the C, M, Y, and K nozzles are arranged in two rows in the width direction of the line head 70. The nozzles in each row are shifted by 1600 dpi as shown in the enlarged view at the bottom of the figure. Therefore, the nozzles for C, M, Y, and K are arranged at intervals of 1600 dpi. That is, in this embodiment, the density of the nozzles that eject the special color ink is ½ that of the nozzles that eject the general ink. In this embodiment, as shown in the enlarged view at the bottom of the figure, for each nozzle of C, M, Y, and K, nozzles for two rows are arranged, but each nozzle is spaced at an interval of 1600 dpi. It is good also as arranging on a straight line.

  FIG. 4 is a diagram showing how dots are formed by R and G inks. When the R and G nozzles have the arrangement shown in FIG. 3, the R ink and the G ink are ejected onto the print medium P as shown in FIG. That is, as shown in FIG. 3, R and G are each arranged at an interval of 800 dpi, but these are arranged by being shifted by 1600 dpi in the width direction of the line head 70, so that R dots And G dots are alternately ejected onto the print medium P in a direction intersecting the transport direction of the print medium P. Since the C, M, Y, and K inks are arranged at intervals of 1600 dpi, dots can be formed in all of the portions indicated by “R” and “G” in FIG.

C. Printing process:
FIG. 5 is a flowchart of printing processing executed by the CPU 30 of the printer 10. This printing process is a process executed when the user performs a predetermined printing operation using the operation panel 96 of the printer 10.

  When this printing process is executed, the CPU 30 inputs the basic dither mask DM1 from the ROM 50 to the RAM 40 (step S10). When the basic dither mask DM1 is input, the CPU 30 executes a special color dither generation process for generating a special color dither mask from the basic dither mask DM1 by the action of the special color dither generation unit 31 (step S20). Details of the special color dither generation processing will be described later. When the special color dither generation process is completed, the special color dither mask DM2 is input to the RAM 40.

  When the basic dither mask DM1 and the spot color dither mask DM2 are input to the RAM 40 by the processing described above, the CPU 30 inputs RGB format image data from the memory card MC via the memory card slot 92 (step S30). ). In this embodiment, the image data is input from the memory card MC. However, the image data may be input from a digital camera or a personal computer via the USB interface 94.

  When the image data is input, the CPU 30 performs color conversion processing for converting the RGB format image data into the CMYKRG format image data by the function of the color conversion unit 32 (step S40). In this color conversion processing, a correspondence relationship between a combination of R, G, and B color densities (color data) and a combination of C, M, Y, K, R, and G color densities (color data) is defined. Color conversion is performed by referring to the lookup table. In this embodiment, this color conversion process converts C, M, Y, and K to a color density of 0 to 100 (%), and R and G have a color density of 0 to 50 (%). It will be converted.

  When the CMYKRG format image data is obtained by the color conversion processing, the CPU 30 performs halftone processing for each color of C, M, Y, K, R, and G by the operation of the halftone processing unit 33, and prints. Data is generated (step S50). In such halftone processing, the basic dither mask DM1 is used for colors (C, M, Y, K) corresponding to general ink, and the dither mask for special colors is used for colors (R, G) corresponding to special color inks. DM2 is used.

  When the halftone process ends, the CPU 30 drives the paper feed motor 84 by the action of the print processing unit 34, and ejects ink droplets from the line head 70 based on the print data in accordance with this movement (step S60). . As a result, ink dots of appropriate colors are formed at appropriate positions, and image data is printed.

  In the present embodiment, the above-described printing process is executed when a predetermined printing operation is performed by the user. That is, every time the printing process is executed, the basic dither mask is input in step S10 and the spot color dither mask is generated in step S20. On the other hand, the input of the basic dither mask and the generation of the spot color dither mask may be performed only once when the printer 10 is turned on. By doing so, the processing speed of the printing process can be improved.

D. Special color dither mask generation:
Next, the details of the spot color dither generation process executed in step S20 of the printing process will be described.
FIG. 6 is a flowchart of the spot color dither generation process. As shown in the figure, in the spot color dither generation process, first, a process called a replacement process is executed by the CPU 30 (step S100), and then the first adjustment process (for the dither mask obtained by the replacement process ( By performing the step S200) and the second adjustment process (step S300), the spot color dither mask DM2 is generated. When the CPU 30 thus generates the spot color dither mask DM2, the CPU 30 records the generated spot color dither mask DM2 in the RAM 40 (step S400). Hereinafter, details of the replacement process, the first adjustment process, and the second adjustment process will be described.

(D-1) Replacement process:
FIG. 7 is a flowchart showing details of the replacement process. When this replacement process is executed, the CPU 30 first extracts the dither value of the coordinates (Y, X) of the basic dither mask DM1 (see FIG. 2) (step S102). At the start of the replacement process, the initial values of Y and X are both “1”. Therefore, referring to FIG. 2, when step S102 is executed first, “26” is extracted as the dither value.

  Subsequently, the CPU 30 extracts a dither value (Y + 1, X) (step S104). Referring to FIG. 2, when this step S104 is executed first, “93” is extracted as the dither value.

  When the dither values (Y, X) and (Y + 1, X) are extracted from the basic dither mask DM1, the CPU 30 compares the dither value (Y, X) with the dither value (Y + 1, X), and (Y , X) is determined whether the dither value is larger (step S106). If it is determined by this determination that the (Y, X) dither value is larger, the CPU 30 replaces the (Y, X) dither value with the (Y + 1, X) dither value (step S108). On the other hand, if it is determined that the dither value of (Y, X) is not greater, the CPU 30 skips step S108.

  Subsequently, the CPU 30 determines whether or not the value of X has reached its maximum value (Xmax) (step S110). In the present embodiment, the value of Xmax is “4” as shown in FIG. If X has not reached Xmax, the CPU 30 adds “1” to X (step S112), and returns the process to step S102.

  When X has reached Xmax, the CPU 30 determines whether the value of Y has reached its maximum value Ymax (step S114). In this embodiment, the value of Ymax is “4” as shown in FIG. If Y has not reached Ymax, the CPU 30 initializes the value of X to “1” and adds “2” to the value of Y (step S116). Then, the process returns to step S102. When Y reaches Ymax, the CPU 30 ends the replacement process.

  FIG. 8 is an explanatory diagram showing a specific example of the replacement process described above. The upper part of FIG. 8 shows the basic dither mask DM1, and the lower part of FIG. 8 shows the result of the replacement process performed on the basic dither mask DM1. As shown in FIG. 8, according to the replacement process of the present embodiment, the basic dither mask is grouped every two columns in step S116 of FIG. 7, and the row direction (Y direction) is set for each group in step S106. ) Compares the magnitude relationship of dither values. As a result of this comparison, if the dither values in the odd-numbered columns are large, the left and right dither values are switched in step S108. That is, according to the replacement process described above, the dither values are replaced so that columns having a large dither value and columns having a small dither value are alternately arranged.

(D-2) First adjustment process:
FIG. 9 is a flowchart of the first adjustment process executed subsequent to the replacement process described above. When the first adjustment process is executed, the CPU 30 first extracts the dither value (2Y, X) in the dither mask subjected to the replacement process (step S202). Then, the extracted dither value is compared with the maximum dot occurrence rate (MaxDuty) of the color corresponding to the special color ink, and it is determined whether the extracted dither value is smaller than MaxDuty (step S204). In this embodiment, at the start of the first adjustment process, the initial values of Y and X are both “1”. Although details of the dot generation rate will be described later, in this embodiment, MaxDuty is “50”.

  If it is determined in step S204 that the dither value extracted in step S202 is smaller than MaxDuty, the CPU 30 temporarily records the dither value in the variable Temp (step S206), and (2Y, X The value of MaxDuty (= 50) is substituted as a new dither value (step S208). Subsequently, the CPU 30 extracts a dither value of (2Y-1, X) from the dither mask subjected to the replacement process (step S210), and MaxDuty as a new dither value of (2Y-1, X), The difference from the variable Temp, which is the original dither value of (2Y, X), is set as a value subtracted from the dither value extracted in step S210 (step S212). If it is determined in step S204 that the dither value extracted in step S202 is not smaller than MaxDuty, the processing from step S206 to step S212 described above is skipped.

  Subsequently, the CPU 30 determines whether or not the value of X has reached its maximum value (Xmax) (step S214). If X has not reached Xmax, the CPU 30 adds “1” to X (step S216), and returns the process to step S202. If X has reached Xmax, the CPU 30 determines whether the value of Y has reached its maximum value Ymax (step S218). If Y has not reached Ymax, the CPU 30 initializes the value of X to “1” and adds “1” to the value of Y (step S220). Then, the process returns to step S202. When Y reaches Ymax, the CPU 30 ends the first adjustment process.

  FIG. 10 is an explanatory diagram showing a specific example of the first adjustment process described above. The upper part of FIG. 10 shows the dither mask subjected to the replacement process, and the lower part of FIG. 10 shows the result of the first adjustment process performed on the dither mask. As shown in FIG. 10, according to the first adjustment process of the present embodiment, a dither value smaller than MaxDuty (= 50) is obtained in step S204 for a column in which large dither values are collected by the replacement process (even column). Judge whether there is. In the example shown in FIG. 10, the dither value of (Y, X) = (2, 2) is “39” that is lower than MaxDuty. If a dither value smaller than MaxDuty (= 50) exists, the dither value is raised to the MaxDuty value (50) in step S208. Furthermore, the difference (MaxDuty-Temp) between the dither values before and after the pulling is subtracted from the adjacent dither values in the same group in step S212. According to such a first adjustment process, it is possible to set all the dither values in the column in which large dither values are collected by the replacement process to a value equal to or greater than MaxDuty. Further, since the difference between the raised dither values is subtracted from the adjacent dither values, it is possible to suppress the variation in the density of the spot color ink as a whole by raising the dither values.

(D-3) Second adjustment process:
FIG. 11 is a flowchart of the second adjustment process executed subsequent to the above-described first adjustment process. When the second adjustment process is executed, the CPU 30 first extracts the (Y, X) dither value in the dither mask subjected to the first adjustment process (step S302). Then, the extracted dither value is compared with the maximum dot occurrence rate (MaxDuty), and it is determined whether the extracted dither value is larger than the MaxDuty (step S304). In this embodiment, at the start of the second adjustment process, the initial values of Y and X are both “1”.

  If it is determined in step S304 that the dither value extracted in step S302 is larger than MaxDuty, the CPU 30 sets a value obtained by subtracting 1 from MaxDuty (= 49) as a new dither value of (Y, X). ) Is substituted (step S306). If the dither value extracted in step S302 is not greater than MaxDuty, the process in step S306 is skipped.

  Subsequently, the CPU 30 determines whether or not the value of X has reached its maximum value (Xmax) (step S308). If X has not reached Xmax, the CPU 30 adds “1” to X (step S310), and returns the process to step S302. If X has reached Xmax, the CPU 30 determines whether the value of Y has reached its maximum value Ymax (step S218). If Y has not reached Ymax, the CPU 30 initializes the value of X to “1” and adds “2” to the value of Y (step S220). Then, the process returns to step S302. When Y reaches Ymax, the CPU 30 ends the second adjustment process. When the second adjustment process is completed, the spot color dither mask DM2 is completed.

  FIG. 12 is an explanatory diagram showing a specific example of the second adjustment process described above. The upper part of FIG. 12 shows the dither mask subjected to the first adjustment process, and the lower part of FIG. 12 shows the dither mask for the special color generated by applying the second adjustment process to the dither mask. The mask DM2 is shown. As shown in FIG. 12, according to the second adjustment process of this embodiment, a dither value exceeding MaxDuty (= 50) is obtained in step S304 for a column (odd column) in which small dither values are collected by the replacement process. Judging whether it exists. In the example shown in FIG. 12, the dither value of (Y, X) = (3, 3) is “52” exceeding the MaxDuty. If there is a dither value exceeding MaxDuty (= 50) as described above, the dither value is reduced to the maximum value (= 49) less than MaxDuty in step S306. According to such a second adjustment process, it is possible to set all dither values in a column in which small dither values are collected by the replacement process to a value less than MaxDuty.

  As described above, in this embodiment, the spot color dither mask DM2 is generated by performing the replacement process, the first adjustment process, and the second adjustment process on the basic dither mask DM1. As shown in the lower part of FIG. 12, the spot color dither mask DM2 has a dither value of less than 50 in the odd-numbered columns (Y = 1, 3) and 50 in the even-numbered columns (Y = 2, 4). The dither mask has a dither value of less than 100.

E. Halftone processing:
Next, halftone processing using the basic dither mask DM1 and the special color dither mask DM2 described above will be described.
FIG. 13 is a flowchart of the halftone process executed in step 50 of the printing process shown in FIG. When this halftone process is executed, the CPU 30 first determines the respective values of the color densities (C, M, Y, K, R, and G) of pixels that are to be subjected to the halftone process (hereinafter referred to as process target pixels). ) Is acquired from the image data subjected to the color conversion process (step S500).

  Subsequently, the CPU 30 refers to a predetermined dot occurrence rate table in which the relationship between the color density and the dot occurrence rate is defined (step S502), and determines the dot occurrence rate corresponding to the color density acquired in step S500 for each color. (Step S504).

  FIG. 14 is a diagram illustrating an example of a dot generation rate table for C, M, Y, and K. FIG. 15 is a diagram illustrating an example of a dot generation rate table for R and G. As shown in these figures, the dot occurrence rate table defines the dot occurrence rate according to the color density. In this embodiment, since the maximum value of the R and G color densities is 50%, the maximum value of the R and G dot occurrence rate is also 50%. In this embodiment, as shown in FIGS. 14 and 15, the dot occurrence rate and the color density of each color have a linear relationship, and “dot occurrence rate = color density”. However, the dot occurrence rate and the color density may have a non-linear relationship. For example, the dot generation rate corresponding to a color density near 50% can be set to about 40% or 60%. Such a relationship between the dot generation rate and the color density can be appropriately changed according to the ink absorption characteristics of the printing medium P and the characteristics of the ink used. That is, the dot occurrence rate can be said to be a value obtained by correcting the color density according to the characteristics of the printing medium P and ink.

  When the dot occurrence rate of each color is acquired in step S504, the CPU 30 refers to the basic dither mask DM1 or the special color dither mask DM2 (step S506), and sets the dither value corresponding to the processing target pixel and each color of the processing target pixel. The dot generation rate is compared (step S508). At this time, the CPU 30 refers to the basic dither mask DM1 for C, M, Y, and K, and refers to the special color dither mask DM2 for R and G, and compares the dither value with the dot occurrence rate. Do.

  In step S508, one dither value corresponding to the pixel to be processed is determined from the dither mask. This dither value is determined by arranging dither masks in tiles in the image data and extracting the dither value present at a position corresponding to the position of the pixel to be processed. The positional relationship between the image data and the dither mask at this time is shown in FIG. For C, M, Y, and K, as shown on the lower left side of FIG. 16, the basic dither mask DM1 is arranged in four directions in the image data, thereby determining the dither value corresponding to the pixel to be processed. Yes. For G, the dither mask DM2 for special colors is arranged in four directions in the image data in the same manner as C, M, Y, and K to determine the dither value corresponding to the pixel to be processed. On the other hand, with respect to R, as shown on the lower right side of FIG. 16, the spot color dither mask DM2 is shifted to the left side (Y direction) by one pixel and applied to the image data. This is because, as shown in FIG. 4, in the nozzle arrangement of the line head of this embodiment, R dots are alternately formed in the horizontal direction with G dots. Further, the reason why the spot color dither mask DM2 is shifted with respect to R instead of G is that, in the line head 70 of this embodiment, G dots are formed in the odd-numbered columns. This is because the dither values that can be compared with the range (0 to 50) of the G dot occurrence rate that can be taken in the odd-numbered columns are collected. That is, the halftone process can be performed on the R dots using the spot color dither mask DM2 by simply shifting the spot color dither mask DM2 by one dot in the horizontal direction.

  When the dot occurrence rate of each color is compared with the dither value as described above, the CPU 30 sets the dot of that color to “on” for the pixel for which the dot occurrence rate is greater than the dither value. (Step S512). On the other hand, for a color whose dot occurrence rate is less than or equal to the dither value, the dot of that color is set to “off” for that pixel (step S514). When the CPU 30 executes the above-described processing for all the pixels of the image data (step S516), the halftone processing ends.

  According to the printer 10 of the present embodiment described above, for general ink essential for full color printing, dots are formed at an output resolution of 1600 dpi and for special color ink used for the purpose of expanding the color reproduction area, Since the influence on is small, dots are formed at an output resolution of 800 dpi. Therefore, as shown in FIG. 3, for the C, M, Y, and K nozzles of the line head 70, two lines of space are required, but for the R and G nozzles, one line each. Space is enough. As a result, the number of colors that can be used in one head can be increased without increasing the size of the line head 70. As a result, for example, in this embodiment, since the two special color inks R and G can be used, the color reproduction region can be expanded in various directions.

  Further, in this embodiment, as the nozzle density of the special color ink is halved from the nozzle density of the general ink, the basic dither mask DM1 for the general ink is not used as it is for the special color ink, but this basic dither mask. Halftone processing is performed using a dither mask DM2 for special colors in which various processing is applied to DM1. In the color conversion process described above, color conversion is performed with the same resolution regardless of the general ink and the special color ink. For this reason, after color conversion, color density data also exists for a portion where no nozzle for special color ink exists. At this time, if a dither value lower than the MaxDuty of the special color ink exists in that portion, the density of the special color ink will fluctuate as a whole unless ink is applied to that portion. Therefore, in this embodiment, the above-described replacement process, first adjustment process, and second adjustment process are performed on the basic dither mask DM1, so that the dot occurrence rate of the special color ink is obtained as shown in FIG. A spot color dither mask DM2 is generated in which columns having dither values in a range to be obtained and columns having more dither values are alternately arranged. When such a special color dither mask DM2 is applied to the image data after color conversion, a halftone processing result that generates dots in a portion where no special color nozzle exists does not occur. Further, if the replacement process, the first adjustment process, and the second adjustment process are performed, as shown in FIG. 12, all odd-number columns to be applied to the special color ink can be set to a value less than the Max Duty of the special color ink. Therefore, when compared with the original basic dither mask DM1, about twice as many dots are formed in the odd-numbered column portion. As a result, even if the nozzle density of the special color ink is half that of the general ink, the number of dots to be output is the same as that of the general ink. For this reason, it is possible to form dots of spot color ink while maintaining a substantial output resolution even at a low nozzle density.

  Further, since the printer 10 of this embodiment dynamically generates the special color dither mask DM2 from the basic dither mask DM1, it is not necessary to store the special color dither mask DM2 in the ROM 50 in advance. As a result, the storage capacity of the ROM 50 can be reduced. Further, since the spot color dither mask DM2 is generated prior to the actual halftone process, there is no delay in the halftone process for the spot color ink.

F. Variations:
As mentioned above, although one Example of this invention was described, it cannot be overemphasized that this invention is not limited to such an Example, but can take a various structure in the range which does not deviate from the meaning. For example, a function realized by software may be realized by hardware. In addition, the following modifications are possible.

(Modification 1):
In the above embodiment, the size of the basic dither mask DM1 is 4 × 4. On the other hand, the size of the dither mask can be various sizes such as 16 × 16 and 512 × 512, for example.

(Modification 2):
In the above embodiment, the printer 10 itself performs all operations from inputting image data to printing. On the other hand, a computer connected to the printer 10 may perform image data input, spot color dither mask generation, color conversion processing, and halftone processing. In this case, the printer 10 receives print data generated by the computer through halftone processing, and controls the line head 70 and the transport mechanism 80 in accordance with the print data to perform printing.

(Modification 3):
In the above embodiment, as shown in FIG. 16, the dither masks are arranged vertically and horizontally equally with respect to the image data. On the other hand, the application of the dither mask to the image data can take various forms. 17 and 18 show other examples of applying a dither mask to image data. FIG. 17 shows an example in which the dither mask is shifted by one pixel in the vertical direction, and FIG. 18 shows an example in which the dither mask is shifted by one pixel in the horizontal direction. As described above, if the dither mask is applied to the image data, it is possible to suppress the appearance of a predetermined pattern on the output image by repeatedly applying the dither mask.

(Modification 4):
In the above embodiment, the maximum dot generation rate (maximum density) of R and G matches. Therefore, the same spot color dither mask DM2 can be applied to R and G. However, the maximum dot occurrence rate of R and G may be different. In this case, different dither masks for spot colors are generated for R and G. As the generation algorithm, the algorithm shown in FIGS. 6 to 11 can be applied as it is only by changing the value of MaxDuty.

(Modification 5):
The line head 70 according to the above embodiment is configured such that nozzles corresponding to the respective colors are arranged on a straight line in the width direction of the head on the lower surface of the substantially rectangular head. On the other hand, the line head can take various modes. 19 and 20 are diagrams showing another configuration example of the line head. FIG. 19 shows a line head 70b in which head modules 71 smaller than the line head 70 are arranged in a staggered manner. In each head module 71, nozzles are formed in the same arrangement as the nozzle arrangement of the line head 70 shown in FIG. FIG. 20 shows a line head 70c in which a plurality of head modules 71 oriented obliquely are arranged in the longitudinal direction. The present invention can be applied to these line heads 70b and 70c in the same manner as in the above embodiment. In the line head 70 c illustrated in FIG. 20, the “width direction” refers to the width direction of the head module 71.

2 is an explanatory diagram illustrating a schematic configuration of a printer. FIG. It is a figure which shows the data structure of basic dither mask DM1. FIG. 3 is a diagram illustrating a nozzle arrangement of a line head 70. It is a figure which shows a mode that a dot is formed with R, G ink. It is a flowchart of a printing process. It is a flowchart of a dither generation process for spot colors. It is a flowchart which shows the detail of a replacement process. It is explanatory drawing which shows the specific example of a replacement process. It is a flowchart of a 1st adjustment process. It is explanatory drawing which shows the specific example of a 1st adjustment process. It is a flowchart of a 2nd adjustment process. It is explanatory drawing which shows the specific example of a 2nd adjustment process. It is a flowchart of a halftone process. It is a figure which shows an example of the dot generation rate table for C, M, Y, K. It is a figure which shows an example of the dot generation rate table for R, G. It is a figure which shows the positional relationship of image data and a dither mask. It is a figure which shows the other positional relationship of image data and a dither mask. It is a figure which shows the other positional relationship of image data and a dither mask. It is a figure which shows the other structural example of a line head. It is a figure which shows the other structural example of a line head.

Explanation of symbols

10 ... Printer 20 ... Control unit 30 ... CPU
31 ... Dither generation unit for special color 32 ... Color conversion unit 33 ... Halftone processing unit 34 ... Print processing unit 40 ... RAM
50 ... ROM
61 ... Ink cartridge 70 ... Line head 80 ... Conveying mechanism 82 ... Paper feed roller 84 ... Paper feed motor 92 ... Memory card slot 94 ... USB interface 96 ... Operation Panel 98 ... Liquid crystal display P ... Print media DM1 ... Basic dither mask DM2 ... Dither mask for special color

Claims (9)

A printing apparatus comprising a line head and a transport mechanism for transporting a print medium relative to the line head,
In the line head, a plurality of nozzles that eject ink of general colors are arranged in the width direction of the line head according to a first nozzle density, and a plurality of nozzles that eject special color ink are arranged in the first nozzle density. Arranged in the width direction of the line head with a lower second nozzle density,
An input unit for inputting image data;
The color data corresponding to the general color ink in the image represented by the image data is subjected to a halftone process using a first dither mask, and the color data corresponding to the special color ink is set to the first color data. A halftone processing unit that performs a halftone process using a second dither mask generated by converting the first dither mask based on the difference between the nozzle density of 1 and the second nozzle density;
A printing apparatus comprising: a print processing unit that controls the line head and the transport mechanism to print the image subjected to the halftone process. The printing apparatus according to claim 1, further comprising:
A storage unit in which the first dither mask is stored in advance;
A printing apparatus comprising: a generation unit that generates the second dither mask based on the first dither mask stored in the storage unit prior to the halftone process. The printing apparatus according to claim 2,
The second nozzle density is one half of the first nozzle density, and the upper limit value of the color data corresponding to the special color ink is the color data corresponding to the general color ink. Half of the upper limit,
The generation unit divides the first dither mask into groups every two columns, and replaces the dither values in the row direction for each group based on the magnitude relationship of the dither values, and has a small dither value. A printing apparatus that generates the second dither mask by forming one dither row and a second dither row having a large dither value. The printing apparatus according to claim 3,
If the dither value that does not reach the upper limit value of the color data corresponding to the spot color ink is included in the second dither row, the generation unit sets the dither value of the color corresponding to the spot color ink. A printer that raises the upper limit of data. The printing apparatus according to claim 4,
When the dither value is increased, the generation unit calculates a difference between the upper limit value of the color data corresponding to the spot color ink and the value before the dither value is increased from the adjacent dither value of the same group. Subtract printing device. A printing apparatus according to any one of claims 3 to 5,
When the first dither row includes a dither value that is equal to or greater than the upper limit value of the color data corresponding to the special color ink, the generation unit uses the dither value of the color corresponding to the special color ink. A printer that reduces the maximum value that is less than the upper limit of the data. A printing method by a printing apparatus including a line head and a conveyance mechanism that conveys a print medium relative to the line head,
In the line head, a plurality of nozzles that discharge ink of general colors are arranged in the width direction of the line head according to a first nozzle density, and a plurality of nozzles that discharge special color ink is more than the first nozzle density. Are arranged in the width direction of the line head by a second nozzle density that is lower than
An input process for inputting image data;
The color data corresponding to the general color ink in the image represented by the image data is subjected to a halftone process using a first dither mask, and the color data corresponding to the special color ink is set to the first color data. A halftone processing step of performing a halftone process using a second dither mask generated by converting the first dither mask based on the difference between the nozzle density of 1 and the second nozzle density;
A printing step of controlling the line head and the transport mechanism to print the image subjected to the halftone process. A computer program for controlling a printing apparatus comprising a line head and a transport mechanism for transporting a print medium relative to the line head,
In the line head, a plurality of nozzles that eject ink of general colors are arranged in the width direction of the line head according to a first nozzle density, and a plurality of nozzles that eject special color ink are arranged in the first nozzle density. Arranged in the width direction of the line head with a lower second nozzle density,
An input function for inputting image data;
The color data corresponding to the general color ink in the image represented by the image data is subjected to a halftone process using a first dither mask, and the color data corresponding to the special color ink is set to the first color data. A halftone processing function for performing halftone processing using a second dither mask generated by converting the first dither mask based on the difference between the nozzle density of 1 and the second nozzle density;
The computer program which controls a line head and the conveyance mechanism, and makes a computer realize the printing function which prints the image by which the halftone process was made.   The recording medium which recorded the computer program of Claim 8 so that computer reading was possible.

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