Patent Publication Number: US-2023150271-A1

Title: Printing control apparatus and printing control method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a technique for printing an image by performing a plurality of print scans with respect to a unit area. 
     Description of the Related Art 
     For inkjet printing apparatuses, what is called a multi-pass printing method is known, in which all pixels in a region printable with a single scan are divided into a plurality of groups, and printing of the region is completed by a plurality of scans. 
     Japanese Patent Laid-Open No. 2005-169940 (hereinafter referred to as Literature 1) describes a technique for reducing color unevenness caused in multi-pass printing due to the order of ink ejection being different between a forward scan and a backward scan. Literature 1 describes using mutually complementary mask patterns so that out of two print heads that eject the same ink, a printing duty for the print head that precedes in the scanning direction may be higher than that for the following print head. A printing duty is a percentage of ink ejection. 
     The actual ejection amount of each ink color may change depending on a color to be outputted. For example, the ejection amount of magenta is different between a case where the hue of a color to be outputted is red (magenta+yellow) and a case where the hue of a color to be outputted is blue (cyan+magenta). In their comparison with the same lightness value, red requires a larger amount of magenta to be ejected than blue does, and blue requires a smaller amount of magenta to be ejected than red does. In this way, a printing duty required of each ink color (ink ejection percentage) is not determined in a fixed manner, but may change depending on the color to be outputted. Because the ratio of the print duties is determined by mask patterns in a fixed manner in the technique in Literature 1, the mask patterns need to be changed in order to adjust the print duties. 
     SUMMARY OF THE INVENTION 
     A printing control apparatus according to one aspect of the present disclosure is a printing control apparatus configured to control a printing apparatus including a first nozzle array having ejection ports configured to eject an ink of a predetermined color and arranged in a sub scanning direction and a second nozzle array having ejection ports configured to eject an ink of the predetermined color and arranged in the sub scanning direction, the printing apparatus printing an image in a predetermined region on a print medium by scanning the first nozzle array and the second nozzle array N times (where N is an integer of 2 or greater) in a main scanning direction intersecting with the sub scanning direction, the printing control apparatus including: a control unit configured to perform control so that a distribution ratio based on which color separation data separated as having the predetermined color is distributed into data for the first nozzle array and a distribution ratio based on which the color separation data is distributed into data for the second nozzle array are different depending on a tone value in the color separation data, and a generation unit configured to generate printing data to be used for printing by the first nozzle array based on the distributed data for the first nozzle array and generate printing data to be used for printing by the second nozzle array based on the distributed data for the second nozzle array. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an outer appearance view of a printing apparatus; 
         FIG.  2    is a side view of a main body of the printing apparatus; 
         FIG.  3    is a diagram showing a print head; 
         FIG.  4    is a block diagram showing a schematic configuration of a printing system; 
         FIG.  5    is a diagram illustrating a multi-pass printing method; 
         FIG.  6    is a diagram illustrating the flow of image data conversion processing in the printing system; 
         FIGS.  7 A and  7 B  are diagrams illustrating printing percentages of masks; 
         FIGS.  8 A and  8 B  are diagrams showing how the way streaks at connection portions in the pass formation process look different; 
         FIG.  9    is a diagram illustrating a method for allotting tone values in input data to a first nozzle array and a second nozzle array; 
         FIGS.  10 A and  10 B  are diagrams illustrating a specific method of distributing color separation data; 
         FIGS.  11 A and  11 B  are diagrams illustrating pass data processing method; 
         FIG.  12    is a flowchart showing pass data generation processing; 
         FIG.  13    is a diagram illustrating the configuration of a print head; and 
         FIGS.  14 A to  14 D  are diagrams illustrating a specific method of distributing color separation data. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present disclosure are described in detail below with reference to the drawings attached hereto. Note that the following embodiments are not to limit the matters disclosed herein and that not all the combinations of features described in the present embodiments are essential to the solutions provided by the present disclosure. Note that the same reference numeral is used to denote the same constituents to omit descriptions. 
     First Embodiment 
     (1) Configuration of the Inkjet Printing Apparatus 
       FIG.  1    is a diagram showing an outer appearance of an inkjet printing apparatus (hereinafter also referred to as a printing apparatus or a printer) according to the present embodiment. A printing apparatus  100  in  FIG.  1    is what is called a serial scanning printer and prints an image by scanning a print head in an X-direction (a scanning direction) orthogonal to a Y-direction (a conveyance direction) of a print medium P.  FIG.  2    is a side view of the main body of the printing apparatus  100 . 
     Using  FIGS.  1  and  2   , an overview of the configuration of the printing apparatus  100  and the operation of the printing apparatus  100  during printing is described. First, a conveyance roller driven via gears by a conveyance motor (not shown) conveys the print medium P in the Y-direction from a spool  6  holding the print medium P. Meanwhile, at a predetermined conveyance position, a carriage unit  2  is caused by a carriage motor (not shown) to scan back and forth (reciprocate) along a guide shaft  8  extending in the X-direction. Then, in the process of this scanning, at a timing based on a position signal obtained by an encoder  7 , a print head  9  (to be described later) attachable to the carriage unit  2  performs an ejection operation from its ejection ports to perform printing of a band with a width corresponding to the range over which the ejection ports are arranged. In the configuration in the present embodiment, the ejection operation is performed with scans being made at a scan speed of 30 inches per second and with a printing resolution of 1200 dpi (the interval of 1/1200 inches). After that, the print medium P is conveyed, and printing for the next band width is performed. 
     Note that a carriage belt can be used to transmit the driving force from the carriage motor to the carriage unit  2 . A driving mechanism other than the carriage belt can also be used, such as, for example, one including a lead screw and an engagement part, the lead screw extending in the X-direction and being driven and rotated by a carriage motor, the engagement part being provided at the carriage unit  2  and engaging with a groove on the lead screw. 
     The print medium P being fed is sandwiched and conveyed by a paper feed roller and a pinch roller and is led to a printing position on a platen  4  (a region scanned by the print head). Usually, the face surface of the print head  9  is capped in idle mode, and thus, the cap is opened before printing to make the print head  9  (the carriage unit  2 ) ready to scan. Then, after one scan worth of data is accumulated in a buffer, the carriage unit  2  is scanned by the carriage motor, and printing is performed as described above. 
     A flexible wiring substrate  19  is mounted to the print head  9  to supply, e.g., a drive pulse for ejection driving and a head temperature adjustment signal. The other end of the flexible wiring substrate  19  is connected to a controller (not shown) including a control circuit such as a CPU that executes control of the printer. A UI screen  50  is configured so that a user can input or check, e.g., information on a pause of the printing operation or information on the print medium P. 
     In a curing region located downstream, in the sub scanning direction Y, of a location where the print head  9  mounted to the carriage unit  2  reciprocates and scans in the main scanning direction X, a heater  10  supported by a frame (not shown) is situated. The heater  10  thermally dries an ink in liquid form on the print medium P. The heater  10  is covered with a heater cover  11 . The heater cover  11  has a function to efficiently irradiate the print medium P with heat from the heater  10  and a function to protect the heater  10 . After being printed by the print head  9 , the print medium P is wound up by a wind-up spool  12  and forms a roll-shaped would-up medium  13 . Specific examples of the heater  10  include a sheathed heater and a halogen heater. A heating temperature for a heating portion in the curing region is set considering the film formability and productivity of water-soluble resin fine particles and heat resistance of the print medium P. Note that examples of heating unit usable for the heating portion in the curing region include heating by hot air blowing from above and heating by a contact heat conductive heater from the underside of a print medium. In the example shown in the present embodiment, the heating unit for the heating portion at the curing region is provided at a single location. However, the heating unit may be provided at two or more locations and used together as long as temperatures measured by a radiation thermometer (not shown) above the print medium P do not exceed a set value for a heating temperature. 
     The printing apparatus  100  of the present embodiment can perform what is called multi-pass printing, in which an image is printed onto a predetermined region (a 1/n band) on the print medium P with a plurality of scans (n scans) by the print head. Details of the multi-pass printing will be described later. 
     (2) Configuration of the Print Head 
       FIG.  3    is a diagram showing the print head  9  according to the present embodiment. The print head  9  includes a first nozzle array  90  (K 1 ) and a second nozzle array  90  (K 2 ) that eject an ink of the same color (a black ink (K) as an example) as an ink containing a color material. Because the ink for the first nozzle array  90  (K 1 ) and the second nozzle array  90  (K 2 ) contains a color material and is therefore also referred to as a color material ink for brevity in the following description. Although a case of black ink is described as an example here, an ink of a different color may be used. 
     The print head  9  has ejection ports arranged in the sub scanning direction. Also, in the print head  9 , the ejection port arrays are arranged side by side from the left side to the right side of the main scanning direction (the X-direction) in the order of the first nozzle array  90  (K 1 ) and the second nozzle array  90  (K 2 ). The first nozzle array  90  (K 1 ) and the second nozzle array  90  (K 2 ) are each formed by 1280 ejection ports  90 A that are configured to eject an ink and arranged in the Y-direction (the arrangement direction, the sub scanning direction) at a density of 1200 dpi. Note that the amount of ink ejected from a single ejection port  90 A at once is approximately 4.5 pl in the present embodiment. 
     The first nozzle array  90  (K 1 ) and the second nozzle array  90  (K 2 ) are each connected to an ink tank (not shown) storing a corresponding ink and is supplied with the ink. Note that the print head  9  and the ink tanks used in the present embodiment may be configured integrally or configured to be separable from each other. Also, although the print head  9  for one color, namely a black ink, is used and described as an example in the present embodiment, the print head may use inks of a plurality of colors. An example of a print head using inks of a plurality of colors will be described in a second embodiment. 
     (3) Configuration of the Printing System 
       FIG.  4    is a block diagram showing a schematic configuration of a printing system including a host apparatus  312  and a control system in the printing apparatus  100  in the present embodiment. The host apparatus  312  is an information processing apparatus connected to the printing apparatus  100 , such as a personal computer or a digital camera. The host apparatus  312  includes a CPU  400 , a memory  401 , a storage unit  402 , an input unit  403  such as a keyboard or a mouse, and an interface  404  for communications with the printing apparatus  100 . The CPU  400  executes various kinds of processing according to programs stored in the memory  401 . These programs are supplied from an external device such as a CD-ROM to store them in the storage unit  402 . The programs may be prestored in the storage unit  402 . 
     The host apparatus  312  is connected to the printing apparatus  100  via the interface  404  and sends the printing apparatus  100  image processing information including image data expressed by R, G, and B in an image processing step to be described later and a table used in image processing after that (print control information). Based on the image processing information transmitted thereto, the printing apparatus  100  executes, e.g., color processing, image processing such as binaralizing processing, and correction processing for printing properties to be described later. Note that the host apparatus  312  may execute at least one of the color processing, the image processing, and the correction processing. 
     The printing apparatus  100  has a main control unit  300 . The main control unit  300  includes a CPU  301  that executes printing operation and processing operation such as computation, selection, determination, and control. The main control unit  300  also includes, e.g., a ROM  302  that stores, e.g., control programs to be executed by the CPU  301 , a RAM  303  used as, e.g., a buffer for print data, and an input/output port  304 . In a memory  313 , mask patterns to be described later and the like are stored. Connected to the input/output port  304  are drive circuits  305 ,  306 ,  307 , and  308  such as actuators in a conveyance motor (LF motor)  309 , a carriage motor (CR motor)  310 , the print head  9 , and the heater  10 . The main control unit  300  is connected to the host apparatus  312  via an interface circuit  311 . 
     (4) Multi-Pass Printing Method 
     In the present embodiment, an image is printed by what is called multi-pass printing, in which printing of a predetermined region on a print medium is performed by a plurality of scans. The printing apparatus  100  of the present embodiment is a printer having a first nozzle array and a second nozzle array each having ejection ports configured to eject an ink of a predetermined color and arranged in the sub scanning direction. Then, the first nozzle array and the second nozzle array are scanned in the main scanning direction intersecting with the sub scanning direction N times (where N is an integer of 2 or greater) to print an image in a predetermined region on a print medium. Such printing with N scans is called multi-pass printing. The following describes typical multi-pass printing. 
       FIG.  5    is a diagram illustrating a typical multi-pass printing method. In an example described here, each nozzle array  90  is divided in the Y-direction to form eight ejection port groups A 1  to A 8 , from each of which an ink is ejected in a corresponding one of eight scans for a predetermined region to print an image in the predetermined region. In other words,  FIG.  5    shows an example of what is called eight-pass printing, in which printing of an image in a predetermined region is completed by eight print scans. Although the print medium P is actually conveyed downstream in the Y-direction between scans of the print head  9 ,  FIG.  5    depicts that the print head  9  is moved upstream in the Y-direction between scans to facilitate understanding. 
     First, in the first scan (Scan  1 ), the print head  9  is scanned under a positional relation such that a predetermined region  80  on the print medium P and the ejection port group A 1  of the nozzle array  90  face each other. Meanwhile, an ink is ejected from the ejection port group A 1  to the predetermined region  80  according to printing data corresponding to the ink of each type and corresponding to the first scan. After Scan  1 , the print medium is conveyed in the Y-direction by a distance corresponding to one ejection port group. After that, the second scan (Scan  2 ) is performed, ejecting an ink from the ejection port group A 2  to the predetermined region  80  according to printing data corresponding to the ink of each type and corresponding to the second scan. 
     After that, the conveyance of the print medium P and the ejection from the print head  9  are performed alternately to execute ejection from the ejection port groups A 3  to A 8  to the predetermined region  80  in the third to eighth scans. Multi-pass printing on the predetermined region  80  is thus completed. 
     (5) Image Data Conversion Processing 
       FIG.  6    is a diagram illustrating the flow of image data conversion processing performed by the printing system of the present embodiment. As described earlier, the printing system of the present embodiment has the host apparatus  312  and the printing apparatus  100 . In the present embodiment, the host apparatus  312  performs, e.g., generation of image data indicating an image to be printed and setting of a user interface (UI) for the data generation. The printing apparatus  100  performs printing using ink based on data sent from the host apparatus  312 . 
       FIG.  6    shows processing executed by an application  3121  and a printer driver  3123  in the host apparatus  312  and processing performed by the main control unit  300  in the printing apparatus  100 . The processing executed by the application  3121  and the printer driver  3123  is implemented by the CPU  400  of the host apparatus  312  by executing programs stored in the storage unit  402 , the memory  401 , or the like. Also, the processing executed by the main control unit  300  of the printing apparatus  100  is implemented by the CPU  301  of the main control unit  300  by executing programs stored in the ROM  302 , the memory  313 , or the like. Some or all of the functions of the steps in  FIG.  6    may be implemented by hardware such as an ASIC or an electronic circuit. Note that the letter “S” in the description of the processing means that it is a step (this applies to the other processing herein). 
     To execute printing, image data created by the application  3121  is passed to the printer driver  3123  via an OS. With respect to the image data received, the printer driver  3123  executes pre-processing S 601 , post-processing S 602 , γ correction processing S 603 , color separation data distribution processing S 604 , half-toning processing S 605 , and printing data creation processing S 606 . The following gives a brief description of each processing. 
     The pre-processing S 601  is processing for performing color gamut mapping. In this processing, data conversion is performed to map a color gamut reproduced by sRGB-standard image data R, G, B to a color gamut reproduced by the printing apparatus  100 . Specifically, a three-dimensional lookup table (LUT) is used to convert 256-tone data in which R, G, and B are each expressed with 8 bits, into R, G, B 8-bit data reproducible by the printing apparatus  100 . 
     The post-processing S 602  is processing for converting the R, G, B data obtained by the color gamut mapping in the pre-processing S 601  into sets of 8-bit color separation data corresponding to a combination of inks to reproduce the color expressed by the data. Here, similarly with the pre-processing S 601 , the processing is performed using the three-dimensional LUT and interpolation computation together. It goes without saying that the three-dimensional LUT used in the pre-processing S 601  and the three-dimensional LUT used in the post-processing S 602  are different LUTs. In the configuration in the present embodiment, the print head  9  that ejects a black ink (K) is used as shown in  FIG.  3   , and thus, a three-dimensional LUT that converts R, G, B data into K data is used. 
     The γ correction processing S 603  is processing for converting density values (tone values) in the color separation data obtained by the post-processing S 602  for each color (although there is only K in the present embodiment, a term “each color” is used for the sake of illustration convenience). Specifically, using a one-dimensional LUT corresponding to the tone characteristics of each color ink used in the printing apparatus  100 , conversion is performed such that the color separation data is linearly associated with the tone characteristics of the printing apparatus  100 . 
     In the present embodiment, after the density values (tone values) in the color separation data obtained in the post-processing S 602  for each color are converted in the γ correction processing S 603 , the color separation data is distributed to the first nozzle array and the second nozzle array. In other words, the color separation data distribution processing S 604  is performed to distribute the color separation data after the γ correction processing into color separation data for the first nozzle array and color separation data for the second nozzle array. Details of the color separation data distribution processing will be described later. 
     The half-toning processing S 605  is processing for performing quantization processing on the color separation data for the first nozzle array and color separation data for the second nozzle array to convert the 8-bit color separation data into 4-bit data. In the present embodiment, ordered dithering is used to convert 256-tone 8-bit data and generate 9-tone 4-bit data. This 4-bit data is tone-value information indicating one of tones from level 0 to level 8 which are indices for indicating dot arrangement pattern in dot arrangement pattern processing S 607 , which is processing performed by the printing apparatus  100 . 
     The printing data creation processing S 606  is processing for creating printing data by adding printing control information to printing image information which is a collection of pieces of tone-value information. The printing control information is, e.g., the grade of a printed image, the type of print medium, and printing information such as color or monochrome. In the printing data creation processing S 606 , printing data for the first nozzle array and printing data for the second nozzle array are created. 
     Once the host apparatus  312  sends printing data to the printing apparatus  100 , the printing apparatus  100  performs the dot arrangement pattern processing S 607  and mask processing S 608  on each of the printing data for the first nozzle array and the printing data for the second nozzle array inputted. 
     In the dot arrangement pattern processing S 607 , binarization processing is performed by expanding the tone-value information, which is 9-tone 4-bit data that are output values from the half-toning processing S 605 , to a dot arrangement pattern. As a result, binary data indicating whether to print an ink droplet (ejection or non-ejection) can be obtained for a region corresponding to one multi-value pixel. Here, data on one multilevel (4-bit) pixel (hereinafter called a pixel region) is converted to generate binary (1-bit) 2×4 pixel data. 
     The mask processing S 608  is processing for performing a logical AND between the dot arrangement of each color determined in the dot arrangement pattern processing S 607  and a plurality of mask patterns having a mutually complementary relation. As a result, for each color, data for ejecting ink droplets in each of print scans forming multi-pass printing is generated. In the present embodiment, mask processing performed on the data for the first nozzle array is described as first mask processing S 608   a , and mask processing performed on the data for the second nozzle array is described as second mask processing S 608   b . In the present embodiment, as will be described later, in the processing, a first mask to be described later is allocated to the first nozzle array, and a second mask to be described later is allocated to the second nozzle array. 
     The main control unit  300  transfers the binary printing data obtained as a result of the mask processing S 608  to the head drive circuit  307 . The 1-bit data on each color inputted to the drive circuit  307  is converted to a drive pulse for the print head  9 , so that an ink is ejected at an appropriate timing in a plurality of print scans in multi-pass printing. Consequently, ink ejection is performed according to the printing data, and an image is printed on the print medium. Although the processing performed by the printer driver  3123  and the processing performed by the main control unit  300  of the printing apparatus  100  are described as being separate in the example in  FIG.  6   , this description is merely an example. Some or all of the processes in  FIG.  6    may be performed by the printer driver  3123  or by the main control unit  300 . In other words, the printing control apparatus that controls the printing apparatus  100  may be the host apparatus  312  or may be the printing apparatus  100 . 
     (6) Printing Percentages in Masks 
       FIGS.  7 A and  7 B  are diagrams illustrating printing percentages (ink ejection percentages) in masks.  FIG.  7 A  is a diagram showing the printing percentages (printing ratio) of the respective passes for the first mask. Such printing percentages can be set by mask patterns for allotting printing data corresponding to a pixel to the respective passes. A mask pattern is formed by mask elements each defines output or non-output of printing data. Through mask processing using mask patterns, printing data corresponding to ink ejection or non-ejection is allotted to each pass. A “printing percentage” also represents the proportion of mask elements defining output of printing data. In the present embodiment, what is used in mask processing to use printing data in each pass is referred to as a “mask pattern,” and these mask patterns are collectively called a “mask.” 
     For example, the first mask shown in  FIG.  7 A  shows mask patterns that complete multi-pass printing with eight passes. The first mask includes mask patterns such that the printing percentage is constant among the passes forming multi-pass printing, namely the first to eighth passes (the printing percentage of each pass is 12.5% in this example). Using the first mask, printing data on a corresponding ink is allotted to the first to eight passes with the print percentages shown in  FIG.  7 A . Also, the eight mask patterns used in the first to eight passes have a mutually complementary relation, and their printing percentages add up to 100%. 
     Note that the printing percentage 100% in the present embodiment refers to a state where an area factor is filled 100%. Specifically, in a case where the resolution on a paper surface is 600 dpi and the diameter of a landed dot is approximately 30 μm, the printing percentage 100% in the present embodiment is a state where an area factor is filled 100% by forming a total of four dots on a 600 dpi grid (the area of 42.3 μm×42.3 μm). 
       FIG.  7 B  shows the printing percentages of the respective passes for the second mask. Like the first mask, the second mask includes eight mask patterns corresponding to the first to eighth passes. While the first mask has a constant printing percentage among the first to eighth passes, the second mask includes mask patterns such that the printing percentages of the intermediate print passes are high relative to those of the earlier print passes and later print passes. In the second mask as well, the eight mask patterns have a mutually complementary relation, and their printing percentages add up to 100%. Thus, the first mask and the second mask each define printing percentages for a single nozzle array. 
     Note that in the present embodiment, color separation data on an ink color is distributed into data for the first nozzle array and data for the second nozzle array. Thus, a printing duty (the proportion of an ink of a certain ink color being ejected) printed by each pass for each nozzle array is found based on the relation between the ratio of distribution of the color separation data and a printing percentage. For example, in a case where the distribution ratio of distribution between the first nozzle array (the first mask) and the second nozzle array (the second mask) is 50%:50%, the printing duty of the color separation data on each pass in the first mask is 6.25%. 
     In this way, in the present embodiment, the first mask used for the first nozzle array and the second mask used for the second nozzle array include mask patterns having different printing percentages of the passes from each other. Specifically, the first mask includes mask patterns such that the printing percentage is constant among the passes, whereas the second mask includes mask patterns such that the printing percentages of the intermediate print passes are high relative to those of the earlier and later printing passes. 
     Each mask can also be said to have a shape having the characteristics as shown in  FIG.  7 A or  7 B . As an example, consider a mask pattern in which a position at which a dot is to be printed is black and a position at which a dot is not to be printed is white. In this case, the first mask corresponding to a predetermined print region (a unit region) has a mask shape such that black dots are evenly distributed over the entire unit area. Meanwhile, the second mask corresponding to the predetermined print region (a unit region) has a mask shape such that black dots are concentrated in a center portion of the unit region and scattered in upper and lower end portions of the unit region. In the present embodiment, such masks are allocated to the first nozzle array and the second nozzle array in a fixed manner. 
     (7) Principle of Color Separation 
       FIGS.  8 A and  8 B  are diagrams showing how the way streaks at connection portions in the pass formation process look different between the mask shape of the first mask (hereinafter referred to as a first mask shape) and the mask shape of the second mask (hereinafter referred to as a second mask shape). In the case described here, the first mask is allocated to the first nozzle array, and the second mask is allocated to the second nozzle array.  FIG.  8 A  is an example where the first mask is allocated, and  FIG.  8 B  is an example where the second mask is allocated. As shown in  FIG.  8 A , in the case where the first mask is allocated, an abrupt change in density is generated at a connection portion between passes and is easily visible as a streak. Meanwhile, as shown in  FIG.  8 B , in the case where the second mask is allocated, a change in density at the connection portion is gradual, and unevenness in density at a connection portion between passes is reduced. As a result, a streak is less likely to be visible. However, in the second mask, the higher the printing duty (the higher the tone value in the color separation data), the more likely it is that lightness becomes uneven within a band according to the slope of the second mask shape. 
     In the present embodiment, in a case where a printing duty is a predetermined value or below, the color separation data is distributed into data for ejecting an ink from the first nozzle array of the print head and data for ejecting an ink from the second nozzle array of the print head so that a proportion of the color separation data distributed to the second nozzle array may be higher than that distributed to the first nozzle array. The predetermined value is, for example, 50%. By thus distributing the color separation data (printing duty), connection streaks are reduced. Meanwhile, in a case where the printing duty is higher than the predetermined value, the data is distributed as follows. Specifically, the color separation data is distributed into data for ejecting an ink from the first nozzle array of the print head and data for ejecting an ink from the second nozzle array of the print head so that a proportion of the color separation data distributed to the second nozzle array may be higher than or the same as that distributed to the first nozzle array. By thus distributing the printing duty, lightness is less likely to become uneven within a band. 
     (8) Method for Distributing Color Separation Data Based on Tone Values in Input Data 
       FIG.  9    is a diagram illustrating a method in which 256-tone values from “0” to “255” in input data inputted after completion of the γ correction processing S 603  are allotted to the first nozzle array and the second nozzle array in the color separation data distribution processing S 604 . 
     The printing duty (the proportion of an ink of a certain ink color being ejected) is 0% in a case where a tone value in color separation data (input data) is “0” and is 100% in a case where a tone value is “255.” As an example, a case is considered here in which, for a tone value “255,” a tone value “128” is allotted to the first nozzle array, and a tone value “128” is allotted to the second nozzle array. In this case, the color separation data is distributed and allotted to the first nozzle array at a distribution ratio of 50% and to the second nozzle array at a distribution ratio of 50%. In a case where the input data indicates a tone value “128,” a tone value “32” is allotted to the first nozzle array, and a tone value “96” is allotted to the second nozzle array. Then, the total printing duty becomes 50%, with the first nozzle array being allocated a printing duty of 12.5% and the second nozzle array being allocated a printing duty of 37.5%. In this case, the 50% printing duty is distributed at a distribution ratio of 25% to the first nozzle array and 75% to the second nozzle array. 
     (9) Specific Method for Distributing Color Separation Data 
       FIGS.  10 A and  10 B  are diagrams illustrating a specific method for distributing color separation data, or in other words, a specific method for distributing a printing duty. A total printing duty for the same ink color is defined as X (%).  FIG.  10 A  is an example of how a printing duty is distributed to the second mask (i.e., the second nozzle array), and  FIG.  10 B  is an example of how a printing duty is distributed to the first mask (i.e., the first nozzle array). The horizontal axis in  FIGS.  10 A and  10 B  indicates the total printing duty of a printing duty for the first nozzle array (for the first mask) and a printing duty for the second nozzle array (for the second mask). Specifically, a total printing duty of 50% indicates that input data has a tone value of “128”. The vertical axis in  FIGS.  10 A and  10 B  indicates a distributed printing duty. A distributed printing duty is a printing duty after distribution. 
     As shown in  FIG.  10 A , a distributed printing duty for the second mask is expressed as α % with X changing from 0% from 100%. Also, as shown in  FIG.  10 B , a distributed printing duty for the first mask is expressed as (X−α)%. Color separation data (i.e., the total printing duty) is distributed to the first nozzle array and the second nozzle array according to these distributed printing duties. 
     In  FIGS.  10 A and  10 B , a region R 1  indicates a first printing duty region where the total printing duty is relatively low, and a region R 2  indicates a second printing duty region where the total printing duty is relatively high. As shown in  FIGS.  10 A and  10 B , in a case where, for example, the total printing duty for the same color is 50%, color separation data is distributed so that α may be 37.5% and X−α may be 12.5%. Also, in a case where the total printing duty for the same color is 100%, color separation data is distributed so that a may be 50% and X−α may be 50%. These percentages are the same as those described in the example in  FIG.  9   . Thus,  FIGS.  10 A and  10 B  show graph data visualizing the example shown in  FIG.  9   . 
     In this way, for a relatively low tone (the first printing duty region R 1 ), color separation data is distributed so that the distributed printing duty for the second nozzle array may be higher than the distributed printing duty for the first nozzle array. By contrast, for a relatively high tone (the second printing duty region R 2 ), color separation data is distributed so that the distributed printing duty for the second nozzle array may be higher than or the same as the distributed printing duty for the first nozzle array. 
     (10) Method for Processing Pass Data 
       FIGS.  11 A and  11 B  are diagrams illustrating a method for processing pass data for each printing element group. Pass data is data after mask processing. Pass data for each of the first nozzle array and the second nozzle array is obtained by performing a logical AND between color separation data distributed to the first or second nozzle array and the first or second mask shape, or specifically, obtained by performing the following processing: 
     pass data for the first nozzle array=color separation data (X−α)%×the first mask, and 
     pass data for the second nozzle array=color separation data α %×the second mask. Here, “×” indicates performing a logical AND. 
       FIG.  11 A  is a diagram illustrating how pass data is created in a case where the total printing duty is 50% like in  FIGS.  10 A and  10 B .  FIG.  11 A  shows, from the left, color separation data ((X−α)%) for the first nozzle array on a predetermined unit region (1280 pixels vertically×512 pixels horizontally), the first mask shape, and pass data. In other words,  FIG.  11 A  schematically shows that the pass data is a result of a logical AND between the color separation data and the first mask. Similarly,  FIG.  11 B  shows, from the left, color separation data (α %) for the second nozzle array on the predetermined unit region, the second mask shape, and pass data. 
     (11) Pass Data Generation Processing 
       FIG.  12    is a flowchart showing pass data generation processing in the present embodiment. The processing in  FIG.  12    corresponds to a series of processing from the color separation data distribution processing S 604  to the mask processing S 608  described with  FIG.  6   . Note that  FIG.  12    is a flowchart mainly focusing on the color separation data distribution processing S 604  and the mask processing S 608  and omits descriptions of the other processing. Also, to make the description simple, the following processing is described as being performed by the main control unit  300 . 
     Once color separation data is inputted in S 1201 , in S 1202 , the main control unit  300  determines a total printing duty. Specifically, the main control unit  300  determines whether the total printing duty is larger than a predetermined value. This determines whether the total printing duty is, for example, a value in the first printing duty region R 1  or a value in the second printing duty region R 2  in  FIGS.  10 A and  10 B . The total printing duty corresponds to a tone value in the input data. 
     In S 1203 , the main control unit  300  determines how to distribute the color separation data according to the total printing duty. Specifically, with reference to a table having relations as shown in  FIGS.  10 A and  10 B , the main control unit  300  determines distributed printing duties α % and (X−α)% corresponding to the total printing duty. For example, in a case where the total printing duty is small, distributed printing duties are determined with reference to values for the first printing duty region R 1  in  FIGS.  10 A and  10 B . In a case where the total printing duty is large, distributed printing duties are determined with reference to values for the second printing duty region R 2  in  FIGS.  10 A and  10 B . 
     The subsequent processing in and after S 1204  is processing for determining data on each pass (pass data). In S 1204 , the main control unit  300  determines whether data to be processed is data for the first nozzle array or data for the second nozzle array. Then, in S 1205 , the main control unit  300  allocates the first mask to the data for the first nozzle array and allocates the second mask to the data for the second nozzle array. 
     Next, in S 1206 , the main control unit  300  distributes (X−α)% of the color separation data to the first nozzle array and α % of the color separation data to the second nozzle array according to the distribution determined in S 1203 . Next, in S 1207 , the main control unit  300  performs mask processing of multiplying the color separation data and the mask patterns in  FIGS.  7 A and  7 B  to perform a logical AND therebetween. As a result, pass data to be printed by each nozzle array of the print head during a scan of the processing target is completed (S 1208 ). The pass data is transferred to the print head  9  at an appropriate timing to perform printing. 
     In S 1209 , a variable n as the number of scans is incremented by 1, and the main control unit  300  determines whether the value of n has reached a predetermined pass count (S 1210 ). If the value is the predetermined pass count, the main control unit  300  determines that the generation of pass data for scans of the print head has been finished and ends the processing. If the value is below the predetermined pass count, the main control unit  300  proceeds back to S 1204  to repeat the printing data generation processing from S 1204  until the predetermined pass count is reached. 
     As thus described, in the present embodiment, the first mask is allocated for the first nozzle array, and the second mask which is a different from the first mask is allocated for the second nozzle array. These first and second masks allocated for the respective nozzle arrays do not vary among the scans, and fixed masks are used. Meanwhile, the distribution ratio of sets of data distributed to the respective nozzle arrays is controlled according to the total printing duty (i.e., a tone value in data after color separation). This can reduce unevenness in density at a connection portion and reduce unevenness in lightness within a band. Note that as to the printing duty distribution method, although the distribution is performed with reference to information held by a table as shown in  FIGS.  10 A and  10 B , the distribution method may instead be determined using a predetermined formula. 
     Second Embodiment 
     In the first embodiment, an example of using one type of ink has been described. In the present embodiment, an example of using a plurality of types of ink is described. Note that the following omits descriptions of configurations similar to those in the first embodiment and focuses on differences. Also, the subtitles are given the same numbers given to those in the first embodiment. 
     (2) Configuration of the Print Head 
       FIG.  13    is a diagram illustrating the configuration of the print head  9  of the present embodiment. As inks containing a color ink, the print head  9  of the present embodiment uses a cyan ink (C), a magenta ink (M), a yellow ink (Y), and a black ink (K). Specifically, the print head  9  includes a plurality of nozzle arrays  90  (C 1  and C 2 , M 1  and M 2 , Y 1  and Y 2 , K 1  and K 2 ) for each of these colors. Although the following description uses C, M, Y, and K inks as an example, inks of other colors may be used. 
     In the print head  9 , these ejection port arrays are arranged side by side from the left side to the right side of the X-direction in the order of the ejection port arrays C 1 , C 2 , M 1 , M 2 , Y 1 , Y 2 , K 1 , and K 2 . These ejection port arrays C 1 , C 2 , M 1 , M 2 , Y 1 , Y 2 , K 1 , and K 2  are each formed by 1280 ejection ports  90 A configured to eject a corresponding ink and arranged in the Y-direction (the arrangement direction) at a density of 1200 dpi. Note that the amount of ink ejected from a single ejection port  90 A at once is approximately 4.5 pl in the present embodiment. 
     These ejection port arrays C 1 , C 2 , M 1 , M 2 , Y 1 , Y 2 , K 1 , and K 2  are connected to ink tanks (not shown) storing corresponding inks and are supplied with the inks. Note that the print head  9  and the ink tanks used in the present embodiment may be configured integrally or configured to be separable from each other. 
     (5) Image Data Conversion Processing 
     The present embodiment differs from the first embodiment in that by the post-processing S 602 , R, G, B data is converted to data on each ink color of C, M, Y, and K. As to processing in and after the γ correction processing S 603 , except for the color separation data distribution processing S 604 , processing similar to the processing described in the first embodiment is performed on each of the data on the color ink C, the data on the color ink M, the data on the color ink Y, and the data on the color ink K. Thus, parts denoted as “K” in  FIG.  6    only need to be read as “CMYK.” The color separation data distribution processing S 604  of the present embodiment is described below. 
     (12) Method for Distributing Color Separation Data 
     In the present embodiment, for the colors C, M, Y, and K, the first mask is allocated to the first nozzle arrays (C 1 , M 1 , Y 1 , and K 1 ), and the second mask is allocated to the second nozzle arrays (C 2 , M 2 , Y 2 , and K 2 ). 
       FIGS.  14 A to  14 D  are diagrams illustrating a specific method for distributing color separation data (a total printing duty) in the present embodiment.  FIGS.  14 A to  14 D  only show the distributed printing duty α % for the second nozzle arrays of the respective colors. As is similar to the example described in the first embodiment, a total printing duty of each of the colors C, M, Y, and K is defined as X (%), and a distributed printing duty for the second mask for each color is expressed as α % with X changing from 0% from 100%.  FIG.  14 A to  14 D  show distributed printing duties of cyan (C), magenta (M), yellow (Y), and black (K), respectively.  FIGS.  14 A to  14 D  only show α %, which is a distributed printing duty for the second mask, and omit (X−α)%, which is a distributed printing duty for the first mask, because the same principle as that in the first embodiment applies. Although  FIGS.  14 A to  14 D  show a case where α % is the same for all of C, M, Y, and K, α % may be adjusted depending on color. For example, for a hue which is less visually noticeable as a streak after every sheet conveyance (such as yellow), the distributed printing duty for the second mask may be closer to the distributed printing duty for the first mask than the other colors. Conversely, for a hue which is easily visible as a streak (such as cyan, magenta, or black), the distributed printing duty for the second mask may be higher than the distributed printing duty for the first mask to a greater extent. 
     In the present embodiment, like in the first embodiment, for a relatively low tone (the first printing duty region R 1 ), distribution into data for the first nozzle array and data for the second nozzle array is done so that a distributed printing duty for the second nozzle array may be higher than a distributed printing duty for the first nozzle array. For a relatively high tone (the second printing duty region R 2 ), distribution into data for the first nozzle array and data for the second nozzle array is done so that a distributed printing duty for the second nozzle array may be higher than or the same as a distributed printing duty for the first nozzle array. 
     As thus described, the present embodiment too can reduce unevenness in density at a connection portion and reduce unevenness in lightness within a band. Also, the present embodiment makes it possible to make fine adjustments according to a hue because the distributed duty is individually determined for each color. 
     OTHER EMBODIMENTS 
     In the first and second embodiments, an example where an ink is a color ink is described. However, the present disclosure is not limited to this example. The ink may be a reactive liquid that reacts with a color material ink or may be an overcoat liquid. 
     In the second embodiment, an example where each of the colors C, M, Y, and K has a plurality of nozzle arrays has been described. In this respect, all the colors used in the print head may have a plurality of nozzles, or some ink colors may have a single nozzle array. In this case, the processing described in each of the above embodiments may be performed on color separation data on an ink color having a plurality of nozzle arrays. 
     In addition, each of the above embodiments has described an example where a total printing duty is distributed according to distributed printing duties shown in, for example,  FIGS.  10 A and  10 B  and  FIGS.  14 A to  14 D . Alternatively, the total printing duty may be distributed according to data that defines a ratio of allotment to the first nozzle array and the second nozzle array (a distribution ratio). For example, in a case where the total printing duty is 50% in the example in  FIGS.  10 A and  10 B , the total printing duty may be distributed according to data (such as a table) defining a distribution ratio such that 75% is allocated to the second mask and 25% is allocated to the first mask. Advantageous effects similar to those offered by the examples described in the above embodiments can still be obtained in this case. 
     The present disclosure allows easy adjustment of printing duties and printing with less image quality degradation without changing mask patterns. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2021-185101, filed Nov. 12, 2021, which is hereby incorporated by reference wherein in its entirety.