Abstract:
A printer is disclosed. One printer includes a first head unit being elongate in a longitudinal direction. The first head unit has a first nozzle group having a plurality of first nozzles arrayed with a first pitch along the longitudinal direction. The printer includes a second head unit being elongate in the longitudinal direction. The second head unit has a second nozzle group having a plurality of second nozzles arrayed along the longitudinal direction. The second nozzle group includes a plurality of nozzle sets. Each of the plurality of the nozzle sets includes some of the plurality of second nozzles. The second nozzles in each of the plurality of the nozzle sets arrayed with the first pitch along the longitudinal direction. The plurality of the nozzle sets are arrayed with a second pitch along the longitudinal direction. The second pitch is different from the first pitch.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2016-073671 filed on Mar. 31, 2016, the content of which is incorporated herein by reference in its entirety. 
       Field of Disclosure 
       [0002]    Aspects disclosed herein relate to a printer a head unit. 
       Background 
       [0003]    There have been known printers including line-type ejection heads. Some of the known line-type ejection heads includes a plurality of head units positioned along a width direction of a recording medium. 
         [0004]    In such an ejection head, one head unit partially overlaps another head unit in a conveyance direction. If nozzles of the one head unit are misaligned with their corresponding nozzles of the another head unit at the overlap area, a streak (e.g., a white streak or a black streak) tends to occur in an image formed by the nozzles positioned at the overlap area. In order to solve this problem, various methods for reducing occurrence of the streak have been proposed. 
         [0005]    In one example, an ejection head includes a plurality of head units. The head units are disposed such that printable ranges of adjacent two of the head units in a width direction of the recording medium partially overlap each other. A nozzle pitch in one of the adjacent head units is greater than a nozzle pitch in the other of the adjacent head units. In this ejection head, at an overlap area where the adjacent head units partially overlap each other, particular nozzles of the one head unit are aligned with particular nozzles of the other head unit. Thus, the nozzles of the one and other head units are appropriately used depending on the locations with respect to the particular nozzles (i.e., a boundary). That is, on one side relative to the boundary, the one head unit is caused to eject ink from one or more of the nozzles thereof. On the other side relative to the boundary, the other head unit is caused to eject ink from one or more of the nozzles thereof. Such an ink ejection manner may reduce occurrence of the streak in an image formed by the nozzles positioned at the overlap area. 
         [0006]    In another example, an ejection head includes a plurality of head units. The head units are disposed such that printable ranges of adjacent two of the head units partially overlap each other. At the overlap area of the ejection head, ink droplets are ejected from nozzles of both of the adjacent head units. At another area of the ejection head, ink droplets are ejected from nozzles of the one or the other of the adjacent head units. Ink droplets ejected from the nozzles of each of the adjacent head units positioned at the overlap area dispersedly land on a recording medium to form a joint of images formed by the nozzles of the one head unit and the nozzles of the other head unit, respectively. Therefore, nozzle misalignment between the head units may less affect the print result. 
       SUMMARY 
       [0007]    Nevertheless, in the known method described as the one example, if a conveying mechanism cannot convey a recording sheet straightly due to its lack of precision in conveyance, it may be difficult to prevent occurrence of the streak in the image formed by the nozzles positioned at the overlap area. In the other known method described as the other example, if the head units are not positioned at their respective optimum positions, density unevenness may occur at the joint of the images formed by the respective head units. 
         [0008]    Accordingly, some embodiments of the disclosure may minimize relative displacement between a dot and its corresponding dot to be formed by two head units, respectively, and surely reduce occurrence of density unevenness at an overlap portion where two images overlap each other. 
         [0009]    According to one aspect of the disclosure, a printer includes a first head unit being elongate in a longitudinal direction. The first head unit extends from a first end of the first head unit in the longitudinal direction to a second end of the first head unit in the longitudinal direction. The first head unit has a first nozzle group having a plurality of first nozzles arrayed with a first pitch along the longitudinal direction. The first nozzle group is positioned between a center of the first head unit in the longitudinal direction and the second end of the first head unit in the longitudinal direction. The printer includes a second head unit being elongate in the longitudinal direction. The second head unit extends from a third end of the second head unit in the longitudinal direction to a fourth end of the second head unit in the longitudinal direction. The second head unit has a second nozzle group having a plurality of second nozzles arrayed along the longitudinal direction. The second nozzle group is positioned between the third end of the second head unit in the longitudinal direction and a center of the second head unit in the longitudinal direction. The second nozzle group is positioned next to the first nozzle group in a transverse direction orthogonal to the longitudinal direction. The second nozzle group includes a plurality of nozzle sets. Each of the plurality of the nozzle sets includes some of the plurality of second nozzles. The second nozzles in each of the plurality of the nozzle sets arrayed with the first pitch along the longitudinal direction. The plurality of the nozzle sets are arrayed with a second pitch along the longitudinal direction. The second pitch is different from the first pitch. 
         [0010]    According to further aspect of the disclosure, a head unit includes a nozzle group A including a plurality of nozzles A and a nozzle group B including a plurality of nozzle sets, each plurality of nozzle sets including of a plurality nozzles B. The head unit is elongate in a longitudinal direction. The head unit extends from a first end of the head unit in the longitudinal direction to second end of the head unit in the longitudinal direction. The plurality of nozzles A are arrayed with a first pich along the longitudinal direction. The nozzle group B is positioned between the first end of the head unit in the longitudinal direction and the nozzle group A. The plurality of nozzles B in each of the plurality of nozzle sets are arrayed with the first pitch along the longitudinal direction. The plurality of nozzles sets are arrayed with a second pitch along the longitudinal direction. The second pitch is different from the first pitch. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic plan view of a printer in an illustrative embodiment according to one or more aspects of the disclosure. 
           [0012]      FIG. 2A  is a plan view of one of inkjet heads in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0013]      FIG. 2B  is a plan view of one of head units in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0014]      FIG. 3  is a partial enlarged plan view of two of the head units each including nozzle groups in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0015]      FIG. 4  is a graph showing nozzle usage rates in the two head units in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0016]      FIG. 5  is a flowchart of an example printing process in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0017]      FIG. 6A  illustrates dot data on which dot data distribution has not been executed in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0018]      FIG. 6B  illustrates dot data on which the dot data distribution has been executed in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0019]      FIG. 6C  illustrates mask data to be used in masking in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0020]      FIG. 6D  illustrates ejection data generated through the masking in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0021]      FIG. 7  is a block diagram of the printer and a testing system in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0022]      FIG. 8  is a flowchart illustrating a procedure for selecting nozzles to be used for printing in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0023]      FIG. 9A  is a plan view of one of the inkjet heads in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0024]      FIG. 9B  illustrates test patterns printed on a recording sheet in the illustrative embodiment according to one or more aspects of the disclosure. 
           [0025]      FIGS. 10A and 10B  are graphs each showing nozzle usage rates in one alternative embodiment according to one or more aspects of the disclosure. 
           [0026]      FIG. 11  is a partial enlarged plan view of two head units each including nozzle groups in another alternative embodiment according to one or more aspects of the disclosure. 
           [0027]      FIG. 12  is a graph showing nozzle usage rates in the two head units of  FIG. 11  in the another alternative embodiment according to one or more aspects of the disclosure. 
           [0028]      FIG. 13  is a partial enlarged plan view of two head units including nozzle groups in still another alternative embodiment according to one or more aspects of the disclosure. 
           [0029]      FIG. 14  is a graph showing nozzle usage rates in yet another alternative embodiment according to one or more aspects of the disclosure. 
           [0030]      FIG. 15  is a graph showing nozzle usage rates in further alternative embodiment according to one or more aspects of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    An illustrative embodiment will be described with reference to the accompanying drawings. Hereinafter, a direction extending along a conveyance direction in which a recording sheet  100  is conveyed is defined as a front-rear direction of a printer  1 . A width direction of the recording sheet  100  is defined as a right-left direction of the printer  1 . A direction orthogonal to the front-rear direction and the right-left direction is defined as a top-bottom direction of the printer  1 . 
         [0032]    &lt;General Configuration of Printer&gt; 
         [0033]    As illustrated in  FIG. 1 , the printer  1  includes a platen  3 , a plurality of, for example, four inkjet heads  4 , a plurality of, for example, two conveyor rollers  5  and  6 , and a controller  7 , which are accommodated in a housing  2  of the printer  1 . 
         [0034]    The platen  3  is configured to support a recording sheet  100  on an upper surface thereof. The inkjet heads  4  are positioned above the platen  3  and next to one another in the conveyance direction. Each inkjet head  4  is configured to be supplied with ink from a corresponding one of ink tanks (not illustrated). Each inkjet head  4  is supplied with ink of different one of colors (e.g., black, yellow, cyan, and magenta). That is, the inkjet heads  4  are configured to eject ink of respective different colors. 
         [0035]    The controller  7  includes a central processing unit (“CPU”)  15 , a read only memory (“ROM”)  16 , a random access memory (“RAM”)  17 , and an application specific integrated circuit (“ASIC”)  18  including various control circuits. The controller  7  further includes a nonvolatile memory  19  configured to store various control parameters that can be rewritten. The controller  7  is connected to an external device  9 , e.g., a personal computer (“PC”), and is configured to perform data communication with the external device  9 . The controller  7  is further configured to control components of the printer  1 , e.g., the inkjet heads  4  and a conveyor motor, based on image data transmitted from the external device  9 . 
         [0036]    More specifically, the controller  7  controls the conveyor motor to cause the conveyor rollers  5  and  6  to convey a recording sheet  100  along the conveyance direction. While controlling the sheet conveyance, the controller  7  controls the inkjet heads  4  to eject ink onto the recording sheet  100 . Thus, an image is printed on the recording sheet  100 . 
         [0037]    The external device  9  may be, for example, a PC that includes a controller including ICs, such as a CPU, a RAM, and a ROM, and that has a printer driver corresponding to the printer  1  installed therein. In the illustrative embodiment, for example, a user provides an image printing instruction by operating the external device  9 . In response to the image printing instruction through the user operation, the external device  9  transmits RGB image data  300  to the printer  1 . The image data  300  is an example of original image data. 
         [0038]    &lt;Configuration of Inkjet Heads&gt; 
         [0039]    Hereinafter, the inkjet heads  4  will be described in detail. All of the inkjet heads  4  have the same or similar configuration, and therefore, one of the inkjet heads  4  will be described in detail. As illustrated in  FIG. 2A , the inkjet head  4  includes a plurality of, for example, four head units  11  that are positioned along the right-left direction. 
         [0040]    The four head units  11  are alternately aligned in two rows (e.g., a front row and a rear row) with respect to the conveyance direction. That is, the head units  11  are staggered along the right-left direction. Each of the head units  11  has nozzles  21  arrayed along the right-left direction. 
         [0041]    The head units  11  in the front row and the head units  11  in the rear row partially overlap when viewed in the front-rear direction. When distinguishing between the four head units  11 , the head units  11  are referred to as head units  11   a,    11   b,    11   c,  and  11   d  individually from the left in the inkjet head  4 . When not distinguishing between the four head units  11 , the head units  11   a,    11   b,    11   c,  and  11   d  are collectively referred to as the head units  11 . Similar to this, reference numerals for components corresponding to the respective head units  11   a,    11   b,    11   c,  and  11   d  also include appropriate one of letters “a”, “b”, “c”, and “d”, at the respective ends of the reference numerals when distinguishing between the components. Nevertheless, when not distinguishing therebetween, no distinguishing letter is appended thereto. 
         [0042]    Hereinafter, an array pattern of the nozzles  21  included in each head unit  11  will be described. All of the head units  11  have the same or similar configuration, and therefore, one of the head units  11  will be described in detail. In the illustrative embodiment, for example, the head unit  11  has 100 nozzles  21 . For explanatory convenience, as illustrated in  FIGS. 2B and 3 , numbers, e.g., # 1 , # 2 , . . . , and # 100 , are assigned to the nozzles  21  from the left. 
         [0043]    As illustrated in  FIG. 2B , the head unit  11  includes a nozzle group  23  consisting of one-hundred nozzles  21 . The nozzle group  23  further includes nozzle groups  25 ,  26 , and  28 . The nozzle group  28  is positioned between the nozzle groups  25  and  26 . The nozzle group  25  is positioned to the right of the nozzle group  28 . The nozzle group  26  is positioned to the left of the nozzle group  28 . The nozzle group  26  consists of twenty-four nozzles  21  of # 1  to # 24 . The nozzle group  28  consists of fifty-two nozzles  21  of # 25  to # 76 . The nozzle group  25  consists of twenty-four nozzles  21  of # 77  to # 100 . The nozzle groups  25  and  28  are collectively referred to as a nozzle group  29 . 
         [0044]    As illustrated in  FIG. 3 , the total of seventy-six nozzles  21  of # 25  to # 100  included in one or the other of the nozzle groups  28  and  25  are arrayed along the right-left direction with a pitch d 1 . The twenty-four nozzles  21  of # 1  to # 24  included in the nozzle group  26  are arrayed with distinctive pitches. 
         [0045]    More specifically, for example, the nozzle group  26  consists of six nozzle sets  27 , each of which consists of four of the nozzles  21 . In each nozzle set  27 , the four nozzles  21  are spaced from each other at the pitch d 1 . The endmost nozzles  21  that are included in respective adjacent nozzle sets  27  and adjacent to each other are spaced from each other at a pitch d 2  that is greater than the pitch d 1 . 
         [0046]    That is, the nozzle group  26  includes five pairs of the adjacent nozzles  21  spaced from each other at the pitch d 2 : a pair of the nozzles  21  of # 4  and # 5 , a pair of the nozzles  21  of # 8  and # 9 , a pair of the nozzles  21  of # 12  and # 13 , a pair of the nozzles  21  of # 16  and # 17 , and a pair of the nozzles  21  of # 20  and # 21 . In a pair of the adjacent nozzles  21  of # 24  and # 25 , the nozzles  21  are spaced from each other at the pitch d 1 . The head units  11   a,    11   b,    11   c,  and  11   d  each have a plurality of nozzles  21  arrayed in the above-described pattern. 
         [0047]    A relatively large difference between the pitch d 1  and the pitch d 2  may be visible to human eyes. Therefore, it is preferable that the difference be a predetermined amount or smaller. For example, the difference between the pitch d 2  and the pitch d 1  may be one-quarter of the pitch d 1  or smaller. In a case where a single inkjet head is capable of printing at a resolution of 600 dpi, the pitch d 1  is 42 μm. In this case, the difference between the pitch d 2  and the pitch d 1  may preferably be 10 μm or smaller. 
         [0048]      FIG. 3  illustrates an array pattern of nozzles  21   a  in the head unit  11   a  in the rear row and an array pattern of nozzles  21   b  in the head unit  11   b  in the front row at the overlap area where the head units  11   a  and  11   b  overlap each other when viewed in the front-rear direction. The head unit  11   a  has the nozzle group  25   a  at its right end portion, and the head unit  11   b  has the nozzle group  26   b  at its left end portion. The head unit  11   a  and the head unit  11   b  are disposed such that the nozzle group  25   a  of the head unit  11   a  and the nozzle group  26   b  of the head unit  11   b  are positioned at substantially the same relative positions in the right-left direction. In other words, the nozzle group  25   a  of the head unit  11   a  is positioned next to the nozzle group  26   b  of the head unit  11   b  in the front-rear direction. 
         [0049]    In a case where a single inkjet head is capable of printing at a resolution of 600 dpi, the pitch d 1  is 42 μm. In this case, the difference between the pitch d 2  and the pitch d 1  may preferably be 10 μm or smaller. 
         [0050]    For example, in a case where the pitch d 1  is 42 μm and the pitch d 2  is 50.4 μm, the difference between the pitch d 2  and the pitch d 1  is 8.4 μm. In this case, the nozzles  21   b  included in each nozzle set  27   b  are offset every nozzle set  27  by 8.4 μm in the right-left direction with respect to their corresponding nozzles  21   a.    
         [0051]    Therefore, while a distance between the nozzles  21   a  of # 77  and # 100  in the nozzle group  25   a  of the head unit  11   a  is 966 μm, a distance between the nozzles  21   b  of # 1  and # 24  in the nozzle group  26   b  is 1008 μm. That is, a distance difference therebetween is 42 μm. This distance difference corresponds to the pitch d 1 . 
         [0052]    As described above, the distances between the nozzles  21   b  and their corresponding nozzles  21   a  in the right-left direction are different between the nozzle sets  27 . Therefore, the nozzle group  26  includes a nozzle set  27   b  consisting of nozzles  21   b  that are offset minimum with respect to their corresponding nozzles  21   a.  Hereinafter, such a nozzle set  27   b  is referred to as an optimum nozzle set  70   b.    
         [0053]    In  FIG. 3 , the third nozzle set  27   b  from the left located between double-dotted-and-dashed lines corresponds to the optimum nozzle set  70   b.  Four nozzles  21   b  (e.g., the nozzles  21   b  of # 9  to # 12 ) constituting the optimum nozzle set  70   b  are substantially aligned with their respective corresponding nozzles  21   a  (e.g., the nozzles  21   a  of # 85  to # 88 ) in the front-rear direction. Hereinafter, the four nozzles  21   b  constituting the optimum nozzle set  70   b  are referred to as optimum nozzles  72   b,  and the four nozzles  21   a  corresponding to the respective optimum nozzles  72   b  are referred to as optimum nozzles  716   a.    
         [0054]    Similarly to the head unit  11   a,  the head unit  11   b  includes a nozzle group  23   b,  and the nozzle group  23   b  includes a nozzle group  25   b  at the right end portion thereof. The head units  11   c  and  11   d  also each have nozzles  21  that are arrayed in a similar manner to the nozzles  21   b  of the head unit  11   b.  Therefore, the head units  11   c  and  11   d  also include optimum nozzle sets  70   c  and  70   d,  respectively. The optimum nozzle sets  70   b,    70   c,  and  70   d  are also collectively referred to as optimum nozzle sets  70 . The nonvolatile memory  19  stores optimum position information in association with each of the head units  11   b,    11   c,  and  11   d.  The optimum position information represents the position of the nozzle set  27  that corresponds to the optimum nozzle set  70  in the head unit  11  in a sequence from the left. The nonvolatile memory  19  stores three pieces of optimum position information for each inkjet head  4 , and thus, the nonvolatile memory  19  stores a total of 12 pieces of optimum position information therein. 
         [0055]    &lt;Ejection Control for Head Units&gt; 
         [0056]    Hereinafter, an ejection control for the nozzle group  25   a  of the head unit  11   a  and the nozzle group  26   b  of the head unit  11   b  will be described. 
         [0057]    The controller  7  changes nozzles  21  to be used for printing between the nozzles  21   a  and the nozzles  21   b  at a boundary region corresponding to the optimum nozzle set  70   b.  That is, the controller  7  causes both the head units  11   a  and  11   b  to eject ink from their optimum nozzles  71   a  and  71   b,  respectively, at the boundary region. Nevertheless, the controller  7  causes only the head unit  11   a  to eject ink from appropriate nozzles  21   a  on the left with respect to the optimum nozzle set  70   b,  and causes only the head unit  11   b  to eject ink from appropriate nozzles    21   b on the right with respect to the optimum nozzle set  70   b.    
         [0058]    In the boundary region corresponding to the optimum nozzle set  70   b,  the nozzles  21   b  are substantially aligned with their corresponding nozzles  21   a,  respectively, in the front-rear direction. Therefore, this configuration may minimize deviation of landing positions of ink droplets ejected from each nozzle  21   b  and its corresponding nozzle  21   a  relative to each other. Thus, this configuration may effectively reduce density unevenness that may be caused by misalignment of the nozzles  21   a  of the head unit  11   a  and the nozzles  21   b  of the head unit  11   b.    
         [0059]      FIG. 4  illustrates a graph showing a usage rate r 1  of each nozzle  21   a  included in the nozzle group  25   a  or in its adjacent group, and a usage rate r 2  of each nozzle  21   b  included in the nozzle group  26   b  or in its adjacent group. In the graph, a horizontal axis indicates the nozzle number of each nozzle  21  and a vertical axis indicates a usage rate r of each nozzle  21 . The usage rate r is determined based on mask data. 
         [0060]    A solid line represents a usage rate r 1  of each nozzle  21   a,  and a double-dotted-and-dashed line represents a usage rate r 2  of each nozzle  21   b.  The graph shows both the usage rate r 1  of each nozzle  21   a  included in the nozzle group  25   a  and the usage rate r 2  of each nozzle  21   b  included in the nozzle group  26   b  within a range specified by two dotted-and-dashed lines. 
         [0061]    In the illustrative embodiment, both the optimum nozzles  71   a  of the head unit  11   a  and the optimum nozzles  72   b  of the head unit  11   b  are used to eject ink. More specifically, for example, as illustrated in  FIG. 4 , lines representing the respective usage rates r 1  and r 2  change linearly within a range corresponding to the optimum nozzle set  70   b.  That is, the line representing the usage rates r 1  of the nozzles  21   a  declines linearly from the nozzle  21  of # 85  to the nozzle  21  of # 88 , whereas the line representing the usage rates r 2  of the nozzles  21   b  rises linearly from the nozzle  21  of # 9  to the nozzle  21  of # 12 . For example, the usage rate r 1  of the nozzle  21   a  of # 87  is 0.4, and the usage rate r 2  of the nozzle  21   b  of # 11  corresponding to the nozzle  21   a  of # 87  is 0.6. 
         [0062]    Assuming that an average of the usage rates r 1  of the four optimum nozzles  71   a  is an average usage rate R 1  and an average of the usage rates r 2  of the four optimum nozzles  72   b  is an average usage rate R 2 , the average usage rate R 1  satisfies 0&lt;R 1 &lt;1 and the average usage rate R 2  satisfies 0&lt;R 2 &lt;1. More specifically, the average usage rate R 1 =0.5, and the average usage rate R 2 =0.5. In this case, an equal amount of ink is ejected from each of the optimum nozzles  71   a  and the optimum nozzles  72   b    
         [0063]    As described above, the optimum nozzles  72   b  are substantially aligned with their corresponding optimum nozzles  71   a,  respectively, in the front-rear direction. Therefore, this configuration may minimize deviation of landing positions of ink droplets that may be caused by misalignment of the nozzles  21   b  and  21   a.  Nevertheless, if landing positions of ink droplets ejected from each nozzle  21   b  and its corresponding nozzle  21   a  are deviated relative to each other due to another factor, e.g., defective conveyance, this positional deviation may influence a printed image directly. 
         [0064]    In the illustrative embodiment, ink is ejected from each of the optimum nozzles  71   a  and  72   b.  Thus, ink droplets ejected from each of the head units  11   a  and  11   b  land on a recording sheet  100  dispersedly. Accordingly, if the landing positions of ink droplets ejected from each nozzle  21   b  and its corresponding nozzle  21   a  are deviated relative to each other due to another factor, density unevenness may be inconspicuous. 
         [0065]    &lt;Controller Operation&gt; 
         [0066]    Hereinafter, referring to  FIGS. 5 and 6A to 6D , operation executed by the controller  7  of the printer  1  will be described. 
         [0067]    As illustrated in  FIG. 5 , in response to an input of a print instruction to the printer  1  from the external device  9 , the controller  7  acquires image data  300  from the external device  9  (e.g., step S 201 ). The image data  300  includes image data  300 R corresponding to red (“R”), image data  300 G corresponding to green (“G”), and image data  300 B corresponding to blue (“B”). Each image data  300 R,  300 G, and  300 B consists of a plurality of pieces of pixel data that are equal in number to the number of pixels corresponding to the resolution of the printer  1 . Each image data  300 R,  300 G, and  300 B may be represented by 256 color levels and represent a color level value of a corresponding color. The image data  300  is generated based on an electronic file in a predetermined format by cooperation of an application program installed on the external device, a printer driver, and an operation system. 
         [0068]    Subsequent to step S 201 , the controller  7  performs color conversion in which the image data  300  corresponding to RGB is converted into image data  400  corresponding to CMYK (e.g., ink colors) (e.g., step S 202 ). The image data  400  includes image data  400 K corresponding to black, image data  400 Y corresponding to yellow, image data  400 C corresponding to cyan, and image data  400 M corresponding to magenta. Each image data  400 K,  400 Y,  400 C, and  400 M consists of a plurality of pieces of pixel data that are equal in number to the number of pixels corresponding to the resolution of the printer  1 . Each image data  400 K,  400 Y,  400 C, and  400 M may be represented by 256 color levels and represent a color level value of a corresponding color. The image data may be converted from an RGB format to a CMYK format using a lookup table in which a relationship between mean values of color level values of RGB and color level values of CMYK is prestored. 
         [0069]    The controller  7  performs halftoning on each of the K image data  400 K, the Y image data  400 Y, the C image data  400 C, and the M image data  400 M to generate dot data  40  correspondingly. Each dot data  40  corresponds to one of the ink colors of CMYK and represents the necessity or unnecessity of dot formation in each pixel. The dot data  40  may be image data consisting of a plurality of pieces of pixel data that are equal in number to the number of pixels corresponding to the resolution of the printer  1 . The dot data  40  includes dot data  40 K corresponding to black, dot data  40 Y corresponding to yellow, dot data  40 C corresponding to cyan, and dot data  40 M corresponding to magenta. Each pixel data of the dot data  40 K,  40 Y,  40 C, and  40 M may be binary data representing the necessity or unnecessity of ink ejection from a corresponding nozzle  21 . A known data conversion method, for example, an error diffusion method or dithering, is used for the data conversion executed in the halftoning. 
         [0070]      FIG. 6A  illustrates an example of black dot data  40 K.  FIG. 6A  illustrates a portion of the black dot data  40 K, and more specifically, illustrates 40 pieces of pixel data (in the right-left direction) by 5 lines (in the front-rear direction). In  FIG. 6A , a blank or white cell schematically represents pixel data indicating the unnecessity of ink ejection from its corresponding nozzle. A cell with a black dot schematically represents pixel data indicating the necessity of ink ejection from its corresponding nozzle. 
         [0071]    Subsequent to step S 203 , the controller  7  distributes the dot data  40 K, the dot data  40 Y, the dot data  40 C, and the dot data  40 M to the four head units  11 , respectively, corresponding to the respective colors. This dot data distribution will be described using an example in which dot data  40 K is distributed to the head units  11   a  and  11   b  of the black inkjet head  4 . Hereinafter, although an explanation will be made on the black inkjet head  4  only, the dot data distribution is also performed on each of the other inkjet heads  4  in the same or similar manner. 
         [0072]    As a first step, dot data  41 K is generated by duplicating pixel data of the 1st row to the 100th row of the dot data  40 K generated in step S 203 , from the left in the right-left direction. The pixel data included in each of the 1st to 100th rows of the dot data  41 K corresponds to one of the nozzles  21   a  of # 1  to # 100  of the head unit  11   a.    
         [0073]    Then, dot data  42 K is generated by duplicating pixel data of the 77th row to the 176th row of the dot data  40 K generated in step S 203 , from the left in the right-left direction. The pixel data included in the 77th row to the 176th row of the dot data  42 K corresponds to one of the nozzles  21   b  of # 1  to # 100  of the head unit  11   b.  Each of the dot data  41 K and  42 K is an example of intermediate image data. 
         [0074]    In step S 204 , dot data  43 K (not illustrated) and dot data  44 K (not illustrated) are also generated. More specifically, for example, the dot data  43 K is generated by duplicating pixel data of the 153th row to the 252th row of the dot data  40 K from the left in the right-left direction. The dot data  44 K is generated by duplicating pixel data of the 229th row to the 328th row of the dot data  40 K from the left in the right-left direction. 
         [0075]      FIG. 6B  schematically illustrates the dot data  41 K for the head unit  11   a  and the dot data  42 K for the head unit  11   b.  Similarly to the dot data  40 K, a cell with a black dot schematically represents pixel data indicating the necessity of ink ejection from its corresponding nozzle. A blank or white cell schematically represents pixel data indicating the unnecessity of ink ejection from its corresponding nozzle. 
         [0076]    As illustrated in  FIG. 6A , in the dot data  40 K which has not been distributed to the head units  11   a  and  11   b,  partial data consisting of a plurality of pieces of pixel data included in a particular region corresponding to both the nozzles  21   a  of the nozzle group  25   a  and the nozzles  21   b  of the nozzle group  25   b,  is referred to as dot data  40 A. In the dot data  41 K which has been distributed to the head unit  11   a,  partial data consisting of a plurality of pieces of pixel data included in a particular region corresponding to the nozzles  21   a  of the nozzle group  25   a  is referred to as dot data  41 A. In the dot data  42 K which has been distributed to the head unit  11   b,  partial data consisting of a plurality of pieces of pixel data included in a particular region corresponding to the nozzles  21   b  of the nozzle group  26   b  is referred to as dot data  42 A. In the undistributed dot data  40 K, partial data consisting of a plurality of pieces of pixel data included in another particular region corresponding to both of the optimum nozzles  71   a  nd the optimum nozzles  72   b  is referred to as dot data  40 X. In the distributed dot data  41 K, partial data consisting of a plurality of pieces of pixel data included in another particular region corresponding to the optimum nozzles  71   a  is referred to as dot data  41 X. In the distributed dot data  42 K, partial data consisting of a plurality of pieces of pixel data included in another particular region corresponding the optimum nozzles  72   b  is referred to as dot data  42 X. 
         [0077]    At the distribution, the nozzles  21   a  of the nozzle group  25   a  are assigned with the dot data  41 A, and the nozzles  21   b  of the nozzle group  25   b  are assigned with the dot data  42 A. Nevertheless, the dot data  41 A and  42 A of the distributed dot data  41 K and  42 K, respectively, are identical to the dot data  40 A of the undistributed dot data  40 K. 
         [0078]    Subsequent to step S 204 , the controller  7  selects mask data for applying masking to each of the dot data  41 K,  42 K,  43 K, and  44 K (e.g., step S 205 ). 
         [0079]    As described above, the nonvolatile memory  19  of the controller  7  stores 12 pieces of the optimum position information. The nonvolatile memory  19  stores six varieties of mask data each corresponding to the nozzles  21  of # 51  to # 100  that include the nozzles  21  constituting the nozzle group  25  and another six varieties of mask data each corresponding to the nozzles  21  of # 1  to # 50  that include the nozzles  21  constituting the nozzle group  26 , in association with each of the six different optimum positions. That is, a total of 12 varieties of mask data is prepared. Referring to the optimum position information corresponding to each head unit  11  stored in the nonvolatile memory  19 , the controller  7  reads each appropriate mask data for applying masking to a corresponding one of the dot data  41 K,  42 K,  43 K, and  44 K. The controller  7  combines the mask data corresponding to the nozzles  21  of # 1  to # 50  and the mask data corresponding to the nozzles  21  of # 51  to # 100  with each other, and stores the combined mask data for each head unit  11  in the RAM  17 . 
         [0080]      FIG. 6C  illustrates mask data  51  corresponding to the head unit  11   a  and mask data  52  corresponding to the head unit  11   b  of the black inkjet head  4 . The mask data  51  and  52  include regions  51 X and  52 X, respectively, both corresponding to the position of the optimum nozzle set  70   b.  Each of the regions  51 X and  52 X includes both a data piece A and a data piece B. The data piece A represents the allowance of ink ejection. The data piece B represents the disallowance of ink ejection. In  FIG. 6C , the data piece A is represented by a cell with a black dot, and the data piece B is represented by a blank or white cell. The controller  7  also reads mask data  53  (not illustrated) corresponding to the head unit  11   c  and mask data  54  (not illustrated) corresponding to the head unit  11   d.    
         [0081]    The nozzle usage rate r refers to a percentage of data pieces A included in a single data-piece row corresponding to a certain nozzle  21 . For example, the single data-piece row includes five data pieces arrayed in the front-rear direction. In  FIG. 6C , for example, a data-piece row corresponding to the nozzle  21  of # 9  consists of one data piece A and four data pieces B. Therefore, a percentage of the data pieces A to the data-piece row is 0.2. 
         [0082]    The percentage of the data pieces A included in the region  51 X is referred to as an average usage rate R 1  of the optimum nozzles  71   a,  and the percentage of the data pieces A included in the region  52 X is referred to as an average usage rate R 2  of the optimum nozzles  72   b.  In the example of  FIG. 6C , the region  51 X has a total of 20 data pieces including 10 data pieces A. Therefore, the average usage ratio R 1  is 0.5. The average usage ratio R 2  is also 0.5. 
         [0083]    The six varieties of mask data include respective different regions in which the usage rate r is less than 1 (one). That is, in each mask data  51  and  52  according to the illustrative embodiment, the usage rate r is less than 1 (one) in the region corresponding to the third nozzle set  27   b  from the left. Nevertheless, in a case another nozzle set  27   b  corresponds to the optimum nozzle set  70   b,  other mask data is read. 
         [0084]    Subsequent to step S 205 , the controller  7  executes masking on each dot data  41 K,  42 K,  43 K, and  44 K to generate ejection data  61 K corresponding to the head unit  11   a,  ejection data  62 K corresponding to the head unit  11   b,  ejection data  63 K corresponding to the head unit  11   c,  and ejection data  64 K corresponding to the head unit  11   d  (e.g., step S 206 ). 
         [0085]      FIG. 6D  illustrates the ejection data  61 K corresponding to the head unit  11   a  and the ejection data  62 K corresponding to the head unit  11   b  of the black inkjet head  4 . Similar to the dot data  40 , each of the ejection data  61 K and  62 K may be binary data representing the necessity or unnecessity of ink ejection from a corresponding nozzle  21 . The controller  7  also generates the ejection data  63 K (not illustrated) corresponding to the head unit  11   c  and the ejection data  64 K (not illustrated) corresponding to the head unit  11   d.    
         [0086]    The number of dots in each ejection data  61 X and  62 X corresponding to the position of the optimum nozzle set  70 B is half of the number of dots in each unmasked dot data  41 X and  42 X. Each of the ejection data  61 K,  62 K,  63 K, and  64 K stored in the RAM  17  is transmitted to the black inkjet head  4 . The same or similar processes are also performed on the data for each of the other inkjet heads  4 . Then, the controller  7  executes printing by controlling the four inkjet heads  4  to eject ink therefrom (e.g., step S 207 ). 
         [0087]    &lt;Boundary Region Determination in Each Nozzle Group&gt; 
         [0088]    Hereinafter, referring to  FIGS. 7 to 9 , an example procedure for determining a boundary region in each nozzle group  26  will be described in detail. The nozzles  21  to be used for printing are changed with respect to the determined boundary region. The boundary region determination is performed prior to shipping of the printer  1 . 
         [0089]      FIG. 7  is a block diagram of the printer  1  and a testing system  31 . The testing system  31  is used for determining a boundary region in each nozzle group  26   b.  The testing system  31  includes a PC  32  and a scanner  33  connected to the PC  32 . The printer  1  and the testing system  31  is connected to each other via a cable  34  and are capable of communicating with each other. 
         [0090]      FIG. 8  is a flowchart of an example boundary region determining process. As illustrated in  FIG. 8 , the testing system  31  causes the printer  1  to print test patterns on a recording sheet  100  (e.g., step S 101 ). The test patterns are used for determining an optimum nozzle set  70  among the six nozzle sets  27  in each nozzle group  26 . 
         [0091]      FIGS. 9A and 9B  are explanatory diagrams for explaining an example test pattern printing process. The PC  32  of the testing system  31  inputs a print instruction to the printer  1  to cause the printer  1  to print six test patterns P on a recording sheet  100 . The six test patterns P includes test pattaerns P 1 , P 2 , P 3 , P 4 , P 5  and P 6 .  FIG. 9B  shows only three test pattaerns P 1 , P 2  and P 3 . In the illustrative embodiment, the printer  1  includes four inkjet heads  4 . Therefore, as illustrated in  FIG. 9A and 9B , a boundary region is determined in each nozzle group  26  of each inkjet head  4 . 
         [0092]    Hereinafter, an explanation will be made on the leftmost head unit  11   a  and the head unit  11   b  that is the right closest to the head unit  11   a  in the right-left direction. In printing of each of the six test patterns P, a different nozzle set  27   b  in the nozzle group  26   b  functions as a boundary region at which the nozzles  21  used for printing are changed from the nozzles  21   a  to the nozzles  21   b.  For example, during printing of a test pattern P 1 , the nozzles  21  used for printing is changed from the nozzles  21   a  to the nozzles  21   b  with respect to the leftmost nozzle set  27   b  among the six nozzle sets  27   b.  Under the circumstances where the boundary region is tentatively determined as such, the above-described ejection control is executed on the nozzle group  26   b.    
         [0093]    An area in the right-left direction occupied by an image formed on a recording sheet  100  by ink ejected from both of the nozzles  21   a  of the nozzle group  25   a  of the head unit  11   a  and the nozzles  21   b  of the nozzle group  26   b  of the head unit  11   b  while the test pattern P 1  is printed, is referred to as a range  200   b.    
         [0094]    Similarly, another area in the right-left direction occupied by another image formed on the recording sheet  100  by ink ejected from the nozzles  21  of both the head unit  11   b  and the head unit  11   c  while the test pattern P 1  is printed, is referred to as a range  200   c.  Still another area in the right-left direction occupied by still another image formed on the recording sheet  100  by ink ejected from the nozzles  21  of both the head unit  11   c  and the head unit  11   d  while the test pattern P 1  is printed, is referred to as a range  200   d.  Hereinafter, an explanation will be made on the leftmost range  200   b.  The ranges  200   b,    200   c,  and  200   d  are also collectively referred to as ranges  200  as needed. 
         [0095]    In a case where the nozzle set  27   b  that is tentatively determined as the boundary region corresponds to the optimum nozzle set  70   b  in the nozzle group  26   b,  less density unevenness may occur in the image formed by ink ejected from both of the nozzles  21   b  of the nozzle group  26   b  and the nozzles  21   a  corresponding to the nozzles  21   b.  On the other hand, in a case where the nozzle set  27   b  that is tentatively determined as the boundary region does not correspond to the optimum nozzle set  70   b,  the landing positions of ink droplets ejected from the nozzles  21   a  and  21   b  may be deviated relative to each other at the position where the nozzles  21  used for printing are changed from the nozzles  21   a  to the nozzles  21   b.  Therefore, as illustrated in  FIG. 9B , density unevenness  50   b  may occur within the range  200   b  on the recording sheet  100 . 
         [0096]    That is, occurrence or state of density unevenness  50   b  within the range  200   b  of the recording sheet  100  is acquired from each of the six test patterns P. Based on this acquisition, the test pattern P in which the nozzle change has been performed with respect to the optimum nozzle set  70   b  can be recognized. The same or similar determination is also made on the ranges  200   c  and  200   d.    
         [0097]    A single inkjet head  4  includes the nozzle group  26   b,  the nozzle group  26   c  and the nozzle group  26   d.  Assembly precision of two adjacent head units  11  affects the position of the optimum nozzle set  70 . Therefore, between the nozzle group  26   b,  the nozzle group  26   c  and the nozzle group  26 , different nozzle sets  27  may correspond to the optimum nozzle set  70 . Accordingly, even when a particular test pattern P indicates the optimum nozzle set  70   b  for the nozzle group  26   b,  the same test pattern P might not always indicate the optimum nozzle sets  70   b  for the other nozzle groups  26   c  and  26   d.  That is, different test patterns P may indicate the optimum nozzle sets  70  for the respective nozzle groups  26 . 
         [0098]    Subsequent to step S 101 , the testing system  31  reads all the six test patterns P using the scanner  33  to acquire density data of an image corresponding to each of the ranges  200  of the recording sheet  100  in each test pattern P (e.g., step S 102 ). The density data is acquired as a luminance value. A higher density portion in a test pattern P has lower luminance. 
         [0099]    Subsequent to step S 102 , the testing system  31  selects, based on the acquired density data, each test pattern P having the smallest degree of density unevenness  50 , as an optimum pattern, for a corresponding one of the ranges  200  of the recording sheet  100  (e.g., step S 103 ). More specifically, the density data acquired using the scanner  33  is transmitted to the PC  32 , and the PC  32  selects the optimum pattern by referring to the density data. For example, as illustrated in  FIG. 9B , the test pattern P 3  has the smallest degree of density unevenness  50   b  within the range  200   b.    
         [0100]    In the illustrative embodiment, the nozzles  21  used for printing are changed from the nozzles  21   a  to the nozzles  21   b  with respect to a nozzle set  27   b  consisting of four nozzles  21   b  arrayed with the pitch d 1 . Therefore, in a case where the nozzles  21  used for printing are changed with respect a nozzle set  27   b  corresponding to the optimum nozzle set  70   b,  ink droplets ejected from the nozzles  21   b  land on substantially the respective same positions as ink droplets ejected from the nozzles  21   a  within the range corresponding to the width of the nozzle set  27   b.  As opposed to this, in a case where the nozzles  21  used for printing are changed with respect to another nozzle set  27   b  not corresponding to the optimum nozzle set  70   b,  ink droplets ejected from the nozzles  21   b  land on respective different positions from ink droplets ejected from the nozzles  21   a  in the range corresponding to the width of the nozzle set  27   b.  That is, a portion in which density unevenness has occurred has a width equal to a width of a single nozzle set  27   b,  and therefore, the density unevenness may be recognized easily. Thus, the test pattern P in which the image has been formed by the nozzles  21   a  and the nozzles  21   b  that are aligned most precisely with each other may be found easily, and this may cause less misdetermination of such a test pattern P. Even if misdirection of ink ejection occurs in one or more of the nozzles  21   a  or one or more of the nozzles  21   b  included in the nozzle set  27   b  that is tentatively determined as the boundary region, the nozzle set  27   b  still has normal nozzles  21   a  and  21   b.  Therefore, the optimum nozzle set  70   b  may be determined based on an image formed using the normal nozzles  21   a  and  21   b.    
         [0101]    Subsequent to step S 103 , the testing system  31  determines the nozzle set  27   b  with respect to which the nozzles change has been performed in the optimum test pattern P selected in step S 103 , that is, positional information on the optimum nozzle set  70   b,  as a boundary region for the nozzle group  26   b  of the head unit  11   b  (e.g., S 104 ). More specifically, the positional information on the optimum nozzle set  70   b  is stored in the ROM  12  of the controller  7  or the nonvolatile memory  19 . 
         [0102]    As described above, the nozzle group  26   b  includes a plurality of locations at which the nozzles  21   a  are aligned with the nozzles  21   b,  respectively. Therefore, recognizability of the test pattern P may be increased. Consequently, this may facilitate selection of the test pattern P having the smallest degree of density unevenness, which enables to readily recognize the optimum nozzle set  70   b  with respect to which the nozzle change has been performed in the selected test pattern P. 
         [0103]    In the illustrative embodiment, as illustrated in  FIG. 9B , the six printed test patterns P are scanned by the scanner  33  and the optimum pattern is selected for each nozzle group  26  based on the acquired density data. Nevertheless, in other embodiments, for example, an operator may visually check the density unevenness  50  in each of the test patterns P to select the optimum pattern for each nozzle group  26 . 
         [0104]    Hereinafter, alternative embodiments in which various changes or modifications are applied to the illustrative embodiment will be described. An explanation will be given mainly for the elements different from the illustrative embodiment, and an explanation will be omitted for the common elements by assigning the same reference numerals thereto. 
         [0105]    (1) In the illustrative embodiment, the pitch d 2  between the nozzle sets  27   b  included in the nozzle group  26   b  is greater than the pitch d 1  between the nozzles  21   a  included in the nozzle group  25   a.  Nevertheless, in other embodiment, the pitch d 2  may be smaller than the pitch d 1 . 
         [0106]    (2) The lines representing the nozzle usage rates r 1  and r 2  of the optimum nozzles  71   a  and  72   b  might not necessarily change linearly. In other embodiments, for example, as illustrated in  FIG. 10A , the lines representing the nozzle usage rates r 1  and r 2  may change curvedly. In still other embodiments, for example, as illustrated in  FIG. 10B , the lines representing the nozzle usage rates r 1  and r 2  may change step by step. 
         [0107]    (3) In other embodiments, for example, ink may be ejected from both of the nozzles  21   a  and the nozzles  21   b  included in another nozzle set  27   b  not corresponding to the optimum nozzle set  70   b.  For example, as illustrated in  FIG. 11 , ink is ejected from both of the nozzles  21   b  included in a nozzle set  80   b  and their corresponding nozzles  21   a  and from both of the nozzles  21   b  included in a nozzle set  90   b  and their corresponding nozzles  21   a.  The nozzle set  80   b  is positioned to the left, adjacent to the optimum nozzle set  70   b,  and is referred to as the adjacent nozzle set  80   b.  The nozzle set  90   b  is positioned to the right, adjacent to the optimum nozzle set  70   b,  and is referred to as the adjacent nozzle set  90   b.  Hereinafter, the nozzles  21   b  of # 5  to # 8  constituting the adjacent nozzle set  80   b  are referred to as adjacent nozzles  82   b,  and the nozzles  21   a  of # 81  to # 84  corresponding to the adjacent nozzles  21   b  are also referred to as adjacent nozzles  81   a.  Hereinafter, the nozzles  21   b  of # 13  to # 16  constituting the adjacent nozzle set  90   b  are referred to as adjacent nozzles  92   b,  and the nozzles  21   a  of # 89  to # 92  corresponding to the adjacent nozzles  21   b  are also referred to as adjacent nozzles  91   a.    
         [0108]      FIG. 12  is a graph showing usage rates r 1  of the nozzles  21   a  and usage rates r 2  of the nozzles  21   b  in the alternative embodiment. A line representing the usage rates r 2  rises linearly from the nozzle  21   b  of # 5  toward the nozzle  21   b  of # 16 . Assuming that an average of the usage rates r 2  of four adjacent nozzles  82   b  is an average usage rate R 2   x  and an average of the usage rates r 2  of four adjacent nozzles  92   b  is an average usage rate R 2   y,  the average usage rates R 2   x  and R 2  satisfies R 2   x&lt; R 2 &lt;R 2   y.  Thus, this configuration may further surely reduce occurrence of density unevenness that may be caused by deviation of landing positions of ink droplets ejected from each nozzle  21   b  and its corresponding nozzle  21   a  relative to each other. 
         [0109]    In  FIG. 12 , the usage rates of the nozzles  21   b  positioned to the right of the optimum nozzle set  70   b  are higher than the usage rate r 2  of the rightmost optimum nozzle  72   b  of the optimum nozzles  72   b,  i.e., the usage rate r 2  of the nozzle  21   b  of # 12 . The usage rates of the nozzles  21   b  positioned to the left of the optimum nozzle set  70   b  are lower than the usage rate r 2  of the leftmost optimum nozzle  72   b  of the optimum nozzles  72   b,  i.e., the usage rate r 2  of the nozzle  21   b  of # 9 . 
         [0110]    In other words, the usage rates r 2  of the nozzles  21   b  positioned to the right of the optimum nozzle set  70   b  are higher than the average usage rate R 2 , and the usage rates r 2  of the nozzles  21   b  positioned to the left of the optimum nozzle set  70   b  are lower than the average usage rate R 2 . 
         [0111]    That is, in the nozzle group  26   b, between any two of the nozzles  21   b  arrayed along the right-left direction, the usage rate r 2  of the right nozzle  21   b  is not lower than the usage rate r 2  of the left nozzle  21   b.    
         [0112]    (4) As illustrated in  FIG. 13 , the rightmost nozzle set  27  in the nozzle group  26   b  may correspond to the optimum nozzle set  70   b.  In this case, no adjacent nozzle set  90   b  is present to the right of the optimum nuzzle set  70   b.  Therefore, ink is ejected from both of the nozzles  21   b  included in the optimum nozzle set  70   b  and their corresponding nozzles  21   a,  and from both of the nozzles  21   b  included in the adjacent nozzle set  80   b  and their corresponding nozzles  21   a.  The adjacent nozzle set  80   b  is positioned to the left of the optimum nozzle set  70   b.  (4) In a case where the leftmost nozzle set  27  in the nozzle group  26   b  corresponds to the optimum nuzzle set  70   b,  ink is ejected in a similar manner to the above case. 
         [0113]    (5) In a case where ink is ejected from both of the nozzles  21   a  of the head unit  11   a  and the nozzles  21   b  of the head unit  11   b,  an image formed on a recording sheet  100  may tend to have lower density due to influence of deviation of landing positions of ink droplets ejected from each nozzle  21   b  of the head unit  11   b  and its corresponding nozzle  21   a  of the head unit  11   a  relative to each other, as compared with a case where ink is ejected from the one or the other of the nozzles  21   a  of the head unit  11   a  and the nozzles  21   b  of the head unit  11   b  only. Therefore, an amount of ink to be ejected from the nozzles  21   b  of the nozzle set  27   b  and their corresponding nozzles  21   a  may be increased. 
         [0114]    Referring to  FIG. 11 , this will be described using an example case where ink droplets are ejected from both of the nozzles  21   b  in the optimum nozzle set  70   b  and their corresponding nozzles  21   a  and from both of the nozzles  21   b  in nozzle sets  80   b  and  90   b  adjacent to the optimum nozzle set  70   b  and their corresponding nozzles  21   a.  In this example case, as illustrated in  FIG. 14 , usage rates r 1  of the nozzles  21   a  and usage rates r 2  of the nozzles  21   b  change step by step. In  FIG. 14 , a dashed line indicates a sum of a usage rate r 1  of a nozzle  21   b  (e.g., one of the nozzles  21   b  of # 5  to # 16  included in the optimum nozzle set  70 , the adjacent nozzle set  80   b,  or the adjacent nozzle set  90   b ) and a usage rate r 2  of its corresponding nozzle  21   a  (e.g., its corresponding nozzle  216   a  of # 81  to # 92 , i.e., the dashed line indicates r 1 +r 2 . 
         [0115]    In this case, a sum of the average usage rate of the nozzles  21   b  and the average usage rate of the nozzles  21   a  may exceed one (1). More specifically, a sum of the average usage rate of the nozzles  21   b  included in the optimum nozzle set  70   b  and the average usage rate of their corresponding nozzles  21   a  may exceed one. A sum of the average usage rate of the nozzles  21   b  included in the adjacent nozzle set  80   b  and the average usage rate of their corresponding nozzles  21   a  may exceed one. A sum of the average usage rate of the nozzles  21   b  included in the adjacent nozzle set  90   b  and the average usage rate of their corresponding nozzles  21   a  may exceed one. That is, the number of ink droplets to be ejected may be greater than the number of ink droplets determined based on image data. This may be implemented using mask data in which a percentage of the data pieces A included is increased as compared with the mask data  51  and  52  of  FIG. 6C . Thus, this configuration may reduce occurrence of insufficient density in an image formed at an area onto which ink droplets are ejected from both of the nozzles  21   a  and the nozzles  21   b.    
         [0116]    (6) The degree of misalignment or positional difference between the nozzles  21   b  included in the optimum nozzle set  70   b  and their corresponding nozzles  21   a  is smaller than the degree of misalignment or positional difference between the nozzles  21   b  included in the adjacent nozzle sets  80   b  and their corresponding nozzles  21   a  and between the nozzles  21   b  included in the adjacent nozzle sets  90   b.  Therefore, as illustrated in  FIG. 15 , the sum of the average usage rate R 1  and the average usage rate R 2  in the optimum nozzle set  70   b  may be smaller than the sum of the average usage rate R 1  and the average usage rate R 2  in each of the adjacent nozzle sets  80   b  and  90   b.  That is, the number of ink droplets to be ejected from the nozzles  21   b  of the optimum nozzle set  70   b  and their corresponding nozzles  21   a  in the optimum nozzle set  70   b  may be smaller than the number of ink droplets determined based on image data, as compared with the adjacent nozzle sets  80   b  and  90   b.  This may be implemented using mask data in which a percentage of the data pieces A included is reduced as compared with the mask data  51  and  52  of  FIG. 6C . 
         [0117]    (7) In the illustrative embodiment, as illustrated in  FIG. 5 , subsequent to color conversion (e.g., step S 202 ), halftoning (e.g., step S 203 ) is executed. Thereafter, dot data distribution (e.g., step S 204 ) and masking (e.g., step S 206 ) are executed independently. Nevertheless, in other embodiments, for example, masking and halftoning may be executed simultaneously on the density data acquired in color conversion. 
         [0118]    (8) In the illustrative embodiment, the controller  7  acquires, as the original data, the image data  300  from the external device  9 . Nevertheless, in other embodiments, for example, in response to a user&#39;s operation for instructing printing of an image, the external device  9  may generate data described in page description language and transmit the generated data to the printer  1 . In this case, the controller  7  of the printer  1  may generate image data  300  represented by RGB values based on the data described in page description language. Subsequent to this, the controller  7  may perform steps S 202  to S 207 . In this case, the data represented by page description language or the image data generated based on page description language may correspond to the original image data. In one example, in a case where the printer  1  includes an interface for reading data from an external memory, e.g., a memory card or a USB memory, or an interface for enabling the printer  1  to communicate with a network, e.g., a local area network, the printer  1  may be configured as described below. The printer  1  may acquire an electronic file directly from the external memory or via the network to which the printer  1  is connected, and the printer  1  may generate the image data  300  corresponding to the resolution of the printer  1 , based on the acquired electronic file. In this case, the electronic file or the image data  300  is another example of the original image data. 
         [0119]    (9) In other embodiments, for example, the nozzle group  28  may include more than fifty-two nozzles  21 . 
         [0120]    (10) In other embodiments, for example, the nozzles  21  may be arrayed in two or more rows. 
         [0121]    (11) In other embodiments, for example, the pixel data of the dot data  40  may be represented by multiple color levels.