JP2010162772A - Printing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a serial type inkjet printer of a bidirectional printing system capable of suppressing the degradation of printing image quality due to a difference in dot forming timings. <P>SOLUTION: A printer 20 carries out printing by performing a halftone process by using a systematic dither method using a dither mask 62. The size of the dither mask 62 is a positive integral multiple of a minimal repetition unit RU of a nozzle pattern indicating a nozzle that is used to form a dot in each position on the printing medium, and correspondence relation between each dot forming position in the minimal repetition unit RU of the nozzle pattern and threshold of the dither mask 62 that is applied to determination of ON or OFF of a dot located in the position is set to be constant. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a printing technique for performing printing while moving a print head relative to a print medium in a main scanning direction and a sub-scanning direction.

  In recent years, serial inkjet printers that perform printing by ejecting ink while moving a print head relative to a print medium in a main scanning direction and a sub-scanning direction have become widespread. In a serial inkjet printer, in order to increase the printing speed, it is common to perform bidirectional printing in which ink is ejected in both forward and backward movements of the reciprocating movement of the print head (for example, Patent Document 1) below.

JP 2000-52543 A

  In such a serial ink jet printer, ink blurring occurs due to the difference in dot formation timing at the joint in the sub-scanning direction for the dots formed in the Nth main scan and the dots formed in the N + 1th main scan. The ease changes and density unevenness occurs. Also, in a bidirectional inkjet type serial ink jet printer, when the print head is reciprocated in the main scanning direction from the left end of the print medium, the right end of the print medium is restored immediately after the dot is formed by the forward movement of the print head. A dot is formed by the movement. On the other hand, at the left end of the print medium, after a dot is formed immediately after the start of the forward movement, a dot is formed by a backward movement after the reciprocation time of the print head has elapsed. Such a difference in dot formation timing also changes each time the print head moves in the sub-scanning direction, and thus causes density unevenness along the sub-scanning direction in units of feed amounts in the sub-scanning direction. (These phenomena are also described in detail in the examples).

  In view of the above-described problems, the problem to be solved by the present invention is to suppress a decrease in print image quality caused by the mechanism in a serial ink jet printer.

  The present invention has been made to solve the above-described problems, and can be realized as the following forms or application examples.

Application Example 1 A printing apparatus that performs printing while moving a print head relative to a print medium in a main scanning direction and a sub-scanning direction,
A nozzle array provided in the print head and arranged with a plurality of nozzles for discharging ink in the sub-scanning direction;
A halftone processing unit for performing halftone processing by comparing each threshold value of a dither mask composed of a plurality of threshold values with image data constituting the image;
A printing unit that performs printing by controlling the ejection of ink from each nozzle of the nozzle row using the result of the halftone processing;
Each position in a minimum unit of repetition of a nozzle pattern indicating which of the plurality of nozzles forms a dot at each position on the print medium, and each of the dither masks applied to each position A printer that has a fixed correspondence with the threshold value.

  In the printing apparatus having such a configuration, the correspondence between each position in the minimum unit of repetition of the nozzle pattern and each threshold value of the dither mask applied to each position is fixed. The ease with which dots are formed can be controlled. As a result, it is possible to control the dot formation timing, and it is possible to suppress a decrease in print image quality caused by the dot formation timing.

Application Example 2 The printing apparatus according to Application Example 1, wherein the dither mask has a size in the main scanning direction and the sub-scanning direction that is a positive integer multiple of the minimum unit of repetition of the nozzle pattern.

  In the printing apparatus having such a configuration, the correspondence between each position in the minimum repeating unit and each threshold value of the dither mask applied to each position can be easily fixed by simply adjusting the dither mask size. It can be. Moreover, a special apparatus structure is unnecessary and versatility is high.

[Application Example 3] In the printing apparatus according to Application Example 1, the feed amount that the print head relatively moves in the sub-scanning direction by one sub-scanning has a predetermined periodicity, and the size in the main scanning direction is The number of dots equal to the product of the number of nozzle pitches, which is the number of dots between the centers of nozzles adjacent in the sub-scanning direction among a plurality of nozzles, and the number of main scans for completing one raster, and the sub-scanning direction In the local region that is the continuous dot formation position on the print medium with the number of dots equal to the number of nozzle pitches, the filling order that is the dot formation order at each dot formation position is one sub-scan. The dither mask has a size in the main scanning direction that is a positive integer multiple of the minimum unit of the repetition of the nozzle pattern, and a size in the sub-scanning direction. Simplest of The feed amount of the periodic and filling the order of one-size positive integer fraction is determined based on the period printing apparatus.

  In the printing apparatus having such a configuration, the feed amount in the sub-scanning direction has a predetermined periodicity, and the filling order changes based on regularity having a predetermined period. Periodicity appears in the pattern. Therefore, even when the size of the dither mask in the sub-scanning direction is set to a size of a positive integer determined based on the cycle of the feed amount and the cycle of the filling order, the regularity is used to repeat the nozzle pattern. The correspondence between each position in the minimum unit and each threshold value of the dither mask applied to each position can be fixed, and the capacity of the dither mask can be reduced. As a result, the storage capacity of the printing apparatus can be reduced or effectively utilized.

Application Example 4 In the printing apparatus according to Application Example 3, the regularity of the filling order is a combination of the relative positional relationships between the dot formation positions of the dots formed continuously in the local region. A printing apparatus in which the relative position pattern shown has periodicity according to a change in the filling order.

  In the printing apparatus having such a configuration, since the filling order has a periodicity of the relative position pattern, the minimum unit of repetition of the nozzle pattern in the main scanning direction is a predetermined amount for each nozzle pattern. A regularity of parallel movement appears. Therefore, even if the size of the dither mask in the sub-scanning direction is set to a size of a positive integer determined based on the period of the feed amount and the period of the filling order, the dither mask is translated to each position of the nozzle pattern. In this way, the correspondence between the positions in the minimum unit of the repetition of the nozzle pattern and the threshold value of each dither mask applied to each position is fixedly determined by spreading and applying in the sub-scanning direction. be able to.

Application Example 5 The printing apparatus according to Application Example 3, wherein the regularity of the filling order is that the changing filling order periodically shows mirror image symmetry.
In the printing apparatus having such a configuration, since the filling order periodically exhibits mirror image symmetry, the nozzle pattern exhibits mirror image symmetry for each period of the feed amount in the sub-scanning direction. Therefore, even if the size of the dither mask in the sub-scanning direction is a size of a positive integer determined based on the period of the feed amount and the period of the filling order, the dither mask is mirror-inverted at each position of the nozzle pattern. By spreading in the sub-scanning direction, the correspondence between each position in the minimum unit of repetition of the nozzle pattern and each threshold value of the dither mask applied to each position can be set constant.

Application Example 6 The printing apparatus according to any one of Application Example 3 to Application Example 5 in which the periodicity of the feed amount is that the feed amount is constant.
In the printing apparatus having such a configuration, since the feed amount is constant, that is, the cycle of the feed amount is minimized, the capacity of the dither mask in the sub-scanning direction can be efficiently reduced.

[Application Example 7] The printing apparatus according to any one of Application Example 3 to Application Example 5, wherein the cycle of the feed amount is the number of dots corresponding to about a few of the minimum unit size of the nozzle pattern repetition in the sub-scanning direction.
In a printing apparatus having such a configuration, if a plurality of dither masks are laid out in the sub-scanning direction, the size in the sub-scanning direction matches the minimum unit of repetition of the nozzle pattern. And the least common multiple of the feed rate cycle, the dither mask while maintaining the constant correspondence between each position in the minimum unit of repetition of the nozzle pattern and the threshold value of each dither mask applied to each position. The size of the mask can be reduced.

The present invention can also be realized as the dither mask of application example 8 and the printing method of application example 9.
Application Example 8 A dither mask that includes a plurality of threshold values and is used for halftone processing for performing printing while moving the print head relative to the print medium in the main scanning direction and the sub-scanning direction.
The dot pattern at each position on the print medium is arranged in the sub-scanning direction on the print head, and is a repeated nozzle pattern that indicates which of a plurality of nozzles that eject ink is formed. A dither mask in which a correspondence relationship between each position in a minimum unit and each threshold value of the dither mask applied to each position is fixed.

[Application Example 9]
While moving the print head relative to the print medium in the main scanning direction and the sub-scanning direction, a plurality of nozzles that are provided in the print head and that eject ink are arranged from each nozzle in the nozzle row arranged in the sub-scanning direction. A printing method in which printing is performed using a printing apparatus that performs printing by controlling ink ejection, and is a dither mask having a plurality of threshold values. Half using a dither mask in which the correspondence between each position in the minimum unit of repetition of the nozzle pattern indicating which nozzle is formed and the threshold value of each dither mask applied to each position is fixed. A printing method in which printing is performed by performing tone processing and controlling the ease of dot formation in units of nozzles in a nozzle row.

A. First embodiment:
A first embodiment of the present invention will be described.
A-1. Device configuration:
FIG. 1 is a schematic configuration diagram of a printer 20 as an embodiment of the present invention. The printer 20 is a serial ink jet printer. As illustrated, the printer 20 reciprocates the carriage 80 in the axial direction of the platen 75 by a mechanism that transports the print medium P by a paper feed motor 74 and the carriage motor 70. The mechanism, the mechanism for driving the print head 90 mounted on the carriage 80 to discharge ink and forming dots, and the exchange of signals with the paper feed motor 74, carriage motor 70, print head 90, and operation panel 99 And a control unit 30 that manages the above.

  A mechanism for reciprocating the carriage 80 in the axial direction of the platen 75 is installed in parallel with the axis of the platen 75 and is driven endlessly between the slide shaft 73 slidably holding the carriage 80 and the carriage motor 70. A pulley 72 and the like for stretching the belt 71 are included.

  On the carriage 80, ink cartridges 82 to 87 for color ink respectively containing cyan ink C, magenta ink M, yellow ink Y, black ink K, light cyan ink Lc, and light magenta ink Lm are mounted as color inks. . In the print head 90 below the carriage 80, nozzle rows corresponding to the above-described color inks are formed. When these ink cartridges 82 to 87 are mounted on the carriage 80 from above, ink can be supplied from each cartridge to the print head 90.

  The control unit 30 includes a CPU 40, a ROM 51, a RAM 52, and an EEPROM 60 that are connected to each other via a bus. The control unit 30 develops a program stored in the ROM 51 or the EEPROM 60 in the RAM 52 and executes it, thereby controlling the overall operation of the printer 20 and also functions as the input unit 41, the halftone processing unit 42, and the printing unit 43. To do. Details of this functional unit will be described later.

  A dither mask 62 is stored in the EEPROM 60. The dither mask 62 is used for halftone processing by a systematic dither method, and has a so-called blue noise characteristic in this embodiment.

  A memory card slot 98 is connected to the control unit 30, and image data ORG can be read and input from the memory card MC inserted into the memory card slot 98. In this embodiment, the image data ORG input from the memory card MC is data composed of three color components of red (R), green (G), and blue (B).

  The printer 20 having the above hardware configuration drives the carriage motor 70 to reciprocate the print head 90 with respect to the print medium P in the main scanning direction, and drives the paper feed motor 74. Thus, the print medium P is moved in the sub-scanning direction. The control unit 30 performs printing by driving the nozzles at an appropriate timing based on the print data in accordance with the movement of the carriage 80 in the reciprocating motion (main scanning) and the paper feeding movement of the printing medium (sub scanning). Ink dots of appropriate colors are formed at appropriate positions on the medium P. Thus, the printer 20 can print the color image input from the memory card MC on the print medium P.

  Details of the print head 90 described above are shown in FIG. This figure schematically shows the bottom surface of the print head 90 (the surface facing the print medium P). As illustrated, the print head 90 includes nozzle rows 92 to 97 in which a plurality of nozzles are formed side by side in the sub-scanning direction. In this embodiment, each nozzle row is formed of 30 nozzles arranged at a nozzle pitch K. It is formed from 30 nozzles. These nozzle arrays 92 to 97 correspond to the ink colors of the cartridge mounted on the carriage 80, and are respectively cyan ink C, magenta ink M, yellow ink Y, black ink K, light cyan ink Lc, and light magenta ink Lm. Is discharged. In this embodiment, the nozzle row corresponding to each ink color is configured by arranging the nozzles in a single row, but the arrangement of the nozzles in one nozzle row is not particularly limited. The nozzles may be arranged in a plurality of rows with respect to the ink color, and the nozzles in the plurality of rows may be configured in a staggered pattern.

A-2. Printing process:
A printing process in the printer 20 will be described. FIG. 3 is a flowchart of the printing process in this embodiment. The printing process here is started when the user performs an instruction to print a predetermined image stored in the memory card MC using the operation panel 99 or the like. When the printing process is started, the CPU 40 first reads and inputs RGB format image data ORG to be printed from the memory card MC via the memory card slot 98 (step S110).

  When the image data ORG is input, the CPU 40 refers to a look-up table (not shown) stored in the EEPROM 60 and performs color conversion from RGB format to CMYKLcLm format for the image data ORG (step S120).

  When the color conversion process is performed, the CPU 40 performs a halftone process for converting the image data into ON / OFF data of each color dot using the dither mask 62 by the systematic dither method as the process of the halftone processing unit 41. This is performed (step S130). The systematic dither method is a well-known technique and will not be described in detail. In short, however, the recording rate corresponding to the gradation data of the target pixel is compared with the threshold value in the dither mask corresponding to the position of the target pixel. If the recording rate is higher, a dot is formed on the pixel, and if the recording rate is lower, it is determined that no dot is formed. Details of the dither mask 62 used here will be described later. The halftone process is not limited to the binarization process of dot ON / OFF, but may be a multi-value process such as ON / OFF of large dots and small dots.

  When the halftone processing is performed, the CPU 40 performs interlace processing for rearranging the dot pattern data to be printed in one main scanning unit in accordance with the nozzle arrangement of the printer 20 and the paper feed amount (step S140). When the interlace process is performed, the CPU 40 drives the print head 90, the carriage motor 70, the motor 74, and the like as the process of the printing unit 42, and executes printing (step S150).

A-3. Nozzle pattern characteristics:
In the printer 20 of the present embodiment, periodicity appears in the nozzle pattern indicating which of the plurality of nozzles included in the print head 90 forms dots at each position on the print medium P. Hereinafter, such periodicity will be described.

  In the present embodiment, in the printing process described above, the number of overlaps is “2”, the nozzle pitch is “2”, the paper feed amount is “15”, and the print head 90 is driven. Bidirectional printing that ejects ink both during forward movement and during backward movement is performed. The number of overlaps refers to the number of main scans necessary to fill all the rasters formed in the main scan direction (horizontal direction) with dots. That is, when the overlap number is “2”, one raster in the main scanning direction is completed in two main scans. The nozzle pitch is the number of dots between the centers of nozzles adjacent in the sub-scanning direction, and is the number of rasters (dots) existing between two nozzles adjacent in the sub-scanning direction plus one. That means. In this embodiment, since the nozzle pitch is set to “2”, dots are formed every other raster in one main scan of the print head 90. The paper feed amount is an amount (raster number) by which the print head 90 is conveyed in the sub-scanning direction for each main scanning. In this embodiment, since the paper feed amount is “15”, that is, an odd number of paper feed amounts, a new dot is formed in the next main scan in the gap between dots formed in advance every other raster. It will follow.

  4 and 5 are diagrams showing how dots are formed by the printer 20. 4 and 5 are continuous views, but are divided into two parts in FIGS. 4 and 5 for the sake of space. Here, the manner in which dots are formed by the nozzle row 92 will be representatively described, but the same applies to other nozzle rows. 4A and 5A show how the nozzle row 92 moves in the sub-scanning direction each time main scanning is performed. Each nozzle is indicated by a number from 0 to 29 for convenience of explanation. In the present application, nozzles arranged at both ends of the nozzle row, that is, nozzles with nozzle numbers 0 and 29 are referred to as the most advanced nozzles. As shown in the figure, in this embodiment, the paper feed amount is set to “15”, so the print head 90 moves in the sub-scanning direction by 15 rasters for each main scanning. Further, the illustrated nozzle position (in the main scanning direction) corresponds to the position of the main scanning number (see the table at the top of FIG. 4A) indicating the relative number of main scanning. For example, the position of the nozzle row illustrated on the leftmost side corresponds to the main scanning number “−3”. The main scanning number is expressed as a relative number with the fourth main scanning shown in the drawing as a reference (0th main scanning).

  FIG. 4B and FIG. 5B indicate the number of main scans for each dot formed on the print medium by the main scan number. Each grid shown in FIGS. 4B and 5B represents the odd-numbered and even-numbered dots in each raster, and the numerical values in the grid are shown in FIGS. 4A and 5A. This corresponds to the main scanning number shown in the upper part. That is, according to FIGS. 4B and 5B, in the uppermost raster, the odd-numbered dots are formed by the 0th main scan, and the even-numbered dots are formed by the -2nd main scan. You can see that

  As shown in FIGS. 4B and 5B, in this embodiment, when attention is paid to a 2 × 2 local area (hereinafter referred to as a local area), the main scanning number is the uppermost two. In the raster, “0”, “−1”, “−2”, and “−3” are in the order of upper left, lower left, upper right, and lower right. That is, in this local region, each dot is filled in the order of lower right, upper right, lower left, and upper left. This order is called “filling order”. Regarding the size of the local area, the horizontal direction (main scanning direction) matches the number of overlaps (“2” in this embodiment), and the vertical direction (sub-scanning direction) corresponds to the nozzle pitch (“2” in this embodiment). Match. In FIG. 4B and FIG. 5B, each lattice is displayed by being separated by a solid line for each local region. The filling order has a property of changing every time the print head 90 is moved in the sub-scanning direction (that is, every time main scanning is performed). In this embodiment, when the filling order changes four times. Return to the original filling order. The number of repeating units in the filling order is the product of the nozzle pitch and the number of overlaps. In FIGS. 4B and 5B, the main scanning number at the position where the filling order changes is displayed in reverse video. Such setting of the filling order is performed in step S140 of the printing process described above. In the present application, the width in the sub-scanning direction for each feed amount of the print head 90 is also referred to as a band, and the dot formation position along the sub-scanning direction of the print medium P is also indicated by a band number. For example, as shown in FIGS. 4A and 5A, the upper end portion of the dot formation position is the band 0, and the band numbers increase in the downward direction, such as band 1 and band 2.

  FIGS. 4C and 5C show nozzle patterns indicating which nozzles form dots at respective positions on the print medium. Numerical values in each grid correspond to the nozzle numbers shown in FIGS. 4 (a) and 5 (a). For convenience of explanation, a dot row number is displayed at the upper end of the figure, and a dot row number is displayed at the left end. 4 (c), 5 (c), FIG. 4 (b), and FIG. 5 (b), the odd-numbered dots in the uppermost raster in the figure are the 0th time. It can be understood that the even-numbered dots are formed by the 15th nozzle in the -2nd main scan, and are formed by the 0th nozzle in the main scan. For the second raster, odd-numbered dots are formed by the eighth nozzle in the -1st main scan, and even-numbered dots are formed by the 23rd nozzle in the -3rd main scan. I can understand that.

  In such a nozzle pattern, on the same raster (in the main scanning direction), the dot formation positions of the odd and even rows correspond to the same nozzle number. In the sub-scanning direction, the nozzle numbers are repeated with the first to 60th rows as one unit. In other words, the nozzle pattern of the printer 20 is configured by repeating a repetitive minimum unit RU composed of the first column to the second column and the first row to the 60th row in the scanning direction and the sub-scanning direction. Such periodicity of the nozzle pattern is related to the repetition of the above-described filling order. In the main scanning direction, the repetition occurs in units of the overlap number, and in the sub-scanning direction, the paper feed amount and the above-described filling order are repeated. Repeats occur with the product of the number of repeat units as the unit.

A-4. Problems to be solved in the first embodiment:
In the above-described nozzle pattern, at the dot forming position where the filling order is switched, that is, at the dot forming position where the dot is formed by the most advanced nozzle, the main scanning number is switched simultaneously in even-numbered rows or odd-numbered rows in the sub-scanning direction. Become. For example, in the even-numbered columns of the 10th, 12th, and 14th rows, dots are formed in the third main scan by the nozzle numbers 27, 28, and 29, respectively, but the even-numbered rows of the 16th, 18th, and 20th rows In the columns, dots are formed in the first main scan by nozzle numbers 0, 1, and 2, respectively. That is, in the 14th row and the 16th row, the main scanning numbers are switched simultaneously in all even-numbered columns. In this application, the position at which the main scanning number for forming the dot in the sub-scanning direction is switched in the sub-scanning direction is also referred to as the most advanced dot forming position. The most advanced dot formation position is the dot formation position indicated by hatching in FIGS. 4C and 5C.

  In this way, if the main scanning numbers are switched simultaneously in all even-numbered rows or odd-numbered rows, there is a high possibility that the dot landing position will shift due to the difference in the main scanning numbers, and the difference in the ink formation timing due to the difference in dot formation timing. Since the ease of bleeding changes, the density unevenness between the most advanced dot formation positions is conspicuous, leading to a decrease in print image quality.

A-5. Nozzle pattern control using a dither mask as the first embodiment:
The first embodiment suppresses a decrease in print image quality due to the nozzle pattern characteristics described above, and its principle will be described below. The size of the dither mask 62 used in the halftone process of the printer 20 (see step S130 in FIG. 3) in the main scanning direction and the sub-scanning direction is a positive integer multiple of the minimum repeating unit RU of the nozzle pattern described above. In the present embodiment, the dither mask 62 is 6 times the minimum repeating unit RU in the main scanning direction and 1 times the minimum repeating unit RU in the sub-scanning direction, that is, the first to twelfth columns and the first row to the first row. The size is such that ON / OFF of the dot at the dot formation position in the 60th row can be determined. By setting the dither mask 62 to such a size, each dot formation position of the repetitive minimum unit RU and the threshold value of the dither mask 62 applied to the dot ON / OFF determination at that position can be easily obtained. The correspondence relationship can be fixed. In this way, since the nozzle pattern has a periodicity of the repetitive minimum unit RU, the ease of dot formation for each nozzle can be controlled depending on the setting of the threshold value of the dither mask 62. .

  The threshold characteristics of the dither mask 62 having such a size will be described with reference to FIGS. 6 and 7 show the nozzle pattern at the dot formation position corresponding to the size of the dither mask 62. FIG. In the dither mask 62, among the dot formation positions shown in the figure, the threshold value applied to the position indicated by the highlighted nozzle number is lower in dot formation priority than the threshold values applied to other positions. That is, the threshold is set to be relatively large. In the present application, such a relatively large threshold is referred to as a dot suppression threshold.

  The dot formation position to which the dot suppression threshold is applied is set from among the dot formation positions where dots are formed by a nozzle having a predetermined width from the most advanced nozzle. In the present application, a nozzle having a predetermined width from such a state-of-the-art nozzle is also referred to as a tip nozzle, and a dot formation position where dots are formed by the tip nozzle is also referred to as a tip dot formation position. In the present embodiment, two nozzles from the most advanced nozzle (nozzles with nozzle numbers 0 and 29) have a predetermined width. That is, the tip nozzles are nozzles with nozzle numbers 0, 1, 2, 29, 28, and 27.

  Further, in the dither mask 62 of the present embodiment, as the threshold value applied to the tip dot formation position that is closer in the sub-scanning direction from the most advanced dot formation position, the number of threshold values set as the dot suppression threshold increases. The number of dot suppression thresholds is changed stepwise. Specifically, for example, the thresholds applied to the even-numbered columns of the 14th row and the even-numbered columns of the 16th row, which are the most advanced dot formation positions, have 6 dot suppression thresholds, and the most advanced dot formation For the thresholds applied to the even-numbered columns of the 12th row and the even-numbered rows of the 18th row separated by 2 rasters (1 nozzle) from the position, the number of dot suppression thresholds is 4, Regarding the thresholds applied to the even-numbered column of the 10th row and the even-numbered column of the 20th row separated by 4 rasters (2 nozzles) from the leading dot formation position, the number of dot suppression thresholds is two. The dot suppression thresholds are also set in the same manner for the other leading edge dot formation positions shown in the figure, and the total number of dot suppression thresholds is 96.

  The dither mask 62 described above can be produced as follows. First, the dither mask threshold is optimally arranged except for the dot formation position to which the dot suppression threshold is applied. Then, the threshold is optimally arranged for the dot formation position to which the dot suppression threshold is applied, using a threshold (dot suppression threshold) that has a lower dot formation priority than the dot formation position. For example, if the threshold value of the dither mask 62 is a value from “1” to “960”, the number of dot suppression threshold values is 96, so “1” to “864” excluding the dot suppression threshold value. Are optimally arranged at each dot formation position excluding the position where the dot suppression threshold is set, and then the 96 dot suppression thresholds from “865” to “960” are set as the dot suppression threshold. Is optimally arranged at each of the dot formation positions for which is set. Since various methods are known as threshold arrangement optimization methods, the description thereof is omitted. For example, the arrangement of each threshold is set in order from the smallest threshold based on a predetermined granularity evaluation value. A method of determining can be used.

  If the characteristics of the dither mask 62 are shown by the nozzle effective rate, which is the ratio of the number of dot formation positions excluding the position to which the dot suppression threshold is applied to the number of all dot formation positions for each nozzle number, FIG. become that way. That is, the nozzles with nozzle numbers 0 to 2 and 27 to 29 set as the tip nozzles are nozzles that form dots at the tip of the nozzle row, that is, at a position closer to the leading-edge dot formation position in the sub-scanning direction. As the nozzle efficiency increases, the nozzle effectiveness decreases.

  If halftone processing is performed using such a dither mask 62, dots are less likely to be formed at the leading dot formation position as the leading edge dot formation position is approached. In particular, if printing is performed with an ink duty of 90% (864/960) or less, no dot is formed at the dot formation position to which the dot suppression threshold is applied, so the effect becomes significant.

  The printer 20 having such a configuration performs halftone processing by a systematic dither method using the dither mask 62 having a size that is a positive integer multiple of the minimum repetition unit RU of the nozzle pattern, and thus each position in the minimum repetition unit RU of the nozzle pattern. And the correspondence relationship with each threshold value of the dither mask 62 applied to each position can be kept constant. Further, in the printer 20, the dither mask 62 is configured such that the number of dot suppression thresholds increases toward the most advanced dot formation position in the vicinity of the most advanced dot formation position where density unevenness occurs, that is, at the most advanced dot formation position. Is set. Therefore, at the tip dot formation position, it is always difficult to gradually generate dots toward the most advanced dot formation position. In particular, when the ink duty is equal to or less than the average value of the nozzle effective ratios of the respective nozzles, the number of dots to be formed always decreases toward the most advanced dot formation position at the tip dot formation position. Accordingly, the density change of the dots can be visually moderated in the vicinity of the most advanced dot formation position where the density unevenness occurs, and as a result, the density unevenness can be made inconspicuous and the deterioration of the print image quality can be suppressed.

B. Second embodiment:
A second embodiment of the present invention will be described. Since the apparatus configuration of the printer 20, the printing process by the printer 20, and the nozzle pattern characteristics are the same as those in the first embodiment, description thereof will be omitted and only differences will be described below.
B-1. Problems to be solved in the second embodiment:
9 and 10 show the dot formation timings at both ends of the print medium P in the main scanning direction for the nozzle patterns shown in FIGS. 4 and 5. Here, for the sake of space, the paper width is set to 16 rows of dots. Focusing on the above-mentioned local area, in the local area at the left end of the print medium P, for example, the local area from the first row and the first column to the second row and the second column at the left end of the print medium P, in the third main scan (rightward) The first dot is formed by the nozzle of nozzle number 23. Then, in the second main scanning (leftward), the second dot is formed by the nozzle of nozzle number 15. Such dots are formed after the reciprocating time of the print head 90 has elapsed since the first dot was formed, and are formed after a relatively long time has elapsed since the previous dot was formed. Therefore, it is also referred to as a long dot L in the present application.

  Then, in the first main scanning (rightward), the third dot is formed by the nozzle of nozzle number 8. Such a dot is a dot that is formed immediately after the second dot is formed and the print head 90 is folded back at the left end of the print medium P, and after a relatively short time has elapsed since the previous dot was formed. Since the dots are formed, they are also referred to as short dots S in the present application.

  Then, in the zeroth main scan (leftward), the last dot is formed by the nozzle with nozzle number 0. Such a dot is a long dot L. Although description is omitted, the long dots L and the short dots S are similarly formed in other local regions at both ends of the print medium P. 9C and 10C, the dot formation position where the long dot L is formed is displayed by hatching, and the dot formation position where the short dot S is formed is displayed by reverse display.

  As described above, in the local region from the first row and the first column to the second row and the second column, after the first dot is formed, the second to fourth dots are the long dot L, the short dot S, and the long dot. The dots are formed in the order of L. In FIG. 9C and FIG. 10C, “LSL” is displayed as the dot formation timing 1 on the left side of the nozzle pattern.

  On the other hand, in the local region at the right end of the print medium P, for example, in the local region of 1 row 15 columns to 2 rows 16 columns, dots are formed in the order of short dots S, long dots L, and short dots S, contrary to the left end. The In FIG. 9C and FIG. 10C, “SLS” is displayed as the dot formation timing 1 on the right side of the nozzle pattern.

  As shown in dot formation timing 1 in FIGS. 9C and 10C, the dot formation timing is set for each dot formation position where the print head 90 moves in the sub-scanning direction along the sub-scanning direction. “LSL” and “SLS” are interchanged. For example, at the left end of the print medium P, the dot formation timing in the local region belonging to the 1st row 1st column to the 14th row 2nd column of the dot formation position is “LSL”, but from the 15th row 1st column to the 30th row 2nd column. The dot formation timing in the local region to which it belongs is “SLS”. Such a change in dot formation timing has the same result as shown in dot formation timing 2 in FIGS. 9C and 10C even when the local area is shifted by one raster and attention is paid.

  Thus, in bi-directional printing, the dot formation timing changes along the sub-scanning direction each time the print head 90 moves relatively. Such a difference in dot formation timing causes a difference in density of ink and causes uneven density, and appears as a striped pattern for each feed amount of the print head 90 in the sub-scanning direction of the print image. This problem is particularly noticeable in large format printers that require a long time for a single reciprocation of the print head 90.

B-2. Nozzle pattern control using a dither mask as a second embodiment:
In the second embodiment, density unevenness of a printed image resulting from the above-described bidirectional printing is suppressed using the characteristics of the nozzle pattern, and the principle thereof will be described below. The size of the dither mask 62 used in the halftone process of the printer 20 (see FIG. 3, step S130) in the main scanning direction and the sub-scanning direction is the same as in the first embodiment. It is an integer multiple.

  The threshold characteristics of the dither mask 62 will be described with reference to FIG. FIG. 11 shows the nozzle pattern shown in FIGS. In FIG. 11, among the dot formation positions with even band numbers, the dot formation positions where dots are formed by the nozzle numbers 23 to 29 and the nozzles with nozzle numbers 0 to 6 among the dot formation positions with odd band numbers. The dot formation position where the dot is formed is shown in reverse display. Each threshold value of the dither mask 62 is set so that dots are not formed when the ink duty is a predetermined value or less at the dot formation position of the reverse display. In this embodiment, the predetermined value is 50%.

  The nozzles having the nozzle numbers 0 to 6 described above have a predetermined number of times for forming the continuous raster for any of the arbitrarily selected continuous rasters equal to the number of nozzle pitches (here, 2). This is a nozzle that forms dots in the first main scan of the four main scans (here, four times), and is also referred to as a front end nozzle in the present application. For example, in the nozzle pattern shown in FIG. 9C, the nozzle number 19 is used both in the formation of the 13th and 14th row rasters and in the formation of the 14th and 15th row rasters. The nozzle is to form dots in the first main scan among the four main scans forming these continuous rasters. Further, the nozzles with the nozzle numbers 23 to 29 described above are nozzles that form dots in the last main scan of the predetermined number of main scans for any of the above-described continuous rasters. Also called a rear end nozzle.

  When the halftone process of step S130 is performed using the dither mask 62 having such characteristics, when printing with an ink duty of 50% or less, dots are formed at the dot formation positions shown in reverse display in FIG. Will not be. The effect of the result on the print image quality will be described with reference to FIGS. 12 and 13 show how the dot formation timing 1 at both ends of the print medium P shown in FIGS. 9 and 10 changes due to the halftone process using the dither mask 62. For example, if attention is paid to the local region from the first row and the first column to the second row and the second column belonging to the even band, when printing with an ink duty of 50% or less, dots are not formed by the front end nozzle of the even band (the local band). In the region, no dot is formed by the nozzle of nozzle number 23), so the first dot is formed in the local region by the nozzle of nozzle number 15 in the second main scan. Thereafter, in the first main scan, the short dot S is formed by the nozzle of nozzle number 8, and then, the long dot L is formed by the nozzle of nozzle number 0 in the zeroth main scan. That is, the dot formation timing in the local region is “SL”.

  If attention is paid to the local region from the 15th row and the 1st column to the 16th row and the 2nd column belonging to the odd band, when printing with an ink duty of 50% or less, dots are not formed at the rear end nozzles of the odd band (the concerned In the local area, no dot is formed with the nozzle of nozzle number 0). Therefore, in the local area, the first dot is formed by the nozzle with nozzle number 22 in the second main scan. Thereafter, the short dot S is formed by the nozzle No. 15 in the 0th main scan, and then the long dot L is formed by the nozzle No. 7 in the 0th main scan. That is, the dot formation timing in the local region in this case is “SL”.

  As described above, if halftone processing is performed using the dither mask 62, when printing with a predetermined ink duty or less (here, 50% or less), a part of the region at the left end of the print medium P is formed. Except for this, the dot formation order can be unified with “SL”. This partial area will be described later. Although not described, the dot formation order can be unified with “LS” at the right end of the print medium P in the same manner. That is, it is possible to suppress the occurrence of density unevenness in the sub-scanning direction by suppressing the dot formation timing from changing along the sub-scanning direction at both ends of the print medium P. Of course, if halftone processing is performed using such a dither mask 62, a certain effect can be obtained even when printing with an ink duty exceeding 50% is performed.

  In this embodiment, the partial area where the difference in dot formation timing cannot be unified is, for example, a local region from 29 rows 1 column to 30 rows 2 columns in FIGS. 12 (c) and 13 (c). An area or a local area from 59 rows 1 column to 60 rows 2 columns. This is because such a local region belongs to the band boundary. For example, in the local region from 29 rows 1 column to 30 rows 2 columns, if dots are not formed with the nozzle of nozzle number 7 in the first main scanning that is the last main scanning for forming dots in the local regions, As in the case described above, the dot formation order is “SL” in the local region, but from the 30th row and the first column to the 31st row and the second column based on the division of the dot formation timing 2 shown in FIG. Focusing on the local region (LSL), the dot formation order does not become “SL” even if the nozzle number 7 is not used to form dots. In order to set the dot formation order to “SL” in the local region, it is necessary to prevent dots from being formed by the nozzle of nozzle number 22 that forms dots in the first main scanning. In other words, the nozzles No. 7 and No. 22 are located at the band boundary, and depending on how the local area is noticed, it may or may not be a nozzle that forms dots in the first or last main scan, In this place, the difference in dot formation timing cannot be completely unified.

  Further, since the above-described printer 20 does not require a special configuration just considering the setting of the threshold value of the dither mask 62, in addition to the effect of suppressing the density unevenness described above, the effect of high versatility. Also play.

  A specific example of such a dither mask 62 is shown in FIG. The dither mask 62 shown in the drawing is an excerpt of a part in the main scanning direction and the sub-scanning direction, and the threshold value is set in the range of 0-255. The matrix numbers given to the upper end and the left end of each threshold indicate the matrix number of the nozzle pattern to which the threshold is applied. That is, the dither mask 62 is applied in such a positional relationship that the upper left end of the nozzle pattern shown in FIG. In FIG. 14, for each threshold value, the threshold value at which the dot is turned on is displayed in reverse when printing with an ink duty of 50%, and the threshold value at which the dot is turned off is indicated by normal black character display. In particular, the threshold value applied to a dot formation position (a reverse display position in FIG. 11) where dots are not formed below a predetermined ink duty shown in FIG. 11 is displayed in gray. In FIG. 14, the threshold value in which the background color is displayed in gray is a value greater than 128, and no dots are formed at the dot formation position during printing with an ink duty of 50% or less.

  The dither mask 62 can be created as follows. First, the dither mask threshold values from 0 to 128 are optimally arranged except for the position where the background of FIG. 14 is displayed in gray. Then, threshold values from 129 to 255 are optimally arranged at the remaining positions including the position where the background is displayed in gray. Various methods for optimizing the threshold arrangement are known and will not be described. For example, the arrangement of each threshold is determined in order from the smallest threshold based on a predetermined granularity evaluation value. Can be used.

  FIG. 15 shows the characteristics of the dither mask 62 shown in FIG. 14 in terms of the nozzle usage rate (when the ink duty is 50%), which is the ratio of ejecting ink for each nozzle. As shown in the figure, in the even band, the nozzle usage rate is zero for the front end nozzles (nozzle numbers 23 to 29), and the nozzle usage rate is an average nozzle usage rate for the nozzles of nozzle numbers 0 to 21. . In this embodiment, the nozzle usage rate of the nozzle of nozzle number 22 in the even band is set to take an intermediate value between the nozzle usage rate of the front end nozzle and the nozzle usage rate of other nozzles.

  On the other hand, in the odd-numbered band, the nozzle usage rate is zero for the rear end nozzles (nozzle numbers 0 to 6), and the nozzle usage numbers of the nozzle numbers 8 to 29 are average nozzle usage rates. In this embodiment, the nozzle usage rate of the nozzle No. 7 in the odd band is set to take an intermediate value between the nozzle usage rate of the rear end nozzle and the nozzle usage rate of the other nozzles. Yes. The reason why the nozzle usage rates of the nozzles Nos. 7 and 22 are set to an intermediate value is that the nozzle is located at the band boundary as described above, so that the stripe pattern at the band boundary is not conspicuous. Because. However, the nozzle usage rate of the nozzles of nozzle numbers 7 and 22 is not limited to an intermediate value, and may be any value.

  In the above-described embodiment, the example in which the threshold value of the dither mask 62 is set so that ink is not ejected from the front end nozzle in the even band and from the rear end nozzle in the odd band is shown. Therefore, it is a matter of course that the same effect can be obtained even if the threshold value of the dither mask 62 is set so that ink is not ejected from the front end nozzle in the odd band. In this case, for example, the dot formation timing at the left end of the print medium P is unified with “LS”.

  In short, the dither mask 62 has different values for the front end nozzle usage rate and the rear end nozzle usage rate, and each time the print head 90 moves relative to each other (for each band), the front end nozzle usage rate and the rear end nozzle usage rate. It suffices if the threshold value is set so that the magnitude relationship with the nozzle usage rate is switched. In other words, in main scanning in one direction of reciprocation of the print head 90, ink ejection from either one of the front end nozzle and rear end nozzle is suppressed, and in main scanning in the other direction. Therefore, it is only necessary to suppress ink ejection from the other of the front end nozzle and the rear end nozzle.

  In the above-described example, the threshold value of the dither mask 62 is set so that dots are not formed at the dot formation positions shown in reverse display in FIG. 11 when printing with an ink duty of 50% or less. The obtained ink duty is not limited to 50% and may be set as appropriate. For example, the ink duty may be 30% or 75%. However, since the above-described density unevenness tends to be noticeable in a printing area where dots are densely formed to some extent, it is more desirable to set the ink duty corresponding to the intermediate gradation area.

C. Third embodiment:
A third embodiment of the present invention will be described. In the third embodiment, a configuration for reducing the size of the dither mask 62 described above using the characteristics of the nozzle pattern will be described.
C-1. First example:
FIGS. 16 and 17 are diagrams illustrating how dots are formed by the printer 20 as in FIGS. 4 and 5 described above. Here, in order to simplify the explanation, the number of overlaps is “4”, the nozzle pitch is “2”, and the paper feed amount is “5” as a drive control mode of the print head 90 and the like. Since the display methods of FIGS. 16A, 17A, 16B, and 17B are the same as those of FIGS. 4 and 5 described above, the description thereof is omitted here.

  FIGS. 16C and 17C show how the above-described filling order changes every time the print head 90 is moved in the sub-scanning direction. Since the number of repeating units in the filling order is the product of the nozzle pitch and the number of overlaps as described above, it is 8 in this example (nozzle pitch 2 × overlap number 4). In order to facilitate the description, the filling order numbers (1 to 8) are assigned to the left side of the filling order in FIGS. 16C and 17C in order from the filling order displayed at the top. Further, FIG. 16D and FIG. 17D show nozzle patterns. As described above, the minimum unit RU of the nozzle pattern is the product of the number of overlaps in the main scanning direction and the product of the paper feed amount and the number of repeating units in the filling order in this example. This is a nozzle pattern of row 1 column to 40 row 4 column.

  Here, the filling order shown in FIGS. 16C and 17C has regularity having a predetermined period. This regularity will be described with reference to FIG. FIG. 18A shows the filling order 1 with the filling order number 1 shown in FIG. As shown in the figure, in this filling order, the relative positional relationship between the dot formation position where the dot is formed in the nth main scan and the dot formation position where the dot is formed in the n + 1 main scan is a positive number. Is expressed as dn (x, y) (x is a displacement amount in the main scanning direction, and y is a displacement amount in the sub-scanning direction), d1 (x, y) = (3, 1 ), D2 (x, y) = (2, 1). FIG. 18B shows the result of arranging such relative positional relationships as n = 1 to 8. As shown in the figure, the relative displacement amount dn in the filling order 1 has a pattern in which (3, 1) and (2, 1) are alternately repeated. In the present application, a combination of relative displacement amounts dn in one filling order, that is, a combination of relative positional relationships between dot formation positions of dots successively formed in a local region is also referred to as a relative position pattern. The relative position pattern shown in FIG.

  Similarly, the relative position pattern obtained for the filling order 2 shown in FIG. 16C is shown as relative position pattern 2 in FIG. In the relative position pattern 2, as shown in the drawing, (2, 1) and (3, 1) are alternately repeated. In this way, when the relative position patterns are obtained for the filling orders 1 to 8 shown in FIGS. 16C and 17C, the relative position patterns of each filling order are the relative position pattern 1, The filling order 2 has a relative position pattern 2, the filling order 3 has a relative position pattern 1, the filling order 4 has a relative position pattern 2, the filling order 5 has a relative position pattern 1, and has a periodicity every two filling orders. It will be.

  In this example, since the paper feed amount is a constant value “5”, the feed amount for each cycle of the relative position pattern is always constant. Thus, when the paper feed amount is constant and the filling order has the regularity described above, the nozzle pattern has a periodicity equal to the filling order period along the sub-scanning direction. Become. Specifically, the nozzle pattern for 10 rows (dots), which is the amount of displacement in the sub-scanning direction corresponding to the period of the relative position pattern (the amount in which the filling order changes twice), is the main scanning direction corresponding to the period of the filling order Appearing repeatedly in the sub-scanning direction while shifting in the main scanning direction by one column (dot) of the size of. Specifically, as shown in FIG. 19, for example, the nozzle pattern from 1 row to 1 column to 10 rows and 8 columns is a nozzle pattern of 11 rows and 2 columns to 20 rows and 9 columns shifted to the left by one column, It becomes equal to the nozzle pattern of 21 rows 3 columns to 30 rows 10 columns shifted by two columns. Accordingly, as shown in FIG. 19, for example, a dither mask having the same size as the nozzle pattern from 1 row to 1 column to 10 rows and 8 columns is shifted in the main scanning direction by one column, and every 10 rows in the sub scanning direction. If applied in a spread manner, the relationship between each dot formation position of the repetitive minimum unit RU and the threshold value of the dither mask 62 applied to the determination of ON / OFF of the dot at that position is maintained. The size of the dither mask 62 in the sub-scanning direction can be reduced to a quarter.

  The shift amount of the dither mask 62 in the main scanning direction is not limited to one column, and the size in the main scanning direction of the repetitive minimum unit RU of the nozzle pattern is four columns, so that 1 + 4N (N is Of course, it is sufficient if it is an integer of 0 or more. For example, when N = 1, as shown in FIG. 20, the dither mask 62 is spread and applied along the sub-scanning direction while shifting in the main scanning direction by 5 columns.

  The size of the dither mask 62 in the sub-scanning direction is not limited to the size in the sub-scanning direction corresponding to the period of the relative position pattern, and may be any size multiple in the sub-scanning direction corresponding to the period of the relative position pattern. . For example, as shown in FIG. 21, it may be double (20 columns) in the sub-scanning direction corresponding to the period of the relative position pattern.

  The printer 20 having such a configuration can reduce the size of the dither mask 62 in the sub-scanning direction and effectively use the storage capacity of the printer 20. Alternatively, the storage capacity of the printer 20 can be reduced. In the above-described example, for the sake of simplicity, the configuration of the printer 20 with a small number of nozzles and overlaps is shown. Therefore, the size of the repeated minimum unit RU of the nozzle pattern in the sub-scanning direction is 40 columns (40 However, if the number of nozzles and the number of overlaps increase, the effect becomes remarkable.

C-2. Second example:
22 and 23 are diagrams showing how dots are formed by the printer 20 as a second example in the third embodiment. Here, the overlap number, the nozzle pitch, and the paper feed amount are the same as those in the first example described above, but the filling order is different from that in the first example. The filling order in this example is as shown in FIGS. 22C and 23C, filling order 1 and filling order 5, filling order 2 and filling order 6, filling order 3 and filling order 7, and filling order 4 And the filling order 8 have a mirror image symmetry relationship. That is, each filling order has mirror image symmetry periodically.

  In this example, since the paper feed amount is a constant value “5”, the feed amount for each cycle of the relative position pattern is always constant. Thus, when the paper feed amount is constant and the filling order has the regularity described above, the nozzle pattern has a period equal to the period of mirror image symmetry of the filling order along the sub-scanning direction. This shows mirror image symmetry. Specifically, the nozzle pattern for 20 rows, which is the size in the sub-scanning direction corresponding to the cycle of mirror image symmetry (for which the filling order changes four times), changes while exhibiting mirror image symmetry. Specifically, as shown in FIG. 24, for example, the nozzle pattern from 1 row 1 column to 20 rows 8 columns and the nozzle pattern from 21 row 1 column to 40 rows 8 columns are in a mirror image symmetry relationship. . Therefore, as shown in FIG. 19, for example, a dither mask having the same size as the nozzle pattern from 1st row 1st column to 20th row 8th column is spread and applied in the sub-scanning direction every 20th row while mirror image inversion is applied. The dither mask 62 has a fixed correspondence relationship between each dot formation position of the minimum repeating unit RU and the threshold value of the dither mask 62 applied to the ON / OFF determination of the dot at that position. The size in the sub-scanning direction can be halved. Even in such a case, the same effect as the first example can be obtained.

  Since the size of the repeated minimum unit RU of the nozzle pattern in the main scanning direction is 4 columns, it is needless to say that the dither mask 62 may be applied while shifting by 4N (N is an integer of 0 or more) columns. It is. For example, when N = 1, the dither mask 62 may be spread and applied along the sub-scanning direction while shifting in the main scanning direction by four columns as shown in FIG.

C-3. Third example:
In the first example and the second example described above, a configuration in which the size of the dither mask 62 in the sub-scanning direction can be reduced when the paper feed amount is constant and the filling order has a predetermined regularity is shown. However, the paper feed amount does not necessarily have to be constant. The product of the number of nozzle pitches, the number of overlaps, and the average value of the paper feed amount, that is, the size of the repeated minimum unit RU of the nozzle pattern in the sub-scanning direction. The number of dots (raster number) may be approximately several minutes. A specific example of such a configuration will be described with reference to FIGS.

  22 and 23 are diagrams showing how dots are formed by the printer 20 as a third example in the third embodiment. In this example, the number of overlaps is “4”, the nozzle pitch is “2”, and the paper feed amount is repeated in the order of “6 → 5 → 4 → 5”. Although the detailed description is omitted, in the filling order in this example, as in the first example, the relative position pattern of each filling order has a periodicity for every two filling orders.

  As described above, since the paper feed amount is a repetition of “6 → 5 → 4 → 5”, it has a periodicity with a period of 20 dots (raster). When the paper feed amount is unequal interval feed, the size in the sub-scanning direction of the repetitive minimum unit RU of the nozzle pattern is obtained as the product of the number of nozzle pitches, the number of overlaps, and the average value of the paper feed amount. Therefore, the paper feed amount in this example has a cycle of the number of dots corresponding to about a few of the size (40 dots) in the sub-scanning direction of the repetitive minimum unit RU of the nozzle pattern.

  As described above, the paper feed amount has a period of the number of dots corresponding to the size of the repetitive minimum unit RU of the nozzle pattern in the sub-scanning direction (40 dots), and the filling order has some periodicity. In this case, the nozzle pattern has a periodicity with the period being the least common multiple of the period of the paper feed amount and the period of the filling order. Specifically, the least common multiple of 20 rows (dots) that is the cycle of the paper feed amount and 10 rows (dots) that is the size in the sub-scanning direction corresponding to the cycle of the filling order (the amount that the filling order changes twice) is Since there are 20 rows (dots), the cycle has 20 rows (dots). Specifically, as shown in FIG. 28, for example, the nozzle patterns from 1 row 1 column to 20 rows 8 columns are each two columns that are displacement amounts in the main scanning direction in the filling order corresponding to 20 rows. It becomes equal to 21 rows 3 columns to 40 rows 10 columns shifted in the main scanning direction. Therefore, as shown in FIG. 28, for example, a dither mask having the same size as the nozzle pattern from 1st row 1st column to 20th row 8th column is shifted by 2 columns in the main scanning direction, and every 20th row in the sub scanning direction. If applied in a spread manner, the relationship between each dot formation position of the repetitive minimum unit RU and the threshold value of the dither mask 62 applied to the determination of ON / OFF of the dot at that position is maintained. The size of the dither mask 62 in the sub-scanning direction can be halved. Even in such a case, the same effect as the first example can be obtained.

D. Variation:
D-1. Modification 1:
In the above-described embodiment, an example in which the nozzle pattern is formed with an equal number of nozzles has been described. However, the present invention is not limited to such an aspect, and for example, an aspect in which dots are formed by partial overlap. Also good. Partial overlap means that two groups of nozzles capable of forming dots at the same dot formation position share the dot formation at the dot formation position. 29 and 30 show a first specific example of a nozzle pattern with partial overlap. This example is different from the above-described embodiment (30 nozzles) in that the nozzle arrays 92 to 97 are composed of 37 nozzles having nozzle numbers 0 to 36. In the present modification, the nozzles with nozzle numbers 30 to 36 can form dots at the same dot formation positions as the nozzles with nozzle numbers 0 to 6 and share the dot formation between the two, so that the excess nozzle Also called.

  FIG. 29 and FIG. 30 are diagrams showing how dots are formed by a printer having surplus nozzles. FIGS. 29 and 30 are continuous views, but are divided into two parts in FIGS. 4 and 5 for the sake of space. FIGS. 29A and 30A show how the nozzle row 92 moves in the sub-scanning direction each time main scanning is performed. In FIGS. 29B and 30B, the main scanning number indicates how many main scans each dot formed on the print medium is formed. Each grid shown in FIGS. 29B and 30B represents the Nth, N + 1, N + 2, and N + 3 (N is an integer equal to or greater than 1) -th dot in each raster in order from the left. Since these drawings are based on the same concept as in FIGS. 4 and 5 described above, detailed description thereof will be omitted. FIG. 29C and FIG. 30C show nozzle patterns. The difference from the nozzle pattern of the first embodiment (see FIGS. 4C and 5C) is that the surplus nozzles (nozzle number 30) are used for half of the positions where dots are formed by the nozzles having nozzle numbers 0 to 6. To 36), dots are formed. In FIG. 29 (c) and FIG. 30 (c), dot formation positions where dots are formed by the surplus nozzles are shown in gray. In such a nozzle pattern, the repetitive minimum unit RU is composed of the first column to the fourth column and the first row to the 60th row as shown in FIGS. This is because a printer having such surplus nozzles can make the banding inconspicuous because the joints of dots having different main scanning numbers are dispersed.

  In the above-described nozzle pattern, in the first and second columns, the main scanning number is switched between the dot formation position of nozzle number 29 and the dot formation position of nozzle number 0 (for example, 14th row 2 Column and 16th row and 2nd column). In the third and fourth columns, the main scanning number is switched between the dot formation position of nozzle number 36 and the dot formation position of nozzle number 7 (for example, the 13th row and the 3rd column and the 15th row). Row 3 column). As described above, the positions at which the main scanning numbers are switched are different depending on the rows. In the first row and the second row, the nozzles 0 and 29 are arranged at both ends of the nozzle rows in order not to form dots with the surplus nozzles. In order to form dots with the surplus nozzles in the third and fourth rows (no dots are formed with nozzle numbers 0 to 6), nozzle numbers 7 and 36 are arranged at both ends of the nozzle row. This is because it works as a nozzle.

  Even in such a nozzle pattern, if the nozzles that serve as nozzles arranged at both ends of the nozzle row are handled as the most advanced nozzles in the embodiment and the dot suppression threshold is set in the same manner as in the embodiment, the first implementation The same effect as the example can be achieved.

  Moreover, the 2nd specific example of the nozzle pattern using a surplus nozzle is shown in FIG.31 and FIG.32. In this example, the sharing ratio between the nozzles with nozzle numbers 0 to 6 and the nozzles with nozzle numbers 30 to 36 is changed according to rows. The minimum repeating unit RU of the nozzle pattern is composed of the first column to the eighth column and the first row to the 60th row. As described above, in the nozzle pattern using the surplus nozzles, the size of the repetitive minimum unit RU in the main scanning direction is different due to the regularity of the change of the sharing ratio.

  In addition, a dot suppression threshold may be set for a nozzle pattern using surplus nozzles by treating it as having no partial overlap. That is, among the illustrated nozzle patterns, the dot suppression threshold may be set by replacing nozzle numbers 30 to 36 with nozzle numbers 0 to 6 or replacing nozzle numbers 0 to 6 with nozzle numbers 30 to 36. . In other words, the dither mask 62 shown in the first embodiment may be used as it is in the printer having the nozzle patterns shown in FIGS. Even in this case, a certain degree of effect can be obtained as compared with the case where the dot suppression threshold is not set.

  In such a case, the size of the dither mask 62 in the main scanning direction does not necessarily have to be a positive integer multiple of the minimum repeating unit RU. For example, although the size of the repetitive minimum unit RU of the nozzle pattern shown in FIGS. 31 and 32 in the main scanning direction is 8 columns, the dither mask 62 to be applied is set to a threshold value ignoring partial overlap. It may be a positive integer multiple (for example, two columns) of the minimum repeating unit RU of the nozzle pattern when there is no partial overlap.

D-2. Modification 2:
In the printer using the surplus nozzle shown in the first modification, it is possible to perform the same control as in the second embodiment. For example, in the nozzle pattern shown in FIGS. 29 and 30, the repetitive minimum unit RU is composed of the first column to the fourth column and the first row to the 60th row. If the size of the scanning direction is set to a positive integer multiple of the repetitive minimum unit RU and the method of the first embodiment is applied, the usage rates of the front end nozzle and the rear end nozzle can be easily controlled.

  Further, as shown in FIGS. 31 and 32, when the sharing ratios of the front end nozzle and the rear end nozzle are not equal, similarly to the first modification, it is treated that there is no surplus nozzle and each threshold value is set. May be. Even in this case, a certain degree of effect can be obtained. In this case, similarly to the first modification, the size of the dither mask 62 in the main scanning direction is not necessarily a positive integer multiple of the repetitive minimum unit RU.

D-3. Modification 3:
In the above-described embodiment, the printer 20 is configured to execute all of the print processing of FIG. 2, but when a computer is connected to the printer 20, even if the computer executes a part of the print processing. Good. In such a case, the printing system constituted by the computer and the printer 20 can be regarded as a printing device in a broad sense.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to such embodiment, Of course, in the range which does not deviate from the summary of this invention, it can implement in a various aspect.

1 is a schematic configuration diagram of a printer 20 as an embodiment of the present invention. FIG. 4 is an explanatory diagram showing nozzle arrangement of a print head 90. 4 is a flowchart of print processing by the printer 20. FIG. 4 is an explanatory diagram showing how dots are formed by a printer. FIG. 4 is an explanatory diagram showing how dots are formed by a printer. It is explanatory drawing which shows the characteristic of the threshold value of the dither mask. It is explanatory drawing which shows the characteristic of the threshold value of the dither mask. It is explanatory drawing which shows the nozzle effective rate for every nozzle by the dither mask 62. FIG. It is explanatory drawing about the cause of the density nonuniformity in bidirectional printing. It is explanatory drawing about the cause of the density nonuniformity in bidirectional printing. It is explanatory drawing which shows the characteristic of the threshold value of the dither mask 62 as 2nd Example. FIG. 10 is an explanatory diagram illustrating an effect of suppressing density unevenness by the printer. FIG. 10 is an explanatory diagram illustrating an effect of suppressing density unevenness by the printer. It is explanatory drawing which shows the specific example of the dither mask 62 as 2nd Example. It is explanatory drawing which shows the example of a nozzle usage rate. It is explanatory drawing which shows a mode that a dot is formed by the printer 20 as the 1st example of 3rd Example. It is explanatory drawing which shows a mode that a dot is formed by the printer 20 as the 1st example of 3rd Example. It is explanatory drawing which shows the regularity of the filling order as a 1st example. It is explanatory drawing which shows the method of applying the dither mask 62 as a 1st example. It is explanatory drawing which shows the method of applying the dither mask 62 as a 1st example. It is explanatory drawing which shows the method of applying the dither mask 62 as a 1st example. It is explanatory drawing which shows a mode that a dot is formed by the printer 20 as the 2nd example of 3rd Example. It is explanatory drawing which shows a mode that a dot is formed by the printer 20 as the 2nd example of 3rd Example. It is explanatory drawing which shows the method of application of the dither mask 62 as a 2nd example. It is explanatory drawing which shows the method of application of the dither mask 62 as a 2nd example. It is explanatory drawing which shows a mode that a dot is formed by the printer 20 as the 3rd example of 3rd Example. It is explanatory drawing which shows a mode that a dot is formed by the printer 20 as the 3rd example of 3rd Example. It is explanatory drawing which shows the method of application of the dither mask 62 as a 3rd example. It is explanatory drawing which shows the nozzle pattern as a modification. It is explanatory drawing which shows the nozzle pattern as a modification. It is explanatory drawing which shows the nozzle pattern as a modification. It is explanatory drawing which shows the nozzle pattern as a modification.

20 ... Printer 30 ... Control unit 40 ... CPU
41 ... Halftone processing unit 42 ... Printing unit 51 ... ROM
52 ... RAM
60 ... EEPROM
62 ... Dither mask 70 ... Carriage motor 71 ... Drive belt 72 ... Pulley 73 ... Sliding shaft 74 ... Paper feed motor 75 ... Platen 80 ... Carriage 82-87 ... Ink cartridge 90 ... Print head 92-97 ... Nozzle array 98 ... Memory Card slot 99 ... Operation panel P ... Print medium MC ... Memory card RU ... Minimum repeat unit

Claims (9)

A printing apparatus that performs printing while moving a print head relative to a print medium in a main scanning direction and a sub-scanning direction,
A nozzle array provided in the print head and arranged with a plurality of nozzles for discharging ink in the sub-scanning direction;
A halftone processing unit for performing halftone processing by comparing each threshold value of a dither mask composed of a plurality of threshold values with image data constituting the image;
A printing unit that performs printing by controlling the ejection of ink from each nozzle of the nozzle row using the result of the halftone processing;
Each position in a minimum unit of repetition of a nozzle pattern indicating which of the plurality of nozzles forms a dot at each position on the print medium, and each of the dither masks applied to each position A printer that has a fixed correspondence with the threshold value.   The printing apparatus according to claim 1, wherein the dither mask has a size in the main scanning direction and the sub-scanning direction that is a positive integer multiple of a minimum unit of repetition of the nozzle pattern. The printing apparatus according to claim 1,
The feed amount that the print head relatively moves in the sub-scanning direction by one sub-scanning has a predetermined periodicity,
The size in the main scanning direction is a product of the number of nozzle pitches, which is the number of dots between the centers of nozzles adjacent in the sub-scanning direction among the plurality of nozzles, and the number of main scanning times for completing one raster. The dot formation order at each dot formation position in a local region that is a continuous dot formation position on the print medium, in which the number of dots is equal and the size in the sub-scanning direction is equal to the number of nozzle pitches Is changed based on regularity having a predetermined period for each sub-scanning,
The dither mask has a size of the feed amount in which the size in the main scanning direction is a positive integer multiple of the minimum unit of repetition of the nozzle pattern, and the size in the sub-scanning direction is the minimum unit of repetition. And a printing apparatus having a size of a fraction of a positive integer determined based on the period of the filling order. The printing apparatus according to claim 3,
The regularity of the filling order is such that the relative position pattern indicating the combination of the relative positional relationships between the dot formation positions of the dots formed continuously in the local region follows the change in the filling order. Printing device that has periodicity.   The printing apparatus according to claim 3, wherein the regularity of the filling order is that the changing filling order periodically exhibits mirror image symmetry.   6. The printing apparatus according to claim 3, wherein the periodicity of the feed amount is that the feed amount is constant.   6. The printing apparatus according to claim 3, wherein the cycle of the feed amount is the number of dots corresponding to about a few of the minimum unit size of the nozzle pattern in the sub-scanning direction. A dither mask comprising a plurality of threshold values and used for halftone processing for performing printing while moving the print head relative to the print medium in the main scanning direction and the sub-scanning direction;
The dot pattern at each position on the print medium is arranged in the sub-scanning direction on the print head, and is a repeated nozzle pattern that indicates which of a plurality of nozzles that eject ink is formed. A dither mask in which a correspondence relationship between each position in a minimum unit and each threshold value of the dither mask applied to each position is fixed. While moving the print head relative to the print medium in the main scanning direction and the sub-scanning direction, a plurality of nozzles that are provided in the print head and that eject ink are arranged from each nozzle in the nozzle row arranged in the sub-scanning direction. A method of performing printing using a printing apparatus that performs printing by controlling ink ejection,
A dither mask having a plurality of threshold values, each position in a minimum unit of a nozzle pattern repetition indicating which nozzle of each of the plurality of nozzles forms a dot at each position on the print medium; and A halftone process is performed using a dither mask in which the correspondence relationship with each threshold value of the dither mask applied to each position is fixed, and dots are easily formed in units of nozzles of the nozzle row. A printing method that controls printing and prints.

Images