JP2015199230A - Print control unit, program and printing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a print control unit suppressing a reduction in image quality of an image printed on a printing medium supported by an uneven shaped platen.SOLUTION: A print control unit controls a printer comprising: a head discharging ink onto a printing medium; and a platen supporting the printing medium and having a plurality of protrusion portions disposed side-by-side at intervals in a predetermined direction on an opposite surface to the head. The print control unit uses a first dither mask with respect to input image data corresponding to locations of the printing medium positioned at the plurality of protrusion portions and uses a second dither mask having characteristics where dispersion properties of dots in a gradation value within a predetermined range with respect to input image data corresponding to locations of the printing medium positioned at recess portions among protrusion portions are less than at those of the first dither mask when performing halftone processing converting input image having a gradation value indicating light and shade of an image printed on the printing medium into data indicating a pattern of the dots constituting the image.

Description

  The present invention relates to a print control apparatus, a program, and a printing method.

There is known a printing apparatus that prints an image composed of a large number of dots by ejecting ink from a head toward a printing medium. A print control apparatus that controls such a printing apparatus, for example, a computer connected to the printing apparatus, uses a dither mask to generate multi-gradation data indicating the density of an image when creating print data. Halftone processing is performed to convert the data to the number of gradations that can be expressed by the printing apparatus (data indicating the dot formation pattern).
In addition, in a printing apparatus, the more the gap (paper gap) between the ink ejection surface of the head and the print medium increases, the more easily it is affected by variations in the ink ejection direction. Will fall. Therefore, when the paper gap is large, a dither mask in which dots are generated in a direction that cancels the influence of the ink ejection characteristics generated by the increase in the paper gap, for example, a dither in which dots are generated in an oblique direction different from the conveyance direction of the printing medium. A method of creating print data by switching to a mask has been proposed.

JP 2006-205389 A

  In a printing apparatus, generally, a portion of a print medium facing the head is supported by a platen, and a recess is formed in the platen for receiving ink that has been removed from the print medium during borderless printing. There is. When such a platen is used, the print medium is supported by the surface of the platen that forms the concavo-convex shape. Therefore, when ink is ejected onto the printing medium and the printing medium absorbs the ink, the paper gap does not change at the portion of the printing medium supported by the convex part of the platen, and the dot landing position does not shift. The portion of the printing medium located in the concave portion of the printing medium bends, the paper gap changes, and the dot landing position shifts. Therefore, density unevenness along the uneven shape of the platen occurs, and the image quality deteriorates.

  In view of the above, an object of the present invention is to suppress deterioration in image quality of an image printed on a printing medium supported by a platen having an uneven shape.

A main invention for achieving the above object includes: a head that ejects ink onto a print medium; and a platen that supports the print medium and has a plurality of protrusions arranged at intervals in a predetermined direction on a surface facing the print medium. A print control apparatus for controlling a printing apparatus comprising: input image data having a gradation value indicating a shade of an image to be printed on the print medium into data indicating a pattern of dots constituting the image. When the halftone process is performed, a first dither mask is used for the input image data corresponding to the portion of the print medium located at the convex portion, and the printing located at the concave portion between the convex portions. A second dither mask having a characteristic that the dispersibility of dots in the gradation value within a predetermined range is lower than that of the first dither mask with respect to the input image data corresponding to a portion of the medium; A print control apparatus which is characterized in that use.
Other features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

FIG. 1 is a block diagram illustrating a configuration of a printing system in which an inkjet printer and a computer are connected. 2A is a schematic cross-sectional view of the printer as viewed from the X direction, and FIG. 2B is a schematic plan view of the periphery of the platen as viewed from above. FIG. 3A is a diagram illustrating the arrangement of dots formed in different passes. FIG. 3B is a diagram for explaining dot arrangement by data half-tone processed with a blue noise dither mask, and FIG. 3C is a diagram for explaining dot arrangement by data half-tone processed by a pair dot dither mask. It is a figure explaining the landing position shift of the dot by a cockling phenomenon. It is a flow of the halftone process in 1st Embodiment. It is a flow which shows the preparation method of a pair dot dither mask. FIG. 7A is a diagram for explaining a region where the concave portion of the platen is partitioned, and FIG. 7B is a flow of the halftone process of the second embodiment. FIG. 8A is a diagram for explaining the platen of the third embodiment, and FIG. 8B is a flow of halftone processing of the third embodiment. FIG. 9A is a diagram for explaining the peripheral region of the target pixel, and FIG. 9B is a flow of halftone processing according to the fourth embodiment. It is a flow of the halftone process of 5th Embodiment.

=== Overview ===
At least the following matters will become clear from the description of the present specification and the drawings.

A printing control apparatus for controlling a printing apparatus comprising: a head that ejects ink onto a printing medium; and a platen that supports the printing medium and has a plurality of convex portions arranged at intervals in a predetermined direction on a surface facing the printing medium When the halftone process for converting input image data having gradation values indicating the density of an image to be printed on the print medium into data indicating a pattern of dots constituting the image, the convex portion is performed. A first dither mask is used for the input image data corresponding to the portion of the print medium located at the position, and the input image data corresponding to the portion of the print medium located in the concave portion between the convex portions is used. On the other hand, the printing control apparatus uses a second dither mask having a characteristic that the dispersibility of dots in the gradation value in a predetermined range is lower than that of the first dither mask. That.
According to such a printing control apparatus, an image with good graininess is printed on the portion of the print medium located on the convex portion of the platen where the dot landing position is not likely to be shifted, and the platen where the dot landing position may be shifted. An image having a small change in density due to the deviation of the dot landing position can be printed on the portion of the print medium located in the concave portion. Therefore, it is possible to suppress the density unevenness along the uneven shape of the platen while improving the graininess of the image, and to suppress the image quality deterioration of the image.

In the print control apparatus, when the halftone process is performed, the first image data corresponding to a portion of the print medium located in a central region of the concave portion in the predetermined direction is selected with respect to the input image data. Using two dither masks, and for the input image data corresponding to the portion of the print medium located in the end region on the convex side in the predetermined direction with respect to the central portion of the concave portion, A printing control apparatus using a third dither mask having a characteristic that the dispersibility of dots in the gradation value in a predetermined range is lower than that of the first dither mask and higher than that of the second dither mask.
According to such a printing control apparatus, it is possible to suppress density unevenness along the uneven shape of the platen while improving the graininess of the image. Further, it is possible to make the boundary between the image based on the data halftoned using the first dither mask and the image based on the data halftoned using the second dither mask inconspicuous. Accordingly, it is possible to further suppress the image quality degradation.

In this printing control apparatus, the platen is formed with a plurality of the concave portions having different lengths in the predetermined direction, and when the halftone process is performed, a portion of the print medium located in a certain concave portion is formed. For the corresponding input image data, the input image data corresponding to a portion of the print medium that is located in the recess having a shorter length in the predetermined direction than the certain recess using the second dither mask. On the other hand, a printing using a third dither mask having a characteristic that the dispersibility of dots in the gradation value in the predetermined range is lower than that of the first dither mask and higher than that of the second dither mask. It is a control device.
According to such a printing control apparatus, it is possible to suppress density unevenness along the uneven shape of the platen while improving the graininess of the image.

In this printing control apparatus, the first dither mask is a blue noise dither mask, and the second dither mask is configured to change the gradation value of the predetermined range by changing the arrangement of threshold values of the blue noise dither mask. The printing control apparatus is a mask in which the dispersibility of dots is adjusted.
According to such a printing control apparatus, it is possible to print an image with a small density change due to a deviation in the dot landing position while printing an image having as good graininess as possible on a portion of the printing medium located in the concave portion of the platen. In addition, the image quality difference between the image based on the halftone processed data using the first dither mask and the image based on the halftone processed data using the second dither mask is reduced, and the image boundary is reduced. It can be inconspicuous. Accordingly, it is possible to further suppress the image quality degradation.

In this print control apparatus, the second input is performed with respect to the input image data corresponding to the portion of the print medium located in the recess according to the amount of ink ejected to the print medium based on the input image data. A printing control apparatus that determines whether or not to use a dither mask.
According to such a printing control apparatus, when there is a large amount of ink discharged to the printing medium and there is a possibility of occurrence of a cockling phenomenon, an image with good graininess is formed on the portion of the printing medium located on the convex portion of the platen. While printing, it is possible to print an image with a small density change due to the deviation of the dot landing position on the portion of the print medium located in the concave portion of the platen. On the other hand, when the amount of ink discharged to the print medium is small and there is no risk of occurrence of a cockling phenomenon, the first dither mask is used for the input image data corresponding to the portion of the print medium located in the recess. It is possible to print an image with better graininess.

In this print control apparatus, the second dither is applied to the input image data corresponding to the portion of the print medium located in the recess according to the distance from the ink ejection surface of the head to the print medium. A print control apparatus that determines whether or not to use a mask.
According to such a printing control apparatus, when the distance from the ink ejection surface of the head to the printing medium is long and there is a possibility of occurrence of a cockling phenomenon, the granularity is generated in the portion of the printing medium located on the convex portion of the platen. While printing a good image, it is possible to print an image with a small change in density due to the deviation of the dot landing position on the portion of the print medium located in the concave portion of the platen. On the other hand, when the distance from the ink ejection surface of the head to the print medium is short and there is no risk of occurrence of a cockling phenomenon, the first dither mask is applied to the input image data corresponding to the portion of the print medium located in the recess. By using it, an image with better graininess can be printed.

A control device for a printing apparatus, comprising: a head that ejects ink onto a print medium; and a platen that supports the print medium and includes a plurality of convex portions arranged at intervals in a predetermined direction on a surface facing the head. The input image data having a gradation value indicating the density of an image to be printed on the print medium is positioned on the convex portion when performing halftone processing for converting the input image data into data indicating a pattern of dots constituting the image. For the input image data corresponding to the portion of the print medium, using the first dither mask, for the input image data corresponding to the portion of the print medium located in the concave portion between the convex portions, This is a program for executing the use of a second dither mask having a characteristic that the dispersibility of dots in the gradation value in a predetermined range is lower than that of the first dither mask.
According to such a program, there is a possibility that the dot landing position may be shifted by causing the printing apparatus to print an image with good graininess on the portion of the print medium located on the convex portion of the platen where the dot landing position is not likely to be shifted. An image having a small change in density due to the deviation of the dot landing position can be printed by the printing apparatus on the portion of the print medium located in the concave portion of the platen. Therefore, it is possible to suppress the density unevenness along the uneven shape of the platen while improving the graininess of the image, and to suppress the image quality deterioration of the image.

A printing method by a printing apparatus comprising: a head that ejects ink onto a printing medium; and a platen that supports the printing medium and has a plurality of convex portions arranged at intervals in a predetermined direction on a surface facing the printing medium. The input image data having a gradation value indicating the density of an image to be printed on the print medium is positioned on the convex portion when performing halftone processing for converting the input image data into data indicating a pattern of dots constituting the image. For the input image data corresponding to the portion of the print medium, a first dither mask is used, and for the input image data corresponding to the portion of the print medium located in the concave portion between the convex portions, Based on the data in which the second dither mask having the characteristic that the dispersibility of dots in the gradation value in the predetermined range is lower than that of the first dither mask, the printing apparatus performs the previous operation. A printing method characterized by printing an image on a printing medium.
According to such a printing method, an image with good graininess is printed on a portion of the printing medium located on the convex portion of the platen where the dot landing position is not likely to be shifted, and the dot landing position may be shifted. An image having a small change in density due to the deviation of the dot landing position can be printed on the portion of the print medium located in the recess. Therefore, it is possible to suppress the density unevenness along the uneven shape of the platen while improving the graininess of the image, and to suppress the image quality deterioration of the image.

=== First Embodiment ===
<Configuration of printing system>
FIG. 1 is a block diagram illustrating a configuration of a printing system in which an inkjet printer 1 (hereinafter referred to as a printer) as a “printing apparatus” and a computer 2 as a “printing control apparatus” are connected. 2A is a schematic cross-sectional view of the printer 1 viewed from the X direction (moving direction of the head 41), and FIG. 2B is a schematic plan view of the periphery of the platen 42 viewed from above. The printer 1 includes a controller 10, a transport unit 20, a carriage unit 30, a head unit 40, and a detector group 50. The controller 10 performs overall control of the printer 1, and includes an interface unit 11 that transmits and receives data to and from the computer 2, a CPU 12, a memory 13, and a unit control circuit 14. The detector group 50 monitors the situation in the printer 1 and outputs the detection result to the controller 10.

  The transport unit 20 feeds a print medium S on which an image is to be printed (for example, paper or cloth) to a printable position and transports it to the downstream side in the Y direction. And a roller 22. In FIG. 2A, continuous paper wound in a roll shape is exemplified as the print medium S, but the present invention is not limited to this, and cut paper may be used. The carriage unit 30 moves the head 41 mounted on the carriage 31 along the guide rail 32 in the X direction that is a direction intersecting the Y direction that is the conveyance direction of the print medium S (the direction that is orthogonal here). Is for.

  The head unit 40 includes a head 41 that ejects ink onto the printing medium S, and a platen 42 that supports the printing medium S from the back surface (the side opposite to the printing surface). On the lower surface of the head 41 (the surface facing the print medium S), a plurality of nozzle rows are formed in which openings of a large number of nozzles that eject ink are arranged at predetermined intervals in the Y direction. For example, yellow ink (Y), magenta ink (M), cyan ink (C), and black ink (K) are ejected from the plurality of nozzle rows, respectively. The ink discharge method from the nozzle may be a piezo method in which a voltage is applied to the drive element (piezo element) to expand and contract the ink chamber, or a bubble is generated in the nozzle using a heating element, A thermal method in which ink is ejected from the nozzles may be used.

  The platen 42 is a substantially rectangular parallelepiped member, and on the surface facing the head 41, a plurality of convex portions 422 extending in the Y direction beyond the length of the nozzle row are arranged at intervals in the X direction. That is, the surface of the platen 42 facing the head 41 has an uneven shape along the X direction. The concave portions 421 between the convex portions 422 also extend in the Y direction more than the length of the nozzle row, and the concave portions 421 receive ink that has been removed from the print medium S during borderless printing, whereby the print media S Dirt can be prevented.

  In the printer 1 having the above configuration, the ejection operation (pass) in which the head 41 ejects ink while moving in the X direction and the conveyance operation in which the print medium S is conveyed downstream in the Y direction are repeated. As a result, a two-dimensional image is printed on the print medium S.

  Then, in the computer 2 that is communicably connected to the printer 1, programs such as various application programs 60 and the printer driver 70 operate under the operating system installed in the computer 2, and computations according to the programs are performed on the computer 2. CPU (not shown) is executing. The printer driver 70 is a program for causing the computer 2 to realize a function of converting input image data input from the application program 60 into print data. The printer driver 70 includes a resolution conversion processing unit 71, a color conversion processing unit 72, and a half A tone processing unit 73 and a rasterization processing unit 74 are provided. The printer driver 70 is recorded on a recording medium readable by the computer 2 such as a CD-ROM, or can be downloaded to the computer 2 through various communication means such as the Internet.

  When the printer driver 70 (program) receives input image data to be printed from the application program 60, the resolution conversion processing unit 71 converts the resolution of the received input image data into the resolution used when the printer 1 prints. . The resolution-converted data is data represented in the RGB color space, and is data in which pixels having a size corresponding to the print resolution are arranged in two dimensions for each color. Each pixel has a multi-tone value (for example, 0 to 255) indicating the density of the pixel area which is an area on the print medium S corresponding to each pixel. In this embodiment, it is assumed that the larger the gradation value, the higher the density. The direction of data corresponding to the X direction in the printer 1 is also referred to as the X direction, and the direction of data corresponding to the Y direction in the printer 1 is also referred to as the Y direction. After the resolution conversion processing, the color conversion processing unit 72 refers to the color conversion table 75 and converts the data represented in the RGB color space into data corresponding to the ink color (YMCK) that can be ejected by the printer 1. To do.

  Next, the halftone processing unit 73 converts the multi-gradation data represented in the YMCK color space into data of the number of gradations that can be expressed by the printer 1, that is, data indicating the pattern of dots constituting the image. To do. For example, when an image is configured by whether or not dots are formed in each pixel area on the print medium S, the gradation value of each pixel is converted into data of two gradations, and the printer 1 has three types of sizes. When the dots can be formed, the gradation value of each pixel is converted into 4-gradation data. In this embodiment, it is assumed that halftone processing is performed by a “dither method” using a dither mask in which threshold values are two-dimensionally arranged. In the dither method, the halftone processing unit 73 repeatedly associates the data to be processed with, for example, a dither mask in order from the upper left part, and corresponds to the gradation value of the pixel constituting the input image data and the pixel. The threshold value of the dither mask is compared. When the gradation value of the pixel is larger than the dither mask threshold, the halftone processing unit 73 determines that a dot is generated in the pixel, and conversely, the pixel gradation value is equal to or smaller than the dither mask threshold. In this case, it is determined that no dot is generated in the pixel. Note that an error diffusion method may be performed in which the difference between the gradation value of the pixel being processed and the threshold value of the dither mask is diffused to the peripheral pixels of the pixel being processed while performing halftone processing using the dither mask.

  Finally, the rasterization processing unit 74 performs processing for rearranging the halftone-processed data in the order to be transferred to the printer 1. The data processed in this way is transmitted to the printer 1 as print data together with other data related to printing. The printer 1 prints an image on the print medium S based on the received print data.

<Concentration unevenness due to cockling phenomenon>
FIG. 3A is a diagram illustrating the arrangement of dots D1 and D2 formed in different passes. In general, in the printer 1, in order to improve the image quality of a printed image, a printing method (a so-called overlap printing method) in which a raster line that is a dot row along the X direction (movement direction of the head 41) is formed by two different nozzles, In order to increase the printing resolution in the Y direction (the conveyance direction of the print medium S), a printing method (a so-called interlaced printing method) in which raster lines are formed in a later pass between raster lines formed in a previous pass is performed. The When performing such a printing method, an image piece to be printed in a partial area of the print medium S is completed in a plurality of passes. In this embodiment, it is assumed that the image piece is completed in two passes, and as shown in FIG. 3A, the dot D1 formed in the previous pass and the dot D2 formed in the subsequent pass are in the X direction and Assume that they are arranged alternately in the Y direction.

  FIG. 3B is a diagram illustrating the arrangement of dots D1 and D2 formed based on data that has been halftoned with a blue noise dither mask, and FIG. 3C is based on data that has been halftoned with a pair dot dither mask. It is a figure explaining arrangement | positioning of the dots D1, D2 to be formed. When a light image is printed with low gradation value data, the rate at which dots are formed in a plurality of pixel regions constituting the image is low. 3B and 3C, dots D1 and D2 are formed in four pixel regions out of 6 × 6 pixel regions.

  In this embodiment, as described above, halftone processing is performed by a dither method using a dither mask. As shown in the left diagram of FIG. 3B, in a light image printed based on data that has been subjected to halftone processing using a blue noise dither mask with high dot dispersibility (good dot dispersibility), dots D1, D2 is not formed, but dots D1 and D2 are formed in distant pixel regions. By dispersing the dots D1 and D2 in this way, the graininess of the image can be improved. The blue noise dither mask has a noise characteristic in which the distribution of dots formed has a peak on the high frequency region side in the spatial frequency region in consideration of human visual characteristics that the sensitivity is low in the high frequency region. It is a mask with excellent dispersibility. For example, it is a mask having the largest frequency component in a high-frequency region where the length of one cycle is 2 pixels or less.

  However, even if the image is subjected to halftone processing with a blue noise dither mask, if the landing position deviation of dots occurs, the dots D1 and D2 that should originally be formed are adjacent as shown in the right diagram of FIG. 3B. Will be formed. In the printer 1, dots having a size larger than that of the pixel area are generally formed. Therefore, when dots are formed in adjacent pixel areas, the dots overlap each other, and the coverage, which is the ratio of the dots covering the print medium S, is reduced. Resulting in. As a result, the density of the image becomes light. In particular, in an image that has been halftone processed with a blue noise dither mask, the coverage of the print medium S is relatively high if no dot landing position deviation occurs. Therefore, the amount of change in the coverage of the printing medium S is large and the change in the density of the image is large depending on whether or not the dot landing position shift occurs.

  On the other hand, in the dither mask, the characteristic that the dispersibility of dots is lower than the blue noise dither mask in the gradation value in the range where the light image is printed, more specifically, the probability that dots are formed in adjacent pixel areas. There is a pair dot dither mask having a characteristic that becomes higher than the blue noise dither mask (details will be described later). In a light image that has been halftone processed with the pair dot dither mask, dots D1 and D2 are formed in adjacent pixel regions as shown in the left diagram of FIG. Therefore, even if a dot landing position shift occurs, there is a high probability that the dots D1 and D2 overlap each other as shown in the right diagram of FIG. 3C. In other words, when using a paired dot dither mask, the graininess of the image will be lower than when using a blue noise dither mask, but when the dot landing position shift occurs and when it does not occur. The amount of change in the coverage of the print medium S is small, and the change in image density can be reduced.

  FIG. 4 is a diagram for explaining the displacement of the landing positions of dots due to the cockling phenomenon. As described above, the surface of the platen 42 facing the head 41 has an uneven shape. Therefore, before the ink is ejected onto the printing medium S or when the ink ejection amount is small, the printing medium S1 is supported by the platen 42 in a flat state as indicated by the solid line S1. However, when the amount of ink ejected to the print medium S increases and the print medium S that has absorbed the ink swells, as indicated by the alternate long and short dash line S2, the portion of the print medium S2 supported by the convex portion 422 of the platen 42 is provided. On the other hand, the portion of the print medium S2 located in the recess 421 is bent downward (on the side opposite to the head 41). That is, a phenomenon in which the print medium S deforms in a wave shape along the uneven shape of the platen 42, that is, a so-called cockling phenomenon occurs. In particular, when the printing medium S is sucked and attracted to the platen 42 in order to prevent the positional deviation of the printing medium S during printing, the cockling phenomenon is more likely to occur.

  Even if the cockling phenomenon occurs, the distance from the nozzle opening surface of the head 41 to the printing medium S, that is, the so-called paper gap does not change at the portion of the printing medium S supported by the convex portion 422. On the other hand, in the part of the printing medium S located in the concave portion 421, the paper gap PG changes due to the cockling phenomenon. Therefore, the dot landing position shift does not occur in the portion of the print medium S supported by the convex portion 422, but the dot landing position shift occurs in the portion of the print medium S positioned in the concave portion 422.

  Here, it is assumed that halftone processing is performed using only a blue noise dither mask having good dot dispersibility regardless of the uneven shape of the platen 42. In this case, in the portion of the print medium S supported by the convex portion 422, the paper gap does not change even when the amount of ink ejected to the print medium S increases, and the landing position deviation of the dots does not occur. An image is printed at the density shown. On the other hand, at the portion of the print medium S located in the recess 421, the paper gap changes as the amount of ink ejected to the print medium S increases. Therefore, in the first pass, the dot D1 is formed in the pixel area indicated by the print data in a state where the print medium S is not bent, whereas in the latter pass, printing is performed by the ink ejected in the previous pass. The medium S bends and dots D2 are formed at positions deviated from the pixel area indicated by the print data. As a result, in the portion of the printing medium S located in the concave portion 421, the dot D1 of the previous pass that should be originally formed and the dot D2 of the subsequent pass are formed so as to overlap with each other, and the coverage is reduced. An image that is lighter than that of 422 is printed. That is, uneven density along the uneven shape of the platen 42 occurs in the printed image.

  Conversely, it is assumed that halftone processing is performed using only a pair dot dither mask regardless of the uneven shape of the platen 42. In this case, even if the dot landing position shift occurs in the portion of the print medium S located in the concave portion 421 due to the cockling phenomenon, the amount of change in the coverage of the print medium S is small, and the change in image density can be reduced. it can. Therefore, it is possible to suppress the occurrence of uneven density along the uneven shape of the platen 42. However, the graininess of the image printed on the portion of the print medium S located on the convex portion 422 where the dot landing position shift does not occur due to the cockling phenomenon is unnecessarily reduced.

  In view of this, the present embodiment aims to suppress the deterioration of the image quality of an image printed on the print medium S supported by the platen 42 having an uneven shape.

<Halftone processing>
FIG. 5 is a flow of halftone processing in the first embodiment. In the first embodiment, when the halftone processing unit 73 performs the halftone process on the input image data after the color conversion process, the input image corresponding to the portion of the print medium S located on the convex portion 422 of the platen 42. A blue noise dither mask (first dither mask) is used for the data, and a pair dot dither mask (second dither mask) is applied to the input image data corresponding to the portion of the print medium S located in the recess 421 of the platen 42. Mask). For this purpose, a blue noise dither mask 76 and a pair dot dither mask 77 are stored in the memory in the computer 2 for creating print data, as shown in FIG.

  As described above, since the portion of the print medium S positioned in the concave portion 421 of the platen 42 is bent by the ink ejected in the previous pass as the later pass, the dots formed in the previous pass and the rear The positional relationship of the dots formed in the pass is shifted, and the dots easily overlap each other. Therefore, in the pair dot dither mask of this embodiment, compared to the blue noise dither mask, the gradation value in the range in which a light image is printed, for example, the low gradation value 1 out of 256 gradation values (0 to 255). ~ 127, the probability that a dot formed in a different pass occurs in an adjacent pixel, that is, the probability that a dot formed when the deflection amount of the print medium S located in the recess 421 is different in an adjacent pixel (pair Dot generation probability) is increased and dot dispersibility is decreased. In this embodiment, as shown in FIG. 3A, a printing method in which dots D1 formed in the previous pass and dots D2 formed in the subsequent pass are alternately arranged in the X direction and the Y direction is taken as an example. . Therefore, if dots occur in either the target pixel in the data or any of the four pixels arranged vertically and horizontally among the eight pixels around the target pixel, dots formed in different passes are generated in adjacent pixels. It will be. Hereinafter, the four pixels above, below, left, and right of the target pixel are also referred to as adjacent pixels of the target pixel. It should be noted that in the gradation value for printing a dark image, the pair dot occurrence probability increases even when the blue noise dither mask is used. Therefore, the pair dot occurrence probability is adjusted in the gradation value for printing a light image. In addition, the range of gradation values (the gradation value of a predetermined range) for adjusting the pair dot occurrence probability (dot dispersibility) is not limited to the gradation values 1 to 127.

The paired dot dither mask of the present embodiment has a pixel of interest and an adjacent pixel of the pixel of interest within a range of gradation values 1 to 127 when the dot occurrence probability of each pixel is k (0 ≦ k ≦ 1). And the target value of the probability of occurrence of dots (paired dot occurrence probability) K is 0.8 × k ² . For example, if the gradation value indicated by the pixel corresponds to the dot occurrence probability k of each pixel and the gradation value of the data to be halftone processed is a uniform value 26, the dot occurrence probability k of each pixel is k = 26. /255≈0.1, and the pair dot occurrence probability K = 0.8 × k ² ≈0.008. Thus, the reason why the paired dot occurrence probability K is set to a value close to k ² is that, for example, even if the dots are formed as far as possible in the predetermined direction with a blue noise dither mask, This is because the probability that a dot is formed in a pixel region adjacent in a predetermined direction converges to k ² when the value becomes sufficiently large. This is because when the two pixels are sufficiently separated on the data, the correlation between the occurrence of dots in both pixels decreases, so the probability of dots occurring in both pixels is the same as when dots are randomly generated. Similarly, this is because the value k ² is simply multiplied by the dot occurrence probability k of each pixel. Accordingly, if the pair dot occurrence rate K is set to be close to k ² in a state where there is no dot landing position deviation, the amount of change in the pair dot occurrence probability K regardless of the dot landing position deviation that occurs. Can be reduced. As a result, the amount of change in the coverage of the print medium S can be reduced, and the amount of change in image density can be reduced. Note that the coefficient of k ^{2 in} the pair dot occurrence probability K (= 0.8 × k ² ) is not limited to 0.8, but the coefficient is preferably 0.6 or more and 1.4 or less, and a value close to 1 The more the amount of change in the coverage of the printing medium S due to the deviation of the dot landing position is more reliably suppressed.

  In the present embodiment, halftone processing is performed using the above-described pair dot dither mask. Specifically, as shown in FIG. 5, the halftone processing unit 73 determines a target pixel for converting a gradation value from the input image data that is a target of the halftone process (S001), and the target pixel Is a pixel on the convex portion 422 of the platen 42. That is, it is determined whether or not the pixel of interest is data for forming a dot on a portion of the print medium S located on the convex portion 422 (S002). Note that, based on information indicating the positional relationship in the X direction between the printing medium S and the unevenness of the platen 42 and information indicating the positional relationship in the X direction between the printing medium S and the target pixel, the target pixel is located on the convex portion 422. It can be determined whether or not it is a pixel.

  When the target pixel is a pixel on the convex portion 422 (S002 → Yes), the halftone processing unit 73 performs dot on / off determination of the target pixel using the blue noise dither mask (S003). That is, by comparing the gradation value of the target pixel with the threshold value of the blue noise dither mask corresponding to the target pixel, it is determined whether or not to form a dot in the pixel region on the print medium S corresponding to the target pixel. The multi-gradation gradation value of the pixel of interest is converted into a low gradation gradation value that can be expressed by the printer 1. On the other hand, when the pixel of interest is a pixel on the recess 421 (S002 → No), the halftone processing unit 73 performs dot on / off determination of the pixel of interest using a pair dot dither mask (S004). The above processing is repeated until the processing of all the pixels belonging to the input image data is completed (S005).

  By performing the halftone process as described above, the coverage of the print medium S can be reduced even if a dot landing position shift occurs due to a cockling phenomenon at a portion of the print medium S located in the concave portion 421 of the platen 42. An image with a small change amount, that is, an image with a small density change can be printed. On the other hand, an image with good graininess can be printed on the portion of the print medium S located on the convex portion 422 of the platen 42 where there is no possibility of the dot landing position shift due to the cockling phenomenon. That is, it is possible to print an image having as good a granularity as possible while suppressing the occurrence of uneven density along the uneven shape of the platen 42.

<How to create a paired dot dither mask>
FIG. 6 is a flowchart showing a method of creating a pair dot dither mask. Hereinafter, a method for creating a pair dot dither mask according to the present embodiment will be described. First, a blue noise dither mask having a high dot dispersibility and the same size as the paired dot dither mask to be created, for example, a storage area having a size of 64 × 64 in the X direction × Y direction is from 0 to 254. A blue noise dither mask storing threshold values is prepared (S101).

  The length of the blue noise dither mask to be prepared in the X direction, that is, the length of the paired dot dither mask in the X direction may be set to a length corresponding to the length of the concave portion 421 of the platen 42 in the X direction. Specifically, the number of pixels arranged in the X direction in the data for printing an image on the print medium S on the concave portion 421 may be an integer multiple of the number of thresholds arranged in the X direction in the dither mask. This is because the image density is adjusted by adjusting the dot generation rate for each pixel group having a size corresponding to the dither mask, and therefore an integer number of data for printing the image on the print medium S on the recess 421 is used. This is because an image having a density closer to the gradation value indicated by the pixel can be printed by assigning the paired dot dither mask. Similarly, the length in the X direction of the blue noise dither mask used for the data on the convex portion 422 of the platen 42 may be set to a length corresponding to the length of the convex portion 422 in the X direction. In this case, for example, if the length of the concave portion 421 in the X direction is an integral multiple of the length of the convex portion 422 in the X direction, the blue noise dither mask and the paired dot dither mask can be made the same size. Then, an integer number of blue noise dither masks can be assigned even when a blue noise dither mask is used for data on the recess 421 as in the fourth embodiment described later.

  Next, the prepared blue noise dither mask (working dither mask) is generated in adjacent pixels that are formed in different passes for each gradation value (1 to 127) in the range in which the pair dot occurrence probability K is adjusted. The number of paired dots to be counted is counted (S102). Specifically, the data is the same size as the current dither mask (here, 64 × 64 pixel group), and the gradation value is a uniform value (any one of 1 to 127). The number of paired dots is counted when halftone processing is performed with a dither mask in operation. At this time, while sequentially moving the pixel of interest from the upper left to the lower right of the data after the halftone process, for example, a position where a dot occurs between the pixel of interest and an adjacent pixel on the right side in the X direction of the pixel of interest The number of dots is counted, and the location where dots occur in the target pixel and the adjacent pixel on the lower side in the Y direction of the target pixel is counted as the number of lower adjacent pair dots. By doing so, the number of paired dots can be counted without overlapping. Note that the number of pair dots formed by the adjacent pixel on the left side in the X direction or the adjacent pixel on the upper side in the Y direction with respect to the target pixel may be counted.

Next, the number of right adjacent pair dots and the number of lower adjacent pair dots in each gradation value (1 to 127) are compared with the target range of the number of pair dots (S103). As described above, in the pair dot dither mask of this embodiment, the target value of the pair dot occurrence probability K is set to 0.8 × k ² (k = dot generation probability of each pixel). Therefore, the target value H for the number of paired dots in the 64 × 64 pixel group is H = 0.8 × k ² × 64 × 64. Since k varies depending on the gradation value, the target value H is obtained for each gradation value (1 to 127). The target value H for each gradation value having a width (for example, ± 20%) is set as the target range for the number of paired dots in each gradation value.

  As a result of the comparison, if at least one of the number of right adjacent pair dots and the number of lower adjacent pair dots within each gradation value is within the target range of the number of pair dots (S103 → Yes), the dither mask being worked on is a pair dot The process ends when it is completed as a dither mask. On the other hand, if the number of right adjacent pair dots and the number of lower adjacent pair dots in each gradation value are not within the target range of the number of pair dots for each gradation value (S103 → No), An appropriate number of threshold values (for example, two threshold values) are randomly changed, and the number of right adjacent pair dots and the number of lower adjacent pair dots in each gradation value (1 to 127) are counted again (S104). In S103, it may be determined whether the total number of the right adjacent pair dots and the lower adjacent pair dots is within the target range, or the right adjacent pair dots and the lower adjacent pair. Even when only one of the number of dots falls within the target range, the process may proceed to the step of replacing the threshold (S104). Further, since the pair dot occurrence probability K is adjusted in the range of gradation values 1 to 127, at least one of the threshold values to be replaced is preferably a threshold value within the range of gradation values. Further, since the number of paired dots changes only within the range of the gradation value corresponding to the replaced threshold value, for example, when the threshold value p and q (p <q) are replaced, a pair of gradation values p to q Re-count the number of dots.

After the threshold value is changed, it is determined whether or not the pair dot characteristic is improved (S105). Whether or not the paired dot characteristics are improved can be determined, for example, as in the following (A) to (C).
(A) If the number of right adjacent pair dots and the number of lower adjacent pair dots that have been recounted are both close to k ² , it is determined that the number has been improved.
(B) If one of the re-counted number of right adjacent pair dots and the number of lower adjacent pair dots approaches k ² and the other has not changed, it is determined that the number has improved.
(C) When the re-counted gradation value range (p to q) is improved or partially deteriorated, the number of pair dots generated at each gradation value (p to q) and the target value H If the sum of the differences is small, it is judged that the improvement has been made.
If the above determination is made and the paired dot characteristics are not improved (S105 → No), the process returns to step S104 where the threshold is changed.

  On the other hand, if the paired dot characteristics are improved (S105 → Yes), it is next determined whether there is no problem with the graininess characteristics (S106). This is because the initially prepared blue noise dither mask has high graininess, but the graininess of the dither mask being worked on is lowered by changing the threshold value. Specifically, a granularity index target range that is acceptable from the viewpoint of human visual characteristics is set, and when the granularity index of the dither mask being worked on is within the target range or before changing the threshold When the graininess index is improved, it is better to determine that there is no problem with the graininess characteristics.

Since the graininess index is a value determined for each tone value, it is preferable to compare the graininess index for each tone value with the target range for each tone value. Further, since the granularity index is a known technique (for example, Japanese Patent Application Laid-Open No. 2007-15359), detailed description is omitted, but the power spectrum FS obtained by Fourier transforming the image to obtain the power spectrum FS is obtained. Is an index obtained by integrating each spatial frequency with a weight corresponding to the sensitivity characteristic VTF (Visual Transfer Function) with respect to the visual spatial frequency possessed by humans. It can be said that. A typical empirical formula for obtaining the sensitivity characteristic VTF is shown in Formula (1). The variable L represents the observation distance, and the variable u represents the spatial frequency. The graininess index can be calculated by equation (2) based on VTF. The coefficient τ is a coefficient for matching the obtained value with human senses.
VTF (u) = 5.05exp (-0.138πLu / 180) {1-exp (-0.1πLu / 180)}… Formula (1)
Granularity index = τ∫FS (u) · VTF (u) du (2)

  If there is a problem with the graininess characteristic (S106 → No), the process returns to step S104 where the threshold is changed. On the other hand, if the pair dot characteristics are improved and there is no problem with the graininess characteristics (S106 → Yes), the process returns to step S103. If at least one of the right adjacent pair dot number and the lower adjacent pair dot number is within the target range of the pair dot number (S103 → Yes), the working dither mask is completed as a pair dot dither mask. The process ends.

  As described above, in the present embodiment, a pair dot dither mask is created based on the blue noise dither mask. In other words, by replacing the threshold value of the mask having the same blue noise characteristics as the dither mask used for the data for printing the image on the print medium S positioned on the convex portion 422 of the platen 42, the level of the range where the light image is printed is changed. A pair dot dither mask in which the dot dispersibility (pair dot occurrence probability K) in the tone values (1 to 127) is adjusted is created. Therefore, a pair dot dither mask having the characteristics of suppressing the density change of the image with respect to the deviation of the landing position of the dots and having a dot dispersibility close to that of a blue noise dither mask, for example, as compared with the case where the threshold value is set by a random number. Can be created. Even if the dither mask to be used is switched according to the uneven shape of the platen 42, the image printed on the print medium S positioned in the concave portion 421 of the platen 42 and the print medium S positioned in the convex portion 422 are printed. It is possible to reduce the image quality difference and make the boundary of the image less noticeable.

  However, the present invention is not limited to creating a pair dot dither mask from a blue noise dither mask, but a dither mask having other characteristics, for example, a pair dot dither mask from a green noise dither mask, Alternatively, a pair dot dither mask may be created from a dither mask having different characteristics from the dither mask used.

  Alternatively, a pair dot dither mask may be created from scratch. Specifically, the threshold values are arranged in order from the smaller side or the larger side. At that time, while changing the arrangement position of the threshold value of interest with respect to the already arranged threshold value, the evaluation value such as the number of pair dots and the graininess index is calculated, and the evaluation value becomes the optimum position. A threshold of interest is arranged.

  In addition, when the dot of the subsequent pass is displaced in a certain direction with respect to the dot of the previous pass, for example, the right side in the X direction, the dot of the subsequent pass adjacent to the right of the previous pass in the X direction. Moves away from the dots in the previous pass (increases the coverage), and the dots in the subsequent pass adjacent to the left in the X direction of the dots in the previous pass approach the dots in the previous pass (decrease the coverage). Therefore, when creating a pair dot dither mask, the number of pair dots adjacent to the dot in the subsequent pass on the direction side (right side in the X direction) where the landing position deviation occurs with respect to the dot in the previous pass, and the dot in the previous pass On the other hand, in consideration of the difference with the number of paired dots adjacent to the dots in the subsequent pass on the opposite side (left side in the X direction) to the direction in which the landing position shift occurs, a paired dot dither mask may be created. . By doing so, the amount of change in the coverage of the print medium S with respect to the deviation of the dot landing position can be further reduced.

  Moreover, although the case where the printing method shown to FIG. 3A is implemented is mentioned as an example in the said embodiment, it is not restricted to this. If the printing method is different, adjacent pixels where dots of different passes from the pixel of interest are generated are different. For example, in a printing method in which a raster line composed only of dots in the first pass and a raster line composed only of dots in the subsequent pass are arranged alternately in the Y direction, adjacent to the top and bottom of the eight pixels around the pixel of interest Four pixels diagonally adjacent to the two pixels are adjacent pixels. In the above embodiment, the probability of occurrence of dots formed in different passes in adjacent pixels is adjusted. However, the present invention is not limited to this, and the pass in which dots are formed is not considered, and dots are simply adjacent. It is only necessary to adjust the probability that the pixel occurs. In that case, a common pair dot dither mask can be used regardless of the printing method. In the above embodiment, the probability that a dot is generated in a pixel adjacent to the pixel of interest is adjusted. However, the present invention is not limited to this, and in addition to the pixel adjacent to the pixel of interest, a pixel ( You may make it adjust the probability that a dot will generate | occur | produce in the vicinity pixel of a focused pixel.

=== Second Embodiment ===
FIG. 7A is a diagram illustrating a region where the concave portion 421 of the platen 42 is partitioned, and FIG. 7B is a flow of halftone processing according to the second embodiment. When the amount of ink ejected to the printing medium S increases, the portion of the printing medium S located in the recess 421 bends downward, but compared to the printing medium S located in the central area A1 in the X direction in the depression 421. Therefore, the printing medium S located in the end region A2 on the convex portion 422 side has a smaller amount of deflection (L1> L2). Therefore, in the concave portion 421, the end region A2 has a smaller change amount of the paper gap due to the occurrence of the cockling phenomenon and the dot landing position shift is smaller than the central region A1. Therefore, the end region A2 can suppress the change in the image density due to the cockling phenomenon even when the dot dispersibility is increased, that is, the pair dot occurrence probability is reduced, as compared with the central region A1.

  Therefore, in the second embodiment, when the halftone processing unit 73 performs the halftone processing, the blue noise dither mask is applied to the input image data corresponding to the portion of the print medium S located on the convex portion 422 of the platen 42. (First dither mask) is used, and a pair dot dither mask (second dither mask) is applied to the input image data corresponding to the portion of the print medium S located in the central area A1 in the X direction of the concave portion 421 of the platen 42. Dither mask), blue noise dither is applied to the input image data corresponding to the portion of the print medium S located in the end region A2 on the convex portion 422 side in the X direction with respect to the central region A1. An intermediate dither mask (third dither mask) having intermediate characteristics between the mask and the pair dot dither mask is used.

The intermediate dither mask has a characteristic that the dot dispersibility is lower than that of the blue noise dither mask and higher than that of the pair dot dither mask in a gradation value (for example, 1 to 127) in a range where a light image is printed. This is a mask having a characteristic that the occurrence probability K is higher than that of the blue noise dither mask and lower than that of the pair dot dither mask. Such an intermediate dither mask can be created in the same manner as the pair dot dither mask. At the time of creating the pair dot dither mask, the target value of the pair dot occurrence probability K is set to, for example, 0.8 × k ^2. However, at the time of creating the intermediate dither mask, the target value of the pair dot occurrence probability may be reduced. That is, the coefficient multiplied by k ² may be set to 0.6 or 0.7, for example, which is smaller than 0.8.

  Then, as shown in FIG. 7B, the halftone processing unit 73 determines a target pixel from the image data to be processed (S201), and determines whether or not the target pixel is a pixel on the convex portion 422. (S202). When the pixel of interest is a pixel on the convex portion 422 (S202 → Yes), the halftone processing unit 73 performs dot on / off determination of the pixel of interest using a blue noise dither mask (S203). On the other hand, when the target pixel is a pixel on the concave portion 421 (S202 → No), the halftone processing unit 73 determines whether or not the target pixel is a pixel on the central region A1, and the target pixel is the central portion. If it is a pixel on the area A1 (S204 → Yes), the on / off determination of the dot of the target pixel is performed using the pair dot dither mask (S205), and if the target pixel is a pixel on the end area A2 (S204) → No), the dot on / off of the pixel of interest is determined by the intermediate dither mask (S206).

  By doing so, an image with good graininess is printed on the print medium S located on the convex portion 422 of the platen 42, and the dot landing position shift is caused on the print medium S located in the central area A1 of the concave portion 421. An image that can more reliably suppress the density change due to the ink is printed, and an image that can suppress the density change due to the deviation of the landing position of the dots is printed on the print medium S that is positioned in the end area A2 of the concave portion 421. An image with better graininess can be printed compared to the area A1. That is, it is possible to print an image having as good a granularity as possible while suppressing unevenness in density along the uneven shape of the platen 42. Further, an image based on the data halftoned by the blue noise dither mask (image on the convex portion 422) and an image based on the data halftoned by the pair dot dither mask (on the central region A1 of the concave portion 421). An image having characteristics intermediate between the two images can be printed between the two images and the boundary between the two images can be made inconspicuous. Therefore, the second embodiment can print a higher quality image than the first embodiment. However, the first embodiment can reduce the memory capacity in which the computer 2 stores the dither mask compared to the second embodiment, and can facilitate the halftone process.

=== Third Embodiment ===
FIG. 8A is a diagram for explaining the platen 42 of the third embodiment, and FIG. 8B is a flow of halftone processing of the third embodiment. As described above, the concave portion 421 of the platen 42 plays a role of receiving ink that is removed from the end portion in the X direction of the print medium S during borderless printing. On the other hand, the printer 1 prints print media S of various sizes, but the interval between the X-direction end portions of the print media S of each size is not always constant. Therefore, as shown in FIG. 8A, the convex portions 421 and the concave portions 422 may not be arranged at regular intervals in the X direction, and a plurality of concave portions 421 having different lengths in the X direction may be formed on the platen 42.

  For example, as shown in FIG. 8A, it is assumed that a wide concave portion 421a having a long length W1 in the X direction and a narrow concave portion 421b having a short length W2 in the X direction are formed on the platen 42. Compared with the wide concave portion 421a, the narrow concave portion 421b has a smaller deflection of the print medium S when ink is ejected (L3> L4). For this reason, the narrow concave portion 421b has a smaller change amount of the paper gap due to the cockling phenomenon and the dot landing position deviation is smaller than the wide concave portion 421a. For this reason, in the narrow concave portion 421b, even if the dot dispersibility is increased, that is, the pair dot occurrence probability is reduced, the change in the image density due to the cockling phenomenon can be suppressed as compared with the wide concave portion 421a.

  Therefore, in the third embodiment, when the halftone processing unit 73 performs the halftone processing, the paired dots are applied to the input image data corresponding to the portion of the print medium S located in the wide recess 421a (a certain recess). A blue noise dither mask and a pair dot dither mask are used for input image data corresponding to a portion of the print medium S located in the narrow concave portion 421b having a shorter length in the X direction than the wide concave portion 421a. An intermediate dither mask having intermediate characteristics is used. The intermediate dither mask is the same mask as the intermediate dither mask of the second embodiment.

  Specifically, as shown in FIG. 8B, the halftone processing unit 73 determines a target pixel from the image data to be processed (S301), and the target pixel is a pixel on the convex portion 422. (S302 → Yes), the dot of the pixel of interest is determined on / off using the blue noise dither mask (S303). On the other hand, when the target pixel is a pixel on the recess 421 (S302 → No), the halftone processing unit 73 determines whether the target pixel is a pixel on the wide recess 421a, and the target pixel is the wide recess 421a. If it is the upper pixel (S304 → Yes), the dot on / off determination of the pixel of interest is performed using the pair dot dither mask (S305), and if the pixel of interest is a pixel on the narrow recess 421b (S304 → No), dot on / off determination of the pixel of interest is performed using the intermediate dither mask (S306).

  By doing so, an image with good graininess is printed on the print medium S positioned on the convex portion 422 of the platen 42, and the density change due to the deviation of the dot landing position is applied to the print medium S positioned on the wide concave portion 421b. An image that can be suppressed more reliably is printed, and the print medium S positioned in the narrow concave portion 421b is printed with an image that can suppress a change in density due to a deviation in the landing position of dots, and has a graininess compared to the wide concave portion 421b. A good image can be printed. That is, it is possible to print an image having as good a granularity as possible while suppressing unevenness in density along the uneven shape of the platen 42. When there are three or more types of lengths in the X direction of the recesses 421, for example, if the length in the X direction of the corresponding recesses 421 corresponding to the pixel of interest is equal to or greater than a threshold value, a pair dot dither mask is used. If the length in the X direction is less than the threshold, an intermediate dither mask may be used.

  In the third embodiment, as in the second embodiment, the wide concave portion 421a is divided into a central region and an end region in the X direction, and corresponds to the print medium S located in the central region of the wide concave portion 421a. A pair dot dither mask may be used for the input image data, and an intermediate dither mask may be used for the input image data corresponding to the print medium S located in the end region of the wide recess 421a.

=== Fourth Embodiment ===
FIG. 9A is a diagram for explaining the peripheral region of the pixel of interest, and FIG. 9B is a flow of halftone processing according to the fourth embodiment. If the amount of ink ejected to the printing medium S is small, it is difficult for the portion of the printing medium S located in the concave portion 421 of the platen 42 to be bent, and the landing position deviation of the dots hardly occurs. Therefore, in the fourth embodiment, a pair dot dither mask is used for input image data corresponding to a portion of the print medium S located in the concave portion 421 of the platen 42 according to the amount of ink ejected to the print medium S. Determine whether or not.

  Specifically, as shown in FIG. 9B, the halftone processing unit 73 determines a target pixel from the image data to be processed (S401), and when the target pixel is a pixel on the convex portion 422. Performs on / off determination of the dot of the pixel of interest using the blue noise dither mask (S403). On the other hand, when the target pixel is a pixel on the concave portion 421 (S402 → No), the halftone processing unit 73 performs pairing when the average gradation value in the peripheral region of the target pixel is equal to or greater than the threshold (S404 → Yes). The dot on / off determination of the pixel of interest is performed using the dot dither mask (S405), and if it is less than the threshold (S404 → No), the dot on / off determination of the pixel of interest is performed using the blue noise dither mask (S403). That is, if the amount of ink ejected around the area of the print medium S corresponding to the pixel of interest is large enough to cause a cockling phenomenon, a pair dot dither mask is used, and there is no possibility of the cockling phenomenon occurring. If less, use a blue noise dither mask.

  By doing so, even if the print medium S is located in the concave portion 421 of the platen 42, an image with good graininess is printed in an area where the cockling phenomenon is not likely to occur, and the cockling phenomenon may occur. In the region, an image capable of suppressing a change in density due to a deviation in dot landing position can be printed. That is, it is possible to print an image having as good a granularity as possible while suppressing unevenness in density along the uneven shape of the platen 42.

  It is assumed that the peripheral area of the target pixel is set, for example, as an area in the recess 421 to which the target pixel belongs. In that case, as shown in FIG. 9A, on the image data, pixels up to the number of pixels corresponding to half the length of the concave portion 421 in the Y direction (N1 pixels) are respectively located above and below in the Y direction from the target pixel. ) And pixels (N2) belonging to the pixel from the target pixel to the left convex portion 422a in the X direction and pixels (N3) belonging to the pixel from the target pixel to the right convex portion 422b in the X direction (N3). Switch the dither mask to be used. However, the peripheral area of the target pixel is not limited to the above range, and may be set to an area where ink is ejected in a pass in which the target pixel is printed, or the print medium S is divided in the X direction or the Y direction. An area may be set.

  Also, the amount of ink discharged to the entire print medium S is compared with a threshold value, and if the amount of ink discharged is so small that the cockling phenomenon does not occur, the blue noise dither mask is used regardless of the uneven shape of the platen 42. You may make it do. By doing so, the granularity of the entire printed image can be improved, and since it is not necessary to determine the dither mask to be used for each pixel of interest, halftone processing can be facilitated. On the other hand, if the amount of ink ejected to the entire print medium S is large enough to cause a cockling phenomenon, the dither mask is switched and used in accordance with the uneven shape of the platen 42. By doing so, it is possible to print an image with as good a graininess as possible while suppressing unevenness in density along the uneven shape of the platen 42.

=== Fifth Embodiment ===
FIG. 10 is a halftone process flow according to the fifth embodiment. In general, the printer 1 prints an image on a plurality of types of print media S having different thicknesses. When the thickness of the print medium S is thin and the paper gap is wide, the print medium S located in the concave portion 421 of the platen 42 is easily bent and the landing position deviation of the dots is likely to occur, but the print medium S is thick. When the paper gap is narrow, even the printing medium S located in the concave portion 421 of the platen 42 is difficult to bend and the landing position deviation of the dots is difficult to occur. Also, the smaller the paper gap, the smaller the landing position deviation of dots. Therefore, in the fifth embodiment, input image data corresponding to a portion of the print medium S located in the concave portion 421 of the platen 42 according to the platen gap (distance from the ink ejection surface of the head 41 to the print medium S). To determine whether to use a pair dot dither mask.

  Specifically, as shown in FIG. 10, the halftone processing unit 73 acquires information about a paper gap when printing is performed based on image data to be processed, and the paper gap is less than a threshold (S501). (Yes), the target pixel is determined from the image data to be processed (S502), and the process of determining the dot of the target pixel on / off using the blue noise dither mask is repeated regardless of the uneven shape of the platen 42 (S503). . On the other hand, when the paper gap is equal to or greater than the threshold (S501 → No), the halftone processing unit 73 determines a target pixel from the image data to be processed (S505), and the target pixel is a pixel on the convex portion 422. (S506), if the pixel of interest is a pixel on the convex portion 422 (S506 → Yes), the on / off determination of the dot of the pixel of interest is performed using the blue noise dither mask (S507), If the pixel of interest is a pixel on the recess 421 (S506 → No), on / off determination of the dot of the pixel of interest is performed using the pair dot dither mask (S508).

  By doing so, when the paper gap is small and there is little risk of occurrence of a cockling phenomenon, the granularity of the entire printed image can be enhanced, and it is not necessary to determine the dither mask to be used for each pixel of interest. Halftone processing can be facilitated. On the other hand, when the paper gap is large and the cockling phenomenon may occur, the density unevenness along the uneven shape of the platen 42 is suppressed by switching the dither mask used according to the uneven shape of the platen 42. An image with good graininess can be printed as much as possible.

  The paper gap may be compared with two threshold values (a first threshold value and a second threshold value smaller than the first threshold value). If the paper gap is equal to or larger than the first threshold, the blue noise dither mask and the paired dot dither mask are switched and used in accordance with the uneven shape of the platen 42, and the paper gap is less than the first threshold and the second If it is equal to or greater than the threshold value, a blue noise dither mask and an intermediate dither mask (a mask having intermediate characteristics between the blue noise dither mask and the pair dot dither mask) are used by switching according to the uneven shape of the platen 42, and the paper gap is first. If it is less than the threshold value, a blue noise dither mask may be used regardless of the uneven shape of the platen 42.

=== Modification ===
In the above embodiment, the computer 2 (printing control apparatus) connected to the printer 1 performs halftone processing and creates print data, but the present invention is not limited to this. The controller 10 in the printer 1 may perform halftone processing and create print data. In this case, in the printer 1, the controller 10 corresponds to the print control device, and the part (head unit 40, transport unit 20, etc.) that performs printing corresponds to the printing device.

  In the above embodiment, the operation of ejecting ink while the head 41 moves in the X direction and the operation of transporting the print medium S in the Y direction are described as an example of the printer 1, but the present invention is not limited thereto. Absent. For example, it may be a printer in which the print medium S is conveyed in the Y direction under a fixed head 41 extending in the X direction more than the width of the print medium S. Even in such a printer, for example, when printing an image with a plurality of heads 41 arranged in the Y direction, when ink is ejected from the downstream head 41 in the Y direction, the ink ejected from the upstream head 41 is used. Since there is a possibility that a cockling phenomenon has occurred in the print medium S, the present invention is effectively applied.

  As mentioned above, the said embodiment is for making an understanding of this invention easy, and is not for limiting and interpreting this invention. The present invention can be changed and improved without departing from the gist thereof, and it is needless to say that the present invention includes equivalents thereof.

1 printer, 2 computers, S print media, 10 controllers,
11 Interface section, 12 CPU, 13 Memory, 14 Unit control circuit, 20 Transport unit, 21 Paper feed roller, 22 Paper discharge roller,
30 Carriage unit, 31 Carriage, 32 Guide rail,
40 head units, 41 heads, 42 platens, 421 concave portions, 422 convex portions,
50 detector groups, 60 application programs,
70 printer driver, 71 resolution conversion processing unit, 72 color conversion processing unit,
73 halftone processing unit, 74 rasterization processing unit, 75 color conversion table,
76 Blue Noise Dither Mask, 77 Pair Dither Mask

Claims (8)

A printing control apparatus for controlling a printing apparatus comprising: a head that ejects ink onto a printing medium; and a platen that supports the printing medium and has a plurality of convex portions arranged at intervals in a predetermined direction on a surface facing the printing medium Because
When performing halftone processing for converting input image data having gradation values indicating the density of an image to be printed on the print medium into data indicating a pattern of dots constituting the image,
Using the first dither mask for the input image data corresponding to the portion of the print medium located at the convex portion,
The input image data corresponding to the portion of the print medium located in the concave portion between the convex portions has a characteristic that the dispersibility of dots in the gradation value within a predetermined range is lower than that of the first dither mask. A printing control apparatus using a second dither mask. The print control apparatus according to claim 1,
When performing the halftone process,
Using the second dither mask for the input image data corresponding to the portion of the print medium located in the central region in the predetermined direction of the recess,
The gradation value in the predetermined range with respect to the input image data corresponding to the portion of the print medium located in the end region on the convex side in the predetermined direction with respect to the central region of the concave portion. A printing control apparatus using a third dither mask having a characteristic that the dispersibility of dots in the ink is lower than that of the first dither mask and higher than that of the second dither mask. The print control apparatus according to claim 1 or 2,
The platen is formed with a plurality of the recesses having different lengths in the predetermined direction,
When performing the halftone process,
Using the second dither mask for the input image data corresponding to the portion of the print medium located in a recess,
For the input image data corresponding to the portion of the print medium located in the recess that is shorter in the predetermined direction than the certain recess, the dispersibility of dots in the gradation value in the predetermined range is A print control apparatus using a third dither mask having characteristics that are lower than the first dither mask and higher than the second dither mask. The print control apparatus according to any one of claims 1 to 3,
The first dither mask is a blue noise dither mask;
The printing control apparatus according to claim 2, wherein the second dither mask is a mask in which the dispersibility of dots in the gradation value in the predetermined range is adjusted by changing a threshold arrangement of a blue noise dither mask. The print control apparatus according to any one of claims 1 to 4, wherein:
Whether to use the second dither mask for the input image data corresponding to the portion of the print medium located in the recess according to the amount of ink ejected to the print medium based on the input image data Determining a printing control apparatus. A printing control apparatus according to any one of claims 1 to 5,
Whether to use the second dither mask for the input image data corresponding to the portion of the print medium located in the recess according to the distance from the ink ejection surface of the head to the print medium. A printing control apparatus characterized by determining. A control device of a printing apparatus comprising: a head that ejects ink onto a print medium; and a platen that supports the print medium and has a plurality of convex portions arranged at intervals in a predetermined direction on a surface facing the head.
When performing halftone processing for converting input image data having gradation values indicating the density of an image to be printed on the print medium into data indicating a pattern of dots constituting the image,
Using the first dither mask for the input image data corresponding to the portion of the print medium located at the convex portion,
The input image data corresponding to the portion of the print medium located in the concave portion between the convex portions has a characteristic that the dispersibility of dots in the gradation value within a predetermined range is lower than that of the first dither mask. A program for executing the use of the second dither mask. A printing method by a printing apparatus comprising: a head that ejects ink onto a printing medium; and a platen that supports the printing medium and has a plurality of convex portions arranged at intervals in a predetermined direction on a surface facing the printing medium. ,
When performing halftone processing for converting input image data having gradation values indicating the density of an image to be printed on the print medium into data indicating a pattern of dots constituting the image,
A first dither mask is used for the input image data corresponding to the portion of the print medium located at the convex portion,
The input image data corresponding to the portion of the print medium located in the concave portion between the convex portions has a characteristic that the dispersibility of dots in the gradation value within a predetermined range is lower than that of the first dither mask. Based on the data for which the second dither mask was used,
A printing method, wherein the printing apparatus prints an image on the printing medium.

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