JP2015199231A - Print control unit, program and printing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a print control unit suppressing occurrence of density unevenness in a printed image.SOLUTION: A print control unit controls a printer discharging ink toward a printing medium even at the time of a forward path where a head is moved to one side in a predetermined direction with respect to the printing medium and at the time of a return path where being moved to the other side. The print control unit uses a first dither mask with respect to input image data corresponding to a first region on the printing medium and uses a second dither mask where a relative position between a forward path threshold value corresponding to data at the time of the forward path and a return path threshold value corresponding to data at the time of the forward path is shifted in accordance with dots landing position deviations occurring in the first region with respect to input image data corresponding to a second region on the printing medium where the dots landing position deviations of the forward path and the return path are less in comparison with those in the first region when performing halftone processing converting input image data 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 toward a printing medium while the head moves in a predetermined direction with respect to the 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).
Further, in the printing apparatus, as the amount of ink ejected to the printing medium increases, a cockling phenomenon occurs in which the printing medium undulates along a predetermined direction that is the moving direction of the head, and a gap between the ink ejection surface of the head and the printing medium occurs. As a result, the paper gap changes, and the dot landing position shifts. As a result, the coverage, which is the ratio of the dots covering the surface of the print medium, the brightness of the image, and the color change, resulting in uneven density in the printed image. Therefore, if the ink discharge amount is less than the reference value and the cockling phenomenon is difficult to occur, use a dither mask with good graininess, the ink discharge amount is greater than the reference value, and the cockling phenomenon will occur. In the case where it is easy to do this, the graininess is inferior, but a method of creating print data using a dither mask that suppresses the color difference of the image has been proposed.

JP 2013-103440 A

  In the printing apparatus, there is a case where bidirectional printing is performed in which ink is ejected both during the reciprocation when the head moves to one side in a predetermined direction and during the return path when the head moves to the other side. If the paper gap is changed when bidirectional printing is performed, the dot landing position is shifted in the reverse direction in the forward path and the backward path, so that the reciprocal dot landing position shift becomes large. Therefore, in a region where the paper gap is likely to change due to the occurrence of a cockling phenomenon or the like, in addition to the dot landing position shift caused by the paper gap changing as the ink discharge amount increases, a round-trip dot landing position shift also occurs. As compared with a region where the paper gap is difficult to change, the dot landing position deviation becomes large. Therefore, even if a dither mask that suppresses the color difference of the image as in Patent Document 1 is used, there is a possibility that the density unevenness cannot be suppressed.

  Accordingly, an object of the present invention is to suppress the occurrence of density unevenness in a printed image.

A main invention for achieving the above object is a printing apparatus that discharges ink toward the print medium during the forward path in which the head moves to one side in a predetermined direction with respect to the print medium and during the return path in which the head moves to the other side. A printing control apparatus for controlling, when performing halftone processing for converting input image data having a gradation value indicating the density of an image to be printed on the printing medium into data indicating a pattern of dots constituting the image In addition, a first dither mask is used for the input image data corresponding to the first area on the print medium, and the second dot landing position deviation on the print medium is smaller than that on the first area. For the input image data corresponding to two areas, a relative position between the forward threshold corresponding to the forward data and the backward threshold corresponding to the backward data is generated in the first area. A printing control apparatus, characterized by using a second dither mask shifted in accordance with the dot landing position shift.
Other features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

1 is a block diagram illustrating a configuration of a printing system in which a printing apparatus and a printing control apparatus 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. 3A and 3B are diagrams for explaining the landing position deviation of dots due to the cockling phenomenon. FIG. 4A is a diagram for explaining the arrangement of forward and backward dots, FIG. 4B shows dots formed on the printing medium on the convex part of the platen, and FIG. 4C is formed on the printing medium on the concave part of the platen. Indicates a dot. FIG. 5A is a diagram illustrating a normal dither mask, and FIGS. 5B to 5D are diagrams illustrating a reciprocating shift mask. FIG. 6A is a flow for determining a reciprocal shift mask to be used for data corresponding to the convex portion of the platen, and FIG. 6B is a diagram showing a test pattern printed by the printer. It is a flow of the halftone process of 1st Embodiment. FIG. 8A is a diagram illustrating a region where the concave portion 421 of the platen 42 is partitioned, and FIG. 8B is a flow of halftone processing according to the second embodiment. FIG. 9A is a diagram for explaining the platen of the third embodiment, and FIG. 9B is a flow of halftone processing of the third embodiment. It is a flow of the halftone process of 4th Embodiment. It is a flow of the halftone process of 5th Embodiment. FIG. 12A is a diagram for explaining dots formed based on data half-tone processed with a blue noise dither mask, and FIG. 12B is a diagram for explaining dots formed based on data half-tone processed with a pair dot dither mask. FIG. It is a flow of the halftone process in 6th Embodiment. It is a figure explaining arrangement | positioning of a forward pass dot and a backward pass dot. It is a flow which shows the preparation method of a pair dot dither mask.

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

A printing control apparatus that controls a printing apparatus that discharges ink toward the printing medium during a forward path in which the head moves to one side in a predetermined direction with respect to the printing medium and a backward path in which the head moves to the other side. When performing halftone processing for converting input image data having a gradation value indicating the density of an image to be printed on a medium into data indicating a pattern of dots constituting the image, the first area on the print medium is For the corresponding input image data, the first dither mask is used, and the input image data corresponding to the second area on the print medium is smaller in the reciprocal dot landing position deviation than the first area. The relative position between the forward threshold corresponding to the forward data and the backward threshold corresponding to the backward data is shifted according to the dot landing position deviation occurring in the first region. A print control apparatus, characterized by using a dither mask.
According to such a printing control apparatus, the relative positional relationship between the forward pass dot and the backward pass dot formed in the second area where reciprocal dot landing position deviation is unlikely to occur is changed to the first area due to occurrence of dot landing position deviation. The relative positional relationship between the forward pass dot and the backward pass dot formed can be made closer. By doing so, the difference in the coverage of the print medium between the first region and the second region can be reduced, and the occurrence of density unevenness in the printed image can be suppressed.

In the printing control apparatus, the printing medium includes a plurality of convex portions that are supported by a platen arranged at intervals in the predetermined direction on a surface facing the head, and the convex portions are formed when the halftone process is performed. The first dither mask is used for the input image data corresponding to the first region which is the part of the print medium located in the recess between the print medium and the part of the print medium located in the convex part. In the printing control apparatus, the second dither mask is used for the input image data corresponding to the second area.
According to such a print control apparatus, even if the ink discharge amount increases, the paper gap does not easily change, and the dot landing position is not easily moved back and forth. The relative position relationship between the forward pass dot and the return pass dot is formed in the portion (first region) of the print medium located in the concave portion of the platen where the paper gap changes and the reciprocal dot landing position shift occurs as the ink discharge amount increases. The relative positional relationship between the forward pass dot and the return pass dot is made closer. Therefore, the difference in the coverage of the print medium can be reduced regardless of the uneven shape of the platen, and the occurrence of density unevenness along the uneven shape of the platen can be suppressed.

In this printing control apparatus, the second dither mask is a mask in which a relative position between the forward path threshold and the backward path threshold in the first dither mask is shifted.
According to such a printing control apparatus, the relative positional relationship between the forward pass dot and the backward pass dot formed in the second region can be made closer to the relative positional relationship between the forward pass dot and the backward pass dot formed in the first region.

In this printing control apparatus, the second dither mask is a relative value between the forward pass threshold and the return pass threshold in the dither mask having a characteristic that the dispersibility of dots in the gradation value within a predetermined range is higher than that of the first dither mask. The print control apparatus is a mask whose position is shifted.
According to such a printing control apparatus, the relative positional relationship between the forward pass dot and the backward pass dot formed in the second region can be made closer to the relative positional relationship between the forward pass dot and the backward pass dot formed in the first region.

In the print control apparatus, when the halftone process is performed, the first input image data corresponding to a portion of the print medium located in a central region of the concave portion in the predetermined direction is applied to the first input image data. The dither mask is used, and 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 in which a relative position between the forward pass threshold and the return pass threshold is shifted by a shift amount smaller than a 2 dither mask.
According to such a printing control apparatus, the printing medium located on the convex part of the platen where reciprocal dot landing position deviation hardly occurs, and the printing medium located at the center of the concave part of the platen where reciprocal dot landing position deviation occurs. The forward dot and the backward dot are respectively formed on the print medium located at the end of the concave portion where the positional deviation of the dot landing occurs more reciprocally than the convex portion, but the positional deviation of the dot landing is less likely to occur compared to the central portion of the concave portion. Can be brought closer to each other. Therefore, it is possible to suppress the occurrence of density unevenness along the uneven shape of the platen.

In this printing control apparatus, based on the data obtained by performing the halftone processing using the first dither mask on the input image data corresponding to the first area and the second area, the printing apparatus A pattern is printed, and the forward path threshold and the return path in the second dither mask are based on the read data of the part of the pattern corresponding to the first area and the read data of the part of the pattern corresponding to the second area A print control apparatus that determines a relative shift amount of a threshold value.
According to such a printing control apparatus, it is possible to determine the second dither mask that can suppress the occurrence of density unevenness in the printed image.

Also, printing on the printing medium is performed on the control device of the printing apparatus that ejects ink toward the printing medium during the forward path when the head moves to one side of the predetermined direction and the backward path when the head moves to the other side. The input image data having a gradation value indicating the density of the image to be converted is converted into data indicating the pattern of the dots constituting the image, and the halftone process corresponding to the first region on the print medium is performed. For the input image data, the first dither mask is used, and the input image data corresponding to the second area on the print medium in which the reciprocal dot landing position shift is smaller than that in the first area. A second dither mass in which the relative position between the forward threshold corresponding to the forward data and the backward threshold corresponding to the backward data is shifted according to the dot landing position deviation occurring in the first area. Is a program for executing the use of.
According to such a program, the relative positional relationship between the forward pass dot and the backward pass dot formed in the second region where reciprocal dot landing position deviation hardly occurs is formed in the first region due to occurrence of dot landing position deviation. Data that approximates the relative positional relationship between the forward pass dot and the return pass dot can be generated by the print control apparatus. Therefore, the difference in the coverage of the print medium between the first region and the second region can be reduced, and the occurrence of density unevenness in the printed image can be suppressed.

A printing method using a printing apparatus that ejects ink toward the print medium both during a forward path in which the head moves to one side in a predetermined direction with respect to the print medium and during a return path in which the head moves to the other side. Corresponding to the first area on the print medium when performing halftone processing for converting input image data having gradation values indicating the density of an image to be printed on to data indicating a pattern of dots constituting the image For the input image data, a first dither mask is used, and the input image data corresponding to the second area on the print medium is smaller in reciprocal dot landing position displacement than the first area. The relative position of the forward threshold corresponding to the forward data and the backward threshold corresponding to the backward data is shifted in accordance with the dot landing position deviation occurring in the first area. Based on the dither mask is used data, the printing apparatus is a printing method characterized by printing an image on the print medium.
According to such a printing method, the relative positional relationship between the forward pass dot and the backward pass dot formed in the second region where reciprocal dot landing position deviation hardly occurs is formed in the first region due to occurrence of dot landing position deviation. The relative positional relationship between the forward pass dot and the return pass dot is made closer. Therefore, the difference in the coverage of the print medium between the first region and the second region can be reduced, and the occurrence of density unevenness in the printed image can be suppressed.

=== 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 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.

  The detector group 50 is for monitoring the situation in the printer 1 and outputting the detection result to the controller 10. The detector group 50 includes, for example, an optical sensor 51 that detects an end portion of the print medium S in the X direction. The optical sensor 51 is mounted on the carriage 31 on the downstream side in the Y direction with respect to the head 41, detects a light emitting unit that emits light toward the print medium S, and light reflected by the print medium S, and And a light receiving unit that outputs an electrical signal corresponding to the intensity of the reflected light. The intensity of light detected by the light receiving unit depends on the density of the image printed on the print medium S. Therefore, based on the electrical signal output from the light receiving unit when the optical sensor 51 moves in the X direction on the image printed on the print medium S, the controller 10 determines the density of the image printed on the print medium S. Can be read.

  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 and FIG. 3B are diagrams for explaining the landing position deviation 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.

  However, even if the amount of ink ejected to the print medium S increases and a cockling phenomenon occurs, the area from the nozzle opening surface of the head 41 to the print medium S at the portion of the print medium S supported by the convex portion 422. Since the distance, the so-called paper gap, does not change, the landing position deviation of the dots does not occur. On the other hand, at the portion of the print medium S positioned in the recess 421, the amount of deflection of the print medium S increases as the amount of ink ejected to the print medium S increases. Misalignment will occur.

  Further, in the printer 1, for example, the head 41 is also on the forward path in which the head 41 moves to the right side (one side) in the X direction (predetermined direction) and on the return path in which the head 41 moves to the left side (other side) in the X direction. In some cases, bi-directional printing in which ink is ejected is performed. The ink ejection timings for the forward pass and the return pass are adjusted in a state where no cockling phenomenon occurs. Therefore, in the portion of the print medium S supported by the convex portion 422, as shown in FIG. 3A, the landing position deviation between the forward pass dot formed in the forward pass and the return pass dot formed in the return pass does not occur. However, when the paper gap changes like the portion of the printing medium S located in the concave portion 421, the forward and backward dots are shifted in the opposite direction as shown in FIG. . That is, in the printing medium S on the concave portion 421, in addition to the dot landing position deviation for each pass, a round-trip dot landing position deviation also occurs, and the dot landing position deviation compared to the printing medium S on the convex portion 422. Will become bigger.

  FIG. 4A is a diagram illustrating the arrangement of the forward pass dot D1 and the return pass dot D2. 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 such a printing method is carried out, an image piece printed on a partial area of the print medium S is completed in a plurality of passes, and the image piece is composed of forward pass dots D1 and return pass dots D2. In this embodiment, the image piece is completed in two passes (outward and backward), and as shown in FIG. 4A, a raster line formed only by the forward dot D1 and a raster line formed only by the backward dot D2. Is an example of a printing method alternately arranged in the Y direction.

  4B and 4C are diagrams illustrating dots formed based on data that has been subjected to halftone processing using a normal dither mask. FIG. 4B illustrates dots formed on the printing medium S on the convex portion 422 of the platen 42. FIG. 4C shows dots formed on the print medium S on the concave portion 421 of the platen 42. In FIG. 4A, dots are formed in all the pixel areas, but in FIGS. 4B and 4C, an example is given in which dots are formed in some of the pixel areas. As described above, in the printing medium S on the convex portion 422, the paper gap does not change even when the ink discharge amount increases. Therefore, as shown in FIG. 4B, the forward dot D1 and the backward dot D2 are pixels indicated by the print data, respectively. Formed in the region. On the other hand, in the printing medium S on the concave portion 421, the paper gap changes as the ink discharge amount increases. Therefore, as shown in FIG. 4C, during the forward pass, which is the previous pass, the forward 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 subsequent pass. During a certain return pass, the print medium S is bent by the ink ejected during the forward pass, and the return pass dot D2 is located at a position shifted from the pixel area indicated by the print data (a position shifted by two pixels to the left in the X direction in the figure). Will be formed. Therefore, the relative positional relationship between the forward pass dot D1 and the backward pass dot D2 is shifted between the portion of the print medium S on the concave portion 421 and the portion of the print medium S on the convex portion 422. Then, since the printer 1 generally forms dots having a size larger than the pixel area, the way in which the forward pass dots D1 and the return pass dots D2 overlap changes, and the coverage, which is the ratio of the dots covering the print medium S, changes. End up. As a result, a density difference occurs between the image printed on the printing medium S on the concave portion 421 and the image printed on the printing medium S on the convex portion 422, and density unevenness along the uneven shape of the platen 42 occurs in the printed image. Resulting in. In addition, the dispersibility of the dots also changes, and an image quality difference occurs along the uneven shape of the platen 42.

  Therefore, in the present embodiment, when the halftone processing unit 73 performs the halftone process, when ink is ejected from the head 41 based on the data that has been subjected to the halftone process using a normal dither mask, the dot landing is reciprocated. “Regular dither mask (first dither mask)” for the input image data corresponding to the print medium S located in the concave portion 421 of the platen 42 (first area) on the print medium S where the positional deviation occurs. Is used. On the other hand, when ink is ejected from the head 41 based on halftone processed data using a normal dither mask, an area on the print medium S (second area) where reciprocal dot landing position deviation hardly occurs, that is, A “reciprocating shift mask (second dither mask)” in which the relative positions of the forward pass threshold and the return pass threshold in the normal dither mask are shifted with respect to the input image data corresponding to the print medium S positioned on the convex portion 422 of the platen 42. Is used.

<Reciprocating mask>
FIG. 5A is a diagram illustrating a normal dither mask (hereinafter referred to as a normal mask), and FIGS. 5B to 5D are diagrams illustrating a reciprocating shift mask. An example is a dither mask in which 8 × 8 storage areas are arranged in the X direction × Y direction. The area (squares) where the forward threshold value corresponding to the data at the time of the outgoing path is stored is shaded, and the area where the backward threshold value corresponding to the data at the time of the backward path is stored is a white area. In the present embodiment, the case where the printing method shown in FIG. 4A is performed is taken as an example, and therefore, in the dither mask of FIG. Are alternately arranged in the Y direction.

  In the storage area, the threshold value is represented not by a specific threshold value but by a number indicating the position in the X direction (number on the left side) and a number indicating the position in the Y direction (number on the right side). In the “normal mask” shown in FIG. 5A, numbers (0, 1, 2,... 7) are assigned in order from the left side in the X direction, and numbers (0, 1, 2,... 7) in order from the top in the Y direction. Is attached. Therefore, the forward path threshold value in the upper left storage area of the normal mask is represented by “0, 0”, and the return path threshold value in the lower right storage area is represented by “7, 7”.

  The reciprocating shift mask is a mask in which the relative positional relationship between the forward pass threshold and the return pass threshold in the normal mask is shifted. In FIGS. 5B to 5D, masks in which the return pass threshold is shifted to the left in the X direction with respect to the forward pass threshold are taken as an example. I will give you. FIG. 5B is a mask in which the return pass threshold is shifted by one pixel to the left in the X direction. Therefore, in the normal mask (FIG. 5A), the return path threshold value “0, 1” is located below the forward path threshold value “0, 0” in the upper left. However, in the mask of FIG. The return path threshold “1, 1” is located. Similarly, FIG. 5C is a mask in which the return path threshold is shifted by two pixels to the left in the X direction, so that the return path threshold “2, 1” is positioned below the forward path threshold “0, 0”, and FIG. Since the threshold is shifted by three pixels to the left in the X direction, the return path threshold “3, 1” is located below the forward path threshold “0, 0” at the upper left.

  As described above, in the present embodiment, an image is printed on the print medium S on the concave portion 421 based on the data subjected to the halftone process using the normal mask (FIG. 5A). However, in the printing medium S on the concave portion 421, the paper gap changes due to the occurrence of the cockling phenomenon. For example, when the forward pass dot D1 and the return pass dot D2 are formed as shown in FIG. As shown in FIG. 4C, the backward dot D2 is formed on the left side in the X direction by two pixels with respect to the forward dot D1. However, in this embodiment, halftone processing is performed using a reciprocating shift mask in which the relative positions of the forward pass threshold and the return pass threshold in the normal mask are shifted in accordance with the dot landing position shift that occurs in the printing medium S on the recess 421. Based on the data thus obtained, an image is printed on the print medium S on the convex portion 422. For example, the data corresponding to the print medium S on the convex portion 422 is halftone processed using a reciprocating shift mask (FIG. 5C) in which the return pass threshold is shifted by two pixels to the left in the X direction with respect to the forward pass threshold. . As a result, data with a high probability of occurrence of the backward pass dot D2 is obtained in a pixel shifted by 2 pixels on the left side in the X direction from the pixel in which the backward pass dot D2 is generated in the data subjected to the halftone process using the normal mask. By printing an image on the print medium S on the convex portion 422 based on such data, an image similar to the image printed on the print medium S on the concave portion 421 (FIG. 4C) is printed on the convex portion 422. Printing on the medium S is possible.

  That is, halftone processing is performed on the data corresponding to the concave portion 422 using a normal mask and halftone processing is performed on the data corresponding to the convex portion 422 using a reciprocating shift mask. Such an image can be printed on the print medium S on the convex portion 422. Therefore, the relative positional relationship between the forward pass dots and the backward pass dots formed on the print medium S on the convex portion 422 (how dots overlap) is expressed as the relative relationship between the forward pass dots and the backward pass dots formed on the print medium S on the concave portion 421. It is close to the positional relationship (how dots overlap). As a result, the difference in coverage between the print medium S on the convex portion 422 and the print medium S on the concave portion 421, that is, the difference in image density, can be reduced, and the density unevenness along the uneven shape of the platen 42 can be reduced. Can be suppressed.

<How to determine the reciprocating displacement mask>
6A is a flow for determining a reciprocal shift mask to be used for data corresponding to the convex portion 422 of the platen 42, and FIG. 6B is a diagram illustrating a pattern p printed by the printer 1. As described above, the reciprocating shift mask used for the data corresponding to the convex portion 422 is a mask corresponding to the dot landing position deviation generated in the print medium S on the concave portion 421. Therefore, it is preferable to perform a process for determining a reciprocal shift mask for each printer 1 before shipment of the printer 1 or under the user.

  Specifically, the computer 2 connected to the printer 1 causes the printer 1 to print the pattern p shown in FIG. 6B (S001). The pattern p is an image printed based on input image data represented by a uniform density (gradation value), and the input image data corresponding to the concave portion 421 of the platen 42 and the input image data corresponding to the convex portion 422 are both normal. Assume that the image is printed based on the halftoned data using the mask (FIG. 5A).

  Here, the computer 2 compares the density difference between the read data d1 of the portion of the pattern p printed on the print medium S on the concave portion 421 and the read data d2 of the portion of the pattern p printed on the print medium S on the convex portion 422. On the other hand, it is assumed that information (not shown) in which a reciprocating shift mask capable of suppressing the density difference (or a relative shift amount between the forward pass threshold and the return pass threshold from the normal mask) is stored is stored. Therefore, the computer 2 causes the optical sensor 51 (or a scanner that is an external device) to read the pattern p printed on the print medium S, and acquires the read data (S002). Then, based on the density difference between the read data d1 of the part of the pattern p corresponding to the concave part 421 and the read data d2 of the part of the pattern p corresponding to the convex part 422, the reciprocal shift used for the data corresponding to the convex part 422 is used. A mask (that is, a relative shift amount between the forward pass threshold and the return pass threshold in the reciprocating shift mask) is determined (S003). The determined reciprocating displacement mask is stored in the memory of the computer 2 so that it can be used when the halftone processing unit 73 performs the halftone processing. As described above, by determining the reciprocal shift mask to be used for the data corresponding to the convex portion 422, it is possible to determine the reciprocal shift mask that can suppress the occurrence of density unevenness along the uneven shape of the platen 42.

  For example, a normal mask (FIG. 5A) is used for input image data corresponding to the concave portion 421, and a plurality of reciprocating shift masks are used for input image data corresponding to the convex portion 422. A plurality of patterns may be printed on the printer 1 based on data used by sequentially switching (FIGS. 5B to 5D). In this case, the pattern in which the density unevenness is most difficult to be visually recognized is determined based on the pattern reading result or visual observation by the optical sensor 51, and the reciprocating shift mask used when creating the print data of the pattern corresponds to the convex portion 422. It is preferable to determine a mask to be used for data to be processed. In addition, for the portion of the print medium S on the concave portion 421, the printer 1 prints a line at the time of the forward pass and the return pass, and the data corresponding to the convex portion 422 is based on the deviation amount of the forward pass line and the return pass line. The reciprocating shift mask to be used may be determined. Further, the mask used for the data corresponding to the convex portion 422 is not limited to the mask shown in FIGS. 5B to 5D, for example, a mask in which the return path threshold is shifted to the right in the X direction with respect to the forward path threshold, A mask in which the shift amount of the return path threshold is larger than 3 pixels is also included.

  In this embodiment, the case where an image piece is completed in two passes is taken as an example. However, in the actual printer 1, the image piece is completed in four passes, for example, more than twice. There is. In this case, in the printing medium S on the concave portion 421, not only the reciprocal dot landing position shift but also the relative positional relationship between the forward pass dot in the first pass and the forward pass dot in the third pass is shifted, and the return pass dot in the second pass The relative positional relationship of the return pass dots in the fourth pass is shifted. Even in that case, as described above, the printer 1 actually prints the pattern, and determines the reciprocating shift mask in which the density unevenness is difficult to be visually recognized, so that the reciprocating dot landing position generated in the printing medium S located in the recess 421 It is possible to determine a reciprocating shift mask that takes into account not only the shift but also the dot landing position shift for each pass (for example, the shift of the forward pass dots in the first pass and the third pass). In other words, the coverage of the printing medium S on the concave portion 421 where the dot landing position deviation of each round and the dot landing position deviation for each pass occurs, and the coverage of the printing medium S on the convex portion 422 where no dot landing position deviation occurs. A reciprocating shift mask for forming the forward dots and the backward dots of the printing medium S on the convex portion 422 by shifting them can be determined so as to be equivalent.

<Halftone processing>
FIG. 7 is a flow of halftone processing according to the first embodiment. The halftone processing unit 73 determines a target pixel for converting the gradation value from the input image data to be processed by the halftone process (S101), and the target pixel is a pixel on the convex portion 422 of the platen 42. Determine whether or not. 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 (S102). 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 pixel of interest is a pixel on the convex portion 422 (S102 → Yes), the halftone processing unit 73 turns on / off the dot of the pixel of interest using a “reciprocal shift mask” (mask determined in the flow of FIG. 6A). A determination is made (S103). That is, it is determined whether or not to form a dot in the pixel region on the print medium S corresponding to the target pixel by comparing the gradation value of the target pixel and the threshold value of the reciprocal shift mask corresponding to the target pixel. The multi-gradation gradation value of the target pixel 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 (S102 → No), the halftone processing unit 73 performs dot on / off determination of the pixel of interest using a “normal mask” in which the forward threshold and the backward threshold are not shifted. (S104). The above processing is repeated until the processing of all the pixels belonging to the input image data is completed (S105).

  By performing the halftone process as described above, the relative positional relationship (how dots overlap) formed in the portion of the print medium S located in the convex portion 422 is positioned in the concave portion 421. It can be brought close to the relative positional relationship (how the dots overlap) formed between the forward pass dots and the backward pass dots formed on the portion of the print medium S, and the occurrence of density unevenness along the uneven shape of the platen can be suppressed.

=== Second Embodiment ===
FIG. 8A is a diagram for explaining a region where the concave portion 421 of the platen 42 is partitioned, and FIG. 8B 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). For this reason, the reciprocating dot landing position deviation generated in the printing medium S on the end area A2 of the concave portion 421 is larger than the reciprocating dot landing position deviation generated in the printing medium S on the convex portion 422, but the concave portion 421. This is smaller than the reciprocal dot landing position deviation that occurs in the print medium S on the central area A1.

  Therefore, in the second embodiment, when the halftone processing unit 73 performs the halftone processing, a “reciprocating shift 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. For 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, and the central portion of the concave portion 421. For 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 region A1, the forward pass threshold and the return pass in the normal mask with a shift amount smaller than the reciprocating shift mask. An “intermediate mask (third dither mask)” in which the position relative to the threshold is shifted is used. For example, when the reciprocating shift mask is a mask in which the return pass threshold is shifted by two pixels to the left in the X direction with respect to the forward pass threshold in the normal mask, the intermediate mask has a return pass threshold X with respect to the forward pass threshold in the normal mask. There is a mask shifted by one pixel on the left side in the direction.

  Specifically, as shown in FIG. 8B, the halftone processing unit 73 determines a target pixel from the image data to be processed (S201), and whether or not the target pixel is a pixel on the convex portion 422. Is determined (S202). When the pixel of interest is a pixel on the convex portion 422 (S202 → Yes), the halftone processing unit 73 performs on / off determination of the dot of the pixel of interest using the “reciprocating shift 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 “normal mask” (S205). If the target pixel is a pixel on the end area A2 ( (S204 → No), the on / off determination of the dot of the pixel of interest is performed by the “intermediate mask” (S206).

  By doing so, the part of the print medium S on the convex part 422 of the platen 42, the part of the print medium S on the central area A1 of the concave part 421 of the platen 42, and the print medium on the end part area A2 of the concave part 421 The relative positional relationship between the forward pass dot and the backward pass dot that are respectively formed in the portion S can be made closer, and the difference in coverage can be reduced. Therefore, it is possible to suppress the occurrence of density unevenness along the uneven shape of the platen 42.

=== Third Embodiment ===
FIG. 9A is a diagram illustrating the platen 42 according to the third embodiment, and FIG. 9B is a flow of halftone processing according to 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. For this reason, as shown in FIG. 9A, the convex portions 422 and the concave portions 421 are not 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. 9A, 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 to the wide concave portion 421a, the narrow concave portion 421b has less deflection of the print medium S due to the ejection of ink (L3> L4). For this reason, the reciprocating dot landing position deviation generated in the printing medium S on the narrow concave portion 421b is larger than the reciprocating dot landing position deviation generated in the printing medium S on the convex portion 422, but on the wide concave portion 421a. This is smaller than the reciprocal dot landing position deviation that occurs in the print medium S.

  Therefore, in the third embodiment, when the halftone processing unit 73 performs the halftone processing, the “reciprocating shift 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. Is used for the input image data corresponding to the portion of the print medium S located on the wide concave portion 421a, and the input corresponding to the portion of the print medium S located on the narrow concave portion 421b is used. For the image data, an “intermediate mask” is used in which the relative position between the forward pass threshold and the return pass threshold in the normal mask is shifted by a shift amount smaller than the reciprocating shift mask.

  Specifically, as shown in FIG. 9B, the halftone processing unit 73 determines a target pixel from the image data to be processed (S301), and when the target pixel is a pixel on the convex portion 422. (S302 → Yes), the dot on / off determination of the pixel of interest is performed using the reciprocating shift 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 on / off determination of the dot of the target pixel is performed using the normal mask (S305), and if the target pixel is a pixel on the narrow recess 421b (S304 → No) Then, on / off determination of the dot of the pixel of interest is performed using the intermediate mask (S306).

  By doing so, in the part of the print medium S located on the convex part 422 of the platen 42, the part of the print medium S located in the wide concave part 421a, and the part of the print medium S located in the narrow concave part 421b, respectively. The relative positional relationship between the forward pass dots and the backward pass dots that are formed can be made closer, and the difference in coverage can be reduced. Therefore, it is possible to suppress the occurrence of density unevenness 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 reciprocating shift mask is used. An intermediate mask may be used if the length in the X direction is less than the threshold.

  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 normal mask may be used for the input image data, and an intermediate 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. 10 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 bend, and the landing position deviation of the reciprocating dots is difficult to occur. Therefore, in the fourth embodiment, a reciprocal shift mask is used for input image data corresponding to a portion of the print medium S located on the convex portion 422 of the platen 42 according to the amount of ink discharged to the print medium S. Determine whether or not.

  Specifically, as shown in FIG. 10, the halftone processing unit 73 configures input image data when the amount of ink ejected to the entire print medium S is less than the threshold (S401 → Yes). When the average gradation value or the total gradation value of all gradation values is less than the threshold value, the target pixel is determined from the image data to be processed (S402), and the normal mask is used regardless of the uneven shape of the platen 42. The process (S403) of performing ON / OFF determination of the dot of the target pixel is repeated. On the other hand, when the amount of ink ejected on the entire print medium S is equal to or greater than the threshold (S401 → No), the halftone processing unit 73 determines a target pixel from the image data to be processed (S405), and the target It is determined whether or not the pixel is a pixel on the convex portion 422 (S406). If the target pixel is a pixel on the convex portion 422 (S406 → Yes), the dot of the target pixel is turned on / off by the reciprocating shift mask. A determination is made (S407), and if the target pixel is a pixel on the concave portion 421 (S406 → No), on / off determination of the dot of the target pixel is performed using a normal mask (S408).

  By doing so, when the amount of ink ejected to the printing medium S is large and the dot landing position displacement in the reciprocating portion 421 is large, a dither mask (ordinary mask or reciprocating displacement) used in accordance with the uneven shape of the platen 42. The mask) is switched, and density unevenness along the uneven shape of the platen 42 can be suppressed. On the other hand, when the amount of ink ejected onto the printing medium S is small and the reciprocating dot landing position deviation in the concave portion 421 is small, the normal mask is used regardless of the concave / convex shape of the platen 42. It is not necessary to determine the dither mask to be performed, and halftone processing can be facilitated.

=== Fifth Embodiment ===
FIG. 11 is a flow of halftone processing 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 print medium S is thick and the paper gap is narrow, even the print medium S located in the concave portion 421 of the platen 42 is difficult to bend, and reciprocal dot landing position deviation is difficult to occur. Also, the smaller the paper gap, the smaller the landing position deviation of dots. Therefore, in the fifth embodiment, it is determined whether to use a reciprocation shift mask for input image data corresponding to a portion of the print medium S located on the convex portion 422 of the platen 42 according to the platen gap. .

  Specifically, as shown in FIG. 11, 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 on / off of the target pixel using the normal mask regardless of the uneven shape of the platen 42 (S503) is repeated. 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 dot on / off of the pixel of interest is determined using a reciprocating shift mask (S507). If the pixel is a pixel on the recess 421 (S506 → No), the dot on / off of the pixel of interest is determined using the normal mask (S508).

  By doing so, when the paper gap is large and the reciprocating dot landing position deviation in the concave portion 421 is large, the dither mask (ordinary mask or reciprocating displacement mask) to be used is switched according to the uneven shape of the platen 42. It is possible to suppress the occurrence of density unevenness along the concavo-convex shape of 42. On the other hand, when the paper gap is small and the reciprocating dot landing position deviation in the concave portion 421 is small, the normal mask is used regardless of the uneven shape of the platen 42, and therefore it is necessary to determine the dither mask to be used for each pixel of interest. And halftone processing can be facilitated.

  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 value, the normal mask and the reciprocating mask are switched according to the uneven shape of the platen 42. If the paper gap is smaller than the first threshold value and equal to or larger than the second threshold value, the platen is switched. Switching between a normal mask and an intermediate mask (a mask in which the shift amount between the forward pass threshold and the return pass threshold is smaller than that of the reciprocating shift mask) according to the concave / convex shape of the plate 42 and the paper gap is less than the second threshold value, Regardless, a normal mask may be used.

=== Sixth Embodiment ===
FIG. 12A is a diagram illustrating the arrangement of dots D1 and D2 formed based on data that has been halftone processed with a blue noise dither mask, and FIG. 12B is based on data that has been halftone processed with a pair dot dither mask. It is a figure explaining arrangement | positioning of the dots D1, D2 to be formed. FIG. 13 is a flow of halftone processing in the sixth embodiment.

  As an example of the dither mask, a blue noise dither mask having high dot dispersibility (good dot dispersibility) can be given. The blue noise dither mask has human noise characteristics that the sensitivity is low in the high-frequency region, and the formed dot distribution has a noise characteristic that has a peak in the high-frequency region side in the spatial frequency region. It is an excellent mask. 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. In a light image printed based on halftone processed data with a blue noise dither mask, dots D1 and D2 are not formed in adjacent pixel areas as shown in the left diagram of FIG. 12A. Dots D1 and D2 are formed. By dispersing the dots D1 and D2 in this way, the graininess of the image can be improved.

  However, even in the case of an image that has been halftone processed with a blue noise dither mask, if the paper gap changes in the concave portion 421 of the platen 42 due to the occurrence of a cockling phenomenon or the like, As shown in the figure, the dots D1 and D2 that should be originally formed are formed adjacent to each other, and the coverage of the print medium S is reduced. That is, in an image that has been subjected to halftone processing using a blue noise dither mask, the amount of change in the coverage of the print medium S is large and the change in image density is large depending on whether or not the dot landing position shift occurs. End up.

  In addition, the dither mask has a characteristic that the dispersibility of the dots is lower than that of the blue noise dither mask in the gradation value in the range where the light image is printed, in other words, 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 a pair dot dither mask, dots D1 and D2 are formed in adjacent pixel areas 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. 12B. 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.

  Therefore, a blue noise dither mask is used for the input image data corresponding to the convex portion 422 of the platen 42 in which the paper gap hardly changes and the dot landing position shift hardly occurs, and the dot landing position shift easily occurs because the paper gap easily changes. It is assumed that a pair dot dither mask is used for input image data corresponding to the concave portion 421 of the platen 42 that is likely to occur. By doing so, an image having good graininess is printed on the portion of the print medium S on the convex portion 422, and the density of the print medium S on the concave portion 421 is increased even if a dot landing position shift occurs. Images with small changes can be printed. However, when bi-directional printing is performed, in the portion of the print medium S on the concave portion 421 of the platen 42, the dot landing position reciprocation is shifted in the opposite direction in addition to the dot landing position shift caused by the change in the paper gap. Due to this, the dot landing position shift occurs, so that the dot landing position shift becomes larger than the portion of the print medium S on the convex portion 422. Therefore, even if a pair dot dither mask is used for the input image data corresponding to the concave portion 421, there is a risk that density unevenness along the concave and convex shape of the platen 42 is suppressed and it is not clean.

  Therefore, in the sixth embodiment, when the halftone processing unit 73 performs the halftone processing, the region on the print medium S in which the reciprocal dot landing position shift (first region), that is, the concave portion 421 of the platen 42 is generated. A “pair dot dither mask (first dither mask)” is used for the input image data corresponding to the portion of the print medium S that is positioned. On the other hand, the input image data corresponding to the region (second region) on the print medium S where the reciprocal dot landing position deviation hardly occurs, that is, the portion of the print medium S located on the convex portion 422 of the platen 42 is blue. In a noise dither mask, that is, a mask having a characteristic that the dispersibility of dots in a gradation value in a predetermined range is higher than that of a pair dot dither mask, a “reciprocal shift mask (second dither mask) in which the relative positions of the forward pass threshold and the return pass threshold are shifted. Mask) ”.

  Specifically, as shown in FIG. 13, the halftone processing unit 73 determines a target pixel from the image data to be processed (S601), and whether or not the target pixel is a pixel on the convex portion 422. Is determined (S602). When the pixel of interest is a pixel on the convex portion 422 (S602 → Yes), the halftone processing unit 73 performs on / off determination of the dot of the pixel of interest using a mask in which the forward threshold and the backward threshold of the blue noise dither mask are shifted ( S603). On the other hand, when the pixel of interest is a pixel on the concave portion 421 (S602 → No), on / off determination of the dot of the pixel of interest is performed using a pair dot dither mask in which the forward threshold and the backward threshold are not shifted (S605).

  By doing so, the relative positional relationship (how dots are overlapped) of the forward pass dot and the backward pass dot formed at the portion of the print medium S located at the convex portion 422 is formed at the portion of the print medium S located at the concave portion 421. It is made close to the relative positional relationship between the forward pass dot and the return pass dot (how the dots overlap), and the occurrence of density unevenness along the uneven shape of the platen can be suppressed.

<Paired dot dither mask>
FIG. 14 is a diagram for explaining the arrangement of forward pass dots D1 and return pass dots D2 in a printing method different from that of the first embodiment (FIG. 4A). FIG. 15 is a flowchart showing a method for creating a pair dot dither mask. Here, as shown in FIG. 14, a printing method in which the forward dot D1 and the backward dot D2 are alternately arranged in the X direction and the Y direction and an image piece is completed in two passes will be described as an example.

  In the pair dot dither mask of the present embodiment, compared to the blue noise dither mask, gradation values in a range in which a light image is printed, for example, gradation values 1 to 127 of 256 gradation values (0 to 255). The probability (paired dot occurrence probability) that the forward pass dot and the return pass dot occur in adjacent pixels is increased. In the case of the printing method shown in FIG. 14, if a dot occurs in any of the target pixel on the data and the four pixels arranged vertically and horizontally among the eight pixels around the target pixel, the forward pass dot and the return pass dot are adjacent to each other. Will occur in the pixel that does. 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 paired dot occurrence probability K is set close to k ² in a state where there is no deviation in the dot landing position, the amount of change in the paired dot occurrence probability K regardless of the dot landing position deviation. 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. The pair dot occurrence probability can be adjusted by adjusting the coefficient of k ² (here, 0.8) in the pair dot occurrence probability K (= 0.8 × k ² ). However, the coefficient is preferably 0.6 or more and 1.4 or less, and the closer the value is to 1, the more reliably the amount of change in the coverage of the print medium S due to the landing position deviation of the dots can be more reliably suppressed.

  Hereinafter, a method of creating a pair dot dither mask will be described based on the flow of FIG. 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 a threshold value is prepared (S701).

  Next, for the prepared blue noise dither mask (working dither mask), the forward pass dot and the return pass dot are generated in adjacent pixels 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 (S702). 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. In addition, you may count the location where a dot generate | occur | produces in the adjacent pixel of the X direction left side of an attention pixel, and the adjacent pixel of the Y direction upper side.

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 (S703). 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 of the number of paired dots in the 64 × 64 pixel group is H = 0.8 × k ² × 64 × 64, and k varies depending on the gradation value, and therefore the gradation value (1 to 127). A target value H for each is obtained. 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 (S703 → 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 in each gradation value (S703 → No), An appropriate number of threshold values (for example, two threshold values) are randomly replaced, 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 (S704). It may be determined whether the total number of the right adjacent pair dot number and the lower adjacent pair dot number is within the target range, or the right adjacent pair dot number and the lower adjacent pair dot number Even when only one is within the target range, the process may proceed to the step of replacing the threshold (S704). 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 changing the threshold value, it is determined whether or not the paired dot characteristics are improved (S705). 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 have not improved (S705 → No), the process returns to step S704 where the threshold is changed.

  On the other hand, if the pair dot characteristics are improved (S705 → Yes), it is next determined whether there is no problem with the graininess characteristics (S706). 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 judge that the graininess characteristic is not a problem.

Since the granularity index is a value determined for each gradation value, it is preferable to compare the granularity index of each gradation value with the target range. 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 (S706 → No), the process returns to step S704 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 (S706 → Yes), the process returns to step S703. If at least one of the number of right adjacent pair dots and the number of lower adjacent pair dots falls within the target range of the number of pair dots (S703 → Yes), the working dither mask is completed as a pair dot dither mask. .

  The method for creating a pair dot dither mask is not limited to the above method, and for example, a pair dot dither mask may be created from a green noise dither mask. Alternatively, a pair dot dither mask may be created from scratch. In this case, the threshold values are arranged in order from the smallest side or the larger side. At that time, while changing the placement position of the target threshold relative to the already placed threshold, calculate the evaluation values such as the number of paired dots and the graininess index, and place the target at the position where the evaluation value is optimal. A threshold value may be arranged. In the above-mentioned pair dot dither mask, the probability that the forward pass dot and the return pass dot occur in adjacent pixels is adjusted. However, the present invention is not limited to this, and the dots are simply adjacent without distinguishing the forward pass dot and the return pass dot. You may adjust the probability which generate | occur | produces in a pixel. 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.

=== Modification ===
In the above-described embodiment, the region where the reciprocating dot landing position is likely to occur and the region where it is difficult to occur due to the uneven shape of the platen 42 are generated. Although tone processing is performed, the present invention is not limited to this. For example, when the carriage 31 moves in the X direction, the transport amount of the printing medium S by the pair of paper feed rollers 21 (FIG. 2B) arranged at intervals in the X direction is different. In some cases, a region (first region) where reciprocal dot landing position deviation is likely to occur and a region (second region) where it is difficult to occur may occur. Even in this case, a normal mask is used for data corresponding to an area where reciprocal dot landing position deviation is likely to occur, and a reciprocal displacement mask is used for data corresponding to an area where reciprocal dot landing position deviation is unlikely to occur. By using it, the occurrence of uneven density can be suppressed.

  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.

  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, 51 optical sensors, 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 print control device that controls a printing device that discharges ink toward the print medium both during a forward path in which the head moves to one side in a predetermined direction with respect to the print medium and during a return path in which the head moves to the other side,
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 a first dither mask for the input image data corresponding to the first region on the print medium;
For the input image data corresponding to the second area on the print medium in which the reciprocal dot landing position deviation is smaller than that in the first area, the forward path threshold corresponding to the forward path data and the backward path data A printing control apparatus using a second dither mask in which a relative position with respect to a return path threshold corresponding to is shifted in accordance with a dot landing position shift generated in the first region. The print control apparatus according to claim 1,
The printing medium is supported by a platen in which a plurality of convex portions are arranged at intervals in the predetermined direction on a surface facing the head,
When performing the halftone process,
Using the first dither mask for the input image data corresponding to the first region, which is a portion of the printing medium located in the concave portion between the convex portions,
The printing control apparatus according to claim 1, wherein the second dither mask is used for the input image data corresponding to the second region which is a part of the printing medium located on the convex portion. The print control apparatus according to claim 2,
The printing control apparatus, wherein the second dither mask is a mask in which a relative position between the forward pass threshold and the return pass threshold in the first dither mask is shifted. The print control apparatus according to claim 2,
The second dither mask is a mask in which relative positions of the forward pass threshold and the return pass threshold are shifted in a dither mask having a characteristic that the dispersibility of dots in the gradation value in a predetermined range is higher than that of the first dither mask. A printing control apparatus comprising: The print control apparatus according to any one of claims 2 to 4,
When performing the halftone process,
Using the first 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,
It is smaller than the second dither mask 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 than the central region of the concave portion. A printing control apparatus using a third dither mask in which a relative position between the forward path threshold and the backward path threshold is shifted by a shift amount. A printing control apparatus according to any one of claims 1 to 5,
Based on the data obtained by performing the halftone process using the first dither mask on the input image data corresponding to the first area and the second area, the printing apparatus prints a pattern,
Relative shift amount of the forward threshold and the backward threshold in the second dither mask based on the read data of the part of the pattern corresponding to the first region and the read data of the part of the pattern corresponding to the second region Determining a printing control apparatus. A control device for a printing apparatus that ejects ink toward the print medium both during the forward path when the head moves to one side in a predetermined direction with respect to the print medium and during the backward path when the head moves to the other side,
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 a first dither mask for the input image data corresponding to the first region on the print medium;
For the input image data corresponding to the second area on the print medium in which the reciprocal dot landing position deviation is smaller than that in the first area, the forward path threshold corresponding to the forward path data and the backward path data A program for executing the use of the second dither mask in which the relative position with respect to the return path threshold corresponding to is shifted according to the dot landing position shift that occurs in the first region. A printing method by a printing apparatus that ejects ink toward the print medium both in the forward path in which the head moves to one side in a predetermined direction with respect to the print medium and in the return path in which the head moves to the other side,
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 first region on the print medium;
For the input image data corresponding to the second area on the print medium in which the reciprocal dot landing position deviation is smaller than that in the first area, the forward path threshold corresponding to the forward path data and the backward path data On the basis of data using a second dither mask in which the relative position with respect to the return path threshold corresponding to is shifted according to the dot landing position shift generated in the first region,
A printing method, wherein the printing apparatus prints an image on the printing medium.

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