Image forming apparatus, image forming method, image processing apparatus, and image processing method

Abstract

To provide an image forming method by which image quality is hardly deteriorated due to a difference in positional deviation when dots are formed.SOLUTION: An output image includes a first pixel group including a plurality of pixels whose positions in the output image are determined, and a second pixel group including a plurality of pixels whose positions are different from those of the pixels of the first pixel group. A halftone processing unit that performs halftone processing on an original image causes an arrangement of pixels where dots are formed in the first pixel group to have a distribution having a first characteristic relating to dispersibility, and in the second pixel group to have a distribution having a second characteristic relating to dispersibility and causes the distribution according to the second characteristic not to be correlated with the distribution according to the first characteristic when the input tone value of the original image is equal to or higher than an intermediate value. Then, the on / off of the dots in the arrangement having the first characteristic in the first pixel group is determined, and the on / off of the dots in the arrangement having the second characteristic in the second pixel group is determined.SELECTED DRAWING: Figure 16

Description

[Technical Field]

[0001] The present disclosure relates to image processing and image formation techniques for processing an original image expressed in multiple tones to produce an output image expressed by a distribution of dots. [Background technology]

[0002] In printing technologies that eject droplets onto a print medium, gradation is expressed by the distribution of dots. This dot distribution is often given blue or green noise characteristics to eliminate bias and improve image quality. When forming these dots, multiple dot groups may overlap in a given area. For example, in bidirectional printing with a serial printer, an image is formed by overlapping a group of dots formed during the forward pass with a group of dots formed during the reverse pass. Alternatively, in large printers with multiple print heads of a given length, the ends of two print heads may be arranged so that they overlap, resulting in overlapping groups of dots formed by one print head and another print head.

[0003] When an image is formed by overlapping groups of dots, if the dot formation positions of each group deviate from the correct positions as designed, image quality will deteriorate significantly. To solve this problem, the applicant has proposed dither mask generation technology and image processing and printing methods using this technology, which keep image quality degradation within a specified range even if the dot formation positions of each group deviate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-245618 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-103437 Summary of the Invention [Problem to be solved by the invention]

[0005] These technologies are excellent in that they simultaneously achieve high speeds such as bidirectional printing and dramatic improvements in print quality, but further improvements were required in the following two areas. First, while the adoption of these technologies reduces the degradation of image quality when there is misalignment in the dot formation position between groups, image quality is highest when there is no misalignment, and image quality deteriorates when there is misalignment, so this difference could be perceived as unevenness. Differences in image quality, particularly granularity, occur between areas where all dots can be formed in a single scan (areas where there is basically no misalignment in the dot placement between groups) and areas where dots are formed over multiple scans (areas where there may be misalignment in the dot placement formed with each scan), and this could be perceived as unevenness.

[0006] Another issue is the time and effort required to generate the dither masks used to implement these technologies. This is because, in order to achieve a dot arrangement using dithering that minimizes image quality degradation even when misalignment occurs between groups, not only must each dither mask that determines the dot arrangement for each group have blue or green noise characteristics, but the threshold values ​​in the dither mask must also be determined so that the overlapping dot arrangements of multiple groups have similar characteristics. To locate the thresholds while verifying the positions of the thresholds that make up the dither mask and the resulting dot dispersion (spatial frequency) one by one, the Fourier transform that determines the spatial frequency of the dot arrangement and its evaluation require time and effort. Because ordered dithering requires large dither masks, such as 64 x 256, to achieve high image quality, reducing this effort and computation time is a significant advantage. [Means for solving the problem]

[0007] The present disclosure can be realized in the following forms or application examples.

[0008] (1) One embodiment of the present disclosure is an image forming apparatus that forms an output image corresponding to an original image. The image forming apparatus includes a halftone processing unit that determines whether dot formation is on or off in accordance with an input gradation value of the original image to be formed, and a dot forming unit that forms an output image consisting of a plurality of pixels using the dots based on the results of processing by the halftone processing unit. The output image includes a first pixel group consisting of a plurality of pixels whose positions in the output image have been determined, and a second pixel group consisting of a plurality of pixels whose positions are different from those of the first pixel group. The halftone processing unit determines the arrangement of pixels in the first pixel group on which dots are formed as a distribution having a first characteristic related to dispersibility, and the arrangement of pixels in the second pixel group on which dots are formed as a distribution having a second characteristic related to dispersibility, and determines the distribution based on the second characteristic to be uncorrelated with the distribution based on the first characteristic when the input gradation value of the original image is equal to or greater than a predetermined intermediate value. The image forming apparatus determines the on / off of the dots in the first pixel group based on the arrangement having the first characteristic, and the on / off of the dots in the second pixel group based on the arrangement having the second characteristic.

[0009] (2) Another embodiment of the present disclosure is an image forming method for forming an output image by distributing dots. In this image forming method, the output image includes a first pixel group consisting of a plurality of pixels whose positions in the output image are determined, and a second pixel group consisting of a plurality of pixels whose positions are different from those of the first pixel group. In this image forming method, during halftone processing that determines whether dot formation is to be performed according to the input gradation value of an original image to be formed, the arrangement of pixels in the first pixel group where dots are to be formed is a distribution having a first characteristic related to dispersibility, and the arrangement of pixels in the second pixel group where dots are to be formed is a distribution having a second characteristic related to dispersibility. When the input gradation value of the original image is equal to or greater than a predetermined intermediate value, the distribution based on the second characteristic is made to have no correlation with the distribution based on the first characteristic. The on / off of the dots in the first pixel group is determined by the arrangement based on the first characteristic, and the on / off of the dots in the second pixel group is determined by the arrangement based on the second characteristic. An output image consisting of a plurality of pixels is formed using the dots using the results of the halftone processing.

[0010] (3) The present disclosure can also be implemented as an image processing device or image processing method that processes an original image expressed in multiple gradations to generate an output image expressed by a distribution of dots. In this image processing device or image processing method, the output image includes a first pixel group consisting of a plurality of pixels whose positions in the output image are determined, and a second pixel group consisting of a plurality of pixels whose positions are different from those of the first pixel group. During halftone processing that determines whether to form dots in accordance with the input gradation value of the original image, the arrangement of pixels in the first pixel group where dots are formed is a distribution having a first characteristic related to dispersibility, and the arrangement of pixels in the second pixel group where dots are formed is a distribution having a second characteristic related to dispersibility. When the input gradation value of the original image is equal to or greater than a predetermined intermediate value, the distribution based on the second characteristic is made to have no correlation with the distribution based on the first characteristic. The on / off of the dots in the first pixel group is determined by the arrangement based on the first characteristic, and the on / off of the dots in the second pixel group is determined by the arrangement based on the second characteristic.

[0011] The present disclosure can also be embodied as a program for realizing the image forming method or image processing method by a computer, or as a computer-readable recording medium on which the program is recorded. [Brief explanation of the drawings]

[0012] [Figure 1A] FIG. 1 is a schematic configuration diagram illustrating a printing system according to an embodiment, focusing on the content of image processing. [Figure 1B] FIG. 1 is a block diagram showing the configuration of an inkjet printer used in a printing system. [Figure 2] FIG. 2 is a cross-sectional view of the main part showing the head unit of the inkjet printer and the state of medium transportation. [Figure 3] FIG. 4 is an explanatory diagram illustrating an example of the nozzle arrangement for one color on the bottom surface of the print head. [Figure 4] FIG. 10 is an explanatory diagram conceptually illustrating a part of a dither mask. [Figure 5] An explanatory diagram showing the concept of turning dot formation on and off using a dither mask. [Figure 6] FIG. 10 is an explanatory diagram conceptually illustrating the spatial frequency characteristics of threshold values ​​set for each pixel of a dither mask having blue noise characteristics. [Figure 7] FIG. 1 is an explanatory diagram conceptually showing the visual spatial frequency characteristic VTF (Visual Transfer Function), which is the sensitivity characteristic of human vision to spatial frequencies. [Figure 8] FIG. 4 is an explanatory diagram illustrating an example in which first pixel groups and second pixel groups that form an output image are arranged in an alternating raster fashion. [Figure 9] 6 is a flowchart showing a dither mask generation process in the first embodiment. [Figure 10] FIG. 10 is an explanatory diagram illustrating positions where threshold values ​​are arranged in the divided dither masks for the first pixel group and the second pixel group when dots are arranged alternately in rasters. [Figure 11] 10 is a flowchart showing a dither mask evaluation process. [Figure 12]FIG. 10 is an explanatory diagram showing how dots are formed in four pixels in a divided dither mask, corresponding to elements that store thresholds that make dots first to fourth most likely to be formed. [Figure 13] FIG. 10 is an explanatory diagram showing a dot density mask in which a dot pattern in which dots are formed in five pixels in a divided dither mask is quantified as dot density. [Figure 14] FIG. 10 is an explanatory diagram showing how dots are formed individually in the first pixel group and the second pixel group. [Figure 15] FIG. 10 is an explanatory diagram illustrating an example of how an output image is formed by mixing first pixel groups and second pixel groups. [Figure 16] FIG. 4 is an explanatory diagram showing how an output image is formed by combining dots formed by a first nozzle unit and a second nozzle unit. [Figure 17] 10A and 10B are explanatory diagrams illustrating an example of the relationship between deviation in dot formation position and graininess. [Figure 18] 6A and 6B are explanatory diagrams illustrating an example of the relationship between deviation of dot formation positions and density changes. [Figure 19] FIG. 10 is an explanatory diagram showing an example in which first pixel groups and second pixel groups are arranged alternately in columns in a mixed region, as a second embodiment. [Figure 20] FIG. 10 is an explanatory diagram showing a variation of the second embodiment in which the mixed area is a checkered pattern. [Figure 21] FIG. 10 is an explanatory diagram showing another configuration in which the mixed area is a checkered pattern as one variation of the second embodiment. [Figure 22] FIG. 10 is an explanatory diagram showing a configuration in which the mixed region has a random pattern as another variation of the second embodiment. [Figure 23] FIG. 10 is an explanatory diagram showing another configuration in which the mixed region has a random pattern as another variation of the second embodiment. [Figure 24] FIG. 10 is an explanatory diagram showing a third embodiment in which nozzle rows are arranged in a staggered pattern. [Figure 25] FIG. 11 is an explanatory diagram showing a variation of the third embodiment in which nozzle rows are arranged obliquely with respect to the printing direction. [Figure 26]FIG. 10 is an explanatory diagram showing a configuration in which nozzle rows are arranged obliquely and in a staggered pattern. DETAILED DESCRIPTION OF THE INVENTION

[0013] A. First embodiment: (A1) Overall configuration of the printing device: 1A is a block diagram showing the configuration of a printing system 10 according to a first embodiment. This printing system 10 includes a host computer (hereinafter simply referred to as a computer) 90 that processes an original image and outputs output image data, and an inkjet printer 20 that can form full-color images.

[0014] The computer 90 is equipped with a well-known CPU and memory, and an application program 95 runs under a predetermined operating system. The operating system incorporates a video driver 91 and a printer driver 96, and the application program 95 outputs print data PD via these drivers to be transferred to the inkjet printer 20. The application program 95 performs the desired processing on the image to be processed, and displays the image on the display 11 via the video driver 91.

[0015] When the application program 95 issues a print command, the printer driver 96 of the computer 90 receives image data from the application program 95 and converts it into print data PD to be supplied to the inkjet printer 20. In the example shown in FIG. 1A, the printer driver 96 is equipped with a resolution conversion module 97, a color conversion module 98, a halftone module 99, and a rasterizer 94. In addition to the CPU and memory that perform this series of processes, the computer 90 is also equipped with a storage device 92 such as a hard disk, which stores programs for performing various processes, as well as a color conversion table LUT that the color conversion module 98 references in its color conversion process, and dither masks DM1 and DM2 that the halftone module 99 references in its halftone process.

[0016] The resolution conversion module 97 serves to convert the resolution (i.e., the number of pixels per unit length) of the color image data handled by the application program 95 into a resolution that can be handled by the printer driver 96. The image data that has undergone this resolution conversion is still image information consisting of the three colors of RGB. The color conversion module 98 refers to the color conversion table LUT and converts the RGB image data, pixel by pixel, into multi-tone data of multiple ink colors that can be used by the inkjet printer 20.

[0017] The color-converted multi-tone data has, for example, 256 tone values. The halftone module 99 performs halftone processing to represent these tone values ​​on the inkjet printer 20 by dispersing and forming ink dots. The halftone-processed image data is rearranged by the rasterizer 94 in the data order to be transferred to the inkjet printer 20, and is output as the final print data PD. The print data PD includes raster data that indicates the dot recording status during each main scan, and data that indicates the sub-scan feed amount. The details of the halftone processing will be described later.

[0018] (A2) Inkjet printer configuration: Next, the configuration of an inkjet printer 20 according to this embodiment will be described with reference to Figures 1B and 2. As shown in Figure 1B, the inkjet printer 20 includes a head unit 30 that ejects droplets, a drive signal generation unit 50 that drives the head unit 30, a transport mechanism 70 that transports the printing medium, and a control unit 60 that executes various processes.

[0019] The head unit 30 has M ejection units 35. In this embodiment, M is a natural number equal to or greater than 4, but M may also be 1, meaning there may be only one head unit 30. The drive signal generation unit 50 generates and outputs a drive signal Vin for driving the head unit 30. The transport mechanism 70 changes the relative position of the medium P with respect to the head unit 30. The control unit 60 controls the operation of each unit of the inkjet printer 20, such as the head unit 30 and the drive signal generation unit 50.

[0020] The inkjet printer 20 includes, for example, a display unit and an operation unit, but these components are not shown in the figures. The display unit is composed of a liquid crystal display, an organic EL display, an LED lamp, or the like, and displays the status of the inkjet printer 20, instructions for the user, error messages, and the like. The operation unit includes various switches for inputting instructions from the user, and an operation panel equipped with such switches. The display unit may display the content of the display by voice, and similarly, the operation unit may input instructions using voice recognition or the like. The display unit and operation unit can also easily be realized by a mobile phone or other portable terminal connected via wired or wireless connections, or a computer.

[0021] In this embodiment, the inkjet printer 20 is a line printer that ejects droplets from the head unit 30 onto a medium P transported by a transport mechanism 70 to form an image on the medium P. This process is shown schematically in FIG. 2. As indicated by the arrows X, Y, and Z in the figure, in the following description, the direction in which the medium P is transported is designated X, the width direction of the medium P is designated Y, and the direction perpendicular to the X and Y directions is designated Z. Regarding the X direction, the direction from upstream to downstream of the medium P is referred to as the +X direction. Regarding the Y direction, the direction from the right side (the far side of the page) to the left side (the near side of the page) of the transported medium P as viewed in the +X direction is referred to as the +Y direction. Regarding the Z direction, the direction from the medium P toward the head unit 30 is referred to as the +Z direction. The -X, -Y, and -Z directions are their opposite directions. These directions will also be indicated appropriately in other figures as necessary. Because the medium P moves in the +X direction relative to the head unit 30, the ink dots formed on the medium P are aligned sequentially from downstream to upstream on the medium P as printing progresses.

[0022] The transport mechanism 70, which transports the medium P from the upstream side to the downstream side, includes a transport motor 71 that serves as a drive source for transport, and a motor driver 72 that drives the transport motor 71. As shown in FIG. 2, the transport mechanism 70 also includes a platen 77 provided below the head unit 30 (in the -Z direction in FIG. 2), transport rollers 73 and 74 that rotate in response to the operation of the transport motor 71, and guide rollers 75 and 76 that follow the rotation of the transport roller 73 or 74. The medium P is transported in the +X direction in the figure (from the upstream side to the downstream side) along a transport path defined by the transport roller 73, guide roller 75, platen 77, guide roller 76, and transport roller 74.

[0023] The inkjet printer 20 includes a carriage 32, which houses a head unit 30 equipped with M ejection units 35. In addition to the head unit 30, the carriage 32 also houses a drive signal generation unit 50 (not shown in FIG. 2) and four ink cartridges 31. The carriage 32 is located on the opposite side of the transport path of the medium P from the platen 77, i.e., above the platen 77 (in the +Z direction).

[0024] The four ink cartridges 31 are provided in one-to-one correspondence with the four colors: yellow, cyan, magenta, and black, and each ink cartridge 31 is filled with ink of the color corresponding to that ink cartridge 31. Each of the M ejection units 35 receives a supply of ink from one of the four ink cartridges 31. Each ejection unit 35 is filled with ink supplied from the ink cartridge 31 and ejects the filled ink as droplets toward the medium P. This makes it possible to eject ink of all four colors from the M ejection units 35, thereby achieving full-color printing. The mechanism for ejecting droplets is well known, so a description thereof will be omitted.

[0025] The inkjet printer 20 according to the present embodiment includes four ink cartridges 31 corresponding to the four ink colors. However, this is not limited to four colors; the printer may include three or fewer ink cartridges 31 corresponding to three or fewer or five or more ink colors. The printer may also include ink cartridges 31 filled with ink of colors other than the four colors, or may include ink cartridges 31 corresponding to only some of the four colors. In other words, the inkjet printer according to the present disclosure may be capable of ejecting one or more colors of ink from the ejection units 35. Instead of being mounted on the carriage 32, each ink cartridge 31 may be provided in a separate location on the inkjet printer 20, and ink may be supplied to each ejection unit 35 of the head unit 30 via a tube or the like. The inkjet printer 20 may also print in a single color, such as black. In this case, M may be set to 1, meaning the printer may include a single ejection unit 35.

[0026] The timing of transport of the medium P and the timing of ejection of droplets from each ejection unit 35 of the head unit 30 are controlled by the control unit 60. Under the control of the control unit 60, each ejection unit 35 ejects ink onto the medium P at the timing when the medium P is transported to a desired position on the platen 77 by the transport mechanism 70, thereby forming an image on the medium P.

[0027] As shown in FIG. 1B , the control unit 60 receives data representing the on / off status of dots (hereinafter referred to as dot data) generated by a computer 90, such as a personal computer or digital camera, through halftone processing of image data Img, and controls the drive signal generation unit 50, transport mechanism 70, and other components to form an image on the medium P in accordance with the image data Img. Specifically, the control unit 60 controls the motor driver 72 to drive the transport motor 71 to transport the long medium P in the transport direction (+X direction), and also controls the drive signal generation unit 50 to control whether or not each ejection unit 35 ejects ink and the timing of ink ejection. In this way, the control unit 60 adjusts the arrangement of ink dots formed by ink ejected onto the medium P and executes a printing process to form an image on the medium P based on the dot data. The control unit 60 may also execute a process to transfer information such as error messages and ejection abnormalities to the computer 90 as needed.

[0028] The control unit 60 includes a CPU 61 and a storage unit 62. The storage unit 62 temporarily stores data required for executing various processes such as printing, including dot data supplied from the computer 90 via an interface unit (not shown), or includes RAM (Random Access Memory) in which control programs for executing various processes such as printing are temporarily loaded, and PROM, which is a type of non-volatile semiconductor memory, for storing control programs for controlling each unit of the inkjet printer 20.

[0029] The CPU 61 stores dot data supplied from the computer 90 in the memory unit 62. Halftoning involves binary digitization (whether or not to form an ink dot) when only one ink dot size can be formed by droplets ejected from each ejection unit 35. If two ink dot sizes (small and large) can be formed, halftoning involves ternary digitization (none, small, and large). If three ink dot sizes (small, medium, and large) can be formed, halftoning involves quaternary digitization (none, small, medium, and large). If the ink cartridge 31 contains a light ink such as light magenta or light cyan, halftoning with even more gradations is possible. In this embodiment, as described below, each ejection unit 35 can form only one type of dot, and the control unit 60 performs binarization. Note that such halftoning may be performed on the inkjet printer 20 side, and the inkjet printer 20 may receive unhalftoned image data Img from the computer 90 and perform halftoning before printing.

[0030] The CPU 61 of the control unit 60 generates signals such as a print signal SI and a drive waveform signal Com for controlling the operation of the drive signal generation unit 50 to drive each ejection unit 35 based on various data such as image data Img stored in the memory unit 62, and also generates various signals such as a control signal for controlling the operation of the motor driver 72 based on various data stored in the memory unit 62, and outputs these generated signals. In this way, the control unit 60 (CPU 61) generates various signals such as the print signal SI and drive waveform signal Com and supplies them to each unit of the inkjet printer 20, thereby comprehensively controlling the operation of each unit of the inkjet printer 20. This allows various processes such as printing to be performed.

[0031] The drive signal generation unit 50 generates drive signals Vin for driving each of the M ejection units 35 included in the head unit 30, based on the print signal SI and the drive waveform signal Com supplied from the control unit 60. Details of the generation of these signals will not be explained here.

[0032] FIG. 3 is an explanatory diagram illustrating an example of the arrangement of nozzles that eject black droplets on the bottom surface of the head unit 30 of the inkjet printer 20. Because the inkjet printer 20 is a line printer, the width of the ejection unit 35 of the head unit 30 in the Y direction is greater than the width of the medium P. To form an ejection unit 35 that ejects droplets across this width, multiple short nozzle heads, each with a predetermined number of nozzles N, are arranged so that they partially overlap in the X direction. For ease of understanding, the figure shows only the first nozzle unit 35a and the second nozzle unit 35b, each with 24 nozzles N in the Y direction. The first nozzle unit 35a, the second nozzle unit 35b, etc. are positioned and fixed to the head unit 30 with screws 36. Adjacent nozzle units are arranged alternately, but the illustration shows eight nozzles N at each end overlapping in the X direction. In an actual ejection unit 35, each nozzle unit has several hundred nozzles, with adjacent nozzle units overlapping by approximately 120 nozzles.

[0033] The pitch pt between the nozzles N in each nozzle row can be set appropriately depending on the printing resolution (dpi: dots per inch). The printing resolution of the inkjet printer 20 according to this embodiment is 720 x 720 dpi. The Y-direction resolution of the inkjet printer 20 depends on the configuration of the head unit 30, specifically the spacing between the nozzles N, while the X-direction resolution depends on the spacing between droplets ejected from the ejection unit 35 and the speed at which the medium P is transported by the transport mechanism 70. These can be freely set depending on the design of the inkjet printer 20.

[0034] In this example, dots are formed in region L1 only by the first nozzle unit 35a, and dots are formed in region L2 only by the second nozzle unit 35b. In the mixed region LA where the two nozzle units 35a and 35b overlap, dots are formed by both the first nozzle unit 35a and the second nozzle unit 35b. In regions L1 and L2, the spacing between dots formed on the medium P is equal to the pitch pt between adjacent nozzles N, and the dot spacing does not vary. In contrast, in the mixed region LA, dots formed by the first nozzle unit 35a and dots formed by the second nozzle unit 35b are mixed, so there is a misalignment Δd between the positions of the nozzles N between the first nozzle unit 35a and the second nozzle unit 35b in the dot formation positions in the mixed region LA. In the figure, this misalignment Δd is depicted as being smaller than the pitch pt between adjacent nozzles. However, if the nozzles N are formed at, for example, 720 dpi, the pitch pt between the nozzles N is 25.4 mm / 720 ≒ 35 μm. Therefore, if the first and second nozzle portions 35a, 35b are fixed with screws 36 or the like, a deviation larger than the nozzle pitch pt, for example several times the nozzle pitch pt, may occur depending on the mechanical attachment precision. The configuration of such a discharge portion 35 is similar to that of the discharge portions 35 for inks of other colors.

[0035] In this embodiment, the multiple nozzles N that make up each nozzle row are arranged in a row in the Y-axis direction, but it is also possible to arrange the multiple nozzles N that make up each nozzle row so that the positions of the even-numbered nozzles N and the odd-numbered nozzles N from the left side of the figure are shifted in the X-axis direction, in a so-called staggered pattern.

[0036] In the inkjet printer 20, while droplets are being ejected from the ejection units 35, the medium P is transported by the transport mechanism 70 in the +X direction in FIG. 2 at a predetermined transport speed Vm. The transport speed Vm (printing speed) of the inkjet printer 20 according to this embodiment is 220 m / min or faster. While the medium P is being transported, droplets are ejected from the nozzles N of the head unit 30 to form ink dots on the medium P, thereby recording an image on the medium P. In other words, the image is recorded as a collection of ink dots formed by droplets ejected from each nozzle N arranged at the printing resolution in the Y direction. Focusing on one ejection unit 35, the ink dots formed by droplets ejected from the nozzles that make up that ejection unit 35 are aligned across the width of the medium P (the Y direction). For this reason, in such line printers, the arrangement of ink dots across the width of the medium P is called a "raster."

[0037] (A3) Halftone processing: The inkjet printer 20 having the hardware configuration described above receives halftoned dot data from the computer 90, and drives the head unit 30 in accordance with the dot data while transporting the medium P using the transport motor 71. This causes droplets of each color to be ejected onto the transported medium P, forming a multi-color, multi-tone image on the medium P. The halftoning process that generates this dot data is described below. The halftoning process in this embodiment is performed using an ordered dithering method that uses a dither mask that provides a dot arrangement with excellent dispersion.

[0038] 4 is an explanatory diagram conceptually illustrating a portion of a dither mask. The illustrated mask has 128 elements in the direction in which the nozzles N are arranged (Y direction, hereinafter also referred to as the main scanning direction) and 64 elements in the direction intersecting the nozzle arrangement direction (X direction, hereinafter also referred to as the sub-scanning direction), for a total of 8,192 elements, and thresholds selected evenly from a range of gradation values ​​from 1 to 255 are stored for each of these. Such a dither mask is created in advance and stored in the storage device 92. The size of the dither mask is not limited to the size illustrated in FIG. 4, and various sizes can be used, including masks with the same number of vertical and horizontal elements.

[0039] FIG. 5 is an explanatory diagram illustrating the concept of dot formation using a dither mask. For convenience of illustration, only some elements are shown. To determine whether or not to form a dot, as shown in FIG. 5, the tone value of the image data is compared with the threshold value stored at the corresponding position in the dither mask. If the tone value of the image data is greater than the threshold value stored in the dither mask, a dot is formed; if the tone value of the image data is smaller, a dot is not formed. The hatched pixels in the figure indicate pixels where a dot will be formed. Using a dither mask, therefore, the simple process of comparing the tone value of the image data with the threshold value set in the dither mask can determine whether or not a dot will be formed for each pixel, thereby enabling rapid tone number conversion processing. Furthermore, once the tone value of the image data is determined, whether or not a dot will be formed at each pixel is determined solely by the threshold value set in the dither mask. As is clear from this, in the ordered dither method, the dot formation status and the dispersion of the formed dots can be controlled by the storage location of the threshold value set in the dither mask. Therefore, the arrangement of dots formed by halftoning can be optimized by optimizing the dither mask. The dither mask optimization process will be explained in detail later.

[0040] FIG. 6 is an explanatory diagram conceptually illustrating the spatial frequency characteristics of thresholds set for each pixel of a blue noise dither mask having blue noise characteristics, as a simple example of dither mask adjustment. The spatial frequency characteristics of the blue noise mask are characterized by having the largest frequency components in the high-frequency range, where one cycle is two pixels or less. These spatial frequency characteristics are set with consideration given to human visual characteristics. In other words, the blue noise dither mask is a dither mask in which the storage positions of the thresholds are adjusted so that the largest frequency components occur in the high-frequency range, taking into account the human visual characteristic of low sensitivity in the high-frequency range.

[0041] Column (a) of Figure 7 is an explanatory diagram conceptually illustrating the visual spatial frequency characteristic (VTF) (Visual Transfer Function), which is the sensitivity characteristic of human vision to spatial frequency. Using the visual spatial frequency characteristic (VTF), it is possible to quantify the graininess of dots after halftoning by modeling human visual sensitivity as a transfer function called the visual spatial frequency characteristic (VTF). The value quantified in this way is called the graininess index. Column (b) of Figure 7 shows a representative empirical formula for the visual spatial frequency characteristic (VTF). The variable L in column (b) of Figure 7 represents the observation distance, and the variable u represents the spatial frequency. Column (c) of Figure 7 is the formula for defining the graininess index. The coefficient K in column (c) of Figure 7 is a coefficient used to align the obtained value with human perception.

[0042] When calculating the graininess index of a two-dimensional printed image, the integration in column (c) of Fig. 7 is typically performed on frequency components in all directions on the medium. However, in this embodiment, the range over which the integration in column (c) of Fig. 7 is performed is limited to some directions, thereby calculating the graininess index for each direction. This graininess index for each direction can be used as an index corresponding to the first and second characteristics related to dispersibility, as will be described later.

[0043] This quantification of graininess that appeals to human vision makes it possible to finely optimize dither masks for the human visual system. Specifically, a Fourier transform is performed on the dot pattern that is expected when each input tone value is input into the dither mask to obtain a power spectrum FS, and then filtering is performed by multiplying it by the visual spatial frequency characteristic VTF, after which the resulting granularity evaluation value is integrated over all input tone values ​​(column (c) in Figure 7) and can be used as the evaluation function for the dither mask. In this example, optimization can be achieved by adjusting the storage position of the threshold value so that the evaluation function for the dither mask is reduced.

[0044] If the print resolution is sufficiently high and the peak occurs in a region where there is no visual sensitivity, the dither mask can be adjusted to have green noise characteristics instead of blue noise characteristics. In this case, the green noise characteristics can be imparted to the dither mask by adding a predetermined bias to the VTF function or the low-pass filter described below. The predetermined bias can be configured, for example, by artificially reducing the sensitivity of the VTF function in the peak frequency band of the green noise characteristics.

[0045] (A4) Creating a dither mask assuming dot formation by different nozzles: As described above, in the inkjet printer 20 of this embodiment, when forming one raster, there are regions where only a single nozzle unit is involved in dot formation, and mixed regions where two nozzle units are involved in dot formation. In the example shown in Figure 3, in region L1, dots are formed only by the first nozzle unit 35a, in mixed region LA, dots are formed by the first nozzle unit 35a and the second nozzle unit 35b, and in region L2, dots are formed only by the second nozzle unit 35b.

[0046] Therefore, the pixels of the output image are grouped as follows. The top row of Figure 8 shows 24 pixels in the horizontal direction (Y direction) and 8 pixels in the vertical direction (X direction) of the multiple pixels that make up the output image. This arrangement of pixels in the horizontal direction is also called a raster. Raster numbers are assigned to the left edge of the figure. First pixel group: As shown by the "●" in the middle of the figure, this is a group consisting of pixels that belong to odd-numbered rasters in the X direction, starting from the first raster. Second pixel group: As shown by the "○" in the lower part of the figure, this is a group consisting of pixels that belong to even-numbered rasters in the X direction, starting from the second raster.

[0047] As described above, the multiple pixels that make up the output image are divided into two groups of alternate pixels using rasters as units, and dots are formed as follows when the input tone value of the original image is a predetermined intermediate value: For example, if the input tone value of the original image is in the range of 0-255 and the tone value is 31, which is about 1 / 8 of the maximum value, then, ignoring dot gain, this corresponds to a state in which dots are formed in 1 / 8 of all pixels, that is, on average 12 pixels out of the 24 x 4 pixels that make up the first pixel group shown in Figure 8.

[0048] The arrangement of pixels in the first pixel group where dots are formed is determined to have a first characteristic related to dispersibility. In other words, dot formation is determined using a dither mask with a threshold set so that the distribution of the 12 pixels formed at this time exhibits dispersibility such as the blue noise characteristic shown as an example. Similarly, the arrangement of pixels in the second pixel group where dots are formed is determined to have a second characteristic related to dispersibility. The first characteristic of the distribution of pixels in the first pixel group where dots are formed and the second characteristic of the distribution of pixels in the second pixel group where dots are formed are determined to have no correlation in terms of dispersibility, at least when the input gradation value of the original image is an intermediate value equal to or greater than a predetermined value. "No correlation in pixel distribution" means that the arrangement of dots in one pixel group is not referenced in determining the arrangement of dots in the other pixel group. Even if both pixel groups have blue noise characteristics, they can be said to have no correlation if the arrangement of dots in the other pixel group does not affect the determination of each other's dot arrangement. As a result, the on / off state of dots in the first pixel group is determined by an arrangement having a first characteristic, and the on / off state of dots in the second pixel group is determined by an arrangement having a second characteristic that is not correlated with the first characteristic.

[0049] (A5) Dither mask generation: A method for generating a dither mask having the above characteristics will now be described. Fig. 9 is a flowchart showing the processing routine of the dither mask generation method in the first embodiment. This dither mask generation method is configured to be able to optimize the first pixel group and the second pixel group obtained by dividing the output image by alternating rasters, taking into account the dispersion of dots formed in each group.

[0050] When the dither mask generation process is started, a grouping process is first performed (step S100). The grouping process refers to a process in which a plurality of pixels constituting the output image are virtually divided into a first pixel group and a second pixel group, and each element of the dither mask is correspondingly divided into a first group and a second group. In this embodiment, as already explained, each pixel of the output image is virtually divided by alternating rasters, and therefore each element of the dither mask is also similarly divided by alternating rasters.

[0051] FIG. 10 shows a dither mask M used in dither-based halftone processing, and the divided dither masks M1 and M2 obtained by dividing it into alternating rasters. For ease of explanation, this example illustrates the creation of a small dither mask with 8 rows and 8 columns. The row numbers of the divided dither mask in the figure correspond to the raster numbers in the output image. The numbers written on each element of the divided dither masks M1 and M2 indicate the pixel group to which each element belongs. In this example, elements in odd-numbered rows belong to the first group corresponding to the first pixel group, and elements in even-numbered rows belong to the second group corresponding to the second pixel group.

[0052] The divided dither mask M1 is composed of a plurality of dither mask elements corresponding to pixels belonging to the first pixel group and a plurality of blank elements. On the other hand, the divided dither mask M2 is composed of a plurality of dither mask elements corresponding to pixels belonging to the second pixel group and a plurality of blank elements. This grouping process is set on the assumption that the mixed area LA will be printed by different first nozzle units 35a and second nozzle units 35b in alternating rasters. After being generated, the divided dither masks M1 and M2 are combined to form a single dither mask M.

[0053] Once the grouping process (step S100) is complete, a target threshold determination process (step S200) is performed. In this target threshold determination process, the threshold to be stored in each storage element of each divided dither mask M1, M2 is determined. In this embodiment, the threshold is determined by selecting a relatively small threshold value, that is, a threshold value that makes it easier to form dots in halftone processing using the dither method. The reason for this will be explained later.

[0054] After the threshold value is determined in the target threshold value determination process, a dither mask evaluation process (step S300) is performed. The dither mask evaluation process is a process for quantifying the optimality of the dither mask based on a preset evaluation function. In this embodiment, a two-dimensional graininess index defined by a calculation formula in which the one-dimensional graininess index calculated by the calculation formula in FIG. 7(c) is expressed in a two-dimensional region, taking into account the direction of the printed image, is used as the evaluation function. This two-dimensional graininess index is used to evaluate the divided dither masks M1 and M2. Details of the two-dimensional graininess index will be described later.

[0055] The details of this dither mask evaluation process will be explained using the flowchart in Fig. 11. When the process starts, an evaluation mask selection process is first performed (step S310). In this embodiment, the evaluation mask selection process is a process of selecting one of two divided dither masks M1 and M2 in order. First, the subsequent process will be explained assuming that divided dither mask M1 has been selected as the evaluation mask.

[0056] Next, the dots corresponding to the determined thresholds are turned on (step S320). The determined thresholds refer to the thresholds for which the stored elements have been determined. In this embodiment, as described above, the thresholds are selected in order starting from the value at which dots are most likely to be formed, so when dots are formed for the target threshold, dots are always formed in pixels corresponding to elements for which the determined thresholds have been stored. Conversely, for the smallest input tone value at which dots are formed for the target threshold, dots are not formed in pixels corresponding to elements other than the element for which the determined thresholds have been stored.

[0057] 12 is an explanatory diagram showing a dot pattern DPM in which a dot (● mark) is formed in each of four pixels corresponding to elements storing thresholds for which dots are first to fourth most likely to be formed in the divided dither mask M1. This dot pattern is used to determine at which pixel the fifth dot should be formed. In other words, it is used to determine the element storing the target threshold for which dots are fifth most likely to be formed. The * mark indicates the pixel corresponding to the target element.

[0058] Next, the corresponding dot of the target element is turned on (step S330). In this example, the target element is one of the candidates for the target threshold storage element that is the fifth most likely to form dots. The target element is selected from the elements of the evaluation mask (in this example, the divided dither mask M1), so it is selected from the elements of the odd-numbered rows.

[0059] Next, a graininess index calculation process is performed (step S340). The graininess index calculation process is a process that uses the above-mentioned formula to calculate a graininess index for the dot pattern DPM when it is assumed that dots have been formed in pixels corresponding to the element of interest. This process is performed based on a dot density mask that quantifies the dot pattern shown in Figure 13. The dot density mask shown in the figure is configured so that pixels with dots formed have a value of "1" and pixels with no dots formed have a value of "0".

[0060] The processing of steps S330 and S340 is performed sequentially on all elements in odd-numbered rows, excluding elements for which thresholds for which dots are most likely to be formed have already been stored, as the target element. The processing of changing the target pixel and assuming that a dot will be formed at that pixel (step S330) and the calculation of the graininess index in that state—in this example, five dots turned on—(step S340) are completed for all target pixels (step S350: “Yes”). Next, it is determined whether processing has been completed for all evaluation masks, i.e., divided dither mask M1 and divided dither mask M2 (step S360). Once processing for divided dither mask M1 is complete, the evaluation mask is replaced with divided dither mask M2, and the processing returns to step S320, where steps S320 to S350 are repeated for divided dither mask M2. The processing for the divided dither mask M2 is the same as that for the divided dither mask M1, except that the processing is performed on elements in even rows.

[0061] When the calculation of the graininess index at all possible pixel positions of interest for all evaluation masks, in this case divided dither mask M1 and divided dither mask M2, is completed, the evaluation values ​​for each mask are saved (step S370), and the dither mask evaluation process is terminated.

[0062] Next, the process of step S400 in FIG. 9 is performed. This process is the optimal storage position determination process. The storage element determination process determines the element (storage position in the divided dither mask) that stores the threshold value at which dots are most likely to be formed next. The storage element is determined individually for divided dither mask M1 and divided dither mask M2 based on the saved evaluation values. Here, for each threshold, the pixel of interest that would result in the smallest granularity index if a dot were to be formed at the pixel of interest is set as the optimal storage position. However, it is possible that a pixel position that does not result in the smallest granularity index at a certain threshold may subsequently result in the smallest granularity index at the threshold value at which dots are most likely to be formed next. To prepare for such a case, all evaluation values ​​for each threshold value of each evaluation mask may be saved, and a so-called annealing process may be performed to determine the optimal storage position for the threshold value.

[0063] It is determined whether this process has been performed for all thresholds, from the threshold at which dots are most likely to be formed to the threshold at which dots are least likely to be formed (step S500), and if it is determined that all thresholds have been arranged (step S500: "Yes"), the obtained divided dither mask M1 and divided dither mask M2 are combined to generate a dither mask M to be used in halftone processing (step S600), and the dither mask generation process is completed.

[0064] (A6) Dot formation method: The divided dither masks M1 and M2 are generated using the method described above, and the dither mask M generated by combining these is used to generate a dither mask M. This section explains the dot on / off behavior when halftoning is performed using this dither mask. Figure 14 shows the case where dither mask M is applied to the first pixel group and the second pixel group. In the figure, diagonally hatched dots DD indicate that dots have been determined to be formed during halftoning. The positions of the dots formed are illustrative. Note that divided dither mask M1 or divided dither mask M2 may be used during halftoning instead of dither mask M. Since the first pixel group forms dots in odd-numbered rasters and the second pixel group forms dots in even-numbered rasters, the results are the same regardless of which dither mask is used.

[0065] The bottom row of the figure shows both pixel groups together. For example, if the output image is printed using a serial printer, with dots formed in the first pixel group during the forward movement of the printing head and dots formed in the second pixel group during the return movement of the head, then in the example at the bottom of Figure 14, both the first pixel group and the second pixel group are formed on the medium P by droplets ejected from the same head. On the other hand, in a line printer equipped with multiple ejection units 35, such as the inkjet printer 20 of this embodiment, as shown in Figure 3, there are regions L1 and L2 where dots are formed by either the first nozzle unit 35a or the second nozzle unit 35b, and a mixed region LA where dots are formed by both the first nozzle unit 35a and the second nozzle unit 35b. In this case, even if dots are formed alternately in rasters, as shown in Figure 15, only one of the first nozzle unit 35a or the second nozzle unit 35b can be used outside the mixed region LA.

[0066] FIG. 16 shows a schematic diagram of how an image including a mixed area LA is formed. As shown, the output image is expressed by a halftone module 99 using a dither mask M with on / off transitions, and the dot arrangement is set to a low graininess based on the setting made when the dither mask M was generated. The inkjet printer 20 receives print data PD corresponding to this output image and forms dots using the first nozzle unit 35a and the second nozzle unit 35b based on the dot data. As described with reference to FIG. 15, dots are formed in the mixed area LA using the first nozzle unit 35a and the second nozzle unit 35b in alternating rasters. Otherwise, dots are formed using only the first nozzle unit 35a or only the second nozzle unit 35b. The resulting printed image PI on the medium P is a superposition of an image Pi1 made up of dots formed by the first nozzle unit 35a and an image Pi2 made up of dots formed by the second nozzle unit 35b.

[0067] At this time, because the first nozzle unit 35a and the second nozzle unit 35b are positioned and fixed by screws 36 or the like, the positions where they form dots in the mixed area LA do not completely coincide with each other as shown in FIG. 15. In this embodiment, as already explained using FIG. 3, the pitch pt between the nozzles N is approximately 35 μm. If the mechanical positioning accuracy using screws 36 or the like is approximately 100 μm, the misalignment between the nozzles of the first nozzle unit 35a and the nozzles of the second nozzle unit 35b in the mixed area LA can be several times the nozzle pitch pt. ​​To address this misalignment, in this embodiment, the dot arrangement in the first pixel group where the first nozzle unit 35a forms dots and the dot arrangement in the second pixel group where the second nozzle unit 35b forms dots are each designed to have blue noise characteristics. Therefore, even if the dot formation positions in the mixed area LA are misaligned by several dots in either the X or Y direction, it does not result in a significant decrease in graininess. Therefore, even if there is a relative misalignment in the arrangement of dots formed using the first nozzle section 35a and dots formed using the second nozzle section 35b, there is no significant difference in graininess between the mixed area LA and the areas L1 and L2, and degradation of image quality in the mixed area LA is sufficiently suppressed for the following reasons.

[0068] In the head unit 30 of the inkjet printer 20, the nozzles N are formed at the same pitch as the resolution (e.g., 720 dpi). Therefore, in regions L1 and L2, the dot formation positions do not shift in the X direction, let alone the Y direction, i.e., between rasters. In contrast, in the mixed region LA, the dot formation positions are affected by the positioning accuracy when the first nozzle unit 35a and the second nozzle unit 35b are attached, and in some cases, there is a possibility of a shift of several dots. In this case, if a dither mask M is used in which the arrangement of threshold values ​​used in dither-based halftone processing is simply set to blue noise characteristics, the graininess of the mixed region LA will deteriorate according to the amount of shift in the dot formation values.

[0069] This is shown in column (A) of Figure 17. The horizontal axis of the figure represents the input gradation value of the image on a scale of 0-100%, with the range of 0-50% indicated. The vertical axis represents the graininess index, with the graininess worsening and image quality decreasing as the value increases. The figure graphs the relationship between the input gradation value and the graininess index, using the difference in the amount of shift between the dots formed by the first nozzle unit 35a and the dots formed by the second nozzle unit 35b in the mixed region LA as a parameter (Shift 0-6). As shown in the figure, even when a dither mask with blue noise characteristics is used, the graininess index in the mixed region LA deteriorates at all gradation values ​​as the amount of shift (shift) between the dots formed by the first nozzle unit 35a and the dots formed by the second nozzle unit 35b increases. As already explained, there is no shift in the dot formation position outside of mixed area LA, and the granularity corresponds to Shift 0 in the diagram. Therefore, if there is a shift in the dot formation position in mixed area LA and the granularity deteriorates, the appearance of the image will change between mixed area LA and areas L1 and L2, resulting in a significant degradation in image quality.

[0070] In contrast, in this embodiment, the divided dither masks M1 and M2 each have blue noise characteristics, and their characteristics are not correlated. Column (B) of FIG. 17 shows the relationship between the input gradation value and the granularity index when the characteristics of the divided dither masks M1 and M2 are further correlated. The correlation between the divided dither masks M1 and M2 corresponds to a case in which, in the dither mask evaluation process of FIG. 11, the granularity of dots generated by the divided dither mask M1 is evaluated in total using a threshold set from the side where dots are most likely to be formed by the divided dither mask M1 and a threshold set from the side where dots are most likely to be formed by the divided dither mask M2, and the result is evaluated to have blue noise characteristics. Even when such dither masks are used, the granularity decreases as the amount of misalignment increases. As shown in the figure, the granularity index is higher when there is a misalignment of 2 dots (solid line J2) than when there is no misalignment (solid line J0), and the granularity index is even higher when the misalignment is 3 to 6 dots (solid lines J3-6). However, the graininess index is suppressed to a fraction of that when a single dither mask with blue noise characteristics is used without using the divided dither masks M1 and M2, and when the shift is 3 dots or more, the graininess index is roughly the same, as shown by the solid line J3-6. Moreover, even at the lowest graininess index, the image quality remains sufficiently high.

[0071] Thus, compared to a dither mask in which there is further correlation between the divided dither mask M1 and the divided dither mask M2, a dither mask without correlation between the divided dither mask M1 and the divided dither mask M2 has a graininess index of 3 to 6 dots out of the characteristics shown in column (B) of FIG. 17, even when the amount of misalignment between the dots formed by the first nozzle unit 35a and the dots formed by the second nozzle unit 35b is 0. Therefore, compared to the graininess index in regions L1 and L2, i.e., when there is no misalignment in the dot formation positions, the graininess index in the mixed region LA, where there may be a misalignment of a few dots or less in the dot formation positions, does not drop significantly, and the difference in graininess when viewed as an image is sufficiently suppressed. Therefore, the difference in appearance between regions L1, L2, and the mixed region LA is suppressed to an almost imperceptible level.

[0072] Next, we will explain the density difference between regions L1 and L2 and the mixed region LA. When dots are formed using two different heads or scans, density changes may occur in addition to reduced graininess. Figure 15 illustrates paired dots PD1 and PD2, in which dots formed in the first pixel group and dots formed in the second pixel group are adjacent. In this case, in an image formed by alternating rasters, if the dot positions formed by the second nozzle unit 35b in the mixed region LA are shifted by one in the direction of the adjacent raster (downward in the figure), the dots formed by the second nozzle unit 35b in the paired dots PD1 will completely overlap with the dots formed by the first nozzle unit 35a. This reduces the area covered by the dots on the medium P, thereby reducing the density of the image. On the other hand, in region L2, the dot formation positions are not shifted in the direction of the adjacent raster, so one of the paired dots PD2 will not overlap the other, and no density reduction will occur. As a result, a difference occurs in the appearance of the image between the areas L1 and L2 and the mixed area LA, which can result in a decrease in image quality.

[0073] This phenomenon occurs every time the offset is an odd number of dots. This is shown in Figure 18. Graphs A1 (solid line) and A2 (dashed line) show the case where the occurrence rate of paired dots, in which dots are formed on both adjacent pixels belonging to different pixel groups, is not controlled, while graphs B1 (solid line) and B2 (dashed line) show the case where the occurrence rate of paired dots is controlled. In the figure, the vertical axis shows the percentage of overlap of formed dots with other dots (hereinafter also referred to as coverage rate), with the offset amount set to 0 as the base. Note that graphs A1 and B1 shown as solid lines represent the results of simulations, while graphs A2 and B2 shown as dashed lines represent actual measurements. When dots are actually formed, due to differences in the droplet ejection speed and ejection direction, the increase and decrease in dot coverage rate does not follow a clear two-pixel cycle, but rather follows an average value, as shown by dashed lines A2 and B2.

[0074] Therefore, it is desirable to use a dither mask that controls the occurrence rate of paired dots and obtains the characteristics of graphs B1 and B2. Such characteristics are obtained by approximating the probability of simultaneous dot formation in a pair of a first pixel selected from a first pixel group and a second pixel selected from a second pixel group to a value determined in correspondence with the square of the input tone value when the separation distance between the first pixel and the second pixel is smaller than a predetermined threshold. The method of approximating the probability of simultaneous dot formation in a pair of a first pixel and a second pixel to a value determined in correspondence with the square of the input tone value is disclosed in, for example, Japanese Patent Application Laid-Open No. 2012-204939, and is a well-known technique, so a detailed description will be omitted. Furthermore, it was found that, as with the divided dither masks M1 and M2 already described, if both masks have blue noise characteristics and the characteristics of the two dither masks are not correlated, the resulting dot coverage rates are as shown in graphs B1 and B2. This is because the distribution of dots formed in the two pixel groups has blue noise characteristics and is highly dispersed. When the dots formed in both pixel groups are considered together, the characteristics of the distribution of dots formed in the first pixel group and the characteristics of the distribution of dots formed in the second pixel group are not correlated. Therefore, even when there is no misalignment in the dot formation positions, that is, even in areas L1 and L2, paired dots occur at a certain rate. Note that the reason for not correlating the characteristics of the two dither masks may be limited to when the input tone value of the original image is equal to or greater than a predetermined intermediate value. When the input tone value is small, the distance between dots is already sufficiently far apart, and dot overlap due to misalignment does not occur. In such areas with sufficiently low input tone values, it is often preferable to correlate the characteristics of the divided dither masks M1 and M2 to increase overall granularity.

[0075] As a result, even if there is a misalignment in the dot formation position due to the positioning accuracy of the first nozzle section 35a and the second nozzle section 35b in the head section 30, the image printed by the printing system 10 of this embodiment is able to maintain high quality, with graininess and density fluctuations being sufficiently suppressed between areas L1 and L2 where dots are formed by one of the nozzle sections and the mixed area LA where dots are formed by both nozzle sections.

[0076] B. Second embodiment: In the first embodiment, dots are formed in the first pixel group and the second pixel group in a raster-alternating manner in the mixed region LA. However, the dot formation in the mixed region LA is not limited to a raster-alternating manner and may be any other manner. For example, as shown in FIG. 19, dots may be formed in a column-alternating manner by the first nozzle unit 35a and the second nozzle unit 35b. In the figure, the lower row PxI shows the state in which the multiple pixels constituting the image are virtually divided into the first pixel group and the second pixel group in which dots are formed in a column-alternating manner, and the middle row PxR shows which nozzle unit actually forms the dots. As shown in the figure, dots are formed only by the first nozzle unit 35a in region L1, dots are formed only by the second nozzle unit 35b in region L2, and dots are formed in the mixed region LA by both the first nozzle unit 35a and the second nozzle unit 35b. In this case, the divided dither masks M1 and M2 used in halftone processing are generated to have characteristics such as blue noise characteristics for each virtual pixel group in the lower row PxI, and these characteristics are not correlated with each other. In the following description, the symbols L1, L2, PxI, etc. are used in the same meaning as in the above description.

[0077] Alternatively, the arrangement of the pixel groups in the mixed area LA may be a checkered pattern that combines alternating columns and alternating rasters, as shown in Fig. 20. Alternatively, as shown in Fig. 21, the mixed area LA may be a checkered pattern in which the proportion of dots formed in the other pixel group at both ends of the mixed area LA gradually decreases or increases.

[0078] Alternatively, as shown in Figure 22, the division into the first pixel group and the second pixel group may be performed in a random pattern. In Figure 22, the pixels are divided into the first pixel group and the second pixel group every other pixel in the order of the threshold value that generates blue noise, starting with the smallest value, and the pattern of pixels on which dots are formed in the mixed area is random. Figure 23 also shows a pattern in which the proportion of dots formed in the other pixel group in the mixed area LA gradually decreases and increases.

[0079] Regardless of which of these divisions is used, it is possible to reduce the differences in graininess and dot coverage between the mixed area LA, where dots formed by multiple different nozzle units are mixed, and the areas L1 and L2, where dots are formed by a single nozzle unit, and to maintain the quality of the printed image at a level that is high enough for practical use, just like in the first embodiment.

[0080] C. Third embodiment: In the above embodiments, the nozzles that form dots are arranged in a single row parallel to the raster in each of the first nozzle section 35a and the second nozzle section 35b. However, as shown in FIG. 24, the nozzles may be arranged in two rows in each of the first nozzle section 135a and the second nozzle section 135b. In this case, the nozzles in one nozzle row NL1 may be offset by half the nozzle pitch pt in the nozzle row arrangement direction (Y direction) relative to the nozzles in the other nozzle row NL2. When the nozzle rows are arranged in a staggered pattern, the offset between the staggered nozzle rows NL1 and NL2 is smaller than the offset between the first nozzle section 135a and the second nozzle section 135b, so the positional offset due to the staggered arrangement is ignored. However, if the first nozzle section 135a and the second nozzle section 135b arranged in this staggered pattern are used, and an alternating column pattern is adopted in the mixed area LA, the areas into which the pixel groups are virtually divided will coincide with the division into the staggered nozzle rows NL1 and NL2, so it is possible to divide the pixel groups by taking both into account and not ignoring the misalignment between the nozzle rows NL1 and NL2.

[0081] In the first embodiment and the like, the nozzles arranged in a row in the first nozzle section 35a are arranged in a direction that coincides with the raster direction. However, as shown in FIG. 25, they may be arranged at an angle to the raster direction. In this example, the first nozzle section 135a and the second nozzle section 135b are each arranged at an angle of 45 degrees to the direction in which the raster is formed (the Y direction). By forming the first nozzle section 135a and the second nozzle section 135b at an angle, the dot pitch that forms the raster can be narrower than the pitch due to the nozzle processing formed in the nozzle section. Furthermore, since the timing of forming adjacent dots on the same raster can be made different, when dots are formed using droplets, the possibility of droplets coming into contact with each other and mixing colors can be reduced.

[0082] Furthermore, as shown in Figure 26, two or more diagonal nozzle rows NI1 and NI2 may be provided for each of the first nozzle section 235a and the second nozzle section 235b. In this case, the pitch of the formed dots in the raster direction can be made even finer. In either case, two nozzle sections are used to form dots in the mixed area LA, and one nozzle section is used in the areas L1 and L2. In this case, the granularity and variations in coverage between dots can be reduced, thereby suppressing degradation of image quality, as in the first and second embodiments.

[0083] D. Fourth embodiment: The fourth embodiment differs from the above embodiments in the method of generating a dither mask. In the fourth embodiment, the dither mask evaluation process shown in FIG. 11 is performed only on the first pixel group, and the generated divided dither mask M1 is shifted to generate a divided dither mask M2 to be applied to the second pixel group. This is because if the dither mask is shifted vertically and horizontally by a sufficient number of pixels, the correlation in characteristics between the two dither masks disappears. In this way, a divided dither mask can be easily generated. Note that the generated dither mask can be applied to form an image using any of the discharge units 35 configured in the first to third embodiments.

[0084] E. Other Embodiments: (1) In addition to the above, the present disclosure can be implemented as an image forming apparatus that forms an output image corresponding to an original image. The image forming apparatus includes a halftone processing unit that determines whether dot formation is on or off in accordance with an input gradation value of the original image to be formed, and a dot forming unit that forms an output image consisting of a plurality of pixels using the dots based on the results of processing by the halftone processing unit. The output image includes a first pixel group consisting of a plurality of pixels whose positions in the output image have been determined, and a second pixel group consisting of a plurality of pixels whose positions are different from those of the first pixel group. The halftone processing unit determines whether the pixels in the first pixel group on which dots are formed have a distribution having a first characteristic related to dispersibility, and whether the pixels in the second pixel group on which dots are formed have a distribution having a second characteristic related to dispersibility, and determines whether the distribution based on the second characteristic is not correlated with the distribution based on the first characteristic when the input gradation value of the original image is equal to or greater than a predetermined intermediate value. The image forming apparatus determines whether the dots are on or off in the first pixel group based on the distribution having the first characteristic, and whether the dots are on or off in the second pixel group based on the distribution having the second characteristic.

[0085] In this image forming device, when determining whether to turn on or off dots in a first pixel group using an arrangement having a first characteristic and determining whether to turn on or off dots in a second pixel group using an arrangement having a second characteristic, the dot distribution based on the second characteristic regarding dot dispersibility is made to have no correlation with the dot distribution based on the first characteristic when the input gradation value of the original image is equal to or greater than a predetermined intermediate value. As a result, even if there is a difference in the formation positions of dots formed corresponding to the first pixel group and dots formed corresponding to the second pixel group in at least a part of the output image, fluctuations in image quality due to the difference in formation positional deviation are suppressed.

[0086] The halftone processing unit may be provided on the side of a device that directly forms dots, such as a printer or display, or on the side of an imaging device, such as a computer or camera, and the results of the halftone processing may be output to these devices in the form of dot data that indicates whether dot formation is on or off. Alternatively, the halftone processing may be performed by a rasterizer or the like provided between the computer that outputs the original image and the printing device.

[0087] The on / off dot formation determined by the halftone processing unit refers not only to binary state changes such as the presence or absence of droplets or the on / off of LED light-emitting cells in a printing device, but also to multi-level processing, including the formation of dots of different sizes such as small, medium, and large dots, as well as the formation of light dots. Light inks include light magenta and light cyan inks. If the image forming device is a display, this includes switching the transmittance of the liquid crystal cell at a number of gradations fewer than the input gradation value of the original image.

[0088] The dot formation unit only needs to be able to express the on / off state of dots for each pixel. If droplets are used, the ejection method and droplet size are not important. The ink constituting the droplets can also be of any hue and brightness. The dot formation unit can form dots directly on a medium such as printing paper, or it can form dots on transfer paper and then transfer them to the medium. As long as dots can be formed, thermal paper can be used, or a stencil can be used to form dots on the medium corresponding to holes in the stencil. In the case of a display, dots can be formed by turning on and off liquid crystal cells placed in front of a light source, or by slightly turning on or off light sources such as LEDs or organic electroluminescent (EL) light sources. If the display is a projector, dots can be formed by turning on and off dots using a liquid crystal shutter combined with a light valve, or by turning on and off micromirrors that make up a digital mirror device.

[0089] (2) In the above configuration, when the separation distance between a first pixel selected from the first pixel group and a second pixel selected from the second pixel group is smaller than a predetermined threshold, the probability that dots will be formed simultaneously on the pair of the first pixel and the second pixel may approximate a value determined in correspondence with the square of the input tone value. This reduces variations in overlapping of the formed dots and reduces differences in density unevenness in the image, even if there is a difference in the positional deviation between the dots formed corresponding to the first pixel group and the dots formed corresponding to the second pixel group in at least a portion of the output image. The method of approximating the probability that dots will be formed simultaneously on the pair of the first pixel and the second pixel to a value determined in correspondence with the square of the input tone value is disclosed in, for example, JP 2012-204939 A and is a well-known technique, so detailed description thereof will be omitted. Furthermore, when the halftone processor performs halftone processing using a dithering method, a dither mask with a first characteristic for the first pixel group and a dither mask with a second characteristic for the second pixel group are separately generated. When the input tone value of the original image is equal to or greater than a predetermined intermediate value, the distribution based on the second characteristic is set to have no correlation with the distribution based on the first characteristic. This achieves the above-mentioned relationship, i.e., when the separation distance between a first pixel selected from the first pixel group and a second pixel selected from the second pixel group is less than a predetermined threshold, the probability of simultaneous dot formation for the pair of the first pixel and the second pixel approximates a value determined in correspondence with the square of the input tone value. Such a dither mask can be easily prepared by first creating one for the first pixel group and then shifting it by a sufficient distance both horizontally and vertically for use in the second pixel group. The sufficient distance can be approximately half the size of the dither mask.

[0090] (3) In the above configuration, the first characteristic and the second characteristic may each be either a blue noise characteristic or a green noise characteristic. This increases the dispersion of the dot arrangement, particularly in the frequency range to which the human eye is highly sensitive, thereby reducing the graininess of the output image and improving image quality. Note that other characteristics, such as pink noise characteristics, may be used as long as they do not have an advantage over the characteristics of the human eye.

[0091] (4) In the configurations (1) to (3) above, the first pixel group and the second pixel group may include pixels corresponding to regions obtained by dividing the output image in one of an alternating raster pattern, an alternating column pattern, or a checkerboard pattern. This allows the ratio of dots formed by the first pixel group to dots formed by the second pixel group to be closer to equal in at least a portion of the output image. The division pattern does not necessarily have to be an alternating pattern for each pixel; it can be an alternating pattern in units of two pixels, or it can be a division in which dots are not arranged alternately, as described below.

[0092] (5) In the configurations (1) to (3) above, each of the first pixel group and the second pixel group may include discontinuous pixels at positions where dots are formed in sequence according to one of white noise characteristics, blue noise characteristics, and green noise characteristics as the input tone value of the output image monotonically increases. This makes it less likely that a specific pattern will emerge in the arrangement of the pixels belonging to the first pixel group and the pixels belonging to the second pixel group, further suppressing degradation of image quality.

[0093] (6) In the configurations (1) to (5) above, the image forming device may be a printing device in which the dot forming unit forms dots on a medium on which the output image is formed, or a display device in which the dot forming unit forms dots on a medium on which the output image is formed that are brighter than the medium. In the case of a printing device, this is not limited to devices that form dots by ejecting ink that is brighter than the medium onto the medium, but also includes devices that eject ink that is brighter than the medium if the medium is not white, devices that eject chemical solutions onto the medium before ejecting ink to improve the ink penetration into the medium surface, and devices that eject chemical solutions to cover up or harden the ink after ejecting the ink for image formation. Since these chemical solutions are also involved in the formation of the output image, they are also included in devices that form output images by dots.

[0094] Printing devices are not limited to inkjet printers that print by ejecting ink droplets or chemicals from nozzles, but can also be applied to various configurations, such as thermal transfer printers and stencil printers, as long as they reproduce multi-tone original images by distributing dots. They can also be applied to a variety of printing formats, including serial printers in which a dot-forming head moves back and forth across the width of the medium to form an image, line printers in which dot-forming elements such as nozzles are arranged to form dots across the width of the medium, and page printers in which the dot arrangement for a page of images is prepared using a stencil or other method. This image forming device configuration is not limited to printers and other printing devices, but can also be applied to devices that form images by turning each pixel on and off, such as LEDs, organic EL displays, and liquid crystal displays.

[0095] (7) In the configurations (1) to (6) above, the dot forming unit may include a first dot forming unit that forms dots in a first portion, which is a part of the output image, and a second dot forming unit that forms dots in a second portion, which is a part of the output image and includes an image area that partially overlaps the first portion. In the image area that is a part of the output image, dots formed by the first dot forming unit and dots formed by the second dot forming unit may be mixed, and in the area of ​​the output image other than the image area, dots may be formed without using either the first dot forming unit or the second dot forming unit. This configuration reduces variations in the overall dot dispersion and image quality degradation, even if there are variations in the misalignment of dot formation positions due to the installation accuracy of the first dot forming unit and the second dot forming unit. Furthermore, in many cases, variations in the overlap state between dots are also reduced, thereby reducing the occurrence of uneven density in the image.

[0096] (8) In the configurations (1) to (7) above, the first pixel group and the second pixel group included in the output image may be divided according to a factor that causes a shift in the formation positions of dots corresponding to pixel positions. For example, if the shift in the formation positions of dots corresponding to the first pixel group and the second pixel group occurs in the direction in which nozzles of the nozzle rows corresponding to the first pixel group and the second pixel group are aligned, i.e., in the raster direction in which dots are formed, the first pixel group and the second pixel group can be divided in an alternating raster pattern. If the shift in the dot formation positions occurs in a direction perpendicular to the raster direction, the first pixel group and the second pixel group can be divided in an alternating column pattern. If the shift occurs in both directions, the first pixel group and the second pixel group can be divided in a checkered pattern. This further reduces fluctuations in image quality due to differences in the formation position shift.

[0097] (9) In the configurations (1) to (8) above, if the deviation in the dot formation positions between the first dot-forming unit and the second dot-forming unit is equal to or less than a predetermined threshold, the arrangement of the first pixel group and the second pixel group in the image area may be arbitrary, thereby increasing the degree of freedom in the arrangement of the pixel groups.

[0098] (10) In the configurations (1) to (9) above, if there are multiple factors that cause deviation in the dot formation positions by the first dot-forming unit and the second dot-forming unit, the arrangement of the first pixel group and the second pixel group in the image area may be determined according to the factor that causes the greatest deviation. This allows the deviation in the dot formation positions to be addressed in accordance with the factor that causes the greatest deviation, thereby efficiently suppressing degradation of image quality.

[0099] (11) In the above configuration, the dot forming unit may be provided on a dot forming head that is relatively movable when forming the dots, with the width direction of the medium on which the output image is formed being the main scanning direction, and the first dot forming unit may form dots in a first scan in the main scanning direction, and the second dot forming unit may form dots in a second scan that is different from the first scan. This makes it possible to deal with fluctuations in the misalignment of dot formation positions that occur between the first scan and the second scan.

[0100] (12) In the above configuration, the first scan and the second scan may be scans performed when the dot-forming head moves forward and backward relative to the medium without moving the dot-forming head in the sub-scanning direction, or two scans out of multiple scans of the dot-forming head performed when the dot-forming head moves in the sub-scanning direction relative to the medium. This makes it possible to suppress variations in dispersion and degradation of image quality due to variations in the misalignment of the image formation position, even in bidirectional printing, in which dots are formed by both the forward and backward movements of the dot-forming head, and in interlaced printing, in which an image is completed by combining rasters from multiple main scans while shifting the dot-forming head by a predetermined number of dots.

[0101] (13) In the above configuration, the first dot-forming unit and the second dot-forming unit may include a plurality of dot-forming elements arranged in a predetermined direction, and may be arranged so that their ends overlap in a direction intersecting the predetermined direction in which the dot-forming elements are arranged, and the dots are formed in the pixels of the image area by the dot-forming elements arranged in the overlapping positions. This reduces the variation in dispersibility between a state in which there is misalignment of dot formation positions that occurs in overlapping positions and a state in which there is no misalignment of dot formation positions that occurs in non-overlapping areas, and also reduces degradation of image quality.

[0102] (14) The present disclosure can also be implemented as an image forming method for forming an output image by distributing dots. In this image forming method, the output image includes a first pixel group consisting of a plurality of pixels whose positions in the output image are determined, and a second pixel group consisting of a plurality of pixels whose positions are different from those of the first pixel group. In this image forming method, during halftone processing that determines whether dot formation is to be performed according to the input gradation value of an original image to be formed, the arrangement of pixels in the first pixel group where dots are to be formed is a distribution having a first characteristic related to dispersibility, and the arrangement of pixels in the second pixel group where dots are to be formed is a distribution having a second characteristic related to dispersibility, and when the input gradation value of the original image is equal to or greater than a predetermined intermediate value, the distribution based on the second characteristic is made to have no correlation with the distribution based on the first characteristic. The on / off of the dots in the first pixel group is determined by the arrangement having the first characteristic, and the on / off of the dots in the second pixel group is determined by the arrangement having the second characteristic. An output image consisting of a plurality of pixels is formed using the dots using the results of the halftone processing.

[0103] In this image forming method, when determining whether to turn on or off dots in a first pixel group using an arrangement having a first characteristic and determining whether to turn on or off dots in a second pixel group using an arrangement having a second characteristic, the dot distribution based on the second characteristic regarding dot dispersibility is made to have no correlation with the dot distribution based on the first characteristic when the input gradation value of the original image is equal to or greater than a predetermined intermediate value. As a result, even if there is a difference in the formation positions of dots formed corresponding to the first pixel group and dots formed corresponding to the second pixel group in at least a part of the output image, fluctuations in image quality due to the difference in formation positional deviation are suppressed.

[0104] (15) The present disclosure can also be implemented as an image processing device that processes an original image expressed in multiple gradations to generate an output image expressed by a distribution of dots. The output image includes a first pixel group consisting of a plurality of pixels whose positions in the output image are determined, and a second pixel group consisting of a plurality of pixels whose positions are different from those of the first pixel group. During halftone processing that determines whether to form dots in accordance with an input gradation value of the original image, the arrangement of pixels in the first pixel group where dots are formed is a distribution having a first characteristic related to dispersibility, and the arrangement of pixels in the second pixel group where dots are formed is a distribution having a second characteristic related to dispersibility. When the input gradation value of the original image is equal to or greater than a predetermined intermediate value, the distribution based on the second characteristic is made to have no correlation with the distribution based on the first characteristic. The on / off of the dots in the first pixel group is determined by the arrangement based on the first characteristic, and the on / off of the dots in the second pixel group is determined by the arrangement based on the second characteristic.

[0105] In this image processing device, when determining whether to turn on or off dots in a first pixel group using an arrangement having a first characteristic and determining whether to turn on or off dots in a second pixel group using an arrangement having a second characteristic, the dot distribution based on the second characteristic regarding dot dispersibility is made to have no correlation with the dot distribution based on the first characteristic when the input gradation value of the original image is equal to or greater than a predetermined intermediate value.As a result, when dots are formed based on an output image that has been halftoned and expressed using a dot distribution, even if there is a difference in the formation positions of dots formed corresponding to the first pixel group and dots formed corresponding to the second pixel group in at least a part of the output image, fluctuations in image quality due to the difference in formation positions are suppressed.

[0106] (16) The same effects can be obtained with an image processing method corresponding to this image processing device.

[0107] (17) In each of the above embodiments, some of the configurations realized by hardware may be replaced with software. At least a portion of the configurations realized by software may also be realized by a discrete circuit configuration. Furthermore, when some or all of the functions of the present disclosure are realized by software, the software (computer program) may be provided in a form stored on a computer-readable recording medium. The term "computer-readable recording medium" is not limited to portable recording media such as floppy disks and CD-ROMs, but also includes internal storage devices within a computer, such as various RAMs and ROMs, and external storage devices fixed to a computer, such as a hard disk. In other words, the term "computer-readable recording medium" has a broad meaning, including any recording medium capable of fixing data packets, not just temporarily.

[0108] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0109] 10...printing system, 11...display, 20...inkjet printer, 30...head unit, 31...ink cartridge, 32...carriage, 35...ejection unit, 35a...first nozzle unit, 35b...second nozzle unit, 36...screw, 50...drive signal generation unit, 60...control unit, 61...CPU, 62...storage unit, 70...transport mechanism, 71...transport motor, 72...motor driver, 73, 74...transport roller, 75, 76...guide roller, 77...platen, 90...computer, 91...video drive driver, 92...storage device, 94...rasterizer, 95...application program, 96...printer driver, 97...resolution conversion module, 98...color conversion module, 99...halftone module, 135a...first nozzle section, 135b...second nozzle section, 235a...first nozzle section, 235b...second nozzle section, A1, A2, B1, B2...graph, LA...mixed area, M1, M2...divided dither mask, N...nozzle, NI1, NI2...nozzle row, NL1, NL2...nozzle row, P...medium

Claims

1. 1. An image forming apparatus for forming an output image corresponding to an original image, comprising: a halftone processing unit that determines whether to form dots in accordance with the input tone value of the original image to be formed; a dot forming unit that forms an output image made up of a plurality of pixels using the dots by using a result of the processing of the halftone processing unit; Equipped with the output image includes a first pixel group consisting of a plurality of pixels whose positions in the output image have been determined, and a second pixel group consisting of a plurality of pixels whose positions are different from those of the pixels in the first pixel group; The halftone processing unit The arrangement of pixels in which dots are formed in the first pixel group is a distribution having a first characteristic related to dispersibility, the arrangement of pixels in which dots are formed in the second pixel group is a distribution having a second characteristic related to dispersibility, and when the input tone value of the original image is equal to or greater than a predetermined intermediate value, the distribution according to the second characteristic is not correlated with the distribution according to the first characteristic; determining whether the dots are on or off in an arrangement having the first characteristic in the first pixel group, and determining whether the dots are on or off in an arrangement having the second characteristic in the second pixel group; Image forming device.

2. 2. The image forming apparatus according to claim 1, wherein when a distance between a first pixel selected from the first pixel group and a second pixel selected from the second pixel group is smaller than a predetermined threshold, the probability that dots will be formed simultaneously on the pair of the first pixel and the second pixel approximates a value determined in correspondence with the square of the input gradation value.

3. 2. The image forming apparatus according to claim 1, wherein the first characteristic and the second characteristic are either a blue noise characteristic or a green noise characteristic, respectively.

4. 2. The image forming apparatus according to claim 1, wherein the first pixel group and the second pixel group include pixels corresponding to respective regions obtained by dividing the output image in one of alternating raster, alternating column, and checkered patterns.

5. 2. The image forming apparatus according to claim 1, wherein each of the first pixel group and the second pixel group includes discontinuous pixels at positions where dots are formed in sequence according to one of white noise characteristics, blue noise characteristics, and green noise characteristics as the input gradation value of the output image monotonically increases.

6. 6. The image forming apparatus according to claim 1, wherein the first pixel group and the second pixel group included in the output image are divided according to a factor that causes a deviation in the formation positions of dots corresponding to pixel positions.

7. the image forming apparatus, a printing device in which the dot forming unit forms dots on a medium on which the output image is formed; or a display that forms dots on a medium on which the output image is formed that are brighter than the medium; The image forming apparatus according to claim 1 .

8. The dot forming unit a first dot forming unit that forms dots in a first portion that is a part of the output image, and a second dot forming unit that forms dots in a second portion that is a part of the output image and includes an image area that partially overlaps with the first portion, In an image region that is a part of the output image, dots formed by the first dot forming unit and dots formed by the second dot forming unit are mixed, dots are formed in an area of ​​the output image other than the image area without using either the first dot forming unit or the second dot forming unit; The image forming apparatus according to claim 1 .

9. 9. The image forming apparatus according to claim 8, wherein the first pixel group and the second pixel group included in the output image are divided according to a factor that causes a deviation in the formation positions of dots corresponding to pixel positions.

10. 10. The image forming apparatus according to claim 9, wherein, when a deviation in the formation positions of the dots by the first dot forming unit and the second dot forming unit is equal to or less than a predetermined threshold, the arrangement of the first pixel group and the second pixel group in the image area is an arbitrary arrangement.

11. 10. The image forming device according to claim 9, wherein, when there are multiple factors that cause deviation in the formation positions of the dots by the first dot forming unit and the second dot forming unit, the arrangement of the first pixel group and the second pixel group in the image area is determined according to the factor that causes the greatest deviation.

12. the dot forming unit is provided in a dot forming head that is relatively movable when forming the dots, with the width direction of the medium on which the output image is formed being the main scanning direction; the first dot forming unit forms dots during a first scan in the main scanning direction, the second dot forming unit forms dots in a second scan, which is different from the first scan; The image forming apparatus according to claim 8 .

13. The first scan and the second scan are Scanning of the dot forming head when it moves forward and backward relative to the medium, which is performed without movement of the dot forming head in the sub-scanning direction; or two scans of the dot formation head on the medium, the scans being performed while the dot formation head is moving in the sub-scanning direction; The image forming apparatus according to claim 12,

14. The first dot forming unit and the second dot forming unit are a plurality of dot-forming elements arranged in a predetermined direction to form dots; the dot-forming elements are arranged at positions where their ends overlap in a direction intersecting the predetermined direction in which the dot-forming elements are arranged, forming the dots in the pixels of the image area by the dot forming elements arranged in the overlapping positions; The image forming apparatus according to claim 8 .

15. An image forming method for forming an output image by a distribution of dots, comprising: the output image includes a first pixel group consisting of a plurality of pixels whose positions in the output image have been determined, and a second pixel group consisting of a plurality of pixels whose positions are different from those of the pixels in the first pixel group; During halftone processing, which determines whether dot formation is on or off according to the input tone value of the original image to be formed, The arrangement of pixels in which dots are formed in the first pixel group is a distribution having a first characteristic related to dispersibility, the arrangement of pixels in which dots are formed in the second pixel group is a distribution having a second characteristic related to dispersibility, and when the input tone value of the original image is equal to or greater than a predetermined intermediate value, the distribution according to the second characteristic is not correlated with the distribution according to the first characteristic; determining whether the dots are on or off in an arrangement having the first characteristic in the first pixel group, and determining whether the dots are on or off in an arrangement having the second characteristic in the second pixel group; forming an output image consisting of a plurality of pixels using the dots using the result of the halftone processing; Image forming method.

16. An image processing device that processes an original image expressed in multiple gradations to generate an output image expressed by a distribution of dots, the output image includes a first pixel group consisting of a plurality of pixels whose positions in the output image have been determined, and a second pixel group consisting of a plurality of pixels whose positions are different from those of the pixels in the first pixel group; During halftone processing, which determines whether dot formation is on or off according to the input tone value of the original image, The arrangement of pixels in which dots are formed in the first pixel group is a distribution having a first characteristic related to dispersibility, the arrangement of pixels in which dots are formed in the second pixel group is a distribution having a second characteristic related to dispersibility, and when the input tone value of the original image is equal to or greater than a predetermined intermediate value, the distribution according to the second characteristic is not correlated with the distribution according to the first characteristic; determining whether the dots are on or off in an arrangement having the first characteristic in the first pixel group, and determining whether the dots are on or off in an arrangement having the second characteristic in the second pixel group; Image processing device.

17. An image processing method for processing an original image expressed in multiple gradations to generate an output image expressed by a distribution of dots, comprising: the output image includes a first pixel group consisting of a plurality of pixels whose positions in the output image have been determined, and a second pixel group consisting of a plurality of pixels whose positions are different from those of the pixels in the first pixel group; During halftone processing, which determines whether dot formation is on or off according to the input tone value of the original image, The arrangement of pixels in which dots are formed in the first pixel group is a distribution having a first characteristic related to dispersibility, the arrangement of pixels in which dots are formed in the second pixel group is a distribution having a second characteristic related to dispersibility, and when the input tone value of the original image is equal to or greater than a predetermined intermediate value, the distribution according to the second characteristic is not correlated with the distribution according to the first characteristic; determining whether the dots are on or off in an arrangement having the first characteristic in the first pixel group, and determining whether the dots are on or off in an arrangement having the second characteristic in the second pixel group; Image processing methods.

Images