JP2010214962A - Dither matrix generating method and printing using dither matrix - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form an ink dot by scanning a common area on a printing medium more than once, and thereby to suppress degradation of image quality due to printing of an image. <P>SOLUTION: A printer creates dot data by executing a half-tone processing, forms dots in a plurality of printing pixels belonging to each of a plurality of pixel groups presumed to have mutually physical differences in the formation of the dots, and creates a printing image by mutually combining the formed dots in a common printing area. In this half-tone processing, at least one of dot patterns formed in the plurality of printing pixels belonging to each of the plurality of pixel groups is constituted to have prescribed spatial frequency characteristics set in advance, at least in some gradation of input gradation values, and in a specified direction set in advance on the printing medium. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a technique for printing an image by forming dots on a print medium.

  2. Description of the Related Art Printing apparatuses that form dots on a printing medium and print images are widely used as output apparatuses for images created by computers and images taken by digital cameras. In such a printing apparatus, since the gradation value of dots that can be formed with respect to the input gradation value is small, gradation expression is performed by halftone processing. As one of the halftone processes, a systematic dither method using a dither matrix is widely used. The systematic dither method greatly affects the image quality depending on the contents of the dither matrix. For example, as disclosed in Patent Document 1, analysis such as simulated annealing or genetic algorithm using an evaluation function taking human vision into consideration The dither matrix has been optimized by techniques.

JP-A-7-177351 JP-A-7-81190 Japanese Patent Laid-Open No. 10-329381

  However, in such dither matrix optimization processing, ink dots are formed by scanning a common area on a printing medium a plurality of times, and thus image quality deterioration due to printing an image is not taken into consideration. . Further, such image quality degradation is not limited to halftone processing using a dither matrix, but generally occurs in printing using halftone processing.

  The present invention has been made to solve the above-described problems in the prior art, and forms an ink dot by scanning a common area on a print medium a plurality of times, thereby printing an image. An object of the present invention is to provide a technique for suppressing the deterioration in image quality caused by the above.

In order to solve at least a part of the problems described above, the present invention provides a printing apparatus that performs printing on a print medium. This printing device
By performing halftone processing on the image data representing the input gradation value of each pixel constituting the original image, the dot formation state on each print pixel of the print image to be formed on the print medium is represented. A dot data generator for generating dot data;
According to the dot data, dots are formed on a plurality of print pixels belonging to each of a plurality of pixel groups assumed to be physically different from each other in dot formation, and the formed dots are used as a common print area. A print image generation unit that generates the print image by combining with each other;
With
The dot data generation unit includes a plurality of print pixels belonging to each of the plurality of pixel groups in a specific direction set in advance on the print medium for at least some of the input gradation values. At least one of the dot patterns formed in the above is configured to have a predetermined spatial frequency characteristic set in advance.

  In the printing apparatus of the present invention, a print image is generated by combining dots with each other in a plurality of print pixels belonging to each of a plurality of pixel groups assumed to be physically different from each other in dot formation. In printing, at least one of dot patterns formed on a plurality of print pixels belonging to each of a plurality of pixel groups has a predetermined predetermined spatial frequency characteristic in a specific direction set in advance on a print medium. Since the conditions of the halftone process are set so as to have a fine adjustment of the halftone process in consideration of the direction on each print medium in the optimization of the halftone process focusing on each of the plurality of pixel groups. It becomes possible. Thereby, for example, when the influence of the dispersibility of each dot of a plurality of pixel groups on the image quality changes or differs depending on the direction, the adjustment of the halftone processing is performed in consideration of such changes or differences. It can be performed.

  The mechanism of image quality degradation resulting from such a physical difference and the organic relationship between halftone processing is a finding that was first discovered by the inventors. In other words, since the conventional halftone processing is configured by paying attention to the spatial frequency distribution of the print image, for example, the relative position of a plurality of pixel groups combined with each other in a common print region is a physical error of the printing apparatus. It is now clear for the first time that the relative positional relationship collapses and the image quality deteriorates excessively when shifting as a whole.

  On the other hand, such a shift occurs remarkably in the main scanning direction in, for example, bidirectional printing, but does not occur remarkably in the sub scanning direction. Focusing on these points, if halftoning with a focus on shifting in the main scanning direction is realized, unnecessary adjustments due to the assumption of shifting in the subscanning direction are eliminated, and optimum halftone processing is achieved. The inventor of the present application has conceived that the property can be further improved.

  Furthermore, the inventor has also found the following phenomenon. In other words, if low-density sparseness exists in dots formed in a plurality of pixel groups, and ink droplets are ejected with overlapping dot formation timing, ink droplets are ejected at positions where the dot density is high. This causes a state such as agglomeration, excessive gloss, and bronze phenomenon, and causes an image difference from a position where the dot density is low. This difference in images causes a problem that it is easily recognized as image unevenness for human vision.

  On the other hand, such a phenomenon occurs more markedly as the pitch of the pixels on which dots are formed in each main scan is smaller, but the pitch of the pixels on which dots are formed in each main scan is different between main scan and sub scan. It can be different. Focusing on these points, if halftone processing that focuses on the direction in which the pitch of the pixels for which dots are to be formed is preferentially assumed in each main scan is realized, unnecessary adjustment due to the assumption of the direction in which the pitch is large The present inventor has also conceived that the optimality of halftone processing can be further improved by eliminating the above-mentioned.

In the above printing apparatus,
The dot data generation unit includes a dot pattern formed on each of a plurality of print pixels belonging to each of the plurality of pixel groups and a dot pattern formed on a plurality of print pixels belonging to the plurality of pixel groups. May be configured to have predetermined spatial frequency characteristics set in advance.

In the above printing apparatus,
The partial gradation is a gradation value included in a dot density range of 40% to 60% in which low frequency components are relatively high when it is assumed that dots are evenly arranged on the print medium. There may be.

  In such a dot density range, many low-frequency components appear as the spatial frequency characteristics of the dots of each of the plurality of pixel groups, and image quality deterioration is likely to occur due to aggregation of ink droplets. There is an effect.

In the above printing apparatus,
The predetermined spatial frequency characteristic falls within a predetermined low frequency range of 4 cycles per millimeter or less, which is a spatial frequency region in which human visual sensitivity is relatively high on a print medium arranged at an observation distance of 300 mm. A spatial frequency characteristic in which a predetermined frequency characteristic of a dot pattern formed on a printing pixel belonging to each of the plurality of pixel groups has a frequency band that is closest to a predetermined characteristic of the spatial frequency of the dot pattern of the printed image. It may be made to be.

  By so doing, it is possible to suppress image quality degradation in a region where human visual sensitivity is high, and therefore it is possible to effectively improve image quality with a focus on human visual sensitivity.

In the above printing apparatus,
The predetermined characteristic is a graininess index calculated by a calculation process including a Fourier transform process,
The graininess index may be calculated based on a product of a VTF function determined based on a visual spatial frequency characteristic and a constant calculated in advance by the Fourier transform process.
Or
The predetermined characteristic may be an RMS granularity calculated by a calculation process including a low-pass filter process.

In the above printing apparatus,
The halftone process is configured such that a dot pattern formed on a print pixel belonging to each of the plurality of pixel groups has a preset two-dimensional spatial frequency characteristic;
The two-dimensional spatial frequency characteristic changes in the one-dimensional spatial frequency characteristic according to the direction on the print medium, and the one-dimensional spatial frequency characteristic is the most significant in the spatial frequency characteristic of the print image in the specific direction. You may make it set so that it may approach.

  In this way, when the degree of optimization required for each of the plurality of pixel groups differs depending on the direction on the print medium, it is possible to efficiently realize optimization of halftone processing. Specifically, for example, optimization in only one direction is possible.

In the above printing apparatus,
In the two-dimensional spatial frequency characteristic, the rate of change of the one-dimensional spatial frequency characteristic according to the direction on the print medium peaks at an angle in the range of 30 degrees to 60 degrees with respect to the specific direction. It may be set as such.

  In this way, since the dot pattern change peaks at an angle in the range of 30 degrees to 60 degrees where human visual sensitivity is low, image quality degradation caused by the dot pattern change is less likely to be detected by the human eye. Can be.

In the above printing apparatus,
The print image generation step forms dots on each print pixel both during forward movement and backward movement of the print head while performing main scanning of the print head,
The plurality of pixel groups include a group of print pixels to be formed with dots when the print head is moved forward, and a group of print pixels to be formed with dots when the print head is moved backward.
The physical difference includes a shift in the relative position of dots for each of the plurality of pixel groups that occurs due to main scanning of the print head;
The specific direction may be the main scanning direction,
Or
The print image generation step forms dots on the print pixels while performing main scanning of a print head,
The plurality of pixel groups include a group of a plurality of print pixels to be formed with dots for each main scan of the print head,
The physical difference includes a shift in dot formation timing due to main scanning of the print head,
The specific direction may be a direction in which the pitch of the print pixel that is the target of dot formation in each main scan of the print head is the smallest.

In the above printing apparatus,
The predetermined spatial frequency characteristic may be one of a blue noise characteristic and a green noise characteristic.

The present invention provides the following printing method that exhibits a remarkable effect that can be objectively observed from the outside in bidirectional printing in which dots are formed in both directions of main scanning. This printing method is
A printing method for printing on a print medium,
By performing halftone processing on the image data representing the input gradation value of each pixel constituting the original image, the dot formation state on each print pixel of the print image to be formed on the print medium is represented. A dot data generation process for generating dot data;
In accordance with the dot data, a dot is formed on each print pixel both during the forward movement and the backward movement of the print head while performing a main scan of the print head, and a dot formation target during the forward movement of the print head A print image generation step for generating the print image by combining a print pixel group to be a dot and a print pixel group to be formed with dots when the print head is moved back together in a common print region;
With
In the dot data generation step, for at least some of the input gradation values, the dot patterns formed on the printing pixels belonging to each of the two pixel groups are combined by shifting relative positions. When the shift direction is the same as the main scanning direction, the halftone processing conditions are set so that the granularity index of the combined dot pattern is minimized. It is characterized by.

  The present invention further provides a dither that stores, in each element, a plurality of threshold values for determining a dot formation state on each print pixel of a print image to be formed on a print medium in accordance with input image data. Two methods are provided for generating the matrix.

The first method is
Of the plurality of thresholds, a threshold value determining step of determining a threshold value that is an undetermined threshold value and a dot formation is most likely to be turned on as a focus threshold value,
Assuming the dot formation state when it is assumed that the target threshold value is stored in each of a plurality of storage candidate elements that are candidate elements for storing the target threshold value, A storage element determination step for determining a storage element of the threshold value of interest from the plurality of storage candidate elements based on a matrix evaluation value representing a correlation;
For at least a part of the plurality of threshold values, repeating the focus threshold value determination step and the storage element determination step while changing the focus threshold value;
With
The storage element determination step includes:
Based on the first dot formation state that is the dot formation state of the print image, the overall evaluation is an evaluation value that represents the correlation between the first dot formation state and the predetermined target state in a preset direction. An overall evaluation value determining step for determining a value;
The second in a predetermined direction based on a second dot formation state that is a dot formation state corresponding only to an element belonging to a pixel position group to which the storage candidate element belongs among the plurality of pixel position groups. A group evaluation value determination step for determining a group evaluation value, which is an evaluation value representing a correlation between the dot formation state of the predetermined target state, and
In accordance with the overall evaluation value and the group evaluation value, a comprehensive evaluation value determining step for determining the matrix evaluation value;
Including
The second method is
A preparation step of preparing a dither matrix as an initial state for storing each of a plurality of threshold values for determining the presence or absence of dot formation for each pixel according to an input gradation value;
It is assumed that the target threshold value to be evaluated among the plurality of threshold values is changed, and the target threshold value is replaced with another threshold value stored in another element of the dither matrix, and dot formation based on the assumption is performed. A storage element replacement step for repeatedly determining whether or not to perform the replacement based on a matrix evaluation value representing a correlation with a predetermined target state calculated assuming a state;
With
The storage element replacement step includes:
Based on the first dot formation state that is the dot formation state of the print image, the overall evaluation is an evaluation value that represents the correlation between the first dot formation state and the predetermined target state in a preset direction. An overall evaluation value determining step for determining a value;
The second in a predetermined direction based on a second dot formation state that is a dot formation state corresponding only to an element belonging to a pixel position group to which the storage candidate element belongs among the plurality of pixel position groups. A group evaluation value determination step for determining a group evaluation value, which is an evaluation value representing a correlation between the dot formation state of the predetermined target state, and
In accordance with the overall evaluation value and the group evaluation value, a comprehensive evaluation value determining step for determining the matrix evaluation value;
It is characterized by including.

In the above dither matrix generation method,
The matrix evaluation value includes a graininess index calculated by a calculation process including a Fourier transform process,
The graininess index may be calculated based on a product of a VTF function determined based on a visual spatial frequency characteristic and a constant calculated in advance by the Fourier transform process.
Or
The matrix evaluation value may be an evaluation value including RMS granularity calculated by a calculation process including a low-pass filter process.

  Halftone processing using such a dither matrix is, for example, intermediate data (number data) for specifying a dot formation state as disclosed in JP-A-2005-236768 and JP-A-2005-269527. In the technique using (), halftone processing using a conversion table (or correspondence table) generated using a dither matrix is also included.

In the present invention, various forms such as a printing apparatus, a dither matrix, a dither matrix generation apparatus, a printing apparatus and a printing method using the dither matrix, and a printed material generation method, or functions of these methods or apparatuses are realized in a computer. The present invention can be realized in various forms such as a computer program for recording the program, a recording medium storing the computer program, and a data signal including the computer program and embodied in a carrier wave.
In addition, the use of a dither matrix in the printing device, printing method, and printed material generation method is based on whether or not dots are formed for each pixel by comparing the threshold value set in the dither matrix and the gradation value of the image data for each pixel. However, for example, the presence or absence of dot formation may be determined by comparing the sum of the threshold value and the gradation value with a fixed value. Furthermore, the presence / absence of dot formation may be determined according to the data generated in advance based on the threshold value and the gradation value without directly using the threshold value. In general, the dither method of the present invention only needs to determine the presence or absence of dot formation according to the gradation value of each pixel and the threshold value set at the corresponding pixel position of the dither matrix.

1 is a block diagram showing the configuration of a printing system as an embodiment of the present invention. FIG. 3 is an explanatory diagram illustrating a configuration of a printer. FIG. 3 is an explanatory diagram showing a nozzle arrangement on the lower surface of the print head. Explanatory drawing which illustrated a part of dither matrix conceptually. Explanatory drawing which shows the idea of the presence or absence of the dot formation using a dither matrix. Explanatory drawing which illustrated notionally the spatial frequency characteristic of the threshold value set to each pixel of the blue noise dither matrix which has a blue noise characteristic. Explanatory drawing which showed notionally the visual spatial frequency characteristic VTF (Visual Transfer Function) which is a sensitivity characteristic with respect to the visual spatial frequency which a human has. Explanatory drawing which shows the dot pattern formed using the conventional dither matrix. Explanatory drawing which shows a mode that the image quality of the printing image formed using the conventional dither matrix deteriorates by bidirectional printing. Explanatory drawing which shows a mode that the image quality degradation of the printing image formed by bidirectional | two-way printing is suppressed by the dither matrix of the Example of this invention. The flowchart which shows the processing routine of the production | generation method of the dither matrix in 1st Example of this invention. Explanatory drawing which shows the dither matrix M and the two division | segmentation matrices M1 and M2 by which the grouping process in 1st Example of this invention was performed. Explanatory drawing which shows the dot formation object pixel of each main scanning in the bidirectional | two-way printing of 1st Example of this invention. The flowchart which shows the process routine of a dither matrix evaluation process. FIG. 9 is an explanatory diagram showing a state in which dots are formed in each of eight pixels corresponding to elements storing threshold values at which dots are likely to be formed first to eighth in the dither matrix M; FIG. 4 is an explanatory diagram showing a dot density matrix in which a dot pattern in which dots are formed in each of nine pixels in the dither matrix M is digitized as a dot density. Explanatory drawing which shows the dot pattern in which five dots were formed in the division | segmentation matrix M1. Explanatory drawing which shows the dot density matrix which digitized the dot pattern in which five dots were formed in the division | segmentation matrix M1 as dot density. Explanatory drawing which shows the two-dimensional filter characteristic extended to the two-dimensional area | region used in order to calculate the two-dimensional granularity index | exponent used by evaluation of division | segmentation matrix M1, M2. Explanatory drawing which shows a mode that the anisotropy of the two-dimensional filter characteristic used in 1st Example of this invention was observed from two places in three-dimensional space. The flowchart which shows the processing routine of the production | generation method of the dither matrix in 2nd Example of this invention. Explanatory drawing which shows the group evaluation value matrix DF0, DF1, DF3 produced | generated by performing a low pass filter process with respect to all the dot density matrix DD0, DD1, DD3. Explanatory drawing which shows the calculation formula for calculating RMS granularity used by 1st Example of this invention. Explanatory drawing which shows a mode that a printing image is produced | generated on a printing medium by forming an ink dot, performing main scanning and subscanning in the comparative example of this invention. Explanatory drawing which shows a mode that a printing image is produced | generated on the printing medium by the dot formed in the printing pixel which belongs to each of several pixel groups in the comparative example of this invention being mutually combined in a common printing area | region. FIG. 4 is an explanatory diagram illustrating a printing method in which a pixel pitch in a main scanning direction, which is a dot formation target in each main scanning, is smaller than a pixel pitch in a sub scanning direction. 4 is an explanatory diagram illustrating a printing method in which a pixel pitch in a sub-scanning direction, which is a dot formation target in each main scanning, is smaller than a pixel pitch in a main scanning direction. Explanatory drawing which shows the flowchart of the example of application of this invention to an error diffusion method. FIG. 3 is an explanatory diagram showing a Jarvis, Judice & Ninke type error diffusion matrix and an error diffusion whole matrix Mg for performing additional error diffusion. Explanatory drawing which shows the printing state by the line printer which has a some print head in the modification of this invention. Explanatory drawing which shows an example of the actual printing state in a bidirectional | two-way printing system.

Below, in order to demonstrate the effect | action and effect of this invention more clearly, embodiment of this invention is described in the following orders.
A. Configuration of a printing apparatus according to an embodiment of the present invention:
B. Generation of optimal dither matrix assuming bi-directional printing:
B-1. Image quality degradation due to bidirectional printing and its suppression mechanism:
B-2. Generation of optimal dither matrix based on granularity index (first embodiment):
B-3. Generation of optimal dither matrix based on RMS granularity (second embodiment):
C. Generation of optimal dither matrix assuming the dot pattern formed in each main scan:
D. Variation:

A. Configuration of a printing apparatus according to an embodiment of the present invention:
FIG. 1 is a block diagram showing the configuration of a printing system as an embodiment of the present invention. This printing system includes a computer 90 as a printing control device and a color printer 20 as a printing unit. The combination of the color printer 20 and the computer 90 can be called a “printing apparatus” in a broad sense.

  In the computer 90, an application program 95 operates under a predetermined operating system. A video driver 91 and a printer driver 96 are incorporated in the operating system, and print data PD to be transferred to the color printer 20 is output from the application program 95 via these drivers. The application program 95 performs desired processing on the image to be processed, and displays the image on the CRT 21 via the video driver 91.

  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 color printer 20. In the example shown in FIG. 1, the printer driver 96 includes a resolution conversion module 97, a color conversion module 98, a halftone module 99, a rasterizer 100, and a color conversion table LUT.

  The resolution conversion module 97 plays a role of converting the resolution of the color image data handled by the application program 95 (that is, the number of pixels per unit length) into a resolution that can be handled by the printer driver 96. The image data subjected to resolution conversion in this way is still image information composed of three colors of RGB. The color conversion module 98 converts RGB image data into multi-tone data of a plurality of ink colors that can be used by the color printer 20 for each pixel while referring to the color conversion table LUT.

  The color-converted multi-gradation data has, for example, 256 gradation values. The halftone module 99 executes halftone processing for expressing the gradation value by the color printer 20 by forming the ink dots in a dispersed manner. The halftone processed image data is rearranged in the order of data to be transferred to the color printer 20 by the rasterizer 100 and output as final print data PD. The print data PD includes raster data indicating the dot recording state during each main scan and data indicating the sub-scan feed amount. The contents of the halftone process will be described later.

  The printer driver 96 corresponds to a program for realizing a function for generating print data PD. A program for realizing the function of the printer driver 96 is supplied in a form recorded on a computer-readable recording medium. Such recording media include flexible disks, CD-ROMs, magneto-optical disks, IC cards, ROM cartridges, punch cards, printed matter on which codes such as bar codes are printed, computer internal storage devices (such as RAM and ROM). A variety of computer-readable media such as a memory) and an external storage device can be used.

  FIG. 2 is a schematic configuration diagram of the color printer 20. The color printer 20 includes a sub-scan feed mechanism that transports the printing paper P in the sub-scan direction by the paper feed motor 22 and a main scan feed that reciprocates the carriage 30 in the axial direction (main scan direction) of the platen 26 by the carriage motor 24. Mechanism, a head drive mechanism that drives a print head unit 60 (also referred to as a “print head assembly”) mounted on the carriage 30 to control ink ejection and dot formation, and these paper feed motor 22, carriage motor 24, a control circuit 40 that controls the exchange of signals with the print head unit 60 and the operation panel 32. The control circuit 40 is connected to the computer 90 via the connector 56.

  The sub-scan feed mechanism for transporting the printing paper P includes a gear train (not shown) that transmits the rotation of the paper feed motor 22 to the platen 26 and a paper transport roller (not shown). Further, the main scanning feed mechanism for reciprocating the carriage 30 has an endless drive belt 36 between the carriage motor 24 and a slide shaft 34 that is installed in parallel with the axis of the platen 26 and slidably holds the carriage 30. And a position sensor 39 for detecting the origin position of the carriage 30.

  The print head unit 60 has a print head 28 and can be mounted with an ink cartridge. The print head unit 60 is attached to and detached from the color printer 20 as one component. That is, when the print head 28 is to be replaced, the print head unit 60 is replaced.

FIG. 3 is an explanatory diagram showing the nozzle arrangement on the lower surface of the print head 28. The lower surface of the print head 28, the black ink nozzle group K _D for ejecting black ink, a dark cyan ink nozzle group C _D for ejecting dark cyan ink, for ejecting light cyan ink light cyan yellow ink for ejecting the ink nozzle group C _L, and dark magenta ink nozzle group M _D for ejecting dark magenta ink, light magenta ink nozzle group M _L for ejecting light magenta ink, a yellow ink a nozzle group Y _D is formed.

  The capital letter of the first alphabet in the code indicating each nozzle group means the ink color, and the subscript “D” indicates that the ink has a relatively high density, and the subscript “L” This means that the ink has a relatively low density.

  The plurality of nozzles of each nozzle group are aligned at a constant nozzle pitch k · D along the sub-scanning direction SS. Here, k is an integer, and D is a pitch (referred to as “dot pitch”) corresponding to the printing resolution in the sub-scanning direction. In this specification, it is also referred to as “nozzle pitch is k dots”. The unit [dot] at this time means the dot pitch of the printing resolution. Similarly, the unit of [dot] is used for the sub-scan feed amount.

  Each nozzle is provided with a piezo element (not shown) as a drive element for driving each nozzle to eject ink droplets. During printing, ink droplets are ejected from each nozzle while the print head 28 moves in the main scanning direction MS.

  In the color printer 20 having the hardware configuration described above, the carriage 30 is reciprocated by the carriage motor 24 while the paper P is transported by the paper feed motor 22, and simultaneously the piezo elements of the print head 28 are driven, so that each color ink Droplets are ejected to form ink dots on the paper P to form a multicolor / multi-tone image. In this embodiment, halftone processing is performed using a dither matrix.

  FIG. 4 is an explanatory diagram conceptually illustrating a part of the dither matrix. In the illustrated matrix, 128 elements in the horizontal direction (main scanning direction), 64 elements in the vertical direction (sub-scanning direction), a total of 8192 elements, were selected uniformly from the range of gradation values 1 to 255. A threshold value is stored. Note that the size of the dither matrix is not limited to the size illustrated in FIG. 4, and may be various sizes including a matrix having the same number of vertical and horizontal elements.

  FIG. 5 is an explanatory diagram showing the concept of the presence or absence of dot formation using a dither matrix. For the sake of illustration, only some elements are shown. In determining the presence or absence of dot formation, as shown in FIG. 2, the gradation value of the image data is compared with the threshold value stored at the corresponding position in the dither matrix. If the gradation value of the image data is larger than the threshold value stored in the dither table, a dot is formed, and if the gradation value of the image data is smaller, no dot is formed. The hatched pixels in FIG. 2 mean pixels on which dots are formed. In this way, if the dither matrix is used, it is possible to determine the presence or absence of dot formation for each pixel by a simple process of comparing the gradation value of the image data and the threshold value set in the dither matrix. The gradation number conversion process can be performed quickly. Furthermore, when the gradation value of the image data is determined, whether or not dots are formed in each pixel is determined solely by the threshold value set in the dither matrix. It is possible to positively control the dot generation state by the threshold storage position set in the matrix.

  As described above, since the systematic dither method can positively control the occurrence of dots according to the threshold storage position set in the dither matrix, the dot dispersion can be achieved by adjusting the threshold storage position setting. And other image quality can be controlled. This means that the halftone process can be optimized for various target states by the dither matrix optimization process.

  FIG. 6 is an explanatory diagram conceptually illustrating the spatial frequency characteristics of threshold values set for each pixel of a blue noise dither matrix having blue noise characteristics as a simple example of dither matrix adjustment. The spatial frequency characteristic of the blue noise matrix is a characteristic having the largest frequency component in a high frequency region in which the length of one cycle is 2 pixels or less. Such spatial frequency characteristics are set in consideration of human visual characteristics. That is, the blue noise dither matrix is a dither matrix in which the threshold storage position is adjusted so that the largest frequency component is generated in the high frequency region in consideration of the human visual characteristic that the sensitivity is low in the high frequency region.

  FIG. 7A is an explanatory diagram conceptually showing a visual spatial frequency characteristic VTF (Visual Transfer Function) which is a sensitivity characteristic with respect to a visual spatial frequency of a human. By using the visual spatial frequency characteristic VTF, the human visual sensitivity is modeled as a transfer function called the visual spatial frequency characteristic VTF, thereby quantifying the graininess of the dots after the halftone process appealing to the human visual sense. It becomes possible. The value quantified in this way is called the graininess index. FIG. 7B shows a typical empirical formula representing the visual spatial frequency characteristic VTF. The variable L in FIG. 7B represents the observation distance, and the variable u represents the spatial frequency. FIG. 7C is an equation that defines the graininess index. A coefficient K in FIG. 7C is a coefficient for matching the obtained value with human senses.

  Note that the calculation of the graininess index of a two-dimensional print image is usually performed by performing the integration of FIG. 7C on frequency components in all directions on the print medium. However, in the present invention, the graininess index for each direction is calculated by limiting the range in which the integration of FIG. The graininess index for each direction can be used as a scale for numerically evaluating “one-dimensional spatial frequency characteristics” in the scope of claims as will be described later.

  Such quantification of the granularity that appeals to human vision enables fine optimization of the dither matrix for the human visual system. Specifically, a Fourier transform is performed on a dot pattern assumed when each input gradation value is input to the dither matrix to obtain a power spectrum FS, and a filter process for multiplying the visual spatial frequency characteristic VTF is performed. The granularity evaluation value obtained by integrating (FIG. 7C) with all input gradation values after that can be used as an evaluation function of the dither matrix. In this example, the optimization can be achieved by adjusting the threshold storage position so that the evaluation function of the dither matrix becomes small.

  Note that when the print resolution is sufficiently high and a peak appears in a region having no visual sensitivity, the dither matrix may be adjusted so as to have a green noise characteristic instead of the blue noise characteristic. In this case, a green noise characteristic can be given to the dither matrix by applying a predetermined bias to the VTF function or a low-pass filter described later. The predetermined bias can be configured by, for example, artificially reducing the sensitivity of the VTF function in the peak frequency band of the green noise characteristic.

B. Generation of optimal dither matrix assuming bi-directional printing:
Bidirectional printing is a method in which dots are formed on the printing pixels both in the forward movement and in the backward movement of the main scanning feed (in this specification, simply referred to as main scanning) of the print head 28 to generate a printed image. It is printing. The dither matrix optimized for bidirectional printing is generated as follows in order to suppress image quality deterioration caused by bidirectional printing.

B-1. Image quality degradation due to bidirectional printing and its suppression mechanism:
FIG. 8 is an explanatory diagram showing a dot pattern formed using a conventional dither matrix. In FIG. 8, three dot patterns Dpall, Dpf, and Dpb are respectively the dot pattern Dpall of the print image, the forward dot pattern Dpf formed during the main scanning forward movement of the print head 28, and the main pattern of the print head 28. The dot pattern Dpb at the time of backward movement formed at the time of backward movement of scanning is shown. The dot pattern Dpall of the print image is formed by combining the forward movement dot pattern Dpf and the backward movement dot pattern Dpb in a common print region.

  As can be seen from FIG. 8, the dot pattern Dpall of the printed image shows a relatively uniform dot dispersibility, whereas the forward dot pattern Dpf and the backward dot pattern Dpb are dot density. Has occurred. Such density of dots is recognized by the human eye as significant image quality degradation. Such image quality degradation is caused by the fact that the conventional dither matrix is configured to improve the image quality of the dot pattern Dpall of the printed image, but the forward movement dot pattern Dpf and the backward movement dot pattern Dpf. If the dot pattern Dpb can be combined without causing an error in the dot formation position as assumed in advance, the dot pattern Dpb is not originally apparent.

  FIG. 9 is an explanatory diagram showing a state in which the image quality of a print image formed using a conventional dither matrix deteriorates due to bidirectional printing. In FIG. 8, four dot patterns Dp11, Dp12, Df1, and Db1 are a dot pattern Dp11 (no dot misalignment) of the print image, a dot pattern Dp12 (dot misalignment) of the print image, and the print head, respectively. 28 shows a forward movement dot pattern Df1 formed during the 28 main scanning forward movement and a backward movement dot pattern Db1 formed during the backward movement of the print head 28 in the main scanning.

  The dot pattern Dp11 (no dot displacement) of the print image is the same as the dot pattern Dpall in FIG. The forward movement dot pattern Df1 is the same as the dot pattern Dpf of FIG. The backward movement dot pattern Db1 is the same as the dot pattern Dpb of FIG.

  In the dot pattern Dp12 (with dot misalignment) of the print image, the image quality is significantly degraded due to the relative misalignment between the forward dot pattern Df1 and the backward dot pattern Db1. The relative displacement of the dot formation position is caused by the displacement of the dot formation positions of the dot patterns Df1 and Db1 in the main scanning direction due to the difference (forward or backward) in the main scanning direction during dot formation. . As described above, the image quality is significantly degraded due to the relative displacement of the dot pattern. As described above, the conventional dither matrix does not cause such displacement, and the dots are formed at accurate positions. This is because of the configuration. In other words, if there is no misalignment, the sparse and dense portions of the dot patterns Df1 and Db1 match each other with high precision, so that uniform dot dispersibility matches. In other words, the density of the dots may be matched with each other, and the density of the dots may be emphasized on the contrary, and the image quality is deteriorated.

  Based on such a hypothesis, the inventors of the present application have confirmed that such image quality degradation is caused by bidirectional printing by conducting experiments on various images. Further, the inventor of the present application has come up with a dither matrix that is resistant (robust) to the misalignment of dots based on this hypothesis.

  FIG. 10 is an explanatory diagram showing a state in which image quality deterioration of a printed image formed by bidirectional printing is suppressed by the dither matrix according to the embodiment of the present invention. In FIG. 10, four dot patterns Dp21, Dp22, Df2, and Db2 are respectively a dot pattern Dp21 (no dot displacement) of the print image, a dot pattern Dp22 (dot displacement) of the print image, and the print head. 28 shows a forward movement dot pattern Df2 formed at the time of 28 main scanning forward movements, and a backward movement dot pattern Db2 formed at the time of backward movement of the print head 28 by main scanning.

  The dither matrix according to the embodiment of the present invention is configured so that the dot dispersibility of the forward movement dot pattern Df2 and the backward movement dot pattern Db2 is improved, and the dot patterns Df2 and Db2 are less dense. This is different from the dot patterns Df1 and Db1. In the dot pattern Dp22 (with dot misalignment) of the printed image formed by combining such sparse and small dot patterns Df2 and Db2, the sparse parts or the dense inevitably caused by the dot misalignment are inevitably generated. Since the overlap between the portions is also reduced, the density of the dots is reduced and the dispersibility is preferable.

  Thus, the inventor of the present invention does not improve the image quality by improving the accuracy of the dot formation position which has been conventionally performed, but the idea of reversal of the configuration of a dither matrix having robustness against the error of the dot formation position. I came up with it. The inventor of the present application has also succeeded in realizing practical generation of a dither matrix having such characteristics.

B-2. Generation of optimal dither matrix based on granularity index (first embodiment):
FIG. 11 is a flowchart showing the processing routine of the dither matrix generation method in the first embodiment of the present invention. This dither matrix generation method is configured to be able to optimize in consideration of the dispersibility of dots formed during both forward movement and backward movement in the print image formation process. In this example, a small dither matrix of 8 rows and 8 columns is generated for easy understanding.

  In step S100, a grouping process is performed. In the present embodiment, the grouping process is a dither matrix for each element corresponding to a pixel group in which dots are formed during forward movement and a pixel group in which dots are formed during backward movement in the print image formation process. This is a process of dividing.

  FIG. 12 is an explanatory diagram showing the dither matrix M and the two divided matrices M1 and M2 after performing the grouping process in the first embodiment of the present invention. In this grouping process, the pixel group is divided into two pixel groups in FIG. The numbers described in each element of the dither matrix M indicate the pixel group to which each element belongs. In this example, the elements in the odd rows belong to the first pixel group, and the elements in the even rows belong to the second pixel group.

  The division matrix M1 includes a plurality of elements corresponding to the pixels belonging to the first pixel group among the elements of the dither matrix M, and blank elements that are blank elements. On the other hand, the division matrix M2 includes a plurality of elements corresponding to the pixels belonging to the second pixel group among the elements of the dither matrix M, and blank elements that are blank elements. Such a grouping process is set assuming the following printing method.

FIG. 13 is an explanatory diagram showing dot formation target pixels for each main scanning in bidirectional printing according to the first embodiment of the present invention. In this printing, dots are formed in each pixel by bidirectional printing using the nozzle row 10. The nozzle array 10 is a nozzle array that represents the nozzle array K _D , C _D , C _L , M _D , M _L , and Y _D on the lower surface of the print head 28 (FIG. 3). The nozzle pitch k · D is 2D. Here, D is a pixel pitch corresponding to the printing resolution in the sub-scanning direction.

  In this bidirectional printing, dots are formed in each pixel as follows. In pass 1, which is the first main scanning, the nozzle row 10 performs main scanning sending in the forward direction, and dots are formed at each pixel position. As a result, dots are formed at the pixel positions in the odd-numbered rows of circles surrounding the numeral “1”. The group of pixels in which dots are formed in this way is the first pixel group. Sub-scan feed is performed after pass 1 is completed, and pass 2 main-scan feed is performed. In pass 2, the nozzle row 10 performs main scanning in the backward direction, and dots are formed at each pixel position. As a result, dots are formed at pixel positions in even-numbered rows of circles surrounding the numeral “2”. The group of pixels in which dots are formed in this way is the second pixel group.

  In this way, when the grouping process (FIG. 11) in step S100 is completed, the process proceeds to the threshold value determination process (step S200).

  In step S200, a target threshold value determination process is performed. The target threshold value determination process is a process for determining a threshold value to be a storage element determination target. In this embodiment, the threshold value is determined by selecting in order from a threshold value having a relatively small value, that is, a threshold value having a value at which dots are likely to be formed. The reason for this will be described later.

  In step S300, a dither matrix evaluation process is performed. The dither matrix evaluation process is a process for digitizing the optimality of the dither matrix based on a preset evaluation function. In this embodiment, the evaluation function is defined by a one-dimensional graininess index calculated by the calculation formula of FIG. 7C and a calculation formula expressed in a two-dimensional area in consideration of the direction of the print image. 2D graininess index. The one-dimensional graininess index is used for the evaluation of the dither matrix M. The two-dimensional graininess index is used for evaluation of the division matrices M1 and M2. Details of the two-dimensional graininess index will be described later.

FIG. 14 is a flowchart showing the processing routine of the dither matrix evaluation process.
In step S310, a rating matrix selection process is performed. In the present embodiment, the rating matrix selection process is a process of sequentially selecting one of the two divided matrices M1 and M2. For example, when the division matrix M1 is selected, the division matrix M1 and the dither matrix M are the evaluation targets of the evaluation function.

  In step S320, the corresponding threshold value corresponding dot is turned on. The determined threshold means a threshold at which the storage element is determined. In this embodiment, since the threshold value is selected in order from the value at which dots are likely to be formed as described above, when a dot is formed as the threshold value of interest, the pixel corresponding to the element storing the determined threshold value is not used. Dots are always formed. Conversely, at the smallest input tone value at which dots are formed at the threshold value of interest, no dots are formed at pixels corresponding to elements other than the element storing the determined threshold value. In this example, it is assumed that the division matrix M1 is selected as the rating matrix.

  FIG. 15 is a diagram illustrating a dot pattern DPM in which dots (● marks) are formed in each of eight pixels corresponding to elements storing threshold values at which dots are likely to be formed first to eighth in the dither matrix M. FIG. This dot pattern is used to determine in which pixel the ninth dot is to be formed. That is, it is used to determine the storage element of the target threshold value at which the ninth dot is most likely to be formed. * Indicates a pixel corresponding to the element of interest.

  In step S330, the corresponding dot of the element of interest is turned on. In this example, the target element is one of candidate storage elements for the target threshold value in which the ninth dot is most likely to be formed. Since the element of interest is selected from the elements of the rating matrix (in this example, the division matrix M1), it is selected from the elements in the odd rows.

  In step S340, a graininess index calculation process is performed. The graininess index calculation process is a process for calculating the graininess index when it is assumed that dots are formed in the pixel corresponding to the element of interest with respect to the dot pattern DPM. This process is performed based on a dot density matrix (FIG. 16) obtained by digitizing the dot pattern. The dot density matrix (FIG. 16) is configured by setting the value of a pixel in which dots are formed to “1” and the value of a pixel in which dots are not formed to “0”.

  The processes in step S330 and step S340 are performed for all elements other than the element in which the threshold value at which dots are likely to be formed in the first to eighth rows is stored among the elements in the odd-numbered rows while changing the element of interest.

  The processing from step S320 to step S350 is performed in the same manner for the divided matrix M1. However, the dot pattern to be evaluated is a dot pattern (FIG. 17) composed of only dots corresponding to each element of the divided matrix M1 and dots corresponding to the element of interest. The corresponding dot density matrix is shown in FIG.

  In step S400 (FIG. 11), a storage element determination process is performed. The storage element determination process is a process for determining a storage element of the target threshold value (threshold in which 9th dot is most likely to be formed in this example). In the present embodiment, the storage element is determined from elements having the smallest overall evaluation value. In this embodiment, the overall evaluation value is calculated by multiplying the evaluation value of the dither matrix M and the evaluation values of the divided matrices M1 and M2 by multiplying them by a predetermined weight (for example, 2: 1).

  When such processing is performed for all threshold values from the threshold at which dots are most likely to be formed to the threshold at which dots are hardly formed, the dither matrix generation processing is completed (step S500).

  FIG. 19 is an explanatory diagram showing a two-dimensional filter characteristic extended to a two-dimensional region used for calculating a two-dimensional granularity index used in the evaluation of the divided matrices M1 and M2. FIG. 19A shows how the filter coefficient with respect to the spatial frequency changes according to the direction on the print medium. FIG. 19B is an explanatory diagram showing the definition of the direction on the print medium. As can be seen from FIGS. 19A and 19B, the two-dimensional filter used in the embodiment of the present invention is configured such that the filter coefficient increases as it approaches the main scanning direction.

  This two-dimensional filter characteristic is obtained by giving directionality to the VTF function used in the conventional graininess index. In the conventional graininess index, by using the VTF function, the weighting of the power spectrum FS in the frequency region with high human visual sensitivity is increased, thereby quantifying the granularity appealing to human vision. This VTF function is premised on isotropy. That is, it is configured on the assumption that human visual sensitivity does not change according to the direction of the printed image.

  FIG. 20 is an explanatory diagram showing a state where the anisotropy of the two-dimensional filter characteristic used in the embodiment of the present invention is observed from two places in the three-dimensional space. This anisotropy is due to bidirectional printing. In bidirectional printing, the forward movement dot pattern formed during forward movement and the backward movement dot pattern formed during backward movement are relatively misaligned in the main scanning direction. Density in the scanning direction is a major factor in image quality degradation. On the other hand, in the sub-scanning direction, although there is a relative shift due to vibration of the print head 28 or the like, it is significantly smaller than the shift in the main scanning direction.

  As described above, the two-dimensional filter characteristics are configured to have anisotropy that makes the dispersibility of dots in the main scanning direction of the divided matrices M1 and M2 better than the dispersibility of dots in the sub-scanning direction. I understand that. Such anisotropy increases the degree of freedom of design in all directions of the dither matrix M and in the main scanning direction of the divided matrices M1 and M2 by relatively reducing the weight of the dot dispersibility in the sub-scanning direction. The optimality of the dither matrix can be enhanced with the effect of.

  As described above, in the first embodiment of the present invention, since the dither matrix is optimized by using such a two-dimensional filter having anisotropy, a print image formed by bidirectional printing is human. It is possible to effectively suppress the graininess that appeals to the visual perception.

  The optimality of the dither matrix generated in the first embodiment of the present invention can be confirmed by the following method, for example. With such a confirmation method, it can be confirmed that the present invention is mounted on a printing apparatus and exhibits the original effects of the present invention.

  The first method focuses on the correlation coefficient of the spatial frequency distribution of the dot pattern. According to this method, in the measurement of the spatial frequency distribution of the dot pattern, the closer the sampling direction of the image data is to the main scanning direction, the spatial frequency distribution of the dot pattern of the printed image, the forward dot pattern, The tendency that the correlation coefficient with the spatial frequency distribution of the moving dot pattern increases is objectively observed. This is because the two-dimensional graininess index is configured to have the same or approximate characteristic to the spatial frequency of the printed image in the main scanning direction.

  The second method focuses on the graininess index in each direction of the forward movement dot pattern and the backward movement dot pattern. According to this method, in the measurement of the spatial frequency distribution of the dot pattern, the one-dimensional granularity index of the forward movement dot pattern and the backward movement dot pattern becomes closer as the image data sampling direction becomes closer to the main scanning direction. The tendency to decrease becomes objectively observed. This is because the two-dimensional granularity index is configured so that the weight is the largest in the main scanning direction, so that the dither matrix optimized based on the evaluation of the two-dimensional granularity index is the main scanning direction. This is because a dot pattern having the smallest one-dimensional graininess index is formed.

  The third method is a method that pays attention to the combination (with shift) of the forward movement dot pattern and the backward movement dot pattern. According to this method, when the forward movement dot pattern and the backward movement dot pattern are captured by the scanner and combined by shifting in the main scanning direction or the sub-scanning direction, the image quality is not deteriorated even if shifted in the main scanning direction. Although it is small, the tendency that image quality deterioration is remarkable when shifted in the sub-scanning direction is objectively observed. This is based on objective characteristics that are directly related to the effects of the present invention. In bidirectional printing, the present invention can achieve the original effects based on such characteristics.

  Thus, in the first embodiment of the present invention, in bi-directional printing, the dispersibility in the main scanning direction of the dot pattern formed during forward movement and the dot pattern formed during backward movement is controlled intensively. By forming a new print image different from the conventional one, it is possible to realize printing that is robust against a shift in the main scanning direction of the relative position of both dot patterns caused by bidirectional printing. it can. In this embodiment, the main scanning direction corresponds to a “specific direction” in the claims.

B-3. Generation of optimal dither matrix based on RMS granularity (second embodiment):
FIG. 21 is a flowchart showing the processing routine (step S300a) of the dither matrix generation method in the second embodiment of the present invention. The generation method of the second embodiment is different from the generation method of the first embodiment in the evaluation function of the dither matrix. That is, the generation method of the second embodiment is different from the generation method of the first embodiment in that the storage element is determined based on a specific RMS granularity instead of the one-dimensional or two-dimensional granularity index.

  The generation method of the second embodiment can be realized by replacing the process of Step S340 (granularity calculation process) with the process of Step S342 (low-pass filter process) and the process of Step S345 (RMS granularity calculation process). It is.

  In step S342, low-pass filter processing is performed on the dot density matrix (FIGS. 16 and 18) corresponding to the dither matrix M and the division matrix. The dither matrix M is subjected to low-pass filter processing using an isotropic low-pass filter LPFa. On the other hand, for the divided matrices M1 and M2, low-pass filter processing is performed using an anisotropic low-pass filter LPFg having a large matrix size in the main scanning direction. The reason why the anisotropic low-pass filter LPFg having a large matrix size in the main scanning direction is used for the divided matrices M1 and M2 is to further improve the dispersibility in the main scanning direction of the dot patterns corresponding to the divided matrices M1 and M2. is there.

  In step S335, RMS granularity calculation processing is performed. The RMS granularity calculation process is a process for calculating a standard deviation after low-pass filtering the dot density matrix. The standard deviation can be calculated using the calculation formula of FIG. The standard deviation is not necessarily calculated for the dot pattern corresponding to all elements of the dither matrix M. In order to reduce the amount of calculation, the standard deviation is calculated for pixels belonging to a predetermined window (for example, a 5 × 5 partial matrix). You may make it carry out using only a dot density. Such processing is performed for all the target pixels (step S350).

  By storing the value calculated by such processing in the same manner as in the first embodiment, the storage element for the target threshold value is determined (S400 (FIG. 11)). As described above, the same processing can be performed even when the RMS granularity is used by using the anisotropic low-pass filter.

C. Generation of optimal dither matrix assuming the dot pattern formed in each main scan:
FIG. 24 is an explanatory diagram showing how a print image is generated on a print medium by forming ink dots while performing unidirectional main scanning and sub-scanning in a comparative example of the present invention. The main scanning means an operation of moving the nozzle row 10 relative to the print medium in the main scanning direction. The sub scanning means an operation of moving the nozzle row 10 relative to the print medium in the sub scanning direction. The nozzle array 10 is configured to form ink dots by ejecting ink droplets onto a print medium. The nozzle row 10 is equipped with ten nozzles (not shown) at intervals of twice the pixel pitch k. In this example, since unidirectional printing is performed, the shift in the main scanning direction is small and hardly causes deterioration in image quality. However, image quality deterioration as will be described later occurs due to a shift in dot formation timing.

  The print image is generated as follows while performing main scanning and sub-scanning. In the main scan of pass 1, the pixel position number is 1, 3, 5, out of 10 main scan lines with raster numbers 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 Ink dots are formed on 7 pixels. The main scanning line means a line formed by pixels that are continuous in the main scanning direction. Each circle indicates a dot formation position. The numbers in each circle indicate a pixel group composed of a plurality of pixels on which ink dots are formed simultaneously. In pass 1, dots are formed in the print pixels belonging to the first pixel group.

  When the pass 1 main scan is completed, the sub-scan feed is performed in the sub-scan direction with a movement amount L that is three times the pixel pitch. In general, the sub-scan feed is performed by moving the print medium. However, in this embodiment, the nozzle row 10 is moved in the sub-scanning direction for easy understanding. When the sub-scan feed is completed, the main scan of pass 2 is performed.

  In the pass 2 main scan, among the 10 main scan lines with raster numbers 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, the pixel position numbers are 1, 3, 5, Ink dots are formed on 7 pixels. In this way, in pass 2, dots are formed in the print pixels belonging to the third pixel group. The two main scanning lines with raster numbers 22 and 24 are not shown. When the pass 2 main scan is completed, the sub-scan feed similar to that described above is performed, and then the pass 3 main scan is performed.

  In the main scan of pass 3, ink is applied to pixels having pixel position numbers 2, 4, 6, and 8 out of 10 main scan lines including the main scan lines having raster numbers 11, 13, 15, 17, and 19. Dots are formed. In the pass 4 main scan, ink dots are applied to pixels having pixel position numbers 2, 4, 6, and 8 out of 10 main scan lines including three main scan lines with raster numbers 16, 18, and 20. Is formed. In this way, it can be seen that ink dots can be formed without gaps in the sub-scanning positions with raster numbers of 15 and later. In pass 3 and pass 4, dots are formed on the print pixels belonging to the second and fourth pixel groups, respectively.

  Observing the generation of such a printed image by paying attention to a certain area, it can be seen that the following is performed. For example, if an area having raster numbers 15 to 19 and pixel position numbers 1 to 8 is set as a target area, it can be seen that a print image is formed in the target area as follows.

  In pass 1, it can be seen that the same dot pattern as the ink dots formed at the pixel positions with the raster numbers 1 to 5 and the pixel position numbers 1 to 8 is formed in the region of interest. This dot pattern is formed of dots formed on pixels belonging to the first pixel group. That is, in pass 1, dots are formed in the pixels belonging to the first pixel group in the region of interest.

  In pass 2, dots are formed in the pixels belonging to the third pixel group in the region of interest. In pass 3, dots are formed in the pixels belonging to the second pixel group in the region of interest. In pass 4, dots are formed in the pixels belonging to the fourth pixel group in the region of interest.

  Thus, in this embodiment, it can be seen that the dots formed on the print pixels belonging to each of the first to fourth pixel groups are formed by being combined with each other in the common print region.

  FIG. 25 shows a state where a print image is generated on a print medium by combining dots formed on print pixels belonging to each of a plurality of pixel groups in a comparative print example in a common print region. It is explanatory drawing shown. In the example of FIG. 25, the print image is a print image of a predetermined intermediate gradation (single color). The dot patterns DP1 and DP1a indicate dot patterns formed on a plurality of pixels belonging to the first pixel group. The dot patterns DP2 and DP2a indicate dot patterns formed on a plurality of pixels belonging to the first and third pixel groups. The dot patterns DP3 and DP3a indicate dot patterns formed on a plurality of pixels belonging to the first to third pixel groups. The dot patterns DP4 and DP4a indicate dot patterns formed on a plurality of pixels belonging to all pixel groups.

  The dot patterns DP1, DP2, DP3, and DP4 are dot patterns when a conventional dither matrix is used. The dot patterns DP1a, DP2a, DP3a, and DP4a are dot patterns when the dither matrix of the present invention is used. As can be seen from FIG. 25, when the dither matrix of the present invention is used, the dot dispersibility is more uniform in the dot patterns DP1a and DP2a where the dot pattern overlap is smaller than when the conventional dither matrix is used. It is.

  Since the conventional dither matrix does not have the concept of pixel groups, optimization is performed focusing only on the dispersibility of dots in the finally formed print image (dot pattern DP4 in the example of FIG. 25). . In other words, since the dispersibility of the dots formed in the pixels belonging to each pixel group is not considered, the dispersibility of the dots formed in the pixels belonging to each pixel group is not good, and the dot density is sparse and dense. .

  The dither matrix of the present invention is formed on the pixels belonging to each pixel group because the dispersibility of the dots formed on the pixels belonging to each pixel group is considered in addition to the dispersibility of the dots in the printed image. The dispersibility of both the dot dispersibility and the dot dispersibility in the printed image is improved.

  The dither matrix of the present invention has been optimized by focusing on not only the finally formed dot pattern but also the dot pattern in the dot formation process. Such a focus has not existed in the past. This is because, conventionally, it has been common technical knowledge that even if the dot pattern dispersion in the dot formation process is poor, the image quality is good if the dispersibility of the finally formed dot pattern is good.

  However, the inventor of the present application dared to analyze the image quality of the printed image by paying attention to the dot pattern in the dot formation process. As a result of this analysis, it has been found that image unevenness occurs due to the density of the dot pattern in the dot formation process. The inventors of the present application have also found out that the unevenness of the image is perceived remarkably by human eyes due to the physical phenomenon of ink such as unevenness of ink aggregation, unevenness of gloss and bronze phenomenon. Note that the bronze phenomenon is a phenomenon in which the state of light reflected on the surface of the printing paper changes, for example, the printing surface changes to a bronze color depending on the viewing angle due to aggregation of dyes in ink droplets.

  For example, ink aggregation and bronzing can occur even when a printed image is formed in a single pass. However, even if ink agglomeration or the like occurs uniformly on the entire surface of the printed image, it is unlikely to be close to the human eye. This is because, since it occurs uniformly, ink non-aggregation or the like does not occur as non-uniform “unevenness” including low-frequency components.

  However, in a dot pattern formed in a pixel group in which ink dots are formed almost simultaneously in the same main scan, if the unevenness occurs in a low frequency region that is easily recognized by the human eye due to ink aggregation or the like, significant image quality degradation occurs. Will become apparent. In this way, when forming a print image by forming ink dots, it is possible to optimize the dither matrix by focusing on the dot pattern formed in the pixel group in which the ink dots are formed almost simultaneously. It was first discovered by the inventor that it leads to

  In addition, since the dither matrix of the prior art is optimized on the premise that the mutual positional relationship of each pixel group is assumed in advance, if the mutual positional relationship is shifted As a result, the optimality is not guaranteed and the image quality is significantly deteriorated. However, according to the dither matrix of the present invention, since the dispersibility of the dots is ensured even in the dot pattern of each pixel group, it is possible to secure a high robustness against the displacement of the mutual positional relationship. First confirmed by experiment.

  Further, the inventor of the present application has found that the above-described image quality deterioration is more likely to occur as the pixel pitch that is the target of dot formation in each main scan is smaller. This is because the smaller the pixel pitch for dot formation in each main scan, the easier the aggregation and bronzing. Further, the inventor of the present application also paid attention to the fact that the pixel pitch that is the target of dot formation in each main scan often differs between the main scanning direction and the sub-scanning direction. For example, in the printing method shown in FIG. 26, the above-described pixel pitch is small in the main scanning direction, and in the sub-scanning direction, the pixel pitch is twice that in the main scanning direction. On the other hand, in the printing method shown in FIG. 27, the above-described pixel pitch is small in the sub-scanning direction, and has a pixel pitch twice as large as that in the sub-scanning direction in the main scanning direction.

  In such a printing method, the dither matrix can be optimized by performing the same processing by replacing the direction in which the pixel pitch is small with the main scanning direction in the first embodiment. For example, in the printing method shown in FIG. 26, the first embodiment may be carried out as it is, and in the printing method shown in FIG. 27, the main scanning direction and the sub scanning direction may be switched.

  As described above, the present invention can form dots in each main scan in the main scan direction and the sub scan direction even in unidirectional printing in which dots are formed only in one of the forward direction and the sub direction of the main scan of the print head, for example. Even when the target pixel pitch is different, it is possible to generate an optimum dither matrix assuming such a difference.

D. Variation:
As mentioned above, although several embodiment of this invention was described, this invention is not limited to such embodiment at all, and implementation in various aspects is possible within the range which does not deviate from the summary. It is. For example, the present invention can optimize a dither matrix for the following modifications.

D-1. In the embodiment described above, halftone processing is performed using a dither matrix, but the present invention can also be applied to the case where halftone processing is performed using error diffusion, for example. The use of error diffusion can be realized, for example, by performing error diffusion processing for each of a plurality of pixel groups.

  Specifically, in addition to normal error diffusion, processing for separately diffusing errors may be performed for each of a plurality of pixel groups, or weighting of errors to be diffused for pixels belonging to a plurality of pixel groups may be performed. You may make it enlarge. Even with this configuration, according to the inherent characteristics of the error diffusion method, all the dot patterns formed on the printing pixels belonging to each of the plurality of pixel groups have predetermined characteristics at each gradation value. Because it can be done. Furthermore, by additionally diffusing an error diffusing into a plurality of pixel groups in the main scanning direction, it is possible to focus on improving the dot dispersibility in each pixel group in the main scanning direction.

  FIG. 28 is an explanatory diagram showing a flowchart of an application example of the present invention to the error diffusion method. The error diffusion method is a kind of halftone processing method configured to diffuse a difference between an input gradation value and an output gradation value to surrounding pixels so that the output gradation value approaches the input gradation value. In the error diffusion method, the dot formation state of all print pixels is determined by shifting the pixel of interest, which is a pixel for which dot formation is determined, one by one. As a shift method, for example, the pixel of interest is shifted one by one in the main scanning direction, and when the processing of all the pixels of the main scanning line is completed, the pixel of interest is shifted to the adjacent unprocessed main scanning line. Is.

  In step S500, a diffusion error diffused from a plurality of other processed pixels for the pixel of interest is read. In this embodiment, the diffusion error includes an overall diffusion error ERa and a group diffusion error ERg.

  The total diffusion error ERa is an error diffused using the error diffusion total matrix Ma shown in FIG. In this embodiment, errors are diffused using a well-known Jarvis, Judice & Ninke type error diffusion matrix. Such error diffusion is performed as general error diffusion. Similar to the error diffusion method of the prior art, such error diffusion enables the final dot pattern to have predetermined characteristics as an inherent characteristic of the error diffusion method.

  However, in this embodiment, in order to give each of the two pixel groups 1A and 1B (FIG. 26) predetermined characteristics, the group diffusion error ERg is additionally diffused. Different from the law. This additional error diffusion is performed using the error diffusion whole matrix Mg. The entire error diffusion matrix Mg is configured to diffuse errors only in the main scanning direction in order to improve the dispersibility of dots in the main scanning direction. In order to improve the dispersibility of dots in the sub-scanning direction as in the main scanning direction, the error diffusion whole matrix Mgc may be used.

  As described above, in this embodiment, the final dot pattern has predetermined characteristics by error diffusion by the error diffusion whole matrix Ma, and each of the plurality of pixel groups by error diffusion by the error diffusion same main scanning group matrix Mg. The error is diffused so that the dot pattern has predetermined characteristics.

  In step S510, an average error ERave which is a weighted average value of the total diffusion error ERa and the group diffusion error ERg is calculated. In the present embodiment, as an example, the weights of the overall diffusion error ERa and the group diffusion error ERg are set to “4” and “1”, respectively. The average error ERave is calculated as a value obtained by dividing the sum of the value obtained by multiplying the total diffusion error ERa by the weight “4” and the value obtained by multiplying the group diffusion error ERg by the weight “1” by the total weight “5”. The

  In step S520, the input gradation value Dt and the average error ERave are added to calculate the positive data Dc.

  In step S530, the calculated correction data Dc is compared with a preset threshold value Thre. If the result of this comparison is that the correction data Dc is greater than the threshold value Thre, it is determined that dots are to be formed (step S540). On the other hand, when the correction data Dc is smaller than the threshold value Thre, it is determined not to form a dot (step S550).

  In step S560, the tone error is calculated and the tone error is diffused to surrounding unprocessed pixels. The gradation error is a difference between the correction data Dc and an actual gradation value generated by determining whether or not dots are formed. For example, if the gradation value of the correction data Dc is “223” and the gradation value actually generated by the dot formation is 255, the gradation error is “−32” (= 223−255). . In this step (S560), error diffusion is performed using the error diffusion whole matrix Ma.

  Specifically, for the pixel adjacent to the right of the pixel of interest, the coefficient “7/48” corresponding to the pixel adjacent to the right of the error diffusion overall matrix Ma with respect to the gradation error “−32” generated in the pixel of interest. A value “−224/48” (= −32 × 7/48) multiplied by is diffused. Further, for the two pixels adjacent to the right of the pixel of interest, the coefficient “5/48” corresponding to the two pixels adjacent to the right of the error diffusion overall matrix Ma with respect to the gradation error “−32” generated in the pixel of interest. The value “−160/48” (= −32 × 5/48) multiplied by “is diffused. Similar to the error diffusion method of the prior art, such error diffusion is to impart predetermined characteristics to the final dot pattern as an inherent characteristic of the error diffusion method.

  In step S570, unlike the conventional error diffusion, error diffusion using the same error diffusion main scanning group matrix Mg (FIG. 29) is additionally performed. As described above, this is because each of the two pixel groups 1A and 1B (FIG. 26) has a predetermined characteristic particularly in the main scanning direction.

  Thus, in the first application example of the present invention to the error diffusion method, the object of the present invention can be achieved by additional error diffusion focused on the main scanning direction to the same pixel group as the pixel of interest. it can. The error may be diffused at once using an error diffusion matrix obtained by combining the error diffusion overall matrix Ma and the error diffusion group matrix Mg.

D-2. In the above-described embodiment, the threshold storage elements are determined in order. However, for example, the dither matrix may be generated by adjusting the dither matrix as an initial state prepared in advance. . For example, a dither matrix is prepared as an initial state for storing in each element a plurality of threshold values for determining the presence or absence of dot formation for each pixel according to the input gradation value, and a plurality of threshold values stored in each element A part of this is replaced with a threshold value stored in another element in a randomly or systematically determined manner, and the dither matrix is adjusted by determining whether or not to replace based on the evaluation values before and after the replacement. May be generated. It should be noted that “for each storage element candidate” in the claims corresponds to “for each set of candidates for a plurality of replaced storage elements” in the present modification.

D-3. In the above-described embodiments and modifications, low-pass filter processing is performed and the optimality of the dither matrix is evaluated based on dot density uniformity and RMS granularity. For example, Fourier transform is performed on the dot pattern. You may comprise so that the optimality of a dither matrix may be evaluated using a VTF function. Specifically, the evaluation scale (Grainess scale: GS value) used by Dooley et al. Of Xerox may be applied to the dot pattern, and the optimality of the dither matrix may be evaluated based on the GS value. Here, the GS value is obtained by performing a predetermined process including a two-dimensional Fourier transform on the dot pattern to digitize the dot pattern, and performing integration after performing a filter process that multiplies the visual spatial frequency characteristic VTF. (Reference: Fine Imaging and Hardcopy, Corona, Japan Photographic Society, Japanese Imaging Society Joint Publishing Committee, P534). However, the former has an advantage that complicated calculation such as Fourier transform is unnecessary.

D-4. In the above-described embodiments and modifications, the presence or absence of dot formation is determined for each pixel by comparing the threshold value set in the dither matrix and the gradation value of the image data for each pixel. Further, the sum of the gradation values may be compared with a fixed value to determine the presence or absence of dot formation. Furthermore, the presence / absence of dot formation may be determined according to the data generated in advance based on the threshold value and the gradation value without directly using the threshold value. In general, the dither method of the present invention only needs to determine the presence or absence of dot formation according to the gradation value of each pixel and the threshold value set at the corresponding pixel position of the dither matrix.

D-5. In the above-described embodiment, the relative position shift of the dots occurs in the main scanning direction, but it may occur in the sub-scanning direction like a line printer having the following configuration, for example. FIG. 30 is an explanatory diagram showing a printing state by a line printer 200L having a plurality of print heads 251 and 252 in a modification of the present invention. A plurality of print heads 251 and print heads 252 are arranged on the upstream side and the downstream side, respectively. The line printer 200L is a printer that outputs only at high speed by performing only sub-scan feed without performing main scanning.

  A dot pattern 500 formed by the line printer 200L is shown on the right side of FIG. Numbers 1 and 2 in the circles indicate which of the print heads 251 and 252 is responsible for dot formation. Specifically, the dots with the numbers “1” and “2” in the circle are formed by the print head 251 and the print head 252, respectively.

  The inside of the thick line of the dot pattern 500 is an overlap region where dots are formed by both the print head 251 and the print head 252. The overlap region is provided in order to make the joint between the print head 251 and the print head 252 smooth and to make the dot formation position error generated at both ends of the print heads 251 and 252 inconspicuous. is there. This is because the manufacturing individual difference between the print heads 251 and 252 increases at the both ends of the print heads 251 and 252 and the error of the dot formation position also increases, which is required to be inconspicuous.

  Even in such a case, a phenomenon similar to that in the case where the dot formation position is shifted between the forward movement and the backward movement due to an error in the positional relationship between the print heads 251 and 252 occurs in the sub-scanning direction. Therefore, the image quality can be improved by performing the same processing as in the above-described embodiment for the pixel group formed by the print head 251 and the pixel group formed by the print head 252.

D-6. In the above-described embodiment, the dither matrix is optimized by focusing on the dot pattern formed in the pixel group. For example, the following method is used to optimize the dither matrix without focusing on such a dot pattern. It is also possible to carry out.

  FIG. 31 is an explanatory diagram showing an example of an actual printing state in the bidirectional printing method. The characters in the circles indicate in which reciprocal main scanning the dots are formed. FIG. 31A shows a dot pattern when there is no deviation in the main scanning direction. FIG. 31B and FIG. 31C show dot patterns when there is a deviation in the main scanning direction.

  In FIG. 31B, the position of the dot formed in the print pixel belonging to the pixel group where the dot is formed when the print head moves forward belongs to the pixel group where the dot is formed when the print head moves backward. The position of the dot formed on the print pixel is shifted by one dot pitch to the right. On the other hand, in FIG. 31C, a pixel group in which dots are formed when the print head is moved backward relative to the positions of dots formed in print pixels belonging to a pixel group in which dots are formed when the print head moves forward. The positions of the dots formed in the print pixels belonging to are shifted leftward by one dot pitch.

  In the above-described embodiment, the spatial frequency distribution of blue noise or green noise is given to both the dot pattern of the pixel group in which dots are formed during forward movement and the dot pattern of the pixel group in which dots are formed during backward movement. Therefore, image quality deterioration due to such a shift is suppressed.

  On the other hand, in this modification, the dot pattern formed in the pixel group formed in the forward movement and the dot pattern formed in the pixel group formed in the backward movement are only one dot pitch in the main scanning direction. The dot pattern synthesized by shifting has a spatial frequency distribution of blue noise or green noise, or has a small granularity index.

  The structure of the dither matrix focusing on the graininess index is, for example, the average value of the graininess index when the shift in the main scanning direction is shifted by one dot pitch on one side, by one dot pitch on the other side, and when there is no shift May be configured to be minimized, or the spatial frequency distribution in these cases may be configured to have a high correlation coefficient. The amount of shift may be 1 dot pitch or less, or 2 dot pitch or more, depending on the printing resolution or other printing environment.

  In this modification, the robustness of the image quality with respect to the deviation of the dot formation position in the forward movement and the backward movement can be increased, so the dot formation positions in the forward movement and the backward movement are collectively. In addition, the image quality deterioration is suppressed even when an unspecified shift occurs between a pixel group in which dots are formed during forward movement and a part of pixel groups in which dots are formed during backward movement. be able to. For example, even when the gap between the print head and the printing paper partially fluctuates between forward movement and backward movement due to periodic deformation caused by the main scanning of the main scanning mechanism of the print head, the image quality deteriorates. Can be suppressed.

10 ... Nozzle array 20 ... Color printer 22 ... Motor 24 ... Carriage motor 26 ... Platen 28 ... Print head 30 ... Carriage 32 ... Operation panel 34 ... Slide Axis 36 ... Drive belt 38 ... Pulley 39 ... Position sensor 40 ... Control circuit 56 ... Connector 60 ... Print head unit 90 ... Computer 91 ... Video driver 95. Application program 96 ... Printer driver 97 ... Resolution conversion module 98 ... Color conversion module 99 ... Halftone module 100 ... Rasterizer

Claims (23)

A printing device for printing on a print medium,
By performing halftone processing on the image data representing the input gradation value of each pixel constituting the original image, the dot formation state on each print pixel of the print image to be formed on the print medium is represented. A dot data generator for generating dot data;
According to the dot data, dots are formed on a plurality of print pixels belonging to each of a plurality of pixel groups assumed to be physically different from each other in dot formation, and the formed dots are used as a common print area. A print image generation unit that generates the print image by combining with each other;
With
The dot data generation unit includes a plurality of print pixels belonging to each of the plurality of pixel groups in a specific direction set in advance on the print medium for at least some of the input gradation values. The printing apparatus is characterized in that the halftone processing conditions are set so that at least one of the dot patterns formed on the substrate has a predetermined spatial frequency characteristic set in advance. The printing apparatus according to claim 1,
The dot data generation unit includes a dot pattern formed on each of a plurality of print pixels belonging to each of the plurality of pixel groups and a dot pattern formed on a plurality of print pixels belonging to the plurality of pixel groups. The halftone processing conditions are set so that all of the above have predetermined spatial frequency characteristics set in advance. The printing apparatus according to claim 1 or 2, wherein
The partial gradation is a gradation value included in a dot density range of 40% to 60% in which low frequency components are relatively high when it is assumed that dots are evenly arranged on the print medium. There is a printing device. The printing apparatus according to any one of claims 1 to 3,
The predetermined spatial frequency characteristic falls within a predetermined low frequency range of 4 cycles per millimeter or less, which is a spatial frequency region in which human visual sensitivity is relatively high on a print medium arranged at an observation distance of 300 mm. A spatial frequency characteristic in which a predetermined frequency characteristic of a dot pattern formed on a printing pixel belonging to each of the plurality of pixel groups has a frequency band that is closest to a predetermined characteristic of the spatial frequency of the dot pattern of the printed image. Is a printing device. The printing apparatus according to claim 4, wherein
The predetermined characteristic is a graininess index calculated by a calculation process including a Fourier transform process,
The printing apparatus, wherein the graininess index is calculated based on a product of a VTF function determined based on visual spatial frequency characteristics and a constant calculated in advance by the Fourier transform process. The printing apparatus according to claim 4, wherein
The printing apparatus, wherein the predetermined characteristic is an RMS granularity calculated by a calculation process including a low-pass filter process. The printing apparatus according to any one of claims 1 to 6,
In the dot data generation unit, the conditions of the halftone process are set so that a dot pattern formed on a print pixel belonging to each of the plurality of pixel groups has a preset two-dimensional spatial frequency characteristic. ,
The two-dimensional spatial frequency characteristic changes in the one-dimensional spatial frequency characteristic according to the direction on the print medium, and the one-dimensional spatial frequency characteristic is the most significant in the spatial frequency characteristic of the print image in the specific direction. A printing device that is set to approach. The printing apparatus according to claim 7, wherein
In the two-dimensional spatial frequency characteristic, the rate of change of the one-dimensional spatial frequency characteristic according to the direction on the print medium peaks at an angle in the range of 30 degrees to 60 degrees with respect to the specific direction. The printing device is set up as follows. The printing apparatus according to any one of claims 1 to 8,
The print image generation unit forms dots on the print pixels both during forward movement and backward movement of the print head while performing main scanning of the print head,
The plurality of pixel groups include a group of print pixels to be formed with dots when the print head is moved forward, and a group of print pixels to be formed with dots when the print head is moved backward.
The physical difference includes a shift in the relative position of dots for each of the plurality of pixel groups that occurs due to main scanning of the print head;
The printing apparatus, wherein the specific direction is a direction of the main scanning. The printing apparatus according to any one of claims 1 to 8,
The print image generation unit forms dots on the print pixels while performing main scanning of a print head,
The plurality of pixel groups include a group of a plurality of print pixels to be formed with dots for each main scan of the print head,
The physical difference includes a shift in dot formation timing due to main scanning of the print head,
The printing apparatus according to claim 1, wherein the specific direction is a direction in which a pitch of a printing pixel to be a dot formation target in each main scanning of the printing head is the smallest. The printing apparatus according to any one of claims 1 to 10,
The printing apparatus, wherein the predetermined spatial frequency characteristic is one of a blue noise characteristic and a green noise characteristic. A printing method for printing on a print medium,
By performing halftone processing on the image data representing the input gradation value of each pixel constituting the original image, the dot formation state on each print pixel of the print image to be formed on the print medium is represented. A dot data generation process for generating dot data;
According to the dot data, dots are formed on a plurality of print pixels belonging to each of a plurality of pixel groups assumed to be physically different from each other in dot formation, and the formed dots are used as a common print area. A print image generation step of generating the print image by combining with each other;
With
The dot data generation step includes a plurality of print pixels belonging to each of the plurality of pixel groups in a specific direction preset on the print medium for at least some of the input gradation values. The printing method is characterized in that the conditions of the halftone process are set so that at least one of the dot patterns formed on has a predetermined spatial frequency characteristic set in advance. A method of generating dots by forming dots on a print medium,
By performing halftone processing on the image data representing the input gradation value of each pixel constituting the original image, the dot formation state on each print pixel of the print image to be formed on the print medium is represented. A dot data generation process for generating dot data;
According to the dot data, dots are formed on a plurality of print pixels belonging to each of a plurality of pixel groups assumed to be physically different from each other in dot formation, and the formed dots are used as a common print area. A print image generation step of generating the print image by combining with each other;
With
The dot data generation step includes a plurality of print pixels belonging to each of the plurality of pixel groups in a specific direction preset on the print medium for at least some of the input gradation values. A method for generating a printed matter, wherein the halftone processing conditions are set so that at least one of the dot patterns formed on the substrate has a predetermined spatial frequency characteristic set in advance. A method of generating print data to be supplied to a printing unit that generates dots by forming dots on a print medium,
By performing halftone processing on the image data representing the input gradation value of each pixel constituting the original image, the dot formation state on each print pixel of the print image to be formed on the print medium is represented. A dot data generation process for generating dot data is provided.
The printing unit forms dots on a plurality of print pixels belonging to each of a plurality of pixel groups assumed to be physically different from each other in the formation of dots according to the dot data, and the formed dots Generating the print image by combining them in a common print area,
The dot data generation step includes a plurality of print pixels belonging to each of the plurality of pixel groups in a specific direction preset on the print medium for at least some of the input gradation values. The print control method is characterized in that the conditions of the halftone process are set so that at least one of the dot patterns formed on has a predetermined spatial frequency characteristic set in advance. A computer program for causing a computer to generate print data to be supplied to a printing unit that forms dots in print pixels and generates a print image,
By performing halftone processing on the image data representing the input gradation value of each pixel constituting the original image, the dot formation state on each print pixel of the print image to be formed on the print medium is represented. A program for causing the computer to realize a dot data generation function for generating dot data,
The printing unit forms dots on a plurality of print pixels belonging to each of a plurality of pixel groups assumed to be physically different from each other in the formation of dots according to the dot data, and the formed dots Generating the print image by combining them in a common print area,
The dot data generation function includes a plurality of print pixels belonging to each of the plurality of pixel groups in a specific direction set in advance on the print medium for at least some of the input gradation values. The computer program characterized in that the halftone processing conditions are set so that at least one of the dot patterns formed on has a predetermined spatial frequency characteristic set in advance. A printing method for printing on a print medium,
By performing halftone processing on the image data representing the input gradation value of each pixel constituting the original image, the dot formation state on each print pixel of the print image to be formed on the print medium is represented. A dot data generation process for generating dot data;
In accordance with the dot data, a dot is formed on each print pixel both during the forward movement and the backward movement of the print head while performing a main scan of the print head, and a dot formation target during the forward movement of the print head A print image generation step for generating the print image by combining a print pixel group to be a dot and a print pixel group to be formed with dots when the print head is moved back together in a common print region;
With
In the dot data generation step, for at least some of the input gradation values, the dot patterns formed on the printing pixels belonging to each of the two pixel groups are combined by shifting relative positions. When the shift direction is the same as the main scanning direction, the halftone processing conditions are set so that the granularity index of the combined dot pattern is minimized. A printing method characterized by the above. For generating a dither matrix for storing each of a plurality of threshold values for determining a dot formation state on each print pixel of a print image to be formed on a print medium in each element according to input image data A method,
Of the plurality of thresholds, a threshold value determining step of determining a threshold value that is an undetermined threshold value and a dot formation is most likely to be turned on as a focus threshold value,
Assuming the dot formation state when it is assumed that the target threshold value is stored in each of a plurality of storage candidate elements that are candidate elements for storing the target threshold value, A storage element determination step for determining a storage element of the threshold value of interest from the plurality of storage candidate elements based on a matrix evaluation value representing a correlation;
For at least a part of the plurality of threshold values, repeating the focus threshold value determination step and the storage element determination step while changing the focus threshold value;
With
The storage element determination step includes:
Based on the first dot formation state that is the dot formation state of the print image, the overall evaluation is an evaluation value that represents the correlation between the first dot formation state and the predetermined target state in a preset direction. An overall evaluation value determining step for determining a value;
The second in a predetermined direction based on a second dot formation state that is a dot formation state corresponding only to an element belonging to a pixel position group to which the storage candidate element belongs among the plurality of pixel position groups. A group evaluation value determination step for determining a group evaluation value, which is an evaluation value representing a correlation between the dot formation state of the predetermined target state, and
In accordance with the overall evaluation value and the group evaluation value, a comprehensive evaluation value determining step for determining the matrix evaluation value;
A dither matrix generation method comprising: For generating a dither matrix for storing each of a plurality of threshold values for determining a dot formation state on each print pixel of a print image to be formed on a print medium in each element according to input image data A method,
A preparation step of preparing a dither matrix as an initial state for storing each of a plurality of threshold values for determining the presence or absence of dot formation for each pixel according to an input gradation value;
It is assumed that the target threshold value to be evaluated among the plurality of threshold values is changed, and the target threshold value is replaced with another threshold value stored in another element of the dither matrix, and dot formation based on the assumption is performed. A storage element replacement step for repeatedly determining whether or not to perform the replacement based on a matrix evaluation value representing a correlation with a predetermined target state calculated assuming a state;
With
The storage element replacement step includes:
Based on the first dot formation state that is the dot formation state of the print image, the overall evaluation is an evaluation value that represents the correlation between the first dot formation state and the predetermined target state in a preset direction. An overall evaluation value determining step for determining a value;
The second in a predetermined direction based on a second dot formation state that is a dot formation state corresponding only to an element belonging to a pixel position group to which the storage candidate element belongs among the plurality of pixel position groups. A group evaluation value determination step for determining a group evaluation value, which is an evaluation value representing a correlation between the dot formation state of the predetermined target state, and
In accordance with the overall evaluation value and the group evaluation value, a comprehensive evaluation value determining step for determining the matrix evaluation value;
A dither matrix generation method comprising: The dither matrix generation method according to claim 17 or 18,
The matrix evaluation value includes a graininess index calculated by a calculation process including a Fourier transform process,
The dither matrix generation method, wherein the granularity index is calculated based on a product of a VTF function determined based on visual spatial frequency characteristics and a constant calculated in advance by the Fourier transform process. The dither matrix generation method according to claim 17 or 18,
The dither matrix generation method, wherein the matrix evaluation value is an evaluation value including RMS granularity calculated by a calculation process including a low-pass filter process. A printing device for printing on a print medium,
A halftone process is performed on the image data representing the gradation value of each pixel constituting the original image, thereby determining a dot formation state on each print pixel of the print image to be formed on the print medium. A dot data generation unit that generates dot data representing the determined dot formation state, and
According to the dot data, a print image generation unit that generates a print image by forming dots in the print pixels;
With
21. The dot data generation unit is configured to determine a dot formation state on each print pixel using a dither matrix generated by the method according to claim 17. A printing device that is characterized. A printing method for printing on a print medium,
A halftone process is performed on the image data representing the gradation value of each pixel constituting the original image, thereby determining a dot formation state on each print pixel of the print image to be formed on the print medium. A dot data generation step for generating dot data representing the determined dot formation state,
In accordance with the dot data, a print image generation step for generating a print image by forming dots in the respective print pixels;
With
21. The dot data generation step includes a step of determining a dot formation state on each print pixel using a dither matrix generated by the method according to claim 17. , Printing method. A method for generating a printed material for forming a print image on a print medium,
A halftone process is performed on the image data representing the gradation value of each pixel constituting the original image, thereby determining a dot formation state on each print pixel of the print image to be formed on the print medium. A dot data generation step for generating dot data representing the determined dot formation state,
In accordance with the dot data, a print image generation step for generating a print image by forming dots in the respective print pixels;
With
21. The dot data generation step includes a step of determining a dot formation state on each print pixel using a dither matrix generated by the method according to claim 17. , Method for generating printed matter.

Images