JP2011124744A - Method for generating dither mask, printing apparatus, and program thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress occurrence of density unevenness of a printed image. <P>SOLUTION: In generating a dither mask 61 to be mounted on a printing apparatus 20, thresholds to be stored in a plurality of storage elements are first prepared. Then, one threshold to be stored in a blank storage element being a storage element in which a prepared threshold has not been stored yet is selected as a target threshold. Next, when the target threshold is stored in one blank storage element, a dot shape to be actually formed is assumed on the basis of a dot formation pattern expressed by the arrangement of storage elements in which the threshold has been already stored, and a coverage C is calculated for each of the blank storage elements. Then, the target threshold is stored in a blank storage element corresponding to the coverage C that is the closest to a set target value. The target value is set within a range in which the degree of dot overlapping is larger than the degree of dot overlapping assumed from the dot formation pattern in a state where a positional deviation does not occur. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for generating a dither mask in which a threshold value is stored in each of a plurality of storage elements.

  In a serial inkjet printer, a print head having a plurality of nozzles is moved relative to the print medium in the main scanning direction and the sub-scanning direction, and ink is ejected from the nozzles to form dots on the printing medium. Form and print. In such an ink jet printer, the landing position of the ink ejected from the nozzles of the print head on the print medium may be displaced from the target position. As a cause of such misregistration, there are a problem of operation accuracy of the print head, a problem of paper feed accuracy, and the like. For example, if the ink ejection timings of the forward and backward movements of the print head are not strictly constant, the relative positions of the dot group formed by the forward movement of the print head and the dot group formed by the backward movement The relationship will deviate from the target position.

  When such a positional deviation occurs, a local density unevenness of dot arrangement occurs. If the density unevenness is increased, local density unevenness of the print image is caused, and the print image quality is deteriorated. Such a phenomenon of density unevenness becomes more prominent as the printing speed is increased. The problem of density unevenness is a problem common to various printers that print while moving the print head, such as a dot impact printer, as well as a serial ink jet printer. Even in a line printer in which a plurality of print heads are arranged in the entire width direction of the print medium and a part of adjacent print heads overlap each other, due to the problem of print head installation accuracy, Misalignment occurred and similar problems could occur.

JP 2007-49443 A

  Based on at least a part of the above-described problems, a problem to be solved by the present invention is to suppress the occurrence of density unevenness in a printed image.

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

Application Example 1 A method for generating a dither mask in which a threshold value is stored in each of a plurality of storage elements,
The dither mask is used for halftone processing for generating dots in a predetermined formation pattern on a print medium based on an arrangement of storage elements in which the threshold value is stored with respect to a predetermined gradation value of an image.
A dither mask generation method for determining an arrangement of the storage elements in which at least a part of the threshold value is stored, using as an index the degree of dot overlap on the print medium assumed from the dot formation pattern.

  In such a dither mask generation method, the arrangement of storage elements for storing threshold values is determined using the degree of dot overlap assumed from the dot formation pattern of the dither mask as an index. Therefore, the dither mask can be generated so as to suppress fluctuations in the degree of dot overlap that occur when the dot formation position is locally deviated from the target position. If fluctuations in the degree of dot overlap are suppressed, the uneven density of dots due to the above-described deviation is suppressed. Therefore, if halftone processing is performed using a dither mask generated by such a method, the dot formation position Even when the position is deviated from the target position, it is possible to obtain a printed image in which the uneven density of dots is suppressed, and the occurrence of density unevenness can be suppressed.

[Application Example 2] A dither mask generation method according to Application Example 1, wherein a first threshold value corresponding to at least a part of the plurality of storage elements is prepared. A step, a second step of selecting one of the thresholds to be stored in a blank storage element that is a storage element in which the prepared threshold is not yet stored as a target threshold, and one of the blank storage elements When the target threshold value is stored, a predetermined evaluation value regarding the degree of dot overlap assumed from the dot formation pattern represented by the arrangement of storage elements in which the threshold value has already been stored is used as the index. A third step of calculating for each of the blank storage elements, and a fourth step of determining the blank storage element for storing the target threshold value based on the calculated predetermined evaluation value, Serial The second to fourth steps are repeated for a predetermined number of times, a method of generating a dither mask for generating the dither mask.

  Such a dither mask generation method selects one of the prepared threshold values as a target threshold value, and stores the threshold value already stored when one of the blank storage elements stores the target threshold value. A blank storage element for storing the target threshold value is determined using a predetermined evaluation value regarding the degree of dot overlap assumed from the dot formation pattern represented by. Therefore, the degree of dot overlap can be evaluated, and a dither mask that suppresses fluctuations in the dot overlap can be suitably generated.

Application Example 3 In the application example 2, the predetermined evaluation value is calculated based on a dot coverage that indicates a ratio of the actually formed dots covering the print medium, which is assumed from the dot formation pattern. Dither mask generation method.

  In such a dither mask generation method, a predetermined evaluation value for determining a blank storage element for storing a target threshold value is calculated based on a ratio of dots that are actually formed to cover a print medium. Since the ratio of the dots covering the print medium varies depending on the degree of dot overlap, this method can favorably evaluate the degree of dot overlap. In addition, the ratio of the dots covering the print medium can be easily calculated, which is efficient.

[Application Example 4] The method for generating a dither mask according to Application Example 2 or Application Example 3, wherein the dither mask is used for printing by a printing apparatus that ejects ink onto the print medium to form the dots. And the third step assumes the spread of the ink on the print medium when the ink has landed on the print medium, and the assumed spread is the dot. A dither mask generation method for calculating the predetermined evaluation value by reflecting it in the formation pattern.

  In such a dither mask generation method, a predetermined evaluation value is calculated by reflecting the spread of ink on the print medium when the ink has landed on the print medium. Can be evaluated. If printing is performed by performing halftone processing using the dither mask generated by such a method, the effect of suppressing density unevenness can be enhanced.

Application Example 5 The method of generating a dither mask according to Application Example 4 in which the ink spread is assumed to be higher in resolution than the dot size when it is assumed that there is no spread.

  Such a dither mask generation method assumes that the ink spread is at a resolution higher than the dot size when it is assumed that there is no spread, so that the evaluation accuracy of the degree of dot overlap can be improved. If printing is performed by performing halftone processing using the dither mask generated by such a method, the effect of suppressing density unevenness can be enhanced.

[Application Example 6] The dither mask generating method according to any one of Application Example 2 to Application Example 5, wherein the dither mask moves the print head relative to the print medium in the main scanning direction and the sub-scanning direction. In the third step, the dot is formed by any of the main scans for use in the halftone process for printing with a printing apparatus that performs printing by forming dots on the printing medium. A plurality of misalignment patterns indicating a state in which the formation positions on the print medium of the dot groups divided by paying attention to the difference are deviated by a predetermined amount in the predetermined direction between the divided dot groups. If the target threshold value is stored for one of the blank storage elements for each of the plurality of types of misalignment patterns, and the step of assuming the type, the arrangement of the storage elements in which the threshold value is already stored represents the threshold value. Dot A step of calculating an evaluation value indicating the degree of overlapping of the dots, which is assumed from the composition pattern, and quantifying the variation between the plurality of types of deviation patterns of the evaluation value calculated for each of the plurality of types of deviation patterns A dither mask generating method comprising: calculating a converted evaluation value as the predetermined evaluation value.

  Such a dither mask generation method assumes a plurality of types of misalignment patterns, calculates an evaluation value indicating the degree of dot overlap for each misalignment pattern, and uses a target threshold value based on an evaluation value obtained by quantifying the variation of the evaluation value. Determine the blank storage element that stores. Therefore, when a deviation assumed as a deviation pattern occurs, fluctuations in the degree of dot overlap can be suitably suppressed. If printing is performed by performing a halftone process using a dither mask generated by such a method, the occurrence of density unevenness can be suppressed even if a shift assumed as a shift pattern occurs.

Application Example 7 The method of generating a dither mask according to Application Example 6, in which the plurality of types of shift patterns include a state in which the shift does not occur.

  Such a dither mask generation method includes a state in which no misalignment occurs in the assumed plural types of misalignment patterns. Therefore, the degree of dot overlap between when no misalignment occurs and when misalignment occurs. Can be prevented from greatly fluctuating. As a result, it is possible to generate a dither mask that can obtain a printing result in which density unevenness is suppressed, regardless of whether the deviation occurs or not.

[Application Example 8] The dither mask generation method according to Application Example 4 or Application Example 5, wherein the dither mask moves the print head relative to the print medium in the main scanning direction and the sub-scanning direction, The third step is used for the halftone process for printing with a printing apparatus that performs printing by forming dots on the print medium, and the third step sets the target threshold value for one of the blank storage elements. In the case of storing, a step of calculating an evaluation value indicating a degree of overlap of the dots, which is assumed from the dot formation pattern represented by the arrangement of the storage elements in which the threshold is already stored, A step of setting a target value of the evaluation value indicating the degree of overlap of the dots, an evaluation value indicating the degree of overlap of the dots, and an evaluation value indicating the degree of separation between the set target value Predetermined A step of calculating as an evaluation value, and the target value is the formation position on the print medium of the dot group that is classified by focusing on the difference in which main scanning the dot is formed. Dither that is set in a range in which the degree of dot overlap is larger than the degree of dot overlap assumed from the dot formation pattern in a state that is not shifted in any direction between the divided dot groups How to generate a mask.

  In such a dither mask generation method, the target position of the predetermined evaluation value is shifted from the formation position on the printing medium of the dot group that is classified by focusing on the difference in which main scan is performed for dot formation. It is set in a range where the degree of dot overlap is larger than the degree of dot overlap assumed from the dot formation pattern in the absence. Then, using the evaluation value indicating the degree of dot overlap and the evaluation value indicating the degree of separation between the set target values as the predetermined evaluation value, the target threshold value is stored in the blank storage element that stores the target threshold value. Therefore, the target threshold value can be stored in the blank storage element so that the degree of dot overlap is close to the target value. Usually, the dither mask is optimized so that the dot overlap is minimized when there is no deviation. Therefore, when the deviation occurs, the degree of dot overlap varies greatly. On the other hand, since the target value of this application example has a relatively large degree of dot overlap, it is possible to generate a dither mask that suppresses fluctuations in the degree of dot overlap when a deviation occurs.

  In addition to the dither mask generation method, the present invention can also be realized as a printing apparatus according to application example 9, a program according to application example 10, a dither mask generation program, a storage medium storing these programs, and the like.

Application Example 9 A printing apparatus that stores a dither mask generated by the method according to any one of Application Examples 1 to 8, and a unit that performs halftone processing using the stored dither mask. Printing device with

Application Example 10 A program for causing a computer to realize a function of performing halftone processing using the dither mask generated by the method according to any one of Application Example 1 to Application Example 8.

1 is a schematic configuration diagram of a printer 20 as a first embodiment of the present invention. 3 is a flowchart showing a flow of printing processing in the printer 20. 3 is an explanatory diagram illustrating a mechanism of dot formation in the printer 20. FIG. It is explanatory drawing which shows the pixel group which comprises a printing image. It is explanatory drawing which shows the pixel group which comprises a printing image. It is process drawing which shows the procedure of a dither mask production | generation process. It is process drawing which shows the procedure of the 1st dither mask evaluation process. It is explanatory drawing which shows the state etc. in which the threshold value was stored in a part of storage element of the dither mask 61. It is explanatory drawing which shows the calculation method of the coverage C. It is explanatory drawing which shows the calculation method of the coverage C. It is explanatory drawing which shows the target value of the coverage C notionally. It is explanatory drawing which shows notionally the fluctuation | variation of the coverage C when a position shift arises at the time of printing. It is process drawing which shows the procedure of the 2nd dither mask evaluation process. It is explanatory drawing of a blue noise characteristic and a green noise characteristic. It is explanatory drawing which shows an example of the sensitivity characteristic VTF used as the calculation base of evaluation value E2. It is process drawing which shows the procedure of the 1st dither mask evaluation process as 2nd Example. It is explanatory drawing which shows the specific example of the shift pattern assumed. It is explanatory drawing which shows the calculation method of the coverage C as 2nd Example.

A. First embodiment:
A first embodiment of the present invention will be described.
A-1. Device configuration:
FIG. 1 is a schematic configuration diagram of a printer 20 as a first embodiment of the present invention. The printer 20 is a serial inkjet printer that performs bi-directional printing. As illustrated, the printer 20 includes a mechanism that transports the print medium P by a paper feed motor 74 and a carriage 80 that moves the carriage 80 by the carriage motor 70. A mechanism for reciprocating in the direction, a mechanism for driving the print head 90 mounted on the carriage 80 to eject ink and forming dots, and the paper feed motor 74, carriage motor 70, print head 90, and operation panel 99. The control unit 30 is responsible for exchanging signals with the control unit 30.

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

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

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

  A dither mask 61 is stored in the EEPROM 60. The dither mask 61 is used for dot dispersion type halftone processing by a systematic dither method, and is configured by storing a plurality of threshold values in the same number of storage elements.

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

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

A-2. Printing process:
A printing process in the printer 20 will be described. FIG. 2 is a flowchart showing the flow of printing processing in the printer 20. The printing process here is started when the user performs a print instruction operation for a predetermined image stored in the memory card MC using the operation panel 99 or the like. When printing processing is started, the CPU 40 first reads and inputs RGB format image data ORG to be printed from the memory card MC via the memory card slot 98 as processing of the input unit 41 (step S110).

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

  When the color conversion process is performed, the CPU 40 performs a halftone process for converting the image data into ON / OFF data of dots of each color as a process of the halftone processing unit 42 (step S130). This process compares the input data with the threshold stored in the storage element at the position corresponding to the input data among the plurality of thresholds constituting the dither mask 61. If the input data is greater than the threshold, It is determined that the dot is OFF if the input data is less than the threshold value. The dither mask 61 is repeatedly applied in the main scanning direction and the sub-scanning direction for each input data arranged in the main scanning direction and the sub-scanning direction. The halftone process is not limited to the binarization process of dot ON / OFF, and may be a multi-value process such as ON / OFF of large dots and small dots. Further, the image data provided to step S130 may be subjected to image processing such as resolution conversion processing or smoothing processing.

  When the halftone process is performed, the CPU 40 performs an interlace process for rearranging the dot pattern data to be printed in one main scanning unit in accordance with the nozzle arrangement of the printer 20 and the paper feed amount (step S140). When the interlace process is performed, the CPU 40 drives the print head 90, the carriage motor 70, the motor 74, and the like as the process of the printing unit 43, and executes printing (step S150).

  A mechanism of dot formation in the printing process will be described below with reference to FIG. The print head 90 in this embodiment includes 10 nozzles Nz for each ink color. These nozzles Nz are arranged in a line in the sub-scanning direction with an interval of one nozzle.

  The print image is generated as follows while the print head 90 performs main scanning and sub-scanning. In the following description, one main scan is also referred to as a pass, and in order to distinguish each main scan, it is also referred to as pass 1, pass 2 or the like with a bus number. The arrangement of dots along the sub-scanning direction is also called a raster, and each raster will be described with a raster number in order to distinguish each raster. Further, in order to distinguish the formation position of each dot constituting the raster, the dot formation position in each raster is specified by the pixel position number attached along the main scanning direction. In the present embodiment, as a mode of drive control of the print head 90 and the like, the overlap number is “2”, the nozzle pitch is “2”, the paper feed amount is “5”, and the print head 90 is moved forward and backward. Bidirectional printing is performed to eject ink both during movement. The number of overlaps refers to the number of main scans necessary to fill all the rasters formed in the main scan direction with dots. The nozzle pitch is the number of dots between the centers of adjacent nozzles in the sub-scanning direction, and is a number obtained by adding 1 to the number of rasters (dots) existing between two adjacent nozzles. The paper feed amount is an amount (raster number) by which the print head 90 is conveyed in the sub-scanning direction for each main scanning.

  In pass 1, which is the first pass shown in FIG. 3, the pixel position number is 1 among the 10 rasters whose raster numbers are 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 Dots are formed at 3, 5 pixels (dot formation positions). Pass 1 is the forward movement of the reciprocating movement of the print head 90. The circles shown in FIG. 3 indicate dot formation positions, and the numbers described in the circles are pass numbers.

  When the pass 1 main scan is completed, the print medium is moved in the sub-scanning direction by the paper feed amount Ls (here, 5) (this movement is also referred to as sub-scan feed). In FIG. 3, for convenience of illustration, the print head 90 is illustrated as moved in the sub-scanning direction. When the sub-scan feed is completed, pass 2 as the first pass is performed. In pass 2, dots are formed in the pixels with pixel position numbers 1, 3, and 5 among the 10 rasters with raster numbers 6, 8, 10, 12, 14, 16, 18, 20, 22, 24. Is done. Pass 2 is a reciprocal movement of the reciprocating movement of the print head 90. The two rasters with raster numbers 22 and 24 are not shown.

  When pass 2 is completed, pass 3 is performed after performing the sub-scan feed similar to that described above. In pass 3, dots are formed on the pixels with pixel position numbers 2, 4, and 6 among the 10 main scanning lines including the main scanning lines with raster numbers 11, 13, 15, 17, and 19. Pass 3 is the forward movement of the reciprocating movement of the print head 90.

  When pass 3 is completed, pass 4 is performed after sub-scan feed similar to that described above is performed. In pass 4, dots are formed on the pixels with pixel position numbers 2, 4, and 6 among 10 main scanning lines including three main scanning lines with raster numbers 16, 18, and 20. Pass 4 is the reciprocal movement of the reciprocating movement of the print head 90.

  In this way, if dots are formed by feeding paper while repeating the forward and backward movements of the print head 90, dots can be formed without gaps at the sub-scanning positions with the raster number of 15 or later.

  The print image generated in this way is observed by paying attention to a certain area. For example, if a region having a raster number of 15 to 20 and a pixel position number of 1 to 6 is set as a region of interest, and the region of interest is observed from the viewpoint of which dot is formed, a printed image of the region of interest As shown in FIG. 4, the first to fourth pixel groups corresponding to the pass numbers 1 to 4 are combined with each other. Each of the pixel groups (dot groups) divided based on such a difference in pass is collectively referred to as a pass group.

  Similarly, if the region of interest is observed from the viewpoint of whether dots are formed by forward movement or backward movement, the print image of the region of interest corresponds to the forward movement corresponding to the forward movement as shown in FIG. The moving picture element group and the backward moving pixel group corresponding to the backward movement are formed. Each pixel group (dot group) divided based on the difference between the forward movement and the backward movement is collectively referred to as a reciprocating movement group. From a large viewpoint, the pass group and the reciprocating group are groups (dot groups) that are classified by paying attention to the difference in which main scanning is performed to form dots.

  The reason for dividing the pixel group in this way is that it is used in the dither mask 61 generation method described below. In the above example, the overlap number is “2” and the nozzle pitch is “2”. However, the pixel group divided from the viewpoint of the pass number has a number corresponding to the product of the overlap number and the nozzle pitch. Can be divided into groups. Further, the number of overlaps, the nozzle pitch, and the paper feed amount are not limited to the above example, and may be set as appropriate.

A-3. Generation method of the dither mask 61:
A method for generating the above-described dither mask 61 will be described. The dither mask 61 has storage elements corresponding to the size (the number of threshold values). The storage element is an element that stores a threshold value constituting the dither mask 61. The dither mask 61 is generated by storing threshold values one by one in all of these storage elements. The generation method described below is a process for generating the dither mask 61 by a CPU such as a main frame. It should be noted that some or all of the steps described below may be performed manually by the user.

  In many cases, the size of the dither mask 61 is, for example, 256 pixels × 256 pixels, 512 pixels × 512 pixels, etc. In the following description, the size of the dither mask 61 is set to simplify the description. The description will be made assuming that the vertical size and the horizontal size are 6 pixels, that is, a size applied to a total of 36 pixels (= 6 × 6) image data. Note that the size of the dither mask 61 is preferably a multiple of the repeat unit of the pass group and the reciprocating group. For example, assuming the pixel groups shown in FIGS. 4 and 5, the dither mask 61 has a vertical size of 2M (M is an integer of 1 or more) pixels and a horizontal size of 2N (N is 1). This is an integer) pixel. If the dither mask 61 having such a size is repeatedly applied to the image data in the main scanning direction and the sub scanning direction, the correspondence between each pixel group and the threshold value constituting the dither mask 61 can be kept constant. This is because the desired halftone process can be efficiently performed.

  FIG. 6 is a process chart showing a procedure of a method for generating the dither mask 61. In the generation of the dither mask 61, as shown in the drawing, first, a threshold corresponding to the size of the dither mask 61 is prepared (step S200). In the present embodiment, since the dither mask 61 has 36 storage elements, the same number of threshold values 0 to 35 are prepared.

  Once the threshold value is prepared, a target threshold value selection process is performed (step S300). The target threshold value selection process is a process of selecting one threshold value as a target threshold value from among the prepared threshold values 0 to 35 that are not yet stored in the storage element. In this embodiment, the threshold value of interest is selected in order from the smallest threshold value of the prepared threshold values. As shown in FIG. 8, when threshold values 0 to 4 are already stored in the storage element in the storage element constituting the dither mask 61 by the process described later, the threshold value of interest selected in step S300 is next selected. Is the value 5.

  Once the target threshold value is selected, first dither mask evaluation processing is performed (step S400). In the first dither mask evaluation process, when a threshold value of interest is stored for one storage element for which a prepared threshold value is not yet stored (hereinafter also referred to as a blank storage element), the threshold value is already stored. This is a process of calculating, for each blank storage element, an evaluation value E1 related to the degree of dot overlap, which is assumed based on the dot formation pattern represented by the arrangement of the stored storage elements (hereinafter also referred to as “decision storage elements”). Although the calculation method of the evaluation value E1 will be described later, in this embodiment, the evaluation value E1 is an index of fluctuation of the degree of dot overlap, and the smaller the value, the smaller the fluctuation and the suppression of density unevenness. It can be said that it is excellent from the viewpoint.

  Once the first dither mask evaluation process is performed, a second dither mask evaluation process is then performed (step S500). The second dither mask evaluation process indicates the degree of dot dispersion for the dot formation pattern represented by the arrangement of the determined storage elements when the target threshold value is stored for one of the blank storage elements. This is a process of calculating the evaluation value E2 for each blank storage element. A method for calculating the evaluation value E2 will be described later. In this embodiment, the evaluation value E2 is an index for the degree of dot dispersion. The smaller the value, the better the dot dispersibility. It can be said that it is excellent in terms of graininess.

  Once the second dither mask evaluation process is performed, a storage element determination process is then performed (step S600). The storage element determination process here is a blank storage that stores a threshold value of interest based on the evaluation value E1 calculated in the first dither mask evaluation process and the evaluation value E2 calculated in the second dither mask evaluation process. This is a process for determining an element. Specifically, using the evaluation value E1 and the evaluation value E2 calculated for each candidate storage element (blank storage element), the overall evaluation value CE is calculated for each candidate storage element by the following equation (1). Then, the candidate storage element corresponding to the comprehensive evaluation value CE having the smallest value is determined as a candidate storage element that should store the target threshold value, and the target threshold value is stored in the storage element. In the following equation (1), α and β are weighting coefficients. These weighting factors are experimentally determined as constant values so that good print image quality can be obtained. CE = θ × E1 + ι × E2 (1)

  When the storage element determination process is performed, the above steps S300 to S600 are repeated until the threshold value prepared in step S200 is stored in all the storage elements constituting the dither mask 61 (step S700). Thus, when threshold values are stored in all the storage elements (step S700: YES), the dither mask 61 is completed, and the dither mask generation process ends.

  Details of the first dither mask evaluation process described above will be described. In the first dither mask evaluation process, first, as shown in FIG. 7, the dot of the decision storage element is turned ON (step S410). FIG. 8 shows a state where dots are turned on in this way by single hatching.

  When the dot of the determined storage element is turned ON, candidate storage element selection processing is then performed (step S420). The candidate storage element selection process is a process of selecting a candidate storage element that is a candidate for a storage element in which a target threshold value is to be stored. Since it is possible to store the target threshold value in each of the blank storage elements, one of the blank storage elements is selected as a candidate storage element here. Once the candidate storage element selection process is performed, the dot of the candidate storage element is then turned ON (step S430). In FIG. 8, a state where one of the blank storage elements is selected as a candidate storage element and the dot of the candidate storage element is turned ON is shown by cross hatching.

  If the dot of the candidate storage element is turned on, next, an evaluation value calculation process is performed (step S440). The evaluation value calculated here is a dot coverage C (hereinafter also simply referred to as a coverage C) indicating the ratio of dots covering the print medium. This coverage ratio C is the dot formation pattern (the dot formation pattern represented by the arrangement of the determined storage element, that is, the arrangement of the storage element in which the dot is turned ON in step S430) when the threshold value of interest is stored in one of the blank storage elements. Hereinafter, the dots that are actually formed are assumed based on the dot formation pattern), and the calculation is performed based on the assumed dots. In the halftone process, since the dot is turned on at a pixel whose threshold value of the dither mask 61 is smaller than the input gradation value, when a solid image having the same gradation value of all the pixels is input, If the gradation value is gradually increased, a dot formation pattern corresponding to the arrangement of threshold values in the dither mask 61 appears. In this embodiment, the dot shape based on such dot generation characteristics is called a dot formation pattern.

  The calculation of the coverage ratio C will be described in more detail with reference to FIG. FIG. 9A shows the dot formation pattern of the decision storage element obtained in step S430. Note that the storage elements shown in FIG. 9A are given row numbers and column numbers 1 to 6 for convenience of the following description. In calculating the coverage ratio C, first, an actual dot shape is assumed and defined. The dot shape defined here is the shape of a dot that is actually formed when ink lands on the print medium. In an ink jet printer, ink that has landed on a print medium spreads over a predetermined range and is fixed. For this reason, the dot shape reflecting the spread of the ink on the print medium is defined.

  A definition example of the dot shape is shown in FIG. Of the 3 pixels × 3 pixels shown in the figure, a central pixel (a pixel indicated as A) indicates a dot formation position where ink landing is scheduled. In addition, the pixels (pixels indicated by B) positioned on the top, bottom, left, and right of the center pixel indicate dot formation positions that can be considered to be formed by spreading of the ink when the ink lands on the center pixel. . Since the printer 20 ejects ink while moving the print head 90 in the main scanning direction, the landed ink actually tends to spread in the main scanning direction, but here, for the sake of simplicity of explanation. As shown in FIG. 9B, a symmetrical shape is defined vertically and horizontally.

  When the dot shape is defined in this way, next, a dot formation pattern for calculating coverage is obtained based on the dot shape pattern of the decision storage element and the defined dot shape. A specific example of the dot formation pattern for calculating the coverage is shown in FIG. This dot formation pattern is obtained by replacing each dot in the dot formation pattern shown in FIG. 9A with the dot shape shown in FIG. 9B. The row numbers and column numbers in the figure correspond to FIG. In FIG. 9C, a pixel indicated by A indicates a dot (dot formation pattern shown in FIG. 9A) formed in a pixel on which ink landing is scheduled. Pixels indicated as B and C indicate dots (pixels indicated as B in FIG. 9B) that can be regarded as dots formed by spreading of ink. Among the pixels displayed as B and C, the pixel displayed as C indicates that dots that can be considered to be formed by spreading of ink are overlapped at the pixel position. On the other hand, the pixel labeled B indicates that dots that can be considered to be formed by spreading of ink are formed without overlapping. Since the dither mask 61 is repeatedly applied to the input data in the main scanning direction and the sub-scanning direction, the dither mask 61 is formed, for example, in the first row and the 0th column (pixels indicated as D in the drawing). The dots that should be formed and the dots that should be formed in the 0th row and the first column (pixels indicated by E in the figure) are formed in the first row, the sixth column, and the sixth row, the first column, respectively. To deal with.

When the dot formation pattern for coverage calculation is obtained, the coverage C is calculated based on the pattern. The coverage C can be obtained by the following equation (2). In the example of FIG. 9C, the coverage C is 77.8% (= 28/36 × 100).
C = number of pixels in which dots are formed as a dot formation pattern / total number of pixels × 100 (2)

  Another method for calculating the coverage C will be described with reference to FIG. In the method described below, the dot shape definition method is different from the above example. FIG. 10A shows a dot formation pattern of a predetermine storage element, which is the same as that shown in FIG. A definition example of the dot shape is shown in FIG. In this example, the actual dot shape is defined with a resolution higher than the dot size assuming that there is no ink spread. Here, the dot shape is defined after converting one pixel into four by performing double resolution conversion in the main scanning direction and the sub-scanning direction, respectively. The meaning of the background of each pixel in FIG. 10B is the same as that in FIG. That is, in FIG. 10B, the pixel shown in black corresponds to the pixel indicated by A in FIG. 9B, and the pixel indicated by hatching is the pixel indicated by B in FIG. 9B. Applicable. By performing resolution conversion in this way and increasing the resolution of the dot shape, the actual dot shape can be accurately reflected. FIG. 10B shows an example in which the ink is defined so as to spread over a wider range in the main scanning direction than in the sub-scanning direction. By increasing the resolution of the dot shape, it becomes easier to faithfully reflect such ink spreading characteristics.

  FIG. 10C shows a dot formation pattern for calculating the coverage when the dot shape is defined as shown in FIG. The method for obtaining the dot formation pattern for calculating the coverage is the same as the method described with reference to FIG. In addition, the meaning of the background of each pixel in FIG. 10C is the same as that in FIG. That is, in FIG. 10C, the pixel shown in black corresponds to the pixel indicated as A in FIG. 9C, and the pixel indicated in single hatching is the pixel indicated as B in FIG. 9C. The pixel indicated by cross hatching corresponds to the pixel indicated as C in FIG. If the coverage ratio C is calculated by the above formula (2) based on the dot calculation pattern for calculating the coverage ratio, the coverage ratio C is 80.6% (= 116/144 × 100).

  Here, the description returns to the first dither mask evaluation process of FIG. Once the coverage C is calculated as described above, next, a target value setting process is performed (step S450). The target value setting process is a process for setting a target value of the coverage C for each input gradation value of the input data to be subjected to the halftone process. In this embodiment, the target value set by the user is set as the CPU. Will be read.

  Such a target value is conceptually shown in FIG. In this example, the input gradation value is set to 0 to 255, and the target value of the coverage C for each gradation value is set. The illustrated line L1 indicates the coverage for each gradation value in the conventional dither mask. The conventional dither mask here refers to a dither mask optimized under a predetermined condition on the assumption that no positional deviation occurs in any direction between the above-described path groups and between reciprocating groups. . The predetermined condition is, for example, that the printed image has a blue noise characteristic. On the other hand, the line L2 shown in the figure indicates the target value of the coverage C to be set.

  FIG. 12 conceptually shows the variation in the coverage C when a positional deviation actually occurs when printing is performed by performing a halftone process using a dither mask having the characteristics of the line L1. As shown in the figure, the coverage C of the printed image to be printed is maximized when there is no positional deviation. For example, when a positional deviation occurs in the main scanning direction, the coverage C decreases according to the positional deviation amount. When the displacement amount reaches one pixel, the coverage C becomes the minimum value, and thereafter, when the displacement amount further increases, the coverage C starts to increase. When the deviation amount reaches 2 pixels, the coverage ratio C starts to decrease, and when the deviation amount reaches 3 pixels, the coverage ratio C starts to increase again.

  For example, when the first pixel group and the third pixel group shown in FIG. 4 are viewed in the positional relationship between the first pixel group and the third pixel group, the first pixel group and the third pixel group are shifted in the main scanning direction. The pixel groups start to overlap each other and are completely overlapped when they are shifted by one pixel. Further, when the shift amount increases, the overlapped portion decreases, and when the two pixel shifts, the overlapped portion This is due to disappearance.

  Actually, the characteristics of the variation of the coverage ratio vary depending on the arrangement of the dots to be formed. Conceptually, the coverage ratio C decreases and increases at a constant cycle according to the shift amount. It will fluctuate repeatedly. Then, the increase / decrease amount in one cycle gradually converges. Such a variation characteristic of the coverage ratio C is not limited to the positional deviation between the path groups, but also appears for the positional deviation between the reciprocating groups. Further, not only a positional shift in the main scanning direction but also a positional shift in the sub-scanning direction or a combined positional shift in the main scanning direction and the sub-scanning direction.

  The target value of the coverage C described above is set to a value smaller than the line L1, as shown as the line L2 in FIG. That is, in the line L2, the formation position on the print medium of the dot group that is divided by focusing on the difference in which main dot is formed is the position between the divided dot groups. It is set in a range where the dot overlap is larger than the dot formation pattern in a state that is not displaced in the direction. Such a target value can be set because, as described above, by defining the spread of the ink, even when no positional deviation occurs, a portion where adjacent dots overlap can occur. This is because the degree of superposition changes by changing (threshold arrangement). Note that in an actual printer, it cannot be guaranteed that no positional deviation will occur, so the value of the coverage C constituting the line L1 is obtained by simulation.

  In the present embodiment, as the target value of the coverage ratio C, the maximum value in the period when the increase / decrease amount in one period has substantially converged in the variation characteristic of the coverage ratio C shown in FIG. . In this way, if the target value is set at the convergence stage value, the correlation between dots in the pass group and the reciprocating group is reduced, and the degree of dot overlap is more important than the degree of dot dispersion described later. The generated dither mask 61 can be generated.

When the target value of the coverage ratio C is set in this way, the dot overlap is then calculated by the following equation (3) based on the coverage ratio C calculated in step S440 and the target value of the coverage ratio C set in step S450. An evaluation value E1 relating to the degree of the above is calculated (step S460).
E1 = | C calculated value−C target value | (3)

  The target value in this step is that the target value of the coverage ratio C is determined for each gradation value, and thus is determined from the target threshold value among the values of the coverage ratio C constituting the line L2. The coverage C corresponds to the gradation value. For example, when the target threshold value is 5, the coverage ratio C is calculated assuming that six threshold values (values 0 to 5) out of 36 threshold values (values 0 to 35) are already stored in the storage element. Therefore, the gradation value obtained from the threshold value of interest is the sixth gradation of the 36 gradations. In step S450, the CPU reads a target value corresponding to the gradation value and performs step S460. If the number of storage elements constituting the dither mask is combined with the number of gradations of the line L2, the target value can be easily set. However, even when the number of storage elements constituting the dither mask does not match the number of gradations of the line L2, the target value can be determined by interpolation. Of course, the coverage ratio C constituting the line L2 does not need to be set for all input gradation ranges, and the coverage ratio C is determined for a part of gradation values, and the gradation values that are not set. The coverage C corresponding to may be obtained by interpolation.

  When the evaluation value E1 is calculated in this way, the processes of steps S420 to S460 described above are repeated until the evaluation value E1 is calculated for all candidate storage elements (blank storage elements) (step S470). When the evaluation value E1 is calculated for all candidate storage elements in this way (step S470: YES), the first dither mask evaluation process ends. As is clear from the calculation method, the evaluation value E1 is an index value indicating how close the calculated coverage C is to the target value. In that sense, it can be said that the smaller the value, the better the evaluation value E1 in terms of the degree of dot overlap. In other words, the evaluation value E1 can also be referred to as an evaluation value representing the degree of separation between the calculated coverage C and the target value. Therefore, for example, instead of the equation (3), the absolute value of the difference between the ratio of the calculated coverage C and the target value and the value 1 (100%) can be used.

  Next, the second dither mask evaluation process will be described with reference to FIG. In the second dither mask evaluation process, first, a grouping process is performed (step S510). In the grouping process, a plurality of storage elements constituting the dither mask 61 are used to form dots at a dot formation position where the threshold values stored in the plurality of storage elements are applied in the halftone process. This is a process of dividing into a plurality of groups by paying attention to the difference in whether or not to do so. That is, it is a process of setting a group of storage elements based on the above-described path group or reciprocating group. In this embodiment, a path group and a reciprocating group are set. That is, four groups, ie, the first to fourth pixel groups and the two groups, the forward pixel group and the backward pixel group, are set in total. However, the group to be set may be one of a path group and a reciprocating group. In this embodiment, since the number of storage elements constituting the dither mask 61 and the number of pixels constituting the region of interest shown in FIGS. 4 and 5 are the same, each set group is shown in FIG. These correspond to the first to fourth pixel groups shown in FIG. 5 and the forward pixel group and backward pixel group shown in FIG.

  When the grouping process is performed in this way, the decision storage element dot is then turned on (step S520), the candidate storage element selection process is performed (step S530), and the candidate storage element dot is turned on (step S540). . The processing in steps S520 to S540 is the same as the processing in steps S410 to S430 (see FIG. 7) in the first dither mask evaluation processing, and detailed description thereof is omitted.

  When the dot of the candidate storage element is turned on, next, group selection processing is performed (step S550). The group selection processing is a group including p groups G1 to Gp (p is an integer of 2 or more, here p = 6) set in step S510 and all the storage elements constituting the dither mask 61. This is a process of selecting one group Gq (q is an integer from 1 to p + 1) from the group Gp + 1.

  When the group Gq is selected, the evaluation value E2q indicating the degree of dot dispersion, that is, how much the dots are uniformly distributed is formed based on the dot formation pattern corresponding to the storage elements belonging to the group Gq. An evaluation value indicating whether or not is calculated (step S560). It is known that a dither mask having blue noise characteristics and green noise characteristics shown in FIG. 14 may be generated in order to form dots in a uniformly dispersed state. In this embodiment, in order to generate a dither mask having such characteristics, the graininess index is used as an evaluation value indicating the degree of dot dispersibility.

  Since the granularity index is a known technique (for example, Japanese Patent Application Laid-Open No. 2007-15359), a detailed description is omitted, but the power spectrum FS is obtained by Fourier transforming the image to obtain the power spectrum FS, This is an index obtained by adding a weight corresponding to a sensitivity characteristic VTF (Visual Transfer Function) with respect to a visual spatial frequency possessed by a human and integrating at each spatial frequency. FIG. 16 shows an example of VTF. Various formulas have been proposed as empirical formulas for giving such VTF, and the following formula (4) shows a typical empirical formula. The variable L represents the observation distance, and the variable u represents the spatial frequency. The graininess index can be calculated based on the VTF by a calculation formula shown in the following formula (5). The coefficient K is a coefficient for matching the obtained value with human senses. As apparent from the calculation method, it can be said that the granularity index is an index indicating whether or not a human feels that dots are conspicuous. It can be said that the smaller the value of the graininess index, the more difficult it is to visually recognize dots in the print image quality, and the more excellent in that respect.

When the evaluation value E2q is calculated, the steps S550 and S560 are repeated until the evaluation value E2q is calculated for all the groups G1 to Gp + 1 (here, G1 to G7) (step S570). Thus, when the evaluation value E2q is calculated for all the groups G1 to G7 (step S570: YES), the evaluation value E2 is calculated by the following equation (6) based on the calculated evaluation values E21 to E27 (step S580). . In Expression (6), γ to ι are weighting coefficients. These weighting factors are experimentally determined as constant values so that good print image quality can be obtained. That is, the evaluation value E2 is the dot formation pattern represented by the entire decision storage element of the dither mask 61, the respective dot formation patterns represented by the decision storage elements corresponding to the first to fourth pixel groups, and the reciprocating pixel. This is an evaluation value obtained by comprehensively evaluating the degree of dot dispersion with a predetermined weight for each dot formation pattern represented by the determination storage element corresponding to the group.
E2 = γ × E21 + δ × E22 + ε × E23 + ζ × E24 + η × E25 + θ × E26 + ι × E27 (6)

  When the evaluation value E2 is calculated, the steps S530 to S580 are repeated until the evaluation value E2 is calculated for all candidate storage elements (blank storage elements) (step S590). In this way, when the evaluation value E2 is calculated for all candidate storage elements (step S590: YES), the second dither mask evaluation process ends. Note that the overall evaluation value CE calculated by the expression (1) in step S600 is directly calculated from the evaluation value E1 and the evaluation values E21 to E27 from which the evaluation value E2 is calculated by one operation. It may be calculated.

A-4. effect:
The above-described generation method of the dither mask 61 determines the arrangement of the storage elements for storing the threshold value using the degree of dot overlap assumed from the dot formation pattern represented by the arrangement of the determined storage elements as an index. Therefore, the dither mask 61 can be generated so as to suppress fluctuations in the degree of dot overlap caused by positional deviation. If fluctuations in the degree of dot overlap are suppressed, the unevenness of dot density due to the above-described shift is suppressed. Therefore, if halftone processing is performed using the dither mask 61 generated by such a method, the positional shift will occur. Even if it occurs, it is possible to obtain a printed image in which the uneven density of dots is suppressed, and the occurrence of density unevenness can be suppressed.

  The dither mask 61 is generated by selecting one of the prepared threshold values as the target threshold value and storing the target threshold value for one of the blank storage elements. A blank storage element for storing a target threshold value is determined using a predetermined evaluation value related to the degree of dot overlap assumed from the formation pattern. Therefore, it is possible to appropriately evaluate the degree of dot overlap and generate the dither mask 61 that suppresses fluctuations in the degree of dot overlap. The predetermined evaluation value is calculated based on the coverage C, which is the ratio of dots that are actually formed covering the print medium. Since the ratio of the dots covering the print medium varies depending on the degree of dot overlap, this method can favorably evaluate the degree of dot overlap. Moreover, since the coverage C can be easily calculated, it is efficient.

  In addition, the generation method of the dither mask 61 calculates the coverage C by reflecting the spread of the ink on the print medium when the ink has landed on the print medium. C can be calculated to evaluate the degree of dot overlap. In addition, since the ink spread is assumed to have a resolution higher than the dot size when it is assumed that there is no spread, the evaluation accuracy of the degree of dot overlap can be increased. If printing is performed by performing a halftone process using the dither mask 61 generated by such a method, the effect of suppressing density unevenness can be enhanced.

  Further, the generation method of the dither mask 61 is such that the formation position on the print medium of the dot group divided by focusing on the difference between the target value of the coverage C and the formation of dots in which main scanning is performed is shifted. It is set in a range where the degree of dot overlap is larger than the degree of dot overlap assumed from the dot formation pattern in the non-appearing state. Then, the target threshold value is stored in the blank storage element in which the evaluation value E1 indicating the degree of separation between the coverage C and the set target value is the smallest, that is, the coverage C is closest to the target value. Normally, the dither mask is optimized so that the coverage C is minimized when there is no deviation, and therefore the coverage C varies greatly when a deviation occurs. On the other hand, since the coverage C is relatively small, the target value of this embodiment generates the dither mask 61 that suppresses the variation of the coverage C when a deviation occurs, that is, the variation of the degree of dot overlap. be able to.

  In addition, the generation method of the dither mask 61 stores a threshold value using the degree of dot dispersion represented by the dot formation pattern as an index in addition to the degree of dot overlap assumed from the dot formation pattern of the dither mask 61. Determine the placement of storage elements. Therefore, it is possible to generate a dither mask having excellent dot dispersibility. If halftone processing is performed using a dither mask generated by such a method, it is possible to obtain a high-quality print image in which the graininess of the print image quality is suppressed in addition to the suppression of density unevenness.

  Further, the generation method of the dither mask 61 uses the evaluation value (evaluation value E27 in the above example) indicating the degree of dot dispersion calculated based on the dot formation pattern represented by the arrangement of the decision storage elements. Since the storage element is evaluated, it is possible to generate a dither mask having excellent dot dispersibility when no positional deviation occurs.

  In addition, in the method of generating the dither mask 61, the storage elements are divided into respective pass groups or reciprocating movement groups by paying attention to the difference in which main scanning is performed to form dots. The candidate storage element is also used by using an evaluation value indicating the degree of dot dispersion (evaluation values E21 to E26 in the above example) calculated based on the dot formation pattern corresponding to the determination storage element belonging to the group. Therefore, it is possible to generate the dither mask 61 in which the dot dispersibility for each group is ensured satisfactorily. If halftone processing is performed using the dither mask 61 and printing is performed, even if a positional deviation occurs between the groups, the dispersibility of the dots for each group remains ensured. A decrease in dot dispersibility can be suppressed, and a high-quality print image in which the granularity of the print image quality is suppressed can be obtained. Thus, the dither mask 61 of the present embodiment can achieve both the effect of suppressing density unevenness and the effect of suppressing deterioration of graininess.

B. Second embodiment:
A second embodiment of the present invention will be described. In the second embodiment, the configuration of the printer 20 is the same as that of the first embodiment, and the generation method of the dither mask 61 is different from that of the first embodiment. Specifically, the content of the first dither mask evaluation process (step S400) in the dither mask generation process shown in FIG. 6 as the first embodiment is different from that of the first embodiment. Only differences from the first embodiment will be described below.

  FIG. 16 shows the flow of the first dither mask evaluation process as the second embodiment. In this process, as shown in the figure, first, a group setting process is performed (step S810). The group setting process is a group (dot group) in which each of the storage elements constituting the dither mask 61 is classified by focusing on the difference in which main scanning is performed to form dots, that is, the first embodiment. This is a process of grouping by the path group and the reciprocating group shown in. In the group setting process, grouping is performed in association with either a path group or a reciprocating group. In the present embodiment, it is associated with the reciprocating group. In this embodiment, since the number of storage elements constituting the dither mask 61 and the number of pixels constituting the region of interest shown in FIG. 5 are the same, the result of grouping the storage elements is as follows: This matches the relationship between the forward pixel group and the backward pixel group shown in FIG.

  Once the group setting process is performed as described above, a shift pattern setting process is performed (step S820). The shift pattern setting process is a process in which the formation positions on the print medium of the dot groups that are classified by focusing on the difference in which main scanning is performed are predetermined between the divided dot groups. This is a process of assuming and setting a plurality of types of shift patterns that indicate a state shifted by a predetermined amount in the direction of. Specifically, when printing is actually performed, a plurality of misregistration directions and misregistrations are generated between the groups grouped in step S810 (here, the forward pixel group and the backward pixel group). Assuming type. Note that step S820 of the present embodiment is a process in which the CPU reads the deviation pattern set by the user.

  A shift pattern assumed in this embodiment is shown in FIG. In this example, as shown in the figure, the positional deviation of the backward pixel group with respect to the forward pixel group is set between −2 and 2 pixels at 0.5 pixel intervals along the main scanning direction, and sub-scanning is performed. It is set between −2 and 2 pixels at 0.5 pixel intervals along the direction, and 21 shift patterns P1 to P21 are assumed from combinations of these position shifts. In the present embodiment, the assumed misalignment pattern includes a pattern (pattern P5 in the figure) in which no misalignment occurs. However, it is not essential to include a pattern in which no positional deviation occurs. Such a shift pattern is not limited to the above example, and may be set in consideration of the characteristics of the printer 20. For example, a print image printed by the printer 20 is analyzed, and the direction and amount of misalignment that is likely to occur in practice may be set as a misalignment pattern.

  When the shift pattern setting process is performed in this manner, the dot of the decision storage element is turned on (step S830), the candidate storage element selection process is performed (step S840), and the dot of the candidate storage element is turned on (step S850). ). The processing of steps S830 to S850 is the same as the processing of steps S410 to S430 (see FIG. 7) in the first embodiment, and detailed description thereof is omitted.

  If the dot of the candidate storage element is turned ON, then a shift pattern selection process is performed (step S860). The shift pattern selection process is one shift pattern Pj (j is an integer not smaller than 1 and not greater than n) among n shift patterns (n is an integer greater than or equal to 2, where n = 21) set in step S820. Is the process of selecting.

  If the pattern Pj is selected, next, the coverage Cj on the assumption of the positional deviation of the pattern Pj is calculated (step S870). A specific example of a method for calculating the coverage Cj will be described in detail with reference to FIG. In this example, the coverage C7 is calculated on the assumption of the shift pattern P7 shown in FIG. 17 (shift of 1.0 pixel in the main scanning direction and 0.0 pixel in the sub-scanning direction).

  In calculating the coverage Cj, first, the dot formation pattern of the decision storage element is classified. FIG. 18A shows the dot formation pattern of the decision storage element obtained in step S850. As shown in the figure, the dot storage pattern of the decision storage element is based on the grouping performed in step S810, and the forward movement dot formation pattern constituted by the storage elements corresponding to the forward movement pixel group and the backward movement pixel. It is possible to classify them into backward movement dot formation patterns constituted by storage elements corresponding to groups.

  When the dot formation pattern of the decision storage element is divided, the dot shape is defined next. FIG. 18B shows a dot shape defined similarly to FIG. 10B of the first embodiment. When the dot shape is defined in this way, next, a dot formation pattern for calculating coverage is obtained based on the dot shape pattern of the decision storage element and the defined dot shape. FIG. 18C shows a dot formation pattern for calculating the coverage C7. In this dot formation pattern, each dot of the dot formation pattern shown in FIG. 18 (A) is replaced with the dot shape shown in FIG. 18 (B), and the backward movement dot formation pattern is replaced by the positional deviation of the deviation pattern P7. In other words, that is shifted by 1.0 pixel in the main scanning direction. The display method of each pixel in FIG. 18C is the same as that in FIG. However, the pixels indicated by ● indicate that dots corresponding to the dot formation pattern shown in FIG. 18A overlap with dots that can be considered to be formed by spreading of ink.

  Thus, when the dot formation pattern for calculating the coverage ratio is obtained, the coverage ratio C7 is calculated using Expression (2) as in the first embodiment. As a result, the coverage C7 is 72.9% (= 105/144 × 100). In the above-described example, the dot shape is defined by increasing the resolution of the dot shape by the method described in the first embodiment. In this way, as in the first embodiment, an effect can be obtained in which the ink spreading characteristics can be reflected more faithfully. In addition, as shown in FIG. It becomes possible. As a result, it is possible to accurately estimate the amount of misalignment in accordance with the actual misalignment. In the calculation of the coverage ratio Cj described above, the coverage ratio Cj is obtained by defining a dot shape that reflects the spread of the ink. However, in the second embodiment, the coverage ratio is not reflected without reflecting the spread of the ink. Cj may be calculated.

  Here, the description returns to the first dither mask evaluation process of FIG. Once the coverage Cj is calculated, the steps S860 and S870 are repeated until the coverage Cj is calculated for all the shift patterns P1 to Pn set in step S820 (step S880). When the coverage Cj is calculated for all the shift patterns P1 to Pn (step S880: YES), the evaluation value E1 is calculated based on the calculated coverages C1 to Cn (step S890).

  The evaluation value E1 is an evaluation value related to the degree of dot overlap as in the first embodiment, but here, the calculation method is different from that in the first embodiment. Specifically, the evaluation value E1 is an evaluation value obtained by quantifying fluctuations in the coverage ratios C1 to Cn between the shift patterns P1 to Pn. In the present embodiment, the absolute values of the differences between the coverage ratio C5 calculated on the assumption of the displacement pattern P5 and the other coverage ratios C1 to C4 and C6 to Cn, which indicate a state in which no positional deviation has occurred. The maximum value of the calculated and obtained values is adopted as the evaluation value E1.

  When the evaluation value E1 is calculated, the steps S840 to S890 are repeated until the evaluation value E1 is calculated for all candidate storage elements (blank storage elements) (step S900). Thus, when the evaluation value E1 is calculated for all candidate storage elements (step S900: YES), the first dither mask evaluation process is completed. In step S810, the group setting process is performed in association with the reciprocating group. However, in the case where the group setting process is performed in association with the path group, the first to fourth pixel groups are performed in step S820. A shift pattern may be assumed for all of the combinations, or a shift pattern may be assumed for only some combinations. When group setting processing is performed in association with a pass group, the number of combinations of pixel groups that assume misalignment becomes large. Therefore, a coverage pattern Cj can be obtained by assuming a misalignment pattern for only some combinations. Thus, the processing can be speeded up.

  As is clear from the above description, in this embodiment, attention is paid so that the variation between the coverage when no positional deviation occurs and the coverage when the assumed positional deviation occurs is small. The evaluation value E1 is set to determine the storage element for storing the threshold value. By doing this, it is possible to determine the storage element for storing the threshold value of interest so that the deviation of the density of the dots due to the positional deviation becomes smaller from the reference when the positional deviation does not occur.

  The generation method of the dither mask 61 assumes a plurality of types of misalignment patterns, calculates a coverage C indicating the degree of dot overlap for each misalignment pattern, and uses an evaluation value quantifying the variation of the coverage C. A blank storage element for storing the target threshold value is determined. Therefore, when a deviation assumed as a deviation pattern occurs, fluctuations in the degree of dot overlap can be suitably suppressed. If printing is performed by performing a halftone process using a dither mask generated by such a method, the occurrence of density unevenness can be suppressed even if a shift assumed as a shift pattern occurs. In addition, since a plurality of types of misalignment patterns to be assumed include a state in which no misalignment occurs, the degree of dot overlap varies greatly between when no misalignment occurs and when misalignment occurs. Can be suppressed. As a result, it is possible to generate a dither mask 61 that can obtain a printing result in which density unevenness is suppressed, regardless of whether the deviation occurs or not. Note that the method of generating the dither mask 61 as the second embodiment includes the evaluation method based on the evaluation value E2 described in the first embodiment, and therefore, the granularity of the print image quality is the same as that of the first embodiment. Of course, there is an effect.

C. Variations:
A modification of the above embodiment will be described.
C-1. Modification 1:
In the above-described embodiment, the target threshold value is stored by the comprehensive evaluation value CE calculated based on the evaluation value E1 as an index regarding the degree of dot overlap and the evaluation value E2 as an index regarding the degree of dot dispersion. Although the blank storage element to be determined is determined, the evaluation value E2 does not necessarily have to be taken into consideration in the determination step. That is, the second dither mask evaluation process (step S500) may be omitted, and a blank storage element for storing the target threshold value may be determined based only on the evaluation value E1. Even in this case, the above-described effect of suppressing the density unevenness is exhibited. In addition, since the dither mask 61 is generated using only the degree of dot overlap as an index, the effect of suppressing fluctuations in dot overlap can be enhanced compared to the above-described embodiment. If the calculation of the evaluation value E2 is omitted, the calculation speed related to the generation of the dither mask 61 can be increased.

  In addition, when determining a blank storage element that stores a target threshold value based only on the evaluation value E1, there may be a plurality of candidate storage elements having the smallest evaluation value E1 in the storage element determination process. In some cases, one of the plurality of candidate storage elements may be selected at random, or an evaluation value E2 may be calculated to select an element with excellent dot dispersibility. Or you may select based on a predetermined setting. As such setting, for example, the candidate storage element is scanned along the main scanning direction from the upper right, the first candidate storage element is selected, and candidate storage corresponding to the forward movement of the forward movement and the backward movement is stored. An element can be given priority. Or you may select based on a predetermined law. As such a rule, for example, candidate storage elements corresponding to the first to fourth pixel groups can be selected in order.

C-2. Modification 2:
In the above-described embodiment, when calculating the coverage C by defining the dot shape that reflects the spread of the ink, the coverage is calculated according to Equation (2), focusing only on the ON / OFF of the dots. The coverage that reflects the number of overlapping dots in the pixel may be calculated. For example, in order to calculate the coverage ratio C according to the above-described equation (2), in calculating the number of pixels on which dots are formed, pixels without overlapping dots (pixels on which only one dot is formed, for example, FIG. 9 (C), the pixels indicated as A and B) are pixels formed by overlapping the number of pixels multiplied by the adjustment coefficient 0.6 and two dots (for example, in FIG. 9C, indicated as C). The number of pixels multiplied by the adjustment factor of 0.8 is a pixel formed by overlapping three or more dots (not applicable in FIG. 9C) is the pixel multiplied by the adjustment factor of 1.0 You may count in numbers. The reason why the weighting is increased as the dot overlap increases is that the more the dots formed in one pixel are overlapped, the higher the print density in the pixel is. In short, the coverage that reflects the density difference of each pixel is calculated. The density unevenness of the printed image is not limited to the uneven density of the dots, but can also be caused by the uneven density of the dots. Thus, if the density fluctuation due to the positional deviation is also suppressed in this way, the coverage ratio C is increased. Based on this, it is possible to enhance the effect of suppressing density unevenness. As is clear from the above description, the evaluation value related to the degree of dot overlap is not limited to the simple ON / OFF ratio of dots, but the average gradation value in a predetermined range calculated according to dot overlap. You can also For example, the average gradation value may be calculated by setting the gradation value of a pixel in which only one dot is formed to a value of 128, and the gradation value of a pixel formed by overlapping two dots to a value of 200.

  From the same point of view, when calculating the coverage C by defining a dot shape that reflects the spread of ink, the pixel that is expected to land on the ink (for example, the pixel indicated as A in FIG. 9C). ) Is formed with a relatively higher weight than a pixel that can be regarded as a dot being formed by the spread of ink (for example, pixels shown as B and C in FIG. 9C). The number of pixels may be counted. This is because the ink density is higher at the dot formation position where the ink has landed than at the dot formation position where the dot is formed due to spreading of the ink.

C-3. Modification 3:
In the above-described embodiment, when calculating the coverage C by defining the dot shape reflecting the spread of the ink, all the dot shapes are defined as the same shape, but depending on the characteristics of the dots to be formed, You may define a dot shape by a different shape. For example, when dots are divided into a plurality of sizes, for example, a large dot, a medium dot, and a small dot, depending on the size of each dot, depending on the pixel on which the ink is expected to land or the spread of the ink You may change the magnitude | size of the pixel in which a dot is formed. In this way, the degree of dot overlap can be evaluated with higher accuracy, and as a result, the effect of suppressing density unevenness can be enhanced.

  Alternatively, the shape of the dot formed by the forward movement of the print head 90 and the shape of the dot formed by the backward movement may be defined as different shapes. This is because the ink ejected from the print head 90 and landed on the print medium has a velocity component in the direction in which the print head 90 travels, and therefore has characteristics that are more likely to spread in the direction in which the print head 90 travels. Even in this case, the degree of dot overlap can be evaluated with higher accuracy, and as a result, the effect of suppressing density unevenness can be enhanced.

C-4. Modification 4:
In the above-described embodiment, the degree of dot overlap and the degree of dot dispersion are evaluated for all the gradations, that is, for all threshold values constituting the dither mask 61, and the storage elements for storing the threshold values are determined. Although the embodiment has been described, the storage element determination method may be applied to only a part of the gradation range. For example, the storage element for storing the threshold value may be determined by the method of the first embodiment, the second embodiment, or the modified example 1 only for a high gradation (high density) region in which density unevenness is likely to be remarkable. Alternatively, the above-described method may be switched depending on the gradation. For example, in the intermediate gradation (medium density) region, the graininess is likely to deteriorate due to the positional deviation. Therefore, in the high gradation (high density) region, the method shown in the first or second embodiment is used. Since the deterioration of the graininess due to the positional deviation hardly occurs, the method shown in the first modification may be used.

C-5. Modification 5:
In the above-described embodiment, as the evaluation value E2, the dot formation pattern represented by the entire decision storage element of the dither mask 61, and the respective dot formation patterns represented by the decision storage elements corresponding to the first to fourth pixel groups, For each dot formation pattern represented by the decision storage element corresponding to the reciprocating pixel group, an evaluation value obtained by comprehensively evaluating the degree of dot dispersion with a predetermined weight is used. The dot formation pattern corresponding to the first to fourth pixel groups and the dot formation pattern corresponding to the reciprocating pixel group do not need to be targeted, and at least one of these is not necessary. It may be a target.

C-6. Modification 6:
In the above-described embodiment, the weighting coefficients α to ι used in the expressions (1) and (6) are set to constant values, but these may be set so as to change according to the print gradation. For example, in the low gradation (low density) to intermediate gradation area, which tends to cause deterioration in graininess due to positional deviation, compared to the other gradation areas, the group constituting a part of the dots constituting the image The evaluation value weighting coefficients γ to θ may be larger than the evaluation value weighting coefficient ι for all dots constituting the image. Also, in a high gradation region where density unevenness is likely to occur due to positional deviation, the weighting coefficient α of the evaluation value E1 regarding the degree of dot overlap is set as the evaluation value E2 regarding the degree of dot dispersion compared to other gradation regions. May be larger than the weighting coefficient β. By doing this, it is possible to solve the main problem according to the print gradation, and it is possible to improve the print image quality over a wide range of print gradation. Further, in this way, when the weighting coefficient is changed according to the print gradation, the change may be made stepwise. In this way, the dot formation pattern does not change abruptly due to slight gradation changes. Therefore, the print image quality can be further improved.

C-7. Modification 7:
In the above-described embodiment, the granularity index is used as the evaluation value E2q indicating the degree of dot dispersion. However, the evaluation value E2q may be any value that can evaluate the degree of dot arrangement dispersion. For example, RMS granularity may be used as the evaluation value E2q. Since RMS granularity is a known technique (for example, Japanese Patent Application Laid-Open No. 2007-174272), detailed description is omitted, but the low-pass filter processing is performed on the dot density value using a low-pass filter, and the low-pass filter is used. The standard deviation of the processed density value is calculated. Alternatively, the dot density after the low-pass filter process may be used as the evaluation value E2q. This is an evaluation value used in the so-called potential method.

C-8. Modification 8:
In the second embodiment described above, as the evaluation value E1, the coverage C5 calculated on the premise of the displacement pattern P5 indicating a state in which no positional displacement has occurred, and the other coverages C1 to C4 and C6 to Cn The absolute value of each difference was calculated, and the maximum value among the obtained values was used, but this evaluation value E1 quantifies the variation of the coverage ratios C1 to Cn between the shift patterns P1 to Pn. Any evaluation value may be used. For example, for all combinations of the coverages C1 to Cn, the absolute value of the difference between the combinations may be calculated, and the maximum value obtained may be used as the evaluation value E1. Even in this case, the maximum fluctuation range of the degree of dot overlap can be reduced, so that the effect of suppressing density unevenness can be obtained. Moreover, you may use the standard deviation, dispersion | distribution, etc. of the coverage C1-Cn as the evaluation value E1. In this way, the degree of variation in the coverage ratios C1 to Cn can be reduced, and a stable density unevenness suppressing effect can be obtained.

  Alternatively, for all combinations of the coverage ratios C1 to Cn, the absolute value of the difference between the combinations may be calculated, and a value obtained by adding the obtained values with a predetermined weight may be used as the evaluation value E1. In such a case, the difference from the coverage C5 calculated on the assumption of the shift pattern P5 that indicates a state in which no positional shift has occurred may be increased in weight. The reason is as follows. For example, a positional deviation occurs between a first printing area in which a positional deviation of 2 pixels occurs in a predetermined direction and a second printing area in which a positional deviation of -2 pixels occurs in the predetermined direction. There is a high possibility that a third region that is not present exists. In other words, the first print area and the second print area are rarely adjacent, and in many cases, the third print area is interposed therebetween. For this reason, the coverage C varies between the third print region and the first print region, and the coverage between the third print region and the second print region. If the variation in the rate C is suppressed, that is, if the variation in the coverage with respect to a state in which no positional deviation occurs is emphasized, the density unevenness becomes difficult to be visually recognized.

C-9. Modification 9:
In the embodiment described above, assuming that the target threshold value is stored for one of the blank storage elements, a predetermined evaluation value is calculated for each of the blank storage elements, and the storage location of the target threshold value is determined based on the evaluation value. Although the method of generating the dither mask 61 by sequentially storing the threshold value in the blank storage element by determining it has been described, the generation procedure of the dither mask 61 is not limited to this. For example, the dither mask 61 may be generated as follows. (1) First, a threshold value is randomly stored in the storage element of the dither mask 61. Note that the arrangement of the threshold values may be determined based on the evaluation value E2 described above. Next, (2) the above-described comprehensive evaluation value CE (or evaluation value E1) is calculated for cases where two arbitrary threshold values are selected from among them and the storage locations of the two threshold values are switched and when the two threshold values are not replaced. To do. (3) The storage locations of the two threshold values are switched only when the evaluation value changes in an excellent direction. (4) The above steps (2) and (3) are repeated until the evaluation value converges in a superior direction. Even in this case, it is possible to generate the dither mask 61 having the same characteristics as those of the above-described embodiment.

C-10. Modification 10:
In the above-described embodiment, the generation method of the dither mask 61 mounted on the serial type ink jet printer has been described. However, the dither mask generation method of the present invention and the dither mask generated by the method may be in other formats. It can also be applied to printers. For example, the method shown in the second embodiment can be widely applied to various printers that perform printing while moving the print head. For example, the method can also be applied to a dot impact printer.

  The dither mask generation method of the present invention and the dither mask generated by the method have a plurality of print heads arranged in the entire width direction of the print medium, and overlap a part of adjacent print heads. It can also be applied to a line printer. Specifically, it can also be realized as a method of generating a dither mask applied to the overlap region of the print head. In this case, the misregistration described in the above-described embodiment may be regarded as an ink landing position misalignment between the print heads constituting the overlap region. Further, the dither mask 61 is not limited to be mounted on a printer, but may be mounted on an information terminal when halftone processing is performed on an information terminal such as a personal computer.

  As mentioned above, although embodiment of this invention was described, elements other than the element described in the independent claim among the components of this invention in embodiment mentioned above are additional elements, and are suitably abbreviate | omitted or combined. Is possible. In addition, the present invention is not limited to such an embodiment, and it is needless to say that the present invention can be implemented in various modes without departing from the gist of the present invention. For example, the present invention provides a dither mask generation method, a printing apparatus equipped with a dither mask generated by the method, a program for performing halftone processing using a dither mask generated by the method, a dither mask generation program, It can also be realized as a recorded storage medium.

20 ... Printer 30 ... Control unit 40 ... CPU
41 ... Input unit 42 ... Halftone processing unit 43 ... Printing unit 51 ... ROM
52 ... RAM
60 ... EEPROM
61 ... Dither mask 70 ... Carriage motor 71 ... Drive belt 72 ... Pulley 73 ... Slide shaft 74 ... Paper feed motor 75 ... Platen 80 ... Carriage 82-87 ... Ink cartridge 90 ... Print head 98 ... Memory card slot 99 ... Operation panel P ... Print medium MC ... Memory card Nz ... Nozzle

Claims (10)

A dither mask generation method in which a threshold value is stored in each of a plurality of storage elements,
The dither mask is used for halftone processing for generating dots in a predetermined formation pattern on a print medium based on an arrangement of storage elements in which the threshold value is stored with respect to a predetermined gradation value of an image.
A dither mask generation method for determining an arrangement of the storage elements in which at least a part of the threshold value is stored, using as an index the degree of dot overlap on the print medium assumed from the dot formation pattern. A method of generating a dither mask according to claim 1,
A first step of preparing a threshold value to be stored in at least some of the storage elements corresponding to at least some of the plurality of storage elements;
A second step of selecting one of the thresholds to be stored in a blank storage element, which is a storage element in which the prepared threshold is not yet stored, as a target threshold;
When the target threshold value is stored for one of the blank storage elements, the degree of dot overlap assumed from the dot formation pattern represented by the arrangement of storage elements in which the threshold value is already stored A third step of calculating a predetermined evaluation value for each of the blank storage elements as the index;
A fourth step of determining the blank storage element for storing the target threshold value based on the calculated predetermined evaluation value;
A dither mask generation method for generating the dither mask by repeating the second to fourth steps a predetermined number of times.   The dither mask generation according to claim 2, wherein the predetermined evaluation value is calculated based on a dot coverage that indicates a ratio of dots that are actually formed to cover the print medium, which is assumed from the dot formation pattern. Method. A method of generating a dither mask according to claim 2 or claim 3,
The dither mask is used for the halftone process for performing printing with a printing apparatus that ejects ink onto the print medium to form the dots,
The third step assumes that the ink spreads on the print medium when the ink has landed on the print medium, reflects the assumed spread on the dot formation pattern, and A dither mask generation method for calculating a predetermined evaluation value.   The dither mask generation method according to claim 4, wherein the ink spread is assumed to have a resolution higher than a dot size when it is assumed that there is no spread. A method of generating a dither mask according to any one of claims 2 to 5,
The dither mask is configured to perform printing on a printing apparatus that performs printing by forming dots on the print medium while moving the print head relative to the print medium in the main scanning direction and the sub-scanning direction. Used for tone processing,
The third step includes
Paying attention to the difference in which main scanning the dot is formed, the formation positions on the print medium of the dot groups that are divided are a predetermined amount in a predetermined direction between the divided dot groups. A process of assuming a plurality of types of misalignment patterns, showing a state of misalignment,
Assuming that the threshold of interest is stored for one of the blank storage elements for each of the plurality of types of misalignment patterns, it is assumed from the dot formation pattern represented by the arrangement of storage elements in which the threshold is already stored A step of calculating an evaluation value indicating a degree of overlap of the dots;
Generating a dither mask comprising: calculating an evaluation value obtained by quantifying a variation between the plurality of types of deviation patterns of the evaluation value calculated for each of the plurality of types of deviation patterns as the predetermined evaluation value. Method.   The dither mask generation method according to claim 6, wherein the plurality of types of shift patterns include a state in which the shift does not occur. A method of generating a dither mask according to claim 4 or claim 5,
The dither mask is configured to perform printing on a printing apparatus that performs printing by forming dots on the print medium while moving the print head relative to the print medium in the main scanning direction and the sub-scanning direction. Used for tone processing,
The third step includes
When the target threshold value is stored for one of the blank storage elements, the degree of dot overlap assumed from the dot formation pattern represented by the arrangement of storage elements in which the threshold value is already stored A step of calculating an evaluation value indicating
A step of setting a target value of an evaluation value indicating the degree of overlap of the dots according to the print gradation;
A step of calculating an evaluation value indicating the degree of overlap of the dots and an evaluation value indicating the degree of separation from the set target value as the predetermined evaluation value,
The target value is determined based on the difference in which main scanning is performed in which the dot is formed. A method of generating a dither mask, wherein the dot overlap is set in a range in which the degree of dot overlap is larger than the degree of dot overlap assumed from the dot formation pattern in a state not shifted in the direction. A printing device,
Means for storing a dither mask generated by the method of any of claims 1 to 8;
Means for performing halftone processing using the stored dither mask.   A program for causing a computer to realize a function of performing halftone processing using the dither mask generated by the method according to claim 1.

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