JP2011121248A - Method of generating dither mask and allocation information, printing apparatus and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress fall of a printing image quality in an overlap region printed by overlapping printing heads or overlapping main scans in a line printer or a serial type printer of a band feeding scanning system. <P>SOLUTION: A dither mask 61 used in halftone processing of a printer 20 as the line printer in which preceding heads PH and following heads FH are arrayed, and a preceding head identification mask 62 and a following head identification mask 63 which allot by which of the preceding heads PH and the following heads FH dots are to be formed are generated simultaneously in a series of processes while related to each other by using a total evaluation value CE of the dither mask which becomes a different value by a difference of the allocation information, and by evaluating the dither mask so that the difference of the allocation information is reflected by the total evaluation value CE. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for generating a dither mask or the like used for halftone processing of a printing apparatus.

  In an ink jet line printer, printing is performed by ejecting ink from nozzles of a print head provided over the entire sheet width in a direction intersecting the sheet feeding direction while feeding a printing sheet. Such a print head is usually configured by arranging a plurality of print heads. Further, these print heads are installed so that adjacent print heads partially overlap in the arrangement direction. In such an overlapping area of print heads (hereinafter also referred to as an overlap area), ink is ejected from both adjacent print heads to complete a print image.

  In such a line printer, problems such as occurrence of density unevenness and deterioration of graininess may occur in the overlap region. Such a problem is caused by the local density of the dot arrangement as a result of the landing position of the ink ejected from the nozzles of the print head on the paper being shifted from the target position. As a cause of the deviation of the target position, for example, the installation position of the print head is deviated from the target position due to the problem of installation accuracy, or the amount of paper transport is not strictly constant due to the problem of mechanical accuracy.

  These problems are not limited to line printers, but in a so-called band-feed scanning serial ink jet printer that completes a raster mainly by one main scan, the overlap is partially completed by two main scans. Even when there is a region, it is a common problem. This is because the above-described problem of the accuracy of the paper conveyance amount also occurs in the serial ink jet printer.

JP 2007-15359 A

  Based on at least a part of the problems described above, the problem to be solved by the present invention is to perform printing by overlapping print heads or overlapping main scans in a line printer or a serial printer of a band feed scanning system. This is to suppress a decrease in print image quality in the overlap region.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

[Application Example 1] A plurality of print heads arranged over a print range, wherein adjacent print heads of the plurality of print heads are overlapped with each other in an overlapping direction. Dither used for halftone processing for printing on a printing apparatus that performs printing by ejecting ink while moving a print medium relative to a print head arranged so as to have a single region that does not overlap A dither mask, wherein a plurality of threshold values constituting the dither mask are stored in each storage element;
A method of generating, with which print head of the adjacent print heads each dot of the overlap region is to be formed, and assigned information assigned for each dot formation position,
An evaluation value for evaluating the dither mask, and preparing a predetermined evaluation value that is different depending on the difference in the allocation information,
The dither mask and the allocation information are simultaneously generated in a series of steps in association with each other by evaluating the dither mask with the predetermined evaluation value so that a difference in the allocation information is reflected. A dither mask and allocation information generation method.

  In such a dither mask and allocation information generation method, the dither mask and the allocation information of the overlap area are generated in a series of steps in association with each other so that the difference in allocation information is reflected. The dither mask and the allocation information can be generated while controlling the allocation balance in the dot pattern of each gradation in the generated overlap region. If halftone processing is performed using the dither mask and the allocation information generated in this way, it is possible to suppress a sudden change in the allocation of print heads in the overlap region due to a change in gradation. As a result, when the ink landing position shifts, the amount of change in dot density between gradations becomes small, and it is possible to suppress a decrease in print image quality in the overlap region.

[Application Example 2] The dither mask and allocation information generation method according to Application Example 1, wherein each raster is a dot row arranged in a direction orthogonal to the direction of the arrangement in the overlap region. A first step of determining a printing ratio, which is a ratio occupied by each of the adjacent print heads, for each dot formation, and a blank storage element that is a storage element in which the threshold is not yet stored Based on a second step of selecting one of the plurality of threshold values as a target threshold value, the predetermined print ratio, and a predetermined condition relating to a balance of dot arrangement, the target storage unit is selected from the blank storage elements. A third step of setting a plurality of combinations of candidate storage elements as candidates for storing thresholds and the candidates for allocation in the candidate storage elements; For each of the combinations, a fourth step of calculating the predetermined evaluation value when the target threshold value is stored in the candidate storage element corresponding to the combination, and the calculated evaluation from among the plurality of combinations A fifth step of selecting one combination having the most appropriate value and storing the threshold value of interest in a storage element corresponding to the one combination; a position of the storage element corresponding to the selected one combination; and And a sixth step of updating the allocation information based on the allocation, and the predetermined evaluation value is the print medium between the adjacent print heads due to a difference between the adjacent print heads. Quantifies the change in dot coverage indicating the ratio of dots covering the print medium in the overlap area when the dot landing position on the lens is shifted by a predetermined amount in a predetermined direction Was includes an evaluation value, said second to sixth steps are repeated for a predetermined number of times, a method of generating a dither mask, and allocation information generated and the allocation information and the dither mask.

  Such a dither mask and allocation information generation method sets combinations of candidate storage elements and allocation candidates based on the print ratio and the conditions regarding the balance of the dot arrangement, and sets the evaluation value calculated for each of the combinations. Based on this, the storage location of the target threshold value is determined. Here, since the evaluation value is a quantification of the change in the dot coverage, from the viewpoint that the change in the dot coverage when the landing position deviation of the ink occurs is small, in other words, the dot density From the standpoint that the change of is relatively small, the above combinations can be evaluated. Therefore, it is possible to generate a dither mask and allocation information in which the amount of change in dot density is small even when the landing position deviation of ink occurs. Further, if the evaluation values are calculated only for the combinations narrowed down under the above conditions, the number of evaluation values calculated can be reduced, and the generation process of the dither mask and allocation information can be made efficient.

[Application Example 3] The dither mask and allocation information generation method according to Application Example 2, wherein the predetermined condition is that when the target threshold value is stored in one of the blank storage elements, (1) The number of storage elements for each divided area divided by the difference between the single area and the overlap area, and the difference in which part of the print head is printed. The ratio of storage elements in which the threshold value is already stored is closest to an equal state, and (2) the threshold value for each storage element corresponding to each raster constituting the overlap area is already The number of stored storage elements is closest to an equal state, and (3) the print ratio of the storage elements in which the threshold is already stored for each raster is closest to the determined value. Method of generating a dither mask, and allocation information which is a.

  Such a dither mask and allocation information generation method, when setting a combination of candidate storage elements and allocation candidates, (1) a balance for each divided area, (2) a balance for each raster, and (3) a set print ratio Therefore, the balance of dot arrangement in the overlap region can be controlled with high accuracy. As a result, it is possible to generate a dither mask and allocation information that can further reduce the amount of change in dot density when an ink landing position shift occurs. In addition, since combinations of candidate storage elements and allocation candidates are largely limited by the above conditions, the number of evaluation values calculated can be reduced, and the dither mask and allocation information generation processing can be made more efficient.

[Application Example 4] The dither mask and allocation information generation method according to Application Example 2, wherein the predetermined condition is that when the target threshold value is stored in one of the blank storage elements, (1) The number of storage elements for each divided area divided by the difference between the single area and the overlap area, and the difference in which part of the print head is printed. The ratio of storage elements in which the threshold value is already stored belongs to a predetermined range from a uniform state, and (2) the threshold value for each storage element corresponding to each raster constituting the overlap region is The number of storage elements already stored belongs to a predetermined range from a uniform state, and (3) the print ratio of the storage elements in which the threshold is already stored for each raster is determined from the predetermined value. Method of generating a dither mask and allocation information is to belong to a constant range.

  Such a dither mask and allocation information generation method, when setting a combination of candidate storage elements and allocation candidates, (1) a balance for each divided area, (2) a balance for each raster, and (3) a set print ratio Therefore, the number of combinations of candidate storage elements and allocation candidates is increased as compared with the method of application example 3. Therefore, it is possible to select the optimum evaluation value from a wide range of combinations while accurately controlling the balance of the dot arrangement in the overlap area. The effect of reducing the amount of change can be further enhanced as compared with Application Example 3.

[Application Example 5] The dither mask and allocation information generation method according to Application Example 3 or Application Example 4, wherein the predetermined evaluation value corresponds to the allocation candidate set in the candidate storage element. In the area where dots are formed by the head, due to the difference between the dispersibility evaluation value indicating the dispersibility of the dot arrangement represented by the storage element in which the threshold value is already stored and the adjacent print head, the adjacent print head Assuming that there are a plurality of types of misalignment patterns that indicate a state in which the dot landing positions on the print medium are deviated by a predetermined amount in a predetermined direction, and for each of the multiple types of misalignment patterns, A method of generating a dither mask and allocation information including calculating a dot coverage and including a coverage evaluation value calculated based on the dot coverage.

  Such a dither mask and allocation information generation method is an evaluation value including a dispersibility evaluation value for evaluating the dispersibility of dot arrangement in an area where dots are formed by a print head corresponding to an allocation candidate set in a candidate storage element. Since the combination of the candidate storage element and the allocation candidate is evaluated, it is possible to generate a dither mask and allocation information that can ensure dot dispersion in a single area and also maintain a good print image in the single area. . In addition, since the evaluation value including the coverage evaluation value obtained by quantifying the change of the dot coverage in the overlap region, calculated for each of the plurality of types of shift patterns, the combination of the candidate storage element and the allocation candidate is evaluated. Regardless of which deviation pattern occurs, the dither can be controlled so that the amount of change in the density of the dots in the overlap area is reduced to a certain extent, and the deterioration in print image quality due to the deviation of the ink landing position in the overlap area can be suppressed. Mask and assignment information can be generated.

Application Example 6 The method for generating a dither mask and assignment information according to Application Example 5, in which the plurality of types of shift patterns include a state in which the shift does not occur.
Such a dither mask and allocation information generation method includes a misalignment pattern in which no misalignment occurs. Therefore, even when no ink landing position misalignment occurs, a dither mask and allocation information that produces a good print result can be generated. can do.

[Application Example 7] The dither mask and allocation information according to Application Example 5 or Application Example 6, wherein the coverage evaluation value is an evaluation value indicating the degree of variation in dot coverage calculated for each of the plurality of types of shift patterns. Generation method.
Such a dither mask and allocation information generation method can evaluate combinations of candidate storage elements and allocation candidates based on the degree of variation in the dot coverage calculated for each shift pattern, so what type of shift pattern is assumed. Even if it occurs, it is possible to generate a dither mask and allocation information with a small variation in dot coverage, that is, a small change in dot density. Therefore, even if the ink landing position is deviated, the print image quality in the overlap region is not greatly deteriorated.

The present invention can also be realized as a dither mask and allocation information generation method according to application examples 8 to 14. This generation method also provides the same effects as Application Example 1 to Application Example 7.
Application Example 8 Ink is ejected while the print head is moved relative to the print medium in the main scanning direction and the sub-scanning direction, and a raster that is a row of dots arranged in the main scanning direction is completed in one main scanning. A dither mask used for halftone processing for printing in a printing apparatus that prints a print image that includes a single region to be printed and an overlap region that completes the raster in two consecutive main scans, A dither mask in which a plurality of threshold values constituting a dither mask are stored in each storage element, and which main scan of the two main scans forms each dot in the overlap region, A method for generating allocation information allocated for each dot formation position, wherein the evaluation value is used to evaluate the dither mask, and is different depending on the difference in the allocation information. A series of steps in which the dither mask and the allocation information are correlated with each other by evaluating the dither mask with the predetermined evaluation value so that a difference in the allocation information is reflected. A method of generating a dither mask and allocation information, wherein the dither mask and the allocation information are generated simultaneously.

[Application Example 9] The dither mask and allocation information generation method according to Application Example 8, wherein each of the two rasters in the overlap area is subjected to the two operations for forming each dot constituting the raster. A first step of determining a printing ratio that is a ratio occupied by each of the main scans, and selecting one of the plurality of threshold values to be stored in a blank storage element that is a storage element in which the threshold value is not yet stored as a target threshold value A candidate storage element as a candidate for storing the target threshold value from among the blank storage elements, based on the second step of performing, the predetermined printing ratio, and a predetermined condition regarding the balance of dot arrangement; A third step of setting a plurality of combinations with the allocation candidates in the candidate storage element, and each of the set combinations corresponds to the combination. A fourth step of calculating the predetermined evaluation value when the target threshold value is stored in the candidate storage element, and selecting one combination that is most appropriate for the calculated evaluation value from the plurality of combinations. The allocation information is updated based on the fifth step of storing the threshold value of interest in the storage element corresponding to the one combination, the position of the storage element corresponding to the selected one combination, and the allocation. And the predetermined evaluation value is determined by a dot landing position on the print medium between the adjacent print heads due to a difference between the two consecutive main scans. Including an evaluation value obtained by quantifying a change in dot coverage indicating a ratio of dots covering the print medium in the overlap area when the predetermined amount is shifted in a predetermined direction, and the second to sixth steps are included. Place Repeat count, a method of generating a dither mask, and allocation information generated and the allocation information and the dither mask.

[Application Example 10] The dither mask and allocation information generation method according to Application Example 9, wherein the predetermined condition is that when the target threshold value is stored in one of the blank storage elements, (1) For each divided area divided according to the difference between the single area and the overlap area, and the main scan and the area completed by which part of the print head. The ratio of the storage elements in which the threshold value has already been stored to the number of storage elements is closest to a uniform state. (2) For each storage element corresponding to each raster constituting the overlap area The number of storage elements in which the threshold value has already been stored is closest to an equal state, and (3) the print ratio of the storage elements in which the threshold value has already been stored for each raster. Dither mask and method for generating allocation information is closest becomes possible to value.

[Application Example 11] The dither mask and allocation information generation method according to Application Example 9, wherein the predetermined condition is that when the target threshold value is stored in one of the blank storage elements, (1) For each divided area divided according to the difference between the single area and the overlap area, and the main scan and the area completed by which part of the print head. The ratio of the storage elements in which the threshold is already stored to the number of storage elements belongs to a predetermined range from a uniform state, and (2) the storage elements corresponding to each raster constituting the overlap region The number of storage elements in which the threshold is already stored belongs to a predetermined range from a uniform state, and (3) the printing ratio of the storage elements in which the threshold is already stored for each raster. The method generates the dither mask and allocation information from the set value is that belonging to a predetermined range.

[Application Example 12] The dither mask and allocation information generation method according to Application Example 10 or Application Example 11, wherein the predetermined evaluation value corresponds to the allocation candidate set in the candidate storage element. Due to the difference between the dispersibility evaluation value indicating the dispersibility of the dot arrangement represented by the storage element in which the threshold value has already been stored in the area where dots are formed by scanning, and the difference between the two consecutive main scans, the 2 Assuming that there are a plurality of types of misalignment patterns in which the landing positions of dots on the print medium between the main scans are shifted by a predetermined amount in a predetermined direction, the overshoot is performed for each of the multiple types of misalignment patterns. A method for generating a dither mask and allocation information including a coverage evaluation value obtained by calculating the dot coverage in a wrap region and quantifying a change in the dot coverage.

Application Example 13 The method of generating a dither mask and allocation information according to Application Example 12, in which the plurality of types of shift patterns include a state in which the shift does not occur.

[Application Example 14] The dither mask and allocation information according to application example 12 or application example 13, wherein the coverage evaluation value is an evaluation value indicating the degree of variation in dot coverage calculated for each of the plurality of types of shift patterns. Generation method.

  In addition to the configuration as a dither mask and allocation information generation method, the present invention uses a printing apparatus that stores a dither mask and allocation information generated by the method, and a dither mask and allocation information generated by the method. The present invention can also be realized as a program for causing a computer to perform the function of performing halftone processing, a dither mask and assignment information generation program, a storage medium storing these programs, and the like.

FIG. 2 is an explanatory diagram illustrating a schematic configuration of a printer 20 as an embodiment. 4 is an explanatory diagram showing a detailed configuration of a print head 80. FIG. 3 is a flowchart showing a flow of printing processing by a printer 20. It is explanatory drawing which shows the usage method of the leading head identification mask 62 and the trailing head identification mask 63. FIG. It is explanatory drawing which shows the size of the dither mask 61. FIG. It is explanatory drawing which shows the area | region of the storage element which stores the threshold value of the dither mask 61. FIG. 10 is a flowchart showing a procedure for generating a dither mask 61, a leading head identification mask 62, and a trailing head identification mask 63. 10 is a flowchart showing a procedure for generating a dither mask 61, a leading head identification mask 62, and a trailing head identification mask 63. It is explanatory drawing which shows the specific example of a printing ratio. It is explanatory drawing which shows the determination method of the division area which stores a focus threshold value concretely. It is explanatory drawing which shows the content of a head group determination process concretely. It is explanatory drawing which shows the selection method of the blank storage element in which the threshold value is not stored concretely. It is explanatory drawing which shows a mode that a focus threshold value is stored in a storage element. It is explanatory drawing which shows the update method of the leading head identification mask 62 and the trailing head identification mask 63 concretely. It is explanatory drawing which shows the range of the storage element used as the object which calculates the 1st evaluation value E1 and the 2nd evaluation value E2 for calculating the comprehensive evaluation value CE. It is a flowchart which shows the calculation procedure of 1st evaluation value E1. It is explanatory drawing which shows the calculation method of the coverage Cn for calculating the 1st evaluation value E1. It is explanatory drawing of a blue noise characteristic and a green noise characteristic. It is explanatory drawing which shows the calculation method of 2nd evaluation value E2. It is explanatory drawing which shows the aspect of the dither mask 61 as a modification. It is explanatory drawing which shows the usage method in the halftone process of the dither mask 61 as a modification. FIG. 10 is a schematic configuration diagram of a printer 420 as a modified example. 5 is an explanatory diagram illustrating a detailed configuration of a print head 490. FIG. It is explanatory drawing which shows the effect of this invention. It is explanatory drawing which shows the effect of this invention.

A. Example:
Examples of the present invention will be described.
A-1. Device configuration:
FIG. 1 is an explanatory diagram showing a schematic configuration of a printer 20 as an embodiment of the present application. The printer 20 is an ink jet line printer, and includes a control unit 30, ink cartridges 71 to 74, a print head 80, a paper feed mechanism 90, and the like as illustrated. The ink cartridges 71 to 74 correspond to inks that express cyan (C), magenta (M), yellow (Y), and black (K). Of course, the type and number of inks are not limited to this. For example, only K may be used, light inks such as light cyan (Lc) and light magenta (Lm), blue (B), and red (R). It may include a special color such as.

  The print head 80 is a line head type print head, and includes a plurality of thermal nozzles arranged in a line on the lower surface thereof. Each ink in the ink cartridges 71 to 74 is supplied to nozzles provided on the lower surface of the print head 80 through an introduction pipe (not shown), and ink is ejected from these nozzles to print on the printing paper P. Details of the print head 80 will be described later with reference to FIG.

  The paper feed mechanism 90 includes a paper feed roller 91, a paper feed motor 92, and a platen 93. The paper feed motor 92 rotates the paper feed roller 91 to convey the print paper P disposed between the print head 80 and the plate-like platen 93 in a direction perpendicular to the axial direction of the paper feed roller 91. .

  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 CPU 40 expands and executes programs stored in the ROM 51 and the EEPROM 60 in the RAM 52 to control the overall operation of the printer 20, and also inputs the input unit 41, the halftone processing unit 42, the allocation processing unit 43, and the printing unit 44. Also works. Details of these functional units will be described later.

  In the EEPROM 60, a dither mask 61, a leading head identification mask 62, and a trailing head identification mask 63 are stored. The dither mask 61 is used for halftone processing by a systematic dither method, and includes a plurality of threshold values. The dither mask 61 of this embodiment has predetermined characteristics due to its generation method. Such characteristics will be described later together with a method for generating the dither mask 61. The leading head identification mask 62 and the trailing head identification mask 63 are masks used for assignment processing described later, and details thereof will be described later.

  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). An operation panel 99 is connected to the control unit 30, and a user of the printer 20 can operate the printer 20 via the operation panel 99.

A-2. Detailed configuration of the print head 80:
FIG. 2 is an explanatory diagram showing a detailed configuration of the print head 80. As shown in the figure, the print head 80 according to the present embodiment has a print head formed with nozzle rows 81 to 84 for ejecting inks of C, M, Y, and K, respectively, over the print range of the print paper P. Are arranged in a zigzag pattern. The reason why the print heads are arranged in a staggered manner as described above is to solve the problem of the strength of the end of the print head and the problem of the installation space of the attached device. The number of print heads may be two or more. Further, the method for arranging the print heads is not particularly limited, and may be, for example, stepped. Similarly, in the embodiment, the nozzles constituting the nozzle rows 81 to 84 are formed in a straight line in the print head arrangement direction, but the nozzle arrangement method is not particularly limited. , May be arranged in a staggered pattern.

  Among these print heads, the one disposed on the upstream side in the conveyance direction of the printing paper P is also referred to as the leading head PH, and the one disposed on the downstream side is also referred to as the trailing head FH. The leading head PH and the trailing head FH have a region where both ends thereof overlap each other (hereinafter also referred to as an overlap region ORA) and a region where there is no overlap (hereinafter also referred to as a single region SA). Has been placed. The reason why the overlap area ORA is provided in this way is to suppress the occurrence of banding in the portion corresponding to the joint between the print heads in the print image due to the difference in the characteristics of each print head. .

  The preceding head PH and the trailing head FH form one dot row along the print head arrangement direction by adjusting the paper feed and ink ejection timing. The dot row is formed by either the preceding head PH or the trailing head FH in the single area SA, and is formed by both the leading head PH and the trailing head FH in the overlap area ORA. At each dot formation position in the overlap area ORA, a dot is formed by either the leading head PH or the trailing head FH.

  By continuously performing the dot row forming operation along the arrangement direction of the print heads, dot rows (also referred to as rasters in this embodiment) are also formed in the conveyance direction of the printing paper P. Of the print areas printed in this way, areas corresponding to the respective print heads are also referred to as bands. In the present embodiment, for convenience of explanation, the print area corresponding to the overlap area ORA area is divided at the center as shown in FIG. 2 and given a number (J in the figure is a positive value). )) To distinguish between bands.

A-3. Printing process:
A printing process in the printer 20 will be described. FIG. 3 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 lookup table (not shown) stored in the EEPROM 60 and performs color conversion from RGB format to CMYK 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). In the halftone process in this embodiment, the input data is compared with the threshold value of the dither mask 61 by a systematic dither method using the dither mask 61. If the input data is equal to or greater than the threshold value, it is determined that the dot is ON. Is less than the threshold value, it is determined that the dot is OFF. The halftone process is not limited to the binarization process of dot ON / OFF, but 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 allocation process for allocating whether the leading head PH or the trailing head FH forms a dot at each dot formation position in the overlap area ORA as a process of the allocation processing unit 43. This is performed (step S140). This allocation process will be described in detail with reference to FIG. The final dot arrangement DA shown in FIG. 4 exemplifies the result of the halftone process (step S130) for the overlap area ORA. The lattice shown in the figure indicates pixels of image data. A grid displayed by hatching indicates a pixel determined to be dot ON in the halftone process, and a grid displayed in white indicates a pixel determined to be dot OFF in the halftone process.

  The leading head identification mask 62 records the positions of pixels that form dots by the leading head PH among the pixels printed in the overlap area ORA. Among the illustrated grids, hatched grids indicate pixels that form dots (set to dot ON) with the preceding head PH, and pixels displayed in white do not form dots with the preceding head PH. The pixel which forms a dot with the head FH is shown.

  By masking the final dot arrangement DA using the preceding head identification mask 62, it is possible to determine a pixel in which dots are formed by the preceding head PH in the final dot arrangement DA. Specifically, the overlapping portion of the dot ON pixel of the final dot arrangement DA and the dot ON pixel of the preceding head identification mask 62 is a dot arrangement DA1 in which dots are formed by the preceding head PH.

  In the trailing head identification mask 63, the positions of pixels that form dots by the trailing head FH among the pixels corresponding to the overlap area ORA are recorded. The dot ON / OFF in the trailing head identification mask 63 is obtained by inverting the dot ON / OFF in the preceding head identification mask 62. By using the trailing head identification mask 63 to mask the final dot arrangement DA, the dot arrangement DA2 in which dots are formed by the trailing head FH can be determined. The dot arrangement DA2 can also be determined as an overlapping portion between the dot ON pixel of the final dot arrangement DA and the dot OFF pixel of the preceding head identification mask 62. In the following description, as in the preceding head identification mask 62 and the following head identification mask 63, the dot formation position (pixel) is used to determine whether the leading head PH or the following head FH forms dots in the overlap area ORA. ) Is also referred to as allocation information.

  When the allocation process is performed, the CPU 40 drives the print head 80, the paper feed motor 92, and the like as the process of the printing unit 44, and executes printing (step S150). Thus, the printing process ends.

A-4. Generation method of the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63:
A method for generating the above-described dither mask 61, leading head identification mask 62, and trailing head identification mask 63 will be described. First, the size and configuration of the generated dither mask 61 will be described below. The positional relationship between the preceding head PH and the trailing head FH constituting the print head 80 is a repetitive pattern in a certain unit due to the staggered arrangement. Specifically, as shown in FIG. 5, in the positional relationship between the preceding head PH and the following head FH in the nozzle row direction, the lower end of the following head FH and the upper end of the preceding head PH overlap in the nozzle row direction. The overlapping area ORA1, the single area SA1 where only the leading head PH exists in the nozzle row direction, the overlapping area ORA2 where the upper end of the trailing head FH and the lower end of the leading head PH overlap in the nozzle row direction, and the nozzle A repetitive pattern including the single area SA2 in which only the trailing head FH exists in the column direction is generated.

  It is desirable to set the vertical direction (nozzle row direction) size D1 of the generated dither mask 61 so as to correspond to the number of pixels corresponding to an integral multiple of the period of the repetitive pattern. In the present embodiment, the vertical direction size D1 of the dither mask 61 is set to one time of the cycle of the repeated pattern. For example, when the number of nozzles constituting one nozzle row in one print head is 360 and the number of nozzles in the overlap areas ORA1 and ORA2 is 8, the vertical size D1 is 704 pixels (= 8 + (360− 8 × 2) +8+ (360−8 × 2)).

  In this way, if the vertical size D1 of the dither mask 61 is set and one dither mask 61 is repeatedly applied in the vertical direction, the position of each nozzle in the repeated pattern and the dot ON / OFF determination at each nozzle are used. The correspondence relationship with the threshold value of the dither mask 61 can be kept constant. As a result, it is possible to efficiently perform a desired halftone process based on the characteristics of the dither mask 61 obtained by the generation method described later. Of course, the vertical size D1 is not limited to an integral multiple of the period of the repetitive pattern. For example, if two dither masks are applied alternately, the vertical size D1 is 1/2 of the period of the repetitive pattern. Also good.

  The dither mask 61 has a horizontal direction (paper transport direction) size D2 that is large enough to prevent the repetition cycle of the dither mask 61 from being visually recognized in the print image quality when the dither mask 61 is repeatedly applied in the horizontal direction to perform halftone processing. For example, it is desirable to set to about 256 pixels.

  In the following description, the size of the dither mask 61 corresponds to a vertical size D1 of 18 pixels and a horizontal size D2 of 5 pixels, that is, a total of 90 pixels (= 18 × 5) in order to simplify the description. Explain as size (unless otherwise noted). The dither mask 61 has a number of storage elements corresponding to the pixel size. The storage element is an element that stores a threshold value constituting the dither mask 61. The dither mask 61 is configured by storing threshold values in all of these storage elements. The dither mask 61 of this embodiment has 90 storage elements because the size is 90 pixels. Among these storage elements, the areas for storing the threshold values applied to the overlap area ORA1, the single area SA1, the overlap area ORA2, and the single area SA2 are also referred to as divided areas A1 to A4, respectively. The divided areas A1 to A4 are areas divided based on the positional relationship between the preceding head PH and the trailing head FH in the repeating unit of the positional relationship between the leading head PH and the trailing head FH. It can be said that the area is divided according to the difference between the wrap area ORA and the single area SA and the portion of the leading head PH or the trailing head FH that is printed.

  Details of the divided areas A1 to A4 are shown in FIG. The grid shown in FIG. 6 shows each storage element. In this example, the divided areas A1 and A3 corresponding to the overlap areas ORA1 and ORA2 each have 4 × 5 storage elements, and the divided areas A2 and A4 corresponding to the single areas SA1 and SA2 are Each has 5 × 5 storage elements. “0” in the figure indicates an area for storing a threshold value applied to image data in which dots are formed by the preceding head PH. “1” indicates an area for storing a threshold value applied to image data in which dots are formed by the trailing head FH. “0/1” indicates an area that can be “1” or “0” depending on the generation method of the dither mask 61 described later. Although not shown, the sizes of the leading head identification mask 62 and the trailing head identification mask 63 are 4 pixels × 5 pixels corresponding to the divided areas A1 and A3.

  A method of generating the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 having such a size will be described below. 7 and 8 are flowcharts showing the procedure of these generation methods. The generation method described below is a process for generating the dither mask 61 and the like by a CPU such as a main frame. The user may perform part or all of the steps described below. Since the dither mask 61 in the present embodiment has 90 storage elements, in the following description, the dither mask 61 is completed by storing threshold values of values 0 to 89 in the 90 storage elements. explain.

  In generating the dither mask 61 and the like, first, the printing ratio for each raster (dot row in the transport direction of the printing paper P) in the overlap areas ORA1 and ORA2 is designated (step S210). The print ratio indicates, for each raster in the overlap areas ORA1 and ORA2, whether each dot constituting the raster is formed by the preceding head PH or the following head FH. The ratio with the row head FH, that is, the dot generation rate of the preceding head PH and the succeeding head FH for each raster.

  The printing ratio in this embodiment is shown in FIG. The horizontal axis of FIG. 9 indicates the raster number corresponding to the vertical size D1 of the dither mask 61. The vertical axis indicates the dot occurrence rate of the leading head PH and the trailing head FH. As shown in the figure, the printing ratio of this embodiment is specified so that the dot occurrence rates of the leading head PH and the trailing head FH are both 50% in all the rasters constituting the overlap areas ORA1 and ORA2. It was decided. Since each raster in the single area SA1 is an area in which only the preceding head PH exists, the dot generation rate of the preceding head PH is naturally 100%, and each raster in the single area SA2 has only the trailing head FH. Since this is an existing area, the dot generation rate of the trailing head FH is 100%. The printing ratio is not limited to the above example, and a manufacturer's desired ratio may be specified.

  When the print ratio is designated, a target threshold value is next selected (step S220). The target threshold value is one threshold value selected from threshold values 0 to 89 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. For example, when a threshold value of 0 to 7 is already stored in the storage element by a process described later, the target threshold value selected in step S220 is the value 8.

  When the target threshold value is selected, next, a divided area determination process for determining a divided area for storing the selected target threshold value from the divided areas A1 to A4 is performed (step S230). The divided area determination process is to determine a candidate for a divided area to which the storage element that should store the target threshold value belongs. The selected divided area may be singular or plural. This process will be specifically described with reference to FIG. As shown in FIG. 10, each of the divided areas A1 and A3 has 20 storage elements, and each of the divided areas A2 and A4 has 25 storage elements. Here, description will be made assuming that the segment areas A1 to A4 have three, six, four, and six already stored threshold values, respectively.

The process of step S230 is performed as follows. First, the threshold setting ratio is calculated by the following equation (1) using the number of storage elements for each of the divided areas A1 to A4 and the number of stored thresholds. For example, the threshold setting ratio of the segment area A1 is 15% (= 3/20).
Threshold setting ratio (%) = number of stored thresholds / number of storage elements (1)

  Next, the minimum value of the threshold setting ratio is specified. In this example, the value 15 of the divided area A1 is the minimum. Then, the divided area whose threshold setting ratio is within the predetermined allowable range from the specified minimum value is selected. In this embodiment, the allowable width is set to 5. In this case, the selected divided areas are the divided areas A1 and A3.

  In short, the processing of step S230 is based on the threshold for the number of storage elements for each divided area when the target threshold is stored in a storage element that has not yet been stored (hereinafter also referred to as a blank storage element). Is a process for determining a divided area as a candidate for storing a threshold value of interest so that the ratio of storage elements already stored (hereinafter also referred to as “stored elements”) belongs to a predetermined range from a uniform state. .

  When the divided area is determined in this way, it is next determined whether or not the determined divided area includes the divided areas A1 and A3 corresponding to the overlap areas ORA1 and ORA2 (step S240). As a result, if the determined divided area includes the divided area A1 or A3 (step S240: YES), head group determination processing is performed (step S250). The head group determination process is a process for narrowing down candidates for combinations of blank storage elements that should store the target threshold value and allocation information, corresponding to the overlap area ORA, in consideration of the dot arrangement balance. Specifically, in the divided area A1 or A3 to which the target element belongs, the raster narrowing down to which raster the blank storage element that should store the target threshold belongs, and the dot formation position to which the target threshold is applied, This is a process for narrowing down the allocation information indicating which of the leading head PH and the trailing head FH forms dots.

  The head group determination process will be specifically described with reference to FIG. FIG. 11 shows the number of already stored elements in the divided area A3 and its ideal value for each raster. Note that the horizontal size D2 of the dither mask 61 of the present embodiment corresponds to 5 pixels as described above, but here, in order to clarify the processing contents, it is assumed that it corresponds to 25 pixels, that is, In the description, it is assumed that there are 25 storage elements in the raster direction.

  A column (A) in FIG. 11 shows numbers assigned to distinguish each storage element row corresponding to the raster in the divided area A3. In the description of FIG. 11, each of these storage element rows is also simply referred to as a raster, and the numbered storage element row is also referred to as “raster 1” or the like. The (B) column of FIG. 11 shows the number of stored elements for each raster. As shown in the figure, the rasters 1 to 4 have already stored 21, 22, 20, and 22 threshold values, respectively. In the already stored element, when a threshold value is stored in the element, assignment of the leading head PH and the trailing head FH is set (details will be described later). For example, as shown in the figure, in the raster 1, 10 of the 21 already stored elements are allocated to the trailing head FH, and 11 are allocated to the preceding head PH.

  The column (D) in FIG. 11 shows the final allocation of the leading head PH and the trailing head FH of each raster (hereinafter also referred to as a threshold storage ratio) based on the printing ratio specified in step S210. . As shown in the figure, the threshold storage ratio of each raster is 50%.

  The column (C) of FIG. 11 shows the ideal number of allocations of the leading head PH and the trailing head FH (hereinafter also referred to as an allocation ideal value) when it is assumed that the target threshold value is stored in each raster. Yes. The allocated ideal value can be obtained as a product of the threshold storage ratio and a value obtained by adding a value 1 (assuming that the target threshold is stored) to the number of already stored elements of the corresponding raster. For example, assuming that the target threshold value is stored in the raster 1, the ideal allocation value is 50% × (21 + 1) = 11 for both the leading head PH and the trailing head FH.

  In the head group determination processing of the present embodiment, based on these pieces of information, a blank storage element (raster) for storing a target threshold value by setting an allowable width from an ideal state of dot arrangement balance, A candidate combination with the allocation information is set.

  Specifically, the raster is narrowed down so that the number of stored elements for each raster falls within a predetermined range from a uniform state when it is assumed that the target threshold value is stored in each raster. In this embodiment, the narrowing is performed so that the difference between the raster having the smallest number of stored elements and the raster having the largest number is 2 or less.

  In the example illustrated in FIG. 11, when it is assumed that the target threshold value is stored in raster 1, the number of already stored elements in raster 1 is 22 (= 21 + 1). At this time, since the above-described difference becomes the value 2, the result falls within the above-mentioned predetermined range. Similarly, when the target threshold value is stored in the raster 3, the number of already stored elements of the raster 3 is the value 21, and the above difference is the value 1, so that the result falls within the predetermined range. On the other hand, when the target threshold value is stored in the raster 2 or the raster 4, the number of already stored elements of the raster 2 and the raster 4 becomes the value 23, and the above difference becomes the value 3. Therefore, the result does not fall within the predetermined range. . From the above, the rasters in which the target threshold value is to be stored are narrowed down to raster 1 and raster 3. The raster thus narrowed down is also called a raster candidate.

  Further, the narrowing down of the allocation information is performed when it is assumed that the focused threshold value is stored in each raster and the storage element storing the focused threshold value is allocated to either the preceding head PH or the following head FH. Allocation of already stored elements is performed so as to be within a predetermined range from the allocated ideal value. In this embodiment, the number of storage elements assigned to the preceding head PH and the succeeding head FH as the ideal assignment values, and the number of storage elements assigned to the preceding head PH and the succeeding head FH actually assigned, Is narrowed down so that the difference between the two values is 2 or less.

  In the example shown in FIG. 11, when the focus threshold value is stored in the raster 1, if the storage element storing the focus threshold value is allocated to the trailing head FH, the number of allocations of the trailing head FH and the leading head PH (FH, PH) = (11, 11), which matches the allocated ideal value (11, 11). That is, the difference from the allocated ideal value is 0. On the other hand, if the storage element for storing the target threshold value is allocated to the preceding head PH, the allocation number of the following head FH and the preceding head PH (FH, PH) = (10, 12), and the allocation ideal value (11, 11). The difference value from 1) is the value 1.

  Further, when the target threshold value is stored in the raster 3, if the storage element storing the target threshold value is allocated to the trailing head FH, the number of allocations of the trailing head FH and the leading head PH (FH, PH) = (9, 12), and the difference value from the allocated ideal values (10.5, 10.5) is 1.5. On the other hand, if the storage element for storing the target threshold value is allocated to the preceding head PH, the number of allocation of the following head FH and the preceding head PH (FH, PH) = (8, 13), and the allocated ideal value (10.5) , 10.5) is 2.5. From the above, the allocation information is narrowed down to the preceding head PH and the trailing head FH for the raster 1 and to the trailing head FH for the raster 3. The allocation information thus narrowed down is also referred to as an allocation candidate.

  Thus, when the raster and allocation information are narrowed down by the head group determination process, or if the determined divided area does not include the divided area A1 or A3 (step S240: NO), it belongs to the divided area determined in step S230. One of the blank storage elements is selected (step S260). In step S260, the blank storage element is specified by sequentially scanning the storage elements in the divided areas (here, the divided areas A1 and A3) determined in step S230. In this embodiment, as shown in FIG. 12, the storage element at the upper right of the divided area A1 is scanned, and an unselected blank storage element that has not yet been selected in step S260 (details will be described later) repeatedly executed. The selection was made sequentially. The blank storage element selected in this way is also referred to as a focused element.

  If the blank storage element is selected as the target element, it is next determined whether or not the target element belongs to the divided areas A1 and A3 corresponding to the overlap areas ORA1 and ORA2 (step S270). As a result, if the element of interest belongs to the divided area A1 or A3 (step S270: YES), it is further determined whether or not the element of interest belongs to the raster candidate narrowed down in step S250 (step S280).

  As a result, if the element of interest belongs to the raster candidate (step 280: YES), the allocation candidate associated with the raster candidate to which the element of interest belongs is selected from the combinations of the raster candidate narrowed down in step S250 and the allocation candidate. Is selected (step S300).

  If an allocation candidate is selected or if the target element does not belong to the classification area A1 or A3 (step S270: NO), the comprehensive evaluation value CE for the target element selected in step S260 is calculated (step S310). The comprehensive evaluation value CE is an evaluation value for evaluating a plurality of storage element candidates for storing a target threshold value and determining a storage element for storing the target threshold value from the candidates. The overall evaluation value CE is configured such that the evaluation value reflects the difference between which the storage element candidate is assigned to the preceding head PH or the succeeding head FH. Although details of the calculation method of the comprehensive evaluation value CE will be described later, in this embodiment, it can be said that the smaller the comprehensive evaluation value CE is, the better the storage element for storing the target threshold value.

  On the other hand, if the element of interest does not belong to the raster candidate (step S280: NO), the comprehensive evaluation value CE is set to the maximum value MAX (step S290). The maximum value MAX here is a value determined in advance so as to be always greater than the comprehensive evaluation value CE calculated in step S310 when the element of interest belongs to a raster candidate (step S280: YES). The comprehensive evaluation value CE is set in this way because if the target element does not belong to the raster candidate, it is not desirable to store the target threshold value in the target element. This is because it is substantially excluded from the candidates.

  Thus, when the comprehensive evaluation value CE is calculated or set, the comprehensive evaluation value CE of the element of interest is compared with the comprehensive evaluation value CE of the most prominent element (step S320). As a result, if the total evaluation value CE of the target element is smaller than the total evaluation value CE of the most prominent element (step S320: YES), the target element is set as the most prominent element (step S330). At this time, when the most prominent element belongs to the divided areas A1 and A3, information indicating the position of the most prominent element is stored in association with the allocation candidate selected in step S300. If the comprehensive evaluation value CE of the element of interest is equal to or greater than the comprehensive evaluation value CE of the most prominent element (step S320: NO), the process of step S330 is omitted.

  The processing in steps S300 to S330 is repeated for all allocation candidates associated with the raster candidate to which the element of interest belongs from among the combinations of raster candidates and allocation candidates narrowed down in step S250 (step S340). Of course, if the element of interest does not belong to the sectioned area A1 or A3 (step S270: NO), this process is not repeated.

  Further, the processes in steps S260 to S340 are repeated for all the blank storage elements belonging to the divided area determined in step S230 (step S350). As is clear from the above description, the most prominent element described above is the process in which the comprehensive evaluation value CE is sequentially calculated for each blank storage element, and the comprehensive evaluation value CE is the smallest value at that time. Is a blank storage element.

  When the comprehensive evaluation value CE is obtained for all blank storage elements in this way (step S350: YES), the focus threshold value is stored in the most prominent element (step S360). A specific example of step S360 is shown in FIG. FIG. 13A shows a state in which the threshold values 0 to 18 are already stored in the storage elements, and the final most prominent element for the threshold value of interest is determined by the above-described processing. The numbers in the grid indicate the stored threshold values. In such a case, in step S360, as shown in FIG. 13B, the focus threshold value of 19 is stored in the most prominent element. It should be noted that the threshold arrangement shown in the figure is randomly arranged for explanation, and does not indicate an arrangement optimized by the above-described method. The same applies to the arrangement of the threshold values shown below.

  When the target threshold value is stored, it is determined whether or not the storage element storing the target threshold value belongs to the divided areas A1 and A3 corresponding to the overlap areas ORA1 and OA3 (step S370). As a result, if the storage element storing the target threshold value belongs to the divided area A1 or A3 (step S370: YES), the leading head identification mask 62 and the trailing head identification mask 63 are updated (step S380).

  The process of step S380 will be specifically described with reference to FIG. FIG. 14A shows the leading head identification mask 62 and the trailing head identification mask 63 before being updated. The hatched pixels of the preceding head identification mask 62 indicate pixels that are turned ON by the preceding head PH. The hatched pixels of the trailing head identification mask 63 are pixels that are turned ON by the trailing head FH.

  On the other hand, FIG. 14B shows the updated preceding head identification mask 62 and succeeding head identification mask 63. In step S380, allocation information when the total evaluation value CE is the smallest of the two pixels of the leading head identification mask 62 and the trailing head identification mask 63 corresponding to the most prominent element (the most prominent element in step S330). The pixel of the identification mask that matches the allocation candidate) is updated as dot ON. FIG. 14B shows a case where the storage element (the most prominent element) that stores the target threshold value in FIG. 13 is associated with the trailing head FH in step S330. A state in which the pixel corresponding to the most prominent element is updated to dot ON is shown.

  Thus, when the leading head identification mask 62 and the trailing head identification mask 63 are updated, or if the storage element storing the target threshold value does not belong to the divided area A1 or A3 (step S370: NO), the prepared threshold values 0 to 89 are used. Until all of the above are stored, the processes of steps S220 to S380 are repeated (step S390). In this manner, the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 are simultaneously generated in a series of processes related to each other. In the final stage of threshold value storage, a part of the processing in steps S220 to S380 can be omitted. For example, when the threshold value 89 is stored in the single area SA1, the processing in step S310 and the like can be omitted.

A-5. Calculation method of comprehensive evaluation value CE:
A method for calculating the comprehensive evaluation value CE described above will be described below. The overall evaluation value CE in the present embodiment is calculated by the following equation (2) using the first evaluation value E1, the second evaluation value E2, and the adjustment coefficients α and β. The adjustment coefficients α and β are coefficients that are experimentally determined based on an actual printing result using the dither mask 61.
CE = α × E1 + β × E2 (2)

  The first evaluation value E1 and the second evaluation value E2 are evaluation values having different concepts, and the storage element areas for which the evaluation values are calculated are also different. FIG. 15 is an explanatory diagram showing ranges of storage elements that are targets for calculating the first evaluation value E1 and the second evaluation value E2. The grid on the left side of FIG. 15 shows all the storage elements of the dither mask 61. The lattices indicated by hatching are already stored elements, and include both storage elements to which the preceding head PH is assigned and storage elements to which the succeeding head FH is assigned. That is, hatching indicates a dot pattern formed by both the leading head PH and the trailing head FH. Such a dot pattern is also called final dot arrangement. Note that a dot pattern formed by only one of the preceding head PH and the following head FH is also referred to as a head group dot arrangement, and in particular, a dot pattern formed only by the preceding head PH is provided by the preceding head group dot arrangement, the following line. A dot pattern formed only by the head FH is also referred to as a trailing head group dot arrangement. The grid filled in black indicates that it is the element of interest selected in step S260. That is, this example is an example when the divided area A3 is selected in step S230.

  The grid in the center of FIG. 15 is a storage of the divided area A4 to which the succeeding head FH is allocated and the divided areas A1 and A3 to which the succeeding head FH may be allocated. Indicates an element. The lattices indicated by hatching indicate storage elements assigned to the succeeding head FH among all the existing storage elements in the divided areas A1 to A4. That is, hatching indicates a dot pattern (following head group dot arrangement) formed by the following head FH. The grid filled in black indicates that it is the element of interest selected in step S260. That is, this example shows a case where allocation to the trailing head FH is selected in step S300 for the element of interest selected in step S260.

  The grid on the right side of FIG. 15 shows storage elements of the divided area A2 to which the preceding head PH is allocated and the divided areas A1 and A3 to which the preceding head PH may be allocated. Show. The lattices indicated by hatching indicate storage elements assigned to the preceding head PH among all the existing storage elements in the divided areas A1 to A4. That is, hatching indicates a dot pattern (preceding head group dot arrangement) formed by the preceding head PH.

  Among the dot arrangements grouped in this way, the first evaluation value E1 is an evaluation value calculated for the final dot arrangement of the divided area to which the element of interest belongs (here, the divided area A3). On the other hand, the second evaluation value E2 is an evaluation value calculated with respect to the allocation candidate head group dot arrangement selected in step S300. If the allocation candidate selected in step S300 is the trailing head FH, as shown in FIG. 15, the second evaluation value E2 is calculated for the trailing head group dot arrangement.

  Details of the first evaluation value E1 will be described below. The first evaluation value E1 is a dot on the printing paper P between the preceding head PH and the succeeding head FH due to the difference between the leading head PH and the succeeding head FH. This is a quantification of a change in the dot coverage when the landing position of is shifted by a predetermined amount in a predetermined direction. The dot coverage is the rate at which dots cover the printing paper P in the overlap area ORA. Hereinafter, the dot coverage is also simply referred to as coverage. Assuming that the target threshold value is stored in the target element, the pixel corresponding to the storage element in which the threshold value has been stored in the previous process is dot-on. Is the final dot arrangement.

  A specific example of the first evaluation value E1 will be described with reference to FIGS. As shown in FIG. 16, the first evaluation value E1 is calculated by assuming N types (N is an integer of 2 or more) of shift patterns (step S311). The shift pattern is a pattern that assumes the shift direction and shift amount of the ink landing position described above, and includes a pattern in which no shift occurs in this embodiment. Such a deviation of the landing position may occur as a deviation in the conveyance direction of the printing paper P due to, for example, the accuracy of the conveyance speed of the printing paper P. Alternatively, it may occur as a deviation in all directions, up, down, left, and right, due to the installation accuracy of the leading head PH and the trailing head FH. Note that the assumed misalignment pattern does not necessarily include a pattern in which no misalignment occurs.

  In this embodiment, in step S311, a pattern 1 indicating a state in which there is no shift and a pattern 2 indicating a state in which the trailing head group dot arrangement is shifted by one pixel in the conveyance direction of the printing paper P are assumed. . A specific example of such a deviation pattern will be shown. FIG. 17A shows a head group dot arrangement when it is assumed that a threshold value is stored in the element of interest exemplified in FIG. Assuming the head group dot arrangement shown in FIG. 17 (a), the final dot arrangement in the divided area A3 in the case of pattern 1 is the same as the preceding head group dot arrangement and the following as shown in FIG. 17 (b). The head group dot arrangement is simply superimposed. The storage elements displayed by single hatching belong to the preceding head group dot arrangement, and the storage elements displayed by cross hatching belong to the subsequent head group dot arrangement. On the other hand, the final dot arrangement in the divided area A3 in the case of the pattern 2 is shifted by one pixel in the conveyance direction of the printing paper P from the subsequent head group dot arrangement shown by cross hatching as shown in FIG. Arrangement.

  In the present embodiment, the number of assumed shift patterns is two, but may be three or more. Also, the assumed direction of misalignment is not limited to the paper transport direction, and may be the direction opposite to the paper transport direction, the direction orthogonal to the paper transport direction, etc., and simultaneously shifted in both the paper transport direction and the paper transport direction. It may be in a state. Further, the assumed shift amount is not limited to an integral multiple of one pixel, and may be 1.5 pixels, 1.25 pixels, or the like. These shift patterns are preferably set from patterns that can actually occur by actually measuring the paper feed accuracy of the printer 20 and the installation accuracy of the print head 80.

  If a plurality of misalignment patterns are assumed, then the dot shape is defined (step S312). The dot shape defined here is the shape of a dot that is actually formed when ink lands on the printing paper P. In the ink jet printer, the ink that has landed on the printing paper P spreads over a predetermined range and is fixed. Further, since ink is ejected while the print head and the print paper P are moved relative to each other, the ink tends to spread in the relative movement direction. The reason for defining the dot shape is to calculate the dot coverage more accurately by reflecting the actual shape of the dot.

  A specific example of the definition of the dot shape is shown in FIG. In this example, the dot shape is defined after converting one pixel into four pixels by performing resolution conversion twice in the paper conveyance direction and the direction orthogonal thereto. Pixels indicated by cross-hatching are pixels in which dots are formed when ink spread does not occur, and correspond to one pixel that is hatched in FIGS. 17 (a) to 17 (c). Pixels displayed by single hatching are pixels in which dots are formed by spreading of ink. By performing resolution conversion in this manner and increasing the resolution of the dot shape, the dot shape can be accurately reflected. Further, it is possible to cope with the case where the assumed shift amount is not an integral multiple of one pixel. Such dot shape definition and resolution conversion are not essential.

  Once the dot shape is defined, the preceding head group dot arrangement and the succeeding head group dot arrangement are superimposed for each deviation pattern assumed in step S311, reflecting the dot shape based on the deviation pattern ( Step S313). FIG. 17E shows the dot arrangement after superposition for pattern 1 and pattern 2. The dot pattern shown in the figure is obtained by replacing each pixel with a defined dot shape with respect to the final dot arrangement shown in FIGS. 17 (b) and 17 (c). The pixels displayed by cross-hatching in the figure are dot patterns that match the final dot arrangement shown in FIGS. 17B and 17C, and the pixels displayed by single hatching are shown in FIG. It corresponds to the pixel displayed by single hatching.

If the preceding head group dot arrangement and the succeeding head group dot arrangement are superimposed, then the coverage Cn is calculated for each of N types of shift patterns (step S314). The coverage Cn can be obtained by the following equation (3).
Cn = number of dots ON in the evaluation area / total number of pixels in the evaluation area × 100 (3)

  For example, in the pattern 1 shown in FIG. 17E, the total number of pixels in the evaluation area A3 is 80, and the number of dots ON that are hatched is 34. C1 has the value 42.5 (= 34/80 × 100). In Pattern 2, since the number of dots ON in the divided area A3 is 36, the coverage C2 is a value of 45.0 (= 36/80 × 100).

  When the coverage Cn is calculated, the first evaluation value E1 is calculated based on these (step S315). In the present embodiment, the first evaluation value E1 is calculated by the following equation (4).

  The first evaluation value E1 calculated in this way indicates the degree of variation in the coverage Cn. The first evaluation value E1 means that the smaller the value, the smaller the variation in the coverage Cn due to the assumed ink landing position deviation. In other words, the smaller the value, the lower the occurrence of density unevenness or the deterioration of graininess in the print image quality due to the ink landing position deviation in the overlap area ORA when the assumed ink landing position deviation occurs. It can be said that.

  Details of the second evaluation value E2 will be described below. The second evaluation value E2 is an evaluation value indicating the dispersibility of dots for the allocation candidate head group dot arrangement selected in step S300, specifically, how far the dots are evenly distributed. It is an evaluation value indicating The head group dot arrangement that is the target of evaluation of the second evaluation value E2 is that the pixel corresponding to the storage element in which the threshold value is stored in the previous process, assuming that the target threshold value is stored in the target element is a dot. This is the head group dot arrangement when turned ON. It is known that a dither mask having blue noise characteristics and green noise characteristics shown in FIG. 18 may be generated in order to form dots in a uniformly dispersed state.

  As the second evaluation value E2, for example, a graininess index can be used. 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. 19A shows an example of VTF. Various formulas have been proposed as empirical formulas for giving such VTF. FIG. 19B 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 by the calculation formula shown in FIG. 19C based on the VTF. 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 second evaluation value E2, the more difficult the dots are visually recognized in the print image quality, and the better this point is.

  As described above, the comprehensive evaluation value CE of the present embodiment has a predetermined point that the variation in the coverage Cn in the overlap area ORA is small and the dot dispersibility in the head group dot arrangement is good. It can be said that this is an evaluation value for achieving both levels.

A-6. effect:
The above-described method for generating the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 includes the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63. And the subsequent head FH are generated simultaneously in a series of steps so as to reflect the difference in which one of them is assigned to the subsequent head FH. Therefore, in the overlap area ORA generated by the generated dither mask 61 The dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 can be generated while controlling the balance of allocation in the dot pattern of each gradation. If halftone processing is performed using the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 generated in this way, the leading head PH and the trailing head FH in the overlap area ORA are changed by gradation change. Can be prevented from changing rapidly. As a result, when the ink landing position shifts, the amount of change in dot density between gradations is reduced, and the occurrence of density unevenness and deterioration in graininess are suppressed, and the print image quality in the overlap area ORA is reduced. Reduction can be suppressed.

  In addition, the above-described generation method of the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 uses the stored elements in the divided areas A1 to A4 when setting the combinations of candidate storage elements and allocation candidates. Narrow down the combinations of storage element candidates and allocation candidates that should store the target threshold value on the condition that the balance, the balance of the stored elements for each raster in the overlap area ORA, and the consistency with the set print ratio. Therefore, the balance of dot arrangement in the overlap area ORA can be controlled with high accuracy. As a result, it is possible to generate the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 that can further reduce the amount of change in dot density when the ink landing position shift occurs. Further, since the combinations of the storage element candidates and the allocation candidates are largely limited by the above conditions, the number of calculation of the comprehensive evaluation value CE can be reduced, and the dither mask 61, the preceding head identification mask 62, and the following head identification mask. The generation process 63 can be made efficient. In addition, since an allowable range is provided for the optimal balance and the like for the above conditions, it is possible to select an optimal comprehensive evaluation value CE from a wide range of combinations as compared to the case where the optimal balance is limited. In addition, the effect of reducing the amount of change in the density of dots when the ink landing position shift occurs can be enhanced.

  Further, the above-described generation method of the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 evaluates the dispersibility of the dots in the allocation candidate head group dot arrangement based on the second evaluation value E2. Since the combination of the storage element candidate and the allocation candidate is evaluated, the dispersibility of dots in the single area SA can be ensured. Therefore, it is possible to generate the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 that can maintain a good print image in the single area SA. Further, storage element candidates are set such that the first evaluation value E1 reduces the degree of variation in the coverage Cn calculated for each assumed shift pattern with respect to the final dot arrangement of the divided area to which the element of interest belongs. Since the combination of the allocation candidate and the allocation candidate is evaluated, it is possible to control so that the amount of change in the density of dots in the overlap region is reduced to a certain extent regardless of which deviation pattern occurs. Accordingly, it is possible to generate the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 that can suppress a decrease in print image quality due to the deviation of the ink landing position in the overlap area ORA.

  In addition, since the above-described generation method of the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 includes a pattern in a state where the ink landing position is not shifted as an assumed shift pattern, the ink Even in the case where the landing position is not shifted, fluctuations in the coverage can be suppressed.

  Further, the generation method of the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 described above does not consider the dot dispersibility for the final dot arrangement in the overlap area ORA, and covers the coverage ratio. Since only the variation in the density is evaluated, the occurrence of density unevenness due to the density of dots can be suppressed to the maximum.

  Specific examples of effects of the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 generated by the above-described generation method will be described with reference to FIGS. FIG. 24A shows the positional relationship between the leading head PH and the trailing head FH of the printer 20. The leading head PH and the trailing head FH are arranged so as to have an overlap area ORA. FIG. 24B shows the dot arrangement around the overlap area ORA when the dither mask is generated by the conventional method. As shown in the figure, when there is no deviation in the ink landing position, the dots are appropriately distributed.

  If this final dot arrangement is separated into the preceding head group dot arrangement and the following head group dot arrangement, it can be seen that the density of dots is generated in the overlap area ORA. This is because the assignment of the leading head PH and the trailing head FH in the overlap area ORA is determined after the dither mask is generated so as to ensure the granularity of the final dot arrangement. As a result, the final dot arrangement in the case where the ink landing position is deviated from the target position (here, deviation of 3 pixels in the paper conveyance direction) is a result of local dot density as shown in the figure. .

  On the other hand, FIG. 24C shows a dot arrangement around the overlap area ORA when the dither mask 61 and the like generated by the method of the present invention described above are generated. As shown in the figure, when there is no deviation in the ink landing position, the dots are appropriately distributed. If this final dot arrangement is separated into the preceding head group dot arrangement and the following head group dot arrangement, it can be seen that the dots are appropriately distributed in any group dot arrangement in the overlap area ORA. . This is a result of controlling the dot arrangement in the overlap area ORA by the comprehensive evaluation value CE by assigning the leading head PH and the trailing head FH simultaneously with the generation of the dither mask 61. As a result, even in the final dot arrangement when the ink landing position is deviated from the target position (here, deviation of 3 pixels in the paper conveyance direction), the occurrence of local dot density is suppressed.

  If the printing result in the overlap area ORA is shown for the evaluation image in FIG. 25A, the conventional method, as shown in FIG. 25B, shows the printing result in which the deterioration of graininess and the occurrence of density unevenness are conspicuous. Become. The overlap area ORA shown in the figure shows the overlap area ORA at the joint between the band J and the band J + 1 shown in FIG. On the other hand, in the present invention, as shown in FIG. 25C, it can be seen that in the overlap area ORA, the deterioration of graininess and the occurrence of density unevenness are greatly suppressed.

B. Variation:
A modification of the above embodiment will be described.
B-1. Modification 1:
In the above-described embodiment, the method of simultaneously generating the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 in a series of steps has been described. However, the leading head identification mask 62 and the trailing head identification mask 63 are shown. Is not essential, as long as the dither mask 61 and the allocation information are generated simultaneously. For example, instead of generating the leading head identification mask 62 and the trailing head identification mask 63, as shown in FIG. 20A, in the storage element area of a size corresponding to a pixel capable of dot formation by the leading head PH. A dither mask 61 including a certain preceding head storage area 61a and a trailing head storage area 61b that is a storage element area having a size corresponding to a pixel capable of dot formation by the trailing head FH may be generated. By doing so, both the preceding head storage area 61a and the succeeding head storage area 61b are provided with the divided areas A1 and A3, respectively. As a result, the dither mask 61 including the allocation information can be generated.

  For example, as shown in FIG. 20B, when the target threshold value is 19 and the most prominent element belongs to the divided area A3 of the trailing head storage area 61b, the leading element in the trailing head storage area 61b The threshold value 19 is stored. At this time, at the dot formation position corresponding to the storage element in which the value 19 is stored, a dot is always formed by the trailing head FH, and no dot is formed by the preceding head PH. A threshold at which no dot is formed, that is, image data, in the storage element of the preceding head storage area 61a that has not been stored and at a position corresponding to the most prominent element in the subsequent head storage area 61b. It is only necessary to store a threshold value (value 99 in the figure) that is always greater than the tone value of.

  An example of how to use the dither mask 61 generated in this way in the halftone process is shown in FIG. FIG. 21A shows the positional relationship between the leading head PH and the trailing head FH. FIG. 21B shows an area of the image data D to be subjected to halftone processing. As shown in the figure, the image data area corresponding to the overlap area ORA1 is referred to as image data D1, D11, D21, and the like. Similarly, image data areas corresponding to the single area SA1 are referred to as D2 and D12, image data areas corresponding to the overlap area ORA2 are referred to as image data D3 and D13, and image data areas corresponding to the single area SA2 are referred to as D4 and D14.

  When halftone processing is performed by applying the dither mask 61 to the image data D, as shown in FIG. 21C, the image data areas corresponding to the overlap areas ORA1 and ORA3 (see FIG. 21). Among them, the image data DR is generated by copying the image data D1, D3, D11, D13) and continuing them in the arrangement direction of the print heads. Then, as shown in FIG. 21D, the arrangement of the leading head storage area 61a and the trailing head storage area 61b is rearranged in accordance with the configuration of the image data DR and the arrangement of the leading head PH and the trailing head FH. The synthesized dither mask 61 is repeatedly applied in the print head arrangement direction to perform halftone processing.

B-2. Modification 2:
In the above-described embodiment, the first evaluation value E1 is calculated by the above-described formula (4). However, the calculation formula for the first evaluation value E1 is not limited to this example. For example, the first evaluation value E1 may be a variance of the coverage Cn, a standard deviation, or the like. Alternatively, it may be a value obtained by summing each difference value of all combinations of two coverage ratios Cn extracted from the coverage ratio Cn calculated for each shift pattern, and each of the difference values is given a predetermined weight. It may be a total value of As long as the degree of variation in the coverage Cn can be evaluated as described above.

  Of course, the first evaluation value E1 is not limited to one that can evaluate the degree of variation in the coverage ratio Cn, but is an evaluation that quantifies the variation in the coverage ratio Cn in the overlap area ORA when the ink landing position shifts. Any value is acceptable. For example, the difference value when the difference value between the coverage ratio Cn calculated for each shift pattern in which the shift has occurred and the coverage ratio Cn calculated based on any pattern in which the shift has not occurred is the difference value or the shift The maximum value of the difference values of all combinations of the two coverage ratios Cn extracted from the coverage ratio Cn calculated for each pattern may be set as the first evaluation value E1. Even in this case, it is possible to generate the dither mask 61 and the like in which the variation of the coverage Cn is suppressed, and thus the effect of suppressing the above-described density unevenness and the deterioration of graininess can be achieved.

B-3. Modification 3:
In the above-described embodiment, in the divided area determination process and the head group determination process, from the viewpoint of dot arrangement balance with a predetermined allowable width, the divided area candidate, raster candidate, and allocation to which the storage element that stores the threshold value of interest belongs. Although the candidate has been determined, an allowable width may not be provided. For example, in the divided area determination process (S230), the divided area may be determined on the condition that the ratio of the stored elements to the number of stored elements for each divided area is closest to the equal state.

  In the head group determination process (step S240), raster narrowing is performed under the condition that the number of stored elements for each raster is closest to an equal state when it is assumed that the threshold value of interest is stored in each raster. As well as Further, the narrowing down of the allocation information is performed when it is assumed that the focused threshold value is stored in each raster and the storage element storing the focused threshold value is allocated to either the preceding head PH or the following head FH. It may be performed on the condition that the allocation of the already stored elements is closest to the allocation ideal value. In this way, the number of times of calculating the comprehensive evaluation value CE is reduced, so that the processing of the dither mask 61 and the like can be speeded up. Of course, depending on the type of conditions, a combination of providing an allowable width and not providing an allowable width may be used.

B-4. Modification 4:
In the above-described embodiment, when generating the dither mask 61 and the like, the second evaluation value E2 is calculated for the allocation candidate head group dot arrangement selected in step S300, but not selected in step S300. The allocation candidate head group dot arrangement may also be included. For example, the second evaluation value E2 is obtained by the following equation (5) using the adjustment coefficients γ, δ, the evaluation value E2ph for the preceding head group dot arrangement, and the evaluation value E2fh for the following head group dot arrangement. Also good. In addition to the head group dot arrangement, the second evaluation value E2 may be calculated for the final dot arrangement.
E2 = γE2ph + δE2fh (5)

B-5. Modification 5:
In the above-described embodiment, the comprehensive evaluation value CE includes the first evaluation value E1 and the second evaluation value E2. However, the second evaluation value E2 is not essential. The dither mask 61 generated only by the evaluation value E1 may be evaluated. Even in this case, the dither mask 61 and the like can be generated so as to reduce the variation in the coverage ratio, so that the effect of suppressing the occurrence of density unevenness and the deterioration of graininess can be achieved to a certain extent. Of course, the second evaluation value E2 is not limited to the dot dispersion index, but may be any person who evaluates the characteristics desired to be included in the dither mask 61. For example, the second evaluation value E2 may be a dot concentration index.

B-6. Modification 6:
In the above-described embodiment, the graininess index is used as the second evaluation value E2, but the second evaluation value E2 may be any value that can evaluate the dispersibility of the dot arrangement. For example, RMS granularity may be used as the second evaluation value E2. 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 set as the second evaluation value E2. This is an evaluation value used in the so-called potential method.

B-7. Modification 7:
In the above-described embodiment, in the head group determination process, the rasters are narrowed down by narrowing the candidate rasters so that the threshold setting ratio for each raster of the divided areas A1 and A3 corresponding to the overlap area ORA is substantially uniform within the allowable range. Although the configuration for ensuring the dispersibility of dots in terms of units has been shown, the second evaluation value E2 reflects the difference in dot occurrence rates of the leading head PH and the trailing head FH in the overlap area ORA and the single area SA. Thus, if an index that can evaluate the dispersibility of dots is used, such dot dispersibility control in raster units is not essential.
B-8. Modification 8:
In the above-described embodiment, all of the prepared threshold values 0 to 89 are stored in the threshold storage area of the dither mask 61 by the method shown in FIGS. 7 and 8, but the threshold values stored by the above method are prepared. May be part of the threshold. For example, the threshold value may be stored by using the above-described method only for an intermediate gradation area where occurrence of density unevenness or deterioration of graininess particularly appears in the overlap area ORA area.

B-9. Modification 9:
In the above-described embodiment, the generation method of the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 used for the halftone process of the line printer, the dither mask 61 generated by the generation method, and the leading head identification Although the configuration of the line printer storing the mask 62 and the trailing head identification mask 63 is shown, the present invention ejects ink while moving the print head relative to the print medium in the main scanning direction and the sub scanning direction. Printing a print image consisting of a single area that completes a raster, which is a row of dots arranged in the main scanning direction, in one main scan, and an overlap area that completes the raster in two consecutive main scans. The present invention can also be applied to a printing apparatus to be performed, a so-called band feed scanning type serial printer.

  The configuration of such a band feed scanning serial printer is shown in FIG. FIG. 22 is a schematic configuration diagram of a printer 420 as a modified example. 22, the same components as those of the printer 20 as the embodiment shown in FIG. 1 are denoted by the same reference numerals as those in FIG. The printer 420 is a serial inkjet printer, and as shown in the figure, a mechanism for transporting the printing paper P by a paper feed motor 474, a mechanism for reciprocating the carriage 480 in the axial direction of the platen 475 by a carriage motor 470, and a carriage A control unit that controls the exchange of signals with the paper feed motor 474, the carriage motor 470, the print head 490, and the operation panel 99 by driving the print head 490 mounted on the 480 to eject ink and forming dots. 30.

  A mechanism for reciprocating the carriage 480 in the axial direction of the platen 475 is an endless drive between the carriage motor 470 and a slide shaft 473 that is laid in parallel with the axis of the platen 475 and slidably holds the carriage 480. A pulley 472 and the like for stretching the belt 471 are configured.

  On the carriage 480, ink cartridges 481 to 484 corresponding to C, M, Y, and K are mounted. In the print head 490 below the carriage 480, nozzle rows corresponding to the above-described color inks are formed. When these ink cartridges 481 to 484 are mounted on the carriage 480 from above, ink can be supplied from each cartridge to the print head 490.

  The printer 420 having the above hardware configuration drives the carriage motor 470 to reciprocate the print head 490 in the main scanning direction with respect to the printing paper P, and drives the paper feed motor 474. Thus, the printing paper 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 480 reciprocally (main scanning) and the movement of paper feeding of the printing medium (sub scanning). Ink dots of appropriate colors are formed at appropriate positions on the paper P. By doing so, the printer 420 can print the color image input from the memory card MC on the printing paper P.

  In this modification, the printer 20 performs printing by so-called band feed scanning. Specifically, as shown in FIG. 23, the print head 490 includes nozzle rows 81 to 84 that eject inks of C, M, Y, and K colors, respectively. The nozzle rows 81 to 84 are arranged along the sub-scanning direction. The print head 490 forms a dot row (also referred to as a raster in the present modification example) in the main scanning direction corresponding to the position by the K-th (K is a positive integer) main scanning. Further, after the K-th main scan, the print head P is transported by a predetermined amount in the sub-scanning direction, and when the print head 490 and the print paper P move relative to each other, the print head 490 performs the (K + 1) -th main scan. A raster corresponding to the relative position of the print head 490 in the (K + 1) th main scan is formed. The same applies to the K + 2th main scan. In the figure, for convenience of display, the print head 490 is displayed so as to move for each main scan, but the print paper P actually moves in the sub-scanning direction.

  In such a print head 490, the relative position of the print head 490 with respect to the print paper P in the continuous K-th and K + 1-th main scans is set so that some of the nozzle rows 81 to 84 overlap in the sub-scanning direction. In the drawing, the upper end nozzle row UN arranged at the front end portion in the sub-scanning direction of the nozzle rows 81 to 84 and the lower end nozzle row LN arranged at the front end portion on the opposite side thereof are shown overlapping. ing. Such an overlapping area is also referred to as an overlap area ORA. A non-overlapping area is also referred to as a single area SA.

  The raster corresponding to the single area SA is formed by one main scan, and the raster in the overlap area ORA is formed by two consecutive main scans (for example, the K-th and K + 1-th).

  In such a printer 420, similar to the printer 20 shown in the embodiment, problems such as density unevenness and deterioration in graininess occur in the joint region between the band K and the band K + 1. If the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 are generated by the method, the same effects as in the embodiment can be obtained. When the dither mask 61 and the like are generated by the same method as in the embodiment, the difference between the preceding head PH and the following head FH in the embodiment is the difference between the even-numbered main scan and the odd-numbered main scan. You can replace it.

B-10. Modification 10:
In the embodiment described above, the printer 20 is configured to execute all of the printing processes shown in FIG. 3. However, when the printing process is performed in a printing system (broadly defined printing apparatus) in which the printer and the computer are connected. For example, all or part of the printing process and the halftone process may be performed by either a computer or a printer. Further, the dither mask 61, the leading head identification mask 62, and the trailing head identification mask 63 may be stored in any of the computer and the printer. Or it is good also as a structure acquired from the node etc. which were connected to the said network via a network for every printing.

  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 and assignment information generation method, a configuration of a printing apparatus that stores the dither mask and assignment information generated by the method, and a half-width using the dither mask and assignment information generated by the method. The present invention can also be realized as a program for causing a computer to perform a tone processing function, a dither mask and allocation information generation program, a storage medium storing the program, and the like.

20 ... Printer 30 ... Control unit 40 ... CPU
41 ... Input unit 42 ... Halftone processing unit 43 ... Assignment processing unit 44 ... Printing unit 51 ... ROM
52 ... RAM
60 ... EEPROM
61 ... Dither mask 61a ... Prior head storage area 61b ... Subsequent head storage area 62 ... Prior head identification mask 63 ... Subsequent head identification mask 71-74 ... Ink cartridge 80 ... Print head 81-84 ... Nozzle array 90 ... Paper feed Mechanism 91 ... paper feed roller 92 ... paper feed motor 93 ... platen 98 ... memory card slot 99 ... operation panel 420 ... printer 470 ... carriage motor 471 ... drive belt 472 ... pulley 473 ... sliding shaft 474 ... paper feed motor 475 ... platen 480 ... Carriage 481-484 ... Ink cartridge 490 ... Print head MC ... Memory card P ... Printing paper ORA, ORA1, ORA2 ... Overlap area SA, SA1, SA2 ... Single area A1-A4 ... Section area PH ... Leading head F ... trailing head UN ... upper end of the nozzle array LN ... lower end of the nozzle row

Claims (16)

A plurality of print heads arranged over a print range, and adjacent print heads of the plurality of print heads are overlapped with each other in an overlap region in the direction of the arrangement, and there is no overlap A dither mask used for halftone processing for printing with a printing apparatus that performs printing by ejecting ink while moving a print medium relative to a print head arranged to have a single region, A dither mask in which a plurality of threshold values constituting the dither mask are stored in each storage element;
A method of generating, with which print head of the adjacent print heads each dot of the overlap region is to be formed, and assigned information assigned for each dot formation position,
An evaluation value for evaluating the dither mask, and preparing a predetermined evaluation value that is different depending on the difference in the allocation information,
The dither mask and the allocation information are simultaneously generated in a series of steps in association with each other by evaluating the dither mask with the predetermined evaluation value so that a difference in the allocation information is reflected. A dither mask and allocation information generation method. A method of generating dither mask and allocation information according to claim 1,
For each raster that is a dot row arranged in a direction orthogonal to the direction of the arrangement in the overlap region, printing that is a ratio occupied by each of the adjacent print heads for the formation of each dot constituting the raster A first step of determining a ratio;
A second step of selecting one of the plurality of threshold values to be stored in a blank storage element that is a storage element that has not yet been stored as the threshold value;
A candidate storage element as a candidate for storing the target threshold value among the blank storage elements based on the determined printing ratio and a predetermined condition regarding the balance of dot arrangement, and the allocation in the candidate storage element A third step of setting a plurality of combinations with the candidates,
For each of the set combinations, a fourth step of calculating the predetermined evaluation value when the target threshold value is stored in a candidate storage element corresponding to the combination;
A fifth step of selecting one combination with the calculated evaluation value most appropriate from the plurality of combinations, and storing the threshold value in a storage element corresponding to the one combination;
A sixth step of updating the allocation information based on the location of the storage element corresponding to the selected one combination and the allocation, and
The predetermined evaluation value is obtained when a dot landing position on the print medium between the adjacent print heads is shifted by a predetermined amount in a predetermined direction due to a difference between the adjacent print heads. Including an evaluation value that quantifies a change in dot coverage indicating the ratio of dots covering the print medium in the overlap region;
A dither mask and allocation information generation method for generating the dither mask and the allocation information by repeating the second to sixth steps a predetermined number of times. A method of generating dither mask and allocation information according to claim 2,
The predetermined condition is that when the target threshold value is stored in one of the blank storage elements,
(1) The storage for each divided area divided by the difference between the single area and the overlap area and the difference in which part of the print head is printed. The ratio of the storage elements in which the threshold is already stored to the number of elements is closest to the uniform state;
(2) For each of the storage elements corresponding to each raster constituting the overlap region, the number of storage elements in which the threshold is already stored is closest to an equal state,
(3) A dither mask and allocation information generation method in which the print ratio of the storage element in which the threshold is already stored for each raster is closest to the predetermined value. A method of generating dither mask and allocation information according to claim 2,
The predetermined condition is that when the target threshold value is stored in one of the blank storage elements,
(1) The storage for each divided area divided by the difference between the single area and the overlap area and the difference in which part of the print head is printed. The ratio of the storage elements in which the threshold is already stored to the number of elements belongs to a predetermined range from an equal state,
(2) For each of the storage elements corresponding to each raster constituting the overlap region, the number of storage elements in which the threshold value has already been stored belongs to a predetermined range from an equal state;
(3) The dither mask and allocation information generation method in which the print ratio of the storage element in which the threshold is already stored for each raster belongs to a predetermined range from the predetermined value. A method for generating dither mask and allocation information according to claim 3 or 4,
The predetermined evaluation value is
A dispersibility evaluation value indicating a dispersibility of a dot arrangement represented by a storage element in which the threshold is already stored in an area where dots are formed by the print head corresponding to the candidate for allocation set in the candidate storage element;
Due to the difference between the adjacent print heads, a plurality of misregistration patterns are assumed in which the landing positions of the dots on the print medium between the adjacent print heads are shifted by a predetermined amount in a predetermined direction. And a method for generating a dither mask and allocation information including the coverage evaluation value obtained by calculating the dot coverage in the overlap region for each of the plurality of types of shift patterns and quantifying the change in the dot coverage.   The dither mask and allocation information generation method according to claim 5, wherein the plurality of types of shift patterns include a state in which the shift does not occur.   The dither mask and allocation information generation method according to claim 5 or 6, wherein the coverage evaluation value is an evaluation value indicating a degree of variation in dot coverage calculated for each of the plurality of types of shift patterns. A single region that completes a raster that is a row of dots arranged in the main scanning direction by ejecting ink while moving the print head relative to the print medium in the main scanning direction and the sub-scanning direction; A dither mask used for halftone processing for printing in a printing apparatus that prints a print image including an overlap region that completes the raster by two consecutive main scans, and forms the dither mask A dither mask in which a plurality of threshold values are stored in each storage element;
A method of generating, in which main scan of the two main scans, each dot of the overlap region is to be formed, and allocation information allocated for each dot formation position,
An evaluation value for evaluating the dither mask, and preparing a predetermined evaluation value that is different depending on the difference in the allocation information,
The dither mask and the allocation information are simultaneously generated in a series of steps in association with each other by evaluating the dither mask with the predetermined evaluation value so that a difference in the allocation information is reflected. A dither mask and allocation information generation method. A method of generating dither mask and allocation information according to claim 8,
A first step of determining, for each raster in the overlap region, a print ratio that is a ratio occupied by each of the two main scans for the formation of each dot constituting the raster;
A second step of selecting one of the plurality of threshold values to be stored in a blank storage element that is a storage element that has not yet been stored as the threshold value;
A candidate storage element as a candidate for storing the target threshold value among the blank storage elements based on the determined printing ratio and a predetermined condition regarding the balance of dot arrangement, and the allocation in the candidate storage element A third step of setting a plurality of combinations with the candidates,
For each of the set combinations, a fourth step of calculating the predetermined evaluation value when the target threshold value is stored in a candidate storage element corresponding to the combination;
A fifth step of selecting one combination with the calculated evaluation value most appropriate from the plurality of combinations, and storing the threshold value in a storage element corresponding to the one combination;
A sixth step of updating the allocation information based on the location of the storage element corresponding to the selected one combination and the allocation, and
The predetermined evaluation value indicates that the dot landing position on the print medium between the adjacent print heads is shifted by a predetermined amount in a predetermined direction due to the difference between the two consecutive main scans. Including an evaluation value that quantifies the change in dot coverage indicating the ratio of dots covering the print medium in the overlap region,
A dither mask and allocation information generation method for generating the dither mask and the allocation information by repeating the second to sixth steps a predetermined number of times. A method of generating dither mask and allocation information according to claim 9,
The predetermined condition is that when the target threshold value is stored in one of the blank storage elements,
(1) Classification according to the difference between the single area and the overlap area and the difference between which main scan is completed in which part of the print head The ratio of storage elements in which the threshold is already stored to the number of storage elements for each area is closest to the equivalent state,
(2) For each of the storage elements corresponding to each raster constituting the overlap region, the number of storage elements in which the threshold is already stored is closest to an equal state,
(3) A dither mask and allocation information generation method in which the print ratio of the storage element in which the threshold is already stored for each raster is closest to the predetermined value. A method of generating dither mask and allocation information according to claim 9,
The predetermined condition is that when the target threshold value is stored in one of the blank storage elements,
(1) Classification according to the difference between the single area and the overlap area and the difference between which main scan is completed in which part of the print head The ratio of storage elements in which the threshold is already stored to the number of storage elements for each area belongs to a predetermined range from an equal state,
(2) For each of the storage elements corresponding to each raster constituting the overlap region, the number of storage elements in which the threshold value has already been stored belongs to a predetermined range from an equal state;
(3) The dither mask and allocation information generation method in which the print ratio of the storage element in which the threshold is already stored for each raster belongs to a predetermined range from the predetermined value. A method for generating dither mask and allocation information according to claim 10 or 11,
The predetermined evaluation value is
A dispersibility evaluation value indicating a dispersibility of a dot arrangement represented by a storage element in which the threshold is already stored in an area where dots are formed in the main scanning corresponding to the allocation candidate set in the candidate storage element;
Due to the difference between the two consecutive main scans, a deviation pattern indicating a state in which the landing positions of dots on the print medium between the two main scans are shifted by a predetermined amount in a predetermined direction Assuming a plurality of types, the dot coverage in the overlap region is calculated for each of the plurality of types of deviation patterns, and a coverage evaluation value obtained by quantifying the change in the dot coverage How information is generated.   The dither mask and allocation information generation method according to claim 12, wherein the plurality of types of shift patterns include a state in which the shift does not occur.   14. The dither mask and allocation information generation method according to claim 12, wherein the coverage evaluation value is an evaluation value indicating a degree of variation in dot coverage calculated for each of the plurality of types of shift patterns.   15. A printing apparatus storing a dither mask and allocation information generated by the method according to claim 1.   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 claims 1 to 14 and allocation information.