JP2012222712A - Generating device of printing data, printer, and printing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that image quality may be deteriorated when printing is performed by the same setting since an element affecting the image quality differs by a size of a printing region in a printing medium.SOLUTION: When a size of a printing region in a printing medium is designated by a size of paper and that of a blank space, for example, a parameter affecting image quality stored in a storing section is changed by the size of the printing region. When image data to be printed is received, the data is generated into dot data showing presence or absence of formation of the printing dot by switching a dither mask, or the like, in accordance with setting based on the parameter so as to perform printing.

Description

  The present invention relates to an apparatus for generating printing data, a printing apparatus, and a method thereof.

  Conventionally, a technique for reproducing a multi-tone image by recording one kind or several kinds of dots on a printing medium is used in a printing apparatus such as a printer. In recent years, this multi-gradation technology has been remarkably developed, and there are about two types of inks of several hues such as cyan (C), magenta (M), yellow (Y) and black (K). By combining dots and controlling their distribution, it is possible to form a so-called photographic image. How to properly control the distribution of dots when trying to reproduce a multi-gradation image with high image quality, with or without dots (such as dot ON / OFF). Is a problem. With the development of technology to analyze the distribution of such dots in the spatial frequency region, at present, in the spatial frequency region, by giving the dot distribution a noise characteristic that reduces the components below a predetermined frequency as much as possible, It is known that image quality can be improved.

  A typical such noise characteristic is a blue noise characteristic. Blue noise means, for example, a characteristic that the spatial frequency of an image in which dots are uniformly formed in order to reproduce an image having a constant gradation value has almost no component below a predetermined frequency. The human eye is sensitive to low frequency components below a certain level, but the visibility of high frequency components is not high. For this reason, an image having such a blue noise characteristic is felt to be smooth and high quality. Patent Document 1 listed below is known as a technique for forming an image having such a blue noise characteristic.

US Pat. No. 5,341,228

  However, since there are a wide variety of printing conditions, it has been found that, depending on the printing conditions, simply forming an image having blue noise characteristics cannot provide the highest image quality. The following reasons can be considered for this. The image quality of the image having the blue noise characteristic is highest when the dots are correctly formed at the dot formation positions obtained by the image processing. In an actual printing apparatus, the dot formation position may not be able to form a dot at the original formation position due to various factors. For example, in an ink jet printer that performs dot formation by ejecting ink droplets from nozzles, the landing positions of the ink droplets differ depending on individual differences for each nozzle. In addition, it is known that in a printer that forms dots while moving a print head that performs dot formation relative to a print medium, for example, printing paper, an error occurs in the dot formation position due to a positioning error of the print head. It has been. As a typical error, there is an error in bidirectional printing in which dots are formed respectively when the print head moves forward and backward. As one of similar errors, an error caused by a multi-pass printing method in which one raster is formed by a plurality of main scans is also known.

  In addition, there is also known a phenomenon in which the landing position of ink droplets, that is, the dot formation position shifts due to the bending of the printing medium, for example, the printing paper absorbing and bending the ink (so-called cockling). Of course, this misalignment in dot formation is not limited to printing devices that use ink droplets, but other types of printing, such as thermal transfer printing devices, thermal sublimation types, or so-called line printers with print heads arranged in the width direction of the paper. Even in the apparatus, when dots are formed in the same region and divided into a plurality of pixel groups, this is a problem that may occur. Due to the presence of such misalignment of dot formation positions, there is still room for further improvement in the technique for realizing printing with high image quality.

  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 data generation device that generates data for printing,
A quality storage unit for storing parameters affecting print quality;
A print area designating part for designating the size of the print area on the print medium;
According to the size of the print area, the quality change unit for changing the stored parameter and the image data to be printed are received, and dot data indicating the presence or absence of the formation of printing dots is received according to the setting based on the parameter. A dot data generation unit to generate;
A data generation apparatus comprising:

  According to this data generation device, the parameter that affects the print quality is changed depending on the size of the print area, and the dot data generation unit determines whether the dot for printing from the image data according to the setting based on the parameter. It is generated in dot data indicating the presence or absence of formation. Therefore, when the print quality is affected by the size of the print area, the print quality can be maintained sufficiently high in an appropriate manner.

[Application Example 2]
A data generation device according to Application Example 1,
The parameter stored in the quality storage unit uses a first dither mask having characteristics corresponding to a dot arrangement giving priority to granularity in a gradation range of a predetermined gradation or less, or dots are formed in adjacent pixels. A parameter for specifying whether to use a second dither mask having a higher probability of being performed than the first dither mask,
The quality changing unit changes the parameter so that the second dither mask is used instead of the first dither mask when the size of the print area on the print medium is equal to or larger than the first predetermined value. Data generator.

  According to the data generation apparatus of the application example 2, the first dither mask having characteristics corresponding to the dot arrangement in which the granularity is prioritized in the gradation range of the predetermined gradation or less depending on the size of the printing area on the printing medium. Instead, print data is generated from the image data using a second dither mask in which the probability of forming dots in adjacent pixels is higher than that of the first dither mask. Therefore, when the print area on the print medium is large, it is possible to generate dot data in which the dot formation position does not shift and the rate at which dots are formed in adjacent pixels does not vary greatly.

[Application Example 3]
A data generation device according to application example 2,
The quality changing unit changes the parameter so that the first dither mask is used when the print area is less than the predetermined value or a second predetermined value smaller than the predetermined value. .
According to this application example, when the print area is less than the second predetermined value, the process is performed using the first dither mask, so that the process giving priority to the graininess can be performed. If the second predetermined value is smaller than the predetermined value, another halftone process may be performed during that time. As another halftone process, a halftone process using a dither mask having an intermediate property between the first dither mask and the second dither mask can be considered.

[Application Example 4]
4. The data generation device according to any one of application examples 1 to 3, further including a print area acquisition unit that acquires the size of the print area.
According to this application example, the size of the print area can be acquired, and it is not necessary to instruct the size of the print area. The size of the print area can be acquired from a variable (member) managed by an operating system, a printer driver, or the like. Of course, a dedicated variable may be provided and managed individually by the application or printer driver. Furthermore, a button for directly specifying the size of the print area may be provided on the operation panel of the printing apparatus, and the button may be obtained from the setting of this button.

[Application Example 5]
The data generation apparatus according to application example 4, wherein the print area acquisition unit acquires the size of the print area from at least one of a print medium size, a print area width, and a margin width in the print medium.
According to this application example, the size of the print area can be easily acquired.

[Application Example 6]
The data generation apparatus according to application example 4, wherein the print area acquisition unit acquires the size of the print area from a member managed by an operating system or a printer driver.
According to this application example, the size of the print medium, the width of the print area, or the margin width of the print medium can be easily obtained by referring to the members managed by the operating system, the printer driver, and the like.

[Application Example 7]
The data generation device according to any one of application examples 2 to 6, wherein the first dither mask is a mask having blue noise characteristics or green noise characteristics.
According to this application example, it is possible to sufficiently increase the dispersibility of dots particularly in a low gradation region.

[Application Example 8]
The data generation device according to any one of Application Example 2 to Application Example 7,
The printing data is printed by forming the dots in a plurality of pixel groups having different printing conditions and performing at least a part of the dot formation by the plurality of pixel groups in a common area. Dot data used for
The first dither mask is a data generation device in which, for each of a plurality of pixel groups, a dot arrangement having a predetermined gradation or less has a blue noise characteristic or a green noise characteristic.

  According to this application example, since the distribution of dots formed in a plurality of pixel groups has a blue noise characteristic or a green noise characteristic, each pixel is formed in a common area where dots are formed together by these pixel groups. Even if the positions of the dots formed in the group are shifted, dot data that can suppress deterioration in image quality can be output.

[Application Example 9]
A printing device for printing on a print medium,
A quality storage unit for storing parameters affecting print quality;
A print area designating part for designating the size of the print area on the print medium;
According to the size of the print area, the quality change unit for changing the stored parameter and the image data to be printed are received, and dot data representing the presence / absence of dot formation for printing is received according to the setting based on the parameter. A dot data generation unit to generate;
And a printing unit that performs printing by forming dots on the print medium using the dot data.

  According to the printing apparatus of this application example, the parameter that affects the print quality is changed depending on the size of the print area, and the dot data generation unit uses the image data for printing according to the setting based on the parameter. It is generated in dot data representing the presence or absence of dot formation. Therefore, when the print quality is affected by the size of the print area, the print quality of the image can be improved appropriately.

Each of the above application examples can also be realized as the following method.
[Application Example 9]
A method for generating data for printing,
Parameters that affect print quality are stored in the storage unit,
Accept the specification of the size of the print area on the print medium,
The stored parameter is changed according to the size of the print area that has received the designation,
A printing data generating method for receiving image data to be printed and generating dot data representing the presence or absence of formation of printing dots in accordance with the setting based on the parameters.

[Application Example 10]
A printing method for printing on a print medium,
Parameters that affect print quality are stored in the storage unit,
Accept the specification of the size of the print area on the print medium,
The stored parameter is changed according to the size of the print area that has received the designation,
Receiving image data to be printed, and generating dot data representing the presence or absence of printing dots according to the settings based on the parameters,
A printing method for performing printing by forming dots on the print medium using the dot data.

1 is a schematic configuration diagram of a printer 20 according to an embodiment of the present invention. It is explanatory drawing which illustrates the nozzle row of the print head 90 in an Example. It is explanatory drawing which shows the variation of the combination of the dot formed at the time of forward movement, and the dot formed at the time of backward movement. 6 is a flowchart illustrating a printing process in the embodiment. It is a flowchart which shows the dot data generation process in an Example. It is explanatory drawing which shows the dot formed at the time of a forward movement and a backward movement, and its combination. It is explanatory drawing which illustrates the case where the shift | offset | difference arises in the dot formation position at the time of a forward movement and a backward movement. It is explanatory drawing which shows the example of a dot arrangement | positioning at the time of using a distributed dither mask, and a pair dot. It is explanatory drawing which shows the adjacent pixels NR and ND with respect to the pixel of interest OJ. It is a graph which shows the relationship between the dot generation rate k and the pair dot generation rate K. It is a graph which illustrates the coverage rate fluctuation | variation when the shift | offset | difference arises in the dot formation position at the time of forward movement and backward movement. Is a graph showing the relationship between deviation from k ² shift amount paired dot generation rate of the dot formation positions in units of pixels. It is a flowchart which shows the production | generation method of a pair pixel control mask. It is explanatory drawing which shows the relationship between the gradation value S and the pair dot target value M. FIG. It is explanatory drawing which shows an example of the sensitivity characteristic VTF (Visual Transfer Function). It is explanatory drawing which shows the relationship between the focused pixel in another printing method, and an adjacent pixel. It is explanatory drawing which illustrates the distribution of the dot at the time of forward movement, the distribution of the dot at the time of backward movement, and the distribution of the dot at the time of composition. It is a flowchart which shows the production | generation process of the dither mask in 2nd Example. It is explanatory drawing explaining the arrangement | positioning of the storage element in the selection process of a focused pixel. It is a flowchart which shows a 1st dither mask evaluation process. It is explanatory drawing which shows an example of a blue noise characteristic and a green noise characteristic.

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

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

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

  As shown in FIG. 2, the print head 90 is provided with a nozzle row in which a plurality of nozzles that eject ink droplets are arranged in the sub-scanning direction corresponding to each ink color. The nozzle array pitch R in the nozzle row is an integral multiple of the dot formation pitch (raster interval r). During printing, the paper is relatively moved in the sub-scanning direction with respect to the print head 90 for each main scanning. By repeating main scanning a plurality of times while moving, printing by so-called interlace is performed to complete each raster. In addition, so-called overlap printing, in which one raster is completed by a plurality of main scans, can also be performed. For this reason, by combining interlacing and overlapping, each raster or each column is unified with dots formed either when the print head 90 moves forward or backward, so-called alternate dot arrangement of columns ( Printing can be performed in FIG. 3 (A)) or in raster alternate dot arrangement (FIG. 3 (B)). Alternatively, it is also possible to print the dots formed during forward movement and the dots formed during backward movement in a so-called slashed dot arrangement (FIG. 3C) in which each raster and each column are arranged alternately. It is. In the first embodiment, printing is performed using the slashed dot arrangement shown in FIG. Since a method for realizing a desired dot arrangement using such interlace or overlap is well known, detailed description thereof is omitted.

  The control unit 30 that executes printing by controlling the print head 90, the carriage motor 70, the paper feed motor 74, and the like is configured by connecting a CPU 40, a ROM 51, a RAM 52, and an EEPROM 60 to each other via a bus. The control unit 30 develops a program stored in the ROM 51 or the EEPROM 60 in the RAM 52 and executes it to control the overall operation of the printer 20 and also functions as a dot data generation unit 42 and a printing unit 43 in claims. . Details of these functional units will be described later.

  Two dither masks 61 and 62 are stored in the EEPROM 60. The first and second dither masks 61 and 62 used in the present embodiment have a size of 64 × 64, and threshold values from 0 to 256 are stored in 4096 storage elements. Each threshold value is used in halftone processing described later. The first and second dither masks 61 and 62 are determined so that the arrangement of the threshold values has a characteristic that emphasizes the dispersibility of dots basically. The characteristics of the first and second dither masks 61 and 62 used in this embodiment will be described in detail later. The first dither mask 61 is a so-called blue noise mask, and the second dither mask 62 is used. Is configured as a dither mask that realizes high image quality by suppressing deterioration of image quality particularly in bidirectional printing.

  In this embodiment, printing is performed by the printer 20 alone. A memory card slot 98 is connected to the control unit 30, and image data ORG is read from the memory card MC inserted into the memory card slot 98 and input. In this embodiment, the image data ORG input from the memory card MC is data composed of three color components of red (R), green (G), and blue (B). The printer 20 performs printing using the image ORG in the memory card MC. Of course, it is possible to adopt a configuration in which an external computer is connected using a USB or a LAN, halftone processing is performed on the computer side, and the result is received and printed by the printer 20.

  The printer 20 having the above hardware configuration drives the carriage motor 70 to reciprocate the print head 90 in the main scanning direction with respect to the printing paper P, and drives the paper feed motor 74. 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 80 in the reciprocating motion (main scanning) and the paper feeding movement of the printing medium (sub scanning). Ink dots of appropriate colors are formed at appropriate positions on the paper P. By doing so, the printer 20 can print the color image input from the memory card MC on the printing paper P.

A-2. Printing process:
A printing process in the printer 20 will be described. 4A and 4B are flowcharts showing the flow of the printing process 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 the printing process 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 (step S110).

  When the image data ORG is input, a process for accepting the setting of the paper to be printed is performed (step S115). The paper setting is a process in which the user sets the paper size using the paper setting dialog box PDB displayed on the operation panel 99. Of course, a predetermined paper size (for example, A4) is set by default, and the default paper size is used unless otherwise specified by the user. The paper size setting is stored in the variable PageSize and is referred to in later processing. After setting the paper size, the CPU 40 refers to a look-up table (not shown) stored in the EEPROM 60 and performs a color conversion process on the image data ORG from the RGB format to the CMYKLcLm format (step S120). ).

  Once the color conversion process is performed, the paper size is then determined (step S125). The paper size is determined by referring to the previously stored variable PageSize. As a result of the determination of the paper size, the CPU 40 sets a value 1 to the image quality parameter DF if the paper size is A4 size or smaller, for example, a photo size (cabinet plate, L size, 2L size, etc.) (step S130). On the other hand, if the paper size is A4 or larger, a value 2 is set in the image quality parameter DF (step S135). Thereafter, the CPU 40 performs dot data generation processing (step S140), and then overlaps the dot pattern data to be printed in one main scanning unit in accordance with the nozzle arrangement of the printer 20 and the paper feed amount. Then, interlace processing is performed (step S150). When the overlap and interlace processing is performed, the CPU 40 performs printing by driving the print head 90, the carriage motor 70, the motor 74, and the like as processing of the printing unit 43 (step S160). Thus, the printing process is completed.

  The dot data generation process (step S140) described above is a so-called halftone process, and is a process for generating ON / OFF (dot formation / no dot) data of printing dots from image data after color conversion. is there. This process will be described with reference to FIG. 4B. When the dot data generation process (step S14) is started, the CPU 40 first refers to the image quality parameter DF (step S141). If the image quality parameter DF is 1, the first halftone process (step S143) is performed. If the paper size is A4 or larger, the second halftone process (step S145) is performed. In both the first and second halftone processes, the CPU 40 performs a halftone process for converting the image data into dot data of each color as the process of the dot data generation unit 42, but the processing contents are different. In the present embodiment, the first and second halftone processes are performed using a dither method. That is, the input data is compared with the threshold value stored in the storage element at the position corresponding to the input data among the plurality of threshold values constituting the first dither mask 61 or the second dither mask 62, and the input data Is greater than the threshold value, it is determined that a dot is to be formed (dot O), and if the input data is equal to or less than the threshold value, it is determined that no dot is to be formed (dot OFF). The first and second dither masks 61 and 62 used in this process are repeatedly applied in the main scanning direction and the sub-scanning direction to the respective input data arranged in the main scanning direction and the sub-scanning direction. The first and second halftone processes in this embodiment are controlled so that the generated dot data has a predetermined characteristic. The contents of this control depend on the properties of the first and second dither masks 61 and 62. In particular, the characteristics of the second dither mask 62 will be described later. The first and second halftone processes are not limited to the dot ON / OFF binarization process, but may be a multi-value process such as ON / OFF of large dots and small dots. Further, the image data to be subjected to the halftone process may be subjected to image processing such as resolution conversion processing or smoothing processing.

  When such printing processing (FIGS. 4A and 4B) is executed, dots are formed as a result using either the first dither mask 61 or the second dither mask 62 according to the size of the paper. Will be. Therefore, the arrangement of dots formed by the processing of this embodiment will be described next. As is clear from the above description, the printer 20 has a print head at a plurality of different timings (ie, forward and backward movements) while changing the ink ejection position with respect to the print medium in a common print area of the print medium. The ink is ejected from the ink to form dots, and the dot formed by forward movement (hereinafter also referred to as forward movement dot) and the dot formed by backward movement (hereinafter also referred to as backward movement dot) are combined with each other. The printed image is output. In the first embodiment, since the dot arrangement is a cross arrangement (FIG. 3C), the dots formed when the print head 90 moves forward are hatched in FIG. 5A. The dots formed at the pixel positions of the staggered array at the time of the backward movement of the print head 90 are 1 in the column direction from the dot position at the time of forward movement as shown by shading in FIG. It is formed at the pixel position of the staggered arrangement shifted by the pixel. A set of pixels corresponding to dots formed during forward movement is referred to as a first pixel group, and a set of pixels corresponding to dots formed during backward movement is referred to as a second pixel group. In FIGS. 5A and 5B, the actually formed dots are indicated by “●” marks and hatched “◯” marks, respectively. The dot size is usually set larger than the diagonal size of the pixels so that the print density can be 100% covered with the maximum density even if there is some deviation in the dot formation position. As shown in FIG. 5C, the printed image is a combination of dots formed in each of the first and second pixel groups.

  Since the printing conditions for dot formation differ between the forward movement and the backward movement, the actually formed dots may be different from those in FIG. For example, assuming that the dot formation position during forward movement is shifted by about one pixel in the raster direction (main scanning direction) with respect to the dot formation position during backward movement, in the example shown in FIG. As shown in FIG. 6A, as a result of the dots formed during forward movement being displaced in the main scanning direction, the overlapping area of the dots increases. Further, as shown in FIG. 6B, in this example, when the shift amount increases to 2, the overlapping area further increases. As can be seen from FIG. 5C, there is very little overlap between dots if there is no misalignment. This is because the dither mask having the blue noise characteristic tries to arrange the dots as far apart as possible. On the other hand, in the actual printing, when a positional deviation occurs in the dot formation position, as shown in FIGS. 6A and 6B, the first pixel group in which dots are formed during forward movement is used. The overlapping amount of dots belonging to the second pixel group in which dots are formed during backward movement increases. As the dot overlap amount increases, the coverage, which is the ratio of dots covering the printing paper P, varies. In addition, if there is no deviation in the dot formation position, it is possible that dots that were not adjacent to each other may be formed in adjacent positions due to the deviation in the dot formation position. In this case, the coverage does not change, but the feeling of graininess changes because the dots are close to each other.

A-3. Halftone processing:
Based on the above points, the characteristics of the halftone process in the first embodiment will be described. In the first embodiment, the first or second halftone process (step S143 or S145) is performed according to the set paper size. First, the first halftone process shown as step S143 in FIG. 4B will be described. In the first halftone process, the gradation values of the pixels belonging to the first pixel group and the pixels belonging to the second pixel group are compared with the first dither mask 61 stored in the EEPROM 60 to thereby obtain the respective pixels. Whether or not to form a dot at a position is determined. Data indicating ON / OFF of the determined dot is referred to as dot data. Since the dither method is a well-known technique, a detailed description thereof will be omitted, but the first dither mask 61 used in the first halftone process is a so-called blue noise mask, and the gradation value of the image data and the first dither mask are used. By sequentially comparing the threshold value of the corresponding position of the dither mask 61, ON / OFF of the dot is determined.

  The blue noise mask is a dither mask in which thresholds are arranged so that the arrangement of dots generated thereby has a blue noise characteristic. The blue noise characteristic is one of the noise characteristics in which the distribution of dots formed has a peak in the high frequency side of the low frequency region below the predetermined spatial frequency in the spatial frequency region. Has excellent properties. Dot data is generated by the first halftone process using the blue noise mask. As a result, if the paper size is less than A4, for example, a photo size (L size), dot data for high-quality printing with excellent graininess is generated with priority on dot dispersibility. In the first halftone process, the first and second pixel groups are compared with the threshold value of the dither mask without particular distinction.

  Next, the second halftone process will be described. Also in the second halftone process (step S145), dot data is generated by comparing the dither mask thresholds, but in the second halftone process, the second dither mask 62 is used. As described above, the second dither mask 62 is set to have high dispersibility, so that the dot arrangement is sparse in the region where the image density is low. From the viewpoint of dispersibility, dots are hardly arranged at two pixels adjacent to each other vertically or horizontally. This state is shown in FIG. FIG. 7A illustrates an 8 × 8 region, and illustrates a case where the gradation value of the image is 26/255 uniformly. In this case, dots are formed in about 10% of the pixels, that is, about 6 pixels in the 8 × 8 region.

  On the other hand, in the second halftone process (step S145) in the first embodiment, the threshold value of the second dither mask 62 is set so that dots are arranged in adjacent pixels with a significant probability. Is set. FIG. 7B shows an example in which dots formed together in adjacent pixels are formed. In the first embodiment, such a state that dots are formed in adjacent pixels occurs with a significant probability even in a region where the gradation value of the image is low (for example, a region where the gradation value is 1 to 127/255). Thus, the second dither mask 62 is made.

Here, the significant probability is a probability set as follows. In the second dither mask 62 used in the first embodiment, when the gradation value of the image data is in the range of 0 to 127/255, dots are arranged in each pixel belonging to the first and second pixel groups. Where k (0 ≦ k ≦ 1) is a pixel adjacent to the right in the raster direction (main scanning direction) or a pixel adjacent in the column direction (sub-scanning direction) of one pixel in which dots are formed. The probability K of forming crab dots is about 0.8 × k ² , respectively.

  Of the pixels adjacent to one pixel of interest, those in which the group in which the dots are formed are different are hereinafter referred to as “adjacent pixels”. There are four pixels adjacent to the pixel of interest in the vertical and horizontal directions in the arrangement shown in FIG. 3C. A large shift in the dot formation position occurs between the dot formed during the forward movement and the dot formed during the backward movement. Therefore, the probability of dot generation is adjusted not only for whether or not they are adjacent to each other, but also for pixels that are adjacent and belong to different pixel groups. In the first embodiment, the dots formed at the time of forward movement and the dots formed at the time of backward movement are staggered as shown in FIG. Pixels belonging to the pixel group are present at four locations on the top, bottom, left and right of the target pixel. In the present embodiment, only the pixels adjacent to the right in the raster direction (main scanning direction) and the column direction (sub-scanning direction) are set as “adjacent pixels” for the pixel of interest. This is because it is only necessary to count bare dots (dots formed on both of the adjacent pixels) in consideration of only one of the adjacent pixels that are point-symmetric with respect to the target pixel. For all the pixels that form an image, if you move the pixel of interest sequentially from the upper left to the lower right of the image, and count only one of the adjacent pixels that are point-symmetric, all pairs will not overlap. Can count dots.

  In FIG. 8A, when the position of the pixel of interest OJ is (0, 0) and the right direction of the main scanning and the downward direction of the sub scanning are positive, the position (1, 0) is the adjacent pixel NR on the right side and the position ( 0, 1) indicates that it is the lower adjacent pixel ND. When specifying the relationship between the pixel of interest OJ and any one of the adjacent pixels NR and ND, these are collectively referred to as “pair pixels”. In the first embodiment, as described above, the adjacent pixels constituting the pair pixel together with the target pixel are limited to the pixels NR and ND on the right or lower side of the target pixel OJ. The number of paired dots can be counted only for the pixels. In FIG. 8A, the pair pixel is limited to the pixel adjacent to the pixel of interest, but the pixel that considers the occurrence probability as the pair pixel is not necessarily limited to the adjacent pixel. As shown in FIGS. 8B and 8C, even a pixel separated from the target pixel can be treated as an adjacent pixel, and a detailed description of such a case will be described later.

The probability that a dot is formed in a pair pixel will be described. Here, the gradation value is treated as corresponding to the probability that the dot is turned on (formed). If the image ORG to be halftone processed is a uniform image having a gradation value of 26/255, the number of dots arranged is about 1 in 10 pixels (k = 0.1). On the other hand, the probability K that dots are formed in the paired pixels is about K = 0.8 × k ² ≈0.008 in the dither mask of the first embodiment. In a conventional dither mask with high dispersibility, priority is given to dot dispersibility in a low density region, and dots are formed in paired pixels that are adjacent pixels, and the probability is as close as possible to zero. In fact, in a dither mask having characteristics known as a blue noise mask, no example was found in which dots were formed on paired pixels with a gradation value of 26/255.

On the other hand, in the second halftone process of the first embodiment, the dot value is 0 to 127/255, that is, the dot formation probability k is in the range of about 0 to 0.5. The probability K of formation is about 0.8 × k ² . That is, for example, if the gradation value is 52/255 (k≈0.2), the probability K that dots are formed on the paired pixels is 0.032, that is, about 3 sets per 100 paired pixels. Dots are formed at a rate.

  FIG. 9 schematically shows the rate at which dots are formed in a pair of pixels. In FIG. 9, the horizontal axis represents the probability that dots are formed in a pixel and corresponds to the gradation value of the image. In addition, the vertical axis in FIG. 9 indicates the rate at which dots are formed in the paired pixels. In FIG. 9, a solid line JD1 shows a case where halftone processing is performed using the dither mask of the present embodiment, and an alternate long and short dash line BN1 uses a blue noise mask as in the first halftone processing. A case where halftone processing is performed is shown. A broken line WN1 indicates a case where halftone processing is performed using a white noise mask. Here, the white noise mask is set so that the mask size is sufficiently large, and each threshold value of the dither mask is set with a random number so that a result equivalent to the random dither method in which the threshold value is generated with a random number every time can be obtained. To the dither mask. The blue noise mask exhibits blue noise characteristics that do not include low-frequency components, whereas the white noise mask exhibits white noise characteristics that evenly include low-frequency components to high-frequency components.

As shown in the figure, when the blue noise mask is used, in the region where the gradation value of the image is low (gradation value 0 to 51, dot generation probability k = 0 to 0.2), both of the pair pixels are dotted. The probability of forming is almost zero. In contrast, in the case of using a white noise mask, since the formation position of dots random, the probability that relative probability k of formation of dots, paired pixel dot is formed is substantially matched to the k ² ing. With respect to these characteristics, the dither mask adopted in this embodiment is a distributed dither mask, but the probability K that dots are formed on the paired pixels is the gradation value as shown by the solid line JD1. In the range of 0 to 127 (dot generation probability k = 0 to 0.5), it is approximately 0.8 × k ² . In other words, the dither mask used in this example shows the dispersibility close to that of the blue noise mask for the distribution of dots to be formed, and the white noise mask for the probability K that both dots are formed on the paired pixels. When it is close, it shows the characteristic. A method of creating a distributed dither mask with an increased dot formation rate in such a pair of pixels will be described later.

A-4. Effects of the embodiment:
In the printer 20 of the first embodiment having the above configuration, the image data ORG is received, and the control unit 30 performs the processing shown in FIGS. 4A and 4B to print an image on the printing paper P. At this time, when the printing paper P is less than A4 size, for example, a photograph size (L size, etc.), halftone processing using the first dither mask 61 having blue noise characteristics is performed to generate dot data. On the other hand, if the size of the printing paper P is A4 or larger, dot data is generated by performing halftone processing using the second dither mask 62 that increases the probability that dots are formed in the paired pixels. , Both are finally converted into dot distributions. As a result, when the paper size is small, an image having excellent graininess can be formed particularly in a low gradation region in which the dot distribution is sparse. Further, since the second dither mask 62 is used when the paper size is large, even if there is a shift in the dot formation position due to bidirectional printing, the pair dot formation ratio in the gradation range 0 to 127 varies. The appearance quality of the A4 size image, which is small and less likely to cause density unevenness and is often viewed farther than the photo size image, can be improved. I will supplement this point.

  This is because, in the image processed by the second halftone process, the pixels of the first pixel group to which the dots formed when the print head 90 moves forward and the second to which the dots formed at the time of backward movement belong. The probability that dots are formed in the pixels adjacent to the pixels in the pixel group, that is, the paired pixels, is set to be higher than that in the blue noise mask. This is because the image quality is deteriorated, particularly, the density unevenness is hardly generated even when the image is generated. This point will be described with reference to FIG.

  FIG. 10 is a graph showing a simulation result of the variation in coverage when the image data ORG having a gradation value with a dot formation ratio of 96/255 is processed. In the figure, the horizontal axis shows the amount of deviation of the dot formation position during forward movement and backward movement in units of pixels, and the vertical axis shows the coverage rate variation rate. In the graph of FIG. 10, the solid line JE1 is the case where the second dither mask 62 of the first embodiment is used, and the broken line BB1 is a typical blue noise mask created so that dots are generated as discretely as possible. The case where is used is shown respectively. Here, the coverage means the ratio of the formed dots covering the paper P, and the fluctuation of the coverage is the ratio of the dots covering the printing paper P when the original dot formation position is not displaced. As a reference, it means the degree to which the dot formation position has changed, so that the dot overlap occurs and the ratio of covering the paper changes.

  In FIG. 10, the dot size is set to be slightly larger than the pixel size in order to approximate the actual printing situation with the printer. For this reason, even if the dots do not overlap, if the dots are in contact with each other, dot overlap occurs, and the coverage is reduced. Since a typical blue noise mask tries to disperse the dots as far as possible from each other, contact between dots without any deviation, that is, a factor of lowering the coverage rate is minimized. Therefore, when a deviation occurs in the dot formation position in the actual printer 20, for example, during reciprocating printing, the dot formation position is deviated from the optimal arrangement, the contact and overlap between the dots increase, and the coverage rate generally decreases. When the data of the same gradation value is printed, if the coverage ratio fluctuates, the image becomes uneven and the image quality deteriorates. Such unevenness in image quality due to the variation in coverage is particularly noticeable when printed on a large size printing paper. This is because a printed matter with a large size is usually viewed at a distance from a printed matter with a photographic size, and when viewed at a distance, low-frequency printing irregularities are easily noticed.

  As shown in FIG. 10, when the second dither mask 62 of this embodiment is used, even if the dot formation position is deviated between the forward movement and the backward movement, an ordinary distributed dither mask is used. It can be seen that the variation in the coverage is less likely to occur than when it is used. In FIG. 10, when the shift amount Δd of the reverse printing position relative to the forward printing position is an odd number (Δd = 1, 3,...), Even when the pixel is a unit (Δd = 2, 4,...). ..) The rate of decrease in the coverage rate increases, and there is a fluctuation with the shift amount 2 as a cycle. The variation of the deviation amount in two cycles occurs when printing with the slash arrangement shown in FIG. 3 (C), when the horizontal deviation amount is an odd number, dots formed during forward movement and dots formed during backward movement. This is because the positions of are completely overlapped. For this reason, in the simulation performed on the assumption that all the dot formation positions are shifted in the same manner by the reciprocation without considering the shift of the dot formation positions due to other factors, when the horizontal shift amount is an odd number, FIG. As shown in the above, a decrease in the coverage rate becomes obvious. In the actual printer 20, since a small positional deviation in units of pixels is superimposed on the positional deviation of dots in reciprocating printing, the variation in coverage shown in FIG. 10 is flattened. The change in the coverage ratio when the second dither mask 62 of this embodiment is used becomes even more flat at the solid line JF1 in FIG. On the other hand, the fluctuation in the case of using the distributed dither mask is slightly flattened from the broken line BB1 in FIG. 10, but cannot be completely eliminated, and the fluctuation in the coverage ratio remains.

Therefore, in the printer 20 of the present embodiment, when dot data is generated using the second dither mask 62, the image quality with respect to the deviation of the dot formation position during the reciprocating motion is higher than that using the conventional distributed dither mask. Can be suppressed, and in reality, high print quality can be realized. In addition, even when a dot formation position is shifted while printing an image having a low gradation value, the number of paired dot formations does not vary greatly. This is originally a ratio of formation of paired dots in the low tone area, because close to the ratio k ² caused when taking a random dot placement. As described with reference to FIG. 9, if the probability K that dots are formed on the paired pixels in the low gradation region is small or substantially zero as in the blue noise mask, the dot is displayed in the low gradation region. If there is a large shift in the dot formation position between the forward and backward movements or the dot formation position after sub-scanning, the dots that were not supposed to be paired dots are adjacent to each other. Or it will overlap, the fluctuation | variation of a coverage will arise, and a density nonuniformity will generate | occur | produce in an image. In the second halftone process using the second dither mask 62 of this embodiment, the probability that dots are originally formed on the paired pixels is increased. Therefore, even if the dot formation position is shifted, the paired pixels The probability that dots are formed in both of them does not vary so much. For this reason, even if there is a deviation in the dot formation position between forward movement and backward movement, it is not said that uneven density is perceived.

The reason why the paired dot formation ratio is made closer to k ² is based on the following new knowledge. In other words, even if the dot spacing is dispersed and arranged as far as possible with a blue noise mask, etc., the dot formation position of a specific pixel group shifts, and if the shift amount becomes sufficiently large, dots in a specific direction are adjacent to each other. Thus, the knowledge that the probability of pair dots converges to k ² was obtained. When checking actual blue noise mask, as shown in FIG. 11, when the shift amount is greater than or equal to 5 pixels from the 4, the incidence of Bae Adotto was found to converge substantially constant value k ^2. This is because, when the shift amount is large, two pixels that are originally separated from each other are adjacent to each other. If the distance between the two pixels is sufficiently large, the correlation between the presence or absence of dot formation in both pixels decreases, so the probability that dots are simultaneously formed in both pixels is simply the tone value of both (the probability of dot formation) k) is a value ^{k 2} obtained by multiplying. Therefore, if the pair dot occurrence rate in a state where there is no deviation is made close to k ² in advance, the pair dot occurrence rate does not change much when any deviation occurs, and the variation in coverage can be suppressed.

In the first embodiment, the probability K that both dots are formed on the paired pixels is
K = 0.8 × k ²
It is said. In this equation, coefficients are multiplied by the k ² is for adjusting the probability of pair dots, meaning to say that if the coefficient is 0.8, the incidence of Beadotto is suppressed 80% is doing. Of course, the coefficient can be appropriately set as long as it is in the range of, for example, about 0.6 to 1.4. If the coefficient is in the range of 0.8 to 1.2, it is possible to suitably suppress the variation in the probability of occurrence of paired dots with respect to the deviation of the dot formation position, and the closer the coefficient is to the value 1.0, the generation of bare dots. This is desirable from the viewpoint of suppressing the fluctuation of probability. Further, if priority is given to the dispersibility of dots in the low gradation region, the coefficient can be adjusted to a value of 0.8 or less, for example, about 0.6 to 0.8.

B. Dither mask generation method:
The dither mask used in the first embodiment described above was generated by the following method. FIG. 12 is a flowchart illustrating an example of a dither mask generation method used in the first embodiment. In this embodiment, prepared blue noise mask, from the blue noise mask to produce a dither mask close the probability that both dots are formed in the pair of pixels to K ^2. The generated dither mask is hereinafter referred to as a “pair pixel control mask”. The dither mask being created is referred to as a “working mask”.

  When generating a pair pixel control mask, a blue noise mask is first prepared (step S200). In this example, a 64 × 64 blue noise mask was used. In the blue noise mask of this example, 255 threshold values from 0 to 254 are stored in a matrix having a size of 64 × 64. Next, a process of counting the number of pair dots for each gradation value over the entire gradation range is performed for the current working mask (step S210). Specifically, this process is a process of counting the right adjacent pair dot number RPD [1, 2,... 127] and the lower adjacent pair dot number UPD [1, 2,. is there. In the following description, when parentheses are used as in (S), the value at the gradation value S is indicated, and when [] is used as in [a,. It is assumed that the sequence from a to x is represented. Further, the arrangement from the gradation range a to x is expressed as [a: x].

  Since all the threshold values are known for the working mask, the dot formation position at each gradation value can be examined in the gradation value range of 1 to 127/255. For this reason, it is easy to count the right adjacent pair dot number RPD (S) and the lower adjacent pair dot number UPD (S) for each gradation value S. Here, the count of the number of paired dots is limited to the gradation values of 1 to 127/255. The paired pixel control mask used in the first embodiment, that is, the paired dots in the gradation range of 1 to 127/255. This is to generate a mask having a predetermined characteristic of the occurrence probability. As the gradation value S increases, the number of pair dots also approaches the probability of occurrence here in the blue noise mask, so instead of counting the number of adjacent pair dots for the entire range, gradation values 1 to 127/255 As described above with reference to FIG. 9, it is only necessary to adjust the probability of occurrence of paired dots within this range. Of course, the method described below is also applicable to the case where the number of pair dots is counted and the probability of occurrence is adjusted for the entire gradation range.

In step S210, after counting the right adjacent pair dot number RPD [1: 127] and the lower adjacent pair dot number UPD [1: 127] in a predetermined gradation range (here, 1 to 127/255), each gradation value is counted. It is determined whether the number of paired dots for each S is within the target range M (S) (step S220). Here, the target range M (S) is a range set as follows. If the dither mask has white noise characteristics, the dots are generated randomly. If the probability that a dot is formed in one pixel is k, the right or lower adjacent pixel The probability that dots are formed (this is called the occurrence probability of pair dots) is k ² , respectively. When the gradation value of the image is 1,
k = 0.00392156 (= 1/255)
And the probability of occurrence of paired dots is
k ² = 0.0000154
It becomes. Therefore, when it is assumed that dots are randomly formed, a value H (hereinafter referred to as a predicted value) H predicted that a pair pixel exists in 64 × 64 pixels is:
H = k ² × 4096 = 0.126≈0
It is. This calculation is repeated in advance in the range of gradation values 1 to 127/255 to obtain a theoretical predicted value H [1: 127] of a paired dot, and this is multiplied by a coefficient of 0.8 for each gradation. The target value m [1: 127] of the paired dot in the value S is obtained in advance. In the present embodiment, the target value m (S) has a width of ± 20%, and this is referred to as a target range M (S).

  FIG. 13 shows the predicted value H [1:32] of the paired dots and the target value m [1:32] when the gradation value S is 1-32. As shown in the figure, the gradation value S = 10, the prediction value H (10) = 6, the target value m (10) = 5, the gradation value S = 20, the prediction value H (20) = 25, the target value. It can be seen that m (20) = 20.

  In step S220, the theoretical pair dot target range M [1: 127] thus obtained is compared with the right adjacent pair dot number RPD [1: 127] and the lower adjacent pair dot number UPD [1: 127]. To do. As a result of comparison, if it cannot be determined that both the paired dot numbers RPD [1: 127] and URD [1: 127] are within the target range M [1: 127], then the threshold value in the work mask is set. Among them, an appropriate number of threshold values (for example, two threshold values) are randomly replaced (step S230). Since the pixel groups are randomly switched, the thresholds corresponding to the same pixel group may be interchanged, or may be interchanged between different pixel groups.

  When the threshold value in the work mask is changed, the number of pair dots in each gradation value changes, so the number of pair dots is corrected by changing the threshold value (step S240). Since the number of paired dots changes only within the range of gradation values corresponding to the replaced threshold value, it is not counted again in the gradation range of 1 to 127/255. For example, the threshold value p and the threshold value q ( If p <q) is exchanged, only the right adjacent pair dot number RPD [p: q] and the lower adjacent pair dot number UPD [p: q] may be recounted. Although the threshold value to be replaced is selected at random, since the generation characteristics of the paired dots are to be adjusted in the range of gradation values 1 to 127/255, at least one of the threshold values to be replaced is within this range. It is desirable to use a threshold value.

The number of pair dots thus counted is checked, and then it is determined whether or not the pair dot characteristics have been improved (step S250). Here, whether or not the paired dot characteristics are improved is determined as follows.
(A) If the right and lower adjacent pair dot numbers RPD [p: q] and UPD [p: q] are close to k ² by replacing the threshold values, it is determined that the number has improved.
(B) When one of the right and lower adjacent pair dot numbers UPD [p: q] and [p: q] approaches k2 and the other has not changed due to the replacement of the threshold, it is determined that the improvement has occurred.
(C) When the gradation range [p: q] is improved or partially deteriorated, the number of pair dots generated in each gradation value of the gradation range and the predicted value at the gradation value If the sum of the differences is small, it is judged as an improvement.

If the above determination is made and it is determined that the paired dot characteristics are not improved, the process returns to step S230, and the process is executed again from the process of randomly changing the threshold value. If the replacement of the threshold is to replace two thresholds, the combination is the entire range of gradations,
₄₀₉₆ C ₂ ways will exist. Even if it is limited to the range of gradation value 1 to 127/255,
There will be ₂₀₄₈ C ₂ ways. Accordingly, a considerable number of combinations of thresholds are replaced, and although it takes a considerable amount of time to complete all cases, replacement is found to improve paired dot characteristics if performed sequentially (step S250, “YES”). .

  Therefore, if it is determined that the paired dot characteristics have been improved, it is next determined whether or not the graininess characteristics are satisfactory (step S260). Here, that there is no problem with the graininess characteristic means that the graininess index shown below is within the target range, or is not within the target range, but is improved from before the threshold replacement. 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. 14 shows an example of VTF. Various formulas have been proposed as empirical formulas that give such VTF, and the following formula (1) shows typical empirical formulas. 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 the following formula (2) based on the VTF. The coefficient τ is a coefficient for matching the obtained value with human senses. As apparent from the calculation method, it can be said that the granularity index is an index indicating whether or not a human feels that dots are conspicuous. It can be said that the smaller the value of the graininess index, the more difficult it is to visually recognize dots in the print image quality, and the more excellent in that respect.

  The initially prepared blue noise mask is configured to have the smallest granularity index, but if the threshold value is randomly changed in step S230, the granularity of the work mask is lower than that of the blue noise mask. To do. Therefore, a target range of the graininess index is set within an allowable range from the viewpoint of human visual characteristics, and it is determined from this range whether there is no problem. Of course, since the granularity index is a value determined for each gradation value, an upper limit value is prepared for each gradation value, and if the granularity index for each gradation value is less than or equal to this upper limit value, the granularity characteristic is within the target range. You may judge that it is in.

  If there is a problem with the graininess characteristic, that is, it is not within the target range and has not been improved compared to before the replacement of the threshold (step S260, "NO"), the process returns to step S230, and the threshold The above process is repeated from the replacement. As a result of repeating the processing of steps S230 to S260, when it is determined that the pair dot characteristics are improved and the graininess characteristics are satisfactory (steps S250 and S260: “YES”), the loop of steps S230 to S260 is temporarily performed. The process returns to step S220, and it is determined whether the pair dot generation characteristic is within the target range.

  If it cannot be determined that the pair dot generation characteristic is within the target range (“NO” in step S220), the processing from step S230 described above is repeated. In the process shown in FIG. 12, steps S220 to S260 are repeatedly executed while changing the threshold value until the condition is satisfied. Therefore, while the number of times the processing from step S230 to S260 is executed (hereinafter referred to as the number of loops) is small, the upper limit value of the graininess index in step S260 is increased, and as the number of loops increases, Processing such as bringing the upper limit value closer to the final target value may be performed. Thus, by changing the upper limit value according to the number of loops, it is possible to prevent the granularity index from falling into a local minimum value.

  In this way, the loop process of steps S230 to S260 is executed several times. Eventually, there is no problem in the graininess characteristics, and the right adjacent pair dot number RPD [1: 127] and the lower adjacent pair dot number UPD [1: 127] are the targets. If it can be determined that it falls within the range M [1: 127] (step S220, “YES”), the paired pixel control mask is completed, and the current working mask is saved as a paired pixel control mask (step S270). END "is exited, and the pair pixel control mask generation routine (FIG. 12) is terminated. In the above description, whether or not the generation characteristics of the paired dots are within the target range is determined in the range of 1 to 127/255 out of the entire range of gradation values where the generation of dots is possible. The pixel control mask may be performed in a gradation range in which the pair dot occurrence probability is to be controlled. For example, it may be performed only in a lower density range (tone range corresponding to dot generation probability k = 0 to 0.25, 0.2 to 0.5, etc.).

By the method described above, a pair pixel control mask can be obtained based on the blue noise mask. This dither mask is the dither mask used in the determination of dot formation in the first embodiment. Since this pair pixel control mask is based on a blue noise mask, analyzing the distribution of dots formed in the range where the gradation value of the image is low as a spatial frequency, it is almost in the low frequency region where human visual sensitivity is high. Have no ingredients. Therefore, it is possible to provide a dither mask that can realize high image quality. In addition, in the above-described pair pixel control mask, the probability of occurrence of a pair dot in which dots are formed in adjacent pixels is set to about k ² × 0.8 in the dot formation probability k at the gradation value. Has been. For this reason, a dither mask that can suppress the occurrence of uneven image density due to the deviation in the dot formation position is small even if the dot formation position in the forward and backward movements is displaced. Can be provided.

In the present embodiment, the pair pixel control mask is generated using the blue noise mask as a starting point, but it can also be generated from a dither mask having other arbitrary characteristics. As described above, the time required for generation can be shortened by generating from a blue noise mask or a green noise mask that is excellent in dispersibility and whose original dispersibility is close to the desired characteristics. Further, when generating a dither mask from scratch, it is also possible to generate a pair pixel control mask by applying the following rule.
(1) The threshold values are sequentially arranged in the matrix from either the small side or the large side.
(2) When the next threshold value is arranged with respect to the threshold value already arranged at a certain position, the evaluation value such as the granularity index is used to associate the arrangement value of the next threshold value with the evaluation value in that case. . Then, candidates for the arrangement position of the next threshold value are identified in order of high evaluation.
(3) The above candidates are taken out in order from the highest evaluation side, and the number of paired dots in that case is counted. When a candidate having the required number of paired dots (for example, the number shown in FIG. 12) is found, the next threshold value is arranged at that position.
(4) Repeat (1) to (3) above until the threshold is exhausted.
Using such a rule, the arrangement of threshold values may be determined from the beginning, and a pair pixel control mask may be generated.

C1. Modification 1 of the first embodiment 1:
A modification of the first embodiment described above will be described. In the first embodiment, in the forward movement and the backward movement, the dot formation positions are staggered in both the main scanning and sub-scanning directions, and the positions of adjacent pixels constituting the paired pixels are shown in FIG. As described above, the two pixels on the right side in the main scanning direction and the lower side in the sub-scanning direction are used. However, the number of adjacent pixels is not limited to these two, and the right and left adjacent pixels in the raster may be included on the lower side in the sub-scanning direction. . When the position of the pixel of interest OJ is (0, 0), not only the pixels at the positions (1, 0) and (0, 1) as adjacent pixels, but also (-2, 1), (2, 1) The pixel at the position is also regarded as an adjacent pixel, and a total of four pairs of paired dots are counted. This is shown in FIG. Of course, it is conceivable to further expand this range to 8 pixels in FIG. 8C. If the range of the paired pixels is wide, it is possible to suppress the occurrence of density unevenness even when the shift of the dot formation position is changed. It should be noted that it is desirable that the range of adjacent pixels be wide in the direction in which the deviation of the dot formation position in printing tends to occur. As shown in FIG. 8B, if adjacent pixels are set widely in the main scanning direction, it is effective to suppress the density caused by the deviation in the main scanning direction.

Further, as shown in FIG. 3 (A), the dots formed during the backward movement of the dots formed during the forward movement can be the alternating columns or the raster alternating shown in FIG. 3 (B). Even in these cases, various settings can be made for the range of adjacent pixels. In the case of alternating columns, as shown in FIGS. 15A to 15C, one, three, and eight pixels can be set as adjacent pixels for the pixel of interest OJ. Alternatively, in the case of alternating rasters, as shown in FIGS. 15D to 15F, one, four, and eight pixels can be set as adjacent pixels for the pixel of interest OJ. Also in these cases, the technique shown in FIG. 12 is applied, and the probability K of occurrence of bare dots is calculated based on the blue noise mask.
K = 0.8 × k ²
The paired pixel control mask can be generated.

  In the above-described embodiments and modifications, the resolution of the gradation value of the image is set to 8 bits and the threshold range is set to 0 to 255 in order to simplify the description, but the threshold value arranged in the dither mask is set to 0 to 0. If the number of pits representing the gradation value of the image is increased to 4095, for example, 10 bits, the number of arranged dots can be reduced with respect to the minimum gradation value 1, and the gradation value is 1 The number of dots that increase with each increase can be reduced. Therefore, the bare dot generation probability can be controlled more finely. Of course, if the dither mask is further increased to 128 × 128, 256 × 512, or the like, the number of dots formed when the gradation value expressed by 10 bits is the value 1, the former is 4, and the latter It becomes about 32 pieces. The size of the dither mask, the number of bits representing the gradation value, the type of threshold value arranged in the dither mask, and the like are the purpose of the halftone processing to be executed (whether image quality priority or processing speed priority or large format printing is used). Or the like) and the processing time.

  In the first embodiment, bare pixel control is performed in the range of gradation values 0 to 127/255, in other words, the pair pixel control mask is set assuming that the dot generation probability k is in the range of 0 <k <0.5. Although prepared, for this range, the upper limit value may be further limited to the low gradation value side. For example, pair dot control may be performed only in a range of 0 <k <0.2. In general, as the actual dot size with respect to the pixel size increases, dot overlap due to misalignment is more likely to occur, so that the gradation region where density variation is a problem moves to the low density side. Therefore, it is realistic to change the range to be adjusted according to the actual dot size with respect to the pixel size. Further, the lower limit value may be further limited to the high gradation value side. In general, in the low gradation region near the gradation value 0, the dot formation positions are originally far apart, and the density unevenness problem does not become so obvious even if the dot formation positions are shifted. Therefore, the pair pixel control mask may be generated only in the range of 0.1 <k <0.4 or 0.2 <k <0.5. Further, the ratio of the dots formed during the forward movement and the ratio of the dots formed during the backward movement are made different from the beginning, and different dot formation probabilities k1 and k2 are set for each to control the pair dots. There is no problem.

C2. Modification 2 of the first embodiment:
In the first embodiment, printing is performed from input of image data to printing by the printer 20 shown in FIG. 1, but the printer 20 is connected to the computer PC, and the computer PC side is shown in FIGS. 4A and 4B. It is also possible to perform steps S110 to S160. In this case, the determination of the paper size (step S125) can be performed by checking the member values managed by the printer driver.

In a general-purpose OS such as Windows (trademark), information necessary for printing is managed as a set of parameters called “members” in order to smoothly perform printing between the computer PC and the printer. For example, some members of Windows (TM)
dmOrientation (paper orientation))
dmPaperSize (paper size)
dmPaperLength (paper length)
dmPaperWidth (paper width)
dmPosition (paper position)
dmScale (scaling)
dmCopies (number of copies)
dmDefaultSource (default paper tray)
dmPrintQuality (print resolution)
dmColor (Color printing or monochrome printing on a color printer)
dmDuplex (whether double-sided printing is possible)
Such members are included. Since these members are set through processing such as “print setting”, the printer driver or the like can know the set value of the paper size and the like at any time by referring to these members.

Therefore, when the first and second halftone processes of the printing process shown in FIG. 4B are performed on the computer PC side, for example, “dmPaperSize (paper size)” among these members is set in the determination in step S141. It is only necessary to call and determine the paper size from this value and execute either the first or second halftone process. It should be noted that the sense of distance when a person looks at a printed material often depends on the paper size, but may vary depending on the size of the area of the actually printed image. Therefore, in the determination in step S141, the following factors may be considered in place of the paper size alone. Note that these elements may be used alone or in combination with other elements. As other elements, for example, the following can be considered.
(B) Paper size (b) Paper direction (portrait or landscape)
(C) Margin for paper
(D) Print resolution (width direction resolution, length direction resolution)
(E) The aspect ratio of the area where the image is printed (f) The aspect ratio (aspect) of the image to be printed
(G) Paper type (plain paper, fine paper, photo paper, OHP sheet, etc.)

  By combining these elements alone or in combination, the first dither mask 61 is generally used when the print area is small, and the second dither mask 62 is used when the print area is large. Alternatively, the first dither mask 61 may be used when the resolution is coarse, and the second dither mask 62 may be used when the resolution is high. In general, the higher the resolution, the longer it takes to print. During printing with ink, the ink is absorbed by the print medium (for example, paper), the print medium expands, cockling occurs, and the ink droplets are displaced to the landing position. It is because it is easy to occur.

  Also, as one of these members, prepare a member that describes the type of dither mask to be used or a reference pointer for the dither mask, and when setting the paper size member, the type of dither mask is set according to the value. The member that describes the dither mask or the member that describes the reference pointer of the dither mask may be changed directly.

D. Second embodiment:
D-1. Dither mask:
Next, a second embodiment of the present invention will be described. The hardware of the printer 20 as the second embodiment is the same as that of the first embodiment (see FIG. 1). Further, the printing control process (FIGS. 4A and 4B) in the printer 20 is the same except that the first dither mask 61 used in the first halftone process is different. Also in the second embodiment, the first and second halftone processes are performed by a so-called dither method.

The features of the first dither mask 161 in the second embodiment are as follows as compared with the first dither mask 61 of the first embodiment.
(1) Common points of both:
In both cases, the size of the dither mask is 64 × 64.
-Both are blue noise masks that prioritize dispersibility.
(2) Differences between the two:
The threshold value of the 64 × 64 first dither mask 61 in the first embodiment is prepared as a simple blue noise mask, whereas the dither mask in the second embodiment moves the print head 90 forward. The first pixel group to which the dots that are sometimes formed belong and the second pixel group to which the dots that are formed when the print head 90 moves backward are formed in consideration of dot dispersibility.

  The above differences will be described. The image formation by the printer 20 is based on dots formed when the print head 90 moves forward and dots formed when the print head 90 moves backward. Therefore, the distribution of dots in an image obtained with a certain gradation value is the distribution of both dots formed by the forward and backward movements of the print head 90. For this reason, conventionally, the threshold value of the dither mask has been determined with the goal of increasing the dispersibility of the dots in this state. In contrast, the dither mask used in the second embodiment has a first pixel group to which dots formed when the print head 90 moves forward and a second pixel to which dots formed when the print head 90 moves backward. Dispersibility is considered for each of the pixel groups. That is, as illustrated in FIG. 16, the dither mask used in the second embodiment has the dispersibility of the dots (FIG. 16A) formed during the forward movement when a certain image is formed. This is because the dispersibility of the dots (FIG. 16B) formed during the backward movement is taken into consideration. Although a method of generating such a dither mask will be described later, it is needless to say that a dither mask having the above properties may be used regardless of the method of making.

  In the printer 20 according to the second embodiment using the dither mask having such a property, even when the dot position formed by the forward and backward movements of the print head 90 is displaced, deterioration in image quality is sufficiently suppressed. Can do. This is because dispersibility is considered for each of the first pixel group to which the dots formed when the print head 90 moves forward and the second pixel group to which the dots formed when the print head 90 moves backward. This is because the dither mask that has been used is used. For this reason, even if there is a deviation in the dot formation position between the forward movement and the backward movement, the dispersibility of the dots belonging to each pixel group is guaranteed, so that the dots of both groups are in a common area. The drop in the dispersibility of the dots when superimposed is only slight. This is because the granularity in the case of combining dots belonging to two pixel groups shows a strong correlation with the granularity of a single dot belonging to each pixel group.

D-2. Dither mask generation method:
FIG. 17 shows a procedure of a method for generating the first dither mask 161 having the above properties. In the generation of the first dither mask 161, as shown in the figure, first, a threshold value corresponding to the size of the first dither mask 161 is prepared (step S310). In the second embodiment, the first dither mask 161 has a size of 64 × 64, but in the following, it will be simplified to 8 × 8, that is, having 64 storage elements. In step S310, the same number of thresholds 0 to 63 as the storage elements are prepared.

  Once the threshold value is prepared, a target threshold value selection process is performed (step S320). The target threshold value selection process is a process of selecting one threshold value as a target threshold value from among the prepared threshold values 0 to 63 that are not yet stored in the storage element. In this embodiment, the threshold value of interest is selected in order from the smallest threshold value of the prepared threshold values. As shown in FIG. 18, when the threshold value of 0 to 3 is already stored in the storage element in the storage element constituting the dither mask by the process described later, the target threshold value selected in step S320 is The value is 4.

  Once the target threshold value is selected, first dither mask evaluation processing is performed (step S400). In the first dither mask evaluation process, when a threshold value of interest is stored for one storage element for which a prepared threshold value is not yet stored (hereinafter also referred to as a blank storage element), the threshold value is already stored. This is a process of calculating, for each blank storage element, an evaluation value E1 indicating the degree of dot dispersion for the dot formation pattern represented by the arrangement of the stored storage elements (hereinafter also referred to as “decision storage elements”). Although the calculation method of the evaluation value E1 will be described later, in this embodiment, the evaluation value E1 is better from the viewpoint of the granularity of the printed image because the smaller the value, the better the dot dispersibility.

  Next, a process for determining a storage element is performed using the evaluation value E1 (step S330), and it is determined whether the process for determining all the storage elements has been completed (step S340). If the processing for all the storage elements has not been completed, the process returns to step S320 described above and the above processing is repeated. When the first dither mask evaluation process described above is performed and the threshold values are stored in all the storage elements (step S340, “YES”), the evaluation of the first dither mask 161 is completed, and the dither mask is stored. The generation process ends.

D-3. First dither mask evaluation process:
The first dither mask evaluation process in the above-described dither mask generation process will be described with reference to FIG. In the first dither mask evaluation process, as shown in FIG. 19, first, a grouping process is performed (step S410). The grouping process refers to a plurality of storage elements constituting a dither mask, and dot formation at the dot formation position where the threshold values stored in the plurality of storage elements are applied in the halftone process. This is a process of dividing into a plurality of groups by paying attention to which of them is performed. In other words, storage is performed based on the arrangement mode of the dots formed when the print head moves forward and when the print head moves backward (in the second embodiment, the column alternate mode shown in FIG. 3A is used). This is a process to set a group of elements. Note that the set groups are different timings when dots are formed by ejecting ink from the print head at a plurality of different timings while changing the ink ejection position with respect to the print medium in a common print area of the print medium. It may be set based on. As the plurality of different timings, instead of or in addition to the forward and backward movements, in addition to the main scanning, when dots are formed by N times (N is an integer of 3 or more) main scanning in a common print area. The order (how many main scans are performed) may be used.

  Thus, when the grouping process is performed, the dot of the decision storage element is turned on (step S420). In FIG. 18, the state in which the dots of the decision storage elements storing the threshold values 0 to 3 are turned on is indicated by single hatching. If the dot of the determined storage element is turned ON, candidate storage element selection processing is then performed (step S430). The candidate storage element selection process is a process of selecting a candidate storage element that is a candidate for a storage element in which a target threshold value is to be stored. Since it is possible to store the target threshold value in each of the blank storage elements, one of the blank storage elements is selected as a candidate storage element here. Once the candidate storage element selection process is performed, the dot of the candidate storage element is then turned on (step S440). In FIG. 18, a state in which one of the blank storage elements is selected as a candidate storage element and the dot of the candidate storage element is turned ON is shown by cross hatching.

  When the dot of the candidate storage element is turned on, group selection processing is next performed (step S450). The group selection process is one group Gq (q is an integer from 1 to p) out of the p groups G1 to Gp (p is an integer of 2 or more, here p = 2) set in step S410. ) Is selected.

  When the group Gq is selected, the evaluation value E1q indicating the degree of dot dispersion, that is, how much the dots are uniformly distributed is formed based on the dot formation pattern corresponding to the storage elements belonging to the group Gq. An evaluation value indicating whether or not is calculated (step S460). It is known that a dither mask having blue noise characteristics and green noise characteristics shown in FIG. 20 may be generated in order to form dots in a uniformly dispersed state. In this embodiment, in order to generate a dither mask having such characteristics, the graininess index described in the first embodiment is used as an evaluation value indicating the degree of dot dispersibility.

When the evaluation value E1q is calculated, the steps S450 and S460 are repeated until the evaluation value E1q is calculated for all the groups G1 to Gp (here, G1 to G2) (step S470). Thus, when the evaluation value E1q is calculated for all the groups G1 to G2 (step S470: YES), the evaluation value E1 is calculated by the following equation (3) based on the calculated evaluation values E11 to E12 (step S480). . In Expression (3), de are weighting coefficients. These weighting factors are experimentally determined as constant values so that good print image quality can be obtained. That is, the evaluation value E1 is represented by the dot formation pattern represented by the entire dither mask decision storage element, the respective dot formation patterns represented by the decision storage element corresponding to the forward movement, and the decision storage element corresponding to the backward movement. It is an evaluation value obtained by comprehensively evaluating the degree of dot dispersion for each dot formation pattern with a predetermined weight.
E1 = d × E11 + e × E12 (3)

  When the evaluation value E1 is calculated, the steps S430 to S480 are repeated until the evaluation value E1 is calculated for all candidate storage elements (blank storage elements) (step S490). When the evaluation value E1 is calculated for all candidate storage elements in this way (step S490: YES), the first dither mask evaluation process ends.

  Using the evaluation value E1, the first dither mask DM1 having a dot formation pattern in which dots are dispersedly arranged for both the dots formed during forward movement and the dots formed during backward movement. Can be generated. In the above evaluation, the granularity of the first pixel group and the granularity of the second pixel group are targeted. However, the granularity of the dot including both the first and second pixel groups is also evaluated. It can be used as a target.

  If halftone processing is performed using the first dither mask 161, the dispersibility of the forward and backward dots remains secured even if a positional deviation occurs between the forward and backward dots. Therefore, the dispersibility of the dots of the entire image is ensured, and the deterioration of the graininess of the print image quality can be suppressed.

  Although several embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and can of course be implemented in various modes without changing the gist of the present invention. . For example, it may be performed in a printer that performs monochrome printing or a line printer in which a print head is provided across the paper width direction. Further, the processing shown in FIGS. 4A, 4B, and 22 may be performed not on the printer side but on the computer side that performs image processing (or on the server side on the network). These processes may be realized by hardware (for example, a RIP provided between the printer and the printer).

  In the above embodiment, as the first and second pixel groups, a group of pixels to which dots formed at the time of forward movement when bidirectional printing is performed and a group of pixels to which dots formed at the time of backward movement belong. However, such pixel groups can be set in various forms as long as the printing conditions are different. For example, the dots formed by each pass of so-called multi-pass printing that constitutes a raster by a plurality of main scans are divided into different pixel groups, adjacent pixels between the pixel groups are set, and pair dots are The incidence can be controlled. Alternatively, for each nozzle row that ejects ink, it is possible to divide the groove by the pixel to which the dot formed by each nozzle row belongs.

  In addition to this, the ratio of dots formed during forward movement and backward movement is varied, and the probability of bare dot formation is controlled, and ink droplets with different ink droplet sizes are ejected from noise. The present invention can also be applied to a structure that forms When sorting such large, medium, and small dots, or when sorting multiple types of light and dark dots, one type from the one with the smallest dot diameter (the one with the lightest ink) or the one with the smallest dot diameter (the lightest) It is preferable to control the occurrence probability of bare dots for a plurality of types of dots (from the ink side). That is, among the plurality of types of dots, large-diameter dots (dark dots) are formed in the high gradation region, so the probability of pair dots may not be controlled for these dots. In the above-described embodiment, two dither masks are switched and applied. However, three or more dither masks are prepared, and the two dither masks described above are generated depending on the size of the print area or other factors. It is also possible to switch to other dither masks.

Further, the first halftone process in the first and second embodiments may be performed by another method as long as the same process as that using the dither mask can be performed. For example, the halftone process by the dither method using the first dither mask described above may be realized by the error diffusion method. In the error diffusion method, the dot arrangement approaches a blue noise characteristic, so halftone processing by error diffusion is performed when the size of the printing paper is smaller than a predetermined value or when the printing area is smaller than the predetermined value. when is large or the print area is larger than the size, if the halftone processing using the dither mask closer the occurrence probability of the pair dot k ^2, to obtain the first, the same effect as the second embodiment Can do. Note that the second halftone process can also be realized by an error diffusion method. In this case, a counter that counts the number of dots formed in adjacent pixels is provided separately from the buffer that diffuses the density error that occurs in the pixel of interest, and the excess or deficiency for the predicted value of the paired pixel is calculated. Thus, it is only necessary to correct the gradation values in the peripheral pixels using this.

The printing apparatus, printing method, and dither mask generation method have been described above as the embodiments of the present invention. The printing apparatuses of the first and second embodiments control the number of paired dots by using a dither mask having specific characteristics when the paper size is A4 or larger. Therefore, it is possible to determine whether the present invention is implemented without necessarily analyzing the characteristics of the dither mask. That is, as shown in FIGS. 4A and 4B, the dither mask to be used is changed when the paper size is changed, and a dither mask with high dispersibility is used as shown in FIG. If the incidence K of Beadotto the gradation value is low area of the image (e.g., 0-50 / 255, the dot generation rate 0 to 0.2), the are spaced larger substantially zero for ^{k 2.} Therefore, when the paper size is large, halftone processing different from that when the paper size is small is adopted, and the dispersibility of the image is equal to or greater than a predetermined value due to the granularity index expressed by the equations (1) and (2). If the bare dot formation probability K is, for example, 0.2 · k ² ≦ K ≦ 0.8 · k ² with respect to the dot occurrence rate k, it is determined that the present invention is being implemented. be able to. In addition, when the error diffusion method is used, the noise characteristic can be regarded as a blue noise characteristic. Similarly, by measuring the occurrence rate of pair dots in a predetermined gradation range of an image, The determination can be easily performed.

20 ... Printer 30 ... Control unit 40 ... CPU
42 ... Halftone processing unit 43 ... Printing unit 51 ... ROM
52 ... RAM
60 ... EEPROM
DESCRIPTION OF SYMBOLS 61 ... 1st dither mask 62 ... 2nd dither mask 70 ... Carriage motor 71 ... Drive belt 72 ... Pulley 73 ... Sliding shaft 74 ... Paper feed motor 75 ... Platen 80 ... Carriage 82-87 ... Ink cartridge 90 ... Printing Head 98 ... Memory card slot 99 ... Operation panel 161 ... First dither mask P ... Printing paper MC ... Memory card

Claims (11)

A data generation device that generates data for printing,
A quality storage unit for storing parameters affecting print quality;
A print area designating part for designating the size of the print area on the print medium;
According to the size of the print area, the quality change unit for changing the stored parameter and the image data to be printed are received, and dot data indicating the presence or absence of the formation of printing dots is received according to the setting based on the parameter. A dot data generation unit to generate;
A data generation apparatus comprising: The data generation device according to claim 1,
The parameter stored in the quality storage unit uses a first dither mask having characteristics corresponding to a dot arrangement giving priority to granularity in a gradation range of a predetermined gradation or less, or dots are formed in adjacent pixels. A parameter for specifying whether to use a second dither mask having a higher probability of being performed than the first dither mask,
The quality changing unit changes the parameter so that the second dither mask is used instead of the first dither mask when the size of the print area on the print medium is equal to or larger than the first predetermined value. Data generator. The data generation device according to claim 2,
The quality changing unit changes the parameter so that the first dither mask is used when the print area is less than the predetermined value or a second predetermined value smaller than the predetermined value. .   The data generation apparatus according to claim 1, further comprising a print area acquisition unit that acquires a size of the print area.   The data generation apparatus according to claim 4, wherein the print area acquisition unit acquires the size of the print area from at least one of a print medium size, a print area width, and a margin width in the print medium.   The data generation apparatus according to claim 4, wherein the print area acquisition unit acquires the size of the print area from a member of an operating system or a printer driver.   7. The data generation apparatus according to claim 2, wherein the dither mask is a mask having a blue noise characteristic or a green noise characteristic. A data generation device according to any one of claims 2 to 7,
The printing data is printed by forming the dots in a plurality of pixel groups having different printing conditions and performing at least a part of the dot formation by the plurality of pixel groups in a common area. Dot data used for
The first dither mask is a data generation device in which, for each of a plurality of pixel groups, a dot arrangement having a predetermined gradation or less has a blue noise characteristic or a green noise characteristic. A printing device for printing on a print medium,
A quality storage unit for storing parameters affecting print quality;
A print area designating part for designating the size of the print area on the print medium;
According to the size of the print area, the quality change unit for changing the stored parameter and the image data to be printed are received, and dot data representing the presence / absence of dot formation for printing is received according to the setting based on the parameter. A dot data generation unit to generate;
And a printing unit that performs printing by forming dots on the print medium using the dot data. A method for generating data for printing,
Parameters that affect print quality are stored in the storage unit,
Accept the specification of the size of the print area on the print medium,
The stored parameter is changed according to the size of the print area that has received the designation,
A printing data generating method for receiving image data to be printed and generating dot data representing the presence or absence of formation of printing dots in accordance with the setting based on the parameters. A printing method for printing on a print medium,
Parameters that affect print quality are stored in the storage unit,
Accept the specification of the size of the print area on the print medium,
The stored parameter is changed according to the size of the print area that has received the designation,
Receiving image data to be printed, and generating dot data representing the presence or absence of printing dots according to the settings based on the parameters,
A printing method for performing printing by forming dots on the print medium using the dot data.

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