JP2005081770A - Printer for printing image while converting image data in units of a plurality of pixels - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent deterioration of image quality due to unit conversion of image data. <P>SOLUTION: A plurality of adjacent pixels are collected into a unit and subjected to unit conversion into dot data indicative of presence/absence of dot formation. With regard to pixels in a converted unit, presence/absence of dot formation is stored under a state capable of identifying the pixel position in the unit. When image data is subjected to unit conversion into dot data, a remarked pixel for which presence/absence of dot formation is judged is set and then dot formation conditions of the remarked pixel is detected at a specified pixel position and dot formation is judged while taking account of the detection results. Consequently, deterioration of image quality due to unit conversion of image data into dot data can be avoided. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a technique for printing an image based on image data, and more specifically, prints a high-quality image while quickly converting the image data by processing a predetermined number of adjacent pixels in a unit. Regarding technology.

  A printing apparatus that prints an image by forming dots on a print medium is widely used as an output apparatus for various image apparatuses. Such a printing apparatus can express only the state of whether or not to form dots locally, but by controlling the dot formation density appropriately according to the gradation value of the image, It is possible to express an image in which continuously changes.

  A method called error diffusion is widely used as a method for determining the presence or absence of dot formation for each pixel so that dots can be formed at an appropriate density according to the gradation value of the image in these printing devices. Has been. In the error diffusion method, an error in gradation expression caused by the formation of a dot in a pixel of interest or the absence of a dot is diffused and stored in undetermined pixels around the pixel of interest. When determining the presence or absence of dot formation for a pixel, this is a method for determining the presence or absence of dot formation so as to eliminate the error diffused from the surrounding pixels. By using the error diffusion method, it is possible to determine whether or not dots are formed so as to eliminate the error in gradation expression generated in the peripheral pixels in this way, so that the density is appropriate according to the gradation value of the image. Thus, it is possible to determine the presence or absence of dot formation.

  By using the error diffusion method, high-quality images can be displayed by forming dots with an appropriate density according to the image. When the number of pixels constituting an image increases, it takes time to process and it is difficult to express the image quickly. In order to solve such a problem, the inventor of the present application has developed a technique for collecting a predetermined number of adjacent pixels into a unit and determining the presence or absence of dot formation while diffusing an error in units. (Patent Document 1).

JP 2001-185789 A

  However, when determining whether or not dots are formed on a unit basis, the same processing is repeated for each unit. For this reason, a dot is formed in the pixel of the same position in a unit over a some unit, and this will be visually recognized as a periodic pattern, and there exists a possibility of degrading an image quality. In addition, there is often a phenomenon in which color unevenness occurs and image quality deteriorates even if a periodic pattern does not occur. Furthermore, when the presence / absence of dot formation is determined on a unit basis, such a problem that image quality may be deteriorated is not limited to the error diffusion method, but may also occur when other known methods are used.

  The present invention has been made to solve the above-described problems in the prior art, and enables quick determination by determining the presence or absence of dot formation in units, while avoiding deterioration in image quality and high image quality. The purpose is to provide a technology that enables printing of a simple image.

In order to solve at least a part of the problems described above, the printing apparatus of the present invention employs the following configuration. That is,
A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster A printing apparatus that prints an image by forming a plurality of rasters,
Image data receiving means for receiving, for a plurality of pixels constituting the image to be printed, image data representing a gradation value of each pixel;
Dot data conversion means for grouping a predetermined plurality of pixels adjacent to each other as a unit, and converting the image data into dot data representing the presence or absence of dot formation for each pixel in units.
For at least some of the pixels constituting the converted unit, dot presence / absence storage means for storing the presence / absence of dot formation in a state where the pixel position in the unit can be identified;
And raster forming element driving means for driving the raster forming element according to the dot data,
The dot data conversion means includes
Pixel-of-interest setting means for sequentially setting a pixel of interest for which the presence / absence of dot formation is determined in the unit;
Dot formation presence / absence detection means for detecting the presence / absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among the pixels in which the presence / absence of dot formation is stored in the converted unit;
Dot formation determination means for determining the presence or absence of dot formation for the pixel of interest based on the gradation value of the image data for the pixel of interest and the presence or absence of the detected dot formation; and
The gist is that the unit is a means for collecting consecutive pixels corresponding to an integral multiple of the predetermined number of times the raster forming element reciprocates to form the raster, at least in the raster direction. .

Also, the printing method of the present invention corresponding to the above printing apparatus is
A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster A printing method for printing an image by forming a plurality of rasters,
Receiving (A) image data representing a gradation value of each pixel for a plurality of pixels constituting the image to be printed;
A step (B) of grouping a predetermined plurality of pixels adjacent to each other as a unit, and converting the image data into dot data representing the presence or absence of dot formation for each pixel in units.
For at least some of the pixels constituting the converted unit, a step (C) of storing the presence / absence of dot formation in a state where the pixel positions in the unit can be identified;
(D) driving the raster forming element according to the dot data,
The step (B)
A step (B-1) of sequentially setting a pixel of interest for which it is determined whether or not to form dots in the unit;
A step (B-2) of detecting the presence or absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among pixels in which the presence or absence of dot formation is stored in the converted unit; ,
A step (B-3) of determining the presence / absence of dot formation for the target pixel based on the gradation value of the image data for the target pixel and the detected dot formation presence / absence,
The gist of the present invention is that the unit is a process of collecting consecutive pixels corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster, at least in the raster direction. .

  In such a printing apparatus and printing control method, when a predetermined number of pixels are grouped as a unit and image data is converted into dot data in units of units, the dot formation status in the already converted units is detected and detected. The presence / absence of dot formation is determined in consideration of the result. The dot formation status in the converted unit is detected as follows. First, for at least some of the pixels constituting the converted unit, the presence / absence of dot formation is stored in a state where the pixel positions in the unit can be identified. Next, the pixel of interest for which the presence / absence of dot formation is to be determined is set in the unit, and the dot for the pixel located at a predetermined position with respect to the pixel position of the pixel of interest is stored among the pixels in which the presence / absence of dot formation is stored. The presence or absence of formation is detected. The pixel for detecting the presence or absence of dot formation may be at a predetermined position with respect to the pixel of interest. Therefore, it is good also as detecting about a some pixel, and it is also possible to set the pixel position to detect as a different pixel position according to the pixel position of the pixel of interest.

  If an image is printed while forming a raster based on the dot data obtained in this way, it is possible to avoid the formation of dots with a constant pattern over a plurality of units. Therefore, it is possible to reliably prevent the periodic pattern from being visually recognized and the image quality from deteriorating.

  In the above printing apparatus and printing method, the following plurality of pixels are grouped as a unit. In other words, at least in the direction of the raster, a continuous number of pixels corresponding to an integral multiple of a predetermined number of times that the raster forming element reciprocates to form the raster is collected. By doing this, it is possible to avoid the formation of dots in a constant pattern across a plurality of units, and at the same time, the phenomenon that color irregularity occurs and the image quality deteriorates without the occurrence of a periodic pattern. It can be avoided. Hereinafter, this reason will be described.

  In general, when printing an image while forming a raster by reciprocating a raster forming element on a print medium, the raster is not formed by a single reciprocating motion, but formed by a plurality of reciprocating motions. Is widely practiced. Such a printing method does not lead to the following phenomenon that occurs when a raster is formed by a single reciprocation, that is, a phenomenon called banding in which the ground color of the print medium appears in a streak shape, or banding. Both are used to avoid the deterioration of image quality due to the phenomenon of color unevenness. In such a printing method, assuming that the number of times the raster forming element is reciprocated to form one raster is N, it is normal to form dots in N pixel jumps in one reciprocating motion. Therefore, when printing an area where dots are accidentally formed in N pixels, even if an image is printed while the raster forming element is reciprocated N times, the area can be reciprocated once. It will be printed. For this reason, banding, color unevenness, and the like may occur in such areas, and image quality may be degraded.

  By the way, in the above-described printing apparatus and printing method, as described above, a plurality of adjacent pixels are grouped as a unit, and image data is converted into dot data in units. At this time, the dot formation status in the already converted unit is detected, and the presence or absence of dot formation is determined in consideration of the detection result. For this reason, it is possible to avoid the occurrence of a certain pattern, such as the formation of dots at pixels at the same position in the unit. However, by avoiding the periodic formation of dots in units of units, an area in which dots are formed so as to jump N pixels may occur. Here, the N pixel is an N pixel when a single raster is formed by N reciprocating motions, and pixels are formed in a jumping manner by individual reciprocating motions.

  For example, consider a case where a total of nine pixels, each of three vertical and horizontal, are grouped as a unit. Even if there is an area in which dots are formed in exactly three pixels in the image, the dot formation position is shifted in order to avoid the formation of dots at the same pixel position in each unit. Dots are not formed periodically every three pixels. However, as a result of shifting the dot formation position, for example, a region where dots are formed every four pixels may be generated. If one raster happens to be formed by reciprocating four times, the raster is formed in this region by one reciprocating motion, resulting in image quality deterioration such as color unevenness. obtain.

  On the other hand, in the printing apparatus and printing method of the present application, when one raster is formed by N reciprocations, the number of continuous pixels corresponding to an integer multiple of N is at least in the raster direction. As a unit. For this reason, if it is avoided that dots are periodically formed over a plurality of units, the occurrence of a region where dots are formed by a single reciprocation can be avoided immediately. In other words, a phenomenon in which dots are periodically formed in a constant pattern across a plurality of units is avoided, and at the same time, even if a periodic pattern does not occur, color unevenness occurs and image quality deteriorates. Can also be avoided.

The present invention can also be realized using a computer by causing a computer to read a program that realizes the above-described printing method and printing control method. Therefore, the present invention includes the following program or a mode as a recording medium storing the program. That is, the program of the present invention corresponding to the printing method described above is
A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster A program for realizing a method of printing an image by forming a plurality of rasters using a computer,
A function (A) for receiving image data representing a gradation value of each pixel for a plurality of pixels constituting the image to be printed;
A function (B) for grouping a predetermined plurality of pixels adjacent to each other as a unit, and converting the image data into dot data indicating the presence / absence of dot formation for each pixel in units;
For at least some of the pixels constituting the converted unit, a function (C) for storing the presence / absence of dot formation in a state where the pixel position in the unit can be identified;
A function (D) for driving the raster forming element according to the dot data and
The function (B) is
A function (B-1) for sequentially setting a pixel of interest for which the presence or absence of dot formation is to be determined in the unit;
A function (B-2) for detecting the presence / absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among the pixels in which the presence / absence of dot formation is stored in the converted unit; ,
A function (B-3) for determining the presence / absence of dot formation for the pixel of interest based on the gradation value of the image data for the pixel of interest and the detected dot formation presence / absence,
The gist of the unit is that the raster forming element collects a number of pixels corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster, at least in the raster direction. .

The recording medium of the present invention corresponding to the above program is
A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster A recording medium on which a program for printing an image by forming a plurality of rasters is recorded in a computer-readable manner,
A function (A) for receiving image data representing a gradation value of each pixel for a plurality of pixels constituting the image to be printed;
A function (B) for grouping a predetermined plurality of pixels adjacent to each other as a unit, and converting the image data into dot data indicating the presence / absence of dot formation for each pixel in units;
For at least some of the pixels constituting the converted unit, a function (C) for storing the presence / absence of dot formation in a state where the pixel position in the unit can be identified;
A program for realizing the function (D) for driving the raster forming element according to the dot data, and
The function (B) is
A function (B-1) for sequentially setting a pixel of interest for which the presence or absence of dot formation is to be determined in the unit;
A function (B-2) for detecting the presence / absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among the pixels in which the presence / absence of dot formation is stored in the converted unit; ,
A function (B-3) for determining the presence / absence of dot formation for the pixel of interest based on the gradation value of the image data for the pixel of interest and the detected dot formation presence / absence,
The gist of the unit is that the raster forming element collects a number of pixels corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster, at least in the raster direction. .

Furthermore, the program of the present invention corresponding to the above print control method is as follows:
A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster Using a computer to supply control data for controlling driving of the raster forming element to a printing unit that prints an image by forming a plurality of rasters, and to control the printing unit A program for realizing
A function (a) for receiving image data representing a gradation value of each pixel for a plurality of pixels constituting the image to be printed;
A function (a) that collects a predetermined plurality of adjacent pixels as a unit, and converts the image data into dot data indicating the presence or absence of dot formation for each pixel in units.
For at least some of the pixels constituting the converted unit, a function (c) for storing the presence or absence of dot formation in a state in which the pixel position in the unit can be identified;
A function (d) for supplying the dot data as the control data to the printing unit is realized, and
The function (b)
A function (a-1) for sequentially setting a pixel of interest for which it is determined whether or not to form dots in the unit;
A function (a-2) for detecting the presence / absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among the pixels in which the presence / absence of dot formation is stored in the converted unit; ,
A function (i-3) for determining the presence / absence of dot formation for the target pixel based on the gradation value of the image data for the target pixel and the detected dot formation presence / absence;
The gist of the unit is that the raster forming element collects a number of pixels corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster, at least in the raster direction. .

The recording medium of the present invention corresponding to the above program is
A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster In addition, a computer reads a program for controlling the printing unit by supplying control data for controlling the driving of the raster forming element to a printing unit that prints an image by forming a plurality of rasters. A recording medium capable of recording,
A function (a) for receiving image data representing a gradation value of each pixel for a plurality of pixels constituting the image to be printed;
A function (A) for grouping a predetermined plurality of pixels adjacent to each other as a unit, and converting the image data into dot data indicating the presence / absence of dot formation for each pixel,
For at least some of the pixels constituting the converted unit, a function (c) for storing the presence or absence of dot formation in a state in which the pixel position in the unit can be identified;
A program for realizing the function (d) for supplying the dot data as the control data to the printing unit;
The function (b)
A function (a-1) for sequentially setting a pixel of interest for which it is determined whether or not to form dots in the unit;
A function (a-2) for detecting the presence / absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among the pixels in which the presence / absence of dot formation is stored in the converted unit; ,
A function (i-3) for determining the presence / absence of dot formation for the target pixel based on the gradation value of the image data for the target pixel and the detected dot formation presence / absence;
The gist of the unit is that the raster forming element collects a number of pixels corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster, at least in the raster direction. .

  If such a program or a program stored in a recording medium is read into a computer and the above-described various functions are executed by the computer, the image data is converted into dot data in units without causing image quality deterioration. Thus, it is possible to print an image quickly.

In order to more clearly describe the operation and effect of the present invention, embodiments of the present invention will be described below in the following order.
A. Summary of the invention:
B. Device configuration:
C. Overview of image data conversion process:
D. First embodiment:
D-1. Tone number conversion processing performed in units:
D-2. In-unit multi-value processing:
D-3. In-unit multilevel processing of the first embodiment:
D-4. Variation:
E. Second embodiment:
E-1. In-unit multi-value processing of the second embodiment:
E-2. Modification of the second embodiment:

A. Summary of the invention:
Before starting the detailed description of the embodiments, the outline of the present invention will be briefly described for convenience of understanding. FIG. 1 is an explanatory diagram showing an outline of the present invention, taking a printing system as an example. The printing system shown in FIG. 1 includes a computer 10 as an image processing apparatus, a color printer 20 and the like. When the computer 10 receives color image data expressed by gradation values of RGB colors from an image device such as a digital camera or a color scanner, the computer 10 expresses the image data by the presence or absence of formation of each color dot that can be printed by the color printer 20. Is converted to print data. Such conversion of image data is performed using a dedicated program called the printer driver 12. Note that color image data for each color of RGB can also be created by the computer 10 using various application programs.

  The color printer 20 is provided with a head for forming dots on the print medium for each color. The color printer 20 reciprocates the head on the print medium in accordance with the print data generated by the printer driver 12. The image is printed by forming dots while At this time, the dot row formed on the print medium while the head reciprocates is called “raster”, but the color printer 20 forms one raster by one reciprocation mainly due to the demands on image quality. Instead, it is divided into a plurality of reciprocations. As described above, the number of reciprocations performed to form one raster may be referred to as “the number of passes”.

  The printer driver 12 includes a plurality of modules such as a resolution conversion module, a color conversion module, a gradation number conversion module 14, and an interlace module. Processing performed in each of these modules will be described later. Among these, the gradation number conversion module 14 performs processing for converting the image data into data represented by the presence or absence of dot formation. Here, in order to speed up the processing, the gradation number conversion module of the present embodiment performs processing in units as follows. That is, a unit is generated by grouping a plurality of adjacent pixels, and whether or not dots are formed for each pixel is collectively determined for each unit. A method for determining whether or not to form dots in units will be described later. Further, as described above, the color printer 20 reciprocates the head a predetermined number of times in order to form one raster. However, when a plurality of pixels are combined as a unit, at least in the direction of the raster. Summarizes consecutive pixels by the number corresponding to an integral multiple of the number of passes.

  In FIG. 1, a conceptual diagram indicated by an arrow below a frame representing the gradation number conversion module 14 shows a state in which image data is converted into dot data in the gradation number conversion module 14. . The small squares indicated by broken lines in the conceptual diagram schematically show the pixels. One unit is composed of four pixels in two rows and two columns. The number of pixels collected as a unit is selected as follows based on the number of passes of the color printer 20 (2 in the example of FIG. 1). That is, a plurality of pixels grouped as a unit are selected so as to be a continuous number of pixels that is an integral multiple of the number of passes, at least in the raster direction. The units shown with fine diagonal lines in the figure indicate units that are being processed, that is, units that are converting image data into dot data. A region indicated by a rough oblique line indicates a region converted to dot data after the presence / absence of dot formation is determined. When the determination of dot formation is performed in units, the same processing is performed for each unit. For this reason, particularly when an area where the gradation value of the image data does not change so much is processed, there is a possibility that dots are formed in a constant pattern across a plurality of units. If a region in which dots are formed in a certain pattern is generated, there is a concern that this is visually recognized and the image quality is deteriorated.

  The conceptual diagram in FIG. 1 shows a case where dots are formed at the same pixel position in a continuous unit as an example in which dots are formed in a constant pattern. As shown in the drawing, on the left side of the unit being processed (the unit with fine slashes), two units in which dots are formed in the pixel in the upper left corner are continuous. If a dot is formed in the pixel in the upper left corner even in the unit being processed, dots will be formed in the pixel at the same position in three units in succession, and this will be visually recognized as a pattern and image quality will deteriorate. There is a fear. Of course, an example in which dots are formed in a fixed pattern is not limited to the case where dots are formed at the same pixel position in successive units, but where dots are formed in a fixed pattern with multiple units as a period. Can also happen.

  In order to avoid the occurrence of such a problem, the gradation number conversion module 14 in the printing system shown in FIG. 1 stores, in the gradation number conversion result storage unit, pixels that have been determined to form dots for units that have already been processed. Remember. When determining the presence or absence of dot formation on a unit basis, the determination result is read from the gradation number conversion result storage unit, and determination is made while avoiding the formation of dots in a constant pattern. For example, in the example shown in the conceptual diagram in FIG. 1, dots are formed at the same pixel position in consecutive units. Whether or not dots are formed is determined. In this way, it is determined whether or not dots are formed while avoiding the formation of dots with a pattern that is easily visible. In this way, it is possible to print a high-quality image without deteriorating the image quality while performing quick processing by determining the presence or absence of dot formation in units. As a unit, a plurality of continuous pixels corresponding to an integral multiple of the number of passes of the color printer 20 are selected at least in the raster direction. As will be described in detail later, if such a plurality of pixels are grouped together, it is possible to avoid the formation of dots in a constant pattern across the plurality of units and at the same time generate periodic patterns. It is possible to avoid the phenomenon that color unevenness occurs and image quality deteriorates. Hereinafter, such image processing will be described in detail based on examples.

B. Device configuration:
FIG. 2 is an explanatory diagram illustrating a configuration of a computer 100 as the image processing apparatus according to the present exemplary embodiment. The computer 100 is a well-known computer configured by connecting a ROM 104, a RAM 106, and the like with a bus 116 around a CPU 102.

  Connected to the computer 100 are a disk controller DDC 109 for reading data from the flexible disk 124 and the compact disk 126, a peripheral device interface PIF 108 for exchanging data with peripheral devices, a video interface VIF 112 for driving a CRT 114, and the like. Has been. A color printer 200, a hard disk 118, and the like, which will be described later, are connected to the PIF 108. Further, if the digital camera 120, the color scanner 122, and the like are connected to the PIF 108, it is possible to print an image captured by the digital camera 120 or the color scanner 122. If the network interface card NIC 110 is installed, the computer 100 can be connected to the communication line 300 to acquire data stored in the storage device 310 connected to the communication line.

  FIG. 3 is an explanatory diagram showing a schematic configuration of the color printer 200 of the first embodiment. The color printer 200 is an ink jet printer capable of forming dots of four color inks of cyan, magenta, yellow, and black. Of course, in addition to these four color inks, an ink jet printer capable of forming ink dots of a total of six colors including cyan (light cyan) ink having a low dye density and magenta (light magenta) ink having a low dye density is used. You can also. In the following, cyan ink, magenta ink, yellow ink, black ink, light cyan ink, and light magenta ink are abbreviated as C ink, M ink, Y ink, K ink, LC ink, and LM ink, respectively. There shall be.

  As shown in the figure, the color printer 200 has a mechanism for driving the print head 241 mounted on the carriage 240 to eject ink and forming dots, and the carriage 240 is reciprocated in the axial direction of the platen 236 by the carriage motor 230. A mechanism for moving the printing paper P by a paper feed motor 235, and a control circuit 260 for controlling dot formation, carriage 240 movement, and printing paper conveyance.

  An ink cartridge 242 that stores K ink and an ink cartridge 243 that stores various inks of C ink, M ink, and Y ink are mounted on the carriage 240. When the ink cartridges 242 and 243 are mounted on the carriage 240, each ink in the cartridge is supplied to ink discharge heads 244 to 247 for each color provided on the lower surface of the print head 241 through an introduction pipe (not shown). Each of the ink discharge heads 244 to 247 for each color is provided with one set of nozzle rows in which 48 nozzles Nz are arranged at a constant nozzle pitch k.

  The control circuit 260 includes a CPU 261, a ROM 262, a RAM 263, and the like. The control circuit 260 controls main scanning and sub-scanning of the carriage 240 by appropriately driving the carriage motor 230 and the paper feed motor 235, and each nozzle based on print data supplied from the computer 100. Ink droplets are discharged at an appropriate timing. Thus, the color printer 200 can print a color image by forming ink dots of respective colors at appropriate positions on the print medium under the control of the control circuit 260.

  Note that various methods can be applied to the method of ejecting ink droplets from the ink ejection heads of the respective colors. That is, a method of ejecting ink using a piezo element, a method of ejecting ink droplets by generating bubbles in the ink passage with a heater arranged in the ink passage, and the like can be used. Also, instead of ejecting ink, use a printer that uses a method such as thermal transfer to form ink dots on printing paper, or a method that uses static electricity to attach each color toner powder onto the print medium. It is also possible to do.

  Furthermore, the size of the ink dots formed on the printing paper can be controlled by controlling the size of the ejected ink droplets, or by ejecting a plurality of ink droplets at a time and controlling the number of ejected ink droplets. A so-called variable dot printer capable of controlling the height can also be used.

  The color printer 200 having the above hardware configuration drives the carriage motor 230 to move the ink ejection heads 244 to 247 of the respective colors in the main scanning direction with respect to the printing paper P, and also feeds the paper. By driving 235, the printing paper P is moved in the sub-scanning direction. The control circuit 260 drives the nozzles at appropriate timing to discharge ink droplets while repeating main scanning and sub-scanning of the carriage 240 according to the print data, so that the color printer 200 prints a color image on the printing paper. doing.

C. Overview of image data conversion process:
FIG. 4 is a flowchart showing a flow of processing in which the computer 100 as the image processing apparatus of the present embodiment converts image data into print data by applying predetermined image processing to the received image data. Such processing is started when the operating system of the computer 100 activates the printer driver 12. Hereinafter, the image data conversion processing of the present embodiment will be briefly described with reference to FIG.

  When the image data conversion process is started, the printer driver 12 first starts reading RGB color image data to be converted (step S100). Next, the resolution of the captured image data is converted to a resolution for printing by the color printer 200 (step S102). When the resolution of color image data is lower than the print resolution, new data is generated between adjacent image data by performing linear interpolation. Conversely, when the resolution is higher than the print resolution, the data is thinned out at a fixed rate. Thus, the resolution of the image data is converted into the printing resolution.

  When the resolution is converted in this way, color conversion processing of color image data is performed (step S104). Color conversion processing refers to color image data expressed by a combination of R, G, and B gradation values, by a combination of gradation values of each color used in the color printer 200 such as C, M, Y, and K. This is a process of converting into expressed image data. The color conversion process can be quickly performed by referring to a three-dimensional numerical table called a color conversion table.

  The printer driver 12 starts the tone number conversion process following the color conversion process (step S106). The gradation number conversion process is the following process. By the color conversion process, the RGB image data is converted into gradation data of C, M, Y, and K colors. The gradation data of each color is data having 256 gradations with gradation values 0 to 255. On the other hand, the color printer 200 according to the present embodiment can take only one of the states of “forming dots” and “not forming dots”. Therefore, it is necessary to convert the gradation data of each color having 256 gradations into image data expressed in 2 gradations that can be expressed by the color printer 200. A process for converting the number of gradations is a gradation number conversion process. As will be described later, the printer driver 12 according to the present embodiment enables rapid processing by collecting a predetermined number of pixels into units and performing gradation number conversion processing on a unit basis. For the processed unit, the pixel determined to form a dot is stored, and the number of gradations is converted while reflecting this data. Further, as will be described in detail later, the size of the unit is set to a predetermined size based on the number of passes of the color printer 200. In this way, it is possible to surely avoid the phenomenon that dots are formed in a constant pattern, which is a concern when the gradation number conversion processing is performed in units of units, and at the same time, to generate periodic patterns. However, it is possible to avoid the occurrence of color unevenness and deterioration of image quality. The gradation number conversion process will be described in detail later.

  When the gradation number conversion process is thus completed, the printer driver starts the interlace process (step S108). The interlace process is a process of rearranging the image data (dot data) converted into a format representing the presence / absence of dot formation into an order to be transferred to the color printer 200 in consideration of the dot formation order. The printer driver 12 outputs the image data finally obtained by performing the interlace processing to the color printer 200 as print data (step S110). The color printer 200 forms ink dots of each color on the print medium according to the print data. As a result, a color image corresponding to the image data is printed on the print medium.

D. First embodiment:
Hereinafter, the tone number conversion process of the first embodiment performed in the image data conversion process shown in FIG. 4 will be described. As described above, in the gradation number conversion process of this embodiment, a plurality of pixels are grouped into units, and the gradation number conversion process is performed in units. Therefore, as a preparation for explaining the gradation number conversion process of the first embodiment, first, a method of performing the gradation number conversion process in units will be described, and then, in the gradation number conversion process of the first embodiment. A process of reflecting the determination result of the processed unit in the determination of the presence / absence of dot formation will be described.

D-1. Tone number conversion processing performed in units:
FIG. 5 is a flowchart showing the flow of gradation number conversion processing performed in units. This process is performed by the CPU 102 of the computer 100. Note that the color printer 200 of this embodiment is a printer that can form ink dots of four colors C, M, Y, and K as described above, and the processing shown in FIG. 5 is also performed for each color. However, in the following description, in order to avoid complicated explanation, the explanation will be made without specifying the color. Unless otherwise specified, when the explanation is made without specifying the color, the same processing is performed for each color. It means to be done.

  When the unit-unit gradation number conversion process is started, first, the position of the unit to be processed is set (step S200). In the gradation number conversion process performed in units, a predetermined number of adjacent pixels are grouped as a unit, and it is determined in units of whether or not to form dots in each pixel. Therefore, when the unit-unit gradation number conversion process is started, first, a process of setting the position of a unit (processing unit) for which it is determined whether or not dots are formed in the image is performed.

  FIG. 6 is an explanatory diagram conceptually showing a state in which a predetermined plurality of adjacent pixels are grouped to set the position of the processing unit. In FIG. 6, a plurality of small squares displayed schematically represent pixels. As shown in the figure, the image is composed of a plurality of pixels arranged in a grid pattern. In the example shown in FIG. 6A, a processing unit is set by grouping four adjacent pixels arranged in two vertical and horizontal columns. In FIG. 6A, a thick broken line surrounding four pixels schematically shows that these pixels are set as a processing unit. For convenience of explanation, among the four pixels constituting the unit, the upper left pixel is called “Pa”, the upper right pixel is “Pb”, the lower left pixel is “Pc”, and the lower right pixel is called “Pd”. A distinction shall be made. In the following description, the unit is assumed to be composed of four pixels arranged in two rows and columns, but of course, the unit is not limited to such a configuration. For example, the processing unit may be set by grouping four adjacent pixels as shown in FIG. Further, the number of pixels grouped as a processing unit is not limited to four.

  After the processing unit is set, image data and diffusion error are read for each pixel in the unit (step S202 in FIG. 5). The image data read here is gradation data of each color of C, M, Y, and K obtained by color conversion by the above-described color conversion process (step S104 in FIG. 4). The diffusion error is an error in which the gradation error caused by converting the number of gradations is diffused from the peripheral unit to each pixel of the processing unit. Since the image data is gradation data for each color of C, M, Y, and K, a gradation error occurs for each of these colors, and therefore the diffusion error is also gradation data for each of these colors. As will be described later, a gradation error is generated along with the conversion of the number of gradations, and the generated error is diffused as a diffusion error to surrounding pixels.

  Here, as shown in FIG. 6A, the processing unit is assumed to be composed of four pixels, pixel Pa, pixel Pb, pixel Pc, and pixel Pd. Therefore, in the process of step S202 in FIG. For these pixels, image data DTa, DTb, DTc, DTd and diffusion errors EDa, EDb, EDc, EDd are read. These data read for the processing unit are stored in the RAM 106 of the computer 100.

  When the image data and the diffusion error of each pixel are read in this way, processing for determining whether or not dots are formed is started for each pixel constituting the processing unit (step S204). Here, as shown in FIG. 6A, since the processing unit is composed of four pixels, in the process of step S204, the presence or absence of dot formation is determined for these four pixels in a predetermined order. Go. In the present specification, the process for determining the presence or absence of dot formation for each pixel in the unit is sometimes referred to as “in-unit multi-value processing”. Details of the multi-value processing in the unit will be described later.

  When the determination of the presence / absence of dot formation for each pixel constituting the processing unit is completed, a process of diffusing the gradation error generated in the processing unit to the peripheral units with the determination is performed (step S206). The gradation error generated in the processing unit is the gradation value of the image data to be expressed in the entire processing unit and the gradation value actually expressed by forming dots at each pixel in the unit. It is an error representing a deviation. A method for calculating the gradation error occurring in the processing unit will be described in the explanation of the multi-value processing in the unit. In step S206, the gradation value indicating the deviation is diffused to the surrounding unprocessed units as a gradation error generated in the processing unit.

  FIG. 7 is an explanatory diagram conceptually showing a state in which the gradation error generated in the processing unit is diffused to surrounding unprocessed units. Each of the plurality of small squares shown in FIG. 7 schematically represents a pixel. A large square indicated by a thick broken line indicates a processing unit, and a large square indicated by a thin broken line indicates an unprocessed unit adjacent to the processing unit. The gradation error generated in the processing unit is diffused at a predetermined ratio to the pixels constituting the surrounding unprocessed units. In FIG. 7, the black arrows extending from the processing unit to the surrounding six pixels schematically indicate that the gradation error generated in the processing unit is diffused to these six pixels at a predetermined rate. Yes.

  FIG. 8 illustrates a state in which a ratio of diffusing gradation errors generated in the processing unit to pixels constituting unprocessed units around the processing unit is set. In FIG. 8, a large square indicated by a thick solid line represents a processing unit, and a large square indicated by a thin broken line adjacent to the processing unit represents an unprocessed unit around the processing unit. is there. A plurality of small squares indicated by thin solid lines in these unprocessed units represent pixels in which an error from the processing unit is diffused.

  In the example shown in FIG. 8A, the gradation error generated in the processing unit is diffused to the two pixels on the right side of the processing unit at a ratio of 1/8, and 2 pixels on the lower side of the processing unit. Similarly, the gradation error is diffused to each pixel at a rate of 1/8. Further, the gradation error is diffused at a ratio of 1/4 to the lower left pixel or the lower right pixel of the processing unit. If the gradation error is diffused at such a ratio, it is diffused evenly to the peripheral units of the processing unit at a ratio of 1/4 of the gradation error. Further, in the example shown in FIG. 8B, the gradation error generated in the processing unit is diffused at the same rate to all the pixels constituting the unit around the processing unit. Of course, errors that diffuse to the same unit may be diffused together in one pixel of that unit.

  In step S206 in FIG. 5, the gradation error generated in the processing unit is diffused to the peripheral units as described above. When all the processes described above are completed for the processing unit, it is determined whether or not the process has been completed for all the units constituting the image (step S208). The following series of processes is repeated. When the processing for all the units is completed in this way, the gradation conversion processing for each unit is finished.

  When the gradation number conversion process is performed on a unit basis as described above, the following advantages can be obtained as compared with the case of processing for each pixel. That is, when the gradation number conversion process is performed for each pixel, the diffusion error stored in association with the pixel must be read every time it is determined whether or not a dot is formed for one pixel. It is necessary to diffuse the error caused by the determination of whether or not dots are formed to surrounding undetermined pixels. For this reason, when the gradation number conversion process is performed for each pixel, the processing time tends to be long. On the other hand, when the gradation number conversion process is performed in units, it is possible to read the image data in units and diffuse the gradation errors, and then write the diffusion errors into the memory in units. If data reading and writing are performed in units in this way, the time required for processing can be shortened.

D-2. In-unit multi-value processing:
The in-unit multilevel conversion process (step S204 in FIG. 5) performed in the unit-unit gradation number conversion process described above will be described. FIG. 9 is a flowchart showing the flow of the in-unit multilevel processing. Hereinafter, it demonstrates according to a flowchart.

  When the intra-unit multi-value processing is started, first, one pixel (a pixel of interest) for determining the presence / absence of dot formation is set from the pixels constituting the processing unit (step S300). Since the unit is composed of a plurality of pixels as described above, one pixel of interest is selected by determining whether or not dots are formed pixel by pixel in a predetermined order for the plurality of pixels. Here, the four pixels constituting the unit shown in FIG. 6A are changed in the order of the pixel Pa, the pixel Pb, the pixel Pc, and the pixel Pd from the upper left pixel to the lower right pixel. In step S300, first, the pixel Pa at the upper left corner is set as the pixel of interest.

  Next, correction data Cx for the set target pixel (here, pixel Pa) is calculated (step S302). The correction data Cx can be obtained by adding the image data of the target pixel and the diffusion error diffused and stored in the target pixel. Since the image data and the diffusion error for each pixel constituting the processing unit have already been read in step S202 during the gradation number conversion processing shown in FIG. Adds the image data of the pixel of interest and the diffusion error from these data to calculate the correction data Cx.

  After calculating the correction data Cx of the pixel of interest Pa, the magnitude relationship between the obtained correction data Cx and the predetermined threshold th is determined (step S304). If the correction data Cx is larger, it is determined that a dot is to be formed on the target pixel (here, pixel Pa) (step S3306). Otherwise, it is determined that no dot is formed on the target pixel (step S308). . The result of the determination is stored in a variable indicating the determination result for each pixel.

  When the presence / absence of dot formation for the target pixel is determined in this way, the gradation error generated in the target pixel is calculated in accordance with the determination (step S310). The gradation error is a gradation value expressed in the pixel of interest by forming a dot or not forming a dot (hereinafter, this gradation value is referred to as a result value). It can be calculated by subtracting from Cx.

  After the gradation error generated in the target pixel is calculated, it is determined whether or not the processing of all the pixels in the processing unit has been completed (step S312). If there are unprocessed pixels remaining (step S312: no). Then, a process of diffusing an error to pixels around the target pixel among these unprocessed pixels is performed (step S314). This process will be described with reference to FIG.

  FIG. 10 is an explanatory diagram schematically showing a state in which the presence / absence of dot formation is determined on a unit basis by selecting each pixel of interest from the pixels in the processing unit and determining the presence / absence of dot formation. It is. Four squares in the figure indicate the pixel Pa, the pixel Pb, the pixel Pc, and the pixel Pd, respectively, constituting the processing unit. DTa and Eda shown in the square indicating the pixel Pa conceptually show that the image data DTa and the diffusion error Eda are read into the RAM 106 in association with the pixel Pa. Similarly, image data DTb and diffusion error EDb are associated with pixel Pb, image data DTc and diffusion error EDc are associated with pixel Pc, and image data DTd and diffusion error EDd are associated with pixel Pd, respectively. Stored in the RAM 106.

  As described above, the target pixel is initially set to the pixel Pa among these four pixels. The correction data Cx is calculated by reading out the image data and the diffusion error for the pixel Pa, and the presence or absence of dot formation for the pixel Pa is determined by comparing with the threshold value th. At the time when the presence or absence of dot formation is determined for the pixel Pa, as shown in FIG. 10A, three pixels of the pixel Pb, the pixel Pc, and the pixel Pd remain as undetermined pixels in the same unit. Therefore, the gradation error generated in the pixel Pa is equally distributed to each of these three pixels by 1/3, and the error is added to the diffusion error stored for each pixel. For example, as shown in FIG. 10A, the diffusion error EDb is already stored in association with the pixel Pb. The gradation error generated in the pixel Pa is distributed to the pixel Pb by 1/3 of the gradation value and added to the diffusion error EDb. As a result, the diffusion error EDb stored in association with the pixel Pb is updated to a new diffusion error EDb to which the error from the pixel Pa is added.

  Similar processing is performed for the other pixels Pc and Pd. Briefly, when the error from the pixel Pa is diffused to the pixel Pc, the diffusion error EDc stored in the pixel Pc and the error from the pixel Pa are added to correspond to the pixel Pc as a new diffusion error EDc. It is memorized. Similarly, for the pixel Pd, the diffusion error EDd stored in the pixel Pd and the error from the pixel Pa are added and stored as a new diffusion error EDd in association with the pixel Pd. In FIG. 10A, the white arrow displayed from the pixel Pa toward the other three pixels schematically shows that the error generated in the pixel Pa is diffused to these three pixels. Is. Note that the gradation error does not necessarily have to be evenly distributed to the surrounding undetermined pixels, and can be distributed to each pixel at a predetermined ratio.

  After the gradation error is diffused to the peripheral pixels in the unit as described above, the process returns to step S300 in FIG. 9 to perform a process for setting a new pixel of interest. Since the pixel Pa in the processing unit has already been determined for dot formation, in step S300, the pixel Pb right next to the pixel Pa is set as the pixel of interest. For the new pixel of interest set in this way, the same processing as that described above is performed. Hereinafter, the difference from the processing in the pixel Pa will be briefly described.

  When the process for the pixel Pb is performed, the correction data Cx is first calculated as in the case of the pixel Pa (step S302). Here, the gradation error previously generated in the pixel Pa is diffused into the diffusion error EDb stored in association with the pixel Pb, and updated as a new diffusion error EDb. Therefore, the correction data Cx for the pixel Pb is calculated by adding the image data DTb for the pixel Pb and the new diffusion error EDb in which the gradation error from the pixel Pa is diffused. The magnitude relationship between the correction data Cx thus calculated and the predetermined threshold th is determined (step S304). If the correction data Cx is larger, it is determined that a dot is to be formed in the pixel Pb (step S306), otherwise. It is determined that no dot is formed on the pixel Pb (step S308). Next, the gradation error generated for the pixel Pb is calculated (step S310). Similar to the case of the pixel Pa, the gradation error can be calculated by subtracting the result value from the correction data Cx of the pixel Pb.

  After the gradation error generated in the target pixel Pb is calculated, it is determined whether or not the processing has been completed for all the pixels in the processing unit (step S312). A process of diffusing the gradation error that has occurred to these unprocessed pixels is performed (step S314). As described above, since the determination for the pixel Pa in the unit has already been completed, two pixels, the pixel Pc and the pixel Pd, remain as undecided pixels. Therefore, the gradation error generated in the pixel Pb is diffused to these two pixels. The process for diffusing the error from the pixel Pb will be described again with reference to FIG.

  FIG. 10B is an explanatory diagram conceptually showing a state in which the gradation error generated in the pixel Pb is diffused. In the figure, the pixel Pa is shaded to schematically indicate that the pixel Pa has already been determined for dot formation. At the time when it is determined whether or not dots are formed for the pixel Pb, two pixels of the pixel Pc and the pixel Pd remain as undetermined pixels in the processing unit. Therefore, the gradation error generated in the pixel Pb is diffused by ½ each to the pixel Pc and the pixel Pd as shown in FIG. In the drawing, white arrows extending from the pixel Pb toward the pixels Pc and Pd schematically indicate that the gradation error generated in the pixel Pb is diffused to these pixels. As a result of the error being diffused in this way, the diffusion error of each pixel is updated to a new diffusion error obtained by adding the diffused error. Here, the gradation error is assumed to be evenly diffused to the undetermined pixels in the processing unit, but of course, the error may be diffused at a predetermined rate instead of being uniformly diffused. .

  After the gradation error generated in the pixel Pb is diffused to the surrounding pixels, the process returns to step S300 in FIG. 9 again, this time the target pixel is moved to the pixel Pc, and the same process is performed for the target pixel Pc. Hereinafter, such processing will be described with reference to FIG. When the process for the pixel Pc is started, first, the correction data Cx is calculated by adding the image data DTc of the pixel Pc and the diffusion error EDc. Here, when the dot formation on / off of the pixel Pc is started, the diffusion error EDc of the pixel Pc is obtained by adding the error from the pixel Pa and the error from the pixel Pb in addition to the originally stored diffusion error. And updated as a new diffusion error EDc. Therefore, when calculating the correction data Cx for the pixel Pc, a new diffusion error EDc is calculated for the image data DTc for the pixel Pc. By comparing the correction data Cx thus obtained with a predetermined threshold th, it is determined whether or not dots are formed for the pixel Pc. Thus, when it is determined whether or not dots are formed for the pixel Pc, the undetermined pixel remaining in the processing unit is only the pixel Pd as shown in FIG. Therefore, all the gradation errors generated in the pixel Pc are diffused to the pixel Pd, added to the diffusion error originally stored in the pixel Pd, and updated as a new diffusion error EDd.

  When the gradation error generated in the pixel of interest Pc is diffused, the process returns to step S300 in FIG. 9 to move the pixel of interest to the pixel Pd, and then the determination of dot formation on the pixel Pd is started. That is, the correction data Cx is calculated by adding the new diffusion error EDd associated with the pixel Pd and the image data DTd of the pixel Pd (steps S300 and S302). Next, the correction data Cx is compared with the threshold value th (step S304). If the correction data Cx is larger, it is determined that a dot is formed in the pixel Pd (step S306). Otherwise, the pixel Pd It is determined that no dot is formed (step S308).

  When it is determined whether or not dots are formed for the pixel Pd in this way, the gradation error generated in the pixel Pd is calculated (step S310), and then it is determined whether or not the processing of all the pixels in the processing unit is completed (step S312). ). As shown in FIG. 10D, at the time when the processing for the pixel Pd is started, there is no longer any undetermined pixel in the processing unit, and in step S312 after the determination for the pixel Pd is completed. Is determined to have been processed for all the pixels in the processing unit. Therefore, the multi-value process in the unit shown in FIG. 9 is terminated, and the process returns to the gradation number conversion process performed in units as described above with reference to FIG. As shown in FIG. 5, in the unit-unit gradation number conversion process, after returning from the multi-value process in the unit, a process of diffusing the gradation error generated in the processing unit to surrounding units is performed (FIG. 5). 5 step S206).

  Here, as the value of the gradation error generated in the processing unit, the value of the gradation error obtained for the pixel Pd in step S310 of the intra-unit multi-value processing can be used. This is because the gradation error generated in the unit is determined to be the last pixel Pd to be determined because the gradation error generated in the unit is determined while diffusing the gradation error generated in the target pixel to the undetermined pixels in the unit. Is accumulated as a diffusion error EDd. Hereinafter, this will be described in detail with reference to FIG.

  The gradation error generated in the pixel Pa is diffused to three pixels of the pixel Pb, the pixel Pc, and the pixel Pd in the processing unit. In determining whether or not the pixel Pb has dot formation, the presence or absence of dot formation is determined in consideration of the error from the pixel Pa so that the error diffused in the pixel Pb is eliminated as much as possible. It is assumed that the error diffused to the pixel Pb can be eliminated by determining the presence or absence of dot formation for the pixel Pb. In this case, a new gradation error does not occur in the pixel Pb, and an error diffused from the pixel Pb to the pixels Pc and Pd does not occur. Similarly, regarding the other two pixels Pc and Pd, it is assumed that the error diffused in each pixel can be eliminated by determining the presence or absence of dot formation. There is no error. Eventually, in this case, the gradation error generated in the pixel Pa is eliminated in the surrounding undetermined pixels Pb, Pc, and Pd, so that the entire processing unit avoids the generation of the gradation error. it can.

  Here, it is assumed that the error in the pixel Pb is eliminated by determining whether or not the dot is formed in the pixel Pb. However, when a new gradation error occurs in the pixel Pb, this error is not detected in the surrounding undetermined pixel Pc. As a result, the gradation error generated in the pixel Pa and the gradation error generated in the pixel Pb are diffused in these pixels. However, if it is possible to eliminate the error diffused in each pixel by determining the presence or absence of dot formation for these pixels Pc and Pd, there will be no gradation error in the entire unit.

  Even if it is not possible to eliminate the diffusion error diffused in the pixel Pc by determining the presence or absence of dot formation for the pixel Pc, if the error can be eliminated in the pixel Pd in the same manner, There is no gradation error. In other words, the remaining error that cannot be resolved in the pixel Pd is a gradation error that occurs in the entire unit. As described above, when the presence or absence of dot formation is determined while diffusing the gradation error generated in the target pixel to the undetermined pixels in the unit, the remaining gradation error that cannot be resolved in each pixel is transferred to the subsequent pixels. Since the data is collected, the gradation error generated in the pixel Pd to be determined last in the unit can be used as the gradation error generated in the entire processing unit.

  In the above description, the value of the gradation error generated in the pixel Pd that finally determines the presence or absence of dot formation in the processing unit is used as the gradation error generated in the entire processing unit. The gradation error may be calculated for each pixel constituting the unit, and the sum of these gradation errors may be used as the gradation error of the processing unit.

  Further, in the example shown in FIG. 10, when determining the presence or absence of dot formation of each pixel constituting the target unit, the determination is performed in the order of the pixel Pa, the pixel Pb, the pixel Pc, and the pixel Pd. For example, it may be determined whether or not dots are formed in the order of the pixel Pa, the pixel Pc, the pixel Pb, and the pixel Pd. The order in which the pixel of interest is set in the processing unit can be selected in order of obtaining good image quality.

D-3. In-unit multilevel processing of the first embodiment:
As described above, when the gradation number conversion process is performed in units, the same process is repeated for each unit. From this, a pixel that is determined to form a dot in a certain unit tends to be easily determined that a dot is formed at the same position in the subsequent unit. Alternatively, although there is a unit where dots are not formed as individual units, dots may be formed in a fixed pattern with a plurality of units as a period. For example, when a certain area of image data continues, almost the same processing is repeated with a plurality of units as a cycle, and it is possible that dots are periodically formed in pixels at the same position in the unit. Of course, the size of the dot itself is so small that it is difficult to visually recognize. However, if dots are formed in a periodic pattern across multiple units, the existence of such a region may be visually recognized and the image quality may deteriorate. There is.

  Furthermore, in a printer that forms one raster by a plurality of main scans, such as the color printer 200 of the present embodiment, a specific pattern in a unit is not necessary even if a periodic pattern is not visually recognized. If dots are formed biased at pixel positions, image quality may deteriorate due to color unevenness. That is, although the main scan is performed a plurality of times, dots are formed in a concentrated manner in a specific main scan, and therefore the effect of improving the image quality due to the formation of the raster divided into a plurality of times cannot be obtained. In some cases, image quality problems such as color unevenness and white stripes may occur.

  In order to avoid the occurrence of such a problem, in the first embodiment, multi-value processing in the unit is performed, and when image data is converted into dot data in units, dots are formed in the already processed units. The detected pixel position is detected, and the presence or absence of dot formation is determined while reflecting the detection result. In this way, if the presence / absence of dot formation in the processing unit is determined while considering the history of pixel positions where dots are formed in the processed unit, the image quality that can be generated by converting the image data in units is determined. Deterioration can be avoided. Further, in this embodiment, the unit collects continuous pixels by the number corresponding to an integral multiple of the number of passes of the color printer 200 at least in the main scanning direction. As will be described later, if the unit is set to such a size, it is possible to avoid the occurrence of problems such as color unevenness and white streak by avoiding the generation of periodic patterns. is there. Hereinafter, the in-unit multi-value processing of the first embodiment will be described.

  As described above, in the intra-unit multilevel processing of this embodiment, the unit size is set to be an integral multiple of the number of passes of the color printer 200 at least in the main scanning direction. Specifically, as shown in FIG. 6, a plurality of pixels each having two pixels are collected at least in the main scanning direction. This corresponds to the fact that the color printer 200 of this embodiment forms one raster divided into two main scans (that is, in two passes). This point will be described with some supplementary explanation.

  FIG. 11 is an explanatory diagram conceptually showing how the color printer 200 forms one raster in two passes. The small rectangle shown in the figure schematically represents a pixel. Assume that a dot row (raster) is formed on one row of pixels shown in the upper part of the drawing. For convenience of description, pixel numbers are assigned in FIG. 11 to identify each pixel. When one raster is formed by two passes, dots are formed every other pixel in each pass. In the example shown in FIG. 11, in pass A, dots are formed on pixels with odd pixel numbers, and in pass B, dots are formed on pixels with even pixel numbers. In FIG. 11, for convenience of understanding, the dots formed in pass A are distinguished by hatched circles, and the dots formed in pass B are distinguished by hatched triangles. In this way, while forming dots on the skipped pixels in each pass, one raster is formed as shown in the lower part of FIG. 11 after the end of the second pass.

  As described above, in this embodiment, in correspondence with the color printer 200 forming a raster in two passes, a total of four pixels of two pixels each in the main scanning direction and the sub-scanning direction are combined as a unit. The pixels grouped together as a unit may be any pixel as long as at least the number of pixels in the main scanning direction is a multiple of the number of passes 2. For example, as shown in FIG. 12, it is also possible to collect a total of eight pixels that are continuous in four pixels in the main scanning direction as a unit.

  Here, the description has been made assuming that the number of passes of the color printer 200 is “2”. However, the matter described here can be similarly applied to other numbers of passes. . That is, in the case of a three-pass printer that forms one raster in three passes, each pixel is a multiple of 3 (that is, any of 3, 6, 9,...) In the main scanning direction. If the printer has 4 passes, multiple pixels of 4 in the main scanning direction may be combined as a unit.

  FIG. 13 is a flowchart showing the flow of multi-value processing of such units in units. In this process, in contrast to the multi-value processing in the unit described above with reference to FIG. 9, the pixel position where the dot is formed in the processed unit is detected, and the tone number conversion process is performed while reflecting the detection result. The point of going is very different. Hereinafter, the intra-unit multi-value processing of the first embodiment will be described with a focus on the differences from the above-described intra-unit multi-value processing.

  Also in the intra-unit multi-value processing of the first embodiment, when processing is started, first, one target pixel is set from the processing unit (step S400), and correction data Cx for the set target pixel is calculated. (Step S402). The correction data Cx can be obtained by adding the image data of the target pixel and the diffusion error diffused and stored in the target pixel. Further, the image data and the diffusion error of each pixel constituting the processing unit have already been read in step S202 during the gradation number conversion process shown in FIG. 5, and data on the pixel of interest is used from these data. Correction data Cx is calculated.

  Next, in the intra-unit multi-value process of the first embodiment, a process of detecting a history of pixel positions where dots are formed in a processed unit, and setting the threshold th to an appropriate value based on the detection result Is performed (step S404). This process (history detection / threshold setting process) will be described with reference to FIG.

  FIG. 14 is a flowchart showing the flow of history detection / threshold setting processing. When the history detection / threshold setting processing is started, first, history data reading processing is performed (step S500). The history data is data in which pixel positions determined to form dots for a processed unit are recorded and stored in a dedicated buffer (history detection buffer) secured in a specific area on the RAM 106. .

  FIG. 15A is an explanatory diagram conceptually showing the structure of history data. The history data shown in FIG. 15A has a length of 12 bits, and each bit corresponds to a pixel position. As described above, since one unit is composed of four pixels here, three units of pixels are stored in the 12-bit history data. Here, although it is assumed that the data for three units is stored in the history data, data of a larger number of units may be stored. Hereinafter, a specific description will be given with reference to FIG.

  As shown in the figure, the history data can be interpreted as three blocks with four consecutive bits as one block. These three blocks are, in order from the lower side (right side in the figure), the unit processed immediately before the unit being processed (“the previous unit”) and the unit processed immediately before that (“the two previous units”). Unit ") and the unit processed immediately before that (" three previous units "). Also, the four bits constituting each block are, from the lower side, a pixel in the lower right corner (pixel Pd), a pixel in the lower left corner (pixel Pc), and a pixel in the upper right corner (pixel Pb) in the corresponding unit. , Corresponding to the pixel (pixel Pa) in the upper left corner. The fact that a value “1” is recorded in a certain bit in the history data means that a dot is formed in the pixel corresponding to that bit, and conversely the value “0” is stored in the bit. If it is, it means that no dot is formed in the pixel corresponding to the bit.

  In FIG. 15, the interpretation of 4 bits as one block corresponds to the fact that one unit is composed of four pixels here (see FIG. 6A). Therefore, if one unit is composed of N pixels, the history data is interpreted as a block of N bits that are continuous. In addition, here, the history data is described as data of 12 bits. However, one unit is composed of N pixels, and the dot formation status from the processing unit to the unit processed before M units is referred to. In this case, the history data may be data having a length of N × Mbit.

  As an example, history data as shown in FIG. Focusing on a group consisting of four bits on the lower side of the history data, the value “1” is set for the most significant (leftmost) bit and the least significant (rightmost) bit in this group, and the other bits The value “0” is set for. This is because in the unit processed immediately before the unit being processed (the “previous unit”), dots are formed in the two pixels Pa and Pd, and no dots are formed in the other pixel positions. Means. Similarly, since the value “0” is stored in any bit for the group in the center of the history data, a dot is formed in any pixel in the unit processed immediately before the processing unit. It means not. For the group on the upper side of the history data, since the value “1” is stored in the most significant bit in the group, a dot is formed at the pixel position of the pixel Pa in the unit processed three times before the processing unit. Is meant to be. By interpreting the history data in this way, it is possible to know the dot formation status shown in FIG. 15C from the history data assumed in FIG.

  In step S500 in FIG. 14, such history data is read from the history detection buffer secured in a predetermined area on the RAM 106. After the history data is read out in this way, subsequently, a process for setting history detection data is started (step S502). Such processing will be described with reference to FIG.

  FIG. 16 is an explanatory diagram conceptually showing how history detection data is set. As an example, consider a case in which it is determined whether or not dots are formed for the pixel Pa in the processing unit. In FIG. 16A, a large square with a broken line indicated by hatching indicates a processing unit, and a black square in the processing unit indicates a pixel of interest. As described above, in the intra-unit multi-value processing, since it is determined on a unit basis whether or not dots are formed in each pixel, dots may be easily formed at the same pixel position. The reason for this will be explained in a little more detail.

  For example, it is assumed that a region being processed in an image is a relatively low gradation image data region (bright image region) in which dots are formed at a rate of one pixel per 36 pixels. In such a region, dots are formed at a rate of approximately one in a square region composed of a total of 36 pixels of 6 pixels vertically and horizontally. In such an image data area, when the presence or absence of dot formation is determined in units of 4 pixels (vertical and horizontal 2 pixels) as one unit, the same processing is repeated with a cycle of 3 units (6 pixels) It is possible that dots are formed at the same pixel position in the third unit. When dots are formed at such a constant cycle, the print pass for forming these dots may be biased to a specific pass as will be described later, and color unevenness may occur to deteriorate the image quality.

  Therefore, in the example shown in FIG. 16A, it is detected whether or not dots are formed in the pixel Pa of each unit in the range from the processing unit to the unit three units before. In FIG. 16A, “?” Shown in the pixel Pa of each unit schematically indicates that the dot formation status is detected at these pixel positions.

  FIG. 16B is an explanatory diagram conceptually showing history detection data set for detecting the dot formation status at these pixel positions. The history detection data also has a length of 12 bits, similar to the history data described above, and is interpreted as being divided into three groups, with 4 bits as one group. The history detection data is composed of three groups corresponding to the detection of dot formation status in three units. The reason why one group is formed by 4 bits corresponds to the fact that four pixels are collectively processed as one unit in this embodiment. The correspondence between each group of history detection data and the unit, and the correspondence between each bit and the pixel position in the unit are the same as in the history data shown in FIG. . Here, the history detection data is described as data having the same length as the history data shown in FIG. 15, but the present invention is not limited to this.

  As shown in FIG. 16A, since it is assumed here that the dot formation status is detected in the pixel Pa of each unit, the history detection data corresponds to the pixel Pa of each unit. The value “1” is set only for the bits to be set, and the data “0” is set for the other bits.

  If the pixel of interest is set to the pixel Pb, similarly, if it is detected whether or not a dot is formed on the pixel Pb of each unit in the range from the processing unit to the unit three units before. Good. FIG. 16C is an explanatory diagram showing a pixel position for detecting the dot formation status in each unit when the target pixel is set to the pixel Pb. A pixel displaying “?” In the figure is a pixel that needs to detect the dot formation status. FIG. 16D is an explanatory diagram conceptually showing history detection data set for detecting the dot formation state in such a pixel. As shown in the figure, history detection data in such a case is data in which the value of the bit corresponding to the pixel Pb of each unit is set to “1” and the other bits are set to the value “0”. Good.

  In step S502 in FIG. 14, as described above, processing for setting history detection data according to the position of the pixel of interest is performed.

  When the history detection data is set in this way, the dot detection status (hereinafter referred to as history detection data is set and detected) by applying the set history detection data to the history data read in step S500. The formation status of the dot that is going to be called may be referred to as “target history”) (step S504).

  FIG. 17A is an explanatory diagram conceptually showing how the history detection data is applied to the history data to detect the target history. The upper part of FIG. 17A shows history data, and the lower part of the history data shows history detection data. When detecting the target history, first, a logical product of corresponding bits is calculated between the history data and the history detection data. In this specification, “acting history detection data on history data” means that intermediate data is obtained by taking the logical product of corresponding bits between history data and history detection data. Say to calculate. Next, the target history can be detected by taking the logical sum or arithmetic sum of each bit of the intermediate data obtained in this way. In FIG. 17A, the intermediate data calculated by applying the history detection data to the history data is shown in the lower part of the history detection data. As described above, in the history detection data, the value “1” is set only at the pixel position to be detected in each unit, and the value “0” is set at the pixel position that does not need to be detected. When the logical product of the bits is calculated from the history data and the history detection data, the bit value corresponding to the pixel position is set to “1” only when a dot is formed at the pixel position to be detected. Is done.

  The above-described processing will be specifically described by paying attention to “the previous unit” shown in FIG. As shown in the history data, dots are formed in the pixel Pa and the pixel Pd in the “previous unit” of the processing unit. Here, since the pixel position where the dot formation status is to be detected is only the pixel Pa as shown in the history detection data, the intermediate data is the pixel Pa as shown in FIG. Only the bit corresponding to “1” has the value “1”, and the other bits have the value “0”. Thus, if any bit of the intermediate data has the value “1”, it means that a dot is formed in the unit at the pixel position to be detected. Next, the logical sum of each unit is calculated, and if the value is “1”, a dot is formed at any pixel position to be detected. Further, by calculating the arithmetic sum of each unit, it is possible to detect how many dots are formed at the pixel position to be detected.

  In step S504 in FIG. 14, processing for calculating the detection result by taking the logical sum (or arithmetic sum) of each bit of the intermediate data is performed as described above. Based on the detection result thus obtained, it is determined whether a target history has been detected (step S506). When the value of the detection result is “0”, it is determined that the target history is not detected. That is, the fact that the value of the detection result (that is, the logical sum of each bit of the intermediate data) is “0” means that no dot is formed at the pixel position to be detected in any unit to be detected. It means not. Therefore, if the value of the detection result is “0”, it can be determined that the target history has not been detected. On the other hand, if the value of the detection result is “1”, this means that a dot is formed at the pixel position to be detected in any of the units to be detected. Can be determined to be detected. In addition, when the arithmetic sum of each bit of the intermediate data is taken, it is determined that the target history is detected if the obtained value is equal to or greater than the predetermined value, and if the arithmetic sum value is less than the predetermined value, the target It may be determined that the history is not detected.

  In the intermediate data shown in FIG. 17 (a), the bits in the “one previous unit” and the “three previous units” are set to the value “1”. A value “1” is obtained by calculating the detection result by taking the logical sum of each bit. Therefore, in this case, the target history is detected.

  As a reference, the dot formation status represented by the history data shown in the upper part of FIG. 17A is shown in FIG. As shown in FIG. 17A, the detection result calculated from the intermediate data is the value “1”, and a result indicating that a dot is formed in the pixel Pa is obtained. As shown in FIG. 17B, a dot is surely formed at the position of the pixel Pa of “the previous unit” and “the previous unit” of the processing unit. From this, it can be confirmed that the dot formation status can be accurately detected by using the method of applying the history detection data to the history data.

  In the history detection / threshold setting processing of FIG. 14, processing for setting the value of the threshold th to an appropriate value is performed based on the target history thus detected. That is, when the target history is not detected in step S506, the normal value Vn is set as the threshold th (step S508). Conversely, when the target history is detected, a value larger than the normal value Vn is set. Is set as the threshold th (step S510). The threshold value th set here is referred to in order to determine whether or not to form a dot on the target pixel during the multi-value processing in the unit. When the threshold value th is set in this way, the history detection / threshold value setting process of FIG. 14 is terminated, and the process returns to the intra-unit multi-value process of the first embodiment shown in FIG.

  When returning from the history detection / threshold setting process, the magnitude relationship between the correction data Cx calculated in step S402 and the threshold th is determined (step S406). The threshold th is set to an appropriate value by detecting the target history during the history detection / threshold setting process. If the correction data Cx is larger (step S406: yes), it is determined that a dot is to be formed on the target pixel (step S408). Otherwise (step S406: no), It is determined that no dot is formed (step S410). As described with reference to FIG. 14, when the target history is detected, the value of the threshold value th is changed to a predetermined premium value (Vp) larger than the normally used value (normal value Vn). When the target history is detected, it becomes difficult to form dots on the target pixel. The determination result is stored in a variable indicating the determination result for each pixel.

  When the presence / absence of dot formation for the pixel of interest is determined as described above, the tone error that accompanies the determination is calculated (step S412). The method for calculating the gradation error is the same as the in-unit multi-value processing shown in FIG. Next, it is determined whether or not the processing of all the pixels in the processing unit has been completed (step S414). If unprocessed pixels remain (step S414: no), after performing processing for diffusing the error to those neighboring pixels in the unit (step S416), the process returns to step S400 to move the pixel of interest. Then, the following series of processes is repeated. In step S416, the process of diffusing the error to the pixels in the processing unit can be performed in the same manner as the above-described intra-unit multilevel processing described with reference to FIGS.

  On the other hand, when it is determined in step S414 that the processing of all the pixels in the processing unit has been completed (step S414: no), a process of writing the determination result for each pixel in the processing unit to the history detection buffer is performed. This is performed (step S418). Such processing will be described with reference to FIGS.

  FIG. 18 is a flowchart showing the flow of processing for writing the determination result for each pixel of the processing unit into the history detection buffer. When the process of writing to the history detection buffer is started, first, the process of shifting the data stored in the buffer by one unit in the upper direction is performed (step S600). FIG. 19 is an explanatory diagram schematically showing such processing. The small square column shown in the figure schematically shows the history detection buffer. The history detection buffer shown in the upper side of the figure shows the state before the data is shifted. The history detection buffer shown on the lower side shows a state after the data is shifted. In step S600 of FIG. 18, the data in the history detection buffer is shifted upward by one unit as shown in FIG. At this time, the data of the highest unit in the history detection buffer is discarded. In addition, a value “0” is written in each bit of the lowest unit after the shift as a precaution. In FIG. 19, the history detection buffer is capable of storing data for three units, but of course, data for a larger number of units may be stored.

  After the data for one unit is shifted in the upper direction in this way (step S600 in FIG. 18), a process for setting 4-bit length data according to the determination result of each pixel in the processing unit is performed (step S602). Each bit of the data is associated with each pixel constituting the processing unit. Specifically, the most significant bit is associated with the pixel Pa of the processing unit, the second most significant bit is associated with the pixel Pb, the third bit is associated with the pixel Pc, and the least significant bit is associated with the pixel Pd. In step S602, a value “1” is written in a bit corresponding to a pixel in which a dot is formed in the processing unit, and a value “0” is written in a bit corresponding to a pixel in which no dot is formed. For example, when a dot is formed only in the pixel Pb in a certain processing unit and no dot is formed in other pixels, in the processing in step S602, “0100 (2)” is obtained as 4-bit length data corresponding to the processing unit. "Will be set. Note that (2) indicates that a binary number is displayed.

  In a succeeding step S604, a process for writing the set data in the area of the lowest unit of the history detection buffer is performed. In FIG. 19, the white arrow toward each bit of the lowest unit schematically shows how the set data is written into these bits.

  As described above, when the process of writing the determination result of each pixel in the processing unit to the history detection buffer is finished, the intra-unit multi-value quantization process of the first embodiment shown in FIG. Return to the tone number conversion process.

  In the above description, when the target history is detected, the setting value of the threshold th used for determination of dot formation presence / absence is changed to a large value to make it difficult for dots to be formed on the target pixel. did. Of course, when the target history is detected, it is possible to determine that no dot is formed in the target pixel. In this way, it is possible to easily avoid the formation of dots at the same pixel position with a plurality of units as a cycle.

  In the above description, history detection data is applied to history data to generate intermediate data, and a detection result is calculated by calculating a logical sum or arithmetic sum of each bit of the intermediate data. On the other hand, the logical product of each bit of the intermediate data is taken, and this value may be used as the detection result. In this case, the value of the detection result is “1” only when dots are formed at all the pixel positions where the dot formation status is to be detected. Therefore, for example, in a situation where dots are formed with a certain density, it is possible to reliably detect only dots with a specific pattern that is easily visible.

  As described above, in the intra-unit multilevel processing of the first embodiment, the presence / absence of the target history is detected during the history detection / threshold setting processing, and when the target history is detected, the value of the threshold th is increased. This makes it difficult for dots to be formed on the target pixel. For this reason, even when the gradation number conversion processing is performed in units of units, an area where dots are formed at the same pixel position with a plurality of units as a cycle is generated, and it is avoided that this area is visually recognized to deteriorate image quality. can do.

  The unit size is set to be an integral multiple of the number of passes at least in the main scanning direction. For this reason, if it is avoided that dots are formed at the same pixel position over a plurality of units, it is possible to simultaneously avoid the deterioration of image quality due to the formation of dots in a specific pass. It becomes possible. Hereinafter, this reason will be described.

  FIG. 20 shows that when the unit size is not an integral multiple of the number of passes in the main scanning direction, dots are formed when trying to avoid the occurrence of dots in a constant pattern across a plurality of units. It is explanatory drawing which showed the reason where a path may be biased to a specific path. Small squares shown in the figure indicate pixels, and large rectangles indicated by broken lines indicate units. For convenience of explanation, only one raster is shown in FIG. 20 as a description will be given focusing on one raster. Further, it is assumed that the raster is formed by two passes, and the unit is formed by collecting three pixels at least in the main scanning direction. The value of 3 pixels is exemplified as a value that is not an integer multiple of the number of passes 2, and the description here applies similarly to other values that are not multiples of 2, such as 5 pixels or 7 pixels. be able to.

  Now, it is assumed that there is an area in which dots are formed at a rate of 1 in 9 pixels in an image to be printed. When the unit is grouped in units of 3 pixels in the main scanning direction and it is determined whether or not dots are formed on a unit basis, in such an area, dots are skipped by 3 pixels unless the special control shown in FIGS. 13 and 14 is performed. It is easy to generate a certain pattern that is formed. FIG. 20A shows a state in which dots are formed every three pixels when special control is not performed. In the figure, a circle with a diagonal line displayed in a square indicating a pixel represents a dot formed on the pixel. In this embodiment, in order to avoid the deterioration of the image quality due to the dots being formed in a constant pattern in this way, as shown in FIGS. 13 and 14, whether or not dots have been formed in the already processed unit. When the determination result is detected and dots are formed in a certain pattern, it is difficult to form dots on the pixels.

  FIG. 20B shows how the dots are prevented from being formed in a certain pattern in this way. For example, in the second unit from the left in the figure, when dots are formed at the pixel position at the upper left corner in the unit, dots are continuously formed at the same pixel position as the head unit. Therefore, the pixel in the upper left corner of the second unit is made difficult to form a dot, and as a result, the pixel position where the dot is formed is shifted to the adjacent pixel. Similarly, the fourth unit from the left represents a state in which the pixel position where the dot is formed is shifted to the adjacent pixel.

  As shown in FIG. 20B, it is possible to avoid the formation of dots in a constant pattern by shifting the dot formation position in this way. However, since the dots are shifted in this way, it is possible that the path for forming the dots is biased to a specific path. Here, it is assumed that one raster is formed by two passes of pass A and pass B, and in pass A, dots are formed in odd-numbered pixels, and in pass B, dots are formed in even-numbered pixels. . As shown in FIG. 20A, before the dot formation position is shifted, dots are formed at almost the same rate in pass A and pass B. On the other hand, after shifting the dot formation position, all the dots are formed by pass A as shown in FIG. Thus, if the pass for forming dots is biased to a specific pass, the effect of improving the image quality by forming a raster with multiple passes cannot be obtained, resulting in color unevenness and white streaks. May be reduced. In particular, when a color image is printed using multi-color ink as in the color printer 200, such color unevenness tends to occur. This is because when ink dots of multiple colors are formed, depending on the order in which the ink dots of different colors are formed, the degree of mixing of the inks and the degree of bleeding of the dots formed later will be different. Since a subtle difference occurs, if ink of a certain color is formed in a specific pass in a specific pass, dots are formed in a specific order different from the surrounding area, and the color development is different, which is recognized as color unevenness. It is because it will be done.

  In this embodiment, in consideration of these points, the unit size is set to be an integral multiple of the number of passes at least in the main scanning direction. FIG. 21 is an explanatory diagram showing a state in which the size of the unit in the main scanning direction is set to an integral multiple of the number of passes to prevent the dots forming pass from being biased to a specific pass. . Here, since the color printer 200 forms a raster in two passes, the size of the unit in the main scanning direction is set to 4 pixels. Of course, the size of the unit in the main scanning direction is not limited to 4 pixels, and can be any size as long as it is a multiple of 2 as the number of passes, such as 2 pixels or 6 pixels.

  When the unit is composed of a plurality of pixels of 4 pixels in the main scanning direction, and the presence or absence of dot formation is determined in units of units, a plurality of units are required unless the special control described above with reference to FIGS. 13 and 14 is performed. It can happen that dots are formed in a constant pattern across the unit. FIG. 21A shows, as an example, a state where dots are formed every four pixels. In this embodiment, in order to avoid the occurrence of such a constant pattern, as described with reference to FIGS. 13 and 14, the determination result of the dot formation presence / absence in the already processed unit is detected. When dots are formed with a certain pattern, it is difficult to form dots on the pixels.

  FIG. 21B shows a state where dots are thus prevented from being formed in a certain pattern. In the second unit and the fourth unit from the left in the figure, the pixel position where the dot is formed is shifted from the pixel position at the upper left corner in the unit to the adjacent pixel position. By doing so, it is possible to avoid the formation of dots at the same pixel position continuously from the head unit. In the printing pass as well, in FIG. 21B in which the dot positions are shifted, dots are formed at substantially the same rate in pass A and pass B. On the other hand, in FIG. 21A in which the dot positions are not shifted, dots are formed with a bias toward pass A. As is clear from comparison between FIGS. 21A and 21B, the printing pass is changed from pass A to pass B by shifting the pixel position for forming the dots. The biased print paths are distributed. That is, if the unit size is set to an integral multiple of the number of passes at least in the main scanning direction, dots formed at the same pixel position in the unit are formed in the same pass. For this reason, if the dot position is shifted in an attempt to avoid the generation of a pattern over a plurality of units, this immediately leads to the distribution of the print path. For this reason, if it is avoided that dots are formed at the same pixel position over a plurality of units, it is possible to simultaneously avoid the deterioration of image quality due to the formation of dots in a specific pass. It becomes possible.

D-4. Variation:
Various modifications exist in the history detection / threshold setting process of the first embodiment described above. Hereinafter, these various modifications will be described.

(1) First modification:
In the history detection / threshold setting process of the first embodiment described above, it has been described that the dot formation status is always detected by going back by 3 units, but the number of units to go back can be made variable. Hereinafter, such a first modification will be described.

  FIG. 22 is a flowchart illustrating a flow of history detection / threshold setting processing according to the first modification. Such processing differs greatly from the processing of the first embodiment described above in that the number of units to be traced back can be changed according to the image data in the processing unit. Hereinafter, it demonstrates according to a flowchart centering on difference.

  When the history detection / threshold value setting process of the first modified example is started, first, a total value of image data for each pixel constituting the processing unit is calculated (step S700). The image data of each pixel constituting the processing unit is read in advance in step S202 during the gradation number conversion process shown in FIG. 5, and the total value of the image data of each pixel can be easily calculated.

  In the first modification, how many units before the dot formation status is checked is set based on the total value of the image data of the processing unit. This is due to the following reason. When the total value is small, it is considered that an area where the gradation value of the image data is small (that is, a bright image area) is being processed. In such a region, since dots are formed sparsely, it is considered that a pattern with a long cycle is formed even when a pattern is generated in the formation of dots by processing in units. For this reason, the number of retroactive units is set to a large value so that a pattern with a long period can be detected in a region where the gradation value of the image data is small. For the number of units to be traced back, some appropriate numerical values are set in advance step by step according to the value of the total value. In step S702, after setting the number of units going back in accordance with the total value, a process of reading data having a bit length corresponding to the number of units from the history detection buffer described above is performed.

  When the history data is read from the history detection buffer in this way, history detection data is subsequently set (step S704). This process is substantially the same as the history detection / threshold value setting process of the first embodiment described above with reference to FIG. 14, except that the bit length of the history detection data varies depending on the processing unit. That is, in the first modification, the sum value of the image data of each pixel constituting the processing unit is calculated, and the dot formation status is detected retroactively by the number of units corresponding to the sum value. Corresponding to this, the history detection data is also set to data having a length corresponding to the total value.

  Next, the history detection data is applied to the history data to detect the presence or absence of the target history (the formation status of the dot to be detected). When the target history is detected, the threshold th value is increased to a large value (Vp When the target history is not detected, the threshold value th is set to the normal value Vn (steps S706 to S712). When the above processing is completed, the history detection / threshold setting processing of the first modification is exited, and the processing returns to the in-unit multi-value processing shown in FIG.

  According to the history detection / threshold setting process of the first modification described above, the dot formation status can be detected retroactively by the appropriate number of units according to the gradation value of the image data being processed. . For this reason, it becomes possible to prevent deterioration of image quality more reliably. In the first modification described above, the total value for each pixel in the processing unit is calculated. If the total value is small, a bright image region (region where the gradation value of the image data is small) is being processed. It explained that it was judged that there was. Of course, it may be detected using a different method that the area being processed is a bright image area. For example, it is also possible to detect using the average value of the image data in the peripheral area. Further, in the history detection / threshold value setting process of the first modification described above, the number of units to be detected retroactively for dot formation has been described as being set according to the total value of each pixel in the processing unit. However, the present invention is not limited to this, and the number of units to be traced back or the pixel position in the unit may be set according to the gradation value of each pixel. This is preferable because the number of retroactive units can be set to a finer and appropriate value.

(2) Second modification:
In the various embodiments described above, the description has been made centering on the case where the pixel position in the processed unit for detecting whether or not dots are formed is the same pixel position as the target pixel of the unit being processed. Of course, the pixel position for detecting the dot formation status in the processed unit can be freely set as necessary by appropriately setting the history detection data described above. Hereinafter, such a second modification will be described.

  FIG. 23 is an explanatory view exemplifying how history detection data is set in the second modification. For example, when determining the presence or absence of dot formation for the pixel Pa of the processing unit, as shown in FIG. 23A, the dot formation status of the pixel Pa is detected for the “previous unit” of the processing unit. As for both “second previous unit” and “third previous unit”, the dot formation status of the pixel Pc is detected. The “?” Mark shown in the drawing schematically indicates that the dot formation status for the corresponding pixel is detected. If the pixel position for detecting the dot formation state is set in this way, the pattern of “lower left pixel”, “lower left pixel”, “upper left pixel”, and “upper left pixel” with a period of 4 units. Thus, it is possible to avoid the formation of dots periodically.

  FIG. 23B shows history detection data set to detect the pixel position indicated by “?” In FIG. As described above, the history detection data is composed of three groups of four adjacent bits. As shown in FIG. 23 (b), each group is shown in FIG. 23 (a). It corresponds to three units. The four bits constituting each group correspond to the pixel Pa, the pixel Pb, the pixel Pc, and the pixel Pd in order from the upper side (left side in the figure). The history detection data can be obtained by setting a value “1” for the bit corresponding to the pixel position to be detected and setting a value “0” for the other bits. FIG. 23C shows the history detection data obtained in this way displayed in hexadecimal. As described above, since the history detection data represents one unit with 4 bits, the history detection data can be simply expressed by displaying 4 bits together in hexadecimal. The history detection data shown in FIG. 23B can be expressed as “0x228” by displaying in hexadecimal. Note that “0x” means that hexadecimal numbers are displayed.

  In the above-described embodiment, the number of pixels for detecting the dot formation status is one pixel for each unit. Of course, it is also possible to detect the dot formation status for a plurality of pixel positions per unit. good. FIG. 24 is an explanatory diagram showing how the dot formation status is detected at a plurality of pixel positions. In this case, as shown in FIG. 24A, the dot formation status is detected at three pixel positions for the “previous unit” of the processing unit. FIG. 24B is an explanatory diagram showing history detection data set to detect the dot formation status at such a pixel position. As shown in the drawing, with respect to “the previous unit”, in addition to the pixel Pa, the dot formation status is detected also at the two pixel positions of the pixel Pb and the pixel Pd. Correspondingly, the value “1” is set in three bits corresponding to the pixel Pa, the pixel Pb, and the pixel Pd in the lower four bits of the history detection data. That is, “D” is set in hexadecimal notation. Further, as for the “second previous unit” and the “third previous unit”, the dot formation status of the pixel Pa is detected as in the case of FIG. 23. Therefore, these units are displayed in hexadecimal notation. “8” is set, and as a result, data “0x88D” is set as history detection data as shown in FIG.

  As described above, by appropriately setting the history detection data, it is possible to freely set the pixel position for detecting the dot formation status in the processed unit. Therefore, even when dots are easily formed in a specific pattern over a plurality of units, such a pattern can be reliably avoided. In addition, since the unit size is set to an integral multiple of the number of passes at least in the main scanning direction, dots may be formed biased to a specific pass by avoiding the occurrence of a pattern. It can be avoided at the same time. In addition, as shown in FIG. 24, if the dot formation status at the pixel position in the vicinity of the processing unit is detected, the formation of dots in the vicinity of the pixel of interest can be avoided. For this reason, it is possible to improve the image quality by improving the dispersibility of the dots.

(3) Third modification:
It is also possible to set the pixel position for detecting the dot formation status using a random number. For example, first, the number of units to be detected retroactively may be set using a random number, and then the pixel position to be detected for each unit may be set using a random number. In this way, since there is no regularity in the setting of the pixel position for detecting the dot formation state, it is possible to reliably avoid the occurrence of a periodic pattern over a plurality of units.

E. Second embodiment:
In the history detection / threshold setting processing performed during the unit multi-value processing of the first embodiment described above, history data representing the dot formation status of the processed unit is stored in one history detection buffer. It was explained as being. Since the unit includes a plurality of pixels, in the first embodiment, data indicating the presence or absence of dot formation for each pixel is mixedly stored on the history detection buffer. On the other hand, as many buffers as the number of pixels constituting the unit are provided, and it is determined whether or not dots are formed for each pixel, the determination results may be stored in different buffers in association with the pixel positions in the unit. . Hereinafter, a second embodiment using a dedicated buffer for each pixel position will be described.

E-1. In-unit multi-value processing of the second embodiment:
FIG. 25 is a flowchart showing the flow of the intra-unit multi-value processing of the second embodiment. Such processing is largely different from the intra-unit multilevel processing of the first embodiment described above with reference to FIG. 13 in that a history detection buffer is provided for each pixel position. Further, it is different from the first embodiment in that it is derived from such a difference and every time it is determined whether or not dots are formed for the target pixel, the determination result is written in the corresponding history detection buffer. . In the following, the in-unit multi-value processing of the second embodiment will be described focusing on these differences from the first embodiment.

  When the process is started, first, a target pixel from which a dot formation presence / absence is to be determined is detected from a plurality of pixels constituting the processing unit (step S800), and image data and diffusion error of the target pixel are detected. The correction data Cx is calculated by addition (step S802). As the image data and diffusion error for each pixel constituting the processing unit, data already read during the gradation number conversion processing shown in FIG. 5 is used.

  When the correction data Cx for the pixel of interest is thus calculated, the history detection / threshold setting process of the second embodiment is started (step S804). As described above, in the second embodiment, a dedicated history detection buffer provided according to the pixel position in the unit is used. Therefore, before describing the contents of such processing, the history detection buffer used in the history detection / threshold setting processing of the second embodiment will be described.

  FIG. 26 is an explanatory diagram schematically showing a history detection buffer provided in association with the pixel position in the unit. The “history detection buffer A” shown in the figure is a buffer associated with the pixel Pa in the unit. In this buffer, the dot formation determination result for the pixel Pa is stored. The “history detection buffer B” is a buffer that stores the determination result for the pixel Pb in association with the pixel Pb in the unit. Similarly, the determination result for the pixel Pc in the unit is recorded in the “history detection buffer C”, and the determination result for the pixel Pd is stored in the “history detection buffer D”. Thus, in the second embodiment, dedicated buffers associated with the pixel positions in the unit are provided, and history data is stored in the respective buffers.

  Next, taking the “history detection buffer A” as an example, the data structure of the stored history data will be described. Each bit of the history detection buffer A is associated with the previous unit one by one from the processing unit in order from the lower side (right side in FIG. 26). That is, the least significant bit (the rightmost bit in the figure) is associated with the “first unit” of the processing unit, and the second bit from the lower level is assigned to the “second previous unit” and the lower 3 bits. Each bit is associated with each unit in order, for example, the second bit is “3rd previous unit”. The value “1” being set in these bits indicates that a dot is formed in the pixel Pa of the corresponding unit, and conversely that the value “0” is set corresponds. This indicates that no dot is formed on the pixel Pa of the unit.

  As is clear from the above description, the history data illustrated in FIG. 26 stored in the history detection buffer A is the units one, three, five, eight, and eight before the processing unit. Indicates that a dot is formed in the pixel Pa, and no dot is formed in the pixel Pa for the other units. In FIG. 26, each history detection buffer has a data length of 8 bits, but it may be data having a larger number of bits.

  The history detection / threshold setting process of the second embodiment performed using such history detection data will be described below. The history detection / threshold setting processing of the second embodiment is different from the processing of the first embodiment described above in that a history detection buffer is provided for each pixel position, and the other processing is almost the same. It is. Accordingly, the following description will be made with reference to the flowchart of FIG. 14 describing the history detection / threshold setting processing of the first embodiment. This process is a process executed in step S804 on the target pixel after the target pixel is set (step S800) in the intra-unit multi-value quantization process of FIG.

  When the history detection / threshold value setting process of the second embodiment is started, first, history data is read from the history detection buffer (corresponding to step S500 in FIG. 14). As described with reference to FIG. 26, in the second embodiment, a history detection buffer is provided according to the pixel position, and in this process, a necessary data amount is obtained from the buffer corresponding to the same pixel position as the target pixel. Read the history data. For example, assuming that the pixel of interest is located in the pixel Pa and the dot formation status is detected retroactively by 5 units, data of 5 bits is read from the lower side of the “history detection buffer A”. As a result, in the example shown in FIG. 26, “10101 (2)” is read as the history data. Here, (2) indicates that a binary number is displayed. If the target pixel is set to the pixel Pb, the pixel Pc, and the pixel Pd, the history data recorded in the history detection buffer B, the history detection buffer C, and the history detection buffer D are read out. Good.

  When the history data is read in this way, subsequently, processing for setting history detection data is performed (corresponding to step S502 in FIG. 14). Such processing will be described with reference to FIG. For example, it is assumed that the dot formation status is detected retroactively from the processing unit to the previous unit. FIG. 27A is an explanatory diagram conceptually showing the distribution of pixel positions for which the dot formation status is to be detected. Each small square in the drawing schematically shows a pixel. A large square indicated by a thick broken line schematically shows a unit being processed, and a large square indicated by a thin broken line schematically shows a unit that has already been processed. The hatched pixel in the processing unit indicates the target pixel, and the pixel marked with “?” In the processed unit indicates the pixel for which the dot formation status is to be detected.

  FIG. 27B shows history detection data used to detect the dot formation status for such pixels. Here, since it is assumed that detection is performed retroactively from the processing unit to the previous unit, the value “1” is set in the bits from the least significant bit to the 5th bit in the history detection data, and the higher bit than the 6th bit is set. Is set to the value “0”. In this way, history detection data can be set by setting a value “1” in the bit corresponding to the unit to be detected and setting a value “0” in the other bits. FIG. 27 (c) shows the history detection data set in this way displayed in hexadecimal.

  When history detection data is set in this way, the target history (the dot formation status at the pixel position to be detected) is detected by applying the set history detection data to the previously read history data (FIG. 14). Step S504). The target history is detected in the same manner as in the first embodiment. That is, intermediate data is calculated by taking the logical product of corresponding bits between the history data and the history detection data, and taking the logical sum (or arithmetic sum) of each bit of the obtained intermediate data. The detection result is calculated. If the obtained detection result is the value “1”, it is determined that the target history has been detected, and the premium value Vp is set to the threshold th. If the detection result is the value “0”, it is determined that the target history has not been detected, and the normal value Vn is set as the threshold th. When the threshold value th is set to an appropriate value as described above, the history detection / threshold value setting process of the second embodiment is terminated, and the process returns to the in-unit multilevel conversion process shown in FIG. Here, the detection result is calculated by taking the logical sum (or arithmetic sum) of each bit of the intermediate data, but the detection result may be calculated by taking the logical product of each bit of the intermediate data. Good.

  When the threshold value th is set as described above and the process returns to the in-unit multivalue processing of the second embodiment shown in FIG. 25, the magnitude relationship between the correction data Cx of the pixel of interest and the threshold value th is subsequently determined. (Step S806 in FIG. 25). If the correction data Cx is larger, it is determined that a dot is to be formed on the pixel of interest (step S808). Otherwise, it is determined that no dot is to be formed (step S810). Next, after calculating a gradation error caused by determining the presence or absence of dot formation (step S812), a process of writing the determination result for the pixel of interest into the corresponding history detection buffer is started (step S814). Such processing will be described with reference to FIG.

  FIG. 28 is an explanatory diagram conceptually showing a state in which the determination result for the target pixel is written in the history detection buffer in the unit multi-value quantization process of the second embodiment. Assuming that the pixel of interest is set to the pixel Pa, the determination result of dot formation presence / absence is written in the history detection buffer A. In writing the determination result, the history data stored in the history detection buffer A is first shifted to the upper side (left side in the figure) by 1 bit. The upper part of FIG. 28 shows history data stored in the history detection buffer A before shifting, and the lower part shows history data stored in the buffer after shifting. As the history data is shifted, the data stored in the most significant bit is discarded. Next, a process of writing a value corresponding to the determination result for the pixel of interest in the least significant bit is performed. If it is determined that a dot is to be formed in the pixel of interest, the value “1” is written in the least significant bit, and if it is determined that no dot is to be formed in the pixel of interest, a value “0” is written. In the above description, the pixel of interest has been described as being set to the pixel Pa. However, the history detection corresponding to the pixel position of the pixel of interest is similarly performed when the pixel of interest is set to another pixel position. A value corresponding to the determination result may be written in the buffer. In step S814 in FIG. 25, processing for writing the determination result for the target pixel in this way into the history detection buffer corresponding to the target pixel is performed.

  When the process of writing the determination result of the target pixel is completed (step S814 in FIG. 25), it is determined whether the process for all the pixels in the processing unit is completed (step S816), and unprocessed pixels remain. If so, the process returns to step S800 to repeat the series of processes described above. When the process for all the pixels is completed, the process returns to the gradation number conversion process shown in FIG.

  In the intra-unit multilevel processing of the second embodiment described above, each bit of the history detection buffer is associated with each processed unit. Accordingly, even when the number of units to be examined retrospectively for the dot formation status increases, it is only necessary to read data having the same bit length as the number of units. For example, when detecting the dot formation status by going back 8 units from the unit being processed, it is necessary to read the history data of 32 bits in the first embodiment, assuming that one unit is composed of four pixels. However, in the second embodiment, it is only necessary to read 8-bit data. For this reason, in the second embodiment, even if the number of units to be traced back is large, a quick process can be performed.

E-2. Modification of the second embodiment:
In the second embodiment described above, the pixel position for detecting the dot formation state is described as being the same pixel as the pixel of interest. However, similarly to the first embodiment, the pixel position for detecting the dot formation status can be freely set. In the second embodiment, the pixel for detecting the dot formation status is also a unit. It is possible to set an arbitrary pixel position every time. Hereinafter, a modification of the second embodiment will be described.

  FIG. 29 is an explanatory diagram showing a method of detecting the dot formation status at an arbitrary pixel position in the second embodiment. FIG. 29A is an explanatory diagram illustrating the distribution of pixel positions to be detected. The small squares shown in the figure schematically indicate pixels, and the large squares indicated by broken lines indicate units. The pixel position where the dot formation state is to be detected is indicated by a “?” Mark in the pixel. Although the pixel position and the number of pixels to be detected can be set arbitrarily, the following description will be made assuming that the dot formation status at the pixel position shown in FIG.

  FIG. 29B is an explanatory diagram illustrating a method for detecting a dot formation state at an arbitrary pixel position in the second embodiment. In the second embodiment, as described above, since the determination result of dot formation presence / absence is stored for each pixel position of each unit, each dot formation state at an arbitrary pixel position can be detected. Detection is performed by a method focusing on the pixel position of the unit. That is, in the example shown in FIG. 29A, the dot formation status for the pixel position of the pixel Pa is to be detected by the “first unit”, “second unit” of the processing unit, These are the three units of “3rd previous unit”. Further, the pixel position of the pixel Pb is two units of “the previous unit” and “the second previous unit” of the processing unit. Furthermore, as for the pixel position of the pixel Pc or the pixel Pd, the dot formation status is detected only in the “one unit before” of the processing unit. In this way, after detecting the unit position for detecting the dot formation status for each pixel position, if it is determined for each pixel position whether or not a dot is formed in the detected unit, any pixel position can be determined. It is possible to detect the dot formation status. Hereinafter, specific description will be made in accordance with the case shown in FIG.

  First, the case where the dot formation status of the pixel Pa is detected will be described. FIG. 29B schematically shows how the dot formation status at the pixel position of the pixel Pa is detected in this way. First, the history data for 3 units (data corresponding to 3 bits from the lowest order) is read from the buffer (history detection buffer A) in which the dot formation determination result at the pixel Pa is stored. Of course, when detecting a larger number of units retrospectively, the data length to be read out may be increased accordingly.

  Next, history detection data is applied to the history data. The history detection data is set according to the unit that is trying to detect the dot formation status at the pixel Pa. As shown in FIG. 29 (a), with respect to the pixel Pa, the dot formation status is detected by three units of the processing unit "one previous unit", "two previous units", and "three previous units". In response to this, in the history detection data, the value “1” is set in the three least significant bits. The history detection data set in this way ("111 (2)" in binary notation) is applied to the read history data to calculate intermediate data. “Making history detection data act on history data” means taking a logical product between corresponding bits of two data as described above. When the intermediate data is obtained in this way, a detection result for the pixel Pa can be obtained by calculating a logical sum (or arithmetic sum) between the bits of the intermediate data.

  The detection result thus obtained for the pixel Pa means the following. That is, the detection result having the value “0” means that no dot is formed in the pixel Pa in any of the units to be detected. On the other hand, the logical sum as the detection result being “1” means that a dot is formed in the pixel Pa in any of the units to be detected. When the detection result is obtained by calculating the arithmetic sum, the obtained numerical value represents the number of dots formed. Of course, instead of taking the logical sum (or arithmetic sum) of each bit of the intermediate data, a logical product may be taken. When the logical product of each bit of the intermediate data is taken, the detection result value of “1” means that dots are formed in the pixels Pa of all the units to be detected. Become.

  The detection result for the pixel Pb can be obtained by the same method. FIG. 29C schematically shows how the dot formation status for the pixel Pb is detected. Briefly described with reference to FIG. 29 (b), first, history data for three units is read from a buffer (history detection buffer B) in which the presence or absence of dot formation for the pixel Pb is recorded. Next, history detection data is applied to this data to calculate intermediate data. For the pixel Pb, since the dot formation status of the “one previous unit” and “two previous units” of the processing unit is to be detected, the history detection data corresponding to this is the least significant bit. From which two bits are set to the value “1” (“011 (2)” in binary notation). Thus, by obtaining a logical sum (or arithmetic sum) between each bit of the obtained intermediate data, a detection result for the pixel Pb can be obtained.

  The detection result for each pixel can be calculated in the same manner for the pixels Pc and Pd. For these pixels, the unit that is trying to detect the dot formation status is only “the previous unit” of the processing unit. Therefore, for these pixels, the value “1” is set only for the least significant bit as history detection data. May be used (“001 (2)” in binary display). FIG. 29D shows how the detection result for the pixel Pc is calculated, and FIG. 29E shows how the detection result for the pixel Pd is calculated.

  Once the detection results for each pixel are obtained in this way, the final detection result can be obtained by calculating the logical sum of these detection results. That is, if a dot is formed in any of the pixels indicated by “?” In FIG. 29A, a value “1” is obtained as a detection result, and conversely, a dot is formed in any pixel. If not, the value “0” is obtained as the detection result. Of course, the final detection result may be calculated by taking the logical product of the detection results for each pixel. In such a case, when dots are formed in all the pixels indicated by “?” In FIG. 29A, the final detection result value is “1”.

  By using the method described above, it is possible to freely set a pixel position from which a dot formation state is to be detected from among already processed units. Therefore, even in the case where the presence / absence of dot formation is determined on a unit basis, the dot formation status at a predetermined pixel position is detected, and the presence / absence of dot formation is determined based on the detection result. It is possible to effectively avoid the formation of dots in a predetermined pattern. In addition, since the unit size is set to an integral multiple of the number of passes at least in the main scanning direction, dots may be formed biased to a specific pass by avoiding the occurrence of a pattern. It can be avoided at the same time.

  Although various embodiments have been described above, the present invention is not limited to all the embodiments described above, and can be implemented in various modes without departing from the scope of the invention.

  For example, in the various embodiments described above, a so-called error diffusion method is used as a method for determining the presence or absence of dot formation for each pixel constituting the unit. Techniques can also be applied.

  Further, in the various embodiments described above, when it is detected that a dot is formed at a predetermined pixel position in a processed unit, it is assumed that it is difficult to form a dot for the pixel of interest. did. On the other hand, when a dot is not formed at a predetermined pixel position, the dot may be easily formed at the target pixel. In this way, for example, in a region where dots are formed at a high density, when pixels where dots are not formed periodically occur across a plurality of units, deterioration of image quality can be effectively avoided. it can.

  A software program (application program) that realizes the above-described functions may be supplied to a main memory or an external storage device of a computer system via a communication line and executed. Of course, a software program stored in a CD-ROM or a flexible disk may be read and executed.

  In the various embodiments described above, the image data conversion process including the gradation number conversion process has been described as being executed in the computer. However, part or all of the image data conversion process is performed on the printer side or a dedicated image processing apparatus. It may be executed by using.

  Further, the image display device is not necessarily limited to a printing device that prints an image by forming ink dots on a print medium. For example, bright spots are dispersed at an appropriate density on a liquid crystal display screen. Thus, a liquid crystal display device that expresses an image whose gradation changes continuously may be used.

1 is a schematic configuration diagram of a printing system illustrating an overview of the present invention. It is explanatory drawing which shows the structure of the computer as an image processing apparatus of a present Example. 1 is a schematic configuration diagram of a printer as an image display apparatus according to an embodiment. It is a flowchart which shows the flow of the image data conversion process performed with the image processing apparatus of a present Example. It is a flowchart which shows the flow of the gradation number conversion process performed per unit. It is explanatory drawing which shows a mode that the process unit is set collectively for several pixels. It is explanatory drawing which illustrated a mode that the gradation error which generate | occur | produced in the processing unit was diffused to the surrounding undetermined pixel. It is explanatory drawing which illustrated the ratio which diffuses the gradation error which generate | occur | produced in the processing unit to the surrounding undetermined pixel. It is a flowchart which shows the flow of the process which multi-values the inside of a unit by determining the presence or absence of dot formation about each pixel which comprises a unit. It is explanatory drawing which showed notionally the method of determining the presence or absence of dot formation for every pixel, diffusing an error to each pixel in a unit. It is explanatory drawing which showed notionally the mode that one raster is formed by 2 passes. It is explanatory drawing which illustrated a mode that a total of eight pixels which continued 4 pixels at a time in the main scanning direction were put together as a unit. In the first embodiment, it is a flowchart showing a flow of processing for multi-valued in the unit by determining the presence or absence of dot formation for each pixel in the unit. 6 is a flowchart showing a flow of processing for detecting a dot formation state and setting a threshold value in the first embodiment. It is explanatory drawing which showed notionally the structure of the historical data read in the log | history detection and threshold value setting process of 1st Example. It is explanatory drawing which showed notionally the structure of the data for log | history detection set in order to detect a log | history in the log | history detection and threshold value setting process of 1st Example. It is explanatory drawing which showed notionally the mode that the data for log | history detection act on log | history data in the log | history detection and threshold value setting process of 1st Example, and a target log | history is detected. 6 is a flowchart showing a flow of processing for writing a determination result for each pixel in a unit into a history detection buffer storing history data in the first embodiment. In the first embodiment, it is an explanatory diagram conceptually showing a state in which a determination result for each pixel in the unit is written in a history detection buffer storing history data. If the size of the unit is not an integral multiple of the number of passes in the main scanning direction, if it is attempted to avoid the occurrence of dots in a constant pattern across a plurality of units, the pass for forming the dots is It is explanatory drawing which showed the reason which may be biased to a specific path | pass. It is explanatory drawing which showed a mode that the path | pass which forms a dot is not biased to a specific path | pass by making the magnitude | size of the main scanning direction of the unit into the integral multiple of the number of passes. It is a flowchart which shows the flow of the process which detects the formation condition of a dot and sets the value of a threshold value in the 1st modification of 1st Example. It is explanatory drawing which showed notionally the mode that the data for log | history detection were set in the 2nd modification of 1st Example. It is explanatory drawing which showed notionally the mode that the different data for log | history detection are set in the 2nd modification of 1st Example. In a 2nd Example, it is a flowchart which shows the flow of the process which multi-values the inside of a unit by determining the presence or absence of dot formation about each pixel in a unit. In 2nd Example, it is explanatory drawing which shows notionally the log | history detection buffer provided correspondingly with the pixel position in a unit. In the second embodiment, it is an explanatory diagram conceptually showing a method of setting history detection data according to the pixel position to be detected dot detection status. In the second embodiment, it is an explanatory diagram conceptually showing a method of writing the determination result of dot formation presence or absence in the history detection buffer. In 2nd Example, it is explanatory drawing which showed notionally the method of detecting the dot formation condition in arbitrary pixel positions by making log | history detection data act on log | history data.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Computer 12 ... Printer driver 14 ... Tone number conversion module 20 ... Color printer 100 ... Computer 102 ... CPU
104 ... ROM
106 ... RAM
108 ... Peripheral device interface P / IF
109 ... Disk controller DDC
110: Network interface card NIC
112 ... Video interface V / IF
114 ... CRT
DESCRIPTION OF SYMBOLS 116 ... Bus 118 ... Hard disk 120 ... Digital camera 122 ... Color scanner 124 ... Flexible disk 126 ... Compact disk 200 ... Color printer 230 ... Carriage motor 235 ... Paper feed motor 236 ... Platen 240 ... Carriage 241 ... Print head 242, 243 ... Ink Cartridge 244 ... Ink ejection head 260 ... Control circuit 261 ... CPU
262 ... ROM
263 ... RAM
300 ... Communication line 310 ... Storage device

Claims (6)

A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster A printing apparatus that prints an image by forming a plurality of rasters,
Image data receiving means for receiving, for a plurality of pixels constituting the image to be printed, image data representing a gradation value of each pixel;
Dot data conversion means for grouping a predetermined plurality of pixels adjacent to each other as a unit, and converting the image data into dot data representing the presence or absence of dot formation for each pixel in units.
For at least some of the pixels constituting the converted unit, dot presence / absence storage means for storing the presence / absence of dot formation in a state where the pixel position in the unit can be identified;
And raster forming element driving means for driving the raster forming element according to the dot data,
The dot data conversion means includes
Pixel-of-interest setting means for sequentially setting a pixel of interest for which the presence / absence of dot formation is determined in the unit;
Dot formation presence / absence detection means for detecting the presence / absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among the pixels in which the presence / absence of dot formation is stored in the converted unit;
Dot formation determination means for determining the presence or absence of dot formation for the pixel of interest based on the gradation value of the image data for the pixel of interest and the presence or absence of the detected dot formation; and
A printing apparatus as a unit that collects a number of continuous pixels corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster, at least in the raster direction. A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster A control unit for controlling the printing unit by supplying control data for controlling driving of the raster forming element to a printing unit that prints an image by forming a plurality of rasters. ,
Image data receiving means for receiving, for a plurality of pixels constituting the image to be printed, image data representing a gradation value of each pixel;
Dot data conversion means for grouping a predetermined plurality of pixels adjacent to each other as a unit, and converting the image data into dot data representing the presence or absence of dot formation for each pixel in units.
For at least some of the pixels constituting the converted unit, dot presence / absence storage means for storing the presence / absence of dot formation in a state where the pixel position in the unit can be identified;
Control data supply means for supplying the dot data as the control data to the printing unit,
The dot data conversion means includes
Pixel-of-interest setting means for sequentially setting a pixel of interest for which the presence / absence of dot formation is determined in the unit;
Dot formation presence / absence detection means for detecting the presence / absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among the pixels in which the presence / absence of dot formation is stored in the converted unit;
Dot formation determination means for determining the presence or absence of dot formation for the pixel of interest based on the gradation value of the image data for the pixel of interest and the presence or absence of the detected dot formation; and
The printing control apparatus as a unit that collects a number of continuous pixels corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster, at least in the raster direction. A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster A printing method for printing an image by forming a plurality of rasters,
Receiving (A) image data representing a gradation value of each pixel for a plurality of pixels constituting the image to be printed;
A step (B) of grouping a predetermined plurality of pixels adjacent to each other as a unit, and converting the image data into dot data representing the presence or absence of dot formation for each pixel in units.
For at least some of the pixels constituting the converted unit, a step (C) of storing the presence / absence of dot formation in a state where the pixel positions in the unit can be identified;
(D) driving the raster forming element according to the dot data,
The step (B)
A step (B-1) of sequentially setting a pixel of interest for which it is determined whether or not to form dots in the unit;
A step (B-2) of detecting the presence or absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among pixels in which the presence or absence of dot formation is stored in the converted unit; ,
A step (B-3) of determining the presence / absence of dot formation for the target pixel based on the gradation value of the image data for the target pixel and the detected dot formation presence / absence,
The printing method, which is a step of collecting a number of pixels corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster as at least a raster direction as the unit. A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster A printing control method for controlling the printing unit by supplying control data for controlling driving of the raster forming element to a printing unit that prints an image by forming a plurality of rasters. ,
(A) receiving image data representing the gradation value of each pixel for a plurality of pixels constituting the image to be printed;
(A) converting the image data into dot data indicating the presence or absence of dot formation for each pixel in a unit (a), by combining a predetermined plurality of pixels adjacent to each other as a unit.
For at least some of the pixels constituting the converted unit, a step (c) of storing the presence or absence of dot formation in a state where the pixel positions in the unit can be identified;
And (d) supplying the dot data as the control data to the printing unit.
The step (a)
A step (a-1) of sequentially setting a pixel of interest for which it is determined whether or not to form dots in the unit;
A step (a-2) of detecting the presence / absence of dot formation for a pixel located at a predetermined position with respect to the pixel position of the pixel of interest from the pixels in which the presence / absence of dot formation is stored in the converted unit; ,
A step (i-3) of determining the presence / absence of dot formation for the pixel of interest based on the gradation value of the image data for the pixel of interest and the detected dot formation presence / absence,
The printing control method, which is a step of collecting continuous pixels as a unit corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster, at least in the raster direction. A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster A program for realizing a method of printing an image by forming a plurality of rasters using a computer,
A function (A) for receiving image data representing a gradation value of each pixel for a plurality of pixels constituting the image to be printed;
A function (B) for grouping a predetermined plurality of pixels adjacent to each other as a unit, and converting the image data into dot data indicating the presence / absence of dot formation for each pixel in units;
For at least some of the pixels constituting the converted unit, a function (C) for storing the presence / absence of dot formation in a state where the pixel position in the unit can be identified;
A function (D) for driving the raster forming element according to the dot data and
The function (B) is
A function (B-1) for sequentially setting a pixel of interest for which the presence or absence of dot formation is to be determined in the unit;
A function (B-2) for detecting the presence / absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among the pixels in which the presence / absence of dot formation is stored in the converted unit; ,
A function (B-3) for determining the presence / absence of dot formation for the pixel of interest based on the gradation value of the image data for the pixel of interest and the detected dot formation presence / absence,
As a unit, a program that collects a number of consecutive pixels corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster, at least in the raster direction. A raster forming element for forming a dot row raster by repeating reciprocating movement a predetermined number of times while forming dots on a printing medium, and relatively moving the raster forming element in a direction crossing the raster Using a computer to supply control data for controlling driving of the raster forming element to a printing unit that prints an image by forming a plurality of rasters, and to control the printing unit A program for realizing
A function (a) for receiving image data representing a gradation value of each pixel for a plurality of pixels constituting the image to be printed;
A function (a) that collects a predetermined plurality of adjacent pixels as a unit, and converts the image data into dot data indicating the presence or absence of dot formation for each pixel in units.
For at least some of the pixels constituting the converted unit, a function (c) for storing the presence or absence of dot formation in a state in which the pixel position in the unit can be identified;
A function (d) for supplying the dot data as the control data to the printing unit is realized, and
The function (b)
A function (a-1) for sequentially setting a pixel of interest for which it is determined whether or not to form dots in the unit;
A function (a-2) for detecting the presence / absence of dot formation for a pixel at a predetermined position with respect to the pixel position of the pixel of interest from among the pixels in which the presence / absence of dot formation is stored in the converted unit; ,
A function (i-3) for determining the presence / absence of dot formation for the target pixel based on the gradation value of the image data for the target pixel and the detected dot formation presence / absence;
As a unit, a program that collects a number of consecutive pixels corresponding to an integral multiple of the predetermined number of times that the raster forming element reciprocates to form the raster, at least in the raster direction.

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