JP2005341330A - Threshold matrix forming method for forming dot image, and dot image forming method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form threshold matrixes that are matrixes to be compared with an original image, when a dot image is formed, and capable of forming a polychromatic dot image having less feeling of roughness. <P>SOLUTION: When the threshold matrixes to be compared with the original image, when the dot image is formed are formed, first of all, templates 72 wherein dot centers are arranged differently from each other for individual color components are prepared, and the templates 72 are arranged similarly in a matrix region 720 that is a region where the threshold matrixes are formed, concerning individual color components. Dot centers are added to the regions 722 between the templates 72 at random, and approximately uniformly. A threshold matrix of each color component is formed, by setting dot cells centering dot as centers, and setting threshold so that dots grow centering the dot centers with increase in tone level of the original image. Consequently, threshold matrixes, capable of forming a polychromatic dot image having a less feeling of roughness, can be obtained. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention belongs to the technical field of generating a halftone image representing a multi-tone original image.

  When creating data for a printing plate from multi-tone (that is, continuous tone) original image data, halftone dots are often used. In AM (Amplitude Modulated) screening, which is generally used, gradation expression is performed by changing the size of dots without changing the number of dots constituting a halftone image. On the other hand, in FM (Frequency Modulated) screening, gradation expression is performed by changing the number of dots of a certain size that are arranged moderately (random but without extreme density). In addition, as disclosed in Patent Document 1, there is also a method of performing gradation expression by changing the size of randomly arranged dots.

  In the AM halftone dot, there is a problem that interference moiré occurs when the original image has periodicity, and overlap moiré (also referred to as rosetta moire) occurs when multicolor printing is performed. However, the AM halftone dot has a feature that a rough feeling (irregular unevenness) hardly occurs on a printed matter or a printing plate in single color and multicolor printing. In an FM halftone dot or a halftone dot generated by the method described in Patent Document 1, the distance between dots is not uniform, and the arrangement of dots is not directional, so that no interference moire or overlapping moire occurs. However, when multicolor printing is performed, the rough feeling tends to be noticeable.

  Roughness in a multi-color halftone image in which halftone dots of randomly arranged color components are superimposed is not considered because the arrangement of dots does not take into account the overlapping of multiple color component (color plate) images. It is thought that this is caused by variations in the overlap of images, which are recognized as low-frequency patterns and color unevenness. As a countermeasure against the feeling of roughness, a method of increasing the dot density (increasing the number of mesh lines) can be considered, but if extremely fine dots are drawn, printing stability is impaired. Therefore, it is considered desirable to reduce the rough feeling by preventing the halftone dots of each color component from overlapping locally.

As a method of reducing the feeling of roughness, for example, an error generated as a result of converting the pixels of the original image into binary values is diffused into the surrounding pixels, and then the surrounding pixels are converted into binary values to express gradation Error diffusion methods are known. As a method for generating a halftone image using the error diffusion method, for example, there is a method disclosed in Patent Document 2. Patent Document 3 discloses a method of reducing the feeling of roughness by controlling the arrangement of dots so as to minimize the overlap of dots randomly arranged at FM halftone dots.
Japanese Patent No. 3427026 Japanese Patent No. 2905106 JP 2000-354172 A

  By the way, in the error diffusion method, since an error is propagated to surrounding pixels, a unique pattern may appear, and many operations are required to generate a halftone image, which is not practical.

  In addition, with the method of reducing the rough feeling by controlling the arrangement of dots, an image with very little rough feeling can be obtained when the printing registration (superimposition of images of each color component) is correct. In general, the printing register is slightly misaligned, which may cause an unexpected graininess. That is, in the method of controlling the arrangement of dots, it is essential to strictly match the printing register, and the manufacturing cost of the printing apparatus increases.

  The present invention has been made in view of the above problems, and an object of the present invention is to easily generate a halftone dot image with little moire and less roughness.

  The invention according to claim 1 is a threshold matrix generation method for generating a threshold matrix to be compared with the original image for each color component when generating a halftone image representing a multi-tone original image. A step of preparing a plurality of templates of a plurality of color components in which dot centers corresponding to positions of dots representing a halftone image are arranged differently, and the plurality of templates in the matrix regions of the plurality of color components, respectively A step of randomly and similarly arranging while avoiding overlapping, a step of randomly and approximately uniformly adding a dot center to a region between templates in each of the matrix regions of the plurality of color components, and the plurality of colors In each of the matrix regions of the component, dots grow around the dot center as the gradation level in the original image increases. By setting the threshold value, and a step of generating a threshold matrix.

  The invention according to claim 2 is the threshold value matrix generation method according to claim 1, wherein in one template and the other template of the plurality of templates, dot centers are in two directions perpendicular to each other. Arranged at equal intervals, one arrangement direction of the dot centers of the one template differs from one arrangement direction of the dot centers of the other template by about 60 ° or 30 °.

  A third aspect of the present invention is the threshold value matrix generation method according to the second aspect, wherein in the three templates for cyan, magenta and black among the plurality of templates, the dot centers are perpendicular to each other. Are arranged at equal intervals, and the arrangement direction of one dot center of each of the three templates differs from the arrangement direction of one dot center of another template by approximately 60 ° or 30 °.

  According to a fourth aspect of the present invention, there is provided a halftone dot image generation method for generating a halftone dot image representing a multi-tone original image, wherein the threshold value matrix generation method according to any one of the first to third aspects is used. Generating a plurality of threshold matrixes of the plurality of color components, and generating a halftone image of the plurality of color components by comparing the plurality of threshold matrices with an original image.

  According to a fifth aspect of the present invention, there is provided a halftone image generating device for generating a halftone image representing a multi-tone original image, wherein the threshold value matrix generating method according to any one of the first to third aspects is used. Means for generating a plurality of threshold matrixes for the plurality of color components, and means for generating a halftone image of the plurality of color components by comparing the plurality of threshold matrixes with an original image.

  According to the first to third aspects of the present invention, it is possible to generate a threshold value matrix that generates a halftone dot image with less moire and less roughness. In the inventions of claims 4 and 5, moire does not occur, and a halftone image with little roughness can be easily generated.

  FIG. 1 is a diagram showing a configuration of an image recording system 1 according to an embodiment of the present invention. The image recording system 1 includes a computer 11 and an image recording device 12. The image recording device 12 receives a signal from the computer 11 and uses a light beam from a laser or the like of a plurality of channels on a printing plate as a halftone recording medium. Record points. The image recording device 12 may be an electrophotographic or ink jet printing device using a photosensitive drum or printing paper as a halftone recording medium.

  The computer 11 has a general computer system configuration in which a CPU 101 that performs various arithmetic processes, a ROM 102 that stores basic programs, and a RAM 103 that stores various information are connected to a bus line. The bus line further includes an image memory 104 for storing data of a multi-gradation image (hereinafter referred to as “original image”) of a plurality of color components to be halftone (screened), and a fixed disk 105 for storing information. A display 106 for displaying various information, a keyboard 107a and a mouse 107b for receiving input from an operator, a reading device 108 for reading information from a computer-readable recording medium 91 such as an optical disk, a magnetic disk, and a magneto-optical disk, In addition, a communication unit 109 that communicates with the image recording apparatus 12 is appropriately connected through an interface (I / F) or the like.

  The image recording apparatus 12 includes a drum 121 that holds the printing plate 8 on its side surface, a drawing head 122 that emits a modulated light beam of a plurality of channels toward the printing plate 8, and a halftone image signal sent to the drawing head 122. In addition, a signal generation circuit 123 that generates the image signal, a driving mechanism that rotates the drum 121 to scan the drawing head 122 with respect to the printing plate 8, and moves the drawing head 122 along the rotation axis of the drum 121 are provided. In the following description, “pixel” refers to one unit of recording (also referred to as drawing) of the image recording apparatus 12 and corresponds to one spot by one light beam.

  The computer 11 reads the program 92 from the recording medium 91 via the reading device 108 in advance and stores it in the fixed disk 105. When the program 92 is copied to the RAM 103 and the CPU 101 executes arithmetic processing according to the program in the RAM 103 (that is, when the computer executes the program), the computer 11 generates a threshold value matrix for generating halftone dots, which will be described later. (Also called SPM (Screen Pattern Memory) data) 710 is generated for each color component. Multi-tone original image data stored in the threshold matrix 710 and the image memory 104 is transferred to the image recording device 12 via the communication unit 109, and the signal generation circuit 123 of the image recording device 12 represents the original image. A halftone signal of one color component is generated, and halftone dots are recorded on the printing plate 8 based on the halftone signal while the drawing head 122 is scanned with respect to the printing plate 8.

  In the image recording system 1 which is an apparatus for generating a halftone image, recording halftone dots on the printing plate 8 may be regarded as generation of a (physical) halftone image, and generation of halftone dot signals. May be regarded as the generation of a (non-physical) halftone image. Generation of a halftone dot signal may be performed by software by the computer 11, and in this case, the computer 11 is a device that generates a halftone dot image based on the original image alone.

  FIG. 2 is a block diagram showing the components of the signal generation circuit 123 of the image recording apparatus 12 and the recording mechanism 120. The recording mechanism 120 corresponds to the drum 121, the drawing head 122, a mechanism for driving them, a circuit for controlling them, and the like in FIG.

  The signal generation circuit 123 includes an image memory 21 for storing multi-tone original image data, an X address generator 22a for generating a sub-scanning address (X address) and a main scanning address (Y address) of the original image, and Y Address generator 22b, SPM (Screen Pattern Memory) 23 for storing threshold matrix 710 generated by computer 11, x for generating sub-scanning address (x address) and main scanning address (y address) of threshold matrix 710, respectively An address generator 24a, a y address generator 24b, and a comparator (comparator) 25 are provided.

  At the time of halftoning the original image, as shown in FIG. 3, the original image 70 is divided into a number of areas of the same size, and a repetitive area 71 serving as a unit of halftone dotization is set. The SPM 23 has a storage area for one color component corresponding to one repetition area 71, and stores a threshold value matrix 710 by setting a threshold value for each address (coordinate) of this storage area. Conceptually, each repetition region 71 of the original image 70 and the threshold matrix 710 are overlapped, and the gradation level of each pixel in the repetition region 71 is compared with the corresponding threshold value of the threshold value matrix 710, so that the network Whether or not to draw at the position of the pixel on the dot recording medium is determined. Therefore, if the gradation level of the original image 70 is uniform, drawing is performed on the pixel of the address for which a threshold value smaller than that gradation level is set in the threshold value matrix 710, and is macroscopically uniform. A halftone dot is generated. Actually, since the original image 70 has light and shade (that is, portions of various gradation levels), the state of the halftone dot changes in the repeat area 71 according to the light and shade of the original image 70. Further, the comparison between the threshold matrix 710 and the original image is performed for each color component.

  More specifically, the halftoning process will be described with reference to FIG. 2. One pixel of the original image from the image memory 21 based on the X address and the Y address from the X address generator 22a and the Y address generator 22b. The gradation level (of a specific color component) is read out. On the other hand, the x address and the y address in the repetitive area corresponding to the X address and the Y address in the original image are obtained by the x address generator 24a and the y address generator 24b, respectively, and one threshold value in the threshold value matrix 710 is specified. Read from the SPM 23. Then, the gradation level from the image memory 21 and the threshold value from the SPM 23 are compared by the comparator 25. If the gradation level is larger than the threshold value, a signal for drawing at the position of the pixel is generated.

  Note that the original image is a multicolor image, and a color designation signal 81 indicating the color component of the halftone dot image to be recorded is input to the image memory 21. At this time, the threshold matrix 710 stored in the SPM 23 is also designated. Replaced with ingredients.

  FIG. 4 is a diagram showing an operation flow of the image recording system 1. When halftone dots are recorded on the printing plate 8 by the image recording system 1, first, in the computer 11, a matrix area for storing the threshold matrix 710 is set, and the unit of the area where the halftone dots are formed in the matrix area. The center of the halftone dot cell (corresponding to the approximate center position of the dot representing the halftone dot image, hereinafter referred to as “dot center”) is set (step S11). FIG. 5 is a diagram showing in detail the flow of arrangement of dot centers.

  When arranging the dot centers in the matrix area, first, an area sufficiently smaller than the matrix area is prepared for each color component (for each color plate) as a template, and the dot center is arranged in this template (step S21). . The dot centers are arranged differently between a plurality of templates. As will be described later, a large number of templates are arranged in the matrix area in a later step. The arrangement of dot centers in the template is sequentially performed for all color components for expressing colors (step S22). However, in order to simplify the description, only two color components (cyan and black) exist below. The case where there are four color components will be described later.

  FIG. A shows a template 72C for cyan, and FIG. B shows a template 72K for black. In the templates 72C and 72K (hereinafter, when simply referring to the template, reference numeral 72 is given), ten dot centers 721 are positioned at equal intervals in two directions perpendicular to each other (that is, at the intersections of equally spaced grids). Arranged). Only the upper left corner of the four corner dot centers 721 is included in the template. In the template 72C, the angle θ formed by one arrangement direction of the dot centers 721 and the horizontal direction is approximately 15 ° (tan θ = 1/3), and in the template 72K, it is approximately 75 ° (tan θ = 3). Thereby, the arrangement directions of both templates differ by about 60 ° (30 ° depending on the selection of the arrangement direction).

  Note that the number of dot centers 721 included in the template 72 is set to 10 because it is sufficiently smaller than the number of dot centers 721 assumed in the matrix area (for example, 10,000), and the template is formed in a later step. This is for preventing the regularity of the arrangement of the dot centers 721 from becoming noticeable when 72 is arranged in the matrix region.

  In general, in an AM halftone dot in which dot centers are regularly arranged, if the arrangement direction of the dot centers between color components is different by 60 ° or 30 °, misregistration of halftone dots of a plurality of color components occurs. The occurrence of low-frequency patterns (moire) is suppressed (that is, even if the overlay is shifted). Although details will be described later, the image recording system 1 uses this principle to arrange the arrangement direction of the dot center 721 of the template 72 of one color component and the arrangement direction of the dot center 721 of the template 72 of another color component. Is different by 60 ° or 30 °, it is difficult to produce a rough feeling that is irregular unevenness even if misregistration occurs.

  When the two color component templates 72 are prepared, these templates 72 are arranged in the matrix region (step S23). FIG. 7 is a diagram showing a state where the template 72 is arranged in the matrix area 720. The templates 72 are randomly arranged while avoiding overlapping. Further, the templates 72 are similarly arranged in the matrix region 720 of a plurality of color components. The size of the template 72 in the matrix area 720 is adjusted so that the density of the dot centers 721 assumed in the matrix area 720 (for example, the number of mesh lines (300 per inch) is 300) is the same as the density in the template 72. Is done. In FIG. 7, the template 72 is drawn larger than the actual size.

  In the arrangement of the template 72, first, evaluation values are associated with all positions (all positions that can be specified by coordinate values) in the matrix area 720, and all evaluation values are initialized to zero. The first template 72 is randomly arranged in the matrix region 720, and 1 / square of the distance between each position and the first template 72 is added to the evaluation value corresponding to the position. At this time, since the matrix area 720 corresponds to the repetitive area 71 shown in FIG. 3, the first template 72 for which the evaluation value is to be calculated is also treated as existing repeatedly in the vertical and horizontal directions. That is, when calculating the evaluation value, the one closest to the position serving as the distance calculation reference is selected from among the plurality of first templates 72 assuming that the matrix region 720 is repeated.

  When the evaluation values of all positions are obtained, the second template 72 is arranged at the position associated with the smallest evaluation value. In other words, the second template 72 is arranged at the position farthest from the first template 72. Next, 1 / square of the distance between each position and the second template 72 (the closest one of the plurality of second templates 72 considering the repetition of the matrix region 720) is the corresponding evaluation value. Is added. Then, the third template 72 is arranged at the position associated with the smallest evaluation value. If there are a plurality of positions that are candidates for placement of the template 72, one of them is selected as appropriate.

  Thereafter, 1 / square of the distance between each position in the matrix area 720 and the template 72 arranged last (considering repetition of the matrix area 720) is added to the corresponding evaluation value, The next template 72 is arranged at a position associated with a small evaluation value. Thereby, the next template 72 is arranged at a position away from any template 72, and the template 72 is arranged randomly and approximately uniformly while avoiding overlapping. The arrangement of the template 72 in the matrix area 720 ends in a state where there is a sufficient gap (area 722 in FIG. 7) between the templates 72. In order to arrange the templates 72 more randomly, the templates 72 may be moved slightly at random just after the arrangement.

  Other methods may be employed as long as the templates 72 are arranged randomly (preferably approximately uniform) while avoiding overlapping between the templates 72. For example, in the above method, a plurality of templates 72 that are initially arranged at random may be provided. In addition, by arranging a plurality of templates 72 in the matrix region 720 first and moving each template 72 in a random direction by a random distance within a certain range, the template 72 can be arranged in the matrix region 720. It may be done.

  As another method, the template 72 may be rearranged approximately uniformly using a solution of the facility arrangement problem using a Voronoi diagram by randomly arranging the template 72. The facility placement problem determines the placement of facilities so that the total cost of using the facilities by a large number of users existing in the 2D space is minimized when placing multiple facilities in the 2D space. This is a nonlinear optimization problem. Here, the evaluation function for obtaining “cost” is, for example, the distance from the user to each facility. As for the Voronoi diagram, see, for example, supervised by Masao Iri, “bit separate volume computational geometry and geographic information processing”, Kyoritsu Shuppan, September 10, 1986, p. 163-168.

  When the arrangement of the templates 72 is completed, the dot centers 721 are then added randomly and approximately uniformly in the area 722 between the templates 72 (step S24). Similarly to the arrangement of the template 72, the dot centers 721 are sequentially added at positions where the sum of 1 / square of the distances from all the other dot centers 721 is minimized while considering the repetition of the matrix region 720. . When the density of the dot centers 721 in the area 722 reaches the density of the dot centers 721 assumed in the matrix area 720, the addition of the dot centers 721 is completed.

  For example, the number of halftone lines (halftone dot density) of both color components is 300 lines, halftone dots are recorded by the image recording device 12 having a resolution of 2400 dpi (the width of one pixel is about 10 μm), and the size of the matrix area 720 is In the case of 800 × 800 pixels, the area allocated to one halftone dot is about 64 pixels (8 × 8), and the dot center 721 is added until 10,000 dot centers 721 are included in the matrix area 720. . In this case, the length of one side of one template 72 is about 25 pixels.

  The dot center 721 may be added by other methods. For example, the dot centers 721 are aligned in the region 722, and each dot center 721 is moved in a random direction by a random distance within a certain range. Accordingly, the dot center 721 may be arranged.

  When the arrangement of the dot centers 721 in the matrix area 720 using the template 72 is completed, a halftone dot cell serving as a unit of halftone dot generation is set around each dot center 721 (FIG. 4: step S12). FIG. 8 is a diagram illustrating a state in which a polygonal halftone cell 73 is set around each dot center 721. The halftone cell 73 is set in consideration of repetition of the matrix region 720 in the matrix region 720.

  Such a halftone cell is set as follows, for example. First, in order to determine which dot center 721 a pixel at a certain position in the matrix region 720 belongs to, the square of the distance to each dot center 721 is obtained as an evaluation value. However, when calculating an evaluation value related to one dot center 721 (hereinafter referred to as “target dot center”), a pixel of interest among a plurality of target dot centers in consideration of repetition of the matrix region 720 in the vertical and horizontal directions. The one closest to is selected as an evaluation value calculation target. Then, it is determined that this pixel belongs to the dot center 721 having the smallest evaluation value. By performing the above calculation for all the pixels, the matrix region 720 is divided into halftone cells 73 centering on each dot center 721.

  When the setting of the halftone cell 73 is completed, the computer 11 further generates a threshold value matrix (step S13). FIG. 9 is a diagram showing a flow of generating a threshold matrix. In the generation of the threshold matrix, first, first-stage evaluation values are obtained for all the pixels of each halftone dot cell 73. As the first-stage evaluation value, for example, the distance from the dot center 721 of the halftone cell 73 including the pixel or the distance from the center of gravity of the halftone cell 73 is used. Then, an integer number increasing by 1 is assigned to all the pixels of the halftone cell 73 in order from the smallest evaluation value in the first stage, and the halftone dot is divided by the number of pixels constituting the halftone cell 73. A second-stage evaluation value normalized to the cell size (with a value of 0.0 to 1.0) is assigned again. Thereby, a smaller evaluation value is assigned to a pixel closer to the dot center 721 (or the center of gravity of the halftone dot cell 73) (step S31).

  Furthermore, an integer number that increases by 1 in order from the smallest evaluation value in the second stage for all the pixels in the matrix area 720 (the order of the pixels to be drawn in accordance with the increase in the gradation level of the original image) , Which is the so-called lighting order at the time of exposure.) Is assigned, and each gradation is compressed according to the number of gradations of screening (in this embodiment, equal to the number of gradations of each color component of the original image). A final threshold value is assigned to the pixel, and a threshold value matrix 710 (see FIG. 1) corresponding to the matrix region 720 is generated (step S32). For example, when the number of pixels in the matrix area 720 is M and the number of gradations of one color component of the original image is N (typically 256 (= 8 bits)), the pixel area is assigned to each pixel. The obtained number (0 to (M-1)) is multiplied by ((N-1) / (M-1)), whereby a threshold value in the range of 0 to (N-1) is assigned to each pixel. As a result, threshold values are set so that dots grow around the dot center as the gradation level of the original image increases in each color component, and a plurality of threshold value matrices 710 corresponding to a plurality of color components are generated.

  FIG. A is a diagram illustrating a change in the size of a dot (a group of pixels to be drawn) drawn in the halftone dot cell 73 in accordance with a change in the gradation level of the original image, and regions 73a to 73c are areas of the original image. It shows dots that grow as the gradation level increases. FIG. In A, since the first-stage evaluation value of each pixel in step S31 is obtained as the distance from the dot center 721, the regions 73a to 73c are circular.

  FIG. B and FIG. C is a diagram showing another example of dot growth, and FIG. The same reference numerals as in A are given. FIG. In B, as the first evaluation value in step S31, the distance from the dot center 721 to the pixel and the dot cell 721 from the dot center 721 on the straight line connecting the dot center 721 and the pixel for which the first evaluation value is obtained. The ratio to the distance to the edge is adopted. FIG. In C, an internal polygon that connects the midpoints of each side of the polygonal halftone cell is set, and the distance from the dot center 721 to the pixel and the internal distance from the dot center 721 on the straight line connecting the dot center 721 and the pixel. The ratio to the distance to the square edge is adopted as the first evaluation value. Note that FIG. In C, in the region between the inner polygon and the edge of the halftone cell 73, a first evaluation value that increases in value toward the vertex of the halftone cell 73 is appropriately set.

  When the threshold matrix 710 for each color component is generated in the computer 11, as described above, the threshold matrix 710 for one color component and the original image data stored in the image memory 104 are recorded via the communication unit 109. It is transferred to the device 12 and stored in the SPM 23 and the image memory 21 shown in FIG. Thereby, a threshold value group corresponding to the growth of halftone dots in each halftone dot cell 73 is set in the storage area having the same size as the matrix area 720 in the SPM 23 (FIG. 4: step S14).

  Then, the gradation level of one color component of each pixel of the original image in the image memory 21 and the corresponding threshold value of the threshold value matrix 710 in the SPM 23 are input to the comparator 25, and the signal generation circuit 123 of the image recording device 12. Generates a halftone image signal, and a halftone image of one color component is recorded on the printing plate 8 by the drawing head 122 (step S15). Specifically, when the gradation level of the pixel of the original image is larger than the threshold, the recording mechanism 120 irradiates the pixel position on the printing plate 8 with light, and drawing is performed.

  When image recording of the next color component is necessary (step S16), the halftone dot recording medium is replaced (step S17), and a color designation signal 81 is input to the image memory 21 as shown in FIG. The threshold matrix 710 is replaced with the next color component, and the halftone image of the next color component is recorded on the halftone recording medium.

  FIG. A thru | or FIG. FIG. 12C is a diagram illustrating an example of a halftone image formed when the dot centers 721 are randomly arranged in the matrix region 720 without using the template 72 (comparative example). A thru | or FIG. C is a diagram illustrating an example of a halftone image when a dot center 721 is arranged in the matrix area 720 using the template 72. FIG.

  FIG. A and FIG. A is a cyan halftone image (shown in black and white binary) when the cyan gradation level of the original image is uniformly 50%. B and FIG. B is a black halftone image when the black gradation level of the original image is uniformly 50%. FIG. FIG. A and FIG. B is superimposed, FIG. C is the same as FIG. A and FIG. B is superposed, and in these figures, cyan and black are not distinguished from each other and are represented by monochrome binary values. FIG. C and FIG. As is clear from comparison with C, by using the template 72 (72C, 72K), the overlapping of halftone dots can be adjusted, and a multi-color halftone image can be made rough without causing moire. Less smooth.

  In addition, as described above, in a method in which dots of each color component are randomly arranged (to some extent) while taking care not to cause a rough feeling when the halftone image for each color component is overlaid, If there is a shift in the registration of the halftone image, an unintentional rough feeling will occur in the multicolor halftone image. On the other hand, in the image recording system 1, a large number of minute templates 72 are similarly arranged in the matrix region 720 in each color component. By utilizing this characteristic, it is possible to suppress the occurrence of unintentional roughness when a misregistration occurs in a multicolor halftone image.

  Further, in the image recording system 1, the arrangement of the dot centers 721 in the template 72 is an arrangement that suppresses the occurrence of moire in the AM halftone dot (an arrangement direction of the dot centers 721 is different by 60 ° or 30 ° between the color components). For this reason, even if misregistration occurs in a multi-color halftone image, it is possible to suppress occurrence of regular or random unevenness as a whole.

  Since the image recording system 1 performs halftoning using the threshold matrix 710, a halftone image with less roughness can be obtained without performing enormous calculations as in the error diffusion method. Further, by using the threshold value matrix 710, it is possible to avoid recording with minute dots such as FM halftone dots, so that printing stability and ease are not impaired.

  Next, an example of the template 72 when the color components of the multicolor halftone image are four colors of cyan, magenta, yellow, and black will be described. A cyan template 72C is shown in FIG. A template 72K for black is shown in FIG. In the case shown in B, the magenta template 72M is as shown in FIG.

  Similar to the cyan and black templates 72, the magenta template 72M is arranged with the dot centers 721 at equal intervals in two directions perpendicular to each other (that is, positioned at the intersections of equally spaced grids). The length of the side is about 25 pixels as in the template 72 for cyan and black. However, eight dot centers 721 are arranged in one template 72M, and the number of halftone lines is smaller than in the case of cyan and black. Only the one at the upper left corner among the dot centers 721 at the four corners and the one at the upper side and the left side among the dot centers 721 at the center of each side are included in the template. In the template 72M, the angle θ formed by one arrangement direction of the dot centers 721 and the horizontal direction is 45 ° (tan θ = 1), whereby one dot center 721 of each of the templates 72C, 72M, and 72K. This arrangement direction differs from the arrangement direction of the dot centers 721 of any other templates 72 by approximately 60 ° or 30 °.

  Although not shown, the yellow template 72Y is generated by appropriately arranging the dot centers 721 in a different arrangement from the other templates 72 in accordance with the density of the dot centers 721 in the matrix region 720. For example, θ is 0 °, and the dot centers 721 are aligned in the vertical direction and the horizontal direction.

  The generation method of the threshold matrix 710 for magenta and yellow is the same as the threshold matrix 710 for cyan and black except that the template 72 is different. That is, the magenta and yellow templates 72M and 72Y are arranged in the matrix area 720 as in the case of cyan as shown in FIG. 7, and the dot centers 721 are randomly and approximately uniformly arranged in the area 722 between the templates 72. In addition, a threshold matrix 710 is generated. Thereafter, the original image data is transferred to the image recording device 12 and the halftone image of each color component is sequentially recorded while the data of the threshold matrix 710 of each color component is sequentially transferred to the image recording device 12.

  In the above operation, the arrangement direction of one dot center 721 of each of the three templates 72 for cyan, magenta, and black differs from the arrangement direction of one dot center 721 of the other template 72 by approximately 60 ° or 30 °. Therefore, for the same reason as described for the cyan and black template 72, even if there is a deviation in the overlay of halftone dot images of any color component, regular or random unevenness may occur as a whole. It is suppressed. Since yellow has little influence on the contrast of the color halftone image, it is not necessary to pay much attention to the arrangement of the dot centers 721 in the template 72Y.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications can be made.

  For example, the template 72 for cyan, magenta, and black in the above embodiment is only a preferable example, and a plurality of templates 72 in which the arrangement of the dot centers 721 is different from each other among a plurality of color components are used. Also good. Also by this, it is possible to suppress the occurrence of unexpected rough feeling due to the misregistration of the halftone image to some extent. As another example, FIG. A and FIG. As shown in B, a template 72 having five dot centers 721 may be used (only the upper left one of the four corner dot centers 721 is included in the template 72). FIG. A and FIG. In the template 72 shown in B, the angle θ between the arrangement direction of the dot centers 721 and the horizontal direction is set so that tan θ becomes 1/2 and 2. When the number of mesh lines is 300 and the recording resolution is 2400 dpi, one side of the template 72 is about 18 pixels. Also in this case, one arrangement direction of one template 72 and one arrangement direction of the other template 72 are different by about 60 ° or 30 °, and a rough feeling is generated in a multicolor halftone image due to misregistration. Can be accurately suppressed.

  As described above, generation of a halftone image may be performed by software only by the computer 11, and furthermore, only generation of the threshold matrix 710 may be performed separately by software. In this case, data of the threshold matrix 710 is transferred to the image recording device 12 via a recording medium such as a magnetic disk or an optical disk, or a computer network.

  The image recording device 12 scans a photosensitive drum with a multi-channel light beam, records halftone dots on the photosensitive drum, and further prints on a printing paper, or multi-channel ink ejection. It may be an ink jet printing apparatus that records halftone dots on a sheet while scanning nozzles on the sheet. In this case, the photosensitive drum or the printing paper becomes the halftone recording medium.

  A technique for generating the threshold matrix 710 using the template 72 can also be used in an ink jet printing apparatus or a direct printing apparatus that records a multi-value halftone image in addition to a binary halftone image. In this case, a plurality of threshold matrices having similar threshold distributions but different average threshold values are obtained for each color component, and the gradation levels of the pixels of the original image are compared with a plurality of threshold values from the plurality of threshold matrices. It is determined at which density the drawing is performed. As a result, a composite dot having a low density region 792 is drawn around the high density region 791 at the center in the halftone dot cell 73 as illustrated in FIG. 15, and the region 791, the region 792 and other regions are drawn. The concentrations are 1, 0.5 and 0, respectively. As a result, a multi-color halftone image with less roughness can be obtained.

  In the case where one pixel is drawn in the image recording apparatus 12 and the recording stability is low when drawing is not performed on pixels around this pixel, the minimum dot size (so-called minimum cluster size) is 2 × 2. In the threshold value matrix 710, a plurality of threshold values in the vicinity of the dot center 721 are preferably corrected to the same value so as to be pixels (may be 1 × 2 pixels).

It is a figure which shows the structure of an image recording system. It is a block diagram which shows a signal generation circuit and a recording mechanism. It is a figure which shows a repetition area | region and a threshold value matrix. It is a figure which shows the flow of operation | movement of an image recording system. It is a figure which shows the flow of arrangement | positioning of a dot center. It is a figure which shows the template of one color component. It is a figure which shows the template of another one color component. It is a figure which shows arrangement | positioning of a template. It is a figure which shows the set halftone cell. It is a figure which shows the flow of the production | generation of a threshold value matrix. It is a figure which shows the example of a change of the magnitude | size of a dot. It is a figure which shows the other example of a change of the magnitude | size of a dot. It is a figure which shows the further another example of the change of the magnitude | size of a dot. It is a figure which shows the halftone image of one color component in a comparative example. It is a figure which shows the halftone image of another one color component in a comparative example. It is a figure which shows the image which overlapped the halftone dot image of two color components in a comparative example. It is a figure which shows the halftone image of one color component in an image recording system. It is a figure which shows the halftone image of another one color component in an image recording system. It is a figure which shows the image which overlapped the halftone dot image of two color components in an image recording system. It is a figure which shows the template of another one color component. It is a figure which shows another template. It is a figure which shows another template. It is a figure which illustrates a multi-value halftone dot.

Explanation of symbols

1 Image Recording System 11 Computer 25 Comparator 72 Template 720 Matrix Area 721 Dot Center 722 Area S12 to S17, S21 to S24 Steps

Claims (5)

A threshold matrix generation method for generating a threshold matrix to be compared with the original image for each color component when generating a halftone image representing a multi-tone original image,
Preparing a plurality of templates of a plurality of color components in which dot centers corresponding to the positions of dots representing a halftone image are arranged differently from each other;
Arranging the plurality of templates in a matrix region of the plurality of color components, respectively, randomly and similarly while avoiding overlapping;
In each of the matrix regions of the plurality of color components, adding a dot center randomly and approximately uniformly in a region between templates;
Generating a threshold matrix in each of the matrix regions of the plurality of color components by setting thresholds so that dots grow around a dot center as the gradation level in the original image increases;
A threshold value matrix generation method comprising: The threshold value matrix generation method according to claim 1,
In one template and the other template of the plurality of templates, dot centers are arranged at equal intervals in two directions perpendicular to each other, and one arrangement direction of the dot centers of the one template and the other one are arranged. A threshold value matrix generating method characterized in that the arrangement direction of one of the dot centers of the template differs by about 60 ° or 30 °. The threshold value matrix generation method according to claim 2,
In three templates for cyan, magenta, and black among the plurality of templates, dot centers are arranged at equal intervals in two directions perpendicular to each other, and one arrangement direction of each of the dot centers of the three templates; A threshold value matrix generating method, wherein a direction of arrangement of dot centers of other templates is different by about 60 ° or 30 °. A halftone image generation method for generating a halftone image representing a multi-tone original image,
Generating a plurality of threshold matrixes of the plurality of color components by the threshold matrix generating method according to any one of claims 1 to 3;
Generating a halftone image of the plurality of color components by comparing the plurality of threshold matrices and the original image;
A halftone dot image generation method comprising: A halftone image generating device that generates a halftone image representing a multi-tone original image,
Means for generating a plurality of threshold matrixes of the plurality of color components in the threshold value matrix generating method according to any one of claims 1 to 3;
Means for generating a halftone image of the plurality of color components by comparing the plurality of threshold matrices and the original image;
A halftone dot image generating apparatus.

Images