JP2009188453A - Gradation setting method for dither matrix, image processing method, image processing apparatus, image forming apparatus, program and recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress appearance of a texture pattern and moire in multi-valued dither processing. <P>SOLUTION: The correspondence relation for each pixel between gradation values of an input image and gradation values of an output image is so set that when the gradation values of the input image are increased uniformly, while the gradation values of respective pixels are held constant, a first line group comprising a predetermined number of line images, extending in a predetermined range or part of it, is formed in the output image within a low-gradation range from the lowest gradation to a first gradation; and a second line group comprising a predetermined number of line images, substantially parallel to a predetermined direction or part of it, is formed in the output image in addition to the first line image group within a half-tone range from the first gradation to a second gradation. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention performs the above-mentioned input by applying multi-value dither processing to a multi-tone input image using a dither matrix in which the correspondence between the tone value of the input image and the tone value of the output image is set for each pixel. The present invention relates to a technique for reproducing the gradation of an image as a multi-gradation dither image.

  Conventionally, as a method for outputting an image to a recording medium such as paper, various recording methods such as a thermal transfer method, an electrophotographic method, and an ink jet method have been adopted. In these recording methods, halftone processing (halftone processing) is performed by generating a dot pattern corresponding to the density (gradation) of an input image and printing it on a recording medium.

  In the dither process, which is one of the halftone processes, a dither matrix in which a threshold value is associated with each pixel arranged in an n × m matrix is used, and multi-value image data input in units of pixels is converted into a dither matrix. The threshold value is compared in units of one pixel, and the multi-value image data is quantized and converted into binary or multi-value to obtain a dither image (halftone image, halftone image).

  Dither processing is classified into a dot dispersion type, a dot concentration type, and the like according to a dither matrix to be used.

  In general dot concentration type dither processing, as the tone value of the input image increases, the dot grows thicker with the center of the dither pattern as the core. Examples of the growth shape of the dot include various shapes such as a circle, an ellipse, a line (linear), a square, a cross, and a rhombus.

  However, with the conventional dither processing method using dot growth, the degree of change in the reproduced area (gradation characteristics) differs depending on the shape of dot growth, or the image quality is profound due to instability of density change. There is a problem that appears.

  For example, (1) When using a dither screen with a low number of lines due to the image output characteristics of the image output device, (2) Object (raster data (eg, continuous tone such as photographic paper photograph or photographed image data taken by a digital camera) Data)), vector data (graphs, etc.), or when switching the dither screen for each text (for example, in the case of a printer function of a printer or a multifunction device), (3) depending on the character and the continuous tone area A dither screen with a low number of lines may be used when performing a process of switching the dither screen (in the case of copier). However, when a dither screen with a low number of lines is used, moire patterns tend to occur when an image containing a large amount of high frequency components is processed.

  Therefore, in order to suppress such a decrease in image quality, conventionally, a method of suppressing moire by changing a screen angle (an angle at which a dither screen is superimposed on an input image) is used.

Japanese Patent Application Laid-Open No. 2004-228688 discloses a gradation reproduction method that can obtain good image quality even when high-speed recording is performed at a low resolution, when a multi-gradation image is thresholded at a certain density by a dither matrix. In addition, it is described that a line tone in a predetermined direction is formed every 3 to 5 dots, and a portion other than the tone has a high-pass filter characteristic. More specifically, in the technique of Patent Document 1, a density range with a reduced number of lines is selected, and the dot arrangement of the dither matrix at the boundary between the selected density range and the other density range is changed to a line in a predetermined direction. It has been re-arranged to have the keynote and high-pass filter characteristics.
JP 2004-166163 A (released on June 10, 2004)

  However, in the method of suppressing moiré by changing the screen angle as described above, the moiré pattern may not be appropriately suppressed due to angle restrictions.

  Further, in the technique disclosed in Patent Document 1, there is a possibility that the continuity of gradation is interrupted and a texture pattern appears in the low density portion of the created dither matrix.

  The present invention has been made in view of the above problems, and an object of the present invention is to appropriately reproduce the gradation of an input image in a low gradation range by preventing a texture pattern from appearing in multi-value dither processing. In addition, it is to appropriately reproduce the gradation of the input image in the middle gradation range while suppressing moire.

  In order to solve the above-described problem, the dither matrix gradation setting method according to the present invention corresponds to the correspondence between the gradation value of the input image and the gradation value of the output image for each pixel of the dither matrix used in the multi-value dither processing. A dither matrix gradation setting method for setting a relationship, wherein the gradation value of an input image having the same gradation value of each pixel is maintained while maintaining the condition that the gradation value of each pixel is the same value. When sequentially increasing, in the low gradation range from the lowest gradation to the first gradation, a first line group consisting of a predetermined number of line images extending in a predetermined direction or a part thereof is formed in the output image. In the middle gradation range from the first gradation to the second gradation, in addition to the first line image group, a second line group consisting of a predetermined number of line images substantially parallel to the predetermined direction or a part thereof is an output image. Input image at each pixel to be formed It is characterized by setting the correspondence between the gradation value of the gradation values and the output image.

  According to the above method, when the gradation value of the input image in which the gradation value of each pixel is the same value is sequentially increased while maintaining the condition that the gradation value of each pixel is the same value, In the low gradation range from the gradation to the first gradation, the first line group consisting of a predetermined number of line images extending in a predetermined direction or a part of the first line group is formed in the output image so that the level of the input image in each pixel. The correspondence between the tone value and the tone value of the output image is set. Thereby, it is possible to appropriately reproduce the gradation by preventing the appearance of the texture pattern for the input image in the low gradation range.

  Further, in the middle gradation range from the first gradation to the second gradation, in addition to the first line image group, the second line group consisting of a predetermined number of line images substantially parallel to the predetermined direction or a part thereof is included. Correspondence between the gradation value of the input image and the gradation value of the output image in each pixel is set so as to be formed in the output image. As a result, the number of line images formed in the output image can be increased and the number of lines can be increased in the medium density range than in the case of the low density range, so that moire can be suppressed without changing the screen angle. be able to. That is, when a low-line-number dither matrix is used for an input image of a high-frequency component, moire generally tends to occur. However, according to the above method, by increasing the number of lines in the medium-density range, A high frequency component can be imparted to moire having a low frequency component to suppress the occurrence of moire.

  Also, the gradation of the input image in each pixel so that the line images belonging to the first line image group and the line images belonging to the second line image group are alternately arranged along a direction perpendicular to the predetermined direction. The correspondence between the value and the gradation value of the output image may be set.

  According to the above method, since the line images belonging to the first line image group and the line images belonging to the second line image group can be formed to be distributed substantially evenly, the image quality of the output image can be improved. it can.

  Further, when the gradation value of the input image is sequentially increased while maintaining the condition that the gradation value of each pixel is the same, the gradation value of each pixel constituting the first line image group is The correspondence relationship between the gradation value of the input image and the gradation value of the output image in each pixel may be set so as to sequentially increase by a predetermined number of pixels.

  According to the above method, as the gradation value of the input image increases, the gradation value of each line image constituting the first line image group is sequentially shifted by a predetermined number of pixels without being biased toward a specific pixel. Since it can be increased, the image quality of the output image in the low density range can be improved.

  Further, when the gradation value of the input image is sequentially increased while maintaining the condition that the gradation value of each pixel is the same value, each pixel in each line image constituting the first line image group is set. The correspondence relationship between the gradation value of the input image and the gradation value of the output image in each pixel constituting the first line image group is set so that the sum of the gradation values increases substantially uniformly for each line image. May be.

  According to the above method, according to the increase of the gradation value of the input image, the gradation value of each pixel constituting the first line image group is increased substantially equally for each line image without being biased toward the specific line image. Therefore, the image quality of the output image in the low density range can be improved.

  Further, when the gradation value of the input image is successively increased while maintaining the condition that the gradation value of each pixel is the same value, the gradation value of each pixel constituting the second line image group is The correspondence relationship between the gradation value of the input image and the gradation value of the output image in each pixel may be set so as to sequentially increase by a predetermined number of pixels.

  According to the above method, in accordance with the increase of the gradation value of the input image, the gradation value of the pixels of each line image constituting the second line image group is sequentially sequentially a predetermined number of pixels without being biased toward specific pixels. Since it can be increased, the image quality of the output image in the medium density range can be improved.

  Further, when the gradation value of the input image is sequentially increased while maintaining the condition that the gradation value of each pixel is the same value, each pixel in each line image constituting the second line image group is increased. The correspondence relationship between the gradation value of the input image and the gradation value of the output image in each pixel constituting the second line image group is set so that the sum of the gradation values increases substantially uniformly for each line image. May be.

  According to the above method, as the gradation value of the input image increases, the gradation value of each pixel constituting the second line image group increases substantially equally for each line image without being biased toward a specific line image. Therefore, the image quality of the output image in the middle density range can be improved.

  In order to solve the above problem, the image processing method of the present invention uses a dither matrix in which the correspondence between the gradation value of the input image and the gradation value of the output image is set for each pixel. An image processing method for reproducing the gradation of the input image as a multi-gradation dither image by performing multi-value dither processing on the input image, wherein the correspondence relationship is determined by any one of the gradation setting methods described above. It is characterized by using a set correspondence.

  According to the above method, it is possible to appropriately reproduce the gradation of the input image in the low gradation range while preventing the appearance of the texture pattern. In addition, it is possible to appropriately reproduce the gradation of the input image in the middle gradation range while suppressing moire.

  In order to solve the above problems, the image processing apparatus of the present invention uses a dither matrix in which the correspondence between the gradation value of the input image and the gradation value of the output image is set for each pixel. An image processing apparatus that reproduces the gradation of the input image as a multi-gradation dither image by performing multi-value dither processing on the input image, wherein the gradation of the input image set for each pixel of the dither matrix Storage means for storing the correspondence between the value and the gradation value of the output image, and the gradation value of each pixel in the output image by reading out the gradation value of the output image corresponding to the gradation value of the input image from the storage means And the storage means stores the correspondence set by one of the above-described gradation setting methods as the correspondence.

  In order to solve the above problems, an image forming apparatus according to the present invention includes the image processing apparatus and an image output apparatus that forms the dither image on a print medium.

  According to the above configuration, it is possible to appropriately reproduce the gradation of the input image in the low gradation range while preventing the appearance of the texture pattern. In addition, it is possible to appropriately reproduce the gradation of the input image in the middle gradation range while suppressing moire.

  The image processing apparatus may be realized by a computer. In this case, an image processing program for causing the image processing apparatus to be realized by the computer by causing the computer to operate as the respective means, and recording the program. Such computer-readable recording media are also included in the scope of the present invention.

  As described above, according to the dither matrix gradation setting method of the present invention, when the gradation value of the input image is sequentially increased while the gradation value of each pixel is kept uniform, In the low gradation range up to the gradation, the first line group consisting of a predetermined number of line images extending in a predetermined direction or a part thereof is formed in the output image, and the intermediate gradation from the first gradation to the second gradation. In the range, in addition to the first line image group, the gradation of the input image in each pixel so that a second line group consisting of a predetermined number of line images substantially parallel to the predetermined direction or a part thereof is formed in the output image. The correspondence between the value and the gradation value of the output image is set.

  Also, the image processing method of the present invention uses a dither matrix in which the correspondence between the tone value of the input image and the tone value of the output image is set for each pixel, and multi-value dither processing is performed on the multi-tone input image. The image processing method reproduces the gradation of the input image as a multi-gradation dither image by applying the above, and the correspondence set by any of the gradation setting methods described above is used as the correspondence .

  Further, the image processing apparatus of the present invention comprises a storage means for storing a correspondence relationship between the gradation value of the input image and the gradation value of the output image set for each pixel of the dither matrix, and the gradation value of the input image. And a gradation determining means for determining the gradation value of each pixel in the output image by reading out the gradation value of the output image corresponding to the storage image, and the storage means The correspondence set by the gradation setting method is stored.

  An image forming apparatus of the present invention includes the above-described image processing apparatus and an image output apparatus that forms the dither image on a print medium.

  Therefore, according to each of the above methods and each of the above apparatuses, it is possible to appropriately reproduce the gradation of the input image in the low gradation range while preventing the appearance of the texture pattern. In addition, it is possible to appropriately reproduce the gradation of the input image in the middle gradation range while suppressing moire.

  An embodiment of the present invention will be described.

(1. Overall configuration)
FIG. 2 is a block diagram illustrating a schematic configuration of a digital color multifunction peripheral (image forming apparatus) 1 including the color image processing apparatus 10 according to the present embodiment.

  As shown in FIG. 2, the color image processing apparatus 10 includes an A / D conversion unit 11, a shading correction unit 12, an input tone correction unit 13, a region separation processing unit 14, a color correction unit 15, and a black generation and under color removal unit. 16, a spatial filter processing unit 17, an output tone correction unit 18, and a tone reproduction processing unit 19. A color image input device 20 and a color image output device 30 are connected to the color image processing device 10 to constitute the digital color multifunction peripheral 1 as a whole. The digital color multifunction peripheral 1 is provided with an operation panel 40.

  The operation panel 40 displays the operating state of each unit in the digital color multifunction peripheral 1, information on jobs, information for supporting (guidance) operation of each unit by the user, and receiving an instruction input from the user. It is. As the operation panel 40, for example, a touch panel in which a display unit such as a liquid crystal display and an operation unit such as a setting button are integrated can be used. Information input to the operation panel 40 is sent to a main control unit (not shown) of the color image processing apparatus 10. The main control unit controls the operation of each unit in the color image input device 20, the color image processing device 10, and the color image output device 30 based on information input to the operation panel 40.

  The color image input device 20 is composed of, for example, a scanner unit having a charge coupled device (hereinafter referred to as a CCD), and a reflected light image from a paper on which an original image is recorded is converted into RGB by the CCD. As an analog signal and input to the color image processing apparatus 10.

  An analog signal read by the color image input device 20 passes through the color image processing device 10 through an A / D conversion unit 11, a shading correction unit 12, an input tone correction unit 13, a region separation processing unit 14, and a color correction unit. 15, the black generation and under color removal unit 16, the spatial filter processing unit 17, the output gradation correction unit 18, and the gradation reproduction processing unit 19 are sent in this order, and output to the color image output device 30 as a CMYK digital color signal. Is done.

  The A / D (analog / digital) converter 11 converts an input RGB analog signal into a digital signal. The shading correction unit 12 performs processing for removing various distortions generated in the illumination system, imaging system, and imaging system of the color image input device 20 on the RGB digital signals sent from the A / D conversion unit 11. Is.

  The input tone correction unit 13 adjusts the color balance of the RGB signal (RGB reflectance signal) from which various kinds of distortion have been removed by the shading correction unit 12, and is adopted in the color image processing apparatus 10 for density signals and the like. It is converted into a signal that can be handled easily by the image processing system.

  The region separation processing unit 14 separates an input image expressed by RGB signals into a plurality of regions such as a character edge region, a dot region, and a photo region. Then, the region separation processing unit 14 outputs a region identification signal indicating which region of each pixel of the input image belongs to the color correction unit 15, the black generation and under color removal unit 16, and spatial filter processing based on the separation result. The output signal is output to the unit 17 and the gradation reproduction processing unit 19, and the input signal output from the input gradation correction unit 13 is output to the subsequent color correction unit 15 as it is.

  The color correction unit 15 performs processing for removing color turbidity based on spectral characteristics of CMY color materials including unnecessary absorption components in order to faithfully reproduce colors. As a processing method, there are a method using a look-up table (LUT) in which a correspondence relationship between an input RGB signal and an output CMY signal is stored in advance, and a color masking method using a conversion matrix as shown in the following Equation 1.

For example, when the color masking method is used, an L ^* a ^* b ^* value (CIE 1976 L ^* a ^* b ^* signal (CIE: Commission International de l ' ^{) of} a color output when a certain CMY is given to the image output apparatus. Eclairage: International Lighting Commission, L ^* : Lightness, a ^* , b ^* : Chromaticity))) RGB data when the scanner reads a color patch having the same L ^* a ^* b ^* and color image output device 30 Are prepared, and the coefficients of the transformation matrix from a11 to a33 in the above equation 1 are calculated from these combinations. Color correction processing is performed using these coefficients. When higher accuracy is desired, a higher-order term of second or higher order may be added.

  The black generation and under color removal unit 16 generates black (K) signals from the CMY three-color signals after color correction, and generates a new CMY signal by subtracting a portion where the original CMY signals overlap. By performing the above, the CMY 3-color signal is converted into the CMYK 4-color signal.

  The spatial filter processing unit 17 performs spatial filter processing using a digital filter on the image data of the CMYK signal output from the black generation and under color removal unit 16 based on the region identification signal, thereby correcting the spatial frequency characteristics. The output image is processed so as to prevent blurring and graininess deterioration.

  The output tone correction unit 18 performs output tone correction processing for converting a signal such as a density signal into a halftone dot area ratio that is a characteristic value of the color image output device 30.

  Similar to the spatial filter processing unit 17, the gradation reproduction processing unit 19 performs predetermined processing on the image data of the CMYK signal based on the region identification signal, and finally simulates the gradation of the image. Gradation reproduction processing is performed so that the reproduction is possible.

  For example, a region separated as a character edge region by the region separation processing unit 14 is subjected to sharp enhancement processing in the spatial filter processing by the spatial filter processing unit 17 in order to improve the reproducibility of black characters or color characters, in particular. The tone reproduction processing unit 19 performs multi-value dither processing (multi-value processing) using a high-resolution dither screen (multi-value dither matrix data) suitable for high-frequency component reproduction.

  Further, with respect to the region separated as the halftone dot region by the region separation processing unit 14, the spatial filter processing unit 17 performs low pass filter processing for removing the input halftone dot component, and the tone reproduction processing unit 19 performs Then, multi-value dither processing using a dither screen that emphasizes gradation is performed.

  Further, with respect to the region separated as the photographic region by the region separation processing unit 14, the gradation reproduction processing unit 19 performs multi-value dither processing using a dither screen that emphasizes gradation reproducibility.

  In the digital color multi-function peripheral 1 of the present embodiment, in order to reproduce a multi-tone input image with gradations smaller than the gradation of the input image (gradation that can be expressed by the color image output device 30), The gradation reproduction processing unit 19 is configured to perform multilevel dither processing. The present invention is characterized by a dither screen (multi-value dither matrix data) used when the gradation reproduction processing unit 19 performs multi-value dither processing. Details of the multi-value dither processing and the dither screen will be described later.

  The image data handled in the color image processing apparatus 10 is composed of the density values of each color of CMYK. In the multi-value dither processing, the same processing is performed for each color component. Only the process for the density value (image data) of the component will be described, and the description of the process for other colors will be omitted.

  The image data subjected to the above-described processes is temporarily stored in a storage unit (not shown), read at a predetermined timing, and input to the color image output device 30. The color image output device 30 outputs an image corresponding to input image data onto a recording medium (for example, paper). An image forming method in the color image output device 30 is not particularly limited, and for example, an electrophotographic method, an inkjet method, or the like can be used. The above processing is controlled by a main control unit (CPU (Central Processing Unit)) not shown.

(2. Multi-level dither processing)
(2-1. Configuration and operation of the gradation reproduction processing unit 19)
Next, the configuration of the gradation reproduction processing unit 19 and the multi-value dither processing performed by the gradation reproduction processing unit 19 will be described. In the present embodiment, it is assumed that an input image with 256 gradations (image before dither processing) from 0 to 255 for each pixel is converted into a 16-gradation dither image by multi-value dither processing. Of course, the number of gradations of the input image and the dither image is not limited to these, and the number of gradations M of the input image and the number of gradations N of the dither image may be integers that satisfy M> N (where N ≧ 3). Any value may be used.

  In the present embodiment, since the gradation of the input image subjected to the multi-value dither processing is expressed by the density values of the respective color components of CMYK, when performing the multi-value dither processing, the input image described above is used. After separation into bit planes for each color component, multi-value dither processing is individually performed on each bit plane, and then the bit planes are synthesized. However, since the multilevel dither processing method for each bit plane is common, only the processing for a single bit plane will be described below.

  FIG. 3 is a block diagram showing the configuration of the gradation reproduction processing unit 19. As shown in this figure, the gradation reproduction processing unit 19 includes a threshold output value storage unit 51 and a threshold processing unit 52. FIG. 4 is an explanatory diagram showing the configuration of the dither matrix used in the present embodiment.

  In the present embodiment, as shown in FIG. 4, a dither matrix composed of 40 pixels is used. Note that the shape of the dither matrix is not limited to this shape. By disposing these dither matrices on the input image as shown in FIG. 5, dither processing with a screen angle as a whole is possible.

  In FIG. 4, one square corresponds to one pixel. The numbers given to the squares indicate the order in which the dots spread in the plane direction (the order in which the dots are output), that is, the dot expansion order in the dither pattern generated corresponding to the dither matrix. ing.

  The threshold output value storage unit 51 stores 15 threshold values for each pixel position in the dither matrix and 16 output density values corresponding to 16 sections distinguished by these threshold values. That is, for each position i (i = 1, 2, 3,..., N) in the dither matrix, the threshold Th [i] [j] (j = 0, 1, 2,... (Threshold level), 14; Th; [I] [j] ≦ Th [i] [j + 1]) and output density value Out [j] (j = 0, 1, 2) corresponding to each section distinguished by each threshold Th [i] [j] ,..., 14; Out [j] ≦ Out [j + 1]). As the output density value Out [j], a value corresponding to the dot growth pattern is stored.

  The threshold processing unit 52 compares the density value of each pixel of the input image data with the threshold value assigned to the position corresponding to each pixel in the dither matrix, and the density value of each pixel of the input image data is 16. Which of the sections is included is specified. Then, the output density value associated with the identified section is set as the density value in the dither image. In the present embodiment, when comparing the density value of each pixel with the threshold value of the position in the dither matrix corresponding to each pixel, a dark output density value is assigned to each pixel in the dither matrix. The size comparison is performed in order from the current pixel.

  FIG. 1 is a diagram illustrating an example of a threshold value associated with each square (position corresponding to each pixel) in the dither matrix. Each of these threshold values is set within a range of values (0 to 255) that the density value of the input image can take, and a range of values (0 to 0) that the density value of the input image can take by 15 threshold values for each square. 255) is divided into 16 sections equal to the number of gradations of the dither image. Each of the 16 sections is associated with an output density value (that is, a density value in a dither image). In the present embodiment, output density values from 0 to 15 are associated with each section in order from the smallest threshold value. A method for setting a threshold value for each square will be described later.

  FIG. 6 is a flowchart showing the flow of multi-value dither processing in the gradation reproduction processing unit 19. In the following description, the threshold value of the jth (j = 0, 1, 2,..., 14) associated with the pixel i (i = 1, 2, 3,..., 16) in the dither matrix is set to Th. Let [i] [j]. Here, when a is an arbitrary integer of 0 to 14, Th [i] [a] ≦ Th [i] [a + 1] is always satisfied. That is, the value of the variable j attached to the minimum threshold is 0, and the value of the variable j increases as the threshold increases.

  In this case, if the kth section in the pixel i is a section [i] [k] (k = 0, 1, 2,..., 15), the section [i] [0] is 0 or more and Th [i] [1]. The interval [i] [k] (k = 1, 2,..., 14) is greater than Th [i] [k−1] and less than or equal to Th [i] [k], and the interval [i] [k] 15] is larger than Th [i] [14] and is 255 or less.

  In addition, the output density value associated with the section [i] [k] is D [k]. In the present embodiment, D [k] = k. The threshold output value storage unit 51 stores Th [i] [j] as threshold data and D [k] as output density value data.

  First, the threshold processing unit 52 selects one pixel in the input image data, specifies which pixel (cell) in the dither matrix corresponds to the selected pixel (input pixel), and specifies the specified pixel. The 15 threshold values associated with are read from the threshold output value storage unit 51 (S1).

  Next, the threshold value processing unit 52 compares the density value of the pixel in the input image data with the threshold value read out in step S1, and the density value of the pixel in the input image data is divided into any section divided by the threshold value. It is specified whether it is included (S2). Then, the threshold processing unit 52 outputs the output density value associated with the identified section as the density value (output pixel density value) of the dither image (output image data) (S3).

  Steps S2 and S3 can be expressed as follows. That is, the coordinate in the main scanning direction in the input image data is x, the coordinate in the sub-scanning direction is y, and the density value of the pixel at the coordinate (x, y) in the input image data is In [y] [x]. If the density value of the output image data (dither image) for this pixel is Out [y] [x], Out [y] [x] is Out [y] [x] = Out [j] (j = 0, 1, 2,..., 14; Out [j] ≦ Out [j + 1]).

More specifically,
If [In [y] [x] ≤ Th [i] [0]) Out [y] [x] = D [0] = 0
else if (In [y] [x] ≤ Th [i] [1]) Out [y] [x] = D [1] = 1
else if (In [y] [x] ≤ Th [i] [2]) If Out [y] [x] = D [2] = 2
・
・
・
else if (In [y] [x] ≤ Th [i] [14]) If Out [y] [x] = D [14] = 14
If else, Out [y] [x] = D [15] = 15
(Where i represents the number of the pixel in the dither matrix corresponding to the input pixel at coordinates (x, y)).

Steps S1 to S3 will be described using a specific example. For example, when the pixel in the input image data corresponds to the pixel “1” in the dither matrix, the threshold processing unit 52 from the threshold output value storage unit 51 to Th [1] [0] to Th [1] shown in FIG. Read up to [14]. For example, when the density value of the pixel “1” in the input image data is 14, the threshold processing unit 52
Th [1] [12] ≦ 14 <Th [1] [13]
Therefore, D [13] = 13 is output as the density value of the output image data for the pixel “1”.

  After step S3, the threshold processing unit 52 determines whether or not the multilevel dither processing (quantization processing; the processing of S1 to S3) has been completed for all the pixels in the input image data (S4), and has been completed. If not, the threshold processing unit 52 selects the next pixel and returns to step S1. At this time, the threshold processing unit 52 selects pixels along the main scanning direction indicated by a broken line in FIG. 5 while superimposing the dither matrix on the input image data, and repeats the multi-value dither processing.

  On the other hand, if it is determined in S4 that the quantization process has been completed for all pixels, the multi-value dither process is terminated.

  In this embodiment, the output density value Out [j] (j = 0, 1, 2,..., 14; Out [j] ≦ Out [j + 1]) is a 4-bit integer value from 0 to 15. However, gamma correction (output gradation correction) may be performed based on the output characteristics of the color image output device 30, and 16 values may be selected from 8-bit integer values from 0 to 255. When gamma correction is performed, the relationship between the input density value (input gradation value) and the output density value (output gradation value) is not linear, so the output density values are not set at equal intervals. Characteristic can be expressed.

(2-2. Setting method of threshold value in dither matrix)
Next, a method for setting the dither matrix shape and threshold will be described. In the present embodiment, in a pixel area (dither matrix) having a certain size and shape, the density value is increased when the density value of each pixel in the input image data is kept uniform. Multi-value by setting the order of darkening pixels (planar dot growth order) and the rate of darkening the density level of each pixel (how much the dots are darkened) Visually set the order of dot growth at the level. In other words, the pixel size and shape of the dither matrix are arbitrarily set, the order in which the pixels are to be darkened within this shape is determined, and then the level in which the dots are darkened while using this order ( (Concentration direction) is set (determining the growth method in the depth direction).

  First, a method for setting the order of pixels in which the density is increased (dot growth order) will be described. FIG. 7 is a flowchart for explaining a method of setting the dot growth order in the dither matrix.

  As shown in this figure, first, the pixel size and shape of the dither matrix are determined (S21). In the present embodiment, as described above, a dither matrix having the pixel size and shape shown in FIG. 4 is used.

  Next, when the density of all the pixels in the input image data is kept uniform, the density is changed from the lowest density (gradation value 0) to the highest density (gradation value 255), that is, the density value of each pixel ( When the input image data with the same (gradation value) is sequentially increased from the lowest density to the highest density while maintaining the condition that the density value of each pixel is the same value, which one for each pixel of the dither matrix Whether to increase the output density value in such an order (growth order) is determined as follows.

  First, a first line composed of pixels on a straight line having a predetermined angle with respect to the horizontal direction, and a second line, a third line, and a fourth line composed of pixels on each straight line substantially parallel to the first line. Is selected (S22). The first line, the third line, the second line, and the fourth line are arranged in this order, and each line is selected so that the intervals between these lines are substantially equal. In the present embodiment, as shown in FIG. 4, the first line is formed by the pixels “1”, “5”, “3”, and “7”, and the second line is formed by the pixels “4”, “8”, “2”, and “6”. A third line is formed by pixels “13”, “9”, “15”, and “11”, and a fourth line is formed by pixels “16”, “12”, “14”, and “10”.

  The first line and the second line are sequentially increased so that the output density value of any one of the pixels constituting the first line and the output density value of any of the pixels constituting the second line are increased. The threshold value (dot growth order) of each pixel that constitutes is set (S23). That is, the threshold value is set for each pixel constituting the first line group (first line image group) including the first line and the second line. Specifically, the threshold value of each pixel is set so that the output density value of each pixel constituting the first line and the second line sequentially increases in the order of the pixel numbers shown in FIG.

  Therefore, as shown in FIG. 1, the pixels belonging to the first line and the second line become a threshold set in which the threshold is set to a relatively low value, and the input has the same size and shape as the dither matrix and the density is uniform. When the input density value is sequentially increased in the image data, dots are turned on at an early stage (low density area) as shown in FIG. In FIG. 8, the numbers given to the pixels indicate the density values of the pixels in the dither pattern.

  As a result, in the low density region (for example, the gradation value 0 to 51 of the input image data), two lines of the first line and the second line are formed in the dither matrix, which are substantially parallel to each other and have a substantially constant interval. To go. For example, when the output resolution is 600 dpi, the first line and the second line are formed at an interval of 134 lpi (line per inch). Therefore, when multi-value dither processing is performed using this dither matrix, as shown in FIGS. 9A and 9B, the entire image has a large number of first lines and second lines in the low density region. Will be formed.

  After that, whether or not the threshold value has been set for each pixel constituting the first line and the second line (in this embodiment, whether or not 15 threshold values have been set for each pixel constituting the first line and the second line) (S24). If the setting of the threshold values for each pixel has not been completed, the process of S23 is continued.

  On the other hand, when the setting of the threshold value for each pixel constituting the first line and the second line is finished, one of the output density values of the pixels constituting the third line and the pixels constituting the fourth line The threshold values of the pixels constituting the third line and the fourth line are set so that any one of the output density values sequentially increases (S25). That is, the threshold value is set for each pixel constituting the second line group (second line image group) including the third line and the fourth line. Specifically, the output density values of the pixels (pixels “9” to “16”) constituting the third line and the fourth line are sequentially increased in the order of the pixel numbers shown in FIG. Set the threshold.

  Therefore, as shown in FIG. 1, the pixels belonging to the third line and the fourth line have a threshold set whose threshold is larger than the threshold values of the pixels belonging to the first line and the second line, and the first line and the second line The third line and the fourth line are formed after the output density value of the pixel reaches the maximum value. That is, in the low density region, the dots rise in a line shape along the first line and the second line, and grow while gradually increasing the output density value. When the output density values of the first line and the second line reach the maximum density value, the third line and the fourth line start to grow in the medium density region (for example, the gradation values 52 to 102 of the input image data).

  Thus, when multi-value dither processing is performed using this dither matrix, as shown in FIGS. 9 (c) to 9 (e), the entire image has an intermediate density region (for example, gradation values of input image data). 52 to 102), the third line grows between the first line and the second line in the region corresponding to each dither matrix, and the second line in the region corresponding to each dither matrix and the other adjacent to the dither matrix. A fourth line grows between the first line in the region corresponding to the dither matrix. Therefore, the medium concentration region has a higher number of lines than the low concentration region. For example, when the output resolution is 600 dpi, four lines of the first line to the fourth line are formed at an interval of 268 lpi (line per inch).

  Thus, by setting the threshold value so that the first line and the second line are formed in the low density area of the input density value, and the third line and the fourth line are formed in the medium density area, the highlight area In this case, it is possible to grow dots in a line while maintaining a low line number, and to grow as a line maintaining a high line number in the medium density region. As a result, when halftone processing is applied to an image containing a large amount of high-frequency components in the medium density region using a dither matrix for low line number design, moiré is generally likely to occur in the medium density region. Although there is such a problem, such problems can be reduced.

  Thereafter, it is determined whether or not the threshold value has been set for each pixel constituting the third line and the fourth line (S26). If the setting of the threshold value for each pixel has not been completed, the process of S25 is continued.

  On the other hand, when the setting of the threshold value for each pixel constituting the third line and the fourth line is finished, the threshold values of the remaining pixels are set (S27). In the present embodiment, the output density value of each pixel (pixels “17” to “40”) for which the threshold value has not been set increases in order from the surrounding pixels of each of the first to fourth lines. By setting a threshold value for each pixel, the first to fourth lines are gradually thickened.

  Specifically, as shown in FIG. 4, the pixels “17” to “40” are made up of a part of pixels adjacent to the first line and the second line (pixels “17” to “20”). , A group of pixels adjacent to the third line and the fourth line (pixels “21” to “24”), and pixels adjacent to the first line and the second line (pixels “25” to “25”). 28 ”), a group of pixels adjacent to the third and fourth lines (pixels“ 29 ”to“ 32 ”), and the remaining pixels (pixels“ 33 ”to“ 40 ”). A process of classifying the output density values of each pixel in each group and sequentially setting the threshold value so that the output density value of each pixel becomes darker is sequentially performed for each group.

  Thereafter, it is determined whether or not the setting of the threshold has been completed for all the pixels in the dither matrix (S28). If there is a pixel that has not been set, the process of S27 is continued, and the setting has been completed for all the pixels. Ends the setting process for the dot growth order.

  Next, a method for setting a density level in each pixel, that is, a method for setting an output density value associated with each density classification distinguished by a threshold value will be described.

  As the density level setting method for each pixel, for example, there are three possible methods: pixel sequential, frame sequential, and mixed sequential combining pixel sequential and plane sequential.

  In the pixel sequence, the output density value of each pixel is such that the output density value of one pixel grows from the minimum density to the maximum density and then the output density value of the next pixel grows from the minimum density to the maximum density. This is a method of sequentially increasing the maximum density in pixel units. Therefore, a certain pixel can be intensively grown to a high density.

  In the surface sequence, the output density values of a plurality of pixels included in the selected area in the dither matrix are sequentially grown from the minimum density to the maximum density corresponding to the density level (first density level) having the input density value. Thereafter, the output density values of a plurality of pixels included in the next region are changed from a density level (first density level) having an input density value to a next density level (second density level) from a minimum density to a maximum density. This is a method in which processing such as sequential growth is repeated and sequentially performed in units of regions (plane units) until the maximum density of input density values is finally reached. Therefore, each pixel included in the selected region can be gradually grown, or a plurality of pixels can be gradually grown as a whole in the matrix.

  The mix order specified that multiple pixels in the dither matrix (not necessarily adjacent to each other) are grown alternately from the minimum density to the maximum density, and then another pixel is grown. This is a method of growing in units of pixel groups, and is a method combining two methods of pixel sequential and plane sequential.

  In this embodiment, a mixed sequential growth method is employed. Specifically, as shown in FIG. 1, among the pixels (pixels “1” to “8”) constituting the first line and the second line, the pixels “1” to “4” are assigned to the first group, Pixels “5” to “8” are set as the second group. Then, every time the input density value increases by 1, the output density value of the pixels belonging to the first group grows alternately by 2 pixels (however, every time the input density value increases by 3, it is 3 pixels once). Then, the threshold value of each pixel is set. Then, when the output density value of the pixels belonging to the first group reaches the maximum density, the threshold value is similarly set for the pixels belonging to the second group. As a result, as shown in FIG. 8, every time the input density value increases by 1 from the low density, the threshold value of the pixels in the dither matrix also increases by 2 pixels or 3 pixels.

  Thereafter, for the images (pixels “9” to “16”) constituting the third line and the fourth line, the third group of pixels “9” to “12” and the pixels “13” to “16” are similarly formed. Is set as the fourth group, and the process of setting the threshold value so that the output density value of each pixel in each group is alternately dark is sequentially performed for each group.

  Thereafter, the pixels “17” to “40” are added to the fifth group, the third line, and the fourth line, which are a part of the pixels adjacent to the first line and the second line (pixels “17” to “20”). A sixth group including a part of adjacent pixels (pixels “21” to “24”), a seventh group including a part of pixels adjacent to the first line and the second line (pixels “25” to “28”). An eighth group composed of a group, a part of pixels adjacent to the third line and the fourth line (pixels “29” to “32”), and a ninth group composed of the remaining pixels (pixels “33” to “40”). A process of classifying into groups and setting threshold values so that the output density values of the pixels in each group are alternately dark is sequentially performed for each group. As a result, the threshold value is finally set so that dots are grown until the density of all pixel positions in the dither matrix reaches the maximum density.

  Here, every time the input density value increases by one, the threshold value is set so that the output density value of the pixels belonging to each group increases by two pixels (however, every time the input density value increases by three, three pixels at a time). The reason for setting is described.

  FIG. 10 is an explanatory diagram showing the relationship between the input density value (uniform density value for each pixel in the input image data) and the total output value (added value of the output density value of each pixel) in each dither matrix. .

  As shown in this figure, when the total number of pixels in each dither matrix is 40 pixels and the output density value is 4 bits, the maximum density value in each pixel is 15, so the total number of densities that can be expressed in each dither matrix ( The number of gradations) is 600. Then, when this total density number is divided by 256, which is the level number of the input density value, 600/256 = 2.34 per level in the input density value. This is simply because when the input density value is increased from 256 to 1 by 256 from 0 to 255, each time the input density value increases by 1 level, 40 dither matrices each having 15 threshold values are used. This shows that it is only necessary to increase the pixel by 2.34 pixels. That is, each time the input density value increases by one level, the threshold value of two pixels is increased, and every time the input density value increases by three levels, the threshold value of one pixel is increased.

  In this way, the total output value is divided by the number of levels of the input density value, and the number of pixels whose threshold value should be increased every time the input density value increases by one level is calculated. As a result, a plurality of threshold levels can be set for each pixel in the dither matrix, and dither matrix data for multilevel levels can be generated.

  In this embodiment, every time the input density value increases by 1, the output density value of the pixels belonging to each group increases by 2 pixels (however, every time the input density value increases by 3, it increases by 3 pixels once). Although the threshold is set so as to be, it is not limited to this. For example, the threshold value is increased by two pixels until the input density value reaches a certain density level, and is increased by three pixels after reaching the certain density, and the threshold value is increased according to the level range of the input density value. The number of pixels may be changed.

  In this embodiment, the case where the relationship between the input density value and the output density value is linear has been described. However, the present invention is not limited to this. For example, gamma correction (output) is performed according to the image output characteristics of the color image output device 30. (Gradation correction) may be performed. When performing gamma correction, the threshold value of each pixel is set so that the relationship between the input density value (uniform for each pixel) and the total output value in the dither matrix is a curve corresponding to the image output characteristics of the color image output device 30. Can be assigned.

  The solid line in FIG. 10 shows the relationship between the input density value when the gamma correction is not performed and the total output value of the dither matrix, and the broken line shows the input density value when the gamma correction is not performed and the total output value of the dither matrix. Shows the relationship.

  As shown in this figure, when gamma correction is not performed, the threshold value of each pixel is set so that the relationship between the input density value and the total output value of the dither matrix is linear. Therefore, pixels “1” to “8” (first line and second line) are input density values 1 to 51, and pixels “9” to “16” (third line and fourth line) are input density values 52 to 102. ), Pixels “17” to “24” for input density values 103 to 153, pixels “25” to “32” for input density values 154 to 204, and pixels “33” to “40” for input density values 205 to 255. The threshold values of these pixels are set so that the output density value increases.

  On the other hand, when performing gamma correction, the threshold value of each pixel is set so that the relationship between the input density value and the total output value of the dither matrix is a curve corresponding to the image output characteristics of the color image output device 30. For example, assuming that the curve corresponding to the image output characteristics of the color image output device 30 is the broken line in FIG. 10, when performing gamma correction, the input density value in the curve shown in FIG. Pixels “1” to “8” (first line and second line), and when the input density value is a range corresponding to the total output values 121 to 240, pixel “9”. To “16” (third line and fourth line), pixels “17” to “24” when the input density value is in a range corresponding to the total output values 241 to 230, and the input density value is the total output value 361 When the input density value is in the range corresponding to the total output values 481 to 230, the output density values of “33” to “40” are set. The threshold value of each pixel is set to increase.

  As described above, in this embodiment, when the gradation value of the input image having the same gradation value of each pixel is sequentially increased while maintaining the condition that the gradation value of each pixel is the same value. In addition, in a low gradation range (low density range) from a gradation value 0 (lowest gradation) to a gradation value 51 (first gradation), a first line group consisting of a predetermined number of line images extending in a predetermined direction. Alternatively, a part thereof is formed in the output image (dither image), and in the middle gradation range (medium density range) from the gradation value 52 to the gradation value 102 (second gradation), in addition to the first line image group. The tone value of the input image and the tone value of the output image in each pixel are such that a second line group consisting of a prescribed number of line images substantially parallel to the prescribed direction or a part thereof is formed in the output image. Set the correspondence.

  Thereby, it is possible to appropriately reproduce the gradation by preventing the appearance of the texture pattern for the input image in the low gradation range. In the middle density range, the number of line images formed in the output image can be increased to increase the number of lines compared to the case of the low density range, so the screen angle is changed for the input image in the middle tone range. Moire can be suppressed without doing so.

  In this embodiment, a plurality of threshold values are set for each pixel in the dither matrix, and the output density value is determined by comparing each threshold value with the input density value. A LUT (Look Up Table) in which the correspondence between the input density value and the output density value is stored for each pixel is prepared, and dither processing (halftone processing) is performed using this LUT. It may be.

  In addition, each block of the color image processing apparatus 10, particularly the dither processing unit 190 of the gradation reproduction processing unit 19, may be configured by hardware logic, or realized by software using a CPU or the like as follows. Also good.

  That is, the color image processing apparatus 10 includes a central processing unit (CPU) that executes instructions of a control program that implements each function, a read only memory (ROM) that stores the program, and a random access memory (RAM) that expands the program. ), A storage device (recording medium) such as a memory for storing the program and various data. An object of the present invention is a recording medium in which a program code (execution format program, intermediate code program, source program) of a control program of the color image processing apparatus 10 which is software for realizing the above-described functions is recorded so as to be readable by a computer. Can also be achieved by reading out and executing the program code recorded on the recording medium by the computer (or CPU or MPU).

  Examples of the recording medium include a tape system such as a magnetic tape and a cassette tape, a magnetic disk such as a floppy (registered trademark) disk / hard disk, and an optical disk such as a CD-ROM / MO / MD / DVD / CD-R. Card system such as IC card, IC card (including memory card) / optical card, or semiconductor memory system such as mask ROM / EPROM / EEPROM / flash ROM.

  The color image processing apparatus 10 may be configured to be connectable to a communication network, and the program code may be supplied via the communication network. The communication network is not particularly limited. For example, the Internet, intranet, extranet, LAN, ISDN, VAN, CATV communication network, virtual private network, telephone line network, mobile communication network, satellite communication. A net or the like is available. Also, the transmission medium constituting the communication network is not particularly limited. For example, even in the case of wired such as IEEE 1394, USB, power line carrier, cable TV line, telephone line, ADSL line, etc., infrared rays such as IrDA and remote control, Bluetooth ( (Registered trademark), 802.11 wireless, HDR, mobile phone network, satellite line, terrestrial digital network, and the like can also be used. The present invention can also be realized in the form of a computer data signal embedded in a carrier wave in which the program code is embodied by electronic transmission.

  The configuration of the computer is not particularly limited. For example, an image input device such as a flatbed scanner, a film scanner, or a digital camera, an image such as a CRT display or a liquid crystal display that displays a processing result of the computer. A network card, a modem, or the like as a communication means for connecting to a display device and a server or the like via a network may be provided.

  The present invention can also be applied to software installed in a personal computer. FIG. 11 is a block diagram illustrating a configuration example when the present invention is applied to a printer driver in the computer system 100.

  As shown in FIG. 11, the computer system 100 includes a personal computer 101 and a color printer 130. The color printer 130 may be a digital multifunction machine having a copy function and a fax function in addition to the printer function.

  The personal computer 101 includes an application program 105, a printer driver 110, and a communication port driver 120 as software, and also includes a CPU, a memory, a communication port 125, and the like as hardware. The various types of software described above are stored in a memory and perform various functions by being executed by the CPU. The communication port 125 is, for example, RS232C or LAN, and is connected to the color printer 130.

  Specifically, the printer driver 110 includes a color correction unit 15, a black generation and under color removal unit 16, a gradation reproduction processing unit 19, a printer language translation unit 111, and the like. Here, the color correction unit 15, the black generation and under color removal unit 16, and the gradation reproduction processing unit 19 have substantially the same functions as those provided in the color image processing apparatus 10, and thus description thereof is omitted.

  When printing is instructed by the application program 105, image data is input from the application program 105 to the color correction unit 15 of the printer driver 110. Then, the image data processed by the color correction unit 15 and the black generation and under color removal unit 16 are input to the gradation reproduction processing unit 19. In the gradation reproduction processing unit 19, multi-value dither processing is performed on multi-gradation image data as in the case of the color image processing apparatus 10. The image data that has undergone the multi-value dither processing is then input to the printer language translation unit 111.

  In the printer language translation unit 111, the image data is converted into a printer language interpretable by the color printer 130 and input to the communication port driver 120. Then, the communication port driver 120 controls the communication port 125, and the image data translated into the printer language is transmitted from the communication port 125 to the color printer 130. The color printer 130 forms and outputs an image corresponding to the received image data on a print medium.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims. That is, embodiments obtained by combining technical means appropriately modified within the scope of the claims are also included in the technical scope of the present invention.

  The present invention can be applied to an image processing apparatus that performs gradation reproduction processing by performing multi-level dither processing on input image data using a dither matrix.

It is explanatory drawing which shows an example of the threshold value matched with each pixel in the dither matrix used in the image processing apparatus concerning one Embodiment of this invention. 1 is a functional block diagram illustrating a configuration of an image forming apparatus including an image processing apparatus according to an embodiment of the present invention. It is a block diagram which shows the structure of the gradation reproduction process part with which the image processing apparatus concerning one Embodiment of this invention is equipped. It is explanatory drawing which shows the dither matrix used in the image processing apparatus concerning one Embodiment of this invention. It is explanatory drawing which shows the example of application of the dither matrix in the image processing apparatus concerning one Embodiment of this invention. It is a flowchart which shows the flow of the multi-value dither process in the image processing apparatus concerning one Embodiment of this invention. It is a flowchart which shows the setting method of the threshold value with respect to each pixel of the dither matrix in one Embodiment of this invention. It is explanatory drawing which shows the relationship between the density value of input image data in the image processing apparatus concerning one Embodiment of this invention, and the output density value of each pixel in a dither matrix. (A)-(e) is explanatory drawing which shows an example of the image formed in the image forming apparatus provided with the image processing apparatus concerning one Embodiment of this invention. 4 is a graph showing a relationship between a density value of input image data and a total value (total output value) of output density values of pixels belonging to a dither matrix in the image processing apparatus according to the embodiment of the present invention. It is a block diagram which shows the structure of the computer system concerning one Embodiment of this invention.

Explanation of symbols

1 Digital color MFP (image forming device)
10 Color image processing device (image processing device)
19 gradation reproduction processing unit 20 color image input device (document reading device)
30 Color image output device (image output device)
51 Threshold output value storage unit (storage means)
52 Threshold processing unit (gradation determination means)
100 Computer System 101 Personal Computer (Image Processing Device)
110 Printer driver (multi-value dither processing program)
130 Color printer (image output device)

Claims (11)

A dither matrix tone setting method for setting a correspondence relationship between a tone value of an input image and a tone value of an output image for each pixel of a dither matrix used in multi-value dither processing,
When the gradation value of the input image in which the gradation value of each pixel is the same value is sequentially increased while maintaining the condition that the gradation value of each pixel is the same value,
In the low gradation range from the lowest gradation to the first gradation, a first line group consisting of a predetermined number of line images extending in a predetermined direction or a part thereof is formed in the output image,
In the middle gradation range from the first gradation to the second gradation, in addition to the first line image group, a second line group consisting of a predetermined number of line images substantially parallel to the predetermined direction or a part thereof is an output image. A dither matrix gradation setting method, wherein the correspondence relationship between the gradation value of the input image and the gradation value of the output image in each pixel is set so as to be formed.   A gradation value of an input image in each pixel so that a line image belonging to the first line image group and a line image belonging to the second line image group are alternately arranged along a direction perpendicular to the predetermined direction; The dither matrix gradation setting method according to claim 1, wherein a correspondence relationship with a gradation value of an output image is set.   When the gradation value of the input image is sequentially increased while maintaining the condition that the gradation value of each pixel is the same value, the gradation value of each pixel constituting the first line image group is a predetermined number. The gradation setting of the dither matrix according to claim 1 or 2, wherein the correspondence relation between the gradation value of the input image and the gradation value of the output image in each pixel is set so that the number of pixels increases sequentially. Method.   When the gradation value of the input image is sequentially increased while maintaining the condition that the gradation value of each pixel is the same value, the gradation of each pixel in each line image constituting the first line image group Setting the correspondence between the gradation value of the input image and the gradation value of the output image in each pixel constituting the first line image group so that the sum of the values increases substantially uniformly for each line image. The gradation setting method for a dither matrix according to any one of claims 1 to 3.   When the gradation value of the input image is sequentially increased while maintaining the condition that the gradation value of each pixel is the same value, the gradation value of each pixel constituting the second line image group is a predetermined number. 5. The dither according to claim 1, wherein the correspondence relationship between the gradation value of the input image and the gradation value of the output image in each pixel is set so that the number of pixels increases sequentially. Matrix gradation setting method.   When the gradation value of the input image is sequentially increased while maintaining the condition that the gradation value of each pixel is the same value, the gradation of each pixel in each line image constituting the second line image group Setting a correspondence relationship between the gradation value of the input image and the gradation value of the output image in each pixel constituting the second line image group so that the sum of the values increases substantially equally for each line image. The dither matrix gradation setting method according to claim 1, wherein: By applying multilevel dither processing to a multi-tone input image using a dither matrix in which the correspondence between the tone value of the input image and the tone value of the output image is set for each pixel, the level of the input image is An image processing method for reproducing a tone as a multi-tone dither image,
An image processing method using the correspondence set by the gradation setting method according to any one of claims 1 to 6 as the correspondence. By applying multilevel dither processing to a multi-tone input image using a dither matrix in which the correspondence between the tone value of the input image and the tone value of the output image is set for each pixel, the level of the input image is An image processing apparatus that reproduces a tone as a multi-tone dither image,
Storage means for storing a correspondence relationship between the gradation value of the input image and the gradation value of the output image set for each pixel of the dither matrix;
Gradation determination means for reading out the gradation value of the output image corresponding to the gradation value of the input image from the storage means and determining the gradation value of each pixel in the output image;
7. The image processing apparatus according to claim 1, wherein the storage unit stores a correspondence set by the gradation setting method according to claim 1 as the correspondence. An image processing apparatus according to claim 8,
An image forming apparatus comprising: an image output device that forms the dither image on a print medium.   An image processing program for causing a computer to function as each unit of the image processing apparatus according to claim 8.   A computer-readable recording medium on which the image processing program according to claim 10 is recorded.

Images