JP4987833B2 - Dither matrix creation method, dither matrix creation device, image processing device, image forming device, program, and recording medium - Google Patents

Description

  The present invention relates to a method for creating a dither matrix used for binary dither processing as halftone processing for input image data, a dither matrix creation device, an image processing device, an image forming device, a program, and a recording medium.

  Conventionally, as a method for outputting an image to a recording medium such as paper, various recording methods such as thermal transfer, electrophotography, or an ink jet method have been adopted. In these recording methods, an image is formed by performing a process of creating a dot pattern according to the density of an input image. The above processing includes dither processing.

  In the dither processing, a dither matrix arranged in a matrix having threshold values of n × m in pixel units is used, multi-value image data input in pixel units is compared with each threshold value of the dither matrix in units of one pixel, A halftone image is obtained by quantizing and transforming image data into binary or multi-value. Examples of the dither matrix used for creating the dot pattern include a dot dispersion type and a dot concentration type dither matrix.

  Among the dither matrices, there are various types of dot aggregate types as the gradation value increases. For example, from the point of dot growth shape, there are a circle, an ellipse, a line (linear), a square, a cross and a diamond. When such dot growth is used, there is a problem that in gradation reproduction, the shape of the dot reproduction area changes due to the difference in the shape of the dot growth and the image quality becomes bulky due to the instability of the density change. .

  Further, regarding multi-value dither and binary dither, in the case of multi-value dither, it is possible to set a plurality of threshold values for one pixel and to make the output value multi-tone, so that the size of the dither matrix is not increased so much. Sufficient gradation can be obtained. On the other hand, in the case of binary dither, there is only one set threshold for one pixel, and the output value has only two gradations, that is, whether or not a dot is hit. Therefore, in order to obtain sufficient gradation, it is necessary to increase the dither matrix size and use area gradation.

One example of using this area gradation is a method of creating a super matrix screen by collecting a plurality of basic matrices. One technique for creating this super matrix is disclosed in Patent Document 1. In Patent Document 1, it is described that when a super matrix is created, a basic matrix is used as a nucleus, and the arrangement order of the nuclei is set using blue noise.
JP 2003-259118 A (published on September 12, 2003)

  However, in the supermatrix creation method of Patent Document 1, although the dot growth order follows the blue noise characteristics, constraints are provided when determining the arrangement order of the nuclei (basic matrix) in the supermatrix. . For this reason, the actual dot growth order may deviate from the blue noise characteristics.

  For example, there is a case where a nucleus at a position where a dot is originally desired is shifted to a nucleus located at the periphery due to a constraint condition. When the shift of the arrangement order of the nuclei is repeated under such constraints, the original blue noise characteristics that are well-balanced while maintaining the characteristics in the horizontal and vertical directions may not be sufficiently reflected in the super matrix. There is.

  Accordingly, the present invention provides a dither matrix creation method, a dither matrix creation device, which can improve the dot reproducibility at the highlight portion in binary output image data by reflecting the original blue noise characteristics satisfactorily. An object of the present invention is to provide an image processing apparatus, an image forming apparatus, a program, and a recording medium.

  In order to solve the above problems, a dither matrix creation method according to the present invention is a dither matrix creation method for performing binary dither processing as gradation reproduction processing on input image data. A second matrix creating step of creating a second matrix in which a plurality of first matrices are arranged side by side using a first matrix in which the dot output order is determined for each pixel corresponding position, which is a position corresponding to each pixel. A central axis setting step for setting one central axis in a direction in which the first matrix is arranged in one direction according to the screen angle of the second matrix, and the first axis in a direction intersecting the central axis. For each column of one matrix, an intersecting axis with respect to the central axis is set in the direction of these columns, and the number of the first matrix existing on these intersecting axes is determined for each of the aforementioned intersecting axes. A first matrix number calculating step of obtaining a first calculation result by calculating for each side of both sides of the central axis with respect to the axis, and obtaining a total number of the first matrix in the second matrix as a second calculation result The central axis of the second matrix coincides with the vertical direction of the blue noise mask, and the intersecting axis of the second matrix coincides with the horizontal direction of the blue noise mask. A blue noise mask applying step of determining a blue noise mask range corresponding to the second matrix from the first calculation result and obtaining blue noise data corresponding to each first matrix of the second matrix; The output order of dots set in each blue noise data is set as the selection order of each first matrix, and the first matrix number calculation is performed. A selection order setting step of rearranging the selection order so that the total value of the first matrix indicated by the second calculation result of the step becomes a final order value of the selection order. Yes.

  The dither matrix creating apparatus of the present invention is a dither matrix creating apparatus for creating a dither matrix for performing binary dither processing as gradation reproduction processing on input image data. A second matrix creating unit that creates a second matrix in which a plurality of the first matrices are arranged, using a first matrix in which a dot output order is determined for each pixel corresponding position that is a position corresponding to A central axis setting unit that sets one central axis in a direction in which the first matrix is arranged in one direction according to a screen angle of the second matrix, and the first matrix in a direction intersecting the central axis For each column, a cross axis with respect to the central axis is set in the column direction, and the number of the first matrix existing on the cross axis is determined for each cross column. A first matrix number calculation unit that obtains a first calculation result by calculating for each side on both sides of the central axis with respect to the axis, and that obtains a total number of the first matrix in the second matrix as a second calculation result The central axis of the second matrix coincides with the vertical direction of the blue noise mask, and the intersecting axis of the second matrix coincides with the horizontal direction of the blue noise mask, and the first matrix number calculation unit A blue noise mask applying unit for determining a blue noise mask range corresponding to the second matrix from the first calculation result, and obtaining blue noise data corresponding to each first matrix of the second matrix; The dot output order set in each blue noise data is set as the selection order of each first matrix, and the first matrix number is calculated. And a selection order setting unit for rearranging the selection order so that the total value of the first matrix indicated by the second calculation result is the final order value of the selection order. .

  According to the above configuration, in the second matrix creation (the second matrix creation unit), the first matrix in which the dot output order is determined for each pixel corresponding position that is a position corresponding to each pixel of the input image data. For example, a basic matrix is used, and a second matrix, for example, a super matrix, in which a plurality of the first matrices are arranged side by side is created.

  In the central axis setting step (the central axis setting unit), one central axis is set in the direction in which the plurality of first matrices are arranged in one direction according to the screen angle of the second matrix.

  In the first matrix number calculation step (the first matrix number calculation unit), for each column of the first matrix in the direction intersecting with the central axis, an intersecting axis with respect to the central axis is set in these column directions. And the number of the 1st matrix which exists on these crossing axes is calculated for every side of the both sides of the central axis about each crossing axis, and the 1st calculation result is obtained. Further, the total number of the first matrix in the second matrix is obtained as the second calculation result.

  In the blue noise mask application process (the blue noise mask application unit), the central axis of the second matrix coincides with the vertical direction of the blue noise mask, and the intersecting axis of the second matrix coincides with the horizontal direction of the blue noise mask. Let In this state, the range of each blue noise data of the blue noise mask corresponding to each first matrix of the second matrix is determined from the first calculation result by the first matrix number calculating step (first matrix number calculating unit). decide. Thereby, each blue noise data of the blue noise mask corresponding to each first matrix of the second matrix is obtained.

  In the selection order setting step (the selection order setting unit), the output order of the dots set in each blue noise data is set as the selection order of each first matrix. Then, the selection of each first matrix is performed so that the total value of the first matrix indicated by the first calculation result by the first matrix number calculation step (first matrix number calculation unit) becomes the final order value of the selection order. Rearrange the order.

  In the dither matrix created as described above, the input image data as the entire dither matrix is determined by the mutual relationship between the dot growth order of the pixel corresponding positions with respect to the input image data in the first matrix and the selection order of each first matrix. It is possible to determine the dot growth order at the pixel corresponding position with respect to, that is, the processing order of each pixel of the input image data.

  Further, when the dither matrix is created by the processing of each part as described above, even if the shape of the second matrix (super matrix) is distorted to set the screen angle, the second matrix is used. Thus, a dither matrix in a state where the blue noise characteristic is substantially maintained can be created by appropriately corresponding to the blue noise mask. Thus, output image data with good dot dispersibility in highlights can be obtained in binary dither processing using a dither matrix.

  In the above-described dither matrix creation method, in the blue noise mask application step, the range of each blue noise data of the blue noise mask corresponding to each first matrix of the second matrix can be moved. It is good.

  Further, in the dither matrix generating apparatus, the blue noise mask applying unit inputs the previously determined range of each blue noise data of the blue noise mask corresponding to each first matrix of the second matrix. It is good also as a structure moved based on.

  According to said structure, in the blue noise mask application process (blue noise mask application part), the range of each blue noise data of the blue noise mask previously determined corresponding to each 1st matrix of a 2nd matrix is suitably set | placed. Can be moved. Therefore, when dots are biased in the output image data as a result of performing dither processing using the created dither matrix, the range of the blue noise mask corresponding to the second matrix is appropriately changed to solve the problem. Can do.

  An image processing apparatus of the present invention includes a gradation reproduction processing unit that uses a dither matrix created by any one of the above-described dither matrix creation methods and performs halftone processing by binary dither processing on image data. It is the composition which is.

  According to the above configuration, since the dither matrix used by the gradation reproduction processing unit is in a state in which the blue noise characteristic is substantially maintained, in binary dither processing in the gradation reproduction processing unit, Output image data with good dispersibility can be obtained.

  The image forming apparatus of the present invention includes the image processing apparatus and an image output apparatus that performs printing based on the image data processed by the gradation reproduction processing unit.

  According to the above configuration, the output image data output from the gradation reproduction processing unit has good dot dispersibility in highlights. Therefore, from the image output apparatus that performs printing based on this output image data, A good printed image can be obtained.

  When a dither matrix is created by the dither matrix creation method of the present invention, a blue noise mask is appropriately applied to the second matrix even when the shape of the second matrix is distorted to set the screen angle. Accordingly, it is possible to create a dither matrix that substantially maintains the blue noise characteristics. Thus, output image data with good dot dispersibility in highlights can be obtained in binary dither processing using a dither matrix.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram showing a schematic configuration of a digital color copying machine (image forming apparatus) 40 including a color image processing apparatus (image processing apparatus) 10 according to an embodiment of the present invention.

  As shown in FIG. 1, the digital color copying machine 40 includes a color image processing apparatus 10, a color image input apparatus 20, and a color image output apparatus 30. The color image processing apparatus 10 and the color image output apparatus 30 are connected to the image processing apparatus 10. The digital color copying machine 40 includes an operation panel 41.

  The image processing apparatus 10 includes an A / D conversion unit (analog / digital) 11, a shading correction unit 12, an input tone correction unit 13, a region separation processing unit 14, a color correction unit 15, a black generation and under color removal unit 16, and a space. A filter processing unit 17, an output tone correction unit 18, and a tone reproduction processing unit 19 are provided.

  The image input device 20 includes, for example, a scanner unit including a charge coupled device (hereinafter referred to as a CCD), irradiates light on a document on which an image is recorded, and the reflected light image is applied to the CCD. Are read as RGB analog signals, and the analog signals are input to the image processing apparatus 10.

  An analog signal input to the color image input device 20 passes through the color image processing device 10 within 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 lower 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 are output to the color image output apparatus 30 as CMYK digital color signals.

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

  The input tone correction unit 13 performs a process for adjusting the color balance on the RGB signal (RGB reflectance signal) from which various distortions have been removed by the shading correction unit 12, and a color image such as a density signal. The signal is converted into an easy-to-handle signal of the image processing method employed in the processing apparatus 10.

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

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

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'Eclairage ^{) of} a color output when a certain CMY is given to the image output apparatus. : International Lighting Commission, L ^* : Lightness, a ^* , b ^* : Chromaticity)) The same color patch with L ^* a ^* b ^* as the RGB data when scanned by the scanner and given to the image output device A large number of combinations with CMY data are prepared, and the coefficients of the transformation matrix from a11 to a33 in 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 order or higher may be added.

  The black generation and under color removal unit 16 performs a black generation process for generating a black (K) signal from the CMY three-color signals after color correction, and a process for generating a new CMY signal by subtracting a portion where the original CMY signals overlap. In this case, CMY three-color signals are converted into CMYK four-color signals.

  The spatial filter processing unit 17 performs a spatial filter process using a digital filter on the image data of the CMYK signal input from the black generation and under color removal unit 16 based on the region identification signal. As a result, the spatial frequency characteristics are corrected, and blurring and graininess deterioration of the output image are prevented.

  The output tone correction unit 18 performs output tone correction processing corresponding to the output characteristic value of the color image output device 30 on the CMYK signal that has been subjected to the spatial filter processing.

  Similar to the spatial filter processing unit 17, the gradation reproduction processing unit 19 performs gradation reproduction processing on the image data of the CMYK signal based on the region identification signal, and can finally reproduce the image in a pseudo gradation. Like that.

  Processing in the spatial filter processing unit 17 and the gradation reproduction processing unit 19 will be further described. For example, in order to improve the reproducibility of black characters or color characters particularly for the region separated as the character edge region by the region separation processing unit 14, the spatial filter processing unit 17 performs high frequency by sharp enhancement processing in the spatial filter processing. Emphasize ingredients. On the other hand, the gradation reproduction processing unit 19 performs binarization or multi-value processing on a high-resolution screen suitable for reproducing high-frequency components.

  In addition, the spatial filter processing unit 17 performs low-pass filter processing for removing the input halftone component on the region separated as the halftone region by the region separation processing unit 14.

  The gradation reproduction processing unit 19 performs binarization or multi-value processing on a region separated as a photographic region by the region separation processing unit 14 using a screen that emphasizes gradation reproducibility. In the present embodiment, the gradation reproduction processing unit 19 performs binarization processing.

  As shown in FIG. 2, the gradation reproduction processing unit 19 includes a threshold processing unit 51 and a threshold output value storage unit 52.

  The threshold output value storage unit 52 stores one threshold value and two output values for each position (hereinafter referred to as a pixel corresponding position) corresponding to each pixel of the input image data in the dither matrix. These threshold values and output values are values corresponding to the dot growth pattern. The threshold processing unit 51 uses the threshold value and output value stored in the threshold output value storage unit 52 to perform the threshold processing shown in FIG. 3 on the input image data. FIG. 3 is a flowchart showing the operation of the threshold processing unit 51.

  That is, the threshold processing unit 51 reads the threshold value of each pixel corresponding position from the threshold output value storage unit 52 (S1). Next, the value of each pixel of the input image data is compared with the threshold value of each pixel corresponding position, and one of the two output values is output according to the comparison result (S2). Thereafter, the processes of S1 and S2 are repeated for all the pixels of the input image data (S3).

  As described above, in the present embodiment, the gradation reproduction processing unit 19 performs binary output binary dither processing on pixel values (density values) 0 to 255 of the input image data. Therefore, the threshold processing unit 51 outputs one of the binary output values according to the pixel value (density value) of the input image data.

  Further, the threshold output value storage unit 52 has a threshold Th [i] [j] (j = 0 (one threshold level)) for each position i (i = 1, 2, 3,..., N) in the dither matrix. ), And the threshold value processing unit 51 has two output values Out [j] (j = 0, 1; Out [0] ≦ Out depending on the magnitude relationship between the threshold value and the pixel value (density value) of the input image data. [1]) is output.

  The operation panel 41 includes, 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. The operations of the color image input device 20, the color image processing device 10, and the color image output device 30 are controlled based on information input from the operation panel 41.

  In the color image processing apparatus 10, the image data subjected to the above-described processes is temporarily stored in a storage unit (not shown), read out at a predetermined timing, and output to the color image output apparatus 30. The color image output device 30 outputs (prints) image data on a recording medium (for example, paper). Examples of the color image output device 30 include, but are not limited to, those using an electrophotographic method or an inkjet method. The above processes are controlled by a CPU (Central Processing Unit) (not shown).

  The color image processing apparatus 10 of this embodiment is particularly characterized by a dither screen (dither matrix) used for binary dither processing in the gradation reproduction processing unit 19. The image data is composed of density values of each color of CMYK, and the binary dither process performs the same process on the image data of each color component regardless of the image data of the color component. Therefore, in the following description, only the processing for the density value (image data) of one color component will be described.

  FIG. 4 is an explanatory diagram illustrating an example of a dither matrix used by the threshold processing unit 51 in binary dither processing. The dither matrix 1 shown in FIG. 4 has a super matrix shape composed of a plurality of basic matrices 2. In FIG. 4, the area of the dither matrix 1 is an area not shaded.

  In the processing by the gradation reproduction processing unit 19 using the dither matrix 1, the pixel value (output value) of the output image data is determined while maintaining the smooth gradation of the output image data by the area gradation. The order in which the pixel values (output values) are determined, that is, the dot growth order, is designated according to the density value of the pixels of the input image data, and is the order from the low density pixels.

  The size of the dither matrix 1 is, for example, 128 × 128 pixels, and is determined based on the level of gradation expression required to express the gradation level and the output characteristics of the color image output device 30. Is done. In the present embodiment, the dither matrix 1 having a size corresponding to 70 × 70 pixels is used as will be described later.

  A numerical value indicating the dot growth order for each pixel is set at the pixel corresponding position in each basic matrix (each cell) 2 of the dither matrix 1 shown in FIG. The threshold value of the threshold output value storage unit 52 is set so that dots are output. Further, the growth order (selection order) of each basic matrix 2 itself is set in each basic matrix (each cell) 2. Therefore, in the dither matrix 1, it is possible to define a dot growth pattern, thereby making it possible to create various dot growth patterns.

  In actual binary dither processing, the threshold value processing is performed on the input image data in the main scanning direction to complete the binary dither processing.

Here, if the density value of the pixel of the input image data is In [y] [x], the density value Out [y] [x] of the corresponding pixel of the output image data is
Out [y] [x] = Out [j] (j = 0,1; Out [0] ≦ Out [1])
Either, that is,
If [In [y] [x] ≤ Th [y] [x] [0]) If Out [y] [x] = 0
If else, Out [y] [x] = 1,
(X: main scan direction, y: sub scan direction)
Determined by the relationship.

  In this embodiment, the output value Out [j] (j = 0, 1; Out [0] ≦ Out [1]) is a 1-bit integer value from 0 to 1, but from 0 to 255 Of these 8-bit integer values, two values selected in consideration of the γ characteristic of the image output apparatus may be used. For example, if the γ characteristic of the color image output device 30 is saturated at 240, 240 may be selected instead of 255.

Next, a method for creating the dither matrix 1 shown in FIG. 4 will be described.
As shown in FIG. 4, the dither matrix 1 has a configuration in which a large number of basic matrices 2 are arranged side by side. In creating the dither matrix 1, first, a large number of basic matrices 2 are arranged side by side as shown in FIG. 4 to create a super matrix. In this super matrix, the selection order of each basic matrix 2 is set to the blue noise mask 3 (FIG. 8)), the dither matrix 1 can be obtained.

  FIG. 5 is an explanatory diagram showing an example of the basic matrix (first matrix) 2. The basic matrix 2 shown in FIG. 5 corresponds to 28 pixels. 5 shows a simple shape as an example of the basic matrix 2, the basic matrix 2 is not limited to this shape.

  In the basic matrix 2, the dot growth order is set as indicated by numerical values from 1 to 28. Such a dot growth order setting affects the growth order of the lines on the entire screen (dither matrix 1) when another basic matrix 2 is arranged next to the basic matrix 2, so that the lines are as smooth as possible. It is desirable to set it to grow.

  The dither matrix 1 is formed by arranging and arranging a large number of the basic matrices 2 described above, and has a screen angle as a whole. As a result, the dither matrix 1 can perform dither processing with a screen angle.

  FIG. 6 shows an example in which another basic matrix 2 is arranged next to the basic matrix 2 shown in FIG. Even in a configuration in which a plurality of basic matrices 2 are arranged in this manner, dots are similarly grown in this order for the pixels numbered 1 to 28 in each basic matrix.

  In the configuration in which the plurality of basic matrices 2 are arranged side by side as described above, the screen angle is determined by the central axis that is a straight line connecting the positions of the numbers in the same growth order. In the case of the configuration of FIG. 6, the screen angle is −27 degrees.

  As shown in FIG. 7, when one basic matrix 2 is used as a reference and the other two basic matrices 2 are arranged in different directions with respect to the basic matrix 2, the center axis 4 is the center. The shafts 4a and 4b are generated. In the present embodiment, the screen angle is determined by one central axis 4a. Further, the number of screen lines and the screen angle are determined by the shape of the basic matrix 2 and the growth order of dots set at the positions corresponding to the respective pixels in the basic matrix 2. In the present embodiment, the dither matrix 1 shown in FIG. 4 is configured by arranging a large number of basic matrices 2 in a state where the screen angle is −27 degrees.

  Next, a method for setting the selection order of each basic matrix 2 in the dither matrix 1 will be described.

  As described above, the selection order is set for each basic matrix 2 in the dither matrix 1. The selection order is set using a blue noise pattern (blue noise mask 3).

  Here, the blue noise is pattern data having a spatial frequency that is difficult for human eyes to perceive. Human vision is almost insensitive above a certain spatial frequency, and the MTF (Modulation Transfer Function) of the visual system is known as a kind of low-pass filter (for example, Tsuyoshi Hamada, “High image quality in inkjet printers”). Technology ", Journal of the Imaging Society of Japan, 2001, Vol. 40, No. 3, p.239-243). Blue noise can be obtained by manipulating a pseudo-random pattern and generating a pattern in which the main component of the spatial frequency is distributed in a band equal to or higher than the cutoff frequency of the visual system MTF. The blue noise is usually given as a data matrix of 256 × 256 pixels, and the data matrix is called a blue noise mask.

  FIG. 8 is an explanatory diagram showing an example of the blue noise mask 3 used when setting the selection order of each basic matrix 2 in the dither matrix 1. The blue noise mask 3 shown in the figure has a pattern of blue noise characteristics generated with a 128 × 128 pixel size, and a value normalized to a value from the minimum value 1 to the maximum value 255 is set. Yes.

  In this process, the normalized blue noise data of the blue noise mask 3 is selected and assigned to each basic matrix 2 of the dither matrix 1 so as to match the shape of the dither matrix 1. is there. As described above, the order of selection of each basic matrix 2 can be set in a state where the blue noise characteristics are substantially maintained by sequentially corresponding the basic value 2 of the dither matrix 1 with the small value to the large value of the blue noise data. it can.

  Here, the matrix corresponding to the dither matrix 1 before setting the selection order of each basic matrix 2 will be described as a super matrix. FIG. 9 is an explanatory diagram showing a super matrix 6 for creating the dither matrix 1.

  When setting the selection order of each basic matrix 2, the central axis 4 is set in the super matrix 6 in order to determine the screen angle, as shown in FIG. In the case of the super matrix 6 in FIG. 9, the screen angle is −27 degrees as described above.

  Next, a direction intersecting with the central axis 4 (a direction that is substantially line symmetric) is determined, and how many basic matrices 2 exist on the plurality of intersecting axes 5 set in this direction is determined for each intersecting axis 5. calculate. Further, the total number of basic matrices 2 included in the super matrix 6 is obtained. In this case, as shown in FIG. 9, for each column of the basic matrix 2 in one direction intersecting with the central axis 4, an intersecting axis 5 with respect to the central axis 4 is set in these column directions. Then, the number of basic matrices 2 existing on these crossing axes 5 is calculated for each side on both sides of the central axis 4 for each crossing axis 5.

In the example of FIG. 9, the cross axis 5-1 to the cross axis 5-23 are selected for the central axis 4, and how many basic matrices 2 exist for each cross axis 5 is calculated. The calculation result of two basic matrices is, for example,
Cross axis 5-1: 2 (on axis: 1, above axis: 1)
Cross axis 5-2: 3 (on axis: 1, below axis: 1, above axis: 1)
Cross axis 5-3: 4 (on axis: 1, below axis: 1, above axis: 2)
Cross axis 5-4: 5 (on axis: 1, below axis: 2, above axis: 2)
・
・
Cross axis 5-23: 2 (on axis: 0, below axis: 2)
It becomes.

  The range of the blue noise data of the blue noise mask 3 corresponding to the super matrix 6 is determined based on the number of basic matrices 2 for each cross axis 5 calculated in this way. Specifically, as shown in FIG. 10, when the blue noise mask 3 is associated with the super matrix 6, the central axis 4 of the super matrix 6 is aligned with the vertical direction of the blue noise data in the blue noise mask 3. The super matrix 6 is rotated, and the range of blue noise data corresponding to the super matrix 6 is determined based on the number of the left and right basic matrices 2 of the central axis 4. In this case, with respect to the cross axis 5, the super matrix 6 itself is rotated in a fixed state so that the blue noise data in the blue noise mask 3 is aligned in the horizontal direction.

  The reason why the above processing is performed is as follows. That is, the shape of the super matrix 6 having a certain screen angle is distorted. Therefore, when the blue noise data arranged in the horizontal direction and the vertical direction are made to correspond to the super matrix 6, the central axes 4 of the super matrix 6 are aligned in the vertical direction of the blue noise data. Thereby, when the blue noise data is made to correspond to the super matrix 6, the blue noise characteristic can be maintained in the super matrix 6 (dither matrix 1). The position (range of blue noise data) in the blue noise mask 3 corresponding to the super matrix 6 may be selected according to the above rules. In other words, the position in the blue noise mask 3 corresponding to the super matrix 6 can be moved while maintaining the shape of the super matrix 6 in the blue noise mask 3.

  As described above, the number of basic matrices 2 required in the left-right direction of the central axis 4 around the central axis (basic axis, vertical axis) 4 set in the super matrix 6 is determined in advance. Therefore, the target range is selected for the normalized blue noise data of the blue noise mask 3 according to the number of the basic matrix 2 by the process of adapting the super matrix 6 to the blue noise mask 3 as described above. can do. In FIG. 10, the range is indicated by a solid thick line.

  Next, the blue noise data in the selected range shown in FIG. 10 is reset with the number of basic matrices 2 in the super matrix 6 as the maximum value. That is, the blue noise data arranged in the order of 1 to 255 is rearranged into the data of 1 to 175 so that the same data does not exist within the range of the super matrix 6.

  Specifically, in the present embodiment, the number of basic matrices 2 in the super matrix 6 is 175. Therefore, if there is only one blue noise data displayed as 1 within the range of the super matrix 6 in the blue noise mask 3, the selection order of the basic matrix 2 at that position is 1. On the other hand, if there is other blue noise data displayed as 1, the selection order of the basic matrix 2 at those positions is 1 for one and 2 for the other. Next, if there is blue noise data displayed as 2, since 1 and 2 are set in advance, the selection order of the basic matrix 2 at that position is 3. In this way, the selection order of the basic matrix 2 at the position of each blue noise data is set to the maximum value 175 so that the same data (the same selection order) does not overlap.

  With the above processing, the selection order from 1 to the maximum number 175 is set for the basic matrix 2 in the super matrix 6. In the dither matrix 1 created in this way, the entire dither matrix 1 is determined by the mutual relationship between the dot growth order at the position corresponding to each pixel of the input image data in the basic matrix 2 and the selection order of each basic matrix 2. As described above, the growth order of the pixel corresponding positions with respect to the input image data, that is, the processing order of each pixel of the input image data (growth order of each pixel of the output image data) is determined as one.

  Specifically, a dot is first placed at the first position (pixel corresponding to the first position) in the basic matrix 2 in which the selection order in the dither matrix 1 is set to 1, then A dot is placed at the first position (pixel corresponding to the first position) in the basic matrix 2 where the selection order in the dither matrix 1 is set to 2. Thereafter, similarly, the process proceeds to the processing of the first position (pixel corresponding to the first position) in the basic matrix 2 in which the selection order in the dither matrix 1 is set to 175. Next, a dot is placed at the second position (pixel corresponding to the second position) in the basic matrix 2 in which the selection order in the dither matrix 1 is set to 1, and thereafter each of the dither matrix 1 The processing for all the pixels of the input image data corresponding to the position proceeds, and the dots grow sequentially so that each pixel of the output image data corresponding to each pixel of this input image data is filled.

  The color image processing apparatus 10 of this embodiment includes a dither matrix creation apparatus 100 shown in FIG. 11 in order to create the dither matrix 1 as described above. The dither matrix creation device 100 may be provided in the gradation reproduction processing unit 19, for example. As shown in FIG. 11, the dither matrix creation apparatus 100 includes a super matrix creation unit (second matrix creation unit) 101, a central axis setting unit 102, a basic matrix number calculation unit (first matrix number calculation unit) 103, blue noise A mask application unit 104 and a selection order setting unit 105 are provided.

  The details of the operation of creating the dither matrix 1 by the dither matrix creating apparatus 100 are as described above. The creating operation is shown in the flowchart in FIG. The creation operation of the dither matrix 1 is summarized as follows according to this flowchart.

  First, the super matrix creation unit 101 prepares a basic matrix 2 (first matrix) in which the dot output order is determined for each pixel corresponding position that is a position corresponding to each pixel of the input image data. Next, the super matrix creation unit 101 creates a super matrix 6 (second matrix) in which a plurality of the basic matrices 2 are arranged (S21: super matrix creation step (second matrix creation step)).

  Next, the central axis setting unit 102 obtains the screen angle of the super matrix 6 and sets one central axis 4 in the arrangement direction of the basic matrix 2 in one direction according to the screen angle (S22: central axis setting step) ).

  Next, the basic matrix number calculation unit 103 sets, for each column of the basic matrix 2 in the direction intersecting with the central axis 4, the intersection axis 5 with respect to the central axis 4 in these column directions. Next, the number of basic matrices 2 existing on these crossing axes 5 is calculated as the first calculation result for each crossing axis 5 on each side on both sides of the central axis 4. Further, the total number of basic matrices 2 in the super matrix 6 is calculated as a second calculation result (S23: basic matrix number calculating step (first matrix number calculating step)).

  Next, the blue noise mask application unit 104 matches the central axis 4 of the super matrix 6 with the vertical direction of the blue noise mask and matches the intersecting axis 5 of the super matrix 6 with the horizontal direction of the blue noise mask. From the first calculation result of the number calculation unit 103, each blue noise data of the blue noise mask corresponding to each basic matrix 2 of the super matrix 6 is obtained (S24: blue noise mask application step).

  Next, the selection order setting unit 105 sets the output order of dots set in each blue noise data as the selection order of each basic matrix 2 in the super matrix 6. Next, the selection order setting unit 105 rearranges the selection order so that the total value of the basic matrix 2 indicated by the second calculation result of the basic matrix number calculation unit 103 becomes the final order value of the selection order. (S25: Selection order setting step).

  As described above, in the method of creating the dither matrix 1 of the present embodiment, first, as shown in FIG. 5, the basic matrix 2 in which the dot growth order is set at the pixel corresponding position is designed in a distorted shape. Next, as shown in FIGS. 6 and 4, the basic matrix 2 is arranged side by side to create a super matrix 6 having a screen angle, and then each basic matrix 2 of the super matrix 6 is assigned to each basic matrix 2. A selection order indicating a processing order (dot growth order) between the matrices 2 is set.

  In actual halftone processing using such a dither matrix 1, the matrix data of the dither matrix 1 is expanded into a rectangular size corresponding to, for example, 70 × 70 pixels, and the matrix data is a threshold value for halftone processing. Set as data. Thereby, the data corresponding to each pixel in the dither matrix 1 functions as dither matrix data for binary level output, that is, as a threshold value.

  Specifically, the dither matrix data (threshold value) is stored in the threshold output value storage unit 52 of the tone reproduction processing unit 19 shown in FIG. In the dither processing, the threshold value processing unit 51 compares the pixel value of the input image data with the threshold value, and a predetermined output value (pixel value) is output according to the comparison result.

  As described above, when the super matrix 6 is applied to the dither matrix 1 (dither screen) for binary output, a fixed central axis 4 is set in the super matrix 6 and the central axis 4 is arranged as blue noise data. The blue noise data in the range that matches the direction (for example, the vertical arrangement direction of the blue noise data) and matches the shape of the super matrix 6 is selected and used as the super matrix 6 data. Thereby, also in the distorted dither matrix 1 (super matrix 6) in which the screen angle is set, the dot growth order (selection order) of the basic matrix 2 becomes close to the blue noise growth order. As a result, the output image data dithered by the dither matrix 1 has particularly good reproducibility in the highlight portion.

  In the present embodiment, the size of the dither matrix 1 (super matrix 6) is a size corresponding to, for example, 70 × 70 pixels, and the size of the blue noise mask 3 is the size of the dither matrix 1 (super matrix 6). For example, the size corresponds to 128 × 128 pixels. Therefore, when determining the blue noise data of the blue noise mask 3 corresponding to the super matrix 6, the range of the blue noise data corresponding to the super matrix 6 is moved by moving the range in the blue noise mask 3. It can be easily changed.

  The operation of changing the range of the blue noise data corresponding to the super matrix 6 can be performed by an input operation from the operation panel 41, for example. In this case, the blue noise mask application unit 104 moves the range of the blue noise data corresponding to the super matrix 6 based on, for example, the change input of the range from the operation panel 41.

  For example, when the selection order of the basic matrix 2 is determined by associating the blue matrix data with the super matrix 6 and the dither matrix 1 is created, the dither processing is performed by the dither matrix 1 and as a result, dots are biased in the output image data. There is a case. In such a case, it is effective to change the range of the blue noise data corresponding to the super matrix 6. The process in this case is shown in FIG. FIG. 13 is an explanatory diagram of the process of moving the blue noise data range corresponding to the super matrix 6 on the blue noise mask 3.

  In FIG. 13, the initially set range of the blue noise data corresponding to the super matrix 6 is indicated by a solid thick line, and the range after the movement is indicated by a two-dot chain thick line. The moved range is shifted by 20 pixels in the X direction and 13 pixels in the Y direction with respect to the initially set range. In this way, by appropriately moving the range of the blue noise data corresponding to the super matrix 6, the selection order of the basic matrix 2 can be changed while maintaining the characteristics of the blue noise, and a dither matrix that can output good output image data 1 can be obtained.

  The color image processing apparatus 10 including the gradation reproduction processing unit 19 that performs dither processing using the dither matrix 1 may be realized as software (application program). In this case, a printer driver incorporating software for realizing dither processing can be provided in the computer.

  As illustrated in FIG. 14, a printer driver 201, a communication port driver 202, and a communication port (for example, RS232C / LAN) 203 are incorporated in the computer 200. The printer driver 201 includes a color correction unit 15, a black generation and under color removal unit 16, a gradation reproduction processing unit 19, and a printer language translation unit 31.

  The computer 200 is connected to a printer (image output device) 204. The printer 204 performs printing based on the image data output from the computer 200. The printer 204 may be a digital multifunction machine having a copy function and a fax function in addition to the printer function.

  For example, image data generated by executing various application programs in the computer 200 is processed in the color correction unit 15, the black generation and under color removal unit 16, and the gradation reproduction processing unit 19 as described above. The image data that has undergone the processing of these units is converted into a printer language by the printer language translation unit 31, input to the printer 204 via the communication port driver 202 and the communication port 203, and printed on paper.

  Each block of the color image processing apparatus 10, the printer driver 201, and the dither matrix creation apparatus 100 shown in the embodiment of the present invention may be configured by hardware logic, or software using a CPU as follows. It may be realized by.

  That is, each device includes a CPU (central processing unit) that executes instructions of a control program that realizes each function, a ROM (read only memory) that stores the program, a RAM (random access memory) that expands the program, A storage device (recording medium) such as a memory for storing the program and various data is provided. An object of the present invention is to provide a recording medium in which a program code (execution format program, intermediate code program, source program) of a control program for each of the above devices, which is software that realizes the above-described functions, is recorded in a computer-readable manner This can also be achieved by supplying each of the above devices and reading 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.

  Further, each of the above devices 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 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.

1 is a block diagram illustrating a schematic configuration of a digital color copying machine including a color 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 shown in FIG. 3 is a flowchart illustrating an operation of a gradation reproduction processing unit illustrated in FIG. 2. It is explanatory drawing which shows an example of the dither matrix which the threshold value processing part shown in FIG. 2 uses in a binary dither process. FIG. 5 is an explanatory diagram illustrating an example of a basic matrix constituting the dither matrix illustrated in FIG. 4. It is explanatory drawing which shows the example which has arrange | positioned another basic matrix adjacent to the basic matrix shown in FIG. FIG. 6 is an explanatory diagram showing a plurality of central axes that are generated when two other basic matrices are arranged in different directions with respect to one basic matrix shown in FIG. 5. FIG. 5 is an explanatory diagram showing an example of a blue noise mask used when setting the selection order of each basic matrix in the dither matrix shown in FIG. 4. FIG. 5 is an explanatory diagram showing a super matrix for creating the dither matrix shown in FIG. 4. FIG. 10 is an explanatory diagram showing processing when the blue noise mask shown in FIG. 8 is associated with the super matrix shown in FIG. 9. FIG. 5 is a block diagram showing a configuration of a dither matrix creation device that creates the dither matrix shown in FIG. 4. 12 is a flowchart showing a dither matrix creation operation by the dither matrix creation apparatus shown in FIG. FIG. 12 is an explanatory diagram showing a process of changing a range of blue noise data corresponding to a super matrix in a dither matrix creation operation by the dither matrix creation apparatus shown in FIG. 11. 1 is a block diagram showing a configuration when a printer driver to which a color image processing apparatus according to an embodiment of the present invention is applied is realized by a computer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Color image processing apparatus 19 Gradation reproduction process part 20 Color image input apparatus 30 Color image output apparatus 51 Threshold processing part 52 Threshold output value storage part 100 Dither matrix creation apparatus 101 Super matrix creation part (2nd matrix creation part)
102 Central axis setting unit 103 Basic matrix number calculation unit (first matrix number calculation unit)
104 Blue Noise Mask Application Unit 105 Selection Order Setting Unit 200 Computer 201 Printer Driver

Claims (8)

In a method of creating a dither matrix for performing binary dither processing as gradation reproduction processing on input image data,
A first matrix in which a dot output order is determined for each pixel corresponding position that is a position corresponding to each pixel of the input image data is used to create a second matrix in which a plurality of the first matrices are arranged. 2 matrix creation process;
A central axis setting step of setting one central axis in a direction in which the plurality of first matrices are arranged in one direction according to the screen angle of the second matrix;
For each column of the first matrix in a direction crossing the central axis, an axis of intersection with respect to the central axis is set in the column direction, and the number of the first matrix existing on the crossing axis is set to each of the intersections. A first matrix number calculating step of obtaining a first calculation result by calculating for each side of both sides of the central axis with respect to the axis, and obtaining a total number of the first matrix in the second matrix as a second calculation result When,
The central axis of the second matrix is made to coincide with the vertical direction of the blue noise mask, and the intersecting axis of the second matrix is made to coincide with the horizontal direction of the blue noise mask, so that the first matrix number calculating step A blue noise mask applying step of determining a blue noise mask range corresponding to the second matrix from the calculation result of 1, and obtaining blue noise data corresponding to each first matrix of the second matrix;
The output order of the dots set in each blue noise data is set as the selection order of each first matrix, and the total value of the first matrix indicated by the second calculation result of the first matrix number calculation step And a selection order setting step of rearranging the selection order so that the value becomes the final order value of the selection order.   2. The blue noise mask application step can move a range of each blue noise data of the blue noise mask corresponding to each first matrix of the second matrix determined in advance. To create a dither matrix.   3. A gradation reproduction processing unit that uses the dither matrix created by the dither matrix creation method according to claim 1 or 2 and performs halftone processing by binary dither processing on image data. An image processing apparatus.   An image forming apparatus comprising: the image processing apparatus according to claim 3; and an image output apparatus that performs printing based on the image data processed by the gradation reproduction processing unit. In a dither matrix creating apparatus for creating a dither matrix for performing binary dither processing as tone reproduction processing on input image data,
A first matrix in which a dot output order is determined for each pixel corresponding position that is a position corresponding to each pixel of the input image data is used to create a second matrix in which a plurality of the first matrices are arranged. 2 matrix creation part;
A central axis setting unit that sets one central axis in a direction in which the first matrix is aligned in one direction according to the screen angle of the second matrix;
For each column of the first matrix in a direction crossing the central axis, an axis of intersection with respect to the central axis is set in the column direction, and the number of the first matrix existing on the crossing axis is set to each of the intersections. A first matrix number calculation unit that obtains a first calculation result by calculating for each side on both sides of the central axis with respect to the axis, and that obtains a total number of the first matrix in the second matrix as a second calculation result When,
The central axis of the second matrix is made to coincide with a vertical direction of a blue noise mask, and the intersecting axis of the second matrix is made to coincide with a horizontal direction of the blue noise mask. A blue noise mask applying unit that determines a blue noise mask range corresponding to the second matrix from the calculation result of 1, and obtains blue noise data corresponding to each first matrix of the second matrix;
The output order of the dots set in each blue noise data is set as the selection order of each first matrix, and the total value of the first matrix indicated by the second calculation result by the first matrix number calculation unit And a selection order setting unit for rearranging the selection order so that the value becomes the final order value of the selection order.   The blue noise mask application unit moves the range of each blue noise data of the blue noise mask corresponding to each first matrix of the second matrix determined in advance based on an input instruction. The dither matrix generating apparatus according to claim 5.   A program for causing a computer to function as each unit of the dither matrix creating apparatus according to claim 5.   A computer-readable recording medium on which the program according to claim 7 is recorded.

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