JP2011217417A - Multi-value dither processing method, image processing apparatus, image forming apparatus, composite machine, multi-value dither processing program and computer-readable recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve multi-value dither processing which can properly reproduce gradation of an input image by allowing the natural growth of dots.SOLUTION: A color image processing apparatus 10 includes: a storage part 192 for storing a group of thresholds each associated with each of pixels of a dither matrix; and a quantizing part 191 for reading the group of thresholds stored in the storage part 192, quantizing the density value of an input image on the basis of results of comparison between the density value of pixels in the input image and a group of thresholds of pixels in the dither matrix and creating a dither image. The group of thresholds stored in the storage part 192 is set to values at which the density values of other adjacent output pixels begin to increase before the density value of a certain output pixel forming the dither pattern reaches the upper limit value along with an increase in the density value of the input image when the density value of the input image having uniform density is sequentially increased.

Description

  The present invention relates to an image processing method and an image processing apparatus for performing halftone processing, particularly multi-value dither processing, on an input image, and an image forming apparatus, a multifunction peripheral, a program, and a recording medium including the same. .

  Conventionally, as a method for outputting an image to a printing medium such as paper, various recording methods such as thermal transfer, electrophotography, or an ink jet method have been adopted. In these recording methods, so-called halftone processing, in which a dot pattern corresponding to the gradation of the input image is generated and formed on the print medium, is used frequently in order to artificially improve the gradation reproducibility of the output device. ing.

  In dither processing, which is one type of halftone processing, a dither matrix in which pixels are arranged in an n × m matrix and a threshold value is associated with each pixel is used to dither a multi-valued image input in units of pixels. A dither image is obtained by comparing the threshold value of the matrix in units of one pixel and quantizing and converting the gradation of the multi-value image to binary or multi-value.

  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. Here, the dot growth shape includes various shapes such as a circle, an ellipse, a line (linear), a square, a cross, and a rhombus. However, in the conventional dither processing method, when the dots grow, there is a problem that the dot shape collapses as the dot area expands, the density change becomes unstable, and the dithered image feels rough. .

  In order to solve this problem, Patent Literature 1 discloses a technique for changing the growth mode of dots in a dither pattern by switching a dither matrix according to a reproduction tone range.

JP 2001-257879 A (published on September 21, 2001)

  However, the conventional dither processing method has a point where the dot shape (contour) in the dither pattern changes suddenly with the change in the gradation level of the input image. The problem is that the texture of the dithered image changes. In particular, in the technique described in Patent Document 1, since the dot shape changes greatly according to the gradation level of the input image, this problem becomes significant.

  The present invention has been made in view of the above problems, and an object of the present invention is to realize multi-value dither processing that can suitably reproduce the gradation of an input image by natural dot growth.

  In order to solve the above problem, a multi-value dither processing method according to the present invention is a multi-value dither processing method for reproducing the gradation of a multi-tone input image by a multi-tone dither image, When the gradation value of the input image having a uniform gradation is sequentially increased, the gradation value of the first pixel constituting the dither pattern which is a partial image corresponding to the dither matrix in the dither image. And the gradation value of the fifth pixel constituting the dither pattern alternately increase, and the gradation value of the first pixel and the fifth pixel increases the gradation value of the input image. Accordingly, before reaching the upper limit value, the dither image is adjusted so that the gradation value of the second pixel constituting the dither pattern and the gradation value of the sixth pixel constituting the dither pattern start to increase. It is characterized in that the gradation value of each pixel constituting is determined.

  In order to solve the above-described problem, an image processing apparatus according to the present invention is an image processing apparatus that performs multi-value dither processing that reproduces the gradation of a multi-tone input image using a multi-tone dither image. Storage means for storing a threshold value group associated with each pixel of the dither matrix, and reading out the threshold value group stored in the storage means, and the gradation value of the pixel in the input image and the dither matrix corresponding to the pixel Gradation determination means for determining the gradation value of each pixel constituting the dither image based on the comparison result with the threshold value group of the middle pixel, and the threshold value group stored in the storage means is uniform When the gradation value of the input image having a certain gradation is sequentially increased, the first pixel constituting the dither pattern, which is a partial image obtained by applying a single dither matrix, among the dither images. Tone value and above The gradation values of the fifth pixel constituting the pattern alternately increase, and the gradation values of the first pixel and the fifth pixel increase with the increase of the gradation value of the input image. Before reaching the upper limit value, the gradation value of the second pixel constituting the dither pattern and the gradation value of the sixth pixel constituting the dither pattern are set to values that start to increase. It is characterized by.

  In the image processing apparatus or multi-value dither processing method according to the present invention, the first pixel may be a pixel closest to the center of the dither pattern.

  In the image processing apparatus or the multi-value dither processing method according to the present invention, a plurality of pixels that sequentially increase the gradation value when the gradation value of the input image having a uniform gradation is sequentially increased. If the group consisting of is a gradation increase group, two pixels belong to the gradation increase group first, and when the gradation values of the two pixels belonging to the gradation increase group reach a predetermined value, Preferably, two pixels belonging to the gradation increasing group are added, and when the gradation values of the two added pixels reach a predetermined value, the process of adding two more pixels is repeated.

  In the image processing apparatus according to the present invention, the maximum threshold value among the threshold value groups associated with the pixels in the dither matrix corresponding to the first pixel is the dither matrix corresponding to the second pixel. It is preferable that the threshold value group is larger than the minimum threshold value among the threshold value groups associated with the middle pixel.

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

  A multifunction machine according to the present invention includes the above-described image processing apparatus, an image output apparatus that forms a dither image reproduced by the image processing apparatus on a print medium, and a document reading apparatus that reads a document to obtain a document image. And the image processing apparatus reproduces the original image as a dither image by using the original image obtained by the original reading device as the input image.

  By the way, the image processing apparatus may be realized by hardware or may be realized by causing a computer to execute a program. Specifically, the multi-value dither processing program according to the present invention is a program for operating a computer as each unit of the image processing apparatus, and the multi-value dither processing program is recorded on the recording medium according to the present invention. ing.

  According to the configuration of the present invention, when the gradation values of the input image are sequentially increased, the gradation values of the first pixel and the fifth pixel are alternately increased first in the obtained dither pattern. Then, before the gradation values of the first pixel and the fifth pixel reach the upper limit value, the gradation values of the second pixel and the sixth pixel start to increase. Then, after the gradation value of the second pixel starts to increase, the gradation values of the first, second, fifth and sixth pixels increase. As described above, by increasing and expanding the dot density in the dither pattern in units of two pixels, there is an effect that the dot reproduction can be stabilized in the highlight region.

FIG. 4 is a diagram illustrating an embodiment of the present invention, and illustrating values of a threshold value group associated with each pixel of a dither matrix. 1 is a functional block diagram illustrating a configuration of a digital color multi-function peripheral 1 according to an embodiment of the present invention. FIG. 4 is a diagram illustrating a configuration of a dither matrix according to an embodiment of the present invention. FIG. 4 is a diagram illustrating an application method of a dither matrix according to an embodiment of the present invention. FIG. 3 is a functional block diagram illustrating a detailed configuration of a gradation reproduction processing unit in FIG. 2 according to an embodiment of the present invention. FIG. 6 is a flowchart illustrating a flow of multi-value dither processing according to an embodiment of the present invention. FIG. 5 is a diagram illustrating a dot growth pattern accompanying an increase in density of an input image according to an embodiment of the present invention. FIG. 6 is a flowchart illustrating a procedure for designing a threshold value associated with each pixel of a dither matrix according to an embodiment of the present invention. FIG. 5 is a diagram illustrating a density level set for each group according to an embodiment of the present invention. FIG. 3 is a diagram illustrating an embodiment of the present invention and is a three-dimensional representation of dot growth in a dither pattern. FIG. 7 is a diagram representing a dither pattern in a three-dimensional manner when increasing the density value of pixels up to a fifth group according to an embodiment of the present invention. It is a graph which shows the total output value by which the gamma correction was carried out. It is a graph which shows an example of the color difference data of the gamma table obtained by measuring the color of the output test patch image. It is a figure which shows an example of an output gradation correction curve. It is a figure which shows another embodiment of this invention, and shows the value of the threshold value group matched with each pixel of a dither matrix. FIG. 10 is a diagram illustrating another embodiment of the present invention, and is a diagram illustrating a dot growth pattern accompanying an increase in density of an input image. It is a figure which shows another embodiment of this invention, and shows the density | concentration level set for every group. FIG. 6 is a diagram illustrating another embodiment of the present invention and three-dimensionally representing dot growth in a dither pattern. FIG. 14 is a diagram illustrating another embodiment of the present invention and three-dimensionally representing a dither pattern when increasing the density value of pixels up to a fifth group. It is a figure showing the modification of the present invention and expressing the growth of the dot in the dither pattern three-dimensionally. It is the figure which expressed the density of each pixel when increasing the density value of the pixel to the 3rd group in three dimensions. It is a figure which shows the pattern of a dot concentration type dither matrix. FIG. 24 is a view showing still another embodiment of the present invention and three-dimensionally expressing dot growth in a dither pattern. FIG. 14 is a view showing still another embodiment of the present invention and a three-dimensional representation of a dither pattern. FIG. 24 is a diagram illustrating still another embodiment of the present invention and showing values of a threshold group associated with each pixel of a dither matrix. FIG. 24 is a view showing still another embodiment of the present invention and showing a dot growth pattern accompanying an increase in density of an input image. FIG. 24, showing another embodiment of the present invention, is a functional block diagram illustrating a configuration of a computer system.

[Embodiment 1]
(1. Outline configuration)
The first embodiment of the present invention will be described with reference to FIGS. 1 to 14 as follows. FIG. 2 is a block diagram illustrating a schematic configuration of a digital color multifunction peripheral (image forming apparatus) 1 including a color image processing apparatus 10 according to an embodiment of the present invention.

  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 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.

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

  The analog signal read by the color image input device 20 passes through the color image processing device 10 in 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 process in this order, and output to the color image output device 30 as a CMYK digital color signal. The

  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, the imaging system, and the imaging system of the image input apparatus 20 on the RGB digital signals sent from the A / D conversion unit 11. It is.

  The input tone correction unit 13 adjusts the color balance of the RGB signal (RGB reflectance signal) from which various distortions have been removed by the shading correction unit 12 and is employed in a color image processing apparatus such as a density signal. The signal is converted into a signal that can be easily handled 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, a correspondence relationship between the input RGB signal and the output CMY signal is stored as a lookup table (LUT), and a method using this LUT, a color masking method using a transformation matrix as shown in Equation 1 below, etc. There is.

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 certain CMY data is given to the image output apparatus. Eclairage: International Lighting Commission, L ^* : Lightness, a ^* , b ^* : Chromaticity))) The same color patch with L ^* a ^* b ^* as the RGB data when the scanner reads it, and gives it to the image output device A large number of CMY data sets are prepared, and the coefficients of the transformation matrix from a11 to a33 in the above equation 1 are calculated from these combinations. Then, the color correction unit 15 performs color correction processing using a conversion matrix to which these coefficients are applied. 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, and high frequency components Is emphasized. The region separated as the character edge region is also subjected to multi-value processing on a high-resolution screen suitable for high-frequency component reproduction by the gradation reproduction processing unit 19.

  On the other hand, the region separated as the halftone dot region by the region separation processing unit 14 is subjected to a low-pass filter process for removing the input halftone component by the spatial filter processing unit 17, and the tone reproduction processing unit 19 Then, multi-value processing is performed by a screen that emphasizes gradation. In addition, 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 processing using a screen that emphasizes gradation reproducibility.

  In the digital color multifunction peripheral 1 of the present embodiment, a multi-gradation input image is reproduced with a gradation (gradation that can be expressed by the image output device 30) smaller than the gradation of the input image. The tone reproduction processing unit 19 performs multi-value dither processing. The present invention is characterized by multi-value dither matrix data used when the gradation reproduction processing unit 19 performs multi-value dither processing. The multi-value dither process will be described later.

  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 forms an image corresponding to input image data on a print medium (for example, paper). Examples of the method of forming an image by the color image output device 30 include an electrophotographic method and an inkjet method, but are not particularly limited. The above processing is controlled by a CPU (Central Processing Unit) (not shown).

(2. Multi-level dither processing)
Hereinafter, the multi-value dither process 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 (an image before dithering) 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 to be 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 above-described input image is represented by each color component. After being divided into bit planes, multi-level 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 diagram showing the configuration of the dither matrix used in this embodiment. In the figure, one square corresponds to one pixel, and the number given to each square represents the dot enlargement order in the dither pattern generated corresponding to the dither matrix. In this embodiment, a dither matrix consisting of 16 pixels is used as shown in FIG. Here, a simple shape is adopted as an example, but the shape of the dither matrix is not limited to this shape. By disposing these dither matrices on the input image, it is possible to perform dither processing with a screen angle as a whole.

  When performing multi-value dither processing for converting the gradation of an input image to 16 gradations as in the present embodiment, 15 threshold values are associated with each pixel in the dither matrix. FIG. 1 is a diagram illustrating an example of a threshold value associated with each pixel. These threshold values are set within a range of values (0 to 255) that the density values of the input image can take, and the range of values (0 to 255) that the density values of the input image can take are set by these 15 threshold values. , It is divided into 16 sections which are the same as 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.

  In the multi-value dither processing, a dither matrix is superimposed on an input image, and the density value (0 to 255) of a pixel of the input image is based on a threshold group associated with a pixel in the dither matrix that overlaps the pixel. Quantize to an integer value between 0 and 15. When the dither matrix is configured as shown in FIG. 3, as shown in FIG. 4, this quantization process is repeated along the main scanning direction indicated by the broken line while superimposing the dither matrix on the input image.

  The details of the quantization process are as follows. The density value of the pixel of the input image (hereinafter referred to as “input pixel”) is compared with the threshold value group associated with the pixel in the dither matrix corresponding to (overlapping) the input pixel, and the density value of the input pixel Is included in any of the 16 sections described above. Then, the output density value associated with the identified section is set as the density value in the dither image.

  The gradation reproduction processing unit 19 includes a dither processing unit 190 as a functional block for performing the above multi-value dither processing. FIG. 5 is a functional block diagram showing the configuration of the dither processing unit 190. As shown in FIG. 5, the dither processing unit 190 includes a quantization unit 191 and a storage unit 192. The quantization unit 191 performs the above-described quantization process. On the other hand, the storage unit 192 stores the above-described threshold value and output density value data as multi-value dither matrix data.

  In the following, the threshold value of the j th (j = 0, 1, 2,..., 14) associated with the pixel i (i = 1, 2, 3,..., 16) in the dither matrix is set to Th [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 k-th section in the pixel i is the section [i] [k] (k = 0, 1, 2,..., 15), the section [i] [k] is as follows.
Section [i] [0] ... 0 or more and Th [i] [1] or less Section [i] [k] (k = 1, 2,..., 14) ... Th larger than Th [i] [k-1] [I] [k] or less interval [i] [15]... Greater than Th [i] [14] and 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 storage unit 192 stores Th [i] [j] as threshold value data and D [k] as output density value data.

  FIG. 6 is a flowchart showing the flow of multilevel dither processing by the quantization unit 191. First, the quantization unit 191 selects one pixel in the input image, specifies which pixel in the dither matrix the selected input pixel corresponds to, and 15 pixels associated with the specified pixel. Is read from the storage unit 192 (S1).

  Next, the quantization unit 191 compares the density value of the input pixel with the threshold value group read out in step S1, and specifies in which section the density value of the input pixel is included by the threshold value ( S2). Then, the quantization unit 191 outputs the output density value associated with the identified section as the density value of the dither image (the density value of the output pixel) (S3).

  Steps S2 and S3 can be expressed as follows. That is, the coordinate in the main scanning direction in the input image is x, the coordinate in the sub-scanning direction is y, and the density value of the input pixel at the coordinates (x, y) is In [y] [x]. Then, if the density value of the output pixel corresponding to the input pixel at the coordinates (x, y) is Out [y] [x], Out [y] [x] can be determined as follows.

That is,
If (In [y] [x] ≤ Th [i] [0]) If 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]) Out [y] [x] = D [2] = 2
・
・
・
else if (In [y] [x] ≤ Th [i] [14]) 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 input pixel corresponds to the pixel “1” in the dither matrix, the quantization unit 191 reads from the storage unit 192 from Th [1] [0] to Th [1] [14] shown in FIG. . For example, when the density value of the input pixel is 14, the quantization unit 191
Th [1] [12] ≦ 14 <Th [1] [13]
Therefore, D [13] = 13 is output as the density value of the output pixel.

  After step S3, the quantization unit 191 determines whether or not the quantization process has been completed for all the pixels of the input image (S4). Select a pixel and return to step S1. On the other hand, when the quantization process is completed for all pixels, the multi-value dither process is terminated.

  In this embodiment, the output density value D [k] (k = 0, 1, 2, 3,..., 15) is a 4-bit integer value from 0 to 15, but the output density value D [K] may be 16 values selected from 8-bit integer values from 0 to 255 based on the characteristics of the color image output device 30.

  Next, the dither pattern obtained by setting the threshold value group associated with each pixel of the dither matrix to the values shown in FIG. 1 will be described. When an input image having the same size and shape as the dither matrix and a uniform density is considered, when the density of the input image is increased successively, the dither pattern (image after dither processing) changes as shown in FIG. To do. In FIG. 7, the numbers given to the respective pixels indicate the density values of the pixels in the dither pattern. Hereinafter, an output pixel in the dither pattern corresponding to the pixel “i” in the dither matrix is referred to as an output pixel “i”.

  In this embodiment, since the pixel having the smallest threshold is the pixel “1” closest to the center of the dither matrix, when the density value of the input image is sequentially increased from 0, in the dither pattern, As shown in FIG. 7, the density value of the output pixel “1” corresponding to the pixel “1” of the dither matrix first increases. Accordingly, dot growth starts from the vicinity of the center of the dither pattern.

  When the density value of the input image is further increased, as shown in FIG. 7, the density value of the output pixel “1” reaches the upper limit value (= 15) as the density value of the input image increases. Later, the density value of the output pixel “2” corresponding to the pixel “2” of the dither matrix starts to increase. Then, before the density value of the output pixel “2” reaches the upper limit as the density value of the input image increases, the density value of the output pixel “3” corresponding to the pixel “3” of the dither matrix starts to increase. As a result, the density values of the output pixel “2” and the output pixel “3” increase. Specifically, after the density value of the output pixel “2” becomes 3, the density value of the output pixel “3” increases from 0 to 1, and then the output pixel “2” and the output pixel “3”. Concentration values increase alternately. Similarly, before the density value of the output pixel “3” reaches the upper limit value as the density value of the input image increases, the density value of the output pixel “4” corresponding to the pixel “4” of the dither matrix increases. Start. The same applies to output pixels corresponding to the pixel “4” and subsequent pixels in the dither matrix.

  In the conventional multi-value dither processing, the density value of the next pixel starts to increase after the density value of a certain pixel in the dither pattern reaches the upper limit. Therefore, after the density value of a certain pixel reaches the upper limit, There was a possibility that the shape (contour) would change abruptly. On the other hand, in the present embodiment, except for the output pixel “1”, the density value of the surrounding output pixels starts to increase before the density value of the output pixel reaches the upper limit. Concentration rises simultaneously. Therefore, in the dither pattern, there is an advantage that the dot density rises and expands simultaneously, and the dots grow smoothly.

  In order to grow the dots of the dither pattern as described above, the maximum threshold value (= 76) in the threshold value group associated with the pixel “2” in the dither matrix is associated with the pixel “3”. It is larger than the minimum threshold value (= 20) in the threshold value group. Furthermore, the maximum threshold value among the threshold value groups associated with the pixel “3” in the dither matrix is larger than the minimum threshold value among the threshold value groups associated with the pixel “4”. The same applies to the threshold values associated with the pixel “4” and thereafter. On the other hand, the maximum threshold value (= 16) in the threshold value group associated with the pixel “1” is smaller than the minimum threshold value (= 17) in the threshold value group associated with the pixel “2”. ing.

  In this embodiment, since the threshold value group associated with the pixel “1” in the dither matrix is set to a small value, when the density of the input image is increased, the vicinity of the center portion in the dither pattern Only the output pixel “1” of the output pixel gradually becomes darker in the highlight area first, and after the output pixel “1” reaches the high density level, the pixels after the output pixel “2” located in the periphery are in turn. A dither pattern that is darker is generated. Therefore, even in an image output device in which dot reproducibility in the highlight region is unstable, the input image can be reproduced well.

  In this embodiment, the output pixel “1” in the dither pattern corresponds to the third pixel described in the claims, and the output pixel “2” corresponds to the first pixel described in the claims. Correspondingly, the output pixel “3” corresponds to the second pixel recited in the claims.

(3. Threshold design method)
Hereinafter, a method for designing the threshold shown in FIG. 1 will be described. In binary dither processing, it is only necessary to set the dot enlargement order in the dither pattern, but in multi-value dither processing, not only the dot enlargement order but also what density level the dots become darker is set. There is a need to. Here, for a dither matrix having a certain size and shape, in addition to the dot enlargement order in the plane, the density level such as the percentage of dots to be darkened is set in the height direction of the dither matrix. By doing so, it is possible to effectively visualize the dot growth method at the multi-value level.

  FIG. 8 is a flowchart showing the flow of the threshold design process. As shown in FIG. 8, first, the pixel size and shape of the dither matrix are determined (S51), and then in what order the pixels are darkened for each pixel in the dither pattern corresponding to the dither matrix. Determine (S53). In the present embodiment, as described above, the dither matrix having the pixel size and shape shown in FIG. 3 is set, and in this dither matrix, the dots are enlarged in the order of the numbers given in the drawing.

Next, a pixel group is created (S55). Specifically, first, a pixel group including a pixel (here, pixel “1”) that forms a dot first in the dither matrix is defined as a first group. Next, a pixel group including the pixel “1” and the pixel “2” is defined as a second group. A pixel group including the pixels “1” to “3” is defined as a third group. In this way, a pixel group including all the pixels included in the previous pixel group and including the pixel to be grown next is sequentially created. In this embodiment, since the dither matrix consists of 16 pixels, up to the 16th group of pixel groups is created. In this case, the relationship between each group is
First group ⊂ second group ⊂... ⊂ 16th group.

  In this embodiment, when the density value of the input image is sequentially increased, the density value of the pixels is increased in the order of this group. That is, the density value of the pixel “1” included in the first group is first increased, and when the density value of the pixel “1” reaches a predetermined value, the pixels “1” and “2” included in the second group When the density value is increased and the density value of the pixel “2” becomes a predetermined value, the density values of the pixels “1” to “3” included in the third group are further increased.

  In each group, the density value is increased in the order of pixel numbers. For example, in the third group, the density value is increased in the order of the pixel “1”, the pixel “2”, and the pixel “3”.

  Next, the density level of each group is set (S57). The density level set here is an index indicating how much the density value of the pixel in the group is to be increased before the transition to the next group. This density level can be set freely. For example, when the 255 levels are equally divided by 16 groups, 256/16 = 16, so the density level of each group may be 16.

  In the present embodiment, the density level of the first group is set to a value as large as 240 in order to increase the density of the pixels of the first group in an early stage, and the density levels of the other groups are set to 32 equally. However, only the density level of the 16th group is set to 255. This is so that the density of the output pixel corresponding to the pixel “16” included only in the sixteenth group finally reaches the upper limit value. FIG. 9 is a diagram showing the density level set for each group.

  Here, when the density value is taken in the height direction, the density value of each pixel increases as shown in FIG. That is, by stacking the blocks of each pixel, it is possible to know the density value of each pixel as the height of the block. For example, when the density value of the pixel is increased up to group 5, the density value of each pixel is visually shown as shown in FIG. Thus, by expressing the density value of the pixel by the height of the block, the degree of dot growth in the dither pattern can be grasped at a glance.

  Note that the maximum density level in each pixel is 255, and if the density level exceeds 255, it is discarded. For example, in the case of the pixel “1”, the density level reaches 255 which is the upper limit value in the second group, and thereafter, the density value does not increase. On the other hand, in the case of the pixel “2”, the density level starts to increase from the second group, and the density level reaches the upper limit value in the ninth group.

  Then, the density value is distributed to each pixel based on the pixel group created in step S55 and the density level set in step S57, and the dot growth pattern shown in FIG. 7 is created (S59).

  Specifically, the density value is first distributed to the first group of pixels (output pixel “1”), and the density value of the output pixel “1” is set to the density level “240” (density value “1”) of the first group. 14), the density value is distributed to the second group of pixels (output pixel “1” and output pixel “2”). In the second group, the density values are distributed in the order of the output pixel “1” and the output pixel “2”. Then, among the pixels of the second group, when the density value of the output pixel “2” reaches the density level “32” (corresponding to the density value “3”) of the second group, the density of the third group of pixels is increased. Distribute values. At this time, since the density value of the output pixel “1” has already reached the upper limit value, the density value is distributed to the output pixel “2” and the output pixel “3”. By repeating this up to the 16th group, the dot growth pattern shown in FIG. 7 is obtained.

  Next, a total output value is obtained, and a threshold value is calculated for each pixel of the dither matrix using the partition level (S61). In other words, for each pixel in the dither matrix, the threshold value group is set so that the dots of the dither pattern grow according to the growth pattern created in step S59.

An example of this step S61 will be described as follows. First, the total output value is obtained. The total output value can be obtained by multiplying the total number of pixels “16” of the dither matrix by the maximum density value “15” per pixel,
16 × 15 = 240
It becomes.

  FIG. 12 is a graph showing the total output value subjected to gamma correction. In this embodiment, since there are 16 pixel groups, the range of possible values (0 to 240) of the total output value is equally divided into 16 as shown in FIG. Then, based on the result of dividing the total output value into 16 equal parts, the range of possible values (0 to 255) of the density value of the input pixel is also divided into 16. Here, when the density value of the input pixel is within the range of (1) in the figure, the density value of the first group of pixels (output pixel “1”) is increased. On the other hand, when the density value of the input pixel is within the range of (2) in the figure, the density values of the second group of pixels (output pixel “1” and output pixel “2”) are increased. .

  Then, the dither pattern of FIG. 7 and the density of the input pixel at that time are matched so that the division position of the density value of the input pixel shown in FIG. 12 and the group transition position in the growth pattern shown in FIG. Determine the correspondence with the value. Then, a threshold value is calculated for each pixel in the dither matrix so as to match the above correspondence. Thereby, the threshold value group shown in FIG. 1 can be obtained.

  In this embodiment, since the density level of the first group is set to a high value, in the output pixel “1” in the dither pattern, dots are turned on at an early stage, and only this pixel becomes darker earlier. Further, by setting the density level other than the first group to a small value, the dots are expanded to the peripheral pixels after the dots become darker to the middle density depending on the density level to be set. Thus, the dot growth process can be changed in various ways by setting the density level for each group in various ways.

  Even if the output level of halftone data is designed with 4 bits, an output device with poor performance (for example, an output device with poor reproducibility of low density dots, or a high gamma value and poor gradation linearity) In an output device or the like, 16 gradation levels cannot be expressed sufficiently. On the other hand, in an output device with good gradation reproducibility, the growth method is determined when the growth order of the dots in the dither matrix is pixel sequential (a method in which one pixel reaches the upper limit density value and then the next pixel grows). Therefore, the degree of freedom for setting the threshold of 16 gradations is low at the design stage. Here, according to the configuration of the present embodiment, the intermediate gradation level can be set fairly freely, so that a high-quality gradation reproduction image can be obtained according to the gradation expression ability of the output device.

  As a further preferred embodiment, the threshold group obtained in step S61 may be corrected based on the output characteristics of the color image output device 30 (S63). The threshold value correction process will be described in detail as follows.

  First, the threshold value group (hereinafter referred to as the initial threshold value group) obtained in step S61 is actually used to perform multi-value dither processing on the test patch image data with gradation values (density values) from 0 to 255. The test patch image subjected to the multi-value dither processing is output by the color image output device 30. Subsequently, the color of each output test patch image is measured. Then, based on the colorimetric value, the data of the initial threshold value group is corrected so that the gradation of the output test patch image is linear with respect to the gradation of the input test patch image data. Actually, the above correction processing is performed for each of C, M, Y, and K.

  FIG. 13 is a graph showing an example of color difference data of a gamma table obtained by performing a multi-value dither process to actually output a 256-tone test patch image and measuring the output test patch image. Actually, as shown in FIG. 13, the density value of the input image and the color difference (ΔE) do not have a linear relationship. In the case of this example, it can be seen that it is better to lower the output density value than the theoretical gradation value in the intermediate density range. Therefore, inverse transformation is performed using the measurement results of FIG. 13 so that the two relationships are linear.

  FIG. 14 is a diagram showing an example of an output tone correction curve obtained from the result of FIG. As shown in this figure, an output tone correction value corresponding to the density value of the input image is obtained. Then, the data of the initial threshold value group is corrected based on the output gradation correction curve shown in FIG.

As a result, the initial threshold value group associated with the pixel “1” shown in FIG. 1 is corrected, and the corrected threshold values are in ascending order.
1, 2, 3, 4, 6, 8, 11, 12, 13, 15
It becomes. Here, the number of threshold values after correction is ten. Since the threshold data stored in the storage unit is always 15, when some of the corrected threshold values are less than 15, some data are repeatedly set as threshold values. For example, in the above example, the threshold values are set in ascending order,
1, 1, 2, 2, 2, 3, 3, 4, 4, 6, 8, 11, 12, 13, 15
And As a result, the output result does not shift.

Further, the initial threshold value associated with the pixel “2” shown in FIG. 1 is also corrected, and the corrected threshold values are in ascending order.
16, 17, 19, 21, 24, 27, 33, 40, 48, 56, 66, 75, 86, 101
It becomes. Here, since the number of corrected threshold values associated with the pixel “2” in the dither matrix is 14, some data is repeatedly set as the threshold value as in the case of the pixel “1”. In particular,
16, 17, 17, 19, 21, 24, 27, 33, 40, 48, 56, 66, 75, 86, 101
And

  Thus, by correcting the threshold value group obtained in step S59 based on the output characteristics of the color image output device 30, a suitable dither pattern according to the characteristics of the color image output device 30 can be obtained.

  In the present embodiment, when the density value of the input image is increased, the density value of the output pixel “2” increases after the density value of the output pixel “1” reaches the upper limit. However, the present invention is not limited to this, and the threshold value may be set so that the density value of the output pixel “2” increases before the density value of the output pixel “1” reaches the upper limit. . In order to obtain such a threshold group, the density level of the first group may be set to 200, for example, in step S57.

  Each block of the color image processing apparatus 10, in particular, the dither processing unit 190 of the gradation reproduction processing unit 19 may be configured by hardware logic, or may be realized by software using a CPU as follows. 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. Further, 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.

[Embodiment 2]
A second embodiment of the present invention will be described with reference to FIGS. 15 to 19 as follows. In addition, description is abbreviate | omitted about the same structure as Embodiment 1 mentioned above.

  In the first embodiment described above, when the density value of the input image is sequentially increased from 0, the density value of the surrounding pixels after the density value of the output pixel “1” near the center of the dither pattern reaches the upper limit value. In this embodiment, the density value of the next output pixel starts increasing when the density value of the output pixel “1” becomes a predetermined value smaller than the upper limit value. ing.

  FIG. 15 is a diagram illustrating threshold data stored in the storage unit 192 of the color image processing apparatus 10 according to the present embodiment. In the present embodiment, the dither pattern changes as shown in FIG. 16 when the density of the input image is sequentially increased.

  In the present embodiment, since the pixel having the smallest threshold is the pixel “1” closest to the center of the dither matrix as in the first embodiment, the density value of the input image is sequentially increased from 0. In this case, in the dither pattern, as shown in FIG. 16, the density value of the output pixel “1” corresponding to the pixel “1” of the dither matrix first increases. Accordingly, dot growth starts from the vicinity of the center of the dither pattern.

  When the density value of the input image is further increased, the density value of the first output pixel corresponds to the pixel “2” of the dither matrix before reaching the upper limit value as the density value of the input image increases. The increase of the density value of the output pixel “2” to be started is started. Specifically, as shown in FIG. 16, after the density value of the output pixel “1” becomes 3, the density value of the output pixel “2” increases from 0 to 1. Then, the density values of the output pixel “1” and the output pixel “2” alternately increase. Thereafter, before the density value of the output pixel “2” reaches the upper limit value as the density value of the input image increases, the density value of the output pixel “3” corresponding to the pixel “3” of the dither matrix starts to increase. To do. The same applies to the output pixels corresponding to the pixels “3” and after in the dither matrix.

  In the conventional multi-value dither processing, the density value of the next pixel starts to increase after the density value of a certain pixel in the dither pattern reaches the upper limit. Therefore, after the density value of a certain pixel reaches the upper limit, There was a risk that the shape would change rapidly. On the other hand, in this embodiment, before the density value of a certain output pixel reaches the upper limit, the density values of the peripheral output pixels begin to increase, so that the density of the central portion and the surrounding area increase simultaneously. Therefore, as in the first embodiment, the dot density rises and enlarges simultaneously in the dither pattern, and there is an advantage that the dots grow smoothly.

  In order to grow the dots of the dither pattern as described above, the maximum threshold value (= 61) in the threshold value group associated with the pixel “1” in the dither matrix is associated with the pixel “2”. It is larger than the minimum threshold value (= 4) in the threshold value group. The maximum threshold value (= 76) in the threshold group associated with the pixel “2” in the dither matrix is the minimum threshold value (= 10) in the threshold group associated with the pixel “3”. Is bigger than. The same applies to the threshold values associated with the pixel “3” and thereafter.

  Furthermore, in this embodiment, when the density value of the input image is sequentially increased, the density value of the output pixel “1” when the density value of the output pixel “2” starts to increase is the output pixel “3”. "Is equal to the density value of the output pixel" 2 "when the density value starts to increase. Specifically, the density value of the output pixel “1” when the density value of the output pixel “2” starts to increase and the density value of the output pixel “2” when the density value of the output pixel “3” starts to increase. The value is 3 as shown in FIG. The same applies to output pixels corresponding to the pixel “3” and thereafter.

  Therefore, according to the multi-value dither processing method of this embodiment, when the density value of the input image having a uniform density is sequentially increased, the density value of the output pixel is a predetermined value (here, smaller than the upper limit value). Then, at the point of time 3), the density value of new output pixels starts to increase one after another.

  In general, in order to stabilize the output state of the low density region, dots are grown while concentrating the output dots (that is, isolated dots are not formed). However, in the method of this embodiment, the low density region is Therefore, the dot lines to be formed are gradually thickened while maintaining a low density, so that more stable image formation is possible. Therefore, it is suitable for an image output apparatus that can output low density dots satisfactorily.

  By using the threshold group created in the present embodiment as a reference, it is possible to create an optimum threshold group according to the output characteristics of each image forming apparatus.

  In the present embodiment, in order to set the threshold value group associated with the dither matrix as shown in FIG. 15, the density level of each group is set equal to 40 in step 57 described above. However, only the density level of the 16th group is set to 255. FIG. 17 is a diagram showing the density level set for each group.

  Here, when the density value is taken in the height direction, the density value of each pixel increases as shown in FIG. For example, when the density value of the pixel is increased up to the group 5, the density value of each pixel is visually shown as shown in FIG.

  In the present embodiment, since the density level set for each group has the same value, as the blocks are stacked for each group, the blocks become higher by a certain height. This indicates that the density value of each pixel increases by a certain value for each group.

  As described above, when the density value of the input image is sequentially increased by setting the density levels of the groups equal in step S57, the density value of a certain output pixel is a predetermined value (less than the upper limit value). In this case, the dots can be grown so that the density value of the next output pixel starts to increase sequentially at 3).

  In this embodiment, the output pixel “1” in the dither pattern corresponds to the first pixel described in the claims, and the output pixel “2” corresponds to the second pixel described in the claims. Correspondingly, the output pixel “3” corresponds to the fourth pixel recited in the claims.

[Modification]
The setting of the density level of each group is not limited to the method of Embodiments 1 and 2 described above. The density level may be the same value between groups, or may be set to a different value for each group. When the density level is set to a different value for each group, the density level of a specific group may be increased as in the first embodiment, or the density level increases monotonically as the group number increases. You may do it. By adjusting the density level of each group, the thickness of the screen line obtained by the multi-value dithering can be freely adjusted.

  FIG. 20 is a diagram showing a comparison between a case where the density level of each group is made constant and a case where only the density level of the third group is increased. FIG. 21 is a diagram showing an example of building up blocks when the density level of each group is constant (corresponding to the left side of FIG. 20). FIG. 21 shows an example in which the density value up to the third group of pixels is increased.

  As in the example on the right side of FIG. 20, dot growth can be accelerated or delayed in the dither pattern by making the density level different between groups. As a result, the lines on the screen can be darkened early or thickened slowly. Therefore, the dither pattern can be easily adjusted according to the output characteristics of the color image output device 30.

[Embodiment 3]
The third embodiment of the present invention will be described below with reference to FIGS. In addition, description is abbreviate | omitted about the same structure as Embodiment 1 * 2 mentioned above.

  In the first and second embodiments described above, the pixel group is created in units of one pixel in step S55, but in this embodiment, the pixel group is created in units of a plurality of pixels. Specifically, when the pixel group is created in step S55, in this embodiment, the pixel group including the pixel “1” and the pixel “2” is set as the first group, and the pixels “1” to “4” are set. The pixel group including the pixel group including the pixel “1” to the pixel “6” is referred to as the third group. In this way, a pixel group including all the pixels included in the previous pixel group and including two pixels to be grown next is sequentially created. Since the dither matrix consists of 16 pixels, in this embodiment, up to the eighth group of pixel groups are created.

  Here, the order of increasing the density of the pixels in the same group may be performed in the order of the pixel numbers, or based on a general halftone dot dither method (dot concentrated dither matrix pattern shown in FIG. 22). You may go in order. For example, when increasing the density value of the first group of pixels, the density values of the output pixel “1” and the output pixel “2” are alternately increased. FIG. 23 is a diagram showing dot growth for each group when the pixels of the dither matrix are grouped in units of two pixels. In the example of FIG. 23, the density levels of the groups are equal. 23 is arranged as shown in FIG. 24 when the blocks in FIG. 23 are arranged corresponding to the pixel positions of the dither pattern. And the threshold value matched with each pixel of a dither matrix is obtained by calculating | requiring a threshold value similarly to step S59-S61 mentioned above.

  FIG. 25 is a diagram illustrating threshold data stored in the storage unit 192 of the color image processing apparatus 10 according to the present embodiment. In the present embodiment, the dither pattern changes as shown in FIG. 26 when the density of the input image is sequentially increased.

  In this embodiment, when the density value of the input image is sequentially increased from 0, first, the density values of the output pixel “1” and the output pixel “2” included in the first group are alternately increased. Then, after the density values of the output pixel “1” and the output pixel “2” reach a predetermined value as the density value of the input image increases, the output pixel “3” and the output pixel “4” included in the second group. ”Starts increasing, and the density values of the output pixel“ 1 ”to the output pixel“ 4 ”increase in order. Then, before the density values of the output pixel “3” and the output pixel “4” reach the upper limit value as the density value of the input image increases, the density values of the output pixel “5” and the output pixel “6” increase. Starts. Thereafter, similarly, the increase of the density value after the output pixel “7” starts.

  According to the configuration of the present embodiment, even a line screen can generate a screen in which dots grow from the center in units of two pixels, and the dot reproduction in the highlight area can be stabilized. Can do.

  In the present embodiment, the output pixel “1” in the dither pattern corresponds to the first pixel recited in the claims, and the output pixel “2” corresponds to the fifth pixel recited in the claims. Correspondingly, the output pixel “3” corresponds to the second pixel recited in the claims, and the output pixel “4” corresponds to the sixth pixel recited in the claims.

[Embodiment 4]
The following describes the fourth embodiment of the present invention with reference to FIG. In addition, description is abbreviate | omitted about the same structure as Embodiment 1-3 mentioned above.

  In the above-described embodiment, the example in which the present invention is applied to the color image processing apparatus 10 constituting the digital color multifunction peripheral 1 has been described. However, the present invention can also be applied to software installed in a personal computer. In this embodiment, an example in which the present invention is applied to a printer driver will be described.

  FIG. 27 is a functional block diagram showing the configuration of the computer system 100. The computer system 100 includes a personal computer 101 and a color printer 130 as shown in FIG. 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 the same functions as those in the first embodiment, 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. The image data processed by the color correction unit 15 and the black generation and under color removal unit 16 is input to the gradation reproduction processing unit 19. The tone reproduction processing unit 19 performs multi-value dither processing on the multi-tone image data as in the first embodiment. 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 personal computer 101 in which the printer driver 110 of this embodiment is installed can exhibit the same functions as the color image processing apparatus 10 of the first embodiment.

  Another object of the present invention is to record a program code (execution format program, intermediate code program, source program) of a program for causing a computer to execute the multi-value dither processing described above on a computer-readable recording medium. It can also be achieved. Accordingly, it is possible to provide a portable recording medium on which a program code for performing multi-value dither processing with improved reproducibility of characters and thin lines is recorded.

  The computer system 100 includes an image input device such as a flatbed scanner, a film scanner, and a digital camera, an image display device such as a CRT display and a liquid crystal display for displaying the processing results of the computer, and a network. A network card or a modem as a communication means for connecting to a server or the like may be provided.

  In each of the above-described embodiments, a multi-value dither image is generated for output by various image output devices such as a multifunction peripheral and a printer. However, the purpose of generating the multi-value dither image is not limited to this. The multi-value dither image obtained by the present invention may be displayed on various image display devices such as a CRT or a liquid crystal display.

  In the above-described embodiment, the case where the input image and the dithered output image are multi-valued colors has been described as an example. However, the present invention is not limited to this. The input image and the output image may be monotone as long as they have multiple gradations.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

  Needless to say, the present invention includes values other than those shown in the present specification as long as they are within a reasonable range that does not contradict the spirit of the present invention.

[Summary of embodiment]
The multi-level dither processing method according to the above embodiment is a multi-level dither processing method for reproducing the gray level of a multi-tone input image by a multi-tone dither image, and the input of uniform gray levels. When the gradation value of the image is sequentially increased, the gradation value of the first pixel constituting the dither pattern that is a partial image corresponding to the dither matrix is the gradation value of the input image. The gradation value of each pixel constituting the dither image is set to each pixel of the input image so that the gradation value of the second pixel constituting the dither pattern starts to increase before reaching the upper limit value with the increase of And a gradation determining step for determining the gradation based on the gradation value.

  The image processing apparatus according to the above embodiments is an image processing apparatus that performs multi-value dither processing for reproducing the gradation of a multi-tone input image with a multi-tone dither image, and each pixel of the dither matrix. A threshold value group stored in the storage means, a gradation value of a pixel in the input image, and a threshold value group of the pixel in the dither matrix corresponding to the pixel; Gradation determination means for determining the gradation value of each pixel constituting the dither image based on the comparison result, and the threshold value group stored in the storage means is a uniform gradation input image. When the gradation values are sequentially increased, the gradation value of the first pixel constituting the dither pattern which is a partial image obtained by applying a single dither matrix among the dither images is the input image. As the gradation value increases Before reaching the upper limit value, characterized in that the tone value of the second pixel constituting the dither pattern is set to a value to increase. Specifically, the maximum threshold value among the threshold value group associated with the pixel in the dither matrix corresponding to the first pixel is associated with the pixel in the dither matrix corresponding to the second pixel. The threshold value group is larger than the minimum threshold value.

  According to the above configuration, when the gradation value of the input image is sequentially increased, in the obtained dither pattern, the gradation value of the first pixel is increased first, and the gradation value of the first pixel is increased. Before the value reaches the upper limit value, the gradation value of the second pixel starts to increase. Then, after the gradation value of the second pixel starts to increase, the gradation values of both the first and second pixels increase. Therefore, the dot density increase and enlargement in the dither pattern occur simultaneously, and the dots grow smoothly. Therefore, the gradation of the input image can be suitably reproduced by natural dot growth.

  Further, in the gradation determination step, when the gradation value of the input image having a uniform gradation is sequentially increased, the gradation value of the third pixel constituting the dither pattern starts to increase. Later, the gradation value of each pixel constituting the dither image is determined so that the gradation value of the first pixel starts to increase, and the gradation value of the input image having a uniform gradation is sequentially increased. In this case, the gradation value of the third pixel when the gradation value of the first pixel starts to increase is the first pixel when the gradation value of the second pixel starts to increase. The third pixel may be a pixel closest to the center of the dither pattern.

  According to the above configuration, when the gradation value of the input image is sequentially increased, in the obtained dither pattern, the density value of the first pixel is increased after the density value of the third pixel closest to the center is greatly increased. The value starts to increase. Therefore, it is possible to obtain a dither pattern in which dots grow so that the pixel closest to the center of the dither pattern first has a high density and then the density of surrounding pixels gradually increases. Therefore, even in an output device with unstable dot reproducibility in highlights, the gradation of the input image can be suitably reproduced.

  Alternatively, in the gradation determination step, when the gradation value of the input image having a uniform gradation is sequentially increased, the gradation value of the second pixel increases the gradation value of the input image. Accordingly, before reaching the upper limit value, the gradation value of each pixel constituting the dither image is determined so that the gradation value of the fourth pixel constituting the dither pattern starts to increase, and uniform gradation is obtained. The gradation value of the first pixel when the gradation value of the second pixel starts to increase when the gradation value of the input image is sequentially increased is the gradation value of the fourth pixel. It may be equal to the gradation value of the second pixel when the value starts to increase.

  According to the above configuration, when the gradation value of the input image is sequentially increased, the density value of the second pixel is obtained when the density value of the first pixel reaches a predetermined value in the obtained dither pattern. When the density value of the second pixel reaches the predetermined value, the density value of the fourth pixel starts to increase. The density value of a new pixel starts to increase. Therefore, it is possible to obtain a dither pattern in which the dot density rise and enlargement are constant. Therefore, it is possible to preferably reproduce the gradation of the input image in an output device with good dot reproducibility in the low density region.

  Further alternatively, in the gradation determination step, when the gradation value of the input image having a uniform gradation is sequentially increased, the gradation value of the first pixel and the fifth dither pattern constituting the dither pattern are configured. And the gradation values of the first pixel and the fifth pixel before the upper limit value is reached as the gradation value of the input image increases. The gradation value of each pixel constituting the dither image may be determined so that the gradation value of the second pixel and the gradation value of the sixth pixel constituting the dither pattern start to increase. .

  According to the above configuration, when the gradation values of the input image are sequentially increased, the gradation values of the first pixel and the fifth pixel are alternately increased first in the obtained dither pattern. Then, before the gradation values of the first pixel and the fifth pixel reach the upper limit value, the gradation values of the second pixel and the sixth pixel start to increase. Then, after the gradation value of the second pixel starts to increase, the gradation values of the first, second, fifth and sixth pixels increase. In this way, by increasing and enlarging the dot density in the dither pattern in units of two pixels, dot reproduction can be stabilized in the highlight area.

  Note that the first pixel may be a pixel closest to the center of the dither pattern. According to the above configuration, it is possible to obtain a dither pattern in which dots gradually grow from the center to the periphery.

  The image forming apparatus according to the above embodiment includes the above-described image processing apparatus and an image output apparatus that forms a dither image reproduced by the image processing apparatus on a print medium.

  In addition, the multifunction peripheral according to the above embodiments includes the above-described image processing device, an image output device that forms a dither image reproduced by the image processing device on a print medium, and a document reading that reads a document to obtain a document image. And the image processing device reproduces the original image as a dither image by using the original image obtained by the original reading device as the input image.

  According to the above configuration, since the above-described image processing apparatus is provided, it is possible to provide an image forming apparatus and a multifunction peripheral that can appropriately reproduce and output the gradation of the input image by natural dot growth. .

  By the way, the image processing apparatus may be realized by hardware or may be realized by causing a computer to execute a program. Specifically, the multi-value dither processing program according to the above embodiment is a program that causes a computer to operate as each unit of the image processing apparatus, and the recording medium according to the above embodiment includes the multi-value dither processing program. The program is recorded.

  When these multi-value dither processing programs are executed by a computer, the computer operates as the image processing apparatus. Therefore, similar to the image processing apparatus, the gradation of the input image can be suitably reproduced by natural dot growth.

  As described above, in the present embodiment, when the gradation value of the input image having a uniform gradation is sequentially increased, the gradation value of the first pixel constituting the dither pattern is the level of the input image. Before the upper limit value is reached as the tone value increases, the tone value of the second pixel constituting the dither pattern starts to increase. Therefore, there is an effect that the gradation of the input image can be suitably reproduced by natural dot growth.

  The present invention can be suitably applied to a multifunction machine, a computer system, and the like.

1 Digital color multifunction device (image forming device, multifunction 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)
100 Computer System 101 Personal Computer (Image Processing Device)
110 Printer driver (multi-value dither processing program)
130 Color printer (image output device)
190 Dither processing unit 191 Quantization unit (gradation determination means)
192 storage unit (storage means)

Claims (11)

A multi-value dither processing method for reproducing the gradation of a multi-tone input image by a multi-tone dither image,
When the gradation value of the input image having a uniform gradation is sequentially increased, the gradation value of the first pixel constituting the dither pattern which is a partial image corresponding to the dither matrix in the dither image. And the gradation value of the fifth pixel constituting the dither pattern alternately increase, and the gradation value of the first pixel and the fifth pixel increases the gradation value of the input image. Accordingly, before reaching the upper limit value, the dither image is adjusted so that the gradation value of the second pixel constituting the dither pattern and the gradation value of the sixth pixel constituting the dither pattern start to increase. A multi-value dither processing method characterized by determining a gradation value of each pixel constituting.   The multi-value dither processing method according to claim 1, wherein the first pixel is a pixel closest to a center of the dither pattern. When the gradation value of an input image having a uniform gradation is sequentially increased, a group consisting of a plurality of pixels that sequentially increase the gradation value is defined as a gradation increase group. Belong to two pixels,
When the gradation value of two pixels belonging to the gradation increasing group reaches a predetermined value, two pixels belonging to the gradation increasing group are added,
3. The multi-value dither processing method according to claim 1, wherein when the gradation value of the two added pixels reaches a predetermined value, the process of adding two more pixels is repeated. . An image processing apparatus for performing multi-value dither processing for reproducing the gradation of a multi-tone input image with a multi-tone dither image,
Storage means for storing threshold groups associated with each pixel of the dither matrix;
Each threshold value group stored in the storage means is read, and each of the dither images constituting the dither image is based on a comparison result between the gradation value of the pixel in the input image and the threshold value group of the pixel in the dither matrix corresponding to the pixel. Gradation determining means for determining a gradation value of a pixel,
The threshold value group stored in the storage means is a portion obtained by applying a single dither matrix in the dither image when the gradation values of the input image having a uniform gradation are sequentially increased. The gradation value of the first pixel constituting the dither pattern which is an image and the gradation value of the fifth pixel constituting the dither pattern are alternately increased, and the first pixel and the first pixel Before the gradation value of the fifth pixel reaches the upper limit value as the gradation value of the input image increases, the gradation value of the second pixel that constitutes the dither pattern and the sixth value that constitutes the dither pattern An image processing apparatus, wherein the gradation value of a pixel is set to a value that starts to increase.   The image processing apparatus according to claim 4, wherein the first pixel is a pixel closest to a center of the dither pattern. When the gradation value of an input image having a uniform gradation is sequentially increased, a group consisting of a plurality of pixels that sequentially increase the gradation value is defined as a gradation increase group. Belong to two pixels,
When the gradation value of two pixels belonging to the gradation increasing group reaches a predetermined value, two pixels belonging to the gradation increasing group are added,
The image processing apparatus according to claim 4 or 5, wherein when the gradation value of the two added pixels reaches a predetermined value, the process of adding two more pixels is repeated.   The maximum threshold among the threshold groups associated with the pixels in the dither matrix corresponding to the first pixel is the threshold among the threshold groups associated with the pixels in the dither matrix corresponding to the second pixel. The image processing apparatus according to claim 4, wherein the image processing apparatus is larger than a minimum threshold value. An image processing apparatus according to any one of claims 4 to 7,
An image forming apparatus comprising: an image output device that forms a dither image reproduced by the image processing device on a print medium. An image processing apparatus according to any one of claims 4 to 7,
An image output device for forming a dither image reproduced by the image processing device on a print medium;
A document reading device that reads a document and obtains a document image;
The multifunction apparatus according to claim 1, wherein the image processing device reproduces the original image by a dither image by using the original image obtained by the original reading device as the input image.   A multi-value dither processing program for causing a computer to function as each unit of the image processing apparatus according to claim 4. A computer-readable recording medium in which the multi-value dither processing program according to claim 10 is stored.

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