JPH09233327A - Dither matrix preparing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method preparing a dither matrix which does not become noisy even if a dither method is used, is reduced in the degradation of resolution and is suppressed in the generation of a pattern. SOLUTION: A binarization processing is performed for the uniform density picture element matrix in which all the picture elements are a uniform density value (i) by an error diffusion method (S110), and the binarization picture element matrix F1 is preserved in a working memory 14 (S120). Subsequently, the binarization processing and the preservation are repeated in the same way, successively making the uniform density value (i) incremental. Afterwards, as for all the preserved binarization matrixes, the binarized values of the picture element of the same location are sunned up and an integrated result matrix M1 in which this total value is defined as an element is formed (S150). Next, threshold for dither matrix is successively set from the picture element location in which the total value of the integrated result matrix M1 is low (S160) and the threshold is preserved in a dither matrix storage memory 16 (S170).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dither matrix creating method for converting a multivalued image into a binary image.

[0002]

2. Description of the Related Art Data that constitutes an image is usually represented by a gray scale of about 8 bits. However, in a device that outputs this image data as an image that can be viewed by a person, such as an output device such as a printer, a dye such as ink is formed as a dot on a recording paper because of its simple mechanism and easy control. So-called binary recording, which is performed only with or without attachment, is general.

When an image represented by 8-bit gray scale is printed using such a binary printing output device, the value of each pixel is simply judged by a threshold value to attach ink. It will be decided whether or not. However, by simply binarizing with a threshold value in this way, the rich gray level of 8-bit gray levels (256 gray levels for one color and 3 primary colors for each 2 levels)
With 56 gradations, it is possible to represent 16,777,216 kinds of colors. ) Becomes impossible to express. For this reason, a method of artificially expressing the light and shade state in such a binary recording output device has been considered. That is, the concept of area gradation is to express light and shade by the number of dots arranged in a predetermined area.

Various proposals have been made as methods for realizing this idea. For example, the error diffusion method, the random number generation type dither method, the systematic dither method, and the like.

[0005]

However, these methods have the following problems. That is, in the error diffusion method, the binarization process is delayed due to the error diffusion or the error distribution calculation, and a unique pattern is easily generated in the binarized image. Although the random number generation type dither method is fast because it uses a dither matrix, it has a problem that the image quality is very noisy, that is, the image quality is such that the image is filled with a lot of noise because it uses random numbers. It was.

As another method using the dither matrix, there is a spiral type of the systematic dither method. In this spiral type, dots are easily clustered and the clustered dots are recognized as large dots, resulting in poor resolution. It was easy to do. Among the systematic dither methods, there is a Bayer method. This Bayer method has good resolution because dots are dispersed, but it has a unique pattern and has poor gradation in dark areas. Met.

It is an object of the present invention to provide a dither matrix forming method which does not cause noise even if the dither method is used, does not deteriorate the resolution, and suppresses the generation of patterns.

[0008]

Means for Solving the Problems and Effects of the Invention According to the dither matrix creation method of the present invention, a plurality of discretely set density values within the range from the minimum value min to the maximum value max of the density values that can be represented in a pixel. The density values are binarized by an error diffusion method for each of the uniform density pixel matrices as the density values of all pixels in each of the plurality of uniform density pixel matrices, and the binarized density values are obtained. Based on this, the threshold value of each pixel in the dither matrix is set.

As a method of setting the threshold value of each pixel in the dither matrix based on the binarized density value, for example, the binarized density value is set at the same position in all the uniform density pixel matrix. Are accumulated for each pixel of, and based on the accumulation result for each pixel at the same position,
Set the threshold for each pixel in the dither matrix.

The minimum value min of the density value is, for example, 0, and the maximum value max is, for example, 255. The plurality of discretely set density values are, for example, max-min + 1 values set by changing the minimum value min to the maximum value max by one.

When the image is binarized by the dither matrix with the threshold value set in this way, it does not become noisy to the human eye, the resolution does not deteriorate, and the binarized image in which the generation of the pattern is suppressed is obtained. Can be generated. As described above, when the dither matrix is created, each uniform density pixel matrix is binarized by using the error diffusion method, the binarization result is integrated for all uniform density pixel matrices, and based on the integration result. The threshold of each pixel in the dither matrix is set. Although the binary sequence generated by the error diffusion method looks irregular,
No bullshit at all. That is, it is difficult to form a pattern, but it is difficult to form a noisy image. As described above, the dither matrix creating method of the present invention is a method of creating the dither matrix by incorporating the binary array property in the error diffusion method by utilizing the binary array generated by the error diffusion method.

Therefore, in the dither matrix which enables the binarization calculation processing at high speed, an appropriate threshold distribution which is irregular but not random is formed by the error diffusion method as described above. , Is not noisy to the human eye, and the resolution does not deteriorate,
Furthermore, it is considered that the binarized image in which the generation of the pattern was suppressed could be generated.

The accumulation result for each pixel at the same position is, for example, the number of one of the binarized density values in the pixel at the same position, and each of the dither matrices corresponds to this number. Pixel thresholds can be set. For example, if a binary value is represented by "1" and "0", "1" existing at the same position in a plurality of binarized uniform density pixel matrixes is counted, and the count number is unchanged. Alternatively, it may be multiplied by a coefficient and set as a threshold value at the same position in the dither matrix. Further, the threshold value may be set from the count value based on a table. Instead of “1”, “0” may be counted.

Further, the uniform density pixel matrix is
A matrix having more pixels than the required dither matrix may be used. Such a uniform density pixel matrix is, for example, a uniform density pixel matrix having both rows and columns larger than the required dither matrix.

When a uniform density pixel matrix having a larger number of pixels than such a dither matrix is used, for example, after binarization by the error diffusion method, the integration is performed on a specific area having the same size as the dither matrix. , The threshold value of each pixel in the dither matrix is set based on the integration result.

In this case, in particular, assuming that the specific area does not include the integration result of the first pixel line portion where the processing of the error diffusion method is started, the error diffusion or the error distribution distortion at the initial stage of the error diffusion will occur. The dither matrix is less likely to be affected, and even if the image is binarized using the completed dither matrix, it is possible to further suppress the deviation of dots and the generation of patterns. In this case, in order to eliminate the influence of the distortion as much as possible, the specific area is a line farthest from the top pixel line portion, and the integration result of the final pixel line portion where the processing of the error diffusion method ends is performed. It is preferable to include it.

According to the present invention, the dither matrix pattern is uniquely obtained by using the binary image obtained by executing the error diffusion method on the uniform density pixel matrix. Therefore, no experience or intuition is required,
Anyone can easily create a dither matrix. Moreover, since the error diffusion processing is performed only once on the uniform density matrix of a certain density in the creation,
A dither matrix can be created at high speed.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION

[Embodiment 1] FIG. 1 is a main block diagram of a dither matrix creating apparatus 2 for executing the dither matrix creating method of the present invention.

The microcomputer section 11 which is the main body of the dither matrix creating apparatus 2 includes a CPU 12 and an RO.
It is composed of a program memory 13 composed of M, a working memory 14 composed of a RAM, a dither matrix storage memory 16 composed of a RAM, and an output image memory 17 composed of a RAM. An input unit 10 and an output unit 19 are connected to the microcomputer unit 11 via a system bus 18.

The input unit 10 is provided with an interface for inputting from a keyboard or an external storage device, and inputs data and instructions necessary for creating a dither matrix. Further, the input unit 10 inputs image data in which each pixel has a gradation represented by 8 bits in order to confirm the effect of the created dither matrix.

The CPU 12 executes the creation of a dither matrix as will be described later, and the program memory 13 stores programs for executing various controls performed by the CPU 12. The working memory 14 temporarily stores data required when the CPU 12 executes the program stored in the program memory 13.

The dither matrix storage memory 16 is C
The dither matrix data formed by the processing of the PU 12 is stored. The output image memory 17 is a CPU
12, the dither matrix stored in the dither matrix storage memory 16 is used to binarize the image data having the gradation represented by 8 bits, and the binarized output image data is stored.

The output unit 19 is a device for printing the binary image data stored in the output image memory 17 by the electrophotographic method with or without dots. Next, the dither matrix creation processing executed by the dither matrix creation device 2 will be described. A flowchart of the dither matrix creation process is shown in FIG.

First, "0" is set to the uniform density value i (S100). Next, the uniform density pixel matrix in which all pixels have uniform density value i = 0 is binarized by the error diffusion method (S110). This uniform density pixel matrix is shown in FIG.
, 256 kinds of uniform density pixel matrices D0 to D255 in which i = 0 to 255 are stored in the working memory 1
It should be read in 4 and used.

In addition to the 256 uniform density pixel matrices being aligned in this way, all pixels in each uniform density pixel matrix have the same density value i, so that the density value of each pixel is used for calculation in the error diffusion method. , All of them can be treated as the density value i, and therefore only 256 values of i = 0 to 255 may be stored. Further, the lower limit value “0”, the upper limit value “255” of the uniform density value i, and the uniform density value i discretely existing between the lower limit value and the upper limit value.
Alternatively, only the step value Sp (a value indicating the interval between the uniform density values i to be processed, here Sp = 1) may be used for determining the value of.

Here, the error diffusion method is used in a broad sense, and when a certain pixel is binarized, a binarization error is distributed to the densities of peripheral pixels which have not been binarized (narrow sense). Error Diffusion Method by Bibliography: Robert W. Floyd and Louis
Steinberg, "An Adaptive Algorithm for Spatial Gray
scale ", Proceeding of the SID Vol.17 / 2,1976
Etc.), or a method of receiving a predetermined ratio of the error generated in the binarization from the already-binarized pixels existing in the surroundings when the binarization is performed (also referred to as the average error minimum method.
Jarvis, CNJudice, and WHNinke, "A Survey of Techn
iques for the Display of Continuous Tone Pictures
on Bilevel Displays ", Computer Graphics and Image P
rocessing.5, 13-40 (1976)) and the like are well known, so a detailed description of the error diffusion method itself will be omitted.

In this embodiment, each pixel is binarized to either "1" or "0" by the error diffusion method. Since the uniform density pixel matrix D0 having the uniform density value i = 0 at first is obtained, as shown in FIG. 4, a binarized pixel matrix F0 in which all pixels are “0” even if binarized. .

The binarized pixel matrix thus obtained is stored in the working memory 14 (S120),
It is determined whether or not the uniform density value i = 255 (S130).
At first, the uniform density value i = 0 (in S130, "N
O ”), and then the uniform density value i is incremented (S
140). Therefore, next, all pixels have uniform density values i = 1.
The uniform density pixel matrix of 1 is binarized by the error diffusion method (S110), and the binarized pixel matrix F1 is stored in the working memory 14 (S120).

Thereafter, the uniform density value i is sequentially incremented (S140), and the uniform density pixel matrix of the corresponding uniform density value i is binarized by the error diffusion method (S1).
10) The process of storing the binarized pixel matrix Fi in the working memory 14 (S120) is repeated.

When the processing (S110, S120) of the uniform density pixel matrix with the uniform density value i = 255 is completed, as shown in FIG. 4, the working memory 14 stores 256 2 pieces of uniform density values i = 0 to 255. Quantized pixel matrices F0 to F255 are formed.

Next, since the uniform density value i = 255, (S
At 130, “YES”), the binarized values of the pixels at the same position are summed for all the binarized pixel matrices, and the integrated result matrix M1 having this sum as an element is formed (S150). That is, as shown in FIG. 4, assuming that the upper left corner is the origin (0, 0) and the horizontal direction is the x-axis and the vertical direction is the y-axis, first, all the binarized pixel matrices F0 to F2.
The values are summed for 55 (0,0). The total result is stored in the same position of the integrated result matrix M1 prepared in the working memory 14 as shown in FIG. This summing process is performed for each pixel position to fill the entire accumulation result matrix M1. This addition process is the same as counting the number of pixel values of "1" in the pixels at the same position, since each pixel is binarized to "1" or "0".

Next, the threshold value for the dither matrix is set in order from the pixel position having the lowest total value in the accumulation result matrix M1 (S160). As a method of setting the threshold value, the total value itself may be set as the threshold value. In this case, since the total process of step S150 corresponds to the threshold setting process, no particular process is performed in step S160. Then, the integrated result matrix M1 itself is stored in the dither matrix storage memory 16 as a dither matrix (S170).

In step S160, instead of simply using the total value itself of the accumulation result matrix M1 as the threshold value as it is, the total value of the accumulation result matrix M1 is determined according to a table of the total value and the threshold value prepared in advance. May be converted into a threshold value. Further, in step S160, the threshold value may be set to “0, 1, 2, ...” in the order of increasing total value, or conversely, the threshold value may be set to “2 in order of increasing total value”.
55, 254, 253, ... ”.
In this case, if there are two or more same total values, they may all be given the same threshold value, or the same total values may be ordered and threshold values may be set similarly to different total values. You may.

A halftone color image is binarized by using the dither matrix thus formed and recorded by a color printer. As a result, the image is not noisy to the human eye and the resolution is high. It was possible to generate a pseudo-halftone binarized image in which pattern generation was suppressed without deterioration. Of course, the dither method does not perform many calculations unlike the error diffusion method, so that it can be processed quickly.

Further, the dither matrix pattern is uniquely obtained by using the binary image obtained by executing the error diffusion method on the uniform density pixel matrix. Therefore, anyone can easily create the dither matrix without requiring experience or intuition. Moreover, since the error diffusion process is performed only once on the uniform density matrix of each density, the dither matrix can be created at high speed.

[Second Embodiment] In the first embodiment,
The uniform result pixel matrix D0 to D255 or the binarized pixel matrix F0 to F255 and the integrated result matrix M1 and the dither matrix having the same size as the size are formed.
, The uniform density pixel matrix D0 to D25
5 and its binarized pixel matrixes F0 to F255 are much larger than the integrated result matrix M1 and the dither matrix, and the binarized pixel matrixes F0 to F255 are binarized in a specific region. The dither matrix is created using the digitized values as the accumulation result matrix M1.

The dither matrix creation processing in the second embodiment will be described. The dither matrix creation process is different from that of the first embodiment in steps S110 and S1.
Since the processing of 20 is different and the others are the same, only the different portions are shown in FIG. 7. After the uniform density value i is initialized to "0" in step S100, the density of the target pixel (equal to the uniform density value i) in the uniform density pixel matrix of uniform density value i = 0 is read (S210). The first pixel of interest is the pixel at the origin position (0,0).

Next, the pixel of interest is binarized by the error diffusion method (S220). Next, if the current pixel of interest is the specific area A
It is determined whether it is included in 1 (S230). As shown in FIG. 6, the specific area A1 is an area in which the binarized value sum processing is performed to form the accumulation result matrix M1, and in the second embodiment, it is the same in all uniform density pixel matrices. Exists in the position.

This specific area A1 does not include the first pixel line L0 portion in the error diffusion process, but the last pixel line L0.
x part. This is because, in the head pixel line L0 portion, since there is no preceding line, distortion occurs in diffusion of the binarization error as compared with the following line, and dot concentration or a specific pattern is likely to occur. This is because the influence of the distortion of the binarization error diffusion becomes smaller as the distance from the head pixel line L0 becomes farther backward, and the influence is least at the last pixel line Lx portion.

When it is determined that the current target pixel is included in the specific area A1 ("YES" in S230), the binarized value of the target pixel is stored in the working memory 14 (S).
240). If it is determined that the current target pixel is not included in the specific area A1 (“NO” in S230), the binarized value of the target pixel is not stored.

Next, it is determined whether or not there are unprocessed pixels in the uniform density value i (S250). If there are unprocessed pixels ("YES" in S250), step S210 is executed again.
Returning to, the above processing is continued with the unprocessed pixel as the pixel of interest. The determination of unprocessed pixels in step S250 may be a determination of whether or not there are unprocessed pixels in the specific area A1.

After "NO" is determined in step S250, it is determined in step S130 whether i = 255. If i = 255 is not satisfied ("NO" in S130),
i is incremented (S140), the process returns to step S210 again, the target pixel is set to the origin of the image in the uniform density pixel matrix of uniform density value i, and the above-described processing is repeated.

Therefore, in step S130, "YE
When it is determined to be “S”, the working memory 14
Similar to the case shown in FIG. 4, the binarized pixel matrix F0 to F25 corresponding to the specific area A1 for each uniform density value i.
5 are formed. All the binarized pixel matrices F0 to F255 are summed up for each pixel at the same position to create an integrated result matrix M1 (S150). Next, similarly to the first embodiment, a threshold value is set to form a dither matrix (S160), and the dither matrix storage memory 1
6 is stored (S170).

When the dither matrix obtained in the second embodiment is binarized into a halftone color image and recorded by a color printer, it is not noisy to human eyes and the resolution is deteriorated. I didn't. Moreover, since the dither method does not perform many calculations unlike the error diffusion method, it can be processed quickly. Furthermore, as in the first embodiment, the dither matrix can be created quickly and easily.

Further, in the second embodiment, the binarized pixel matrix F0 to F255 of the specific area A1 not including the first pixel line L0 and including the last pixel line Lx is obtained from the uniform density pixel matrix, and this 2 An integration result matrix M1 smaller than the uniform density pixel matrix is obtained from the binarized pixel matrices F0 to F255, and a dither matrix is formed based on this integration result matrix M1. Therefore, the influence of the distortion of the error distribution occurring in the head pixel line L0 is less likely to occur, and the pseudo halftone binarized image in which the generation of the pattern is further suppressed can be generated.

[Others] In the first and second embodiments, the total of the binarized values at the same position, that is, the number of pixels which have become "1" in the binarization is counted and used as the total value.
The number of binarized pixels that have become “0” is counted and used as a total value, and based on this, an integrated result matrix M1 is created, and the processing of step S160 is executed for this to create a dither matrix. Is also good.

The total value may not be used as it is as the accumulation result matrix M1, but may be multiplied by a predetermined coefficient. Further, for the binarized value Iixy of the uniform density pixel matrix of the uniform density value i, a different coefficient k (i) is set for each uniform density value i as shown in the following equation, and the total value of all uniform density pixel matrices is set. The integrated result matrix M1 may be created by obtaining Cxy.

[0048]

[Equation 1]

It should be noted that x and y represent pixel positions on the uniform density pixel matrix or the integration result matrix M1.
The binarization of the uniform density pixel matrix was binarization to "1" or "0", but any value may be binarized. It suffices to form the matrix M1 and convert this integration result matrix M1 into a dither matrix. Also,
Instead of counting the number as described above, the sum of the binarized values may be used.

In the second embodiment, the specific area A1 for storing the binarized value is at the same position in all uniform density pixel matrices, but it may be different depending on the uniform density pixel matrix. This is because the degree of the backward influence of the distortion of the error diffusion occurring in the head pixel line L0 differs depending on the uniform density value i forming the uniform density pixel matrix.

Further, according to the uniform density value i,
Since the degree of influence of the distortion of the error diffusion generated in the first pixel line L0 portion on the rear is different, in order to eliminate the influence of the distortion of the error diffusion in binarization of all the uniform density pixel matrix, The size may be adjusted according to the value of the uniform density value i.

[Brief description of drawings]

FIG. 1 is a main block diagram of a dither matrix creation device according to a first embodiment.

FIG. 2 is a flowchart of a dither matrix creation process according to the first embodiment.

FIG. 3 is an explanatory diagram showing uniform density value image data according to the first embodiment.

FIG. 4 is an explanatory diagram showing a state in which the uniform density value image data according to the first embodiment is binarized.

FIG. 5 is an explanatory diagram of an accumulation result matrix according to the first embodiment.

FIG. 6 is an explanatory diagram showing a relationship between an image having a uniform density value and a specific region according to the second embodiment.

FIG. 7 is a flowchart showing a part of dither matrix creation processing according to the second embodiment.

[Explanation of symbols]

 2 ... Dither matrix creation device 10 ... Input unit 11 ... Microcomputer unit 12 ... CPU 13 ... Program memory 14 ... Working memory 16 ... Dither matrix storage memory 17 ... Output image memory 18 ... System bus 19 ... Output unit

Claims (9)

[Claims] 1. A plurality of density values discretely set in a range from a minimum value min to a maximum value max of density values that can be represented in a pixel are all pixels in each of a plurality of uniform density pixel matrices. The density value is binarized by the error diffusion method for each uniform density pixel matrix, and the threshold value of each pixel in the dither matrix is set based on the binarized density value. How to create a dither matrix. 2. The binarized density values are integrated for each pixel at the same position for all of the uniform density pixel matrix, and based on the integration result for each pixel at the same position,
2. The dither matrix creating method according to claim 1, wherein a threshold value for each pixel in the dither matrix is set. 3. The dither matrix creating method according to claim 1, wherein the minimum value min of the density values is 0 and the maximum value max is 255. 4. The minimum value min to the maximum value ma of the density value
4. A plurality of density values discretely set in a range up to x are max-min + 1 values which are set by changing from a minimum value min to a maximum value max by 1, respectively. The dither matrix creation method described in any one of. 5. The accumulation result for each pixel at the same position is the number of one of the binarized density values, and the threshold value of each pixel of the dither matrix is set according to this number. 5. The dither matrix creation method according to claim 1. 6. The dither matrix creating method according to claim 1, wherein the uniform density pixel matrix is a matrix having a larger number of pixels than a required dither matrix. 7. The uniform density pixel matrix is a matrix having both rows and columns larger than a required dither matrix, and after binarization by the error diffusion method, the integration is performed on a specific area having the same size as the dither matrix. The dither matrix creation method according to claim 6, wherein the threshold value of each pixel in the dither matrix is set on the basis of the integration result. 8. The dither matrix creating method according to claim 7, wherein the specific area does not include a result of integration of a first pixel line portion where the processing of the error diffusion method is started. 9. The dither matrix creating method according to claim 7, wherein the specific area includes an integration result of a final pixel line portion where the processing of the error diffusion method ends.