JPH05219362A - Processing system for pseudo halftone picture and processing unit - Google Patents

Abstract

PURPOSE:To sharpen the picture quality at a high density portion by providing a threshold level from a low density to a high density to each element of a dither matrix and providing the same threshold level to lots of elements as the density gets higher so as to increase the resolution of a halftone picture. CONSTITUTION:A threshold level of a low density and a threshold level of a high density are respectively arranged to each element of a dither matrix 41 and the same threshold level is set respectively to lots of elements toward the threshold level of higher density. When each picture element of an input picture is binarized by using the matrix, since the threshold level of the low density has less number of the same threshold levels in comparison with that of the high density, a many number of degrees of gradation are taken. Furthermore, since the threshold level of higher density has many number of the same threshold levels, the resolution of the binarized halftone gets higher. Thus, smooth gradation is obtained for the low density part of the input picture and sharpened picture quality is obtained from the high density part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pseudo halftone image processing method for expressing a halftone image in a pseudo manner by a binary image and a processing apparatus using this processing method.

[0002]

2. Description of the Related Art A dither method is one of conventional processing methods for pseudo-halftone images. In this method, an input image signal is compared with a threshold value forming an N × N pixel dither matrix, and binarization is independently performed for each pixel. The threshold distribution pattern of the dither matrix is a dot dispersion type (Bayer type) in which adjacent threshold values are significantly different, or a dot concentration type (Bayer type) in which the threshold values are arranged closer to the center. Fattening type)
and so on. Generally, the gradation is the same for both, but the dot dispersion type is excellent in resolution, and the dot concentration type is excellent in image smoothness. FIG. 7 shows an example of a 4 × 4 pixel dot dispersion type dither matrix pattern.
FIG. 7B shows an example of a dot-concentrated pattern in FIG.

[0003]

However, in the pseudo halftone image processing method using the general dither method, the pattern is periodic regardless of whether it is the dot dispersion type or the dot concentration type. It is easy to interfere with the stripe cycle. Therefore, there is a problem that image quality is deteriorated.

The present invention aims to solve such problems.

[0005]

In order to solve the above-mentioned problems, the pseudo halftone image processing method of the present invention uses the density of each pixel of the input image and the thresholds arranged in each element of the dither matrix. This is a processing method for comparing each value with each other and binarizing each pixel according to the comparison result. Each element of the dither matrix is arranged from a low density threshold to a high density threshold. Therefore, the same threshold value is arranged in many elements as the threshold value becomes higher.

Further, the pseudo-halftone image processing apparatus of the present invention is arranged in each element from a low density threshold value to a high density threshold value, and the number of elements increases as the density becomes higher. A dither matrix in which the same threshold value is arranged, a comparing means for comparing the dither matrix with the density of each pixel of the input image, and a converting means for binarizing each pixel according to the result of the comparing means. Is equipped with.

[0007]

According to the pseudo halftone image processing method and processing apparatus of the present invention, as the threshold value becomes higher in density, the dither matrix in which the same threshold value is arranged in many elements is used to input the input image. Each pixel of is binarized.

That is, since the number of thresholds in the low density portion is smaller than that in the high density portion, the number of gradations can be increased. Therefore, it is possible to obtain smoothness of gradation in the low-density portion (highlight portion) of the input image.

On the other hand, the number of thresholds in the high density portion is larger than that in the low density portion, so that the resolution of the binarized halftone image is high. Therefore, the image quality in the high density portion of the input image becomes clear.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A pseudo halftone image processing system and processing apparatus according to the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a block diagram showing the processing apparatus of this embodiment. As shown in the figure, the processing device includes a CPU 10 for controlling image processing, a memory 20 in which an image processing program 21 such as a comparison unit and a conversion unit is stored, an image scanner 30 for inputting image data, and a dither matrix 41. Large-capacity memory 40 that stores the
A monitor 50 for displaying a binarized halftone image and a keyboard 60 for giving an instruction for image processing are provided.
Further, the memory 20 includes a multi-valued image input unit, a normalization unit, an address calculation unit, a threshold value reading unit, a comparison unit, a binary image output unit, and the like.

Next, a pseudo halftone image processing method using this image processing apparatus will be described with reference to the flowchart of FIG. First, the image processing program 21 is started under the control of the CPU 10 and "0" is substituted for x and y (step 100, step 105). Then, the input image data previously read by the image scanner 30 or the like and stored in the large-capacity memory 40 is accessed, and the horizontal x,
The density d (x, y) of the pixel at the vertical y position is read (step 110). This input image data is, for example, 2
It is a multi-valued image having horizontal L pixels and vertical M pixels having a density of 56 gradations. Next, density conversion is performed so that the number of input gradations of the read input pixel density d (x, y) matches the number of output gradations (step 120). By this density conversion, for example, the density of 256 gradations is converted to the density of 32 gradations. Then, the address (i, j) of the dither matrix 41 corresponding to the pixels at the horizontal x and vertical y positions is calculated (step 130). The horizontal address i can be obtained as the remainder obtained by dividing the horizontal x by the horizontal size n of the dither matrix 41. Similarly, the vertical address j can be obtained as the remainder obtained by dividing the vertical y by the vertical size n of the dither matrix 41. The threshold value t (i, j) located at the address (i, j) of the dither matrix 41 thus obtained is read from the large capacity memory 40 (step 140). Then, the input pixel density d ′ (x, y) after density conversion is compared with the threshold value t (i, j) (step 150), and the input pixel density d ′ (x,
If y) is greater than or equal to the threshold value t (i, j), the value of position (x, y) is set to "1" and written as output image data in the large capacity memory 40 (step 16).
0). In addition, the input pixel density d '(x, y) is the threshold t
If it is smaller than (i, j), the value of the position (x, y) is set to "0" and written as output image data in the large capacity memory 40 (step 170). Next, it is checked whether the horizontal x is smaller than L (step 180), and while it is small, the processing from step 110 to step 180 is repeated to binarize the horizontal image data. When the horizontal x becomes equal to L, the process proceeds to the next step, and it is checked whether the vertical y is smaller than M (step 190). While the vertical y is smaller than M, the processing from step 105 to step 190 is repeated,
Binarize image data. When the vertical y becomes equal to M, the processing of the image processing program 21 ends. The output image data generated by such processing can be displayed on the monitor 50 by instructing using the keyboard 60, for example.

Next, the dither matrix 41 used in the processing method of this embodiment will be described in detail with reference to FIGS. As shown in FIG. 3A, the dither matrix 41 used in the present embodiment is an 8 × 8 matrix in which 2 × 4 matrices surrounded by thick lines are used as a basic unit, and 4 pieces are arranged horizontally and 2 pieces are arranged vertically. It is configured. Figure 3 (b)
~ When the gradation level is 0 to 12 as shown in Fig. 3 (n),
It becomes a (2 × 4) × (4 × 2) matrix. Further, as shown in FIGS. 3 (o) to 3 (v), the gradation levels are 13 to 20.
In the case of, the matrix becomes (2 × 4) × (2 × 2) matrix. Furthermore, as shown in FIGS. 3 (w) to 3 (e '), when the gradation levels are 21 to 29, the matrix is (2 × 4). As described above, the number of gradations can be increased by properly using the three-step dither matrix depending on the gray level of the image. As a result, it is possible to obtain smoothness of gradation in the low density portion (highlight portion), and at the same time, to increase the resolution in the middle density portion or higher.

The processing method of this embodiment is well matched to human visual characteristics and actual image characteristics. In other words, according to human visual characteristics, the gradation discrimination ability is high in the low-density portion (highlight portion) where the image is whitish. Therefore, it is better to have a large number of gradations. Further, in the high density portion where the image is dark, it is sensitive to the discrimination of the edge portion. Therefore, it is better to increase the resolution. Furthermore, according to the actual image characteristics, the high-frequency component in the low-density portion (highlight portion) is relatively small, and there are many flat or gentle images. In this embodiment, the dither matrix is constructed so as to match these characteristics well.

Next, another example of the dither matrix is shown in the plan view of FIG. From the figure, the dither matrix 42
Also, the basic unit is a 2 × 4 matrix surrounded by a thick line, and this is composed of an 8 × 8 matrix in which 4 units are arranged horizontally and 2 units are arranged vertically. The difference from the dither matrix 41 described with reference to FIG. 3 is that the number of gradations in the low density portion is increased. Specifically, when the gradation level is 0 to 16, (2 × 4) × (4 × 2) matrix, the gradation level is 1
When it is 7 to 24, it is a (2 × 4) × (2 × 2) matrix, and when the gradation level is 25 to 32, it is a (2 × 4) matrix. As a result, 33 gradations can be expressed.

Next, an example in which the rows and columns of the dither matrix 42 are interchanged is shown in the plan view of FIG. FIG. 5A shows
It is the figure which divided the dither matrix 42 for every basic unit of 2x4 matrix. Further, FIG. 5B is a diagram in which each of these basic units is further divided into two. In this case, the basic unit a2, the basic unit c2, and the basic unit b2
And the basic unit d2 are equal to each other. Then, by replacing the rows and columns of these basic units, a dither matrix 43 as shown in FIG. 5C is obtained. FIG. 5D is a diagram in which each basic unit in which rows and columns are interchanged is further divided into two. Thus, by performing the row-column conversion, the horizontal lines become inconspicuous, and a very neat impression can be given.

FIGS. 6A to 6C show a dither matrix 42 (A) and a row-column converted dither matrix 4.
This is an example in which the image quality is further improved by combining with 3 (B). This is because the horizontal lines are conspicuous in the output image binarized using the dither matrix 42 and the vertical lines are conspicuous in the binarized output image using the dither matrix 43. Therefore, if two dither matrices are mixed, This is because such a drawback can be eliminated.

[0018]

According to the processing system and the processing apparatus of the present invention, it is possible to obtain a halftone image having a low resolution but a large number of gradations in the low density portion of the input image. Further, for a high density portion of the input image, it is possible to obtain a halftone image having a high resolution but a small number of gradations.

The halftone image obtained in this manner is well matched with the human sensory characteristics such that the gradation discrimination ability is high in the low density portion and the edge portion is sensitive in the high density portion. Further, even in an actual image, a low-density image has relatively few high-frequency components and is in conformity with a general characteristic that it is flat or gentle.

Further, since the density threshold value is non-linearly arranged in the dither matrix, it hardly interferes with the cycle of the moire fringes.

[Brief description of drawings]

FIG. 1 is a block diagram showing a processing apparatus of this embodiment.

FIG. 2 is a flowchart showing a pseudo halftone image processing method of the present embodiment.

FIG. 3 is a plan view showing the structure of a dither matrix.

FIG. 4 is a plan view showing the configuration of another dither matrix.

FIG. 5 is a plan view showing row-column conversion of a dither matrix.

FIG. 6 is a plan view showing an example in which two dither matrices are combined.

FIG. 7 is a plan view showing a dither matrix used in a conventional processing method.

[Explanation of symbols]

10 ... CPU, 20 ... Memory, 21 ... Image processing program, 30 ... Image scanner, 40 ... Large-capacity memory, 4
1 ... Dither matrix, 50 ... Monitor, 60 ... Keyboard.

Claims (2)

[Claims] 1. Comparing the density of each pixel of the input image with a threshold value arranged in each element of the dither matrix,
In a pseudo halftone image processing method for binarizing each pixel according to the comparison result, each element of the dither matrix is arranged from a low density threshold to a high density threshold, A pseudo halftone image processing method characterized in that the same threshold value is arranged for many elements as the threshold value becomes higher. 2. A dither matrix in which each element is arranged from a low density threshold to a high density threshold, and the same threshold is arranged in many elements as the density becomes higher. , A pseudo-halftone image characterized by comprising: a comparison means for comparing the density of each pixel of the input image with the dither matrix; and a conversion means for binarizing each pixel according to the result of the comparison means. Processing equipment.