JPH0824339B2 - Image signal processor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image signal processing device having a function of binary-reproducing image information including a gradation image.

2. Description of the Related Art In recent years, along with the mechanization of office processing and the rapid spread of image communication, there has been an increasing demand for high-quality image reproduction of gradation images and printed images in addition to conventional black and white binary documents.

In particular, many proposals have been made for the pseudo gradation reproduction by the binary image of the gradation image because of the good compatibility with the display device and the recording device.

The dither method is best known as one means for reproducing these pseudo gradations. This method attempts to reproduce gradation in a predetermined fixed area by the number of dots reproduced in that area. While comparing the threshold value prepared in the dither matrix with the input image information for each pixel. Binarization processing is performed. In this method, the gradation characteristics and the resolution directly depend on the size of the dither matrix, and are incompatible with each other. In addition, it is difficult to avoid the occurrence of moire patterns in reproduced images used for printed images.

A random dither method has been proposed as a method that achieves both the above gradation characteristics and high resolution and has a large effect of suppressing the generation of moire patterns. Grayscale "S-ID 75 Digest page 36-37 (Reference: R.FLOYD &L.STEINBERG," An Adaptive Algo
rithm for Spatial Gray Scale ", SID 75 DIGEST, pp36-3
7)] is proposed.

FIG. 4 is a block diagram of a main part of an apparatus for realizing the above error diffusion method.

When the coordinates of the target pixel in the original image are (x, y), 401 is an error storage unit, 402 is an unprocessed pixel area around the target pixel indicated by the error distribution coefficient matrix, and 403 is a target pixel coordinate (x, y location of accumulated error Sxy in y), 404
Is an input terminal of the input level Ixy at the coordinates (x, y) of the pixel of interest, 405 is an input correction means of I′xy (= Ixy + Sxy), 406
Is the output terminal of the binary signal Pxy of output level 0 or R, 407
Is a signal terminal for applying a constant threshold value R / 2, and 408 is an input signal I ′.
xy is compared with a fixed threshold value R / 2, and when I'xy> R / 2, Pxy = R
, And in other cases, a binarizing means for outputting Pxy = 0, 409
Is a difference calculation means for obtaining a binarization error for the target pixel of Exy (= I'xy-Pxy).

Now, the integration error Sxy for the pixel of interest is expressed by equation (1),
It is represented by (2).

Sxy = ΣKij · Ex-i + 2, y-i + 1 (1) (where i and j indicate the coordinates in the error distribution coefficient matrix) This error distribution coefficient Kij is the distribution of the error Exy to the peripheral pixels of the target pixel. In the above literature, Is illustrated.

In the configuration of FIG. 4, the above calculation is performed on the target pixel by 2
Each pixel A in the unprocessed peripheral pixel area 402 is
It is realized by the error distribution calculating means 410 that multiplies the distribution coefficient corresponding to D to add to the value in the error storing means 401 and store again in the corresponding position. However, the error storage means 401
The integration error at the pixel position B is cleared to 0 in advance.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The error diffusion method described above has excellent performance in terms of gradation characteristics and resolution as compared with the dither method, and the appearance of moire patterns is extremely small even when reproducing a printed image. Yes. But 2
Since the arrangement of dots, which is a digitized output, is not taken into consideration, the reproduction of characters and figures lacks continuity, leading to deterioration of character quality.

SUMMARY OF THE INVENTION The present invention provides an image signal processing device which secures continuity of characters and figures in the above-mentioned error diffusion method, has excellent gradation characteristics and resolution, and has good character quality.

Means for Solving the Problems To achieve the above object, the technical solution of the present invention focuses on a character area and a photographic area when binarizing multi-tone density levels sampled in pixel units. An area determining unit that detects a character area based on a density difference between a density level of a pixel and an average density level of surrounding pixels and outputs an area determination signal, and a binarization error of a target pixel are associated with pixel positions of the surrounding area. Error storing means for storing the binarized level, the binarized output storing means for storing the binarized level that has already been processed, the input level of the target pixel, and the integrated error corresponding to the target pixel position in the error storing means. An input correction means for adding and outputting a correction level, and a threshold level for binarization corresponding to a dot arrangement of peripheral output pixels of a target pixel in the binarized output storage means by an area determination signal from the area determination means. From the peripheral output calculation means for outputting, the binarization means for comparing the correction level from the input correction means with the threshold level from the peripheral output calculation means to determine the binarization level of the pixel of interest, and the input correction means. Of the pixel of interest from the difference calculation means for obtaining a binarization error which is the difference between the correction level of 1 and the binarization level from the binarization means, and the correction error from the error calculation means and a plurality of predetermined distribution coefficients. The error distribution value corresponding to the unprocessed pixel, the error distribution value is added to the integrated error of the corresponding pixel position in the error storage means, and the result is stored again. The output calculation means, when the area determination means determines that the area is a character area, sets the threshold level according to the dot arrangement of the peripheral output pixels of the target pixel in the binarized output storage means,
It is configured to output a level lower than the threshold level of the photographic area.

With the above configuration, the threshold level when binarizing only the character region by the region determination is set to a value lower than the threshold level during normal processing, for example, according to the dot arrangement of the peripheral output pixels of the pixel of interest. By facilitating the black level, continuity with peripheral output pixels is ensured and the image quality of characters and graphics is improved.

Embodiment FIG. 1 is a block diagram of the essential parts of an image signal processing apparatus in an embodiment of the present invention.

In the figure, the configuration and operation of each block of 101 to 110 are the same as the respective configuration elements 401 to 410 in the conventional error diffusion method of FIG. Binary output storage means 11 different from the configuration of FIG.
1, the peripheral output calculation means 113 and the area determination means 114 will be described in detail below.

The binary output storage means 111 stores the binarization level Pxy from the binarization means 108 in correspondence with the peripheral output pixel position of the pixel of interest, and when binarizing, the inside of the peripheral output area 112 of the pixel of interest is It is read and output to the peripheral output calculation means 113.

As a relatively simple method of the area determination means 114, for example,
When the density difference determined based on the density difference between the density level of the pixel of interest and the average density level of the peripheral pixels is large, it is determined to be a character / graphic area and an area determination signal 115 is output.

The peripheral output calculation means 113 reads the binarization level in the peripheral output area 112 of the binary output storage means 111, and outputs the peripheral output area 1
When the binarization level in 12 is all pixels or some of the pixels are black levels, the threshold level 107 for binarization is set low, for example, and the binarization level of the pixel of interest is likely to become the black level.

The peripheral output area referred to by the peripheral output calculation means 113 will be described with reference to FIG.

FIG. 2A shows a peripheral output pixel a adjacent to the pixel of interest.
In this example, the threshold levels are controlled by the logical product or logical sum conditions at the binarization level of all or some of the peripheral output pixels a to d.

FIG. 2B shows an example in which the peripheral output pixel area is widened, and the directivity of the dot arrangement of the black pixels is taken into consideration. Θ _{1 to} Θ ₄ are vertical / horizontal directions or diagonal lines. This indicates the directionality of the direction, and the directionality is detected as follows according to the binarization level of the peripheral output pixels a to h.

Further, as the peripheral output calculation means 113, Θ1 to Θ4
Threshold level when binarizing according to the logical condition of the directionality of
It controls Bth low.

FIG. 3 is a block diagram of the area determining means and the peripheral output calculating means. 301 is an output operation circuit for realizing the logical condition of the peripheral output area, 304 is a selector for selecting a threshold level for binarization, 114 is an area determination means, 312 is a register, and 313 is a density difference operation means.

The output arithmetic circuit 301 will be described by taking the logical condition of FIG. 2 (b) as an example: y = (a * b) + (c * d) + (e * f) + (g *
h) (3) If the logical condition is expressed by a logical expression, it becomes as shown in Expression (3), and the output operation circuit can be configured by a gate circuit of the logical product 302 and the logical sum 303.

In order to simplify the description, the area determination unit 114 will be described as an example in which it is detected by the density difference between one-dimensional adjacent pixels.
The pixel of interest input from the terminal 310 is input to the register 312 for delaying by one pixel. Next, the density difference calculation means 313 calculates the density difference between the density level of the pixel of interest and the output of the register 312 delayed by one pixel as a peripheral pixel. The comparison means 315 compares the output from the density difference calculation means 313 with a certain threshold level and outputs the area detection signal 115.

In the selector 304, the select signal from the output arithmetic circuit 301 is prohibited by the gate circuit 309 except for the character area by the area detection signal 115 from the area determination means 114. Therefore, when the gate circuit 309 is at “L” level, the normal threshold level Bth1 is selected, and when the gate circuit 309 is at “H” level, the threshold level Bth2 set lower than the normal threshold level Bth1 is selected. The threshold level Bth10 is applied to the quantizing means 108.
It outputs 7.

It is also a simple method to configure the output operation circuit 301 by ROM (read only memory) instead of random logic such as a gate circuit. It is easily conceivable to use the peripheral output pixels a to d or a to h shown in FIG. 2 as an address and to output a single or a plurality of threshold level select signals in the ROM pattern.

Further, the number of black pixels of the peripheral output pixels a to d in FIG. 2A or Θ ₁ to Θ in the peripheral output pixel area in FIG. 2B.
It is also possible to set a plurality of threshold levels with respect to the threshold level at the time of normal processing, depending on the number of detected ₄ directions.

By setting the level of the threshold level Bth2 to “0”, the black level can be forcibly set to the black level when the logical condition is satisfied, rather than the probability that the black level is easily set.

EFFECTS OF THE INVENTION As described above, according to the present invention, the threshold level when binarizing only the character region by the region determination is set to be lower than the threshold level during normal processing, for example, according to the dot arrangement of peripheral output pixels of the target pixel. Thus, when binarized, the black level is apt to be established, the continuity with the peripheral output pixels can be secured, and the image quality of characters and graphics can be improved.

The same effect can be obtained by setting the second threshold level for binarization to "0" and forcibly setting it to the black level.

[Brief description of drawings]

FIG. 1 is a block diagram of a main part of an image signal processing apparatus according to an embodiment of the present invention, FIG. 2 is a conceptual diagram showing a peripheral region of a peripheral output pixel of a target pixel in the apparatus, and FIG. FIG. 4 is a detailed block diagram of the peripheral output computing means, and FIG. 4 is a block diagram of the essential parts of an image signal processing apparatus for implementing the conventional error diffusion method. 101 ... Error storage means, 110 ... Error calculation means, 111 ...
Binary output storage means, 113 ... Peripheral output calculation means, 114 ...
Area determination means, 301 ... Output arithmetic circuit, 304 ... Selector.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuo Nakazato 3-10-10 Higashisanda, Tama-ku, Kawasaki City, Kanagawa Matsushita Giken Co., Ltd. (56) Reference JP 62-139472 (JP, A) Actual Kaisho 58-184965 (JP, U)

Claims (4)

[Claims] 1. When binarizing multi-tone density levels sampled on a pixel-by-pixel basis, character areas and photo areas are differentiated by the density difference between the density level of a pixel of interest and the average density level of surrounding pixels. A region determining unit that detects a character region and outputs a region determining signal, an error storing unit that stores a binarizing error of a pixel of interest in correspondence with pixel positions around the target pixel, and a binarizing level that has already been processed. Binary output storage means for storing, input correction means for adding an input level of the pixel of interest and an integrated error corresponding to a pixel position of interest in the error storage means, and outputting a correction level; A peripheral output calculation means for outputting a threshold level for binarization according to a dot arrangement of peripheral output pixels of a target pixel in the binarized output storage means by a region determination signal, and a correction level from the input correction means. Of the correction level from the input correction unit and the binarization level from the binarization unit, and a threshold level from the peripheral output calculation unit to determine the binarization level of the pixel of interest. A difference calculation means for obtaining a binarization error which is a difference, and an error distribution value corresponding to an unprocessed pixel around the target pixel is calculated from the correction error from the error calculation means and a plurality of predetermined distribution coefficients, and the error is calculated. Error distribution calculation means for adding the distribution value to the integrated error of the corresponding pixel position in the error storage means and storing again. The peripheral output calculation means is determined to be a character area by the area determination means. In this case, the image signal processing apparatus is characterized in that the threshold level of the photographic region is output as the threshold level according to the dot arrangement of the peripheral output pixels of the target pixel in the binarized output storage means. 2. A low level that is output as a threshold level according to the dot arrangement of peripheral output pixels of a pixel of interest in the binarized output storage means when the peripheral output calculation means determines that the area is a character area. 2. The image signal processing device according to claim 1, wherein is included in zero level. 3. The peripheral output computing means outputs the threshold level according to the dot arrangement of the peripheral output pixels of the pixel of interest in the binarized output storage means in accordance with the area determination signal from the area determining means. 7. A lower level than the threshold level of the photo area is output when the dot arrangement of the output pixels around the pixel of interest in the binarized output storage means is arranged in any of the horizontal, vertical and diagonal directions. 1. The image signal processing device according to 1. 4. When the peripheral output calculation means outputs a threshold level according to the dot arrangement of the peripheral output pixels of the target pixel in the binarized output storage means by the area determination signal from the area determination means, When the dot arrangement of the output pixels around the pixel of interest in the binarized output storage means is arranged horizontally, vertically, or diagonally, the low level output as the threshold level includes zero level. The image signal processing device according to claim 3.