JPH0666876B2 - 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 is 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.

As a method in which the gradation characteristics and high resolution are compatible with each other and the moire pattern generation suppressing effect is large, the error difference diffusion method (reference: R. FROOD & FLOYD &.
L (L). STEINBERG, “Ann
Adaptive Algorithm Forspeech
Gray Scale (An Adaptive Algorithm for Sp
atial Gray Scale) ”, esd eye (SDI) 75
DIGEST, pp36-37) has been proposed.

FIG. 3 is a block diagram of an essential part of an apparatus for realizing the above error diffusion method.

When the coordinates of the pixel of interest in the original image are (x, y), 1 is the error storage means, 2 is the unprocessed pixel area around the pixel of interest indicated by the error distribution coefficient matrix, and 3 is the coordinate (x, y).
Storage position of accumulated error Sxy in y), 4 is coordinate (x, y)
Input terminal of input level Ixy at 5 is I'xy (= Ixy
+ Sxy) input correction means, 6 is an output terminal of an output image signal Pxy of output level 0 or R, 7 is a signal terminal for applying a constant threshold value R / 2, and 8 is an input signal I′xy and a constant threshold value R / 2. In comparison, a binarizing means for outputting Pxy = R when I'xy> R / 2 and Pxy = 0 otherwise, 9 is Exy (= I'xy-
Pxy) is a difference calculation means for obtaining a binarization error for the pixel of interest.

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

Sxy = ΣKij · Ex _{−j, y−i + 1} (1) (where i and j indicate 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, (However, * indicates the position of the pixel of interest).

In the configuration shown in FIG. 3, the above calculation results in the binarization error Exy for the pixel of interest and each pixel A to A in the unprocessed peripheral pixel region 2.
Error distribution calculation means 10 for multiplying the distribution coefficient corresponding to D, adding to the value in the error storage means 1 and storing again in the corresponding position
Is realized by. However, the integration error at the pixel position B of the error storage means 1 is cleared to 0 in advance.

Problems to be Solved by the Invention The error diffusion method described above has excellent performance in terms of gradation characteristics and resolution compared to the dither method, and the appearance of moire patterns is extremely small even when reproducing a printed image. Yes. However, the pattern (texture) peculiar to the method occurs in an image with little change in density or an image with uniform density generated by a computer.
It is hardly popular. The main cause of the occurrence of this texture is that the ratio of the binarization error distribution to the peripheral pixels of the target pixel is always held in a constant relative positional relationship with the target pixel.

Further, with the above-described configuration of the error diffusion method, it is difficult to obtain a high-speed image processing device because one pixel processing is a multi-stage calculation.

The present invention provides an image signal processing apparatus that suppresses the occurrence of texture in the error diffusion method, is excellent in gradation characteristics and resolution, has very few moire patterns even when a printed image is reproduced, and is capable of high-speed processing. is there.

Means for Solving the Problems According to the present invention, when binarizing multi-tone density levels sampled on a pixel-by-pixel basis, the binarization error of the pixel of interest is stored in association with the pixel positions in the surroundings. The error storage means, the input level of the target pixel, the input correction means for temporarily storing the integrated error corresponding to the position of the target pixel in the error storage means in the addition register, and outputting the correction level;
Difference calculation means for obtaining differences in parallel in advance in two states of the binarization level, temporarily storing in each register and outputting difference level A and difference level B, the difference level A, difference level B, and previous pixel The binarization error temporarily stored during the processing is added to each to obtain the error A and the error B, the error A is compared with a predetermined threshold value, and the binarization level of the pixel of interest is determined. Binarization means for selecting the error A or the error B according to the binarization level, temporarily storing the new binarization error in the register, and setting it as the binarization error in the next pixel processing,
A distribution coefficient for distributing the binarization error to unprocessed pixels around the pixel of interest is arbitrarily initialized for each arbitrary sub-scanning line from a plurality of distribution coefficient sets at a predetermined change cycle. An error distribution value corresponding to an unprocessed pixel around the pixel of interest is calculated from the distribution coefficient generating means generated according to random number generation, the difference from the difference calculating means, and the plurality of distribution coefficients from the distribution coefficient generating means, and the error is calculated. An image signal processing device comprising error distribution / update means for adding the distribution value to the integration error of the corresponding pixel position in the error storage means and storing again as a new integrated pixel is attempted to achieve the above object. It is a thing.

Operation The present invention has the above-mentioned configuration so as to make a prediction when a binarization error is obtained.
The error calculation is performed in each of the two states of the binarization level, and the binarization error is selected according to the decision of the binarization level, and the distribution ratio of the binarization error to the peripheral pixels of the pixel of interest is selected from a plurality of distribution coefficient sets. By the function of the distribution coefficient generating means that selects one set according to the random number generation initialized by an arbitrary value or a value obtained by adding + n to the previous line for each arbitrary sub-scanning line, the distribution amount of the binarization error is constant relative to the pixel of interest. This is to prevent the pixels having a positional relationship from being biased, suppress the occurrence of a texture pattern in the processed output image, and enable high-speed pixel processing. Further, the random number generator such as the maximum length counter circuit has a periodicity, so that the texture is not generated in the output image even when the number of pixels in the main scanning direction, that is, the X direction is N times the periodicity. It was done.

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 FIG. 1, the coordinates of the pixel of interest in the original image are (x,
y), 1 is the error storage means, 2 is the unprocessed pixel area around the pixel of interest indicated by the error distribution coefficient matrix, 3
Is the read position of the integration error at the coordinate (x + 2, y). Reference numeral 15 is an input correction means, 18 is a difference calculation means, 23 is a binarization means, 11 is an error distribution / update means, and 12 is a distribution coefficient generation means. These configurations will be described in further detail below.

The input correction means 15 adds the input level I _{x + 2, y} from the input terminal 4 and the integrated error S _{x + 2, y} from the error storage means 1 when the coordinates of the pixel of interest are (x, y), and synchronizes with the surroundings of pixel processing. The sync signal 14 is temporarily stored in the register 17.

The difference calculating means 18 outputs the I ′ output from the input correcting means 15.
Differences between _{x + 1 and y} and the binarization levels of 0 and R which are output levels are respectively stored in the registers 21 and 22 and temporarily output to the difference levels A and B.

The binarizing means 23 adds the difference level A and the difference level B to the error distribution value 47 from the error distribution / update means 11 to obtain the error A and the error B. The error A is compared with a predetermined threshold value to obtain the binarization level Pxy and 2
The error A or the error B is selected by the selector 27 according to the digitization level
Is temporarily stored in the register 28 as a new binarization error and the binarization error Exy is output.

The distribution coefficient generating means 12 prepares a plurality of distribution coefficient sets for unprocessed pixels around the target pixel in advance, and a sync signal synchronized with the pixel processing cycle in the X direction from the sync signal input terminal 13.
2 for pixel positions A to D in the peripheral pixel area 2 by obtaining 14
The distribution coefficients K _{A to} K _D of the binarization error Exy are selected from the plurality of distribution coefficient sets and output to the error distribution / updating means 11.
The error distribution / updating means 11 synchronizes with the synchronization signal 14 and, along with the distribution coefficients K _{A to} K _D , the binarization error Exy for the pixel of interest from the difference calculating means 9 and the peripheral pixel area 2 of the error storing means 1 It is stored in the storage device corresponding to the pixel positions A, C, and D. Integrated error _{S 'A, S'} in the previous pixel processing program C, reads the S _'D, obtaining a new integrated error S _A to S _B by the equation (3).

Further, the error distribution / updating means 11 carries out an updating process of writing new integrated errors S _{A to} S _D in the storage devices corresponding to the pixel positions A to _D in the error storage means 1.

A more specific circuit of the error distribution / update means 11 and the distribution coefficient generation means 12 is shown in FIG. In the same figure, the case where the number of distribution coefficient sets is two will be described.

The distribution coefficient generating means 12 is composed of a plurality of distribution coefficient sets K _{1A to} K _1D.
And a storage device 40 for storing K _{2A to} K _2D in advance.
41 is provided to store the coefficient set prior to the start of pixel processing.

The initial value table 37 stores an initial value for determining a starting point of random number generation for each sub-scanning line prior to the start of pixel processing, and a line which is a synchronizing signal in the sub-scanning direction, that is, the Y direction, given from the line synchronizing signal input terminal 35. An initial value 38 is output by the synchronization signal 36. The initial value table can also be configured by a storage device such as a RAM or a ROM that stores an initial value + n for each sub-scanning line, but when setting the initial value of the previous line + n for each arbitrary sub-scanning line, A counting counter or the like can be easily configured.

The random signal generator 39 sets the starting point of random number generation for the initial value 38 output from the initial value table 37 by the line synchronization signal 36, and corresponds to the pixel processing cycle in the X direction given from the synchronization signal input terminal 13. When the synchronizing signal 14 is input, the select signal 42 is output.

The random signal generator 39 uses a Maxim length counter circuit or the like to generate a select signal 42 and selects two distribution coefficient sets.

The error distribution / updating means 11 multiplies the distribution coefficients K _{A to} K _D input from the distribution coefficient generating means 12 and the binarization error Exy input from the binarization means 23 while synchronizing with the synchronization signal 14, and the error Get a share of 47-50. The error distribution value 47 is output to the binarizing means 23 to obtain a new binarizing error. Since the integration error for the pixel position B occurs in the processing of the pixel of interest 3, the error distribution value 48 is temporarily stored in the internal register 51 (R _B ) as the integration error (S _B ) corresponding to the pixel position B. The error distribution value 49 and the data of the internal register 51 (R _B ) temporarily stored in the previous pixel processing are added, and the data is temporarily stored as the data of the internal register 52 (RC) as the integration error (SC) of the pixel position C. The error distribution value 50 and the data of the internal register 52 (RC) temporarily stored in the processing of the previous pixel are added and stored in the storage device corresponding to the pixel position D of the error storage means 1. With such an error distribution / update means 11, access to the storage device in the error storage means 1 is performed at the pixel position (x +
2, y), and only the read access corresponding to the pixel position D and the write access corresponding to the pixel position D can be realized easily.

As described above, according to the present invention, when the binarization error is obtained, two types of provisional binarization errors are calculated in advance, and when the binarization level is determined, two types of provisional binarization are performed. By selecting from the error, the multi-stage arithmetic circuit configuration becomes a single-stage arithmetic circuit configuration and high-speed processing becomes possible.

The distribution ratio of the binarization error to the peripheral pixels of the pixel of interest is not fixed, but is selected from a plurality of distribution coefficient sets together with the pixel processing to be used. ) Can be significantly suppressed, and by setting the starting point of random number generation for each arbitrary sub-scanning line, the false image (texture) can be significantly suppressed without being affected by the number of pixels in the X direction. It became possible to do.

[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 detailed circuit diagram of an error distribution / update means and a distribution coefficient generation means, which are the main parts of FIG. Three
The figure is a block diagram of a main part for realizing the conventional error diffusion method. 1 ... Error storage means, 11 ... Error distribution / update means, 40-
41 ... Memory device, 39 ... Random signal generator, 43-46 ...
… Selector, 51 to 52 …… Internal register, 37 …… Initial value table.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiharu Kurosawa 3-10-10 Higashisanda, Tama-ku, Kawasaki City, Kanagawa Matsushita Giken Co., Ltd. (72) Kiyoshi Takahashi 3-chome, Higashisanda, Tama-ku, Kawasaki City, Kanagawa Prefecture No. 10 No. 1 Matsushita Giken Co., Ltd.

Claims (1)

[Claims] 1. An error storage means for storing a binarization error of a pixel of interest in association with a pixel position in the vicinity thereof when binarizing a multi-tone density level sampled in pixel units.
The input level of the pixel of interest and the integrated error corresponding to the position of the pixel of interest in the error storage means are temporarily stored in an addition register,
An input correction means for outputting a correction level and a difference operation for predicting the difference between the correction levels in two states of the binarization level and temporarily storing the difference in each register and outputting the difference level A and the difference level B. Means, the difference level A, the difference level B, and the binarization error temporarily stored at the time of the front pixel processing are added to obtain the error A and the error B, and the error A is compared with a predetermined threshold value. 2 of the attention pixel
A binary value that determines the binarization level, selects the error A or the error B according to this binarization level, temporarily stores it in a register that obtains a new binarization error, and uses it as the binarization error in the next pixel processing. And a distribution coefficient that distributes the binarization error to unprocessed pixels around the pixel of interest at a predetermined change cycle.
A distribution coefficient generating means for generating a random number from a plurality of distribution coefficient sets, and an error distribution value corresponding to an unprocessed pixel around the pixel of interest are calculated from the plurality of distribution coefficients from the distribution coefficient generating means, An image signal processing apparatus comprising: an error distribution / update means for adding an error distribution value to an integrated error of a corresponding pixel position in the error storage means and storing again.