JPS63102473A - Image signal processing device - Google Patents

Abstract

PURPOSE:To prevent the occurrence of a texture pattern in a processed output picture, by preventing leaning of the distributing quantity of binary coded errors to a location having a fixed relative positional relation with a noticed picture element. CONSTITUTION:An error distributing and updating means 11 multiplies distribution factors KA-KD from a distribution factor producing means 12 by a binary coded error EXY from a difference calculating means and produces error distributing values 19-22 synchronously to a synchronizing signal 14. Then the distributing value 19 is added to an accumulated error SA' corresponding to the picture element position read from an error storing means 1 and the sum is temporarily stored in an internal register 23. Since the accumulated error corresponding to another picture element position B is produced for the first time when a noticed picture element 3 is processed, the distributing value 20 is stored in a register 24 as an accumulated error SB. The distributing value 20 is added to the data of the register 24 and the accumulated error SC of a picture element position C is obtained. The error SC is added to the data of a register 25 and the sum is stored in the position corresponding to a picture element position D of the error storing means 1 as the accumulated error SD of the picture element position D. Therefore, the access to the storing means 1 becomes the reading access corresponding to the position A and writing access corresponding to the position D only and a false picture is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image signal processing device having a function of performing binary reproduction of image information including gradation images.

Conventional technology In recent years, with the mechanization of office processing and the rapid spread of image communications, 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 originals. . In particular, pseudo gradation reproduction using a binary image of a gradation image is highly compatible with display devices and recording devices, and many proposals have been made.

The dither method is the most well-known method for reproducing these pseudo gradations. This method attempts to reproduce gradations in a predetermined area by the number of dots reproduced within that area, and the threshold value prepared in the dither matrix and input value information are compared for each pixel. Binarization processing is performed from In this method, the gradation characteristics and resolution characteristics directly depend on the size of the dither matrix, and are incompatible with each other. Furthermore, when used for printed images, it is difficult to avoid the occurrence of moiré patterns in the reproduced image.

As a method that achieves both the above gradation characteristics and high resolution, and has a large effect of suppressing the occurrence of moiré patterns, the error expansion 75 DIGES
T, pp36-37) has been proposed.

FIG. 3 is a block diagram of essential parts of an apparatus for realizing the above error diffusion method. 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 storage position of the integrated error Sxy at the coordinates (x, y), and 4 is the coordinates (x.

y), the input terminal of the input level Ixy, 5 is I'x
y (= I xy + 5xy) input correction means, 6
is the output terminal of the binary signal Pxy of output level O or R,
7 is a signal terminal that applies a constant threshold value R/2, 8 is a binarization means that outputs Pxy=R when input signal I'xy > R/2, and Pxy=O in other cases; 9 is Exy ( =
This is a difference calculation means for calculating the binarization error for the pixel of interest (I'xy - Pxy).

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

8xy=ΣKij・Ex−j+2. y-i+
1 -- (1) (However, '+J indicates the coordinate in the error distribution coefficient matrix.) This error distribution coefficient K i j weights the distribution of the error Exy to the surrounding pixels of the pixel of interest, and is as described above. In the literature, (where * is the position of the pixel of interest) is given as an example.

In the configuration shown in FIG. 3, the above calculation adds the binarization error Exy to the pixel of interest to each pixel A in the unprocessed surrounding pixel area 2.
This is realized by the error distribution calculation means 10 which multiplies -D by the corresponding distribution coefficient, adds it to the value in the error storage means 1, and stores it again at the corresponding position. However, the integrated error at pixel position B in 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 superior performance in terms of gradation characteristics and resolution compared to the dither method, and the appearance of moiré patterns is extremely rare even when reproducing printed images. . However, this method is not widely used because it creates a pattern (texture) that is unique to the method in images with little density change or images with uniform density generated by a computer. The main reason for the occurrence of this texture is that the ratio of binarization error distribution to surrounding pixels of the pixel of interest is always maintained in a constant relative positional relationship with the pixel of interest.

The present invention provides an image signal processing device that suppresses the occurrence of texture in the error diffusion method described above, has excellent gradation characteristics and resolution, and has extremely low occurrence of moiré patterns even when reproducing printed images.

Means for Solving the Problems The present invention stores the binarization error of a pixel of interest in correspondence with its surrounding pixel positions when binarizing multi-gradation density levels sampled in pixel units. error storage means; input correction means for adding the input level of the pixel of interest and the accumulated error corresponding to the position of the pixel of interest in the error storage means and outputting a correction level; and comparing the correction level with a predetermined threshold. A binarization means that determines the binarization level of the pixel of interest, and a difference between the correction level and the binarization level (binarization error).
and a distribution coefficient for distributing the binarization error to unprocessed pixels surrounding the pixel of interest, at a predetermined change period, randomly changing the corresponding pixel position of one set of distribution coefficients. calculating an error allocation value corresponding to an unprocessed pixel around the pixel of interest from a distribution coefficient generation means that generates the distribution coefficient while changing it at will, a difference from the difference calculation means and a plurality of distribution coefficients from the distribution coefficient generation means; The above object is achieved by providing error distribution/updating means for adding the error distribution value to the accumulated error at the corresponding pixel position in the error storage means and storing the result again as a new accumulated error. be.

According to the above configuration, the present invention randomly changes the distribution ratio of the binarization error to the surrounding pixels of the pixel of interest by randomly changing the pixel position relative to the pixel of interest of a set of distribution coefficient cents along with pixel processing. The distribution coefficient generating means prevents the distribution amount of the binarization error from being biased toward a fixed relative positional relationship with the pixel of interest, and prevents texture patterns from occurring in the processed output image.

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

In FIG. 1, the structure and operation of each block 1 to 9 are the same as the conventional one shown in FIG. The difference from the configuration in FIG. 3 is that error distribution/updating means 11 and distribution coefficient generation means 12 are provided in place of the error distribution calculation means 10 shown in FIG. 3, and this point will be described in detail below. .

First, the distribution coefficient generating means 12 prepares in advance a set of distribution coefficients for unprocessed pixels around the pixel of interest, and obtains a synchronization signal 14 synchronized with the pixel processing cycle in the X direction or Y direction from the synchronization signal input terminal 13. The distribution coefficient of the binarization error Exy for pixel positions A and D in the peripheral pixel area 2 is
KD is randomly selected from the one set of distribution coefficients and outputted to the error distribution/updating means 11. The error distribution/updating means 11 updates the distribution coefficient KA in synchronization with the synchronization signal 14.
-Ko, the binarization error Exy for the pixel of interest from the difference calculation means 9 and the surrounding pixel area position A of the error storage means 1, 0. Integration error S'A, S6 in the previous pixel processing process stored in the storage device corresponding to D
, S6 are read out, and a new integration error 5A-8D is determined using the following equation (3).

Furthermore, the error distribution/updating means 11 generates a new accumulated error 5A-
An update process is performed to write 8D into the storage device corresponding to the pixel position A-D in the error storage means 1.

Below, error allocation/update means 11 and allocation coefficient generation means 12
A more specific configuration is shown in FIG. In Figure 2,
The distribution coefficient generating means 12 is provided with a storage device 15 for previously storing 1 to 4 in one distribution coefficient set, and stores the coefficient set before starting pixel processing. Furthermore, the random signal generator 16 outputs a select signal 17 in response to input of a synchronization signal 14 corresponding to the pixel processing period in the X direction or Y direction, which is applied from the synchronization signal input terminal 13 . This random signal generator 16 has a maximum length
It can be easily constructed by using a plurality of bit signals such as a counter circuit, and a select signal with high generality can be obtained. The selector 18 has a four-manpower, four-output configuration, uses the select signal 17 as a select signal, and randomly selects and connects 4 to KA-KD from the distribution coefficients stored in the storage device 15. That is, there are 16 ways of connecting the distribution coefficients 1 to 4 and the distribution coefficients 8 to Ko corresponding to the pixel positions A to D in the peripheral pixel area 2 of the error storage means 1, and in order to select one of them, The select signal 17 has a 4-bit configuration. The error distribution/updating means 11 uses the synchronization signal 14
In synchronization with , the distribution coefficient inputted from the distribution coefficient generation means 12 is multiplied by +> and the binarization error Exy inputted from the difference calculation means 9 to generate error distribution values 19 to 22. The error distribution value 19 is added to the accumulated error S'A corresponding to the pixel position A read from the error storage means 1, and is temporarily stored in the internal register 23 (RA) for use as the accumulated error Sxy in the next pixel processing. . Since the accumulated error for pixel position B occurs for the first time in the processing of the pixel of interest 3, the error distribution value 20 is temporarily stored as it is in the internal register 24 (RB) as the accumulated error (SB) corresponding to pixel position B. The error distribution value 20 and the data in the internal register 24 (RB) temporarily stored in the previous pixel processing are added and the integrated error (Sc) at the pixel position C is stored in the internal register 2.
5 (R・) and the integration error (S
o) is stored in the storage device corresponding to the pixel position of the error storage means 1. Such error allocation/updating means 11
Accordingly, access to the storage device in the error storage means 1 is as follows.
There is only a read access corresponding to the pixel position A and a write access corresponding to the pixel position, resulting in an easily realized configuration.

Effects of the Invention As described above, in the present invention, the allocation ratio of binarization error to surrounding pixels of a pixel of interest is not fixed, but multiple allocation coefficients corresponding to surrounding pixel positions are calculated from a set of allocation coefficients during pixel processing. By selecting and using them at random, it has become possible to significantly suppress the false images (textures) seen in conventional error diffusion methods.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an image signal processing device according to an embodiment of the present invention, Fig. 2 is a block diagram of main parts of the same device, and Fig. 3 is a block diagram of an image signal processing device that performs a conventional error diffusion method. It is a wiring diagram. DESCRIPTION OF SYMBOLS 1... Error storage means, 11... Error allocation/updating means, 15... Storage device, 16... Random signal generator, 18... Selector, 23-25... Internal register. Name of agent: Patent attorney Satoshi Nakao and 1 other person 1
Figure M2 Figure 3

Claims (1)

[Claims] An error storage means for storing the binarization error of a pixel of interest in correspondence with the surrounding pixel positions when binarizing multi-gradation density levels sampled in pixel units, and an input level of the pixel of interest. input correction means for adding the accumulated error corresponding to the pixel position of interest in the error storage means and outputting a correction level; and comparing the correction level with a predetermined threshold to determine the binarization level of the pixel of interest. A binarization means, a difference calculation means for calculating a binarization error from the difference between the correction level and the binarization level, and a distribution coefficient for distributing the binarization error to unprocessed pixels surrounding the pixel of interest in advance. Distribution coefficient generation means that randomly changes corresponding pixel positions of one distribution coefficient set at a predetermined change period, and a binarization error from the difference calculation means and the distribution coefficient generation means. An error distribution value corresponding to unprocessed pixels around the pixel of interest is calculated from a plurality of distribution coefficients from the plurality of distribution coefficients, and the error distribution value is added to the accumulated error of the corresponding pixel position in the error storage means and stored again. An image signal processing device comprising distribution/updating means.