JPH0666874B2 - 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, 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 the conventional monochrome binary document. .
In particular, the pseudo gradation reproduction by the binary image of the gradation image has good compatibility with the display device and the recording device, and many proposals have been made.

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 characteristic and the resolution characteristic directly depend on the size of the digit matrix, and are incompatible with each other. Further, when it is used for a printed image or the like, it is unavoidable that a moire pattern is generated in a reproduced image.

The error diffusion method (reference: Earl Frodde & El Steinberg “Unadaptive Algorithm for Special Gray Scale” S75 Day Digest (R .FLOY
D & L. STEINBERG, “An Adaptive Algorithm for spa
tial Gray Scale ", SID 75 DIGEST), pp36-37) is proposed.

FIG. 3 is a block diagram of an essential part 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 a storage position of the integrated error Sxy at coordinates (x, y), 4 is an input terminal of the input level Ixy at coordinates (x, y), and 5 is I '.
xy (Ixy + Sxy) input correction means, 6 is an output terminal for a binary signal Pxy having an 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> R / 2. Hour Px
A binarization unit that outputs y = R and Pxy = 0 in other cases, and a difference calculation unit 9 that obtains 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 the equation (1),
It is represented by (2).

Sxy = ΣKij · Ex−j _{+ 2} , y−i _{+ 1} (1) (However, i and j indicate the coordinates in the error distribution coefficient matrix.) This error distribution coefficient Kij is the pixel of interest of the error Exy. In the above literature, the weighting of the distribution to the peripheral pixels is performed. Is illustrated.

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. . However, in images with little change in density, images with uniform density generated by a computer, etc., a pattern (texture) peculiar to the method is formed, and therefore it is hardly used. 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.

The present invention provides an image signal processing apparatus which suppresses the generation of texture in the above-mentioned error diffusion method, has excellent gradation characteristics and resolution, and has a very small amount of moire patterns even when a printed image is reproduced.

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. Error storage means, an 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 means to output a correction level, and the correction level having a predetermined threshold value. Binarization means for comparing and determining the binarization level of the pixel of interest, difference calculation means for obtaining the difference between the correction level and the binarization level, that is, binarization error, and the binarization error around the pixel of interest. Distribution coefficient generating means for generating a distribution coefficient to be distributed to the unprocessed pixels while selecting the distribution coefficient in a predetermined order from a plurality of distribution coefficient sets in a predetermined change cycle, and the difference calculating means. From the binarization error and the plurality of distribution coefficients from the distribution coefficient generating means, an error distribution value corresponding to an unprocessed pixel around the pixel of interest is calculated, and the error distribution value is stored at the corresponding pixel position in the error storage means. By providing an error distribution / updating means for adding the integrated error of and the stored again as a new integrated error,
It is intended to achieve the above object.

The distribution coefficient generating means randomly selects a distribution coefficient for a plurality of predetermined unprocessed pixels around the target pixel from a plurality of distribution coefficient sets at a predetermined change cycle. The above-mentioned purpose can be achieved even if it is made to function as desired.

Operation The present invention has the above-described configuration, and selects one set of the distribution ratio of the binarization error for the peripheral pixel of the pixel of interest from a plurality of sets of distribution coefficient in a predetermined order or in a random order. With the function of the distribution coefficient generation means, the distribution amount of the binarization error is prevented from being biased to the pixel having a fixed relative positional relationship with the pixel of interest, and the texture pattern is not generated in the processed output image. is there.

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

In FIG. 1, the configuration and operation of each block 1 to 9 are the same as those of the conventional one shown in FIG. In FIG. 1, the third
The difference from the configuration of the figure is the error distribution calculating means shown in FIG.
This point will be described in detail below in that error distribution / update means 11 and distribution coefficient means 12 are provided instead of 10.

First, the distribution coefficient generation means 12 prepares a complex distribution coefficient set for unprocessed pixels around the target pixel in advance, and obtains a synchronization signal 14 synchronized with the pixel processing cycle in the x direction or the y direction from the cyclic signal input terminal 13. Then, the distribution coefficients K _{A to} K _D of the binarization error Exy for the pixel positions A to D in the peripheral pixel area 2 are selected from the plurality of distribution coefficient sets and output to the error distribution / update means 11. Then, the error distribution / update means 11 synchronizes with the synchronization signal 14, and along with the distribution coefficients K _{A to} K _D , the binarization error Exy of the pixel of interest from the difference calculation means 9 and the peripheral pixel area 2 of the error storage means 1. Pixel positions A, C, D
Integrated error _{S 'A,} S' in the previous pixel process stored in the corresponding storage device and C, reads the _{S 'D,} by the following new integrated error S _A ~S _D (3) formula Ask.

The error distribution / updating means 11 further uses these new accumulated errors.
An update process is performed in which S _{A to} S _D are written in the storage device corresponding to the pixel positions A to _D in the error storage means 1.

Specific examples of the error distribution / update means 11 and the distribution coefficient generation means 12 are shown in FIG. In the figure, distribution coefficient generating means
_A storage device 15 and a storage device 16 are provided for storing two sets of distribution coefficient sets K1 _{A to} K1 _D and K2 _A to K2 _D in advance, and store the coefficient sets prior to pixel processing. Further, the selection signal generator 17 receives the synchronization signal 14 corresponding to the pixel processing cycle in the X direction or the Y direction given from the synchronization signal input terminal 13 to select signals 22 for the selectors 18 to 21 at predetermined pixel processing intervals. Is output. The selectors 18 to 21 select a set of (K1 _A , K1 _B , K1 _C , K1 _D ) and a set of (K2 _A , K2) from the two sets of distribution coefficient stored in the storage device 15 and the storage device 16.
2 _B, K2 _C, K2 _D sets of) outputs as a distribution coefficient K _A ~K _D corresponding to the selected pixel position A~D the error storage means 1 on the basis of the select signal 22. This distribution coefficient generation means
The selection signal generator 17 in 12 generates a select signal 22 using a maximum length scouter circuit (M sequence generator) which receives the synchronization signal 14 as an input, and randomly selects two allocation coefficient sets. You may. The error distribution / update means 11 multiplies the distribution coefficients K _{A to} K _D input from the distribution coefficient generation means 12 and the binarization error Exy input from the difference calculation means 9 while synchronizing with the synchronization signal 14 Get distribution values 26-29. The error distribution value 36 and the pixel position A read from the error storage means 1.
Internal register 23 for adding the corresponding integrated error S _'A for use as an integrated error Sxy for the next pixel processing
Temporarily store in (R _A ). Since the integration error for the pixel position B occurs for the first time in the processing of the pixel of interest 3, the error distribution value 27 is temporarily stored in the internal register 24 (R _B ) as the integration error corresponding to the pixel position B. The error distribution value 28 and the data of the internal register 24 (R _B ) temporarily stored in the previous pixel processing are added and the result is temporarily stored in the internal register 25 (R _C ) as an integrated error of the pixel position C. The error distribution value 29 and the data of the internal register 25 (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. Such error distribution / update means 11
Thus, access to the storage device in the error storage means 1 is
A read access is made corresponding to the pixel position A, and a write access is made corresponding to the pixel position D, which is a structure that can be easily implemented.

EFFECTS OF THE INVENTION As described above, in the present invention, the distribution ratio of the binarization error to the peripheral pixels of the pixel of interest is not made constant, but is used by selecting from a plurality of distribution coefficient sets together with the pixel processing.
It has become possible to suppress the false image (texture) seen in the conventional error diffusion method.

[Brief description of drawings]

FIG. 1 is a block connection diagram of an image signal processing apparatus according to an embodiment of the present invention, FIG. 2 is a block connection diagram of main parts of the apparatus, and FIG. 3 is an image signal processing apparatus for realizing a conventional error diffusion method. It is a block connection diagram. 1 ... Error storage means, 11 ... Error distribution / update means, 15-
16 ... Storage device, 17 ... Selection signal generator, 18-21 ... Selector, 23-25 ... Internal register.

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

Claims (2)

[Claims] 1. An error storage unit for storing a binarization error of a pixel of interest in association with pixel positions in the surroundings when binarizing multi-tone density levels sampled in pixel units. The input correction means for adding 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 and outputting the correction level, and the correction level is compared with a predetermined threshold value to obtain 2 pixels of the pixel of interest. Binarization means for determining a binarization level, difference calculation means for obtaining a binarization error by the difference between the correction level and the binarization level, and the binarization error is distributed to unprocessed pixels around the pixel of interest. Distribution coefficient generating means for generating a coefficient to be selected while selecting it from a plurality of distribution coefficient sets at a predetermined change cycle, a binarization error from the difference calculating means, and a plurality of distribution coefficient generating means from the distribution coefficient generating means. Distributor Error distribution / update means for calculating an error distribution value corresponding to an unprocessed pixel around the pixel of interest, adding the error distribution value to an integrated error at a corresponding pixel position in the error storage means, and storing again. An image signal processing device provided. 2. A distribution coefficient generating means randomly selects a distribution coefficient for a plurality of predetermined unprocessed pixels around a target pixel from a plurality of distribution coefficient sets at a predetermined change cycle. The image signal processing device according to claim 1, wherein