JPH0793682B2 - 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, gradation characteristics and resolution are directly dependent 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 that achieves both the gradation characteristics and high resolution and has a large effect of suppressing the generation of moire patterns, the error diffusion method [R.
Floyd and L. Stinberg, "Unadaptive Algorithm Force Spacious Gray Scale" SID 75 Digest 36-37
Page (Reference: R.FLOYD & L.STEINBERG, “An Adaptive Al
gorithm for Spatial Gray Scale ", SID 75 DIGEST, pp36
−37)] has been proposed.

FIG. 3 is a block diagram of a main part of an apparatus for realizing the error extension method.

When the coordinates of the pixel of interest in the original image are (x, y), 3
01 is the error storage means, 302 is the unprocessed pixel area around the pixel of interest indicated by the error distribution coefficient matrix, and 303 is the coordinate (x,
storage position of accumulated error Sxy in y), and 304 is coordinate (x, y)
Input terminal of input level Ixy at 305, I'xy (= Ix
y + Sxy) input correction means, 306 is output level 0 or R
Output terminal of the binary signal Pxy, 307 is a signal terminal for applying a constant threshold value R / 2, 308 is a comparison between the input signal I′xy and the constant threshold value R / 2, and when I′xy> R / 2, Pxy = R, otherwise Pxy =
Binarizing means for outputting 0, 309 is Exy (= I'xy-Pxy)
It 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 + 2 (1) (where i and j represent coordinates in the error distribution coefficient matrix) The error distribution coefficient Kij weights the distribution of the error Exy to the peripheral pixels of the target pixel. In the above literature ... (2) (where * is 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 302.
Error distribution calculation means for multiplying D by a distribution coefficient corresponding to D, adding to the value in the error storage means 301, and storing again in the corresponding position
It is realized by 310. However, the integration error at the pixel position B of the error storage means 301 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. , In principle, it is a method that can reproduce the gradation of black pixel (or white pixel) density according to all the levels of the input level Ixy.

However, if it is attempted to implement the above processing method by a practical integer arithmetic processing circuit, the binarization error Exy and the sum ΣKij · Exy of the error distribution values to the peripheral pixels do not necessarily match.
This means that all the values of the binarization error Exy are not distributed to the peripheral pixels, and the gradation corresponding to all the levels of the input level Ixy cannot be reproduced. At high concentration levels this phenomenon is noticeable,
The reproduction pixel has a narrow gradation reproduction area.

The present invention provides an image signal processing apparatus which has improved tone reproduction characteristics when the above-mentioned error diffusion method is implemented and has an extremely small number of moire patterns.

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, input correction means for adding the input level of the pixel of interest and integrated 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 value. Then, a binarizing unit that determines the binarizing level of the pixel of interest, a difference calculating unit that obtains a binarizing error Exy that is a difference between the correction level and the binarizing level, and the binarizing error Exy.
A binarization error distribution means for extracting the lower n bits and outputting as a remainder error Exyl, and further complementing the lower n bits of the binarization error Exy with “0” and outputting as a new binarization error Exyu; A distribution coefficient generating unit that generates a distribution coefficient that distributes the binarization error to unprocessed pixels around the target pixel, and a new binarization error Exyu from the binarization error distribution unit.
Error distribution value calculating means for calculating an error distribution value corresponding to an unprocessed pixel around the pixel of interest from the distribution coefficient from the distribution coefficient generating means, one of the error distribution values, and
An error updating means for adding the residual error Exyl from the binarizing error distributing means, adding the remaining error distribution value and the integrated error of the corresponding pixel position in the error storing means and storing again is provided. is there.

According to the present invention, the lower n bits of the binarization error Exy are distributed to one of the peripheral pixels of the pixel of interest as the residual error Exyl by the above configuration, and "0" is filled in the lower n bits of the binarization error Exy. By distributing the new binarization error Exyu to the peripheral pixels according to the distribution coefficient, the sum of the binarization error and the error distribution value of the peripheral pixels is made to coincide, and the gradation reproduction characteristics of the low-density and high-density areas of the input level are It has been improved so that moire patterns do not occur.

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 109 is the same as that of each section of 301 to 309 of the conventional error diffusion method of FIG. 1 is different from the configuration of FIG. 3 in that an error distribution value calculating means 110, a distribution coefficient generating means 111, an error updating means 112, and a binarization error distributing means 113 are provided. Describe in detail.

The binarization error distribution unit 113 extracts the lower n bits from the binarization error Exy and outputs it as a residual error Exyl to the error updating unit 112, and supplements the lower n bits of the binarization error Exy with “0” to newly add it. The binary distribution error Exyu is output to the error distribution value calculation means 110.

The distribution coefficient generation means 111 prepares a distribution coefficient set for unprocessed pixels around the target pixel in advance,
The new binarization error Exyu for pixel positions A to D in
The distribution coefficients KA to KD are output to the error distribution value calculation means 110.

Now, in the error distribution value calculation means 110, the error storage means with the distribution coefficients KA to KD and the new binarization error Exyu for the target pixel from the difference calculation means 109 is synchronized with the synchronization signal synchronized with the pixel processing cycle. The error distribution values GA to GD corresponding to the pixel positions A, B, C and D in the peripheral pixel area 102 of 101 are obtained by the following formula (3).

GA = KA × Exyu GB = KB × Exyu GC = KC × Exyu (3) GD = KD × Exyu Further, the error distribution values GA to GD are output to the error updating means 112.

The error updating means 112 synchronizes with the synchronization signal, and the error distribution values GA to GD from the error distribution value computing means 110, the residual error Exyl from the binarization error distribution means 113, and the error storage means 101.
The integrated errors SA ′, SC ′, SD ′ in the previous pixel processing process stored in the storage device corresponding to the pixel positions A, B, C, D in the peripheral pixel region 102 of
SA to SD are calculated by the following formula (4).

SA = SA '+ GA SB = Exyl + GB SC = SC' + GC (4) SD = SD '+ GD Further, the error updating means 112 corresponds the new integrated errors SA to SD to the pixel positions A to D of the error storing means 101. Update processing for writing in the storage device.

However, in the formula (4), the residual error Exyl is set to the peripheral pixel area 10
Although the pixel position B in 2 is added, the pixel position B may be added to any of the pixel positions A, B, C, and D, and the pixel position B will be described later.

FIG. 2 shows a specific configuration of the error distribution value calculating means 110, the distribution coefficient generating means 111, the error updating means 112, and the binarization error distributing means 113. In the figure, the residual error Exyl will be described below as being reflected on the peripheral pixel position B.

The distribution coefficient generation means 205 is provided with a storage means 206 for storing the distribution coefficients KA to KD in advance, and stores them before starting the pixel processing. Further, the storage unit 206 may use a ROM (Read Only Memory) in which the distribution coefficients KA to KD are written in advance.

The binarization error distribution means 203 is a lower order n of the binarization error Exy.
The output of the bit allocating means 208 for compensating the bits with “0” is output to the error allocating value calculating means 207 as a new binarizing error Exyu, and the lower n bits of the binarizing error Exy are extracted and distributed. The output of the means 209 is output to the error updating means 210 as the residual error Exyl.

The error distribution value calculation means 207 uses the new binarization error E.
The error distribution values GA to GD are multiplied by xyu and the distribution coefficients KA to KD, and output to the error updating means 210.

The error updating means 210, while synchronizing with the synchronizing signal 214 synchronized with the pixel processing input from the synchronizing signal input terminal 204,
The internal register 211 for adding the error distribution value GA and the integrated error S′A corresponding to the pixel position A read from the error storage means 201 and using it as the integrated error Sxy in the next pixel processing
Temporarily store in (RA). The integration error for the pixel position B is the error distribution value GB and the residual error E from the binarization error distribution means 203.
It is added to xyl and temporarily stored in the internal register 212 (RB) as an integrated error (SB) corresponding to the pixel position B. Error allocation value G
C and the data of the internal register 212 (RB) temporarily stored in the pixel processing are added to obtain the integration error (S
It is temporarily stored in the internal register 213 (RC) as C). The error distribution value GD and the data of the internal register 213 (RC) temporarily stored in the pixel processing are added to obtain a storage error corresponding to the pixel position D of the error storage means 201 as an integrated error (SD) of the pixel position D. Remember.

By such an error updating means 210, the storage device in the error storage means 201 can be accessed only by the read access corresponding to the pixel position A and the write access corresponding to the pixel position D, which is a readily feasible configuration. .

As described above, according to the present invention, the lower n bits of the binarization error Exy are extracted and distributed to one of the peripherals of the pixel of interest as the remainder error Exyl, and the lower n bits of the binarization error Exy are set to “n”. 0 "
By distributing the new binarization error Exyu, which has been compensated for, to the peripheral pixels according to the distribution coefficient, the truncation error generated during the calculation with the distribution coefficient is suppressed and the binarization error Exy and the peripheral pixels of the target pixel are distributed. The tone reproduction characteristics in the low and high density areas of the input level, which was a problem when the error diffusion method was configured with a practical integer arithmetic processing circuit by matching the sum of error distribution values, was greatly improved. The practical processing became possible.

[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, and FIG. 2 is an error distribution value calculating means, a distribution coefficient generating means, an error updating means and 2 in the apparatus.
FIG. 3 is a block configuration diagram of the binarization error distribution means, and FIG. 3 is a block configuration diagram of a main part of an image signal processing device for implementing the conventional error diffusion method. 101, 102 ... Error storage means, 110 ... Error distribution value calculation means, 11
1 ... Allocation coefficient generating means, 112 ... Error updating means, 113, 203 ...
Binary error distribution means, 206 ... Storage means, 211-213 ... Internal registers.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kiyoshi Takahashi Kiyoshi Takahashi 3-10-1 Higashisanda, Tama-ku, Kawasaki, Kanagawa Matsushita Giken Co., Ltd. (72) Toshiharu Kurosawa 3-chome, Higashisanda, Tama-ku, Kawasaki-shi, Kanagawa No. 10 No. 1 in Matsushita Giken Co., Ltd. (56) Reference JP-A-57-125579 (JP, A) JP-A-61-48275 (JP, A)

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 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 Exy which is a difference between the correction level and the binarization level, and extracting lower n bits from the binarization error Exy. Output as a remainder error Exyl, and further output the lower n bits of the binarization error Exy to "0".
And a binarization error distribution means for generating a new binarization error Exyu to be distributed to unprocessed pixels around the pixel of interest. Means, and an error distribution value for calculating an error distribution value corresponding to an unprocessed pixel around the target pixel from the new binarization error Exyu from the binarization error distribution means and the distribution coefficient from the distribution coefficient generation means. Arithmetic means, one of the error distribution values, and the residual error Ex from the binarization error distribution means
and an error updating unit for adding the accumulated error of the corresponding pixel position in the error storage unit and storing the result again.