JPH1127521A - Color image-processing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To significantly reduce UCR processing amount, while keeping image quality of a reproduced image by using a quantized achromatic color signal and a CMY signal, so as to apply UCR processings with respect to few gradation number. SOLUTION: A black signal K generated by an arithmetic equation K=2×min(C,M,Y)-max(C,M,Y) in a black generating circuit 1 is given to a 1st quantization circuit 2, where a weighted average of a quantization error stored in an error buffer is added to each CMY signal to correct it, and quantized to obtain a K' signal. Then the signal K' is outputted to a printer and to a 2nd quantization circuit 3, where a weighted average of a quantization error of an identical color stored in the error buffer is added to each CMY signal, corrected and quantized, and the signal K' is subtracted from the result, the result is subject to UCR processing, and outputted to the printer or the like as C', M', Y'. Thus, the UCR arithmetic amount is reduced and the signal K is directly quantized so as t make the texture less conspicuous.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image processing apparatus for performing a UCR process on a quantized gradation image.

[0002]

2. Description of the Related Art Conventionally, when CMY performs UCR on a full-color image of 8 bits each, as shown in FIG.
Black generation processing and UCR for each 8-bit CMY data
An operation is performed to calculate a CMYK signal. Thereafter, halftone processing is performed, and the data is requantized to the number of gradations that can be expressed by one dot of the printer.

However, it is wasteful to perform UCR processing on data having a greater number of tones than can be expressed by a printer, and the amount of UCR calculation is large and costs are high. Therefore, if the UCR process can be performed with the same number of gradations as that of the printer while maintaining the image quality, this is a very effective method.

For example, in a color image processing apparatus described in Japanese Patent No. 2603277, the number of gradations of image data is set to one.
When the CMY three colors become 1 in an area where the achromatic color component is large, the CM
The Y signal is replaced with the K signal and output.

[0005]

According to the above-described method, since black does not appear in a highly saturated color area, a reproduced image can be reproduced vividly without dulling. However, if the CMY signal is simply replaced with K, the texture looks dirty in the area where black ink is present, and there is a problem that the image quality is greatly deteriorated.

An object of the present invention is to perform a UCR process on an image signal having a small number of gradations, thereby greatly reducing the UCR operation amount and obtaining an excellent reproduced image without image quality deterioration. Is to provide.

[0007]

According to a first aspect of the present invention, there is provided an image processing apparatus comprising: a means for extracting an achromatic signal from a color multi-tone image signal; First means for quantizing the number of tones smaller than the number of tones of the signal, second means for quantizing the number of tones of the color image signal to a smaller number of tones, and The image processing apparatus further comprises means for correcting the output of the second means based on a coloring signal.

According to a second aspect of the present invention, the correction means subtracts the achromatic signal quantized by the first means from the color image signal quantized by the second means. I have.

According to the third aspect of the present invention, the first and the second
Is characterized in that an error generated by quantization is diffused to peripheral pixels.

[0010] The invention according to claim 4 is characterized in that the first quantization means performs quantization using a predetermined threshold value array.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be specifically described below with reference to the drawings. FIG. 1 shows the configuration of an embodiment of the present invention. In the figure, 1 is a black generation circuit for generating a K signal from a CMY signal, 2 is a first quantization circuit for quantizing the K signal, and 3 is a second quantization circuit for quantizing the CMY signal.

First, the black generation circuit 1 calculates the K
FIG. 2 shows a configuration of a black generation circuit. As shown in FIG. 2, the black generation circuit shifts the minimum value one bit to the left, with the minimum value detection unit 11 detecting the minimum value of the CMY signal, the maximum value detection unit 12 detecting the maximum value of the CMY signal. It comprises a shift circuit 13 and a subtraction circuit 14 for subtracting the maximum value from the output value of the shift circuit 13.

That is, the black generation circuit realizes the following arithmetic expression. K = 2 × min (C, M, Y) -max (C, M, Y) The generated black signal (K) is input to the first quantization circuit 2 in FIG. FIG. 3 shows a configuration of the first quantization circuit. The operation of the first quantization circuit will be described with reference to FIG. 3. The weighted average 22 of the quantization errors of the quantized pixels stored in the error buffer 21 is added to the input black signal (K). In addition, the input black signal (K) is corrected by the detector 23.
FIG. 5 shows an example of the weight for averaging the errors of the quantized pixels (seven pixels) around the target pixel.

Next, the correction signal is quantized by a quantizer 24. The relationship between the input and output of the quantizer has a step-like shape as shown in FIGS. In FIG. 6, an 8-bit input is quantized into a 4-bit output (16 values from 0 to 15),
In FIG. 7, the input of 8 bits is 1 bit (2 of 0 or 1).
Value).

The subtracter 25 calculates the difference between the correction signal and the quantized signal and stores the difference in the error buffer 21. The K 'signal quantized to N bits (N <8) by the quantizer 24 is output to a printer or the like. The first quantization circuit is
When the quantization level is 1 bit, as shown in FIG. 4, means for comparing a threshold array such as a dither mask and a blue noise mask with an image signal (K signal) to perform quantization may be used.

The K ′ signal is input to a second quantization circuit 3 for quantizing the CMY signal, and controls the operation of the second quantization circuit 3. The CMY signal quantized by the second quantization circuit 3 is output to a printer or the like.

FIG. 8 shows a configuration of a second quantization circuit for quantizing the CMY signal. FIG. 8 shows the second quantization circuit for the C signal, but the circuit configuration for the M and Y signals is exactly the same. An adder 33 adds a weighted average 32 of quantization errors of quantized pixels of the same color stored in an error buffer 31 to the C (M, Y) input image signal to correct the C input image signal. Do. The weights used in calculating the weighted average are those shown in FIG. 5, but other weights may be used.

Next, the correction signal 37 is supplied to the quantizer 34.
Then, the quantized K ′ signal is subtracted from the quantized C (M, Y) signal by the subtractor 36 (UCR processing). Further, the C (M, Y) correction signal 3
7 and the already-quantized C ′ (M ′, Y ′) obtained by subtracting the K ′ signal
The difference from the signal 38 is obtained and stored in the error buffer 31. Quantized and UCR-processed C ′ (M ′,
Y ') signal is output to a printer or the like.

As described above, since the UCR is performed with a small number of gradations, the amount of the UCR operation is greatly reduced, the cost is reduced, and the K signal is directly quantized instead of replacing the CMY signal with the K signal. As a result, the texture becomes less noticeable, and the image quality of the reproduced image is maintained.

[0020]

As described above, according to the first and second aspects of the present invention, the UCR process is performed on a small number of gradations using the quantized achromatic signal and CMY signal. The UCR processing amount is greatly reduced as compared with the related art. Moreover, instead of replacing the CMY signal with the K signal,
Since the signal is directly quantized, the texture becomes less noticeable, and the image quality of the reproduced image is maintained.

According to the third aspect of the present invention, since the error caused by the quantization is diffused to the peripheral pixels, the density of the image is preserved, and the high gradation property of the area printed including black is improved. Color reproduction is guaranteed.

According to the fourth aspect of the present invention, the cost of quantizing the K signal is eliminated.

[Brief description of the drawings]

FIG. 1 shows a configuration of an embodiment of the present invention.

FIG. 2 shows a configuration of a black generation circuit.

FIG. 3 shows a configuration of a first quantization circuit.

FIG. 4 shows an example of a dither mask for binarization.

FIG. 5 shows an example of weights for weighted averaging of errors.

FIG. 6 shows a first example of input / output characteristics of a quantizer.

FIG. 7 shows a second example of input / output characteristics of a quantizer.

FIG. 8 shows a configuration of a second quantization circuit.

FIG. 9 is a diagram illustrating conventional black generation and UCR processing.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Black generation circuit 2 First quantization circuit 3 Second quantization circuit

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Claims (4)

[Claims] A means for extracting an achromatic signal from a color multi-tone image signal; a first means for quantizing the achromatic signal into a number of gradations smaller than the number of gradations of the color image signal; A second means for quantizing the number of gradations of the color image signal to a smaller number of gradations; and a means for correcting the output of the second means based on the quantized achromatic signal. A color image processing device characterized by the above-mentioned. 2. The apparatus according to claim 1, wherein said correction means subtracts an achromatic signal quantized by said first means from a color image signal quantized by said second means. Color image processing device. 3. The color image processing apparatus according to claim 1, wherein said first and second quantization means diffuse an error generated by quantization to peripheral pixels. 4. The color image processing apparatus according to claim 1, wherein said first quantization means performs quantization using a predetermined threshold value array.