JPH02126774A - Color picture processing method and its device - Google Patents

Abstract

PURPOSE:To execute excellent compression so as not to generate color blurring by determining a tone curve and a masking coefficient for compression in a system such as B, G and R density so as to compress and reproduce luminance and chromaticavers without changing a hue in a uniform color spatial system. CONSTITUTION:A color picture input part 2 to read an original 1 of color and to output the density signals of blue (B), green (G) and red (R) original colors, a picture processing part 3, etc., are provided. The original 1 is compared with the color reproducing range of a copy in a uniform color spatial system, without converting the signal of the inputted B, G and R density system, etc., to the signal of the other system, the tone curve and masking coefficient for compression obtained by linearly compressing both luminance and chromaticavers without changing a hue are determined. Thus, the color blurring due to compression is eliminated, and the compression characteristic to obtain the excellent picture quality can be easily determined.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a color reproduction method and apparatus when the color reproduction range of an original is wider than the color reproduction range of a copy.

(Conventional technology) The basis of color reproduction is faithful reproduction, but the color reproduction range of the original (
In other words, if the range of colors that can be expressed is wider than that of the copy, part of that wide range cannot be faithfully reproduced and will be distorted.

In the past, this type of problem has been studied primarily as a tone compression problem in the density system, from the perspective of determining the best tone curve, and empirically it has been found that an upwardly convex curve is best. It is said.

However, when considering color reproduction, it is difficult to understand at first glance the correspondence between changes in tone curves and colors. , and had to deal with gradation and color issues separately.

In order to solve this problem, in recent years, a method of converting from the BGR (density) system to a uniform space (L"2*b fist, L*u/V fist, etc.) system and compressing the colors has been studied.

However, this method has the disadvantage that it is necessary to additionally provide a new conversion circuit, which increases the number of circuits and increases the cost. Conversion errors also become a problem.

(Problems to be Solved by the Invention) The purpose of the present invention is to convert the BC;R (concentration) system into a uniform space (L).
・Providing a color image processing method and device that can perform good compression without the above-mentioned collapse without adding a new conversion circuit to convert the image into a ``a''b'' + L''u'' V fist, etc.) system. It's about doing.

(Means for Solving the Problems) The color image processing method of the present invention compresses the brightness and saturation without changing the hue in a uniform color space (L*u*v*) system. B so that I can reproduce it.

It is characterized by determining the compression tone curve and masking coefficient based on other systems such as G and R densities.

According to one aspect of the color image processing method of the present invention, the tone curve for compression is determined so as to linearly compress two points: the highest lightness of gray between the original and the copy or print, and the lowest lightness of gray. .

According to one aspect of the color image processing method of the present invention, the compression masking coefficient is determined based on a tone-compressed signal of the original and an output signal expressed by compressed chromaticity.

The color image processing apparatus of the present invention for carrying out the above method includes isoeugenic density conversion means, tone compression means, and compression masking means.

The isoeugenic density conversion means transforms the grayscale blue, green, and red densities into a grayscale luminous density camphor projection so that when the input image is gray, the densities of blue, edges, and red are equal. It is to be adjusted accordingly.

The tone compression means calculates, for the density signal of the BGR input image subjected to isoeugenic density conversion, the maximum brightness L2 of gray in the color reproduction range of the input system and the maximum brightness L2 of gray in the color reproduction range of the output system, 111, and the tone curve whose characteristics are set so as to linearly compress the two points: the lowest gray lightness L1 in the input system color reproduction range and the lowest gray lightness L1-° in the output system color reproduction range. The concentration is changed accordingly.

The compression masking means uses the chromaticity L ``a'' of the input system.
The output of the tone compression means has a compression masking coefficient determined based on the data of b. and the chromaticity L$1a41b$1 of lightness and saturation compression calculated from the following equations (1) and (2). The tone compressed RGB density signals are compressed using the coefficients to produce compressed yellow (Y) and magenta (M
), the output color material amount of cyan (C) is converted into a density signal.

L"=[:(Ll"-L,・')/(Ll"-L 1 fist)](L・-L +") + L t"
・・・・・・(1) 2*=ka middle, b fist'=k
b skewer (2) Here, L ” a ” b ”: chromaticity of the original.

Cll + chroma Ll of the original; constant L11 representing the minimum brightness of the original; constant L*' representing the minimum brightness of the copy; constant L21 representing the maximum brightness of the original; to a constant representing the maximum brightness of the copy; after brightness compression The average value (effect) of the ratio C''/C of the maximum saturation for each lightness and hue of the original and the maximum saturation of the copy (effect) Figure 4 shows the color reproduction range of the original and the copy in a uniform color space system. In the diagram shown, the horizontal axis represents the saturation CI and the vertical axis represents the lightness L. This shows the difference between the color reproduction range of the original and the color reproduction range of the copy.Compression results in a change in the color reproduction range of the original. However, in actual processing, signal processing using a uniform color space system is not performed, and B, G, R Performed by equivalent signal processing that converts to other color systems such as density.B, G.

Parameters for compression in other color systems such as R density consist of a tone curve for compression and a masking coefficient for compression.

The example of the four-quadrant graph in FIG. 2 is a diagram for determining a compression tone curve, and the first quadrant is the compression tone curve. In the same figure, the second quadrant is the gray scale density of the copy DOLl? The relationship between (END) and brightness L-0° is obtained by measurement and illustrated. The fourth quadrant is the density DIN (END) of the gray scale of the original and the brightness L.
1H relationship is determined by measurement and illustrated. The third quadrant shows a straight line for linearly compressing the original grayscale lightness L"IN to the copy grayscale lightness L"out, which is the maximum lightness of the original. and minimum brightness 1. ．． 11 and the maximum brightness L21 and minimum brightness L21 of the copy gray scale, Fist +.

The tone curve for brightness compression in the first quadrant can be determined based on the data in the second to fourth quadrants.

FIG. 3 is a diagram showing an example of a procedure for determining a compression masking coefficient.

■ Create the chromaticity data of the original by actual measurements in L”2*b equal steps.

■ BGR by interpolation based on the measured data Lsa-b fist
Calculate W degree.

(2) The calculated BGR density is subjected to equal eugenic density conversion (END conversion), and tone compression is performed using the compression tone curve shown in FIG.

■ Compressed chromaticity data L 111a1) lb@1 as described above (
Calculated using equations 1) and 2).

■ Compressed chromaticity data L0゛a$lb obtained in step ■
Calculate YMC% (YMC dot area ratio) from $1.

■ Convert YMC% to YMC concentration amount by inverse TRC conversion.

■ Determine the compression masking coefficient by the least squares method based on the values obtained in steps ■ and ■.

As described above, the present invention is based on a uniform color space (L”2*b*
, L * uda v * ) system to compare the color gamut of the original and the copy, and use the BGR density system signal as it is without converting the human-powered BGR density system signal to another system, but change the hue. Since the tone curve for compression and the masking coefficient for compression are determined by linearly compressing both brightness and saturation without any distortion, the compression error is improved and good image quality can be obtained.

(Embodiment) FIG. 1 is a plot diagram showing an embodiment of a color image processing apparatus that performs compression using a compression tone curve and masking obtained by implementing the method of the invention.

This device reads the color original 1 and reads blue (B), green (
A color image input unit 2 outputs density signals of the three primary colors of G), red (R), and a color image input unit 2 that outputs density signals of the three primary colors of yellow (Y) and magenta (M
), an image processing unit 3 converts bitmap data to YMC corresponding to the amount of color material of cyan (C), and performs compression processing on brightness and saturation without changing the color and phase; , laser xerography, inkjet,
It consists of a color image output section 4 that outputs a color copy 5 using thermal transfer or the like. Note that the color image output section may be a film exposure machine that outputs to black and white film, such as a color scanner for printing.

The color image processing section 3 includes an END (equal eugenic density) conversion circuit 31, a tone compression circuit 32, a compression masking circuit 33, and a TRC (tone area ratio conversion) circuit 34.

The END conversion circuit 31 performs equal-eugenic density processing on the density signal input from the color image input section 2. This isoeugenic density processing transforms grayscale red, green, and blue densities projectively into grayscale visual densities so that when the input document is gray, the red, green, and blue densities are equal. adjust.

The tone compression circuit 32 converts two points, ie, the maximum brightness of gray between the original and the copy, and the minimum brightness of the gray, as shown in FIG.
END conversion times v8 according to the density compression characteristics of the first quadrant of FIG.
3! This compresses the density signal input to the .

Note that the END conversion circuit 31 and tone compression circuit 32 are
Each may be an independent circuit, but their END
The configuration can be simplified by combining the conversion characteristics and tone compression characteristics into one lookup table using ROM.

The compression masking circuit 33 performs masking processing on the brightness-compressed signal supplied by the tone compression circuit 32.

This masking process is a normal masking process for correcting color turbidity caused by the influence of unnecessary absorption density of the output coloring material used in the color image output unit 4, for example, 3
In addition to normal matrix masking processing, which corrects color turbidity by ×3 or 3 × 9 matrix calculations and converts the input signal into signal amounts corresponding to the output color materials of yellow, magenta, and cyan, the present invention has the following features: It also performs compression processing. The compression masking coefficient is determined by the procedure shown in FIG. 3 described above.

The TRC circuit 34 is a circuit that converts the density signal output from the compression masking circuit 33 into a signal amount corresponding to the dot area ratio, and converts the density signal output from the compression masking circuit 33 into the corresponding dot area ratio. It is configured as a look-up table stored in ROM from which the corresponding signals are read.

An example of copying the results of compression by the color image processing apparatus of this embodiment configured as described above from a color slide (here, chromin of a proof print is taken as an example) is shown in Figure t). explain.

Figure 5 (a), (b), and (c) show the color reproduction range of color slide, and Figure 6 (a), (b), and (c) show the color reproduction range of copy (chromalin) in L*a*b space. FIGS. 7(a), 7(b), and 7(C) show the color reproduction range when brightness and saturation are compressed for color slide input using the apparatus of this embodiment. Comparing FIG. 6 with FIG. 7, it can be seen that by the compression of the present invention, most of the brightness and saturation are compressed within the color reproduction range of the copy.

In addition, compressed samples were created from the color slides using the color image processing device of this example, and the reproducibility of low brightness areas was improved.
Visual evaluation was performed focusing on the good appearance and fidelity to the original, and both were found to be good.

(Effects of the Invention) As described above, the method of the present invention provides uniform color space (L
b*, L*u*y*) system to compare the color reproduction range of the original and the copy, and input BGR density system signals as they are without converting them to other systems. Since the tone curve for compression and the masking coefficient for compression are determined by linearly compressing both the brightness and saturation without changing the hue, it is possible to easily determine the compression characteristics to obtain good image quality without any distortion due to compression. It is also possible to easily change and adjust its compression characteristics.

Further, the color image processing device according to the present invention can perform BGR density processing without converting from a BGR density system to a uniform color space system.
Since the compression tone curve and compression masking coefficient are set while maintaining the 21m degree system, there is no need to add a new conversion circuit, and the configuration is simplified.

Furthermore, according to the present invention, deviation from the target chromaticity is small (compression accuracy is also excellent).

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the general configuration of a color image processing apparatus according to an embodiment of the present invention. Figure i2 shows an example of a compression tone curve used in the present invention. Figure 3 is a diagram showing the procedure for determining the compression masking coefficient used in the present invention. FIG. 4 is a diagram showing the color reproduction range of an original and a copy. FIGS. 5(a), 5(b), and 5(c) are diagrams showing the color reproduction range of the color slide. FIGS. 6(a), 6(b), and 6(c) are diagrams showing the color reproduction range of copies. FIGS. 7(a), 7(b), and 7(c) are diagrams showing the color reproduction range obtained by compressing the color reproduction range of a color slide according to the present invention. DESCRIPTION OF SYMBOLS 1... Original document, 2... Color image input section, 3... Color image/processing section, 31... END conversion circuit, 32.
... Tone compression circuit, 33 ... Compression masking circuit, 34 ... TRC circuit, 4 ... Color image output section,
5...Copy Patent Applicant Fuji Xerox Co., Ltd. Agent Patent Attorney Noboru Iwawa Figure Figure C* Figure (a) ■ (a) Figure Figure (a) Umbrella♂ Procedure Procedures Written (spontaneous) March 14, 1989

Claims (4)

[Claims] (1) When copying or printing from an original, reproduce the brightness and saturation in a uniform color space (L^*a^*b^*, L^*u^*v^*) system without changing the hue. An image processing method characterized by determining tone curves and masking coefficients for compression based on other systems such as B, G, and R densities so as to enable image processing. (2) A color image processing method characterized in that the tone curve for compression is a tone curve that linearly compresses two points: the highest lightness of the original and the gray of the copy or print, and the lowest lightness of the gray. . (3) A color image processing method characterized in that the masking coefficient for compression is determined by a signal after tone compression of the original and an output signal expressed by compressed chromaticity. (4) Projectively convert grayscale blue, green, and red densities to grayscale visual densities, and when the input image is gray, blue (B), green (G), and red (R) an equivalent neutral density conversion means that adjusts the densities to be equal; The highest gray lightness L^*_2 in the color reproduction range of the output system, and the lowest gray lightness L^*_1 in the input system color reproduction range and the lowest gray lightness L^*_1 in the output system color reproduction range A tone compression means that changes the density according to a tone curve whose characteristics are set to linearly compress the two points of
That is, L^*'=[(L_2^*'-L_1^*')/(L_
2^*-L_1^*)] (L^*-L_1^*)+L_
1^*'a^*'=ka^*, b^*'=kb^* However, L^*a^*b^*; chromaticity of the original, c^*; saturation of the original L_1^*; Constant L_1^*' representing the minimum brightness of the original; Constant L_2^* representing the minimum brightness of the copy
Constant L_2^*' representing the maximum brightness of the original; Constant k representing the maximum brightness of the copy; Ratio C^*' of the maximum saturation of each brightness and hue of the original after brightness compression and the maximum saturation of the copy Chromaticity L^*'a^ of brightness and saturation compression calculated from the average value of /C^*
The tone compression R output from the tone compression means has a compression masking coefficient determined based on *'b^*'.
Compression masking means for compressing the GB density signal using the coefficient and converting it into density signals of compressed output color material amounts of yellow (Y), magenta (M), and cyan (C). Color image processing device.