JPH0530108B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is characterized in that the saturation and density of chromatic colors are lower than that of the undercolor due to the overlap of black color with other colors and increase in white color after undercolor removal. The present invention relates to an undercolor removing device that prevents the undercolor from being lowered compared to before removal.

[Background technology]

Traditionally, in the field of color printing and color hard copies, color reproduction has been performed using four color materials, by adding K (black) to the three color materials Y (yellow), M (magenta), and C (cyan). There is. This color reproduction method is based on the fact that when three color materials Y, M, and C are reproduced by subtractive color mixing, the portion where Y, M, and C overlap is basically reproducing black. This is an attempt to reproduce the black component using a K colorant, and the parts using K are achieved by subtracting the amount equivalent to K from the amount of Y, M, and C colorants. be able to.

For example, as shown in FIG. Each correction data Y′, M′, by subtracting from the amount
C' can be obtained as shown in Figure 7b.

By the way, the image shown in Fig. 8a is removed by removing the undercolor using the above-mentioned correction means (in order to correct the poor adhesion of afterprint ink during overprinting, the black in the shadow part where three colors overlap is removed). The image shown in FIG. 8b can be obtained by replacing the black ink with black ink and expressing it with a single black color, and the black in the shadow portion is immediately converted to black. In addition, in the figure, Y,
R, K, and W represent yellow, yellow+magenta, yellow+magenta+cyan, and white, respectively, and BK represents black.

Furthermore, regarding this type of device, Japanese Patent Application Laid-Open No. 1983
-There is No. 161981.

[Problem that the invention seeks to solve]

However, in the conventional undercolor removal device, the eighth
As shown in Figure b, when the black color material overlaps with the Y, R, and K pixels, and when the white color increases,
There is a problem that the image quality deteriorates because the saturation of the chromatic color and the density of the chromatic color decrease compared to the color before removing the undercolor.

[Means for solving problems]

The present invention has been made in view of the above, and in order to prevent the decrease in saturation and density of chromatic colors due to removal of undercolor, undercolor removal processing is applied unconditionally to achromatic colors, and undercolor removal processing is applied unconditionally to achromatic colors. An object of the present invention is to provide an undercolor removal device in which an undercolor removal amount is applied to a color so that as the saturation increases, the amount of black component decreases.

[Effect]

According to the undercolor removal device of the present invention, the amount of black component that is reduced from each signal of Y, M, and C is calculated using the black component calculated based on the minimum value and maximum value of the signal amount of Y, M, and C. Undercolor removal is performed to prevent a decrease in saturation and density of chromatic colors.

〔Example〕

Hereinafter, the undercolor removing device according to the present invention will be explained in detail.

FIG. 1 shows an embodiment of the present invention, in which Y ₁ , M ₁ ,
A minimum value detection circuit 1 that detects the minimum value of each color signal of C 1 , a maximum value detection circuit 2 that detects the maximum value of each color signal of _{Y 1 ,} M ₁ , C ₁ , a black component calculation circuit 3 that calculates the black signal amount K ₂ based on the detection output of the value detection circuit 1; and the circuit 3.
A correction value calculation circuit 4 that calculates a black component amount K ₁ to be reduced from each of Y ₁ , M ₁ , and C ₁ based on the output signal K ₂ of
and a subtraction circuit 5 that subtracts the black component amount K ₁ from each signal of Y ₁ , M ₁ , and C ₁ to calculate each of Y ₂ , M ₂ , and C ₂ ,
Consists of 6 and 7.

In the above configuration, each color of B (blue), G (green), and R (red) is converted.
Minimum value K _MIN and maximum value K _MAX of each signal Y ₁ , M ₁ , and C ₁ are detected by minimum value detection circuit 1 and maximum value detection circuit 2 . Here, K _MIN = MIN (Y ₁ , M ₁ , C ₁ ), and K _MAX = MAX (Y ₁ , M ₁ , C ₁ ).

K _MIN and K _MAX are input to the black component calculation circuit 3,
The circuit 3 calculates the following equation.

K ₂ = K _MIN − _{f RI} (K _MAX − K _MIN ) (However, f _RI (K _MAX − K _MIN ) is a function that takes into account the magnitude of the saturation of chromatic colors, and RI ₁ (K _MAX − K _MIN ) is expressed as, for example, r ₁ (K _MAX - _{K MIN} ), where r ₁ indicates a coefficient) The signal amount K ₂ output from the black component calculation circuit 3 is input to the correction value calculation circuit 4, and the corresponding Circuit 4 has Y ₁ ,
Black component amount to be subtracted from each signal amount of M ₁ and C ₁
Calculate K ₁ . This black component amount K ₁ is, for example, K ₁
= signal amount expressed as r ₂ × K ₂ (r ₂ is a coefficient),
Based on this black component amount _K1 , each of the subtraction circuits 5, 6, and 7 produces correction signals of _Y2 = _Y1 - _K1 , _M2 = _M1 - _K1 , and _C2 = _C1 - _K1. Output.

The process of the present invention will now be described in detail with reference to FIGS. 2a to 2d. First, as shown in Figure 2 a,
Among Y ₁ , M ₁ , and C ₁ , the maximum value and the minimum value (K _MAX =Y ₁ , K _MIN =C ₁ ) are detected. Next, as shown in Figure 2b, (K _MAX - K _MIN ) is calculated, and since (K _MAX - _{K MIN} )≠0, it is not an achromatic color, but it is not a very saturated color. to judge. This corresponds to the processing of the black component calculation circuit 3. Furthermore, as shown in Figure 2c,
Set the coefficient of K ₂ as r ₁ =0.5. Since this is a chromatic color, the amount of black component is set smaller than in the case of an achromatic color. Next, the second
As shown in FIG. d, Y ₂ , M ₂ , and C ₂ are calculated by setting the coefficient of K ₁ for calculating the correction value to, for example, r ₂ =0.5.

Figures 3a and 3b show color pixel diagrams when undercolor removal is performed in the present invention, and Figure 3a
corresponds to FIG. 7a. Figure 3b and Figure 7b
As is clear from the comparison, the number of black dots remains the same (3 points), and Y and R dots that do not overlap with the black dots occur separately, resulting in a decrease in the saturation of chromatic colors. can be made less noticeable. Furthermore, since there are fewer pixels in the W (white) portion than in the past, it is possible to suppress a decrease in density.

Figures 4a and b show the case where undercolor removal is performed on an achromatic signal. In this case, Y ₁ = M ₁ = C ₁ , so K _MAX = K _MIN , and K ₂ = K _MIN , K ₁ =
1/3K _MIN . However, as shown in FIG. 4b, since K ₂ is directly output as a black component, there is no change in the number of pixel points, and black is reproduced as shown in FIGS. 5a and 5b.

Figures 6a and b show undercolor removal for signals with high saturation. In the case of highly saturated colors, (K _MAX - K _MIN ) increases as shown in Figure 6 a, so K ₂ =
fR ₁ of the second term in the K _MIN − fR ₁ (K _MAX − K _MIN ) formula
(K _MAX − K _MIN ) increases. Therefore, the amount of K ₂ , that is, the black component, decreases, and the signal amounts of Y ₂ , M ₂ , and C ₂ do not change. Therefore, as the saturation of a chromatic color increases, the amount of black component decreases, and undercolor removal is not performed for colors with high saturation as shown in FIG.

〔Effect of the invention〕

As explained above, according to the undercolor removal device of the present invention, the undercolor removal process is applied unconditionally to achromatic colors, and the amount of black component is applied to chromatic colors as the saturation increases. Since the amount of undercolor removal is changed so that the amount of undercolor is reduced, it is possible to remove undercolor while suppressing a decrease in density and saturation. Furthermore, by varying the coefficients and start points, adjustments can be made depending on the image, and image quality can be improved.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention;
FIGS. 2a, b, c, and d are explanatory diagrams showing the undercolor removal process according to the present invention. FIGS. Explanatory diagram of black component calculation when inputting achromatic color in the invention, No. 5
Figures a and b are illustrations of undercolor removal when achromatic color is input in the present invention, Figures 6a and b are illustrations of black component calculation when high chroma color is input in the present invention, and Figures 7a and b are illustrations of conventional color removal. An explanatory diagram of correction data calculation in color removal processing, and FIGS. 8a and 8b are explanatory diagrams of conventional undercolor removal. Explanation of symbols: 1... Minimum value detection circuit, 2... Maximum value detection circuit, 3... Black component calculation circuit, 4... Correction value calculation circuit, 5, 6, 7... Subtraction circuit.

Claims (1)

[Scope of Claims] 1. An undercolor removal device that performs undercolor removal processing by subtracting a black component from each color other than black when reproducing colors using four colors: yellow, magenta, cyan, and black, a minimum value detection circuit for detecting the minimum value in each of the yellow, magenta and cyan signals; a maximum value detection circuit for detecting the maximum value in each of the yellow, magenta and cyan signals; and the maximum value detection circuit. Saturation is detected based on the difference between the output and the output of the minimum value detection circuit, and an adjustment amount is determined so that the black component amount decreases as the detected saturation increases, and this adjustment amount is a black component calculation circuit that calculates a black component amount by subtracting it from the output of the minimum value detection circuit; and a black component calculation circuit that multiplies the output signal of the black component calculation circuit by a predetermined coefficient and subtracts it from each of the yellow, magenta, and cyan. A correction value calculation circuit that calculates the expected black component amount, and a subtraction unit that subtracts the output value of the correction value calculation circuit from the yellow, magenta, and cyan color signals are provided, and the adjustment value is set to the maximum value and the minimum value. An undercolor removing device characterized in that the difference value from 0 increases from 0 and takes a value from 0 to a minimum value.