JPH09238264A - Color conversion settings - Google Patents

Abstract

(57) Abstract: A color conversion relationship for converting a color signal from a first color system to a second color system can be obtained with high precision and with very little effort, and various color reproductions are possible. Provided is a color conversion relation setting method capable of easily responding to characteristics. SOLUTION: A color patch YMC is measured at predetermined intervals to obtain a relationship between a color signal YMC, a lightness index L ^* , and perceived chromaticities a ^* , b ^* (step S10), and the relationship is subjected to multiple regression. By analyzing and obtaining the color signal for which the interpolation process is preferentially performed in the ascending order of the change (step S20), the interpolation process is performed in accordance with the priority order (step S3
0), create a highly accurate color conversion table (step S
50).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color conversion relationship setting method for setting a color conversion relationship for converting a color signal from a first color system to a second color system.

[0002]

2. Description of the Related Art When a color image is recorded using an output device such as a color printer or a printing machine, for example, a color signal Y
By operating the MCK, it is possible to realize a color image having a desired color. However, the color of the color image depends on the color reproduction method of the output device (the method of generating a gradation image by halftone dot modulation, the method of generating a gradation image by density modulation, etc.) and the characteristics of the output medium. It is necessary to generate the color signal YMCK that has been subjected to the color conversion processing in consideration of the color reproduction characteristics including the above-mentioned methods and characteristics.

Therefore, for example, input color signals RGB
Is converted into a common color space composed of stimulus value signals XYZ of the CIE standard color system, and then the color reproduction characteristics are output by an output device defined in the common color space, so that it does not depend on an input / output device or an output medium. A processing system capable of obtaining a color image is desired.

For example, a color signal YMCK of the color patch is generated by creating a plurality of color patches of different colors using the target output device and output medium and measuring the color.
To a stimulus value signal XYZ in a common color space that does not depend on the output device and the output medium (hereinafter, this conversion relation is referred to as “forward conversion relation”), and then from the forward conversion relation, the stimulus value signal is obtained. A conversion relationship for converting XYZ into a color signal YMCK (hereinafter, this conversion relationship is referred to as “inverse conversion relationship”) is obtained, and the stimulation value signal XYZ is used by using this inverse conversion relationship.
Is converted into the color signal YMCK, the color signal YMCK having the color reproduction characteristics of the output device and the output medium can be obtained.

[0005]

FIG. 8 shows a known color signal YM in a color patch created at regular grid intervals.
The forward conversion relationship and the inverse conversion relationship between the CK and the stimulus value signal XYZ are two-dimensionally displayed for the sake of convenience. The color signal YMCK at each regular grid point and the stimulus value signal X at the intersection of an irregular curve.
It corresponds to YZ. In this case, in the color space of the stimulus value signal XYZ, the intervals of the intersections are very irregular, and therefore, for example, the color signal Y for the stimulus value signal XYZ at the point A
When calculating the MCK, the interpolation calculation should be performed using the color signals YMCK of A1, A2, A3, and A4 points, but the interpolation calculation should be performed using the color signals YMCK of A2, A3, A4, and A5 points close to the A point. There is a risk that In order to avoid such a problem, it is necessary to obtain the relationship between the color signal YMCK and the stimulus value signal XYZ using as many color patches as possible.
In When you create a color patch, it becomes necessary to 11 ^four color patch.

On the other hand, when the number of color patches is increased,
The problem arises that the measurement requires a considerable amount of time. The color reproduction characteristics include not only the characteristics of the output device but also the characteristics of the output medium. That is, the characteristics of the output medium include the type of output paper, the type of color material used, etc.
Since it depends on a very large number of factors, it would take a longer time to measure the color patch in an attempt to address all of them. Moreover, the inverse transformation relationship created in this way is described in, for example, Japanese Patent Laid-Open No. 3-13.
As described in Japanese Patent Laid-Open No. 1920, if it is attempted to hold it as a print color conversion table, a very large amount of data will be required, which is not practical in practice.

In view of the above, the present invention solves the above-mentioned problems, and requires a highly accurate and highly accurate color conversion relationship for converting a color signal from the first color system to the second color system. It is an object of the present invention to provide a color conversion relation setting method capable of achieving various color reproduction characteristics easily.

[0008]

SUMMARY OF THE INVENTION The present invention provides a first color signal.
A method of setting a color conversion relationship for converting from a color system to a second color system, the first step of obtaining a color signal of the first color system for a predetermined color signal of the second color system; , A color conversion relationship between the component colors is obtained by performing interpolation processing in the ascending order of change of each component color of the first color system with respect to a predetermined amount change of each component color of the second color system. It is characterized by comprising a second step and a third step of obtaining a color conversion relation in the remaining color space by interpolation processing using the color conversion relation obtained in the second step.

In the present invention, after the conversion relationship between the color signals of the second color system and the color signals of the first color system is obtained at desired intervals, the change in the component color of each color system in the conversion relationship is obtained. Since the interpolation processing is performed in ascending order to obtain a detailed conversion relationship, it is possible to obtain a highly accurate conversion relationship. Then, the conversion relationship between the first color system and the second color system can be created by performing interpolation processing on the remaining color space using the conversion relationship.

[0010]

1 shows a schematic configuration of a color conversion apparatus 10 to which a color conversion relation setting method of the present invention is applied. This color conversion device 10 converts a color signal such as RGB supplied from an external input device 12 into a lightness index L ^* and a perceptual chromaticity a ^* , b ^* which are color components of a common color space, and then performs color conversion. Using a table, the lightness index L ^* , perceived chromaticity a ^* , b ^*
The color conversion unit 16 converts the color signal YMCK according to the color reproduction characteristic of the output device 14. In this case, the color conversion table is created by the table creation unit 18 and stored in the color conversion table storage unit 20. The color conversion table stored in the color conversion table storage unit 20 is
It is selected by the table selection unit 22 according to the color reproduction characteristic and supplied to the color conversion unit 16.

The color conversion device 10 is basically constructed as described above. Next, a method of creating a color conversion table in the color conversion device 10 will be described with reference to the flow chart shown in FIG. . In the following description, a color conversion table is created for the color signals YMC in the three-dimensional color space, but a color conversion table can be similarly created for a four-dimensional color space including the color signal K.

First, the case of creating a color conversion table for typical color reproduction characteristics will be described. in this case,
If the output device 14 is a printing machine, its color reproduction method is, for example, halftone dot modulation, and the inherent color space is a color signal Y of halftone%.
Represented by MC. Further, the characteristics of the output medium are determined by the characteristics of the printing paper and printing ink used.

Therefore, in the output device 14, the color signal YMC is set at 20% intervals and a color patch is created using the printing paper and the printing ink according to printing conditions such as a predetermined screen ruling.

In this case, in the CMY three-dimensional color space shown in FIG. 3, the color patch is, for example, a plane α (a1
-A2-a3-a4), plane β (a1-a2-a6-a)
5), plane γ (a2-a3-a7-a6), plane δ (a
5-a6-a7-a8), plane ε (a4-a3-a7-
a8), plane ζ (a1-a4-a8-a5), plane η
(A2-a3-a8-a5), plane θ (a2-a4-a
8-a6), the plane ι (a1-a2-a7-a8) and the gray axis κ (a2-a8) on the plane γ (a2-a8).
Create by setting 0% intervals. FIG. 4 shows the plane α (a1-
a2-a3-a4) represents the color signal CY for creating the color patch set on the black circle.

The color patch created as described above is color-measured using a colorimeter, and the correspondence relationship between the color signal YMC and the lightness index L ^* of the common color space and the perceived chromaticities a ^* and b ^*. Is required (step S10).

Next, the tendency of increase / decrease in the lightness index L ^* and the perceived chromaticities a ^* , b ^{* on} each of the planes α to ι and the gray axis κ on which the color patch is created is analyzed by multiple regression analysis or principal component analysis. It is obtained by using (step S20).

For example, FIGS. 5A to 5F show the relationship between the color signal CY on the plane α (CY plane) obtained by multiple regression analysis and the lightness index L ^* and the perceived chromaticities a ^* and b ^*. is there. In FIG. 5A, assuming that the dot% of the color signal K is 0%, the color signal Y
Represents the relationship between the color signal C and the lightness index L ^* when the values are fixed to 10, 30, 50, 70 and 90% respectively from the top. Further, in FIG. 5B, the dot% of the color signal K is 0%,
Color signal C is 10, 30, 50, 70, 90% from top to bottom
The relationship between the color signal Y and the lightness index L ^* in the case of being fixed to each of the above is shown. In this case, comparing FIG. 5A and FIG. 5B, as shown in FIG. 5B, L ^{* with} respect to the increment of the color signal C is shown.
Is larger than that of the color signal Y shown in FIG. 5A. Therefore, regarding the lightness index L ^{* on} the plane α (CY plane), as shown in FIG. 6, after the color signal Y is interpolated first to obtain the correspondence with the lightness index L ^* , the color signal C
If the interpolation process is performed, the lightness index L ^* between each color patch
The relationship with respect to can be obtained with high accuracy. Similarly,
Regarding the perceptual chromaticity a ^* , the interpolation processing of the color signal Y is preferentially performed based on the relationship between FIGS. 5C and 5D, and regarding the perceptual chromaticity b ^* , one of the color signals based on the relationship between FIG. 5E and FIG. 5F. Interpolation processing is performed by arbitrarily selecting CY.

Next, the color signal YM obtained as described above.
In the order of priority of C, the relationship between the color signal YMC on each plane α to ι and the gray axis κ and the lightness index L ^* and the perceptual chromaticities a ^* and b ^* is obtained by interpolation processing (step S30). In this case, in the order of the interpolation processing, the gray axis κ where the influence of the color shift is most significant is given the highest priority, then the interpolation processing of the planes α to ζ is performed, and then the interpolation processing results of the planes α to ζ are obtained. Is used to interpolate the planes η to ι.

After the respective planes α to ι and the gray axis κ are interpolated as described above, the conversion relation obtained in the remaining space portion is obtained using the obtained conversion relation (step S40). This conversion relation is, for example, the plane α to ι.
And the lightness index L ^* and the perceived chromaticities a ^* and b ^* corresponding to the color signal YMC on the gray axis κ can be obtained by volume interpolation. That is, as shown in FIG. 7, the lightness index L ^* _j , the perceived chromaticities a ^* _j , b ^* _j (j = 0 to 0) corresponding to the color signal YMC at the eight lattice points c0 to c7.
7), the volume V of the solid surrounded by the grid points c0 to c7, and the volume V0 divided into eight by any interpolation point c in the solid.
With V7,

[0020]

[Equation 1]

Can be obtained as

The relationship between the color signal YMC and the lightness index L ^* and the perceptual chromaticities a ^* and b ^* obtained as described above is stored in the color conversion table storage section 20 as a typical color conversion table ( Step S50). A plurality of typical color conversion tables obtained as described above are set according to the color reproduction method of the output device 14, the characteristics of the output medium, the printing conditions, and the like.

Here, when the color conversion table is created as described above, it is not necessary to prepare a large number of color patches, so the working efficiency is significantly improved. Further, when a detailed conversion relationship is obtained based on the conversion relationship obtained from the color patch roughly set as described above, priority is given to a color signal that is less likely to cause an error. Therefore, the accuracy of the conversion relationship obtained by the interpolation processing is high. Will be fully guaranteed. As a result, a highly accurate conversion relationship can be obtained with a small amount of labor.

Next, the processing using the color conversion table set as described above will be described. In this case, in FIG. 1, the color signal supplied from the input device 12 is converted into a lightness index L ^* and a perceptual chromaticity a ^* , b ^* which are color signals in a common color space in the color conversion unit 16 of the color conversion device 10. To be converted. On the other hand, the table selection unit 22 reads out the color conversion table according to the color reproduction method of the output device 14, the output medium, the printing conditions, etc. from the color conversion table storage unit 20, and the color conversion unit 1
6 Therefore, the color conversion unit 16 converts the lightness index L ^* and the perceptual chromaticities a ^* and b ^* into the color signal YMC by using the selected color conversion table, and supplies the color signal YMC to the output device 14. The output device 14 outputs a desired image based on the color signal YMC.

Next, another embodiment of the color conversion relation setting method will be described. In this case, a single color signal YMC
It is known that the relationship expressed by the following expression (4) is established with high accuracy between the color signal and the color signal in the common color space.

Rd = (1-a-b) .Rw + a.Rs + b.Rg (4) Here, Rd is a color signal of the common color space to be obtained, Rw is a colorimetric value of the output paper, and Rs is a color used. The colorimetric value of the material, Rg, is the colorimetric value of the portion of the output paper due to the dot gain. Further, a is a dot% of the color material, and b is a dot% of the portion due to the dot gain of the output paper. In this case, the dot gain is proportional to the perimeter of the halftone dot printed on the output paper, and can be approximated by the following equation (5). In addition,
n is a parameter that is set in response to a change in the perimeter of a halftone dot due to a change in printing conditions such as the number of screen lines.

[0027]

[Equation 2]

Therefore, for example, in the equation (4), a is set at 20% intervals for each color material, and Rw for a predetermined output sheet, b and Rg for a predetermined printing condition, and Rs for a predetermined color material. Setting the color signal YM
The correspondence between C and the color signal in the common color space can be obtained. Hereinafter, the color conversion table can be set by performing the conversion-related interpolation processing in the same manner as the processing (steps S20, S30, S40, S50) in the first embodiment.

In this case, by selecting and setting each parameter of the equation (4) according to the color reproduction characteristic, it is possible to easily create the conversion relationship corresponding to various conditions.

In the above embodiment, the lightness index L ^* and the perceived chromaticities a ^* and b ^* (first color system) are used as the color signal YM.
The case of creating a color conversion table for conversion into C (second color system) has been described, but a color conversion table for converting the lightness index L ^* and the perceived chromaticities a ^* , b ^* into the color signal YMCK is also the same. Can be created. In addition, other color systems such as XYZ color system, L ^* u ^* v ^* color system, and Y
Needless to say, the same can be applied to the case of creating a color conversion table between the MCK color system and the RGB color system.

[0031]

As described above, according to the present invention, after obtaining the minimum color conversion relation when obtaining the color conversion relation for converting the color signal from the first color system to the second color system. , The color conversion relationship is interpolated in order from the optimum color relationship with less error, the highly accurate color conversion relationship can be obtained with very little labor, and moreover, various color reproduction characteristics can be easily supported. It becomes possible.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a color conversion device.

FIG. 2 is a processing flowchart of a color conversion relationship setting method.

FIG. 3 is an explanatory diagram of a color space for creating a color patch.

FIG. 4 is an explanatory diagram of colorimetric points of color patches set on a CY plane.

5A to 5F are characteristic explanatory diagrams between a first color system and a second color system obtained from color patches.

FIG. 6 is an explanatory diagram of a case where an interpolation process is performed by giving priority to the Y direction on the CY plane.

FIG. 7 is an explanatory diagram of volume interpolation.

FIG. 8 is an explanatory diagram of a conversion relationship between a first color system and a second color system.

[Explanation of symbols]

10 ... Color conversion device 12 ... Input device 14 ... Output device 16 ... Color conversion unit 18 ... Table creation unit 20 ... Color conversion table storage unit 22 ... Table selection unit

Claims (3)

[Claims] 1. A method for setting a color conversion relationship for converting a color signal from a first color system to a second color system, wherein the first color system for a predetermined color signal of the second color system. A first step of obtaining a color signal of the system, and an interpolation process in the order in which the change of each component color of the first color system with respect to a predetermined amount change of each component color of the second color system is small, A second step of obtaining a color conversion relationship between the component colors, and a third step of obtaining a color conversion relationship in the remaining color space by interpolation processing using the color conversion relationship obtained in the second step. A color conversion relation setting method characterized by. 2. The method according to claim 1, wherein in the first step, a color patch is created based on a predetermined color signal of the second color system, and the color patch is colorimetrically measured to obtain the first color patch. A color conversion relation setting method characterized by obtaining color signals of a color system. 3. The method according to claim 1, wherein in the first step, a color signal Rd of the first color system with respect to a predetermined color signal a of the second color system is Rd = (1-a -B) -Rw + a-Rs + b-Rg Rw: Colorimetric value of the output paper Rs: Colorimetric value of the color material used Rg: Colorimetric value of the part of the output paper depending on the dot gain a: Color material net% b: A color conversion relation setting method characterized in that it is obtained as the dot% of the portion of the output paper depending on the dot gain.