JPH0834546B2 - Color reproduction method in color hard copy - Google Patents

Description

TECHNICAL FIELD The present invention relates to a color reproduction method for expressing a color that is out of a color reproduction area when a color hard copy is made.

[Conventional technology]

Generally, color hard copy has a narrow color reproduction area,
For example, when obtaining a hard copy of a color image such as a CRT, some colors cannot be reproduced. In the past, colors outside the color reproduction area have been reproduced by using a method such as changing the constant of the masking equation by a calculation according to the distribution of image data.

[Problems to be solved by the invention]

However, the conventional color reproduction method by the above method is
It depends on my experience, difficult to reproduce faithful colors,
Also, because it is necessary to change the parameters for each image,
There is a problem that it requires complicated operations and is not practical.

The present invention has been made to solve the above problems in the color reproduction method of the conventional color hard copy, and an object thereof is to provide a color reproduction method of a color hard copy capable of faithful color reproduction by an easy method. To do.

[Means and Actions for Solving Problems]

In order to solve the above-mentioned problems, the present invention provides a color reproduction method in a color hard copy in which a color outside a color area reproducible in a color hard copy is converted into a color in the color area to be expressed, and a predetermined luminance is obtained. On the chromaticity diagram, a polygon with the maximum saturation point of each colorant of multiple colors used for color reproduction at the apex is set, and the area enclosed by this polygon is defined as the reproducible color area. When the chromaticity coordinates on the chromaticity diagram of the input original image data are outside the color region, the straight line connecting the chromaticity coordinates of the original image data and the white chromaticity coordinates and the polygon And a step of replacing the chromaticity coordinate of the original image data outside the color area with the chromaticity coordinate of the intersection point.

By applying the color reproduction method of such a color hard copy, for colors outside the color reproduction area of the color hard copy, only the saturation is added without changing the hue and gradation expression, That is, the color reproduction area is defined as a polygonal area on the color coordinates, and the calculation is performed to calculate the intersection of a straight line connecting the color coordinate points of the colors other than the polygonal area and the white point and one side of the polygonal area. Since the color coordinates after conversion are obtained, color conversion can be performed by solving a simple simultaneous equation, and faithful color reproduction can be performed without complicated operations.

〔Example〕

Examples will be described below. FIG. 1 is a flow chart for explaining an embodiment of a color reproduction method of a color hard copy according to the present invention, and the embodiment will be explained with reference to the flow chart.

As a first step, the image data is read into the color processing means of the copy machine in order to perform a predetermined color hard copy. Image data is XYZ of color D to be expressed
It is shown by tristimulus values (X, Y, Z) of the color system. Next, from the image data (X, Y, Z), the chromaticity coordinates (x, y) in the chromaticity diagram of the color D to be expressed are obtained from the following equations (1) and (2).

x = X / (X + Y + Z) ・ ・ ・ (1) y = Y / (X + Y + Z) ・ ・ ・ (2) In color hard copy, yellow Y, magenta M, and cyan C Since there are 6 combinations of the primary colors and the secondary colors of red R, green G, and blue B, it is necessary to determine which combination the color D to be represented has. In this case, Neugebauer (Neug
ebauer) equation, the hue region consisting of the combination of the primary color and the secondary color is divided into 6 parts by a straight line from the I region to the VI region as shown in FIGS. 2 (A) and (B). be able to.

In this 6-divided hue area (I, II, ... VI), in which hue area, the color D represented by chromaticity coordinates (x, y)
Whether or not is included is determined by the positive / negative of the following (3) color.

However, i = Y, M, C, R, G, B (x _i , y _i ) is the coordinate of the vertex of the hexagonal color reproduction hue area, and (x _w , y _w ) is the coordinate of the white point W. Is.

That is, by substituting the chromaticity coordinates (x, y) into the equation (3),
The hue area of the color D represented by the chromaticity coordinates (x, y) and the combination of the primary color and the secondary color are determined based on the areas that are both positive.

In the color reproduction hue area, as the luminance Y increases,
It becomes smaller as shown by a broken line or a dotted line in FIG.
Therefore, for the luminance Y, the size of the color reproduction hue area is obtained as follows. First, assuming that the combination of the primary color and the secondary color is magenta M and blue B, and the predetermined color reproduction hue region is the IV region, according to the magnitude of the luminance Y,
The area ratios a _M , a _B of the primary color and the secondary color are set based on Table 1, and the maximum saturation point P (x _M ′, y of the primary color M is set using the area ratios a _M , a _{B. M} ′) and the maximum saturation point Q (x _B ′, of the secondary color B,
y _B ′).

To obtain by calculation, the following Neugebauer equation is used.

X = X _W a _W + X _M a _M + X _B a _B Y = Y _W a _W + Y _M a _M + Y _B a _B Z = Z _W a _W + Z _M a _M + Z _B a _B However, X _W , Y _W , Z _W is white stimulus value X _M , Y _M , Z _M is primary color stimulus value X _B , Y _B , Z _B is secondary color stimulus value Black stimulus value is 0 .

Then, by connecting the maximum saturation points P and Q with a straight line PQ, a color reproduction area at the luminance Y is obtained.

Next, it is further determined whether or not the chromaticity coordinates (x, y) of the color D exist in the color reproduction area corrected by the luminance Y. This determination is similarly made by the positive / negative of the following equation (4).

Then, if the chromaticity coordinates (x, y) of the color D are within the corrected color reproduction area, the white, primary color, and secondary color are respectively calculated by the color reproduction equation based on the image data (X, Y, Z). Area ratio a _W , a
_{Find M} , a _B. Then, the area ratio of black a _K = 1−a _W −a _M −a _B is obtained from these area ratios to form a color image.

On the other hand, if the chromaticity coordinate (x, y) of white D is outside the corrected color reproduction area, the white coordinate point is set to W (x _W , y _W ) and a straight line ▲
The coordinates (x ', y') of the intersection D'of ▼ and straight line ▼ are obtained from the following simultaneous equations (5) and (6).

Next, regarding the coordinates (x ', y') of the intersection D'as the chromaticity coordinates of the color D to be expressed, using these coordinates and the luminance Y,
X'and Z'are determined by the following equations (7) and (8).

X ′ = x ′ / y ′ · Y (7) Z ′ = (1-x′-y ′) / y ′ · Y
(8) The (X ′, Y, Z ′) thus obtained is the image data (tristimulus value) corrected so that it stays within the color reproduction area.
This is regarded as original image data, and based on the corrected image data (X ′, Y, Z ′), the area ratios a _W , a _M , and a _B are obtained by a color reproduction equation, and further, Area ratio a _W , a _M ,
The area ratio a _K of black is obtained from a _B , and a color image is formed with the ink which is the corresponding colorant based on each area ratio.

In the above embodiment, the Neugebauer equation is used as the color reproduction equation used to determine the color reproduction area, but the color reproduction equation is not limited to this, and other color reproduction equations are used. You can also

〔The invention's effect〕

As described above based on the embodiment, according to the present invention, it is possible to perform faithful color reproduction without requiring a complicated operation.

[Brief description of drawings]

FIG. 1 is a flow chart for explaining an embodiment of a color reproduction method of a color hard copy according to the present invention, and FIGS. 2 (A) and 2 (B) are diagrams showing combined areas of primary colors and secondary colors. FIG. 3 and FIG. 3 are diagrams showing changes in the color reproduction area based on changes in the luminance Y.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location H04N 1/46 B41J 3/00 B

Claims (1)

[Claims] 1. A color reproduction method in a color hard copy, in which a color outside a color range reproducible by a color hard copy is converted into a color within the color range and expressed, wherein a color is represented on a chromaticity diagram at a predetermined luminance. Set a polygon that has each maximum saturation point of multiple colorants used for reproduction at the apex,
The step of defining the area surrounded by the polygon as the reproducible color area, and when the chromaticity coordinates on the chromaticity diagram of the input original image data are outside the color area, the original image data A line connecting the chromaticity coordinates of the chromaticity coordinates and the chromaticity coordinates of white, and the step of calculating the chromaticity coordinates of the intersection of one side of the polygon, the chromaticity coordinates of the original image data outside the color region of the intersection A color reproduction method in a color hard copy, comprising the step of replacing with chromaticity coordinates.