JPH10210275A - Color reproducing area converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reproduce a primary color as truly as possible in converting color reproducing area between machines with different color reproducing areas and to achieve excellent color reproduction without losing gradation. SOLUTION: An RGB signal inputted from a color display 1 is converted into uniform color space data of a CIELAB surface color system by a color space converting means 2. The color reproducing area of the CIELAB surface color system data is compressed or expanded by using a converting function stored in a color reproducing area storage means 3a by a color reproducing area converting means 3. The converted uniform color space data is converted into a CMY (K) signal of a color printer 5 and outputted by a color space reconverting means 4. The color reproducing area converting means 3 is instructed to select one converting function among plural converting functions stored in the color reproducing area storage means 3a by a selecting instruction means 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to color reproduction for converting a color gamut when inputting / outputting a color image signal between image input / output devices having different color reproduction ranges, such as a color display, a color scanner, and a color printer. The present invention relates to an area conversion device.

[0002]

2. Description of the Related Art Generally, the color gamut of a color display is wider than the color gamut of a color printer, and it is impossible to accurately reproduce all colors represented by the color display with the color printer. For this reason, a method or apparatus has been proposed that assigns the closest color to a color that cannot be reproduced by a color printer, performs a reduction mapping process on the color gamut, and converts the color gamut to fall within the color gamut of the color printer. I have.

For example, in an apparatus disclosed in Japanese Patent Application Laid-Open No. 4-101566, a color compression unit determines whether an input signal is in a color area that can be expressed by a color printer, and determines a hue according to the result. Evenly compresses only the brightness without changing it.

Further, in the method and printer disclosed in Japanese Patent Application Laid-Open No. 4-1155691, when an image displayed on a color monitor is output by a color printer, linear compression is performed for saturation in a uniform color space. For brightness, only tone reproduction is performed.

In the method and apparatus disclosed in Japanese Patent Application Laid-Open No. 4-186968, color reproduction is performed without converting most of the color gamut and partly compressed in the entire color space. Accordingly, the color reproduction faithful to the color printer is almost satisfied, and colors outside the color reproduction range can be recorded and reproduced.

[0006]

However, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 4-101566, saturation compression is not performed, and data outside the color gamut is directly applied to the outer edge outside the color gamut. If they are attached, the gradation will be lost.

In the method and printer disclosed in Japanese Patent Application Laid-Open No. Hei 4-1155691, gradation is maintained because compression is performed uniformly. Outputs colors that deviate from the primary colors.

In the method and apparatus disclosed in Japanese Patent Application Laid-Open No. 4-186968, there is no continuity between a color reproduced without conversion and a color to be compressed. Cannot reproduce.

Accordingly, the present invention has been made to solve the above-mentioned conventional problems. When color gamut conversion is performed between devices having different color gamut, the primary colors are reproduced as faithfully as possible and the gradation is improved. It is an object of the present invention to provide a color gamut conversion device that realizes good color reproduction without losing performance.

[0010]

According to a first aspect of the present invention, there is provided a color reproduction gamut conversion apparatus for converting an input color image signal into a color image comprising lightness, hue, and saturation. Color space conversion means for converting to space data, and color gamut conversion means for compressing or expanding brightness and saturation of the converted color space data without changing the hue to match the color gamut of the output device; And a color space re-conversion unit for converting the color space data whose brightness and saturation have been compressed or decompressed into a color image signal suitable for the output device.

According to a second aspect of the present invention, there is provided a color gamut conversion apparatus according to the first aspect, wherein the color gamut conversion means uniformly compresses or reduces brightness. It is characterized by expanding and compressing or expanding the saturation using a higher-order function.

According to a third aspect of the present invention, there is provided a color gamut conversion apparatus according to the first aspect, wherein the color gamut conversion means uniformly compresses saturation. Alternatively, it is characterized in that the lightness is compressed or expanded using a higher-order function.

According to a fourth aspect of the present invention, in the color gamut conversion device according to the second or third aspect, the high-order function is an elastic curve function. There is a feature.

According to a fifth aspect of the present invention, in the color gamut conversion device according to the second or third aspect, the high-order function is a quadratic function. There is a feature.

According to a sixth aspect of the present invention, in the color gamut conversion device according to the first aspect, the color gamut conversion means includes a plurality of conversion functions. And a selection instructing unit for instructing the color gamut conversion unit to select any one conversion function from the plurality of conversion functions.

With the above arrangement, the color gamut conversion apparatus according to the first aspect of the present invention converts a color image signal into a lightness signal,
Converts color space data consisting of hue and saturation, and compresses or expands the lightness and saturation without changing the hue. And good color reproduction is realized without losing gradation.

In the color gamut conversion device according to the second aspect of the present invention, the brightness is uniformly compressed or expanded, and the saturation is compressed or expanded using a higher-order function. The saturation reproduces the primary colors as faithfully as possible without losing the gradation.

In the color gamut conversion apparatus according to the third aspect of the present invention, the saturation is uniformly compressed or expanded and the brightness is compressed or expanded using a higher-order function. Emphasis is placed on lightness, and primary colors are reproduced as faithfully as possible without losing gradation.

Since the color gamut conversion device according to the fourth aspect of the present invention compresses or expands the lightness or the saturation using the elastic curve function, the color gamut conversion device can also be used for an output device having a narrow color gamut.
It reproduces primary colors as faithfully as possible and achieves good color reproduction without losing gradation.

Since the color gamut conversion device according to the fifth aspect of the present invention compresses or expands the lightness or the saturation using a quadratic function, it is as faithful as possible to an output device having a narrow color gamut. In addition, it reproduces primary colors and achieves good color reproduction without losing gradation.

The color gamut conversion device according to the sixth aspect of the present invention selects an arbitrary one of a plurality of conversion functions, thereby obtaining a color image of a color according to a user's request. Can be.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a functional block diagram of the color gamut conversion device according to the embodiment of the present invention. The color space conversion means 2 outputs the R
(Red), G (green), and B (blue) signals are converted into uniform color space data of the CIE (International Commission on Illumination) LAB color system. The color gamut conversion means 3 converts the color gamut of the CIELAB color system data using the conversion function stored in the color gamut storage means 3a, that is, compresses or decompresses and outputs. The color space re-conversion means 4 converts the converted uniform color space data into C (cyan), M (magenta), and Y (yellow) signals of the color printer 5 as an output device, and outputs the signals. The selection instructing means 6 instructs the color gamut conversion means 3 to select one conversion function from the plurality of conversion functions stored in the color gamut storage means 3a.

In the color gamut conversion device of the present embodiment, the output device is the color printer 5, so that the color system composed of lightness, hue, and saturation used in the color gamut conversion means 3 is as follows.
C useful for devices using color mixture with ink, toner, etc.
The IELAB color system is used. In the case of an apparatus using color mixture by color light, such as a color television, the CIELUV color system is used.

As the color signals output to the color printer 5, C (cyan), M (magenta), Y (yellow), and K (black) signals may be used.

Next, the operation of the above constituent means will be described. The color space conversion means 2 converts the RGB signals input from the color display 1 into Lab data (lightness is L, saturation is √ (a ²
+ B ² ).

The color gamut conversion means 2 compares the Lab data with respect to the distance from the achromatic color point (L = a = b) to the outer edge of the color gamut, ie, the color difference (√ (L ² + a ² + b ² )). And performs compression in accordance with the difference in the distance.

FIG. 2 shows the distance conversion from the achromatic color point to the color reproduction range. Points A and B are stored in color gamut storage means 3 as color gamut information of a color display, a color printer, or the like.
a. The color gamut converter 3 compresses the input Lab data at the point P into Lab data at the point Q.

The achromatic point O from the point A, B, P, respectively distance to _{Q, l A = √ (L} A 2 + a A 2 + b A 2) l B = √ (L B 2 + a B 2 + b B ²⁾ l _P = can be expressed as ^{_{√ (L P 2 + a P}} 2 + b P 2) l Q = √ (L Q 2 + a Q 2 + b Q 2).

When S = l _A −l _B is given by the higher-order function f (x) shown in FIG.

(Equation 1) And the function f (x) is determined.

Further, l _P = √ (L _P ² + a _P ² + b _P ² ) = √ (L _Q ² + a _Q ² +
b _Q ² ) + f (Q), from which the point Q can be determined.

As described above, by performing the color difference compression conversion, the brightness and the saturation are compressed and the point Q
Is obtained. The Lab data compressed and converted by the color gamut conversion unit 3 is output to the color space re-conversion unit 4. Then, the color space re-conversion means 4
The compressed and converted Lab data is transferred to a color printer 5.
Is converted again into a CMY (K) signal and output.

[0032] Next, in the color gamut conversion device of another embodiment, the ratio of the color gamut conversion unit 3, on the input Lab data, the lightness L _B of the brightness of the point A L _A and point B Is the reduction ratio, and the lightness L _{Q of the} point _Q is determined by performing uniform compression based on the lightness L _{P of the} point P by this reduction ratio.

Further, regarding the saturation, assuming that l _A = √ (a _A ² + b _A ² ) l _B = √ (a _B ² + b _B ² ) l _P = √ (a _P ² + b _P ² )

(Equation 2) And l _Q = √ (a _Q ² + b _Q ² ) + f (Q), so that the saturation √ (a _Q ² +
b _Q ² ) is determined.

As described above, the Lab data at the point Q is obtained by performing the compression conversion of the brightness and the saturation.

In the color gamut conversion device of still another embodiment, the color gamut conversion means 3 calculates the saturation √ (a _A ² + b _A ² ) of the point _A with respect to the input Lab data. The ratio of the saturation of point _{B to} (a _B ² + b _B ² ) is defined as the reduction ratio, and the saturation of point P is
From (a _P ² + b _P ² ), the saturation 率 (a _Q ² + b _Q ² ) of the point Q is determined by uniform compression according to this reduction rate.

Regarding the lightness, l _A = L _A , l _B =
If L _B , l _P = L _P ,

(Equation 3) And l _Q = L _Q + f (Q), so that the lightness L _{Q at the} point _Q is determined.

As described above, the Lab data at the point Q is obtained by performing the lightness and saturation compression conversion.

Next, an example in which the color gamut conversion means 3 performs compression using an elastic curve function as a conversion function will be described. The elastic curve function will be described by taking as an example the deflection due to the concentrated load acting on the free end of the cantilever shown in FIG.

The deflection of the cross-section at a distance x from the free end, W a concentrated load, EI _Z beams bending stiffness, when the length of the l cantilever,

(Equation 4) It can be expressed as.

Therefore, f (x) = a (−x ³ + 3lx ² −6l ² x + 6l ³ )

(Equation 5) It can be expressed as.

Since l _B = √ (L _B ² + a _B ² + b _B ² ),

(Equation 6) Can be expressed as

(Equation 7) Becomes

Therefore,

(Equation 8) It can be expressed as. Therefore, the distance l to the point Q
_Q is

(Equation 9) More can be obtained. Here, since (vector OP) = α (vector OQ), a point Q on the compressed Lab color space can be determined.

Next, an example in which the color gamut conversion means 3 performs compression using a quadratic function as a conversion function will be described. This quadratic function will be described with reference to FIG. In this figure, l
Represents the distance L _B of the narrow color gamut, the distance of the color reproduction gamut that S can not be reproduced (L _A -L _B). If f (x) = ax ² ,

(Equation 10) It can be expressed as.

Also,

[Equation 11] Can be expressed as

(Equation 12) Becomes

Therefore,

(Equation 13) It can be expressed as. Therefore, the distance l to the point Q
_Q is

[Equation 14] More can be obtained. Here, since (vector OP) = α (vector OQ), a point Q on the compressed Lab color space can be determined.

As described above, the color gamut conversion apparatus of the present invention, when compressing a wide color gamut such as a color display into a narrow color gamut such as a color printer as shown in FIG. Since the colors outside the gamut are compressed as much as possible and are expressed in a narrow color reproduction range, the primary colors are reproduced as faithfully as possible and good color reproduction is achieved without losing the gradation as compared with the uniform compression.

[0047]

As described above, the color gamut conversion device according to the first aspect of the present invention converts a color image signal into lightness,
Converts color space data consisting of hue and saturation, and compresses or expands lightness and saturation without changing the hue, so that primary colors can be reproduced as faithfully as possible between devices with different color reproduction ranges. And good color reproduction is realized without losing gradation.

In the color gamut converter according to the second aspect of the present invention, brightness is uniformly compressed or expanded, and saturation is compressed or expanded using a higher-order function. The saturation reproduces the primary colors as faithfully as possible without losing the gradation.

The color gamut conversion apparatus according to the third aspect of the present invention uniformly compresses or expands the saturation and compresses or expands the lightness using a higher-order function. Emphasis is placed on lightness, and primary colors are reproduced as faithfully as possible without losing gradation.

Since the color gamut conversion device according to the fourth aspect of the present invention compresses or expands the lightness or the saturation using the elastic curve function, the color gamut conversion device can be applied to an output device having a narrow color gamut.
It reproduces primary colors as faithfully as possible and achieves good color reproduction without losing gradation.

Since the color gamut conversion device according to the fifth aspect of the present invention compresses or expands the lightness or the saturation using a quadratic function, it is as faithful as possible to an output device having a narrow color gamut. In addition, it reproduces primary colors and achieves good color reproduction without losing gradation.

In the color gamut conversion apparatus according to the sixth aspect of the present invention, an arbitrary one of a plurality of conversion functions is selected, so that a color image having a color according to a user's request can be obtained. Can be.

[Brief description of the drawings]

FIG. 1 is a functional block diagram of a color gamut conversion device according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating distance conversion from an achromatic color point to a color reproduction range.

FIG. 3 is a diagram illustrating an example of a higher-order function.

FIG. 4 is an explanatory diagram of deflection caused by a concentrated load acting on a free end of a cantilever.

FIG. 5 is an explanatory diagram of a quadratic function.

FIG. 6 is a diagram showing a comparison of a compression method from a wide color gamut to a narrow color gamut.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Color display 2 Color space conversion means 3 Color gamut conversion means 3a Color gamut storage means 4 Color space re-conversion means 5 Color printer 6 Selection instruction means

Claims (6)

[Claims] 1. A color space conversion means for converting an input color image signal into color space data comprising lightness, hue, and saturation, and brightness and chroma of the converted color space data without changing the hue. Color gamut conversion means adapted to compress or expand the degree to match the color gamut of the output device; and convert the color space data whose brightness and saturation have been compressed or expanded to a color image signal suitable for the output device. A color gamut conversion device comprising: a color space re-conversion unit. 2. The color gamut conversion apparatus according to claim 1, wherein the color gamut conversion means compresses or expands the lightness uniformly and compresses or expands the saturation using a higher-order function. apparatus. 3. The color gamut conversion apparatus according to claim 1, wherein the color gamut conversion means compresses or expands the saturation evenly and compresses or expands the lightness using a higher-order function. apparatus. 4. The color gamut conversion apparatus according to claim 2, wherein the higher-order function is an elastic curve function. 5. The color gamut conversion apparatus according to claim 2, wherein the higher-order function is a quadratic function. 6. The color gamut conversion means includes a plurality of conversion functions, and selection instructing means for instructing the color gamut conversion means to select an arbitrary one of the plurality of conversion functions. The color gamut conversion device according to claim 1, further comprising: