Color conversion apparatus, and color conversion method Number:7,146,038 from the United States Patent and Trademark Office (PTO) owispatent A method and apparatus for converting color includes converting the hue, lightness, and/or saturation of first color data representing each of plurality of colors forming a color image, to produce second color data corresponding to the first color data. The second color data is converted to third color data suitable for the color space which can be expressed by the image output device used for the output of the color image. Color contraction which is conventionally associated with color conversion that increases lightness or saturation can be prevented.<H conversion, apparatus, Color, conversio, 7146038.html, Patent, pto, 7146038

Title: Color conversion apparatus, and color conversion method

Abstract: A method and apparatus for converting color includes converting the hue, lightness, and/or saturation of first color data representing each of plurality of colors forming a color image, to produce second color data corresponding to the first color data. The second color data is converted to third color data suitable for the color space which can be expressed by the image output device used for the output of the color image. Color contraction which is conventionally associated with color conversion that increases lightness or saturation can be prevented.

Patent Number: 7,146,038 Issued on 12/05/2006 to Kagawa, et al.

Inventors: Kagawa; Shuichi (Tokyo, JP), Sugiura; Hiroaki (Tokyo, JP), Takahashi; Mariko (Tokyo, JP), Matoba; Narihiro (Tokyo, JP)
Assignee: Mitsubishi Denki Kabushiki Kaisha (Tokyo, JP)

Appl. No.: 10/212,108

Filed: August 6, 2002

Foreign Application Priority Data


Feb 15, 2002 [JP]			2002-037642

Current U.S. Class: 382/166 ; 358/523

Current International Class: G06K 9/00 (20060101); G03F 3/08 (20060101); G06F 13/00 (20060101)

Field of Search: 382/162,165,166,167 358/1.9,515,518,520,523,521 345/589,600,603,604

References Cited [Referenced By]

U.S. Patent Documents


4740833	April 1988	Shiota et al.
4887150	December 1989	Chiba et al.
4989079	January 1991	Ito
5436733	July 1995	Terada et al.
5659406	August 1997	Imao et al.
5933252	August 1999	Emori et al.
5937089	August 1999	Kobayashi
6125202	September 2000	Kagawa et al.
6297826	October 2001	Semba et al.
6434268	August 2002	Asamura et al.
6771813	August 2004	Katsuyama
6781716	August 2004	Yoda
6829062	December 2004	Asamura et al.
6865292	March 2005	Kagawa et al.
2003/0165266	September 2003	Kagawa et al.
2003/0228055	December 2003	Kanagawa et al.

Foreign Patent Documents


1028586	Aug., 2000	EP
63-39188	Aug., 1988	JP
63-227181	Sep., 1988	JP
2-30226	Jul., 1990	JP
5-48885	Feb., 1993	JP
5-183742	Jul., 1993	JP
7-023245	Jan., 1995	JP
7-170404	Apr., 1995	JP
8-321964	Dec., 1996	JP
11-17974	Jan., 1999	JP
11234531	Aug., 1999	JP
2001-307080	Nov., 2001	JP

Primary Examiner: Alavi; Amir
Attorney, Agent or Firm: Birch, Stewart, Kolasch & Birch, LLP

Claims

What is claimed is:

1. A color conversion apparatus comprising: a second-color-data generating unit for converting a hue, lightness, and/or saturation of first color data representing respective colors forming a color image to generate second color data corresponding to the first color data; and a third-color-data generating unit for generating third color data suitable for the color space which can be expressed by an image output unit used for outputting the color image, wherein said third-color-data generating unit generates the third color data based on a maximum value of the gradation levels of the respective colors represented by the second color data.

2. The color conversion apparatus as set forth in claim 1, wherein said third color data generating unit reduces the gradation level of each of the colors of the second color data according to the maximum value in excess of a predetermined value.

3. The color conversion apparatus as set forth in claim 1, further comprising a complementary-color-data outputting unit for outputting complementary color data representing the respective complementary colors of a plurality of colors represented by the second color data, wherein said third-color-data generating unit generates the third color data based on the complementary color data.

4. The color conversion apparatus as set forth in claim 2, wherein said third-color-data generating unit generates the third color data based on a maximum value of the gradation levels of the respective complementary colors represented by the complementary color data.

5. The color conversion apparatus as set forth in claim 4, wherein said third-color-data generating unit generates the third color data by reducing the gradation level of each of the complementary colors of the complementary color data according to the maximum value in excess of a predetermined value.

6. The color conversion apparatus as set forth in claim 1, further comprising a ratio calculating unit for calculating a ratio between the hue components of magenta and red, red and yellow, yellow and green, green and cyan, cyan and blue, or blue and magenta, said third-color-data generating unit generates the third color data based on the ratio between the hue components.

7. A color conversion apparatus comprising: a first-calculation-term generating unit responsive to first color data representing respective colors of red, green and blue, for generating a first calculation term which is effective for at least one of the hues of red, green, blue, cyan, magenta and yellow; a second-color-data generating unit for generating second color data corresponding to the first color data; and a third-color-data generating unit responsive to the first calculation term, and the second color data, for generating third color data suitable for the color space expressed by the image output unit used for outputting the color image, wherein said third-color-data generating unit generates the third color data based on a maximum value of the gradation levels of the respective colors represented by the second color data.

8. The color conversion apparatus as set forth in claim 7, wherein said third color data generating unit reduces the gradation level of each of the colors of the second color data according to the maximum value in excess of a predetermined value.

9. The color conversion apparatus as set forth in claim 7, further comprising a second-calculation-term generating unit for generating a second calculation term effective for a predetermined hue included in one of inter-hue zones of red to yellow, yellow to green, green to cyan, cyan to blue, blue to magenta, and magenta to red, based on the first calculation term; wherein said second-color-data generating unit generates the second color data also by multiplying the second calculation term by a predetermined matrix coefficient; the third-color-data generating unit generates the third color data based also on the second color data.

10. A color conversion method comprising the steps of: converting a hue, lightness, and/or saturation of first color data representing the respective colors forming a color image to generate second color data corresponding to the first color data; and generating, based on the second color data, third color data suitable for the color space which can be expressed by an image output unit used for outputting the color image, wherein said step of generating the third color data generates the third color data based on a maximum value of the gradation levels of the respective colors represented by the second color data.

11. The color conversion method as set forth in claim 10, wherein said step of generating the third color data generates the third color data by reducing the gradation level of each of the colors of the second color data according to the maximum value in excess of a predetermined value.

12. The color conversion method as set forth in claim 10, wherein said step of generating the third color data generates the third color data based on the complementary color data representing the respective complementary colors of a plurality of colors represented by the second color data.

13. The color conversion method as set forth in claim 12, wherein said step of generating the third color data generates the third color data based on a maximum value of the gradation levels of the respective complementary colors represented by the complementary color data.

14. The color conversion method as set forth in claim 13, wherein said step of generating the third color data generates the third color data by reducing the gradation level of each of the complementary colors of the complementary color data according to the maximum value in excess of a predetermined value.

15. The color conversion method as set forth in claim 10, further comprising the step of: calculating a ratio between the hue components of magenta and red, red and yellow, yellow and green, green and cyan, cyan and blue, or blue and magenta, wherein said step of generating the third color data generates the third color data based on the ratio between the hue components.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a color conversion apparatus in an image output device, such as an image display device, color printer, or color scanner, and in particular, to a color conversion apparatus, and a color conversion method converting color data representing images, in accordance with the characteristics of the output device reproducing or expressing the image.

2. Prior Art

When input color data is displayed by an image display device, color conversion is performed to achieve desired color rendition or reproducibility in accordance with the conditions in which the device is used. Also in printers, color conversion is performed to achieve good color reproducibility for compensating for mixed-color property due to the fact that inks are not of pure colors, and picture quality degradation due to non-linearity of the input-output characteristics.

Two typical color conversion methods are of a table conversion method and a matrix calculation method. The table conversion method uses a memory such as a ROM for storing conversion coefficients corresponding to the input color data representing each of the colors of R, G, B, and performs color conversion in accordance with the stored conversion coefficients. An advantage of this method is that color conversion can be performed based on an arbitrary conversion characteristics.

However, it is necessary to store conversion coefficients for respective combinations of the color data, and a memory of a large capacity is required, so that implementation in an integrated circuit is difficult. Another problem is that it cannot be adapted to changes in the condition in which the device is used.

The matrix calculation method performs a matrix calculation in accordance with the following formula (24) using the color data Ri, Gi, Bi representing colors of R, G, B, to output new color data Ro, Go, Bo.

.function. ##EQU00001##

In the above formula, Aij (i=1 to 3, j=1 to 3) are matrix coefficients determining the conversion coefficients for the color conversion.

There is a concept of "preferred color reproduction" as opposed to the concept of "exact color reproduction." "Exact color reproduction" means exact or faithful reproduction of colors of the original, and color conversion which matches the method of generating the input color data is performed. For example, the color reproduction using standard color space, such as that of NTSC, or sRGB is performed. "Preferred color reproduction" does not necessarily coincide with "exact color reproduction," and is a color reproduction taking into consideration the characteristics of human visual sense, and memorized colors, and produces colors preferred by human beings.

Often, "preferred color reproduction" is performed in connection with moving pictures of television images. For instance, there is a tendency that the color of the sky, or green of the lawn is often memorized as colors which are more vivid, and of a higher saturation. Accordingly, in "preferred color reproduction," a general practice is to perform the color conversion in which the saturation or lightness (value) of the colors are increased. In "exact color reproduction" as well, it is not rare that color conversion which increases the saturation or lightness is performed. This is because, the range or gamut of the colors which can be expressed by the image display device is narrower than the gamut of the standard color space or the color space used in the generation of the image data.

FIG. 36 shows a configuration of a conventional color conversion apparatus. A color converter 1 performs color conversion on first color data R1 G1, B1 to generate second color data R2, G2, B2. A data limiter 101 limits the second color data R2, G2, B2, to output third color data which can be displayed by the image display device, not shown, provided in a succeeding stage.

In the conventional color conversion apparatus shown in FIG. 36, due to the function of the data limiter 101, the color contraction (a phenomenon in which a fine difference between colors of high lightness or saturation is eliminated (two different input color data are translated into the same color) may occur. A specific example of color contraction due to the limiting operation will next be described.

It is presumed that the first color data R1, G1, B1 is 8-bit digital data representing an integer without a sign, assuming a value of 0 to 255, and the second color data R2, G2, B2 is 10-bit digital data with a sign, assuming a value of -512 to 511. It is also presumed that the number of gradation levels which can be displayed by the image display device used for displaying the image responsive to the second color data R2, G2, B2 is 256 (expressed by 8 bits), and the data limiter 101 accordingly converts the second color data into the third color data of 8 bits.

The color converter 1 outputs the second color data by matrix calculation represented by the following formula (25).

.function. ##EQU00002##

The matrix calculation formula (25) is for increasing the lightness of the first color data R1, G1, B1.

For two different sets of first color data with: R1=230, G1=20, B1=20; and R1=240, G1=20, B1=20, the second color data obtained as a result of the color conversion according to the formula (25) will have values: R2=276, G2=24, B2=24; and R2=288, G2=24, B3=24. The third color data output from the data limiter 101, which limits the maximum value to "255," and input the limited data to the image display device, not shown, will have values: R3=255, G3=24, B3=24; and R3=255, G3=24, B3=24. Thus the two sets of different values representing different colors are translated into the color data of identical values, due to the limiting operation. In this way, the input or original color data representing colors of different hue, lightness or saturation (in the input or original data) are translated into and displayed as an identical color, and the difference between colors (as existed in the original image) is eliminated or diminished--this phenomenon is called color contraction.

The color contraction also occurs when a processing for enhancing the saturation is performed in the color converter 1. The following matrix calculation formula (26) is an example used to enhance the saturation of the first color data.

.function. ##EQU00003##

When color conversion using the matrix calculation formula (26) is applied to two sets of different values of color data: R1=230, G1=10, B1=10; and R1=232, G1=20, B1=20, the second color data will have values: R2=228, G2=-14, B2=-14; and R2=228, G2=-5, B3=-5. The data limiter 101 limits the minimum value of the second color data to "0" in accordance with the display characteristics of the image display device. The third color data supplied from the limiter 101 to the image display device (not shown) in a succeeding stage will have values: R3=228, G3=0, B3=0; and R3=228, G3=0, B3=0. Thus, the two sets of color data of different values are output as the color data of identical values.

As has been described, a conventional color conversion apparatus, which performs processing for enhancing lightness and/or saturation of the color data, is associated with a problem of color contraction in which fine differences between colors of high lightness or saturation diminishes.

SUMMARY OF THE INVENTION

The invention has been achieved in view of the above problem, and its object is to provide a color conversion apparatus and a color conversion method which can perform desired color conversion without color contraction.

According to the present invention, there is provided a color conversion apparatus including a second-color-data generating unit and a third-color-data generating unit. The a second-color-data generating unit converts a hue, lightness, and/or saturation of first color data representing respective colors forming a color image to generate second color data corresponding to the first color data. The third-color-data generating unit generates third color data suitable for the color space which can be expressed by an image output unit used for outputting the color image.

With the above arrangement, the third color data suitable for the color space which can be expressed by the image output unit is generated based on the second color data, so that desired color conversion can be achieved without color contraction.

The third-color-data generating unit may be adapted to generate the third color data based on a maximum value of the gradation levels of the respective colors represented by the second color data. In this case, the third color data generating unit may be adapted to reduce the gradation level of each of the colors of the second color data according to the maximum value in excess of a predetermined value.

The color conversion apparatus may further include a complementary-color-data outputting unit for outputting complementary color data representing the respective complementary colors of a plurality of colors represented by the second color data, and the third-color-data generating unit may be adapted to generate the third color data based on the complementary color data.

In this case, the third-color-data generating unit may be adapted to generate the third color data based on a maximum value of the gradation levels of the respective complementary colors represented by the complementary color data. In this case, the third-color-data generating unit may be adapted to generate the third color data by reducing the gradation level of each of the complementary colors of the complementary color data according to the maximum value in excess of a predetermined value.

The color conversion apparatus may further include a ratio calculating unit for calculating a ratio between the hue components of magenta and red, red and yellow, yellow and green, green and cyan, cyan and blue, or blue and magenta, and the third-color-data generating unit may be adapted to generate the third color data based on the ratio between the hue components.

According to the invention, there is also provided a color conversion apparatus including a first-calculation-term generating unit, a second-color-data generating unit, and a third-color-data generating unit. The first-calculation-term generating unit is responsive to first color data representing respective colors of red, green and blue, for generating a first calculation term which is effective for at least one of the hues of red, green, blue, cyan, magenta and yellow. The second-color-data generating unit generates second color data corresponding to the first color data. The third-color-data generating unit is responsive to the first calculation term, and the second color data, for generating third color data suitable for the color space expressed by the image output unit used for outputting the color image.

With the above arrangement, the second color data is generated based on the calculation term effective for one of the hues of red, green, blue, cyan, magenta, and yellow, and the third color data suitable for the color space which can be expressed by the image output unit is generated based on the calculation term and the second color data, so that it is possible to control each hue component independently, and to achieve desired color conversion without color contraction.

The third-color-data generating unit may be adapted to generate the third color data based on a maximum value of the gradation levels of the respective colors represented by the second color data. In this case, the third color data generating unit may be adapted to reduce the gradation level of each of the colors of the second color data according to the maximum value in excess of a predetermined value.

The color conversion apparatus may further include a second-calculation-term generating unit for generating a second calculation term effective for a predetermined hue included in one of inter-hue zones of red to yellow, yellow to green, green to cyan, cyan to blue, blue to magenta, and magenta to red, based on the first calculation term. In this case, the second-color-data generating unit may be adapted to generate the second color data also by multiplying the second calculation term by a predetermined matrix coefficient, and the third-color-data generating unit may be adapted to generate the third color data based also on the second color data.

With the above arrangement, it is possible, in color conversion, to independently control a certain hue component included in one of the inter-hue zones, and desired color conversion can be achieved without color contraction.

The invention also provides a color conversion method in which a hue, lightness, and/or saturation of first color data representing the respective colors forming a color image is converted to generate second color data corresponding to the first color data, and third color data suitable for the color space which can be expressed by an image output unit used for outputting the color image is generated based on the second color data.

With the above arrangement, the third color data suitable for the color space which can be expressed by the image output unit is generated based on the second color data, so that desired color conversion can be achieved without color contraction.

The third color data may be generated based on a maximum value of the gradation levels of the respective colors represented by the second color data. In this case, the third color data may be generated by reducing the gradation level of each of the colors of the second color data according to the maximum value in excess of a predetermined value.

The third color data may alternatively be generated based on the complementary color data representing the respective complementary colors of a plurality of colors represented by the second color data. In this case, the third color data may be generated based on a maximum value of the gradation levels of the respective complementary colors represented by the complementary color data. In this case, the third color data may be generated by reducing the gradation level of each of the complementary colors of the complementary color data according to the maximum value in excess of a predetermined value.

In the above method of color conversion, a ratio between the hue components of magenta and red, red and yellow, yellow and green, green and cyan, cyan and blue, or blue and magenta may be additionally calculated, and the third color data may be generated based on the ratio between the hue components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of the color conversion apparatus according to Embodiment 1;

FIG. 2 is a block diagram showing the configuration of the color data compressor;

FIG. 3 is a block diagram showing the configuration of the color data multiplication coefficient calculator;

FIG. 4 is a graph showing the characteristics of the color data multiplication coefficient;

FIG. 5 is a block diagram showing the configuration of the color data compressor according to Embodiment 2;

FIG. 6 is a graph showing the characteristics of the color data multiplication coefficient;

FIG. 7 is a block diagram showing the configuration of the color conversion apparatus according to Embodiment 2;

FIG. 8 is a block diagram showing the configuration of the complementary color data compressor;

FIG. 9 is a block diagram showing the configuration of the complementary color data multiplication coefficient calculator;

FIG. 10 is a graph showing the characteristics of the complementary color data multiplication coefficient;

FIG. 11 is a block diagram showing the configuration of the color conversion apparatus according to Embodiment 4;

FIG. 12 is a graph showing the characteristics of the color data multiplication coefficient;

FIG. 13 is a graph showing the characteristics of the complementary color data multiplication coefficient;

FIG. 14 is a block diagram showing the configuration of the color conversion apparatus according to Embodiment 5;

FIG. 15 is a graph showing the characteristics of the complementary color data multiplication coefficient;

FIG. 16 is a graph showing the characteristics of the color data multiplication coefficient;

FIG. 17 is a block diagram showing the configuration of the color conversion apparatus according to Embodiment 6;

FIG. 18 is a block diagram showing the configuration of the color converter according to Embodiment 6;

FIG. 19 schematically illustrates the hue data;

FIG. 20 is a block diagram showing the configuration of the polynomial calculator;

FIG. 21 schematically illustrates the relationship between the polynomial data and the hues;

FIG. 22 schematically illustrates the relationship between the polynomial data and the hues;

FIG. 23 is a block diagram showing the configuration of the matrix calculator;

FIG. 24 is a block diagram showing the configuration of the color data compressor;

FIG. 25 is a block diagram showing the configuration of the color data multiplication coefficient calculator;

FIG. 26 is a block diagram showing the configuration of the selection-type look-up table;

FIG. 27 is a block diagram showing the configuration of the color conversion apparatus according to Embodiment 7;

FIG. 28 is a block diagram showing the configuration of the color conversion apparatus according to Embodiment 8;

FIG. 29 is a block diagram showing the configuration of the color data compressor;

FIG. 30 is a block diagram showing the configuration of the polynomial data generator;

FIG. 31 is a block diagram showing the configuration of the color converter according to Embodiment 9;

FIG. 32 is a block diagram showing the configuration of the polynomial calculator;

FIG. 33 schematically illustrates polynomial data;

FIG. 34 schematically illustrates polynomial data;

FIG. 35 is a block diagram showing the configuration of the matrix calculator;

FIG. 36 is a block diagram showing the configuration of a conventional color conversion apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will now be described with reference to the attached drawings.

Embodiment 1.

FIG. 1 is a block diagram showing the configuration of a color conversion apparatus according to Embodiment 1. As illustrated, the color conversion apparatus of this embodiment includes a color converter 1a, and a color gamut compressing section 2 consisting of a color data compressor 3a. The color converter 1a color-converts first color data R1, G1, B1 in accordance with the following matrix calculation formula (1) to output second color data R2, G2, B2. Here, Aij (i=1 to 3, j=1 to 3) are matrix coefficients determining the color conversion characteristics.

.function. ##EQU00004##

The color data compressor 3a applies color gamut compression processing, to be described later, to the second color data R2, G2, B2, to output third color data R3, G3, B3. The third color data R3, G3, B3 is supplied to an image display device (not shown) provided in a subsequent stage.

The first color data R1, G1, B1 is color data of 8 bits (256 gradation levels) and can assume any value from 0 to 255. Here, the second color data obtained by color-converting the first color data is assumed to be color data of 10 bits with a sign. The second color data can assume a value in the range of from -512 to 511. Accordingly, the second color data represents a gradation level of each of the colors of R, G, B, by means a value in the range of -512 to 511. The color data compressor 3a applies color gamut compression processing to the second color data R2, G2, B2, to produce color data having gradation levels suitable for the color space which can be displayed by the image display device. In the color conversion apparatus according to the present embodiment, the display ability of the image display device is of 8 bits, and accordingly, the color gamut compressing section 2 converts the second color data to the third color data R3, G3, B3 which is color data of 8 bits (256 gradation levels).

The second color data, and the third color data are generated taking into consideration the chromaticities of the primary colors of the image display device used, and the chromaticities of the primary colors of the color spaces expressed by the second and third color data are made equal. When the chromaticities of the primary colors are equal, the magnitude of the color space that is expressed depend on the values of the color data. The color data compressor 3a converts the second color data to the third color data which represents hues equal to the those of the second color data and which can be expressed by the image display device.

The main function of the color data compressor 3a is to perform color gamut compression processing for avoiding color contraction due to color conversion that increases the lightness.

FIG. 2 is a block diagram showing the configuration of the color data compressor 3a. As shown in FIG. 2, the color data compressor 3a comprises a color data multiplication coefficient calculator 4a, and a coefficient multiplier 5. Based on the maximum value of the second color data R2, G2, B2 output from the color converter 1a, the color data multiplication coefficient calculator 4a outputs a color data multiplication coefficient k, and supplies it to the coefficient multiplier 5. The coefficient multiplier 5 multiplies the second color data R2, G2, B2 by the color data multiplication coefficient k to produce the third color data R3, G3, B3. The third color data R3, G3, B3 output from the coefficient multiplier 5 can be represented by the following formula (2): R3=k.times.R2 G3=k.times.G2 B3=k.times.B2 (2)

FIG. 3 shows the configuration of the color data multiplication coefficient calculator 4a. It includes a maximum value calculator 6a and a look-up table (LUT) 7a. The maximum value calculator 6a selects the maximum value of the second color data R2, G2, B2, and outputs it to the look-up table 7a. The look-up table 7a outputs the color data multiplication coefficient k based on the maximum value of the second color data R2, G2, B2. The maximum value of the second color data R2, G2, B2 is input to the look-up table 7a, as a read address, and the look-up table 7a outputs the corresponding color data multiplication coefficient k.

The characteristics of the color data multiplication coefficient k stored in the look-up table 7a is determined in accordance with the matrix coefficients used for the color conversion performed by the color converter 1a. For instance, when the color conversion is performed in accordance with the following matrix calculation formula (3), the characteristics of the corresponding color data multiplication coefficient k is determined as follows:

.function. ##EQU00005##

The above matrix calculation formula (3) is for a conversion which increases the lightness of the first color data R1, G1, B1. When the color conversion is performed in accordance with the formula (3), the values of the second color data will be in the range of 0 (=0.times.1.2) to 306 (=255.times.1.2). The maximum value of the second color data R2, G2, B2 output from the maximum value calculator 6a will also be in the range of 0 to 306.

FIG. 4 shows characteristics of the color data multiplication coefficient k in the present embodiment, and shows the value of the color data multiplication coefficient k against the maximum value of the second color data R2, G2, B2. As will be seen from FIG. 4, the color data multiplication coefficient k is "1" when the maximum value of the second color data R2, G2, B2 is not more than "200." When the maximum value is larger than "200," the color data multiplication coefficient k decreases monotonically with the increase of the maximum value. Accordingly, in the range in which the maximum value of the second color data is not more than "200,", the third color data is equal to the second color data, while in the range in which the maximum value of the second color data is more than "200," the third color data is smaller than the second color data. When the maximum value of the second color data R2, G2, B2 is "306," the color data multiplication coefficient k is set to be 0.833 (=255/306) such that the second color data is converted to the color data of the maximum gradation level which can be displayed by the display device.

A specific example of the operation of the color conversion apparatus shown in FIG. 1 will next be described. For two different sets of color data with: R1=230, G1=20, B1=20; and R1=240, G1=20, B1=20 which are input as the first color data to the color converter 1a, the second color data obtained by the color conversion according the above matrix calculation formula (3) will have values: R2=276, G2=24, B2=24; and R2=288, G2=24, B2=24, respectively.

The color data compressor 3a multiplies the second color data by different values of the color data multiplication coefficient k corresponding to the respective maximum values "276" and "288" of the second color data R2, G2, B2, to output the third color data R3, G3, B3. As shown in FIG. 4, the multiplication coefficient k corresponding to the maximum values "276" and "288" are 0.880 and 0.861, respectively. Accordingly, the third color data will have values: R3=243, G3=21, B3=21; and R3=248, G3=21, B3=21, so that the color conversion which increases the lightness is performed without causing color contraction.

As is illustrated by the above specific example, the color conversion apparatus of the present embodiment multiplies the second color data R2, G2, B2 obtained by the color conversion, by a predetermined coefficient, so as to convert the second color data to color data of the number of gradation levels which can be displayed by the display device, so that it is possible to avoid color contraction accompanying the conventional color conversion which increases the lightness.

Moreover, as shown in FIG. 4, when the maximum value of the second color data R2, G2, B2 is more than a predetermined value, the value of the color data multiplication coefficient k is decreased with the increase of the maximum value, while when the maximum value is not more than the predetermined value, the value of the color data multiplication coefficient k is fixed to "1." Accordingly, reduction in the lightness due to unnecessary color gamut compression processing is avoided, and the color gamut compression is applied only to the color data having a high lightness which may be associated with color contraction.

Moreover, the same color data multiplication coefficient k is used for multiplication with each of the R, G, B components of the second color data in the color gamut compression processing, so that the ratio between the R, G, B components in the second color data is preserved in the third color data. The ratio between the R, G, B components in the color data is the hue information of the color, and the visual sense of the human being is very sensitive to the change in the hue. Because the color conversion apparatus of the present embodiment performs color gamut compression while preserving the hue information of the second color data, desired color conversion can be achieved without causing color contraction.

In the present embodiment, the color data multiplication coefficient calculator 4a is formed of the look-up table 7a. The memory capacity may be reduced if a calculation circuit is used in combination with a look-up table, for interpolation between data points stored in the look-up table.

Also, by rewriting the contents of the look-up table 7a, the characteristics of the color data multiplication coefficient k can be flexibly altered in conformity with the display characteristics of the display unit, and the conversion characteristics of the color converter 1a.

The color conversion apparatus shown in FIGS. 1 to 3 can also be implemented by software. However, when moving pictures are processed in real time, and in particular when a high-speed processing is required, a configuration using hardware is preferred.

The color conversion processing and the color gamut compression processing according to the present embodiment can be applied not only to color data consisting of three primary colors, but also to color data consisting of four or more color components.

Embodiment 2.

FIG. 5 is a block diagram showing another configuration of the color data compressor 3a. As illustrated in FIG. 5, the color data compressor 3a of Embodiment 2 comprises a color data multiplication coefficient calculator 4a, a set value storage 8, a set value subtractor 9, a coefficient multiplier 10, and a set value adder 11.

The set value storage 8 outputs a predetermined set value th, and sends it to the set value subtractor 9 and the set value adder 11. The set value subtractor 9 subtracts the set value th from each of the second color data R2, G2, B2 output from the color converter 1a (FIG. 1). The output data R4, G4, B4 from the set value subtractor 9 are given by the following formula (4). R4=R2-th G4=G2-th B4=B2-th (4)

The color data multiplication coefficient calculator 4a is formed of a maximum value calculator 6a and a look-up table 7a, as in Embodiment 1 (see FIG. 3), and outputs a color data multiplication coefficient k based on the maximum value of the data R4, G4, B4 output from the set value subtractor 9. The coefficient multiplier 10 multiplies the output data R4, G4, B4 of the set value subtractor 9, by the color data multiplication coefficient k output from the color data multiplication coefficient calculator 4a. The output data R5, G5, B5 of the coefficient multiplier 10 is given by the following formula (5). R5=k.times.R4 G5=k.times.G4 B5=k.times.B4 (5)

The set value adder 11 adds the set value th to the output data R5, G5, B5 of the coefficient multiplier 10. The output data of the set value adder 11 are output as the third color data R3, G3, B3. The third color data R3, G3, B3 output from the set value adder 11 are given by the following formula (6). R3=R5+th G3=G5+th B3=B5+th (6)

The characteristics of the color data multiplication coefficient k output from the color data multiplication coefficient calculator 4a is determined in the following manner. When the color converter 1a (FIG. 1) performs the color conversion processing in accordance with the above matrix calculation formula (3), the values of the second color data R2, G2, B2 are in the range of from "0" to "306." It is assumed here that the set value th determined by the set value storage 8 is "100." The values of the output data R4, G4, B4 of the set value subtractor 9 are in the range of from "-100" to "206," so that the maximum value of the output data R4, G4, B4 of the set value subtractor 9 is also within the range of from "-100" to "206."

When the maximum value of R4, G4, B4 is smaller than "0," i.e., when the maximum value of the second color data R2, G2, B2 is smaller than the set value th=100, by setting the color data multiplication coefficient k to be a constant (=1), the amount of data stored in the look-up table 7a (see FIG. 3) for the respective values of R4, G4, B4 can be reduced. Specifically, when the maximum value of R4, G4, B4 is smaller than "0," the read address of the look-up table 7a is fixed to "0," so as to read the color data multiplication coefficient k=1 at the address.

FIG. 6 shows characteristics of the color data multiplication coefficient k in the present embodiment. As shown in FIG. 6, when the maximum value of R4, G4, B4 is not more than "100," the color data multiplication coefficient is "1," while when the maximum value is more than "100," the color data multiplication coefficient decreases with increasing maximum value. When the maximum value of R4, G4, B4 is "206," the corresponding color data multiplication coefficient k is (255-100)/(306-100)=155/206=0.752.

The operation of the color data compressor 3a shown in FIG. 5 will next be described with reference to a specific example. Converting the first color data with R1=230, G1=20, B1=20 in accordance with the above matrix calculation formula (3) results in the second color data with R2=276, G2=24, B2=24. In this case, the output data of the set value subtractor 9 will have values: R4=176, G4=-76, B4=-76. The color data multiplication coefficient calculator 4a outputs the color data multiplication coefficient k based on the maximum value of R4, G4, B4, and the coefficient multiplier 10 multiplies the R4, G4, B4 by the output color data multiplication coefficient k.

As shown in FIG. 6, the color data multiplication coefficient k corresponding to the maximum value 176 of R4, G4, B4 is 0.822. Accordingly, the output data of the coefficient multiplier 10 will have values: R5=145, G5=-62, B5=-62. The set value adder 11 adds the set value th=100 to the color data R5, G5, B5, to produce the third color data with R3=245, G3=38, B3=38.

When similar color conversion is applied to the first color data with R1=240, G1=20, B1=20, the second color data will have values R2=288, G2=24, B2=24. In this case, the output data of the set value subtractor 9 will have values R4=188, G4=-76, B4=-76.

As shown in FIG. 6, the color data multiplication coefficient k corresponding to the maximum value 188 of R4, G4, B4 is 0.794. Accordingly, the output data of the coefficient multiplier 10 will have values R5=149, G5=-60, B5=-60. The set value adder 11 adds the set value th=100 to the color data R5, G5, B5, to produce the third color data with R3=249, G3=40, B3=40.

As will be understood, the two sets of the first color data with values: R1=230, G1=20, B1=20; and R1=240, G1=20, B1=20 representing different colors are converted to the third color data with values: R3=245, G3=38, B3=38; and R3=249, G3=40, B3=40 without causing color contraction.

Embodiment 3.

FIG. 7 is a block diagram showing another configuration of a color conversion apparatus. As shown in FIG. 7, the color gamut compressing section 2 of the color conversion apparatus according to the present embodiment comprises a complementary color data compressing section 12, which in turn includes a first complementary color data calculator 13, a complementary color data compressor 3b, and a second complementary color data calculator 14. The main function of the complementary color data compressing section 12 shown in FIG. 7 is to perform color gamut compression for avoiding color contraction due to color conversion which increases saturation.

The first complementary color data calculator 13 determines the complements of the second color data R2, G2, B2 output from the color converter 1a, in accordance with the following formula (7), to output complementary color data C1, M1, Y1 representing complements of the second color data. The complementary color data C1, M1, Y1 represent the gradation levels of cyan, magenta, yellow (C, M, Y) corresponding to the complements of the second color data R2, G2, Y2. C1=255-R2 M1=255-G2 Y1=255-B2 (7)

When the second color data R2, G2, B2 are color data of 10 bits with a sign, and assumes a value of -512 to 511, the complementary color data C1, M1, Y1 assumes a value of -256 to 767. Accordingly, the complementary color data C1, M1, Y1 represent gradation levels of the complementary colors by a value in the range of -256 to 767. There are 1023 integer values in the range of -256 to 767, and they can be represented by 10-bit data. The complementary color data compressor 3b outputs compressed complementary color data C2, M2, Y2 obtained by conversion of the complementary color data C1, M1, Y1 into complementary color data of 8 bits which can be displayed by the display device. The second complementary color data calculator 14 determines the complements of the complementary color data C2, M2, Y2 in accordance with the following formula (8), to output third color data R3, G3, B3 of 8 bits.

As an alternative to determining the complements in accordance with the formula (7), the second color data R2, G2, B2 may be subtracted from "511" to produce the complementary color data C1, M1, Y1. In this case, the complementary color data C1, M1, Y1 will have values in the range of 0 to 1023. R3=255-C2 G3=255-M2 B3=255-Y2 (8)

FIG. 8 is a block diagram showing the configuration of the complementary color data compressor 3b. The complementary color data C1, M1, Y1 output from the first complementary color data calculator 13 are input to a complementary color data multiplication coefficient calculator 4b, and a coefficient multiplier 5. The complementary color data multiplication coefficient calculator 4b outputs a complementary color data multiplication coefficient h based on the complementary color data C1, M1, Y1, and sends it to the coefficient multiplier 5. The coefficient multiplier 5 multiplies the complementary color data C1, M1, Y1 by the complementary color data multiplication coefficient h, to produce compressed complementary color data C2, M2, Y2. The compressed complementary color data C2, M2, Y2 output from the coefficient multiplier 5 are represented by the following formula (9). C2=h.times.C1 M2=h.times.M1 Y2=h.times.Y1 (9)

FIG. 9 is a block diagram showing the configuration of the complementary color data multiplication coefficient calculator 4b. A maximum value calculator 6b selects the maximum value of the complementary color data C1, M1, Y1, and outputs it to a look-up table (LUT) 7b. The look-up table 7b outputs the complementary color data multiplication coefficient h, based on the maximum value of the complementary color data C1, M1, Y1.

The characteristics of the complementary color data multiplication coefficient h stored in the look-up table 7b is determined in correspondence with the matrix coefficients for the color conversion performed by the color converter 1a. When the color conversion is performed in accordance with the following matrix calculation formula (10), the characteristics of the corresponding complementary color data multiplication coefficient h is determined in the following way.

.function. ##EQU00006##

The above matrix calculation formula (10) is a conversion formula that increases the saturation of the first color data R1, G1, B1, and when the color conversion is performed in accordance with the above matrix calculation formula (10), the value of the second color data will be in the range of -51 to 255. Accordingly, the maximum value of the complementary color data C1, M1, Y1 output from the maximum value calculator 6b shown in FIG. 9 will be in the range of 0 to 306, so that the read address of the look-up table 7b will be from 0 to 306.

FIG. 10 shows characteristics of the complementary color data multiplication coefficient h th in the present embodiment. FIG. 10 shows the value of the complementary color data multiplication coefficient h against the maximum value (read address) of the complementary color data C1, M1, Y1, which is supplied to the look-up table 7b as the read address. As shown in FIG. 10, the complementary color data multiplication coefficient his "1" when the maximum value of the complementary color data C1, M1, Y1 is not more than "240," and monotonically decreases with increase of the maximum value in excess of "240." When the maximum value of C1, M1, Y1 is "306," the complementary color data multiplication coefficient h is set to 0.833 (=255/306) such that the compressed complementary color data C2, M2, Y2 will be the color data of the maximum gradation level which can be displayed by the image display device.

The operation of the color conversion apparatus shown in FIG. 7 will next be described with reference to a specific numerical value example.

For two different sets of the color data with: R1=230, G1=10, B1=10; and R1=230, G1=20, B1=20 that are input to the color converter 1a as the first color data, the second color data obtained in accordance with the color conversion the above matrix calculation formula (10) will have values: R2=228, G2=-14, B2=-14; and R2=228, G2=-5, B2=-5, respectively.

The first complementary color data calculator 13 determines the complements of the second color data in accordance with the above formula (7) to produce complementary color data with values: C1=27, M1=269, Y1=269; and C1=27, M1=260, Y1=260, respectively.

The complementary color data compressor 3b multiplies the first complementary color data with the complementary color data multiplication coefficient h corresponding to the complementary color data M1, C1, Y1, to produce the compressed complementary color data M2, C2, Y2. As shown in FIG. 10, the values of the complementary color data multiplication coefficient h corresponding to the maximum values 269 and 260 are 0.927 and 0.950, respectively. Accordingly, the compressed complementary color data will have values: C2=25, M2=249, Y2=249; and C2=26, M2=247, Y2=247, respectively.

The second complementary color data calculator 14 determines the complements of the above compressed complementary color data C2, Y2, M2 in accordance with the above formula (8), to produce third color data with values: R3=230, G3=6, B3=6; and R3=229, G3=8, B3=8, respectively.

As shown with reference to the above specific example, by applying the color gamut compression processing to the complementary color data of the second color data, it is possible to avoid color contraction due to the color conversion which increases saturation.

Moreover, as shown in FIG. 10, when the maximum value of the complementary color data M1, C1, Y1 is larger than a predetermined value, the value of the complementary color data multiplication coefficient h is reduced with increase of the maximum value, while when the maximum value is smaller, the color data multiplication coefficient h is fixed to "1," so that it is possible to perform color gamut compression on only the color data having a high saturation liable to color contraction, and it is possible to avoid reduction in the saturation due to unnecessary color gamut compression.

Also, in the color gamut compression, an identical complementary color data multiplication coefficient h is used for multiplication with the M, C, Y components of the complementary color data, so that the ratio between the M, C, Y components of the complementary color data is preserved in the compressed complementary color data M2, Y2, C2. As a result, the hue information of the second color data R2, G2, B2 corresponding to the complementary color data M1, C1, Y1 will be preserved in the third color data R3, G3, B3 corresponding to the compressed complementary color data M2, C2, Y2. As has been described, the color conversion apparatus according to the present embodiment performs color gamut compression while preserving the hue information of the second color data, so that it can achieve desired color conversion without causing color contraction, as in the previous embodiments.

Embodiment 4.

FIG. 11 is a block diagram showing another configuration of a color conversion apparatus. As shown in FIG. 11, the color gamut compressing section 2 of the color conversion apparatus according to Embodiment 4 includes a color data compressor 3a, and a complementary color data compressing section 12, which in turn includes a first complementary color data calculator 13, a complementary color data compressor 3b, and a second complementary color data calculator 14. The color gamut compressing section 2 shown in FIG. 11 performs color gamut compression for avoiding color contraction accompanying color conversion that increases lightness and saturation.

The second color data output from the color converter 1a is color data of 10 bits with a sign, and can assume a value of from -512 to 511. The color data compressor 3a has a configuration similar to that shown in the block diagram of FIG. 2. Applying color gamut compression explained in connection with Embodiment 1 to the second color data R2, G2, B2, it outputs compressed color data R6, G6, B6 with the maximum value being set to "255." The range of the value of the compressed color data R6, G6, B6 will be from -512 to 255.

The first complementary color data calculator 13 determines the complements of the compressed color data R6, G6, B6 in accordance with the following formula (11) to produce complementary color data C1, M1, Y1. C1=255-R6 M1=255-G6 Y1=255-B6 (11)

As a result of the above-mentioned calculation of the complements, the value of the complementary color data C1, M1, Y1 will be in the range of 0 to 767. The complementary color data compressor 3b has a configuration similar to that shown in the block diagram of FIG. 8, and applies the color gamut compression explained in connection with Embodiment 3, to the complementary color data C1, M1, Y1, to produce compressed complementary color data C2, M2, Y2 of 8 bits with the maximum being 255. The second complementary color data calculator 14 determines the complements of the compressed complementary color data C2, M2, Y2 in accordance with the above formula (8) to produce third color data R3, G3, B3 of 8 bits.

Characteristics of the color data multiplication coefficient k, and complementary color data multiplication coefficient h used in the color gamut compression in the color data compressor 3a (FIG. 2), and the complementary color data compressor 3b (FIG. 8) are determined in correspondence with the matrix calculation formula used in the color conversion. When the color converter 1a performs color conversion in accordance with the following matrix calculation formula (12), the characteristics of the corresponding color data multiplication coefficient k, and complementary color data multiplication coefficient h are determined in the following manner.

.function. ##EQU00007##

The matrix calculation formula shown in the formula (12) is for performing conversion that increases the lightness and saturation of the first color data R1, G1, B1, and when the color conversion is performed in accordance with the above matrix calculation formula (12), the values of the second color data R2, G2, B2 will be within the range of -51 to 306. The result is that the maximum value of the second color data R2, G2, B2 output from the maximum value calculator 6a (FIG. 3) to the look-up table 7a will be within the range of from -51 to 306. When the maximum value of the second color data R2, G2, B2 is not more than 0, the color data multiplication coefficient k is set to a constant (=1). Specifically, the read address of the look-up table 7a is fixed to `0`, and the color data multiplication coefficient k=1 is output.

FIG. 12 shows characteristics of the color data multiplication coefficient k according to the present embodiment. As shown in FIG. 12, the color data multiplication coefficient k is "1," when the maximum value of the second color data R2, G2, B2 is not more than 200; it monotonically decreases with increase in the maximum value in excess of 200; and it is 0.833 (=255/306) when the maximum value is 306.

By multiplying the second color data R2, G2, B2 with the color data multiplication coefficient k having a characteristics shown in FIG. 12, the second color data R2, G2, B2 are converted to compressed color data R6, G6, B6 assuming a value in the range of -51 to 255. Accordingly, the values of the complementary color data C1, M1, Y1 obtained by determining the complements of the compressed color data R6, G6, B6 in accordance with the above formula (11) will be in the range of 0 to 306, so that the maximum value of C1, M1, Y1 output from the maximum