JPH0846989A - Method and device for color conversion - Google Patents

Abstract

PURPOSE:To effectively correct the variation of color visivility caused by the variation of white points in an image depending on a reproduction system, an observer or perception by deciding the saturation of each picture element of the image after conversion without changing the main wavelength of a color with respect to each picture element before and after the conversion. CONSTITUTION:Color data of each picture element converted to tristimulus values of XYZ by a front color conversion means 2 is stored in a pre-conversion color area storage means, and the chromaticity coordinates of white points of the reproducing system before and after the conversion are stored in storage means 12 and 16 respectively. A conversion color decision means 17 converts the color of each picture element without changing the main wavelength depending on before or after the conversion by the use of each of these data and data from a main wavelength storage means 13. Furthermore, the chromaticity coordinates of each picture element after the conversion is converted within the color area of the post-conversion reproduction system by the use of saturation calculated by a saturation calculation and storage means based on data on a color area and the white points before the conversion and each data on the color area and the white points after the conversion. Thereby, the entire image is subjected to proper color correction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention converts color image data into
For example, the present invention relates to a color conversion method and apparatus for different types of media or different media, such as conversion to color data for CRT display or printing, and more particularly to a color conversion method and apparatus for changing the white point of a reproduction system. It is a thing.

[0002]

2. Description of the Related Art When a color image is displayed on a color CRT or the like, the reference white point of the CRT actually used may be different from the reference white point of the CRT used as a reference when creating the image. For example, the white point of an NTSC standard color CRT is
CIE: On the xy chromaticity diagram defined by the International Commission on Illumination (x,
y) = (0.310, 0.316): correlated color temperature≈65
It is the point of 00K, but the color CR that is actually on the market
The correlated color temperature of the white point of T is often 9000K or higher. As a method of correcting the color of the image due to the difference in the white point, the chromaticity coordinates of the color that will be reproduced by the reference CRT are obtained, and the same chromaticity coordinates are given on the CRT actually used. A method of making the chromaticity coordinates of reproduced colors the same has been adopted, such as obtaining RGB signals for

On the other hand, a color image displayed on a color CRT or the like may be output as a hard copy, or a color image on a reversal film may be used to create a printed matter, so that color data may be exchanged between different media. Even in this case, the white point before conversion and the white point after conversion do not necessarily match. For example, the reference white point of the color CRT is 6500K at the correlated color temperature, as described above, while the reference white point in a printed or photographic image is the correlated color temperature of 5000K. In this case as well, it is fundamental to perform color conversion without changing the chromaticity coordinates of each pixel before and after conversion. For example, even if the color gamuts between media are different and the color gamuts must be compressed, the white points are treated as equal, and compression is performed with this white point as the center.

When the observation environment or the observer changes, the chromaticity coordinates (perceptual white point) of "white" felt there change depending on the adaptation to the observation environment and the difference in the spectral luminosity of the observer. As a correction method for this, for example, JP-A-1-
As shown in No. 318486, a method of simulating the appearance of a hard copy under a certain illumination light by a color CRT is known. This method obtains a correction value by empirically performing correction so that a hard copy placed under a certain illumination light and the image on a CRT have the same appearance, and thereby a correction look under various illumination lights is obtained. An up table (LUT) is defined, and the CRT display image is corrected so that it looks like the same color as the hard copy.

As a method of correcting the color of an image with respect to a change in perceptual white point, a method of moving the white point of the image according to the adaptation state of the observer is known.
Since this method does not change the chromaticity coordinates of the primary colors of the image reproduction system, colors on the outermost edge of the color reproduction area or colors having chromaticity close to the outermost edge are hardly corrected.
As described above, no color conversion device has been proposed so far, which corrects the color of the entire image with respect to the change of the perceptual white point.

[0006]

When converting a color of an image reproduced by a color CRT having a correlated white point of a reference white point of 9000K into a color reproducible on a printed matter having a reference white point of 5000K, before and after conversion. In the conventional method in which the colorimetric values of the respective images are converted to be the same, there is a problem that what was reproduced as white on the CRT is reproduced as a bluish color. Further, in the above-mentioned conventional example, an image of a color CRT having a reference white point correlated color temperature of 6500K is converted into a color CR having a reference white point correlated color temperature of 9000K.
When converting to a T image, if the colorimetric values of each pixel are the same before and after conversion, both CRs are converted.
The images when T are arranged side by side look almost the same, but when the converted image is observed only with a CRT having a reference white point correlated color temperature of 9000K, the white point perceived by the observer is the CRT screen. Since it adapts to the white point of, the appearance of the image is observed as reddish.

Further, in the above-mentioned conventional example, a look-up for monitor image correction is performed in consideration of the influence of the illumination light of the observation environment so that the hard copy placed under the illumination light and the image on the CRT have the same appearance. The method of creating and correcting a table (LUT) has a problem that it requires considerable skill to create an LUT for the correction. In addition, when the observer's perceptual white point changes due to the adaptation effect of the viewing environment light, the reference white point of the color reproduction system is moved to correct the appearance, and colors near the outer edge of the color reproduction range are almost eliminated. There is a problem in that the image is not corrected, and the appearance of the entire image is not always appropriate.

The present invention has been made in view of the above circumstances, and a color capable of effectively correcting the deterioration of the appearance of an image due to a change in white point perceived by an image reproduction system or an observer over the entire image. It is an object of the present invention to provide a conversion method and its device.

[0009]

In order to achieve the above object, the invention of claim 1 provides a white point and a second point of a first image reproduction system.
When the chromaticity point of the white point of the image reproduction system is different, the first
A color conversion method for converting an image before conversion of a color reproducible by an image reproduction system into an image after conversion of a color reproducible by the second image reproduction system, wherein a white point of the first image reproduction system is used as a reference. The main wavelength of the color for each pixel of the pre-conversion image and the main wavelength of each pixel of the post-conversion image based on the white point of the base 2 image reproduction system are set to be the same. Color saturation is determined and color conversion is performed. And claim 2
In determining the saturation, a distance from a white point of the first image reproduction system to a color gamut of the first image reproduction system through a chromaticity point of each pixel of the image before conversion, and The ratio of the distance from the white point of one image reproduction system to the chromaticity point of each pixel of the image before conversion is calculated, and the chromaticity point of each pixel of the image after conversion is calculated from the white point of the second image reproduction system. The ratio of the distance to the color gamut of the second image reproduction system and the distance from the white point of the second image reproduction system to the chromaticity point of each pixel of the converted image is equal to the ratio. It determines the saturation.

According to a fourth aspect of the present invention, there is provided a color conversion apparatus, which comprises a pre-conversion white point storage means for storing a white point of a first image reproduction system for reproducing a pre-conversion image, and the pre-conversion based on the white point. From the white point, a main wavelength detection storage unit that detects the main wavelength of each pixel of the image and stores the chromaticity coordinates thereof, a pre-conversion color gamut storage unit that stores the color gamut data of the first image reproduction system, From the ratio of the distance from the chromaticity point of each pixel of the pre-conversion image to the color gamut of the first image reproduction system to the distance to the chromaticity point of each pixel of the pre-conversion image, Saturation calculation storage means for calculating and storing the saturation of each pixel of the pre-conversion image, and the post-conversion white point for storing the white point of the second image reproduction system for reproducing the post-conversion image obtained by color-converting the pre-conversion image. Storage means and post-conversion color gamut storage means for storing the color gamut data of the second image reproduction system The hue line connecting the chromaticity coordinates stored in the main wavelength detection storage means and the white point in the converted image is determined, and the saturation of the color of the pixel is determined by the saturation data from the saturation calculation storage means, The color of each pixel of the image before conversion is determined based on the white point data from the converted white point storage means and the color gamut data from the converted color gamut storage means, and the color of each pixel of the image before conversion is within the color gamut of the second image reproduction system. And a conversion color determining unit for converting the color into the color.

According to the present invention, the white point before conversion of the first image reproduction system is a white point defined by the first image reproduction system or a perceptual condition depending on an observation condition of the first image reproduction system. The white point after conversion of the second image reproduction system, which is a white chromaticity point, is the white point defined by the second image reproduction system or the perceptual white chromaticity according to the observation condition of the second image reproduction system. It is a point.

[0012]

In the present invention, since the color of each pixel is color-converted without changing the dominant wavelength of the color for each pixel before and after conversion, an image when a color image is reproduced by a reproduction system having different white points. It is possible to appropriately correct the color appearance of the image due to the change of the white point due to the change of the adaptation state of the observer that occurs when the environment changes, or the entire image.

Further, in the apparatus of the present invention, the conversion color determining means determines the color of each pixel as before conversion by the respective data from the pre-conversion white point storage means, the main wavelength detection storage means and the post-conversion white point storage means. After that, color conversion is performed without changing the dominant wavelength,
Further, the saturation calculation storage means uses the respective data from the pre-conversion color gamut storage means and the pre-conversion white point storage means to pass the white point of the pre-conversion image through the chromaticity points of each pixel of the pre-conversion image before conversion. Obtaining the ratio of the distance to the color gamut of the image and the distance from the white point to the chromaticity point of each pixel of the image before conversion,
The saturation of the image before conversion is calculated from this ratio, and the conversion color determination means further uses the respective data from the saturation calculation storage means, the converted color gamut storage means, and the converted white point storage means to determine each pixel of the converted image. Is converted into a color within the color gamut of the second image reproduction system. Therefore, the color correction of the image when reproducing a color image in a reproduction system with a different white point, or the appearance of the image color due to the change of the perceptual white point due to the change of the adaptation state of the observer that occurs when the environment changes It becomes possible to perform the correction properly over the entire image.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a color conversion device according to the method of the present invention. In FIG. 1, 1 is, for example, C
A color conversion unit that corrects image deterioration due to a change in white point when image data reproduced by a reproduction system such as RT is reproduced by a reproduction system such as another CRT (or as a printed matter which is another medium). Is. Reference numeral 2 denotes a pre-color conversion unit, which pre-converts arbitrary image data (for example, RGB gradation value, YMCK halftone dot area ratio or density value) handled in the pre-conversion reproduction system, for example. It is converted into XYZ tristimulus values defined by CIE (International Commission on Illumination). Further, 3 is a rear color conversion means, and this rear color conversion means 3 handles color data (for example, XYZ values by the CIE method) of the image data converted by the color conversion device 1 in a converted reproduction system. Arbitrary data (eg RGB gradation value or Y
MCK halftone dot area ratio or density value).

The color conversion unit 1 calculates a color gamut of a reproduction system before conversion based on the XYZ tristimulus values converted by the pre-color conversion unit 2, and stores the pre-conversion color gamut storage unit 11. Pre-conversion white point storage means 12 for inputting and storing the white point of the previous reproduction system or the perceptual white point under the observation condition;
With the white point stored in the pre-conversion white point storage means 12 as a reference, the line connecting the white point and the chromaticity point of the pixel obtained from the XYZ tristimulus values is extended to extend the spectrum locus or the magenta line. Based on the XYZ tristimulus values from the pre-color conversion unit 2, the main wavelength detection storage unit 13 that stores the chromaticity coordinates of the main wavelength of each pixel is calculated from the intersection that intersects with the pre-conversion white point storage unit 12. And the distance from the white point of the reproduction system before conversion stored in to the color gamut of the reproduction system before conversion through the chromaticity point of the pixel, and the distance from the white point to the chromaticity point of the pixel Is calculated, and from the ratio, the saturation calculation storage unit 14 that calculates and stores the saturation of the pixel in the image of the reproduction system before conversion, and the color gamut storage after conversion that stores the color gamut of the reproduction system after conversion. Means 15 and perceptual in the white point or viewing condition of the converted reproduction system The converted white point storage means 16 for storing color points, the main wavelength data for each pixel from the main wavelength detection storage means 13, the saturation data from the saturation calculation storage means 14, and the converted color gamut storage means 15 Using the converted color gamut data and the converted white point data from the converted white point storage means 16, the color of each pixel in the image of the converted reproduction system is converted without changing the dominant wavelength, and the converted reproduction system is also used. And a conversion color determining means 17 for converting into the color gamut.

Next, the operation of this embodiment configured as described above will be described. First, in the pre-color conversion means 2, for example, the conversion formulas for converting RGB signals handled by a color CRT into XYZ tristimulus values are as follows: chromaticity point of each primary color phosphor of CRT, white chromaticity point, and each fluorescence point. Considering the gamma characteristic of the body, for example, the following conversion formula [Equation 1] is used.

[0017]

[Equation 1]

Each element a _ij (i, j = 1, 1 of the above conversion formula
2, 3) are obtained by measuring the chromaticity points of each primary color phosphor and the white chromaticity points and substituting them in the above equation. For example NT
The conversion formula [Formula 2] from the RGB signal to the XYZ value for the SC standard CRT is shown below.

[0019]

[Equation 2]

The gamma coefficient in this case is 2.2. XY from YMCK halftone dot area ratio handled in printing
The conversion into the Z value may be performed as described below, for example. That is, the CMYK halftone dot area ratio is changed in steps of 10%, and a color chart reproduced by a combination of those halftone dot area ratios is printed. Next, they are color-measured using a colorimeter, and the correspondence between the halftone dot area ratio and the XYZ values is looked up in a lookup table (L
UT). For intermediate colors, corresponding points are obtained by interpolating based on this LUT data. This allows XYZ for YMCK halftone dot area ratio.
Can be converted to a value. In the rear color conversion means 3, for example, the reverse conversion in the case of the front color conversion means 2 may be performed.

First, all the color data of each pixel of the image converted into the XYZ tristimulus values by the pre-color conversion means 2 are input to the pre-conversion color gamut storage means 11. Here, the substantial color gamut required for the image to be handled is stored by the following operation, for example. That is, the input color data is converted into the Yxy color space (here,
It is desirable that Y be normalized to 0 to 1). Then, for example, the maximum and minimum values of x are obtained and stored from all the image data of y = 0 on the plane of equal brightness of Y = 0.05. Next, the value of y is increased by, for example, 0.005, and similarly, the maximum value and the minimum value of x at that time are obtained and stored. This process is repeated until, for example, y is the maximum value that can be adopted by the reproduction system before conversion (in the case of a color CRT, the y coordinate of the G phosphor). Next, Y is increased by 0.05, for example, and the same processing is repeated. In this way, the color gamut of each image plane of equal brightness of the image to be handled is stored.

The post-conversion color gamut storage unit 15 stores color gamut data that can be reproduced by the post-conversion color conversion unit 3 in the post-conversion reproduction system. This may be obtained from the data that can be reproduced by the post-conversion reproducing system by the inverse conversion of the post-color conversion unit 3, for example.

The pre-conversion white point storage means 12 stores the chromaticity coordinates of the white point of the pre-conversion reproduction system, and the post-conversion white point storage means 16 stores the chromaticity coordinates of the white point of the post-conversion reproduction system. Remembered For example, the chromaticity point of D65 is stored as the white point of the monitor of the HDTV standard, and the chromaticity point of D50 is stored in the case of a printed matter. Alternatively, white (R = G = B) is displayed on the monitor actually used, the color of the white is measured by a colorimeter, the white point is obtained, and the white point is stored.

If the effect of adaptation due to the observation environment light is to be corrected, the chromaticity coordinates of the perceptual white point perceived on the pre-conversion reproduction system are stored in the pre-conversion white point storage means 12,
The converted white point storage means 16 stores the chromaticity coordinates of the perceptual white point perceived on the converted reproduction system. in this case,
The white point is calculated by actually measuring the color perceived as white in each observation environment with a telephoto colorimeter. Also, color CRT
If the reference white point before and after the conversion is different, such as when converting the above image to the print image, the chromaticity coordinates of the reference white point may be stored in the respective white point storage means. With the above preparations completed, the color data of each pixel input from the front color conversion unit 2 is converted. Less than,
The operation will be described with reference to the xy chromaticity diagram of FIG.

The color data of each pixel converted by the front color conversion means 2 (here, handled by Yxy) is sent to the main wavelength detection storage means 13 and the saturation calculation storage means 14. In the main wavelength detection storage means 13, the pre-conversion white point storage means 12
A straight line 28 connecting the chromaticity coordinates 25 of the pre-conversion white point and the chromaticity coordinates 20 of the pixel stored in is extended to the chromaticity point side of the pixel and stored in the main wavelength detection storage means 13. An intersection point 27 with the spectrum locus 21 (or the red-purple line 22) is obtained, and the wavelength corresponding to this intersection point 27 is stored in the main wavelength detection storage means 13 as the main wavelength of the pixel. This dominant wavelength is obtained, for example, by the following method.

That is, the wavelength data of the spectrum locus 21 and the magenta line 22 are actually visible light (for example, 380).
(nm to 780 nm) is obtained from the x and y coordinates on the chromaticity diagram of each monochromatic light. For example, since the spectrum locus of 560 nm to 780 nm becomes almost linear, 380 nm to 560 n
In the spectrum locus of m, an appropriate wavelength interval, for example, 1 nm
For each time, the x, y coordinates on the chromaticity diagram of the monochromatic light are stored as wavelength data, and another point, that is, chromaticity coordinate data of monochromatic light of 780 nm is sufficient. Then, the intersection of the line segment between the wavelengths having the wavelength data and the straight line 28 may be solved by a simultaneous equation. At this time, it is not necessary to check whether there is an intersection with respect to the line segment between all wavelength data. For example, when the chromaticity coordinate 20 of the pixel is (x _c , y _c ) and the chromaticity coordinate 25 of the pre-conversion white point is (x _wc , y _wc ), x _c −x _wc ≧ 0,
If y _c −y _wc ≧ 0, the line segment between the wavelength data to be examined is such that at least one of the x coordinates of the end points is x _c or more, and at least one of the y coordinates of the end points is y _c or more. It is a thing. The red-purple line 22 is a straight line connecting the chromaticity coordinate point of 380 nm and the chromaticity coordinate point of 780 nm.

In the saturation calculation storage means 14, the line segment connecting the two points of the chromaticity coordinate 25 of the pre-conversion white point and the chromaticity coordinate 20 of the pixel stored in the pre-conversion white point storage means 12 is defined as the pixel color. The straight line 28 extended to the degree side and the pre-conversion color gamut storage means 11
The color gamut outer edge 23 (here, color CR
It is shown that all the chromaticity points reproduced by T are included in the image to be handled. ), And the distance from the chromaticity coordinate 25 of the white point before conversion to the chromaticity coordinate 20 of the pixel is divided by the distance from the white point chromaticity coordinate 25 to the intersection point 31 obtained on the outer edge of the gamut. The value is stored as the saturation of that pixel.

In the conversion color determining means 17, the main wavelength of the pixel (intersection 27
(Wavelength corresponding to the above) and the chromaticity coordinate 26 of the white point stored in the converted white point storage means 16 is converted into the color of the chromaticity coordinate 20 of the pixel to the point 30 on the straight line 29. That is, the hue line of each pixel is determined. The color saturation at this time is stored in the saturation data sent from the saturation calculation storage means 14, the color gamut data stored in the converted color gamut storage means 15, and the converted white point storage means 16. It is determined by referring to the stored white point data of the white point chromaticity coordinates 26.

For example, the saturation by the reproduction system before conversion (in this case, from the white point chromaticity coordinate 25 to the color gamut 23 of the reproduction system before conversion)
To the chromaticity coordinate 20 of the pixel with respect to the distance to and the saturation by the reconstructed reproduction system (in this case, the color of the pixel with respect to the distance from the white point chromaticity coordinate 26 to the outer edge 24 of the gamut of the reconstructed reproduction system). When the conversion is performed so as to be equal to the distance to the degree coordinate 30), the value stored in the saturation calculation storage unit 14 is connected to the straight line 29 connecting the chromaticity coordinate 26 of the white point to the point 27 of the dominant wavelength. It suffices to multiply by the distance from the upper white point chromaticity coordinate 26 to the intersection 32 with the color gamut 24 of the post-conversion reproduction system. The relationship between the pre-conversion saturation and the converted saturation in this case is shown in FIG.
As shown by the line 40 in the graph of FIG. Further, the value of the saturation data stored in the saturation calculation storage means 14 is set to a curve 41 such as an exponential function or a logarithmic function as shown in FIG.
Alternatively, a straight line 29 connecting the white point chromaticity coordinate 26 and the dominant wavelength point 27 is obtained by conversion according to the relationship shown in 42.
Outer edge 2 of the color gamut of the reproduction system after conversion from the white point chromaticity coordinates 26 above
Proportion of distance to 4 (saturation by the reproduction system after conversion)
May be determined. As described above, color conversion is performed when the white point changes.

In this embodiment, the conversion color determining means 17 converts the color of each pixel based on the dominant wavelength data from the dominant wavelength detecting and storing means 13 and the white point data from the converted white point storing means 16. Color conversion is performed without changing the main wavelength between before and after, and the saturation calculation storage means 14 stores the XYZ tristimulus values from the front color conversion means 2 and the pre-conversion color gamut storage means 1.
On the basis of the color gamut data from No. 1 and the white point data from the pre-conversion white point storage means 12, the white point in the pre-conversion reproduction system passes through the chromaticity point of the pixel to be the color gamut of the pre-conversion reproduction system. The ratio between the distance to the point and the distance from the white point to the chromaticity point of the pixel is calculated, the saturation before conversion is calculated from this ratio, and the conversion color determination means 17 is the saturation calculation storage means. 1
4, the chromaticity coordinates of each pixel after conversion are converted from the color gamut data from the converted color gamut storage unit 15 and the converted white data from the converted white point storage unit 16 into the color of the converted reproduction system. Since it is converted into the range, the color correction of the image when reproducing a color image in a reproduction system with a different white point, or the image corresponding to the change of the white point due to the change of the adaptation state of the observer that occurs when the environment changes The color appearance can be corrected properly over the entire image, and the appearance of the entire image reproduced by the reproduction system having different white points can be made the same.

The present invention is not limited to the configurations described in the above embodiments, and various modifications can be made without departing from the scope of the claims.

[0032]

As described above, according to the present invention, when the white point of the first image reproduction system and the white point of the second image reproduction system have different chromaticity points, a color that can be reproduced by the first image reproduction system is obtained. Is a color conversion method for converting a pre-conversion image into a post-conversion image of a color reproducible by the second image reproduction system, wherein each pixel of the pre-conversion image is based on the white point of the first image reproduction system. To the same wavelength as the main wavelength of each pixel of the converted image based on the white point of the stage 2 image reproduction system, and determines the saturation of each pixel of the converted image to perform color conversion. Is to do. When determining the saturation, the distance from the white point of the first image reproduction system to the color gamut of the first image reproduction system through the chromaticity point of each pixel of the pre-conversion image and the first The ratio of the distance from the white point of the image reproduction system to the chromaticity point of each pixel of the pre-conversion image is calculated, and the ratio from the white point of the second image reproduction system to the chromaticity point of each pixel of the post-conversion image is passed. So that the ratio of the distance to the color gamut of the second image reproduction system and the distance from the white point of the second image reproduction system to the chromaticity point of each pixel of the converted image is equal to the ratio. It determines the saturation.

Further, the color conversion apparatus of the present invention includes pre-conversion white point storage means for storing the white point of the first image reproduction system for reproducing the pre-conversion image, and each of the pre-conversion images with the white point as a reference. A main wavelength detection storage unit that detects the main wavelength of a pixel and stores the chromaticity coordinates thereof, a pre-conversion color gamut storage unit that stores the color gamut data of the first image reproduction system, and the white point before the conversion. From the ratio of the distance to the color gamut of the first image reproduction system through the chromaticity point of each pixel of the image and the distance to the chromaticity point of each pixel of the pre-conversion image, the pre-conversion image Saturation calculation storage means for calculating and storing the saturation of each pixel, and post-conversion white point storage means for storing the white point of the second image reproduction system for reproducing the post-conversion image obtained by color conversion of the pre-conversion image. A post-conversion color gamut storage means for storing the color gamut data of the second image reproduction system; A hue line connecting the chromaticity coordinates stored in the length detection storage means and the white point in the converted image is determined, and the saturation of the color of the pixel is determined by the saturation data from the saturation calculation storage means, The color of each pixel of the pre-conversion image is determined based on the white point data from the white point storage unit and the color gamut data from the post-conversion color gamut storage unit, and the second
And a conversion color determining means for converting the color into a color within the color gamut of the image reproduction system.

Further, according to the present invention, the white point before conversion of the first image reproduction system is a white point defined by the first image reproduction system or a perceptual white color depending on an observation condition of the first image reproduction system. The white point after conversion of the second image reproduction system is a white point defined by the second image reproduction system or a perceptual white chromaticity point according to an observation condition of the second image reproduction system. . Therefore, according to the present invention, it is possible to cope with the color correction of an image when a color image is reproduced by a reproduction system having a different white point, or the change of the white point due to the change of the adaptation state of the observer which occurs when the environment changes. Image color appearance correction
It can be done properly over the entire image,
There is an effect that the appearances of the entire images reproduced by the reproduction systems having different white points can be made the same.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the configuration of a color conversion device according to the method of the present invention.

FIG. 2 is a schematic diagram showing a state of color conversion in the present embodiment on an xy chromaticity diagram.

FIG. 3 is a schematic diagram showing a relationship between pre-conversion chroma and post-conversion chroma when performing color conversion in the present embodiment.

[Description of Reference Signs] 1 color conversion unit 2 pre-color conversion unit 3 post-color conversion unit 11 pre-conversion color gamut storage unit 12 pre-conversion white point storage unit 13 main wavelength detection storage unit 14 saturation calculation storage unit 15 post-conversion Color gamut storage means 16 Post-conversion white point storage means 17 Conversion color determination means 20 Pre-conversion chromaticity points 21 Spectral locus 22 Red purple line 23 Pre-conversion reproduction system color gamut (color CRT) 24 Post-conversion reproduction system color gamut ( Hard copy) 25 White point before conversion 26 White point after conversion 27 Dominant wavelength (chromaticity) point 28 Hue line before conversion 29 Hue line after conversion 30 Chromaticity point of pixel after conversion 31 Hue line before conversion and color gamut Intersection point 32 Intersection point between hue line after conversion and color gamut 40 Relationship between saturation before conversion and saturation after conversion (linear) 41 Relationship between saturation before conversion and saturation after conversion (non-linear) 42 Saturation before conversion Between post-conversion and saturation (non-linear)

Claims (5)

[Claims] 1. When the white point of the first image reproduction system is different from the chromaticity point of the white point of the second image reproduction system, the pre-conversion image of the color reproducible by the first image reproduction system is converted into the second image. A color conversion method for converting a color-converted image that can be reproduced by an image reproduction system, comprising: a main wavelength of a color for each pixel of the pre-conversion image based on a white point of the first image reproduction system; The main wavelength of each pixel of the converted image based on the white point of the platform 2 image reproduction system is made the same, and the color saturation is determined by determining the saturation of each pixel of the converted image. Color conversion method. 2. The determination of the saturation is performed by determining a distance from a white point of the first image reproduction system to a color gamut of the first image reproduction system through a chromaticity point of each pixel of the pre-conversion image, The ratio of the distance from the white point of the first image reproduction system to the chromaticity point of each pixel of the image before conversion is calculated, and the chromaticity point of each pixel of the image after conversion from the white point of the second image reproduction system. The ratio of the distance to the color gamut of the second image reproduction system and the distance from the white point of the second image reproduction system to the chromaticity point of each pixel of the converted image is equal to the ratio. The color conversion method according to claim 1, wherein the saturation is determined as described above. 3. The white point before conversion of the first image reproduction system is a white point defined by the first image reproduction system or a chromaticity point of perceptual white color depending on an observation condition of the first image reproduction system. And the converted white point of the second image reproduction system is a white point defined by the second image reproduction system or a perceptual white chromaticity point according to an observation condition of the second image reproduction system. The color conversion method described in 1 or 2. 4. A pre-conversion white point storage unit that stores a white point of a first image reproduction system that reproduces a pre-conversion image, and a main wavelength of each pixel of the pre-conversion image is detected based on the white point. A main wavelength detection storage unit that stores the chromaticity coordinates, a pre-conversion color gamut storage unit that stores the color gamut data of the first image reproduction system, and a chromaticity point of each pixel of the pre-conversion image from the white point. From the distance to the color gamut of the first image reproduction system and the distance to the chromaticity point of each pixel of the pre-conversion image, the saturation of each pixel of the pre-conversion image is calculated. And a saturation calculation and storage means for storing the converted image, and
Converted white point storage means for storing the white point of the image reproduction system, converted color gamut storage means for storing the color gamut data of the second image reproduction system, and chromaticity stored in the main wavelength detection storage means. A hue line connecting the coordinates and the white point in the converted image is determined, and the saturation of the color of the pixel is determined by the saturation data from the saturation calculation storage means;
The color of each pixel of the pre-conversion image is determined based on the white point data from the post-conversion white point storage unit and the color gamut data from the post-conversion color gamut storage unit, and the color of each pixel of the second image reproduction system is determined. A color conversion device comprising: a conversion color determining unit for converting the color into a color within the range. 5. The white point before conversion of the first image reproduction system is a white point defined by the first image reproduction system or a chromaticity point of perceptual white color depending on an observation condition of the first image reproduction system. , The white point after conversion of the second image reproduction system is
The color conversion device according to claim 4, wherein the color point is a white point defined by the image reproduction system or a perceptual white chromaticity point according to the observation conditions of the second image reproduction system.