1. Field of the Invention
The present invention relates to a color processing apparatus and color processing method and, more particularly, to a color processing apparatus and color processing method, which create a color conversion table by mapping between color gamuts.
2. Description of the Related Art
In recent years, opportunities to output images acquired by a digital camera, scanner, or the like using printers are increasing. These images acquired by the digital camera, scanner, or the like are temporarily stored as data on an RGB color space of an input device. However, since this RGB color space depends on each device, the images are converted into color space data such as an sRGB color space as a standard color space, an AdobeRGB color space, or the like, and the converted data are saved. An x-y chromaticity diagram in FIG. 22 shows the color reproduction range (color gamut) of the sRGB color space. In FIG. 22, a triangular region Rs indicated by the dotted line represents an sRGB color gamut. A horseshoe-shaped region Rv indicated by the solid curve represents a range visible to the human eye, and a curved region Rp indicated by the broken curve represents a color gamut of a general printer. As shown in FIG. 22, since the sRGB color gamut Rs is different from the printer color gamut Rp, for example, when an sRGB image is output using the printer, the sRGB color signal value must be associated with that of the printer so as to correct the color gamut difference. This association processing is called color gamut mapping.
Color gamut mapping will be explained below with reference to FIG. 23 taking color conversion processing of an RGB image as an example. In the following description, sRGB will be exemplified as an input color space. In step S1M, a perceived color space value of a pixel value RGB of an sRGB image is calculated. As the perceived color space, for example, CIELAB, CIELUV, CIECAM02, and the like are available. In the following description, as the perceived color space value, a color values Jab on the CIECAM02 color space is used. In step S2M, conversion is applied to the color value Jab to fall within the color gamut of an output device on the perceived color space. The association processing between the input and output color values in step S2M is generally called color gamut mapping. Finally, in step S3M, the color value after mapping is converted into an output device value R′G′B′.
As techniques for color gamut mapping to be executed in step S2M, various methods such as convergence point mapping, minimum color difference mapping, and the like have been proposed to date.
For example, according to the technique of convergence point mapping, an input color value within an output color gamut is maintained. On the other hand, as for an input color value which falls outside the output color gamut, a certain convergence point PF is set in the output color gamut, a line segment that connects the convergence point and input color value is assumed, and the input color value is shifted to a point between the convergence point and an intersection with the boundary of the output color gamut, as shown in FIG. 24. As a practical example, the following method has been disclosed (e.g., Japanese Patent Laid-Open No. 2001-036757 (U.S. Pat. No. 7,177,465)). That is, a maximum saturation point at an arbitrary hue in an input color gamut is shifted to a predetermined point at that hue along an equi-hue plane, and the color value within the input color gamut at that hue is nonlinearly mapped to those within the output color gamut. According to this method, the color value in the input color gamut can be reproduced by that in the output color gamut while preserving its hue.
According to the technique of minimum color difference mapping, an input color value within the output color gamut is also maintained. On the other hand, as for an input color value outside the output color gamut, the input color value is pasted on a point Pin′ where a three-dimensional (3D) distance between the input color value and the boundary surface is minimum, i.e., a point where the color difference before and after mapping is minimum, as shown in FIG. 25. As a practical example, a method of pasting an input color value on a point vertically drawn from the input color value to the boundary surface upon mapping an input color value outside the output color gamut has been disclosed (for example, see Japanese Patent Laid-Open No. 2005-117096). According to this method, upon outputting an input color value, it can be reproduced by a color value which is perceptually closest to the input color value.
The conventional color gamut mapping is effective to realize satisfactory color reproduction when the color gamut of an output device is smaller than that of an input device. However, depending on the color gamut shapes of the input and output color spaces, when an input color value is expressed by a perceived color space value, a hue (tone) is often reserved upon expression.
FIG. 26 shows an example of hue reversal. In FIG. 26, the solid curve represents an input color gamut boundary, which connects Cyan, Blue, and Magenta maximum saturation points of an input device, and points P1, P2, P3, and P4 represent grid points on the input color gamut boundary. As shown in FIG. 26, on this color gamut, a hue is apparently reversed in a section from the point P1 to the point P4. For this reason, when a color outside the output color gamut is pasted on the boundary surface using the convergence point mapping, regions P1-P2, P2-P3, and P3-P4 are mapped on the identical surface of the output color gamut. Therefore, the tone collapses after mapping, and pseudo edges are generated on an output image, thus causing considerable deterioration of image quality.