1. Field of the Invention
The present invention relates to color processing for generating a profile for color separation, which indicates the relationship between input color values and output signal values to be output to an image forming device.
2. Description of the Related Art
Upon generation of a lookup table (LUT) for color separation, color-separated values of representative colors are manually calculated. Note that the color-separated values are equivalent to color signal values corresponding to amounts of color materials. Using a technique described in, for example, Japanese Patent Laid-Open No. 2002-033930 (patent reference 1), color-separated values of all grid points in a color space are interpolated using those of the representative colors, thus generating an LUT for color separation.
The technique of patent reference 1 performs the interpolation to obtain smooth changes of amounts of color materials, but does not consider obtaining smooth changes of reproduced colors. For this reason, linearity between an input color space such as an RGB space and an output color space such as a CMYK space cannot be maintained. When color separation processing is performed using linear interpolation such as tetrahedral interpolation, pseudo contours may be generated, and color reproduction precision may drop.
Japanese Patent Laid-Open No. 2004-320625 (patent reference 2) discloses a technique which estimates colors from amounts of color materials, and smoothes a color separation LUT so that reproduced colors of respective grid points become more uniform in a uniform color space with respect to an RGB space. However, with the technique of patent reference 2, when a gamut shape is deformed, the smoothing effect is reduced.
The smoothing effect reduction when the gamut shape is deformed will be described below with reference to FIGS. 1A and 1B. FIG. 1A illustrates the shape of a gamut of a certain printer, and shows an L*b* plane in a CIELab space. The gamut of the certain printer has a shape that is deformed to be drastically narrowed in a dark part with low lightness L*. FIG. 1B shows processing for smoothing output values (color-separated values) of grid points in the dark part shown in FIG. 1A. Reference symbols P, O, and M denote grid points, and a broken curve represents a color trace in the CIELab space, which is reproduced in practice. Output values of the grid points P and M are given, and an output value of the grid point O located between the grid points P and M is interpolated by the technique of patent reference 2. In this case, the grid point O is moved along the color trace indicated by the broken curve, and an output value corresponding to a point (color) on the trace where a vector sum of {right arrow over (OP)} and {right arrow over (OM)} is minimized is set in the grid point O.
As shown in FIG. 1B, a line segment which couples the grid points O and M deviates from the color trace that was actually reproduced. When an LUT for color separation is generated in such state, it is difficult to estimate an output value between the grid points O and M from those of surrounding grid points. Especially, when an output value corresponding to an arbitrary point (color) in an input color space is estimated from output values of surrounding grid points by tetrahedral interpolation, a pseudo contour is generated, and color reproduction precision drops in such region.