1. Field of the Invention
This invention relates to a color image processing technique for converting an input color image signal into a color image record signal of a color printer, etc., and more particularly to a technique for converting an input color image signal into a color image record signal of N or more colors including a solid color (N is an integer of five or more).
2. Description of the Related Art
In a print technique, solid colors provided by primary color ink of red (R), green (G), and blue (B) and fluorescent ink are added to four colors of Y (yellow), M (magenta), C (cyan), and K (black) to perform color reproduction as a technique for representing vivid colors that cannot be reproduced in four-color process print of Y, M, C, and K. As color samples of solid colors, color samples of Pantone and the like are known and about 1000 solid colors are defined.
However, up to a maximum of eight colors can be printed at the same time on a printer and thus the number of solid colors that can be used is four at the maximum and is a very small number; there is designer's demand for increasing the number of solid colors that can be used. Further, to prepare solid color ink, basic inks of about 20 colors need to be mixed in accordance with the mixing ratio defined in the color sample. When solid color ink is replaced, ink mixing work, printer cleaning work, and the like are required and thus solid color print involves a large number of steps at the print site; this is a problem.
To solve such a problem, in print technique, a color reproduction method called HiFi color print is proposed for the purpose of enlarging color gamut by perform color reproduction using five to seven colors in which at least one color of RGB is added to four process colors of YMCK, and making it possible to reproduce solid color colorimetrically by performing color separation processing. As HiFi color print, hexachrome print of Pantone is widely used; it is known that about 90% of solid color can be reproduced by performing color reproduction in six colors of four colors of YMCK plus R-based orange (O) ink and G ink. The color reproduction method of the HiFi color print, which will be hereinafter referred to as HiFi color, is general as a technique of enlarging the color gamut since the application range of HiFi color is also widened to ink jet and electrophotographic color printers.
Further, in print and a color printer, a color signal of a display or a digital camera of sRGB color space, etc., may be input as an input signal. Since the color gamut of YMCK four-color process print or a color printer is narrower than the color gamut of the sRGB color space, there is demand for enlarging the color gamut intended for faithful reproduction. Also in such a case, enlarging the color gamut using the HiFi color is effective.
Thus, in the HiFi color, it is possible to enlarge the color gamut of a color printer. At this time, to well reproduce a calorimetric value signal of input solid color and an sRGB color signal, it is necessary to match the tristimulus values of print of output matter with the tristimulus values of the input color signal, namely, realize colorimetric color reproduction in color conversion processing of converting the input color signal into a an image record signal of five to seven colors.
A color management system (CMS) based on the specifications proposed by International Color Consortium (ICC) widely prevalent as industrial standard at present converts an RGB color signal of an input color signal of machine-dependent color space into a device-independent color signal like L*a*b* or XYZ color space and then converts the device-independent color signal into a YMCK color signal of a color printer of machine-dependent color space. As such conversion processing is performed, the input color signal and the output image record signal match indevice-independent color signal and thus it is possible to ensure colorimetric color reproduction. As the CMS, ColorSync installed in the Mac (registered trademark) operating system of Apple and ICM installed in Windows (registered trademark) of Microsoft are representative.
It is also considered in the HiFi color that a CMS in conformity with ICC is adopted for realizing colorimetric color reproduction. To do this, color conversion from a device-independent color signal like L*a*b* or XYZ color space to an image record signal of five to seven colors maybe realized. The following technique is proposed as a related art of color conversion processing for the HiFi color:
U.S. Pat. No. 4,812,899 discloses an image formation apparatus for performing color reproduction in process inks of seven colors of YMCKRGB. In the image formation apparatus, color conversion from an RGB signal of an input machine such as a scanner to YMCKRGB of an output machine such as a printer is determined by UCR (Under Color Removal) of achromatic component and chromatic component, and a technique called Kueppers Technique is proposed. This technique is the first proposed technique as color conversion processing of the HiFi color and has an easy algorithm and thus is widely utilized.
Considering application of Kueppers Technique to color conversion in conformity with ICC, it is possible to adopt a method of converting a device-independent color signal like L*a*b* color space into an RGB color signal by any color conversion unit and performing UCR processing of achromatic component and chromatic component for the RGB signal. However, as with the usual UCR processing, a colorimetric match between the RGB color signal of an input color signal and the YMCKRGB color signal of an image record signal is not considered and therefore the technique cannot realize calorimetric color reproduction and color conversion accuracy is poor; this is a problem. Further, as with the usual UCR processing, the technique involves a problem of incapability of making full use of the color gamut of an output machine and a problem of incapability of making full use of the color gamut widened in the HiFi color.
In contrast, the following technique called a division method is proposed: The color gamut of the HiFi color consisting of five to seven colors is divided into color gamut divisions each involving three or four colors in combination, and color conversion from the colorimetric value is performed in each color gamut division according to a similar technique to that with a usual three-color or four-color printer. For example, JP-A-2000-32284 proposes a technique of using color gamut divisions each involving K and other two colors close to K in hue in combination and determining coefficients in a direct look-up table (DLUT) for color conversion processing for realizing colorimetric color reproduction. For example, JP-A-2001-136401 proposes a technique of using color gamut divisions each involving K and other three colors in combination and determining DLUT coefficients for color conversion processing for realizing colorimetric color reproduction. Further, for example, U.S. Pat. No. 5,563,724 proposes a method of forming k (k<n) color gamut divisions each consisting of m color inks in an image formation apparatus of n (n>4) color inks and creating a printer model for each color gamut division, thereby converting from CIE color space into n-color inks color space so that a calorimetric match is found in the color gamut division and the color difference becomes the minimum outside the color gamut division. Further, for example, U.S. Pat. No. 5,892,891 describes a color conversion method from the colorimetric value in five-colors process of YMCKO; it proposes a method of executing color conversion of the colors in the YMCK color gamut from a colorimetric value. A color in the YMCK color gamut is converted from L*a*b* color space to YMCK. A color outside the YMCK color gamut is color-converted using a color gamut division to which a solid color is added (YOMK color gamut).
Each of the division methods disclosed in the documents has the advantage that the color conversion accuracy is high because colorimetric color reproduction in each color gamut division is insured. Although the computation amount is large as compared with Kueppers Technique, the division method is rarely used directly to perform color conversion; usually, it is used only for determining the lattice point parameter of DLUT for executing color conversion from L*a*b* color space to color signal of five to seven colors of an output machine, and therefore the large computation amount does not introduce any problem.
Thus, the division method has the advantage that the color conversion accuracy is high because colorimetric color reproduction is insured. However, the division method involves an intrinsic problem such that the color separation values become discontinuous on the boundary between the color gamut divisions and a pseudo contour occurs, as described in JP-A-2001-136401. Then, in the method disclosed in JP-A-2001-136401, occurrence of pseudo contours is suppressed by performing smoothing processing of the lattice point parameters of DLUT determined by the division method. However, a new problem is caused, that is, the smoothing processing worsens the color conversion accuracy. That is, in the division method, prevention of a pseudo contour and realizing high color conversion accuracy cannot be made compatible; this is a problem of the division method.
Further, a problem occurs in addition to occurrence of the pseudo contour in the boundary depending on the division method into color gamut divisions. In the color gamut division methods disclosed in JP-A-2000-32284 and U.S. Pat. No. 5,563,724, each color gamut division is made up of two or three adjacent colors containing K. In the division methods, the gray axis becomes a single color of K and thus gray cannot be reproduced in three colors of YMC; this is a problem. Usually, gray reproduced in the K single color is poorer in graininess than gray reproduced in the three colors of YMC and therefore a problem of poor graininess in the proximity of the gray axis occurs in the division method. In the division method, the place where the maximum density is generated becomes a K signal color solid part and the maximum density is lower than that in a YMCK four-color solid part in the YMCK four-color process. Full use of the color gamut in the high-density part that should be able to be reproduced essentially cannot be made; this is a problem.
In contrast, in the methods disclosed in JP-A-2001-136401 and U.S. Pat. No. 5,892,891, one of the color gamut divisions is formed of the YMCK color gamut, whereby gray can be reproduced in three colors of YMC and the place where the maximum density is generated can be made a YMCK four-color solid part, so that the graininess in the proximity of the gray axis can be improved and full use of the color gamut of the high-density part can be made. However, in the method, discontinuousness of the color separation values in the boundary between color gamut divisions containing RGB and the YMCK color gamut is very large as compared with that in the division methods disclosed in JP-A-2000-32284 and U.S. Pat. No. 5,563,724. There is a problem of easy occurrence of a pseudo contour.