1. Field of the Invention
The present invention relates to a technical field in which a color signal located within a first color reproduction gamut (or range) represented by a first color system is subjected to mapping conversion into a color signal located within a second color reproduction gamut represented by the first color system.
2. Related Background Art
In recent years, as personal computers and work stations have spread, DTP (desktop publishing) and CAD (computer-aided design) have come to be widely used. Thus, color reproduction techniques by which a color represented on a monitor by the computer is accurately reproduced by using a coloring agent becomes important. For example, in DTP, a computer system which includes at least a color monitor and a color printer creates, edits and processes a color image on the color monitor, and then outputs the obtained color image by means of the color printer. Here, a user strongly wishes that the color image on the monitor and the printer output image have the same appearance.
However, in the color reproduction technique, achieving this goal is difficult, because of the following reasons.
The color monitor represents the color image by generating light of a specific frequency using a fluorescent body. On the other hand, ink or the like in a print made by a color printer absorbs light of a specific frequency, and the appearance of the color image is based on the remaining (unabsorbed) reflection light. Thus, since an image display form of the color monitor is different from an image display form of the color printer, the color reproduction gamut of the color monitor is very different from the color reproduction gamut of the color printer. Further, even among color monitors, the color reproduction gamut is different as between liquid crystal monitors, CRTs (cathode-ray tube) of electron-gun type, and plasma monitors. Also, even among color printers, the color reproduction gamut is different in accordance with recording sheet quality, difference of ink usage quantity, and the like. For this reason, it is impossible to achieve a complete match of the color of the image on the color monitor with the color of the image output from the color printer calorimetrically. It is also impossible to achieve a complete match of the colors of the images on the plural kinds of sheets output from the plural kinds of color printers calorimetrically. Therefore, when seeing the displayed color image on each output medium, a person observes a serious distinction among the output images.
Here, as a technique to absorb perceivable distinction on the displayed color image and perceivably match the displayed images among the display media of which the reproduction gamuts are different from others, there is known a gamut mapping technique, in which one color reproduction gamut is mapped into another color reproduction gamut by using a uniform color system. As one example of the gamut mapping technique, there is a technique in which, in the uniform color system, linear mapping is performed in a lightness-chroma dimension for each hue. According to this technique, a monitor color reproduction gamut as schematically shown in FIG. 27 is mapped into a printer color reproduction gamut as schematically indicated by the dotted line in FIG. 28.
However, an image corrected by such linear mapping and then output might be undesirable in appearance. This is because the difference between the shape of the monitor color reproduction gamut and the shape of the printer color reproduction gamut causes unnaturalness.
Here, the difference between the shape of the monitor color reproduction gamut and the shape of the printer color reproduction gamut will be simply explained. For example, FIG. 29 schematically shows the monitor and printer color reproduction gamuts in a green hue, the printer color reproduction gamut being indicated by the solid line, while the monitor color reproduction gamut is indicated by the dotted line. As is apparent from FIG. 29, in the green hue, the monitor color reproduction gamut is not similar to the printer color reproduction gamut, and thus the shape of the monitor color reproduction gamut is quite different from the shape of the printer color reproduction gamut. FIG. 30 schematically shows the monitor and printer color reproduction gamuts in a red hue, the monitor color reproduction gamut being indicated by the solid line, while the printer color reproduction gamut is indicated by the dotted line. As is apparent from FIG. 30, in the red hue, the shape of the monitor color reproduction gamut is relatively similar to the shape of the printer color reproduction gamut.
In order to solve the above problem, nonlinear gamut mapping to preserve chroma in the low-chroma part and lightness in the intermediate-lightness part and also absorb the difference between the shape of the monitor color reproduction gamut and the shape of the printer color reproduction gamut is effective.
As such a nonlinear gamut mapping, a method of superposing one- to three-dimensional mapping has been proposed.
However, in this nonlinear gamut mapping, there is room for improvement in the point of gradation. That is, in superposing the one- to three-dimensional mapping according to the conventional gamut mapping method, each mapping is different from the others because of chromaticity, hue and the like of the mapping-target color. Thus, even if there is no problem in the individual mapping, a problem may still occur in the gradation as a result of superposing the mappings.
Here, it should be noted that the term “gradation” is used for the meaning of a proper change rate in a case where color changes according to a certain rule. Further, an operation designed to preserve gradation corresponds to an operation designed to preserve the change rate. Next, FIGS. 31A and 31B will be briefly explained. In a case where a proper change rate has varied greatly, as shown by the part enclosed with the circle in FIG. 31A, in general, such variation frequently causes a pseudo-contour and the like, although this phenomenon is dependent on conditions such as hue, chroma and the like. On the other hand, in a case where a proper change rate can be kept, as shown in FIG. 31B, a perceivable problem does not occur easily.