Image analysis – Color image processing – Color correction
Reexamination Certificate
1997-09-04
2001-03-06
Nguyen, Madeleine (Department: 2722)
Image analysis
Color image processing
Color correction
C358S518000
Reexamination Certificate
active
06198843
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for mapping all colors of an input image to a color gamut that can be reproduced by an output device in the field of color matching between different color reproduction devices. More particularly, the present invention relates to a method and apparatus for color gamut mapping in which an image can be reproduced with an optimum appearance by evaluating color matching using new color differences defined taking spatial frequency characteristics of the visual sense of a person.
The entire contents of Japanese Patent Application No. 9-53566 filed on Mar. 7, 1997 are incorporated herein by reference.
Various types of apparatuses have been developed as media for color reproduction including color displays, dye-sublimation printers, ink-jet printers, photographs, and commercial printers. In general, color gamuts which can be reproduced by such apparatuses significantly vary depending on the methods of color reproduction (color mixture), the methods for tone reproduction (halftone dot method, dithering, error diffusion method, etc), and differences in the spectral characteristics of the primary colors used. For example, it is known that the color gamut of a display is considerably wider than that of a dye-sublimation type printer. When color matching is performed between two apparatus having color gamuts of different sizes as described above, it is inevitable that some colors are unreproducible for one of the devices. Therefore, a problem arises in that it is necessary to find how to allow reproduce such colors out of such a color gamut at a device of interest.
In general, the visual system of a person who observes an image is characterized in that it has a difficulty in detecting a difference of a color from an original color for each picture element because it maintains the relationship between adjoining pixels (spatial frequency characteristics of an image) when such adjoining pixels have some complicated context an the case of a natural image. On the other hand, it detects a difference in one kind of color such as on a color chip more easily than it does on a natural image because the image of such a single color has constant frequency characteristics.
However, methods of mapping a color gamut proposed in the past pays no attention on such spatial frequency characteristics of the human visual system at all. Further, since those methods are used for color gamut mapping between an original image and a reproduced image on a pixel-by-pixel basis, a problem has been pointed out in that there is no way to provide a practical method of performing color gamut mapping on a natural image.
As described, it has been difficult to provide a method of performing color gamut mapping on a natural image properly, and there is a need for a more practical technique for performing color gamut mapping on an image that takes the spatial frequency characteristics of the human visual system into consideration.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and apparatus for mapping a color gamut in which the relationship between adjoining pixels that are in some context such as in a natural image is maintained and in which the color difference is minimized for each pixel.
According to an aspect of the present invention, there is provided a method for color gamut mapping in order to reproduce a target image on a color reproduction device, the method comprising the steps of: performing bi-directional conversion between a colorimetric value of each pixel in the target image and a color separation value which represents a distribution at least three colors and depends on characteristics of the color reproduction device; determining whether or not a color of the each pixel can be reproduced by the color reproduction device according to a difference between the colorimetric value and colorimetric value after the bi-directional conversion; and optimizing the colorimetric value before the bi-directional conversion of the color determined as which can not be reproduced on the color reproduction device such that an evaluation function of the color is minimized, the evaluation function representing a perceptual difference of the determined color.
According to a second aspect of the present invention, there is provided an apparatus comprising:
input means for inputting signals each of which indicates a colorimetric value representing a color of each pixel of a target image; conversion means for performing bi-directional conversion between the colorimetric value input by the input means and a device value of a color reproduction device; color gamut extraction means for performing a thresholding process by calculating norm between the colorimetric value before the bi-directional conversion and the colorimetric value after the bi-directional conversion; filtering means for calculating square norm of an image obtained by performing a spatial frequency filtering process on an image of the difference between the reproduced image obtained by the bi-directional conversion performed by the conversion means and the target image; optimization means for adjusting the colorimetric value to be bi-directional converted so as to minimize an evaluation function expressed by the sum of an output of the color gamut extraction means and the square norm calculated by the filtering means; and output means for outputting signals representing an image which has been subjected to color gamut mapping by the optimization means.
According to a third aspect of the present invention, there is provided a computer-readable memory comprising: first instruction means; for causing a computer to perform a bi-directional conversion between a colorimetric value of each pixel in a target image and a color separation value which represents a distribution at least three colors and depends on characteristics of the color reproduction device; second instruction means for causing a computer to determine whether or not a color of the each pixel can be reproduced by the color reproduction device according to a difference between the colorimetric value and colorimetric value after the bi-directional conversion; and third instruction means for optimize the colorimetric value before the bi-directional conversion of the color determined as which can not be reproduced on the color reproduction device such that an evaluation function of the color is minimized, the evaluation function representing a perceptual difference of the determined color.
Specifically, when bi-directional color conversion is performed between an intermediate color system and color separation values (device values), if the colorimetric values of the input pixels are within the color gamut and, the bi-directional conversion results in identical mapping in which the pixels are mapped to the same points on the intermediate color system that are within allowable ranges of conversion errors. In case the pixels are out of the color gamut, conversion of the internal color system into the device values results in mapping onto values out of the range of the device values (which is, in general, the range from 0 to 255). For example, since the input and output characteristics of an output unit of a neural network are sigmoidal characteristics, the mapped values will be force into the range of the device values. As a result, when those values are remapped onto the intermediate color system, a point is mapped onto a location which is so significantly different from the original location that it goes out of the color gamut of the device of interest. Based on this, it is possible to determine how far is the point from the color gamut (or how deep is it in the color gamut). Further, color gamut mapping is properly achieved by adjusting the colorimetric values of a reproduced image such that the color difference in a pixel of interest between the original and the reproduced images minimizes an evaluation function which is defined by the sum of the color difference obtained taking the sp
Imamura Masahiro
Nakauchi Shigeki
Usui Shiro
Ladas & Parry
Nguyen Madeleine
Toyo Ink Mfg. Co. Ltd.
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