Image analysis – Color image processing
Reexamination Certificate
1999-11-22
2003-06-17
Tran, Phuoc (Department: 2621)
Image analysis
Color image processing
C382S167000, C382S165000, C382S276000, C345S600000, C345S603000, C345S604000
Reexamination Certificate
active
06580822
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a color space conversion device and a color space conversion method for converting RGB data of a color image into CMYK data, by which high quality color printing is realized.
Description of the Prior Art
These days, a wide variety of color printers are used as color image output devices for printing color images on paper. When such a color image output device outputs a color image, parameters called UCR (Under Color Removal) and BG (Black Generation) are set to appropriate or particular values, and the UCR/BG are used for outputting the color image.
For example, in an “Image processing method” disclosed in Japanese Patent Application Laid-Open No.HEI2-9273 (Japanese Patent No.2728208), a line of RGB data is scanned in the horizontal scanning direction, and differential signals R(x)−G(x) and G(x))−B(x) (R(x): red signal, G(x): green signal, B(x): blue signal) are integrated on the line, and thereby an appropriate UCR (UCR characteristic line) for the line is determined depending on the two integrals.
In a “Printing device” disclosed in Japanese Patent Application Laid-Open No.SHO63-100895, when color space conversion is executed for achromatic color areas, the UCR/BG is corrected by use of a gamma table.
Generally, a color image input device such as a scanner, a digital camera, etc. handles and manages color images employing the RGB color space, while a color image output device such as a color printer manages color images employing the CMYK color space in many cases. A monitor (which operates as a color image input device when a color image displayed on the monitor is directly printed by a color printer) also manages color images employing the RGB color space. Therefore, color space conversion from RGB data into CMYK data becomes necessary when a color image inputted by a color image input device is outputted by a color image output device. The color space conversion from RGB data into CMYK data can be executed directly using a table called LUT (Logical Unit Table). However, in many cases, the color space conversion from RGB data into CMYK data is conducted by two steps: a first color space conversion from the RGB data into CMY data, and a second color space conversion from the CMY data into the CMYK data. The first color space conversion from RGB data into CMY data is usually executed by means of complementary color conversion, conversion using a color space conversion table, etc. The second color space conversion from CMY data into CMYK data is usually executed using the UCR/BG. The UCR/BG is usually defined as a value or a table, independently of the position of the CMY data in the CMY color space (independently of the position of the RGB data in the RGB color space).
However, in many cases, the optimum values of the UCR/BG change depending on the position of the CMY data in the CMY color space (depending on the position of the RGB data in the RGB color space). Concretely, when the CMY data is like an achromatic color (when the RGB data is like an achromatic color), it is desirable that most of the CMY data is replaced by “K” (black print), that is, the UCR/BG should be near 100%. On the other hand, when the chroma of the CMY data is high (when the chroma of the RGB data is high), it is desirable that the replacement by the “K” is reduced, that is, the UCR/BG should be smaller.
Therefore, when UCR/BG which have been optimized near the achromatic color area are employed, the “K”-print is used a lot with respect to CMY data in the achromatic color area, and thereby printed output like pure gray is obtained successfully with respect to the CMY data in the achromatic color area. However, with respect to CMY data far from the achromatic color area, the chroma of the CMY data is lowered undesirably due to the heavy use of the “K”-print. On the other hand, when UCR/BG which have been optimized near the achromatic color area are employed, the replacement by the “K”-print is limited with respect to CMY data having high chroma, and thereby printed output having high chroma is obtained successfully with respect to the CMY data having high chroma. However, with respect to CMY data in the achromatic color area, gray is necessitated to be expressed by the combination of C, M and Y since the replacement by the “K”-print is limited, and thus printed output like pure gray can not be obtained successfully.
In the conventional image processing method disclosed in Japanese Patent Application Laid-Open No.HEI2-9273, a plurality of UCRs are prepared, and an appropriate UCR to be used is selected from the UCRs based on the characteristics (the two line integrals) of the RGB data. However, the detection of the characteristics of the RGB data and the determination (selection) of the appropriate UCR based on the characteristics are executed with respect each line in the horizontal scanning direction. Therefore, even when the characteristics of the RGB data changed rapidly on the line (when achromatic areas and high chroma areas appeared alternately on the line, for example), the characteristics are integrated on the line and only one appropriate UCR is determined for the line based on the integrals. Therefore, the detection of the characteristics of the RGB data and the determination of the appropriate UCR can not be executed correctly by the conventional image processing method. Further, when the selection of an appropriate UCR from the prepared UCRs is executed with respect to each line, the selected UCR changes between lines, and thus color discontinuity occurs between the lines in the printed color image. However, no countermeasures for correcting the color discontinuity have been disclosed in the document.
The conventional printing device disclosed in Japanese Patent Application Laid-Open No.SHO63-100895 executes gamma correction to the UCR/BG when color space conversion is executed to achromatic color areas, in order to obtain printed output like pure gray for the achromatic color areas. However, the conventional printing device does not use the “K”-print a lot for expressing achromatic colors, and thus it is impossible for the conventional printing device to express pure gray in the printed output.
SUMMARY OF THE INVENTION
It is therefore the primary object of the present invention to provide a color space conversion device and a color space conversion method, by which correct detection of the characteristics of the RGB data (or the CMY data) and appropriate setting of the UCR/BG table (or the LUT) are executed with respect to each smaller unit of the RGB data so as to realize correct color space conversion from the RGB data to CMYK data, and thereby RGB data in the achromatic color area, for example, can be expressed as pure gray in the CMYK data and in the printed output.
Another object of the present invention is to provide a color space conversion device and a color space conversion method, by which the color discontinuity, which occurs due to the change of the UCR/BG table (or the LUT) which is used for color space conversion, can be corrected and eliminated.
In accordance with a first aspect of the present invention, there is provided a color space conversion device for executing color space conversion from RGB data to CMYK data, comprising a first color space conversion means, a CMY position detection means, two or more conversion tables, a conversion table synthesizer means, and a second color space conversion means. The first color space conversion means converts the RGB data into C′M′Y′ data. The CMY position detection means detects the position of the C′M′Y′ data in the CMY color space with respect to each dot of the C′M′Y′ data. The conversion tables are tables which have been optimized at particular positions in the CMY color space so as to be used for color space conversion from C′M′Y′ data to CMYK data. The conversion table synthesizer means synthesizes a suitable conversion table for each dot of the C′M′Y′ data from
Alavi Amir
Dickstein Shapiro Morin & Oshinsky LLP.
Tran Phuoc
LandOfFree
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