Image analysis – Color image processing – Color correction
Patent
1995-03-07
1997-11-25
Johns, Andrew
Image analysis
Color image processing
Color correction
358523, G06K 900
Patent
active
056920717
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for color output device characterization applicable to a wide range of color hardcopy technologies. In particular, the present invention provides a color image processing method and apparatus for characterizing the image signals of a color output device using device-independent color representations.
In device-independent color communication, input and output devices (such as scanners and printing devices, respectively) have to be characterized by device-independent color representations such as the CIEXYZ or CIEL*a*b* coordinates. The goal of color characterization of output devices is to define the relation between the device-independent color representation and the device-dependent color image signals which control the amount of colorants (inks, pigments; dyes, toners) used in the device to reproduce a color. Based upon this relation, a specific target color or colors of an original image can be reproduced with a high level of color correspondence. Preferably the color characterization also includes information on the color reproduction range or color gamut of the output devices.
DESCRIPTION OF THE PRIOR ART
The European patent application EU.A. 0 264 281 discloses a method for printing a color on paper which matches a color on a CRT display. The technique is based on linear mixing calculations in the CIExyY color space.
The European patent EU.P. 0 124 908 describes a method for obtaining halftone dot percents required to reproduce the color of a color specimen by printing. The method is based on color density measurement.
SUMMARY OF THE INVENTION
The object of the present invention consists in providing a color image processing method and apparatus for characterizing the color image signal values of a color output device using device-independent color representations (e.g. CIEL*a*b* coordinates). This means that the method and the apparatus of the present invention allows determination of the device-independent color values and generation of signals (e.g. digital signals, corresponding thereto) and hence the color image signal values for a color reproduction device, so that the color reproduction device produces colors which are identical or closest to target colors, in such a way that the human observer is not able to distinguish between the target color and the color produced by the color output device by means of the present invention. This system is universal, i.e. applicable to different color hardcopy technologies (offset, electrophotography, engraving, photography, etc.).
Color image signal values are preferably a combination of three values (a triplet ABC) used to generate three or more color control signals. Color control signals are a combination of three or more device-dependent signals such as, for example, RGB (red, green, blue) or CMYK signals, serving to control the processor of a color reproduction output device so that the latter produces the colors corresponding to these color control signals. They are called device-dependent since the same combination of these color control signals will usually cause different color output devices to produce a different color.
Device-independent color coordinates are used to uniquely define each color of the color spectrum. Device-independent coordinates are, for example, CIEXYZ or CIEL*a*b* coordinates.
A further object of the present invention is to achieve a high level of accuracy by distributing the color samples through the color space. This is done by using a high concentration of samples in the areas of the color gamut of the color output device with non-linear responses.
Another object of the present invention is to achieve high accuracy while keeping the number of calculations low enough to provide a fast method and a practical and efficient apparatus. This is done by interpolating only a limited set of intermediate grid points in the ABC space of the device-dependent color image signal coordinate triplets of a color output device.
Still another object of t
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patent: 4941039 (1990-07-01), D'Errico
patent: 4949135 (1990-08-01), Ng
patent: 5121196 (1992-06-01), Hung
patent: 5305119 (1994-04-01), Rolleston et al.
patent: 5471324 (1995-11-01), Rolleston
AGFA-Geveart
Birch Richard J.
Johns Andrew
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