Typewriting machines – Including control of format and selection of type-face by...
Reexamination Certificate
2002-06-06
2004-08-10
Chau, Minh H (Department: 2854)
Typewriting machines
Including control of format and selection of type-face by...
C400S062000, C358S001900, C358S523000
Reexamination Certificate
active
06773175
ABSTRACT:
This application is based on applications Nos. 2001-174547 and 2001-174548 filed in Japan, the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color conversion table generation method, a color conversion method, and a color conversion program. Particularly, the present invention relates to a color conversion table generation method, a color conversion method, and a color conversion program to convert color data represented by a certain color space into color data represented by another color space.
2. Description of Related Art
In order to achieve correct color reproduction on a printer, it is necessary to obtain the correct values of CMYK (Cyan, Magenta, Yellow and Black) that can represent an applied color (for example, L*a*b* that does not depend on the apparatus).
The most conventional CMYK conversion generally effected is CMY conversion+GCR (Gray Component Replacement) (or UCR (Under Color Removal)). This includes the processes of, as shown in
FIG. 28
, converting color data (L*a*b*) into the three colors of CMY, generating K data from the CMY data, and correcting the CMY data from the produced K data.
This conversion method is widely employed due to the fact that conversion to CMY can be conducted easily, and that a moderate image quality can be readily achieved by just adding black to the region of low lightness.
However, various problems are noted as set forth below when color conversion is conducted according to the conventional method.
The color reproduction range by CMY is smaller than the color reproduction range of CMYK (for example, by the combination of the three colors of CMY, umber brown is achieved instead of black). Therefore, in the conversion process from L*a*b* to CMY, there are some colors that cannot be represented by CMY even if reproduction in CMYK is allowed.
As to correcting the values of CMY based on the produced K data, it is difficult to identify the K component amount in CMY, and determine how much the CMY is to be corrected when K data is produced since the actual ink property is complex. In practice, conversion is carried out empirically (or by trial and error).
Also, in the case where an image on a display is to be printed out through a printer in a computer system, the RGB (Red, Green and Blue) signal output with respect to the display must be converted into the CMYK (Cyan, Magenta, Yellow and Black) signal employed for the printer output. Since both the RGB signal of the display and the CMYK signal of the printer depend on the property of the apparatus, it is necessary to first represent colors with the colorimetric system that does not depend on the property of the apparatus for the purpose of achieving accurate color conversion.
Specifically, the RGB signal is converted into the colorimetric system such as L*a*b* that does not depend upon the property of the apparatus, and then further converted into the CMYK signal.
In general, the color range that can be reproduced differs between a display and a printer. Therefore, an operation of substituting a color that cannot be represented through the printer with a color that can be represented (gamut compression) is necessary.
The conventional technique of gamut conversion is disclosed in, for example, Japanese Patent Laying-Open No. 10-84487. This method effects conversion so that the color difference between color data of the image prior to conversion and the color data of the image after conversion is reduced.
FIG. 29
is a diagram to describe a specific method of gamut compression. The L*a*b* space is viewed from the direction of the L* axis. The hatched region in
FIG. 29
represents the gamut.
In the case where image data that is to be represented is located at the initial point of the arrow in
FIG. 29
, that data must be converted into data residing in the gamut since the data is located outside the gamut. Here, a color that cannot be reproduced is converted into a reproducible color by shifting the image data from the initial point of the arrow to the end point of the arrow.
In order to minimize the color difference, the end point of the arrow is set so that the distance from the initial point to the end point of the arrow is as short as possible.
In the above-described gamut compression method, problems called “blue-shift” and “lightning rod effect” are noted. Blue-shift is a phenomenon in which the color of bright blue outside the gamut is converted into the color of purple that has small color difference instead of the desirable conversion biased towards cyan. The lightning rod effect is a phenomenon in which data after conversion is congregated at the acute area of the gamut.
Various methods have been proposed to avoid such problems (for example, refer to Japanese Patent Laying-Open Nos. 10-258548 and 10-290375).
However, these methods require the accurate determination of the boundary of the gamut. The process inside the gamut and the process outside the gamut are independent of each other, so that the amount of calculation is increased. Furthermore, although appropriate conditions of constraint must be set so that there is no discontinuity in the data inside the gamut and outside the gamut, this setting is extremely difficult since the boundary plane to which the constraint condition is to be assigned is a three dimensional free-form surface.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a color conversion table generation method, a color conversion method, a color conversion program, and an image processing apparatus that can effect better color reproduction without having to depend on empirical rules or trial and error.
Another object of the present invention is to provide a color conversion table generation apparatus and color conversion apparatus that can effect conversion readily and of enhanced appearance.
According to an aspect of the present invention, a color conversion table generation method includes the steps of: (a) printing out a plurality of colors determined in a color space formed of a plane indicating a cyan component and a magenta component, a plane indicating a magenta component and a yellow component, a plane indicating a yellow component and a magenta component, and a gray component that is not in parallel with any of these planes; (b) reading out color of each color printed out at step (a); and generating a table in which each colorimetric result obtained at step (b) and each color determined on the color space are in correspondence.
According to another aspect of the present invention, a color conversion table generation apparatus includes a printer for printing out a plurality of colors determined in a color space formed of a plane indicating a cyan component and a magenta component, a plane indicating a magenta component and a yellow component, a plane indicating a yellow component and a cyan component, and a gray component that is not in parallel with any of these planes; a colorimetry portion for reading out color of each color printed out at the printer; and a table generation portion for generating a table in which each colorimetric result obtained at the colorimetry portion and each color printed out by the printer and determined on the color space are in correspondence.
According to a further aspect of the present invention, a color conversion method conducts color conversion using a table in which each point of a first color space and each point of a second color space are set in correspondence. The first color space can represent a color not depending on the apparatus. The second color space is formed of a plane indicating the cyan component and the magenta component, a plane indicating the magenta component and the yellow component, a plane indicating the yellow component and the magenta component, and a gray component that is not in parallel with any of these planes.
According to still another aspect of the present invention, a color conversion apparatus includes a color conversion portion for color conversion
Chau Minh H
Minolta Co. , Ltd.
Sidley Austin Brown & Wood LLP
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