Computer graphics processing and selective visual display system – Computer graphics processing – Graph generating
Reexamination Certificate
1999-01-20
2001-10-02
Brier, Jeffery (Department: 2672)
Computer graphics processing and selective visual display system
Computer graphics processing
Graph generating
C345S156000, C345S156000, C345S156000
Reexamination Certificate
active
06297826
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of converting color data, and more particularly to a method of converting color data when the gamut of a color input/output device for input and that of a color input/output device for output are different.
Furthermore, the present invention relates to a method of modifying the registration data in a color conversion table which is referred to when the registration data are converted to a different expression system of color signals.
2. Description of the Related Art
Along with the recent advent of high-performance personal computers and low-cost color printers, opportunities to print color pictures have remarkably increased. When color pictures are printed, particularly the coincidence in appearance between color pictures displayed on a CRT display connected to a personal computer and those printed by a color printer is required.
There are a wide variety of expression methods of color signals. For example, printers and scanners express color using three primary colors, such as CMY values, RGB values, etc. Taking the output color of a printer in which the minimum signal value is 0 and the maximum signal value is 255, as an example, when color is expressed using CMY, all output colors are expressed by the combinations of CMY values ranging from 0 to 255. In this case, this method has a characteristic that even if data to be printed are prepared using the same CMY values, printed (reproduced) color varies with the kind of printer. For this reason, the CMY values are a way of expressing color, dependent on individual devices. A scope in which the colors of this individual device can be reproduced, is called a gamut.
Besides this expression method of color signals dependent on devices, there is an expression method of absolute colors independent of devices. This is a method of expressing colors using coordinate values, such as L*a*b*, XYZ, etc. Since colors expressed by this method are clearly defined, colors can be expressed independently of individual devices. Color expression methods such as L*a*b, XYZ, etc. are used as an intermediate color expression method when colors are converted between the color spaces of different devices. That is, this method is used in such a case where, for example, RGB values used in a first device are once converted to, for example, L*a*b* values, and then the converted L*a*b* value are converted back to RGB values again. If color signals are accurately converted by these expression methods, inputted colors (in the case of scanners, etc.) and outputted colors (display on display monitors or printing by printers) between different devices can be matched with each other.
In this way, in order to match color outputs between devices, color data have to be converted using a variety of expression methods of color data. For a method of converting color data, there is a method of using the correspondence of color in each color space, that is, a color conversion table in which paired data of first color data in a first device and paired data of second color data in a second device are stored. Generally speaking, in a conversion method of using a color conversion table, when a color registered in the color conversion table is converted, the color can be converted by using the registered correspondence as it is. Therefore, the color conversion is processed at a remarkably high speed. However, since data volume to be registered in the table becomes enormous, sometimes all the color correspondences are not registered in the color conversion table. In this case, when a color which is not registered in the color table is converted, colors in the neighborhood of the color to be converted are selected, by performing an interpolation operation process for color signals in the neighborhood, the color data to be converted is calculated, and the color conversion is performed based on this calculation result.
However, in the conversion of a color which is not registered in a color conversion table, errors occur in the interpolation operation process.
Although in preparing a color conversion table, operations to establish each color correspondence are performed, in these operations too, errors occur. For this reason, even when color conversion is performed using color data registered in the color table, errors often occur as a result of color conversion.
First, a conventional color data conversion method is described.
An example of conventional color data conversion methods and the used apparatuses is disclosed in the Japanese Laid-open Patent Publication No. 60-105376. In this example, first color data included in a first gamut dependent on a first color input/output device are converted toward an achromatic color with the same lightness as the first color data, and second color data included in a gamut dependent on a second color input/output device are calculated.
FIG. 1
shows the L*b* cross section of a L*a*b* space. A field where b* is positive is largely the cross section of a yellow field, and an field where b* is negative is largely the cross section of a blue field. L* indicates lightness, and as a* and b* values increase, the saturation also increase. A field enclosed by straight lines
3701
shown in
FIG. 1
is a gamut in the L*a*b* space of a display using a CRT display being a general-purpose color input/output device. A field enclosed by straight lines
3702
is a gamut of a printer being a general-purpose color input/output device using inks of cyan, magenta and yellow (CMY).
When first color data
3703
being yellow with high saturation which can be displayed on the CRT display are printed, this color cannot be printed because of a characteristic problem on color available from a printer. Therefore, the color data
3703
has to be converted in such a way that they can be printed. In a conventional method, the color data
3703
are converted using an algorithm toward third color data
3704
of an achromatic color with the same lightness value as the color data
3703
so as to be included in a gamut
3702
, and the second color data
3705
are calculated. In the same way, the first color data
3706
being blue with high saturation are converted toward third color data
3707
of an achromatic color with the same lightness value as the lightness value of the color data
3706
so as to be included in a gamut
3702
, and the second color data
3708
are calculated.
Since a printer can print color data
3705
and
3708
, the printer prints the color data
3705
instead of the color data
3703
, and the color data
3708
instead of the color data
3706
. However, as clearly shown in
FIG. 1
, the color data
3705
are obtained with fairly low saturation compared with the color data
3703
, and the color data
3708
are also obtained with fairly low saturation compared with the color data
3706
. Using the same algorithm as described above, second color data
3711
are obtained from first color data
3709
. Although before conversion the color data
3703
has higher saturation than the color data
3709
, after conversion the color data
3711
corresponding to the color data
3709
has far higher saturation than the color data
3705
corresponding to the color data
3703
, and the order in saturation degree is reversed between before and after conversion.
In a technology disclosed in the Japanese Laid-open Patent Publication No. 4-101566, first color data included in a first gamut dependent on a first color input/output device are converted toward an achromatic color of different lightness for each hue angle, and second color data included in a gamut dependent on a second color input/output device are calculated.
Fields and straight lines
3701
and
3702
shown in
FIG. 2
are the same as those shown in FIG.
1
. Since first color data
3721
being yellow with high saturation which can be displayed on a CRT display, cannot be printed by a printer, the color data
3721
are converted using an algorithm toward third color data
3722
of an achromatic color with high li
Murashita Kimitaka
Semba Satoshi
Shimizu Masayoshi
Suzuki Shoji
Brier Jeffery
Fujitsu Limited
Havan Thu-Thao
Staas & Halsey , LLP
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