Color data converting method

Details Color data converting method Color data converting method

1999-06-07

2002-04-16

Williams, Kimberly A. (Department: 2622)

Facsimile and static presentation processing

Static presentation processing

Attribute control

C358S520000, C358S523000, C358S524000, C382S163000, C345S589000

Reexamination Certificate

active

06373595

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color data converting method of converting color data so that the whole color images when they are seen by apparatuses having different color reproducing ranges (color gamuts: hereinbelow, a color reproducing range is simply referred to as a “gamut”) are not different and, more particularly, to a color data converting method of converting color data for image display of a wide gamut into color data for printing of a narrow gamut.
2. Description of the Related Arts
Color image apparatuses such as personal computer, color printer, digital camera, and the like at present start to be widespread owing to the realization of high performance and low costs. Among them, in the color printer, it is important to print a color image displayed on a CRT display without changing an atmosphere of the whole image. In order to make the colors coincide among the different apparatuses, as a chrominance signal showing an absolute color that does not depend on the apparatus, there is a method of processing the chrominance signal by using an L*a*b* space, a CIEXYZ space, or the like of a luminance color difference separation system as a reference. For example, when an image using the color data of the L*a*b* space is outputted to a CRT display, if the color of the L*a*b* space is converted to the color data of the RGB space peculiar to the CRT display in accordance with characteristics of the CRT display and a color image is displayed, fundamentally, any CRT display can output the same color. When an image using the color data of the L*a*b* space is printed by a printer, if the color of the L*a*b* space is converted to the color of the CMY space peculiar to the printer in accordance with printer characteristics and a color image is printed, any printer can output the same color. Further, when the color data of the RGB space to be displayed on the CRT display is converted to the color data of the CMY space of the printer and a color image is printed, the RGB color data is converted into the L*a*b* color data and the L*a*b* color data is subsequently converted into the CMY color data, so that the same color image as that of a display image on the CRT display can be fundamentally printed by the printer. However, a gamut of the CRT display (hereinbelow, referred to as a display gamut) in the L*a*b* space is largely different from a gamut of the color printer (hereinbelow, referred to as a printer gamut) with respect to both of the width and shape. Many colors which can be displayed by the CRT display but cannot be printed by the color printer exist. Therefore, a color data converting method of converting the color of the CRT display which cannot be printed by the color printer into a color which can be printed is needed.
As such a color data converting method, for example, a method disclosed in JP-A-60-105376 (U.S. Pat. Ser. No. 4,675,704) has been known.
FIG. 1
shows a display gamut
100
serving as an L*a*b* gamut corresponding to the RGB space of the general CRT display and a printer gamut
102
serving as an L*a*b* gamut corresponding to the CMY space of the general color printer at a vertical section of a certain hue angle value in the L*a*b* space. An (L*a*b*) value in the display gamut
100
, namely, color data is shown by Oi and an (L*a*b*) value in the printer gamut
102
, namely, color data is shown by Qi. In this case, i denotes an index of an integer indicative of an arbitrary space position. The printer gamut
102
in
FIG. 1
is smaller than the display gamut
100
and, moreover, the positions of a white color Q
1
and a black color Q
2
in the printer gamut
102
are different as compared with those of a white color O
1
and a black color O
2
in the display gamut
100
.
In the method of JP-A-60-105376, as shown in
FIG. 2
, as shown by arrows, saturation values of the color data O
1
to O
9
which are contained in the display gamut
100
and are not contained in the printer gamut
102
are reduced to saturation values in the printer gamut
102
without changing hue angle values and lightness values L*, thereby obtaining the color data Q
1
to Q
9
contained in the printer gamut
102
. In the conventional method, however, even when saturation values a*b* are maximally compressed, for example, from the color data O
1
to Q
1
or from O
9
to Q
9
, the color data is not included in the printer gamut
102
, so that there is a problem that when a color image is printed by the color printer, a color image of the CRT display cannot be reproduced at a high fidelity. As an algorithm to solve the problem, there is a method disclosed in JP-A-61-288662 (U.S. Pat. Ser. No. 4,758,885). In the conventional method, as shown in
FIG. 3
, lightness values of the color data O
1
to O
5
included in the display gamut
100
are first compressed toward the center of the gamut as shown by arrows, thereby obtaining color data O
11
to O
15
. The compression of the lightness results in that the shape of the display gamut
100
is deformed to a display gamut
104
surrounded by a broken line. Subsequently, as shown in
FIG. 4
, saturation values of the color data O
11
to O
15
are decreased and moved to the printer gamut
102
as shown by arrows, thereby obtaining the color data Q
1
to Q
5
. In the-conventional method of
FIGS. 3 and 4
, the problem in the conventional method of
FIG. 2
that a part of the color data on the CRT display cannot be reproduced by the color printer doesn't occur.
The conventional color data converting methods, however, have the following problems. As for the problem in the conventional method of
FIG. 2
that a part of the color data on the CRT display cannot be reproduced by the color printer, as shown in
FIG. 5
, after the color data Q
1
and Q
2
were obtained by compressing the saturation values of the color data O
1
and O
2
, by changing the lightness values of the color data Q
1
and Q
2
toward the center of the printer gamut
102
as shown by arrows, color data Q
11
and Q
12
included in the printer gamut
102
can be obtained. According to the method, however, all of color data locating in an area
106
which is included in the display gamut
100
and in which the lightness is higher than a highest lightness point in the printer gamut
102
and in an area
108
which is included in the display gamut
100
and in which the lightness is lower than the highest lightness point in the printer gamut
102
in
FIG. 6
are concentrated to one point of the color data Q
11
or Q
12
in the printer gamut
102
in
FIG. 5
, so that there is a problem that a gradation deterioration occurs.
Problems of the conventional method in
FIGS. 3 and 4
will now be described. The printer gamut
102
fundamentally has a “<”-character shape as shown in FIG.
1
. However, in a blue region of blue, violet, and purple, the printer gamut
102
has an “L”-character shape as shown by a boundary
110
in FIG.
7
. The bottom side portion
110
of an “L”-character shape is inwardly curved. In a yellow region of the printer gamut, as shown in the printer gamut
102
in
FIG. 8
, it has a reverse “L”-character shape and an upper side portion
112
of the reverse “L”-character shape has a shape that is inwardly curved. When the compression is performed to the “L”-shaped and reverse “L”-shaped printer gamuts
102
in
FIGS. 7 and 8
in the lightness direction so as to concentrate the color data to the center of the gamut in accordance with the conventional method in
FIGS. 3 and 4
, as shown in
FIGS. 9 and 10
, areas
114
and
116
out of the display gamut
100
after completion of the compression occur in a part of the printer gamut
102
. Since the colors included in the printer gamuts
114
and
116
out of the display gamut
100
are not used for printing, a printing of a better quality in which the performance of the printer is fully utilized cannot be obtained. This problem can be solved by changing a compression amount of the lightness value by the saturation value. Specifically speaking, when the co

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