Optics: measuring and testing – By shade or color – Tristimulus examination
Reexamination Certificate
2000-02-23
2003-03-25
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By shade or color
Tristimulus examination
C356S406000, C345S604000
Reexamination Certificate
active
06538742
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-048042, filed Feb. 25, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to a color reproduction system including a color image processing apparatus for obtaining an input color image signal used to display any desired color on a color image display.
In general, display apparatuses such as CRT displays, liquid crystal displays, liquid crystal projectors, etc. are widely used as means for reproducing a digital color image input by an image input device such as a color scanner, a digital camera, etc.
In these display apparatuses, various colors are usually reproduced by additive color mixture of three primary colors RGB (red, green and blue).
The color gamut (color reproduction area) that can be reproduced on a display is limited to a region in which each color is expressed as the sum of color vectors of the three primary colors in the three dimensional color space. For example, in a CRT display for reproducing colors using three kinds of fluorescent substances, i.e. Red, Green and Blue, the color reproduction area corresponds to a hexahedron having corners (0, 0, 0), (Xr, Yr, Zr), (Xg, Yg, Zg), (Xb, Yb, Zb), (Xr+Xg, Yr+Yg, Zr+Zg), (Xg+Xb, Yg+Yb, Zg+Zb), (Xb+Xr, Yb+Yr, Zb+Zr), (Xr+Xg+Xb, Yr+Yg+Yb, Zr+Zg+Zb), assuming that (Xr, Yr, Zr), (Xg, Yg, Zg) and (Xb, Yb, Zb) represent X, Y and Z data which belongs to the CIE1931 color system (XYZ color system) which are obtained when RGB fluorescent substances emit maximum light respectively.
FIG. 16
schematically shows a color reproduction area in the XYZ space of such a three-primary-color display. If this area is expressed using an xy chromaticity diagram, it corresponds to the interior of a triangle formed by the chromaticity of each primary color, as is shown in FIG.
17
.
In the display using the additive color mixture of three primary colors RGB as the color reproduction principle, the relationship between RGB signal values input to the display and XYZ values of each color displayed thereon is uniquely determined in the color reproduction area of the display.
If it is assumed that the emission spectrum of each primary color is independent of the intensity of any other primary color, and the relative spectral distribution does not depend upon the intensity of emission (i.e., the chromaticity does not vary even if the emission intensity varies), the X, Y and Z data of a to-be-displayed color corresponding to input RGB values are given by the following formula:
(
X
Y
Z
)
=
(
Xr
Xg
Xb
Yr
Yg
Yb
Zr
Zg
Zb
)
(
R
′
G
′
B
′
)
R
′
=
γ
r
(
R
)
G
′
=
γ
g
(
G
)
B
′
=
γ
b
(
B
)
(
1
)
where Xr, Xg and Xb represent the respective X data items obtained when the RGB fluorescent substances emit maximum light, similarly, Yr, Yg and Yb represent the respective Y data items obtained when the RGB fluorescent substances emit maximum light, and Zr, Zg and Zb represent the respective Z data items obtained when the RGB fluorescent substances emit maximum light. Further, &ggr;r, &ggr;g and &ggr;b represent functions indicating the relationship between the input signal value and the output luminescence, and R′, G′ and B′ represent signal values normalized so that they will be “1” when the RGB fluorescent substances emit maximum light.
From the reverse relationship of the above, input RGB values for displaying desired X, Y and Z data are given by the following formula:
R
=
γ
r
-
1
(
R
′
)
G
=
γ
g
-
1
(
G
′
)
B
=
γ
b
-
1
(
B
′
)
(
R
′
G
′
B
′
)
=
(
Xr
Xg
Xb
Yr
Yg
Yb
Zr
Zg
Zb
)
-
1
(
X
Y
Z
)
(
2
)
where “−1” indicates inverse function and inverse matrix. Thus, in the three primary color display, it is easy to model the relationship between XYZ data and RGB values, and a conversion method using matrix conversion and gradation correction is generally used as described in “COLOR IMAGE DUPLICATION” (written by Johji Tajima and published by Maruzen Co., Ltd.).
If in the three primary color display, a point (X, Y, Z) falls out of the color reproduction area of the display, one of R′, G′ and B′ obtained from the formula (2) is “negative” or “higher than 1”.
As described above, in the display apparatus using, as the principle, the additive color mixture of three primary colors, the area determined by the chromaticity coordinates of each primary color is the color reproduction area. In order to enlarge the color reproduction area, it is considered to increase the chroma of each primary color or to increase the number of primary colors.
For example, as is disclosed in NHK Technology Research Published Documents (published by NHK Broadcasting Technology Research, Tokyo 1995), an attempt has been made to realize a wider color reproduction area than the conventional three-primary-color display by using four primary colors instead of three primary colors.
FIGS. 18 and 19
show a color reproduction area in the XYZ space of the four primary color display, and a color reproduction area in the xy chromaticity diagram, respectively.
Also in the case of a multi-primary-color display using a number N of primary colors not less than four primary colors, the XYZ data of each to-be-displayed color corresponding to signal values can be given by the following formula that is obtained by generalizing the formula (1):
(
X
Y
Z
)
=
(
Xc1
Xc2
Xc3
⋯
XcN
Yc1
Yc2
Yc3
⋯
YcN
Zc1
Zc2
Zc3
⋯
ZcN
)
(
C1
′
C2
′
C3
′
CN
′
)
C1
′
=
γ
1
(
C1
)
C2
′
=
γ
2
(
C2
)
C3
′
=
γ
3
(
C3
)
CN
′
=
γ
N
(
CN
)
(
3
)
The conversion from XYZ data into signal values, which is considered a reverse relationship with respect to the formula (3), is not directly executed except for the surface of the color reproduction area of the multi-primary-color display.
It is necessary to execute conversion with signal values corresponding to X, Y and Z determined on the basis of certain conditions. For example, Japanese Patent Application KOKAI Publication No. 6-261332 discloses a color conversion method employed in a multi-primary-color display.
In a conversion method as a first invention of the application, color reproduction is executed using the linear sum of three primary colors selected on the basis of the chroma values of input colors. In this method, accurate color reproduction can be executed within a range in which reproduction can be realized on the basis of the selected three primary colors. However, since the method does not consider a color reproduction area in the direction of brightness in the multi-color display, it cannot deal with all reproducible input colors.
Moreover, in a second invention disclosed in the above publication, linear conversion is executed using multiple primary colors. In this case, however, it is not guaranteed whether a solution that imparts a positive value to any primary color signal can be obtained. Accordingly, there is a case where even a reproducible input color cannot accurately be reproduced.
To execute accurate color reproduction, the color conversion method for display apparatuses including a multi-primary-color display that uses four or more primary colors needs to satisfy the following conditions:
First, colorimetrically accurate color reproduction can be executed.
Second, there is continuity between XYZ data and signal values.
Third, conversion that satisfies the above conditi
Font Frank G.
Frishauf Holtz Goodman & Chick P.C.
Lauchman Layla
Olympus Optical Co,. Ltd.
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