Color converting method

Details Color converting method Color converting method

1999-05-04

2002-01-01

Luu, Matthew (Department: 2672)

Computer graphics processing and selective visual display system

Computer graphics processing

Attributes

C345S591000, C345S606000, C348S444000, C348S453000, C348S496000, C358S518000, C358S525000, C382S162000, C382S300000

Reexamination Certificate

active

06335734

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color converting method of converting a chrominance signal of a different color space by using a multidimensional color conversion table and, more particularly, to a color converting method of converting a chrominance signal which is not registered in a conversion table by an interpolation calculation.
2. Description of the Related Arts
Color image apparatuses such as personal computer, color printer, digital camera, and the like are at present being widespread owing to the realization of high performance and low costs. Generally, the apparatus such as printer, display, or the like expresses a color of an RGB space, a CMY space, or the like of a primary color system. In this case, since the color that is outputted from the RGB space or CMY space differs depending on the apparatus such as a display or a printer, it can be regarded as a chrominance signal depending on the apparatus. 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, an XYZ space, or the like of a luminance color difference separation system as a reference. For example, when the color of the L*a*b* space is outputted to a printer, if the color of the L*a*b* space is converted to the color of the CMY space peculiar to the printer and the chrominance signal is printed, fundamentally, any printer can output the same color. A technique for converting the chrominance signal between the different color spaces is called a color converting technique. As such a color converting technique, a color conversion using a multidimensional conversion table (multidimensional look-up table) has been known.
FIGS. 1A
to
1
C are explanatory diagrams of a multidimensional conversion space to realize the multidimensional conversion table.
FIG. 1A
shows an RGB/Lab conversion coordinates space
100
to convert the color from the RGB space to the L*a*b* space.
FIG. 1B
shows an Lab/CMY conversion coordinates space
102
to convert the color from the L*a*b* space to the CMY space. Further,
FIG. 1C
shows an RGB/CMY conversion coordinates space
104
to convert the color from the RGB space to the CMY space. For instance, when the color of the RGB space of a display is converted to the color of the CMY space of a printer, RGB color values are inputted to the RGB/Lab conversion coordinates space
100
of
FIG. 1A
, thereby converting to L*a*b* color values. Subsequently, the L*a*b* color values are inputted to the Lab/CMY conversion coordinates space
102
of
FIG. 1B
, thereby converting to CMY color values. Practically, the RGB/CMY conversion coordinates space
104
in which the conversion to the intermediate L*a*b* space as shown in
FIG. 1B
is omitted is used. The RGB color values are inputted to the RGB/CMY conversion coordinates space
104
and are directly converted to the CMY color values. In such a multidimensional conversion table, output representative colors of the color space on the conversion destination side are set to lattice points which are decided by input representative colors of the color space serving as a converting source. The table is used in combination with an interpolation calculating mechanism, so that the high-speed and accurate color conversion can be realized by a small memory capacity of the table.
As an interpolating method in the conventional color converting method in which the multidimensional conversion table and the interpolation calculating mechanism are combined, a cube interpolating method (Cube Algorithm) has been known. According to the cube interpolating method, as for the multidimensional conversion space constructed by a set of lattice points having a predetermined width constructing the multidimensional conversion table, an output color corresponding to an input color at an arbitrary position existing in a lattice space is obtained from output colors set at eight vertices of the lattice space constructing a cube by an interpolation calculation. The cube interpolating method will be explained with respect to a case where the color is converted from the RGB space of the display to the CMY space of the printer as an example.
FIG. 2
shows the details of the RGB/CMY conversion coordinates space
104
of FIG.
1
C. Color values of the RGB space serving as an input color and color values of the CMY space serving as an output color have values within a range of 0 to 255. In the RGB/CMY conversion coordinates space
104
, for example, a lattice interval w is set to 32 (w=32), each of the input colors (R, G, B) serving as a converting source has representative input colors 0, 32, 64, 96, 128, 160, 192, 224, and 255, respectively. The representative output colors (C, M, Y) after completion of the conversion have been stored at positions of the lattice points which are decided by the representative input colors.
Specifically speaking, the CMY representative output colors are stored at the lattice points of the RGB space in accordance with the following arrangements.
C[r, g, b]
M[r, g, b]
 
Y[r, g, b]
where, r, g, and b denote lattice numbers indicative of a coordinates position in a lattice space serving as a cube surrounded by eight lattice points. Since the lattice space is called a data set, the lattice number is also called a data set number. Explanation will now be made on the assumption that the lattice space is the data set. The data set numbers indicative of a three-dimensional position of the lattice point where an arbitrary input color (R, G, B) is included are obtained by the following equations.
r=
(int)(
R÷w
)
g=
(int)(
G÷w
)
b=
(int)(
B÷w
)  (1)
where, w denotes the lattice interval and, for example, w=32. (int) means a formation of an integer by rounding-off. For instance, the data set numbers corresponding to the minimum value (
0
,
0
,
0
) of the input color (R, G, B) are set to r=0, g=0, and b=0. The data set numbers corresponding to an input color (
32
,
0
,
0
) are set to r=1, g=0, and b=0. Further, the data set numbers corresponding to an input color (
32
,
128
,
128
) are set to r=1, g=4, and b=4.
As shown in
FIG. 3
, in the cube interpolating method, interpolating arithmetic operations are performed by using eight lattice points A
1
to A
8
serving as vertices of a cube surrounding an interpolation point
110
of an input color (Rc, Gc, Bc). First, when coordinates positions shown by the data set numbers of the eight lattice points A
1
to A
8
surrounding the interpolation point
110
are shown by setting the lattice point A
1
to a reference position and setting the other positions to relative positions for the reference position, the following coordinates values are obtained.
A
1
(
r, g, b
)
A
2
(
r
+1,
g, b
)
A
3
(
r, g+
1,
b
)
A
4
(
r+
1,
g
+1,
b
)
A
5
(
r, g, b+
1)
A
6
(
r+
1,
g, b
+1)
A
7
(
r, g
+1,
b
+1)
A
8
(
r+
1,
g
+1,
b
+1)
When output colors (C, M, Y) set in the lattice points A
1
to A
8
are expressed by arrangements of the lattice point coordinates, the following equations are obtained.
C
1
=
C[r, g, b]
C
2
=
C[r
+1,
g, b]
 
C
3
=
C[r, g+
1,
b]
C
4
=
C[r
+1,
g+
1,
b]
C
5
=
C[r, g, b
+1]
C
6
=
C[r
+1,
g, b
+1]
C
7
=
C[r, g
+1,
b+
1]
C
8
=
C[r+
1,
g+
1,
b+
1]
M
1
=
M[r, g, b]
M
2
=
M[r+
1,
g, b]
M
3
=
M[r, g+
1,
b]
M
4
=
M[r+
1,
g+
1,
b]
M
5
=
M[r, g, b
+1]
M
6
=
M[r+
1,
g, b+
1]
M
7
=
M[r, g+

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