Color data conversion method, color data conversion...

Details Color data conversion method, color data conversion... Color data conversion method, color data conversion...

1999-06-21

2003-08-26

Coles, Edward (Department: 2622)

Facsimile and static presentation processing

Static presentation processing

Attribute control

C358S500000, C358S518000, C358S525000, C382S162000, C382S167000

Reexamination Certificate

active

06611356

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color conversion technology (color gamut compression technology) for placing color data inside a specific color gamut.
2. Description of the Related Art
Generally speaking, a picture input/output device, such as a printer, display, etc., processes the colors of a picture using an RGB value, a CMY value, etc. However, even if a pixel with the same RGB value or CMY value is used, the color of it in the varies depending on an input/output device and the model to be used. Therefore, the RGB value and CMY value are color data values dependent on the manufacturer of an input/output device and the model to be used. For example, with a CMY value, the density information of three colors, cyan, magenta and yellow, is used as a numeric value to instruct a printer to print a color. For example, a C value, M value and Y value can have a value between 0 and 255, and by transmitting or outputting the C value, M value and Y value of a pixel composing an outputted picture, to a printer, the amount of cyan, magenta and yellow ink corresponding to the C value, M value and Y value are painted on a printing medium, such as paper, etc., for each pixel through the printing head of a printer under the control of the controller of the printer. Thus, a CMY value is an indexed numeric value for instructing the printing head as to the amount of cyan, magenta and yellow ink that should be painted for each pixel. However, even if the same CMY value is designated, an actually processed color varies depending on the manufacturer of an input/output device and the model to be used, since the kind of ink to be used and the ink painting mechanism varies depending on the manufacturer of an input/output device and the model to be used. The same phenomenon occurs in the case of a display in which an RGB value is used. In the case of a display value, a variety of colors of a color picture are processed by instructing a display as to the degree of lightness (gradation) for each of red, green and blue for each pixel using the numeric values R, G and B.
There is a method for processing color data that is independent of an input/output device and is based on absolute color data values (L*a*b*, XYZ, etc.), which serves to unify output colors among a variety of input/output devices. For example, in order to output a color of a certain L*a*b* value on a printer, it is sufficient to convert the L*a*b* value to an appropriate CMY value according to the color processing characteristics of the CMY value of each printer and to transmit the CMY value to a printer to print the color. Basically, by compensating for color processing characteristics, all printers can print the same picture in the same colors. A technology to convert color data between different color data values (for example, from an L*a*b* value to a CMY value, as described above) like this is termed a color conversion technology. Generally speaking, in a color conversion technology, a table in which the color correspondence between an intermediate color space, such as an L*a*b* space, etc., and a device-dependent color space, such as a CMY space, RGB space, etc., is registered (color conversion table) is prepared, and colors to be actually outputted to a device (CMY value, RGB value, etc.) are obtained by converting colors by referring to the table. Colors which are not registered in the table are converted and obtained by executing an interpolation process using data registered in the table.
In order to put the color conversion technology into practice, it is necessary to take into consideration only a color range for which a device can output colors (color gamut). More specifically, in order to output color data which are outside the range of a color gamut, to a printer or display, it is necessary to convert the values of the color data to values contained within the range of the color gamut. Such a technology to place color data which are outside the range of a color gamut and within the range of the color gamut of a device is called a color gamut conversion technology. Essentially, color conversion technology, it is important to convert color data to a color that is as close as possible to its color data value before before being outputted.
Therefore, to output colors in a wide range, including colors other than those in the color gamut of a device, that are as close to the original color as possible, both an accurate color conversion technology that converts different color data values and a color gamut conversion technology that changes the color as little as possible are required.
Here, first a case where an L*a*b* value is converted to a CMY value using a color conversion table in which CMY values corresponding to the colors of L*a*b* values distributed in a grid shape in an L*a*b* space are registered is described as an example of color conversion technology using an interpolation operation.
Usually, in an L*a*b* space L* takes a value in the range of 0 to 100, and a* and b* take a value in the range of −128 to 127. However, in the following description, 2.55 times an ordinary value is used for an L* value, and an ordinary value plus 128 is used for a* and b* values for convenience (L
255
* value, a
255
* value, b
255
* value). This is because an L* value, a* value and b* value can be handled as a value in the range of 0 to 255.
It is assumed that colors distributed in a grid shape in an L*a*b* space (in the following example, 0, 32, 64, 96, 128, 160, 192, 224 and 255 are the 9 values used for the L
255
* value, a
255
* value and b
255
* value), which are color values after conversion (CMY value) corresponding to colors of a conversion source (L
255
* value, a
255
* value, b
255
* value), are stored in the color conversion table in which colors are distributed in a grid shape. Specifically, it is assumed that a C value, M value and Y value are stored in the following three-dimensional arrays: C [L] [a] [b], M [L] [a] [b] and Y [L] [a] [b]. L, a and b are the numbers of a grid point in the L*a*b* space (grid number). For example, the grid numbers corresponding to the minimum value (0, 0, 0) of (L
255
* value, a
255
* value, b
255
* value) are L=1, a=0, b=0, grid numbers corresponding to (32, 0, 0) are L=1, a=0, b=0, and grid numbers corresponding to (32, 128, 128) are L=1, a=4, b=4.
An example of a color conversion is given below. For the conversion, a method for performing an interpolation operation using eight points (in the shape of a cubic grid) surrounding an L
255
* value, a
255
* value, b
255
* value (Lconv, aconv, bconv) to be converted is adopted. Lconv, aconv and bconv are variables indicating an L
255
* value, a
255
* value and b
255
* value, respectively.
FIG. 1
explains how to perform an interpolation operation.
(1) Selects Grid Points to be Used for Interpolation (Eight Points Surrounding an L*a*b* Value (Lconv, aconv, bconv) to be Converted (Selects a Cube)
In the following description, w is an interval between grids in the L*a*b* space, and is assumed to be 32 as described above. (int) indicates the omission of decimal places. L, a and b are grid numbers.
L
=(int)(
L
conv/
w
),
a
=(int)(
a
conv/
w
),
b
=(int)(
b
conv/
w
)  (1)
In addition to the grid point (L, a, b) calculated by the above equations (1), grid points of (L+1, a, b), (L, a+1, b), (L, a, b+1), (L, a+1, b+1), (L+1, a, b+1), (L+1, a+1, b) and (L+1, a+1, b+1) are used for interpolation operation. These eight grid points are the vertices of a cube
100
shown in FIG.
1
.
(2) Calculates Positions Inside a Cube
Calculates the positions (Lw, aw, bw) inside the cube
100
of an L*a*b* value using the following equations (2).
Lw
=(
L
conv/
w−L
)·
w
aw
=(
a
conv/
w−a
)·
w
bw
=(
b
conv/
w−b
)·
w
  (2)
(3

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