JP2009177285A - Color data conversion apparatus and color data conversion method - Google Patents

Abstract

A first color system using a color conversion table in which gradation values in a second color system are associated with each of a plurality of grid points formed in a color space of the first color system. The input color data represented by the gradation value is interpolated from a plurality of grid points of the color conversion table close to the gradation value, and the output color data of the gradation value in the second color system is obtained. Convert. At this time, the gradation value between the grid points of the color conversion table is interpolated with high accuracy.
Focusing on the interpolation characteristics in a specific color area, the interpolation accuracy suitable for each area is used to improve the interpolation accuracy as a whole. In particular, cubic interpolation is performed for other than the gray axis, and tetrahedral interpolation is performed for the gray axis.
[Selection] Figure 1

Description

  The present invention relates to a technique for converting color data between different color systems, and more particularly to an interpolation technique for obtaining color data of different color systems with reference to a color conversion table.

  As a color system that expresses color data by a combination of numerical values, an RGB color system using color values of red (R), green (G), and blue (B), cyan (C), and magenta (M). The CMY color system using the color values of yellow and yellow (Y) is well known. The RGB color system is used in devices that express colors by combining light emission colors or transmitted light colors, such as CRTs, liquid crystal display devices, and plasma displays. On the other hand, the CMY color system is used for photographic paper and printers. Further, an L * a * b * (hereinafter simply referred to as “Lab”) color system is also used as a color system that corresponds to human perception and does not depend on equipment.

  In order to transfer color data between apparatuses using different color systems, it is necessary to perform color conversion between the respective color systems. For example, when an image displayed on a display device of a computer is printed by a printer, conversion from the RGB color system to the CMY color system is necessary. This conversion is generally performed via the Lab color system in order to remove the device dependency.

  The conversion from the RGB color system to the Lab color system and the conversion from the Lab color system to the CMY color system can be performed analytically, but the processing becomes complicated. Called). In this color conversion table, color data (tone value) in the second color system is associated with each of a plurality of grid points formed in the color space of the first color system. Although the grid points do not represent all colors, colors other than the grid points can be interpolated from surrounding grid points.

  The color conversion table from the RGB color system to the Lab color system will be described as an example. RGB individual gradation values (R, G, B) = (0, 0, 0), (0, 0, 15) , (0,0,30),. . , (0,0,255), (0,15,0), (0,15,15),. . , (0,15,255),. . . , (15,0,0),. . (15, 0, 255),. . , (255, 255, 255) are lattice points, and the gradation values (L *, a *, b *) are associated with the respective points. Here, it is assumed that each gradation value of RGB is represented by 8 bits (0 to 255), and the grid point interval is 15 assuming that the grid points are evenly arranged in the RGB color space. Are design items that can be arbitrarily selected, and are not necessarily equal.

  Cubic interpolation and tetrahedral interpolation are well known as methods for obtaining colors other than grid points by interpolation (see Non-Patent Document 1). In the cubic interpolation, desired color data is obtained from the gradation values of the eight lattice points of the cube including the interpolation point to be obtained. On the other hand, in tetrahedral interpolation, interpolation is performed with reference to four nearby points.

Fuji Xerox Technical Report No.11 1996, http://www.fujixerox.co.jp/company/tr/tr96/Hitoshi_Ogatsu/H_Oga104.html

  Although cubic interpolation has high interpolation accuracy, processing takes time. Further, since the diagonal of the cubic lattice is affected by all reference points, the accuracy of the gray axis is degraded. The tetrahedron interpolation is faster in processing time than the cubic interpolation because there are fewer reference points, but the interpolation accuracy is inferior.

  The present invention pays attention to the interpolation characteristic in a specific color region, and an object thereof is to improve the interpolation accuracy as a whole by using an interpolation method suitable for each region.

  The color data conversion apparatus according to the present invention stores a color conversion table in which a gradation value in the second color system is associated with each of a plurality of grid points formed in the color space of the first color system. The input color data expressed by the color conversion table storage means and the first color system gradation value is interpolated from a plurality of grid points of the color conversion table close to the gradation value, and the second Color conversion means for converting to gradation color output color data in the color system, and the color conversion means determines whether or not the gradation value of the input color data is included in a predetermined specific color area. Means for detecting, first interpolation means for performing cubic interpolation by surrounding eight grid points when the gradation value of the input color data is not included in a specific color area, and gradation values of the input color data Is included in a specific color region, the second interpolation is performed with less than eight surrounding grid points. Characterized in that it comprises an interpolation means.

  The first color system is an RGB color system represented by gradation values of three colors of red, green, and blue, and a specific color region is a color in the vicinity of gray in which the gradation values of all three colors are substantially equal. Preferably, the second interpolation means is a region and includes means for performing tetrahedral interpolation. The present invention can also be used when a specific color region is a color region in which any one of the gradation values of the three colors is zero or near the maximum value.

  The color data conversion method of the present invention detects whether or not the gradation value of the input color data represented by the first color system is included in a predetermined specific color area, and determines the level of the input color data. When the tone value is not included in the specific area, the color in which the gradation value in the second color system is associated with each of the plurality of grid points formed in the color space of the first color system Cubic interpolation is performed from 8 grid points close to the gradation value of the input color data in the conversion table to convert it to output color data of the gradation value in the second color system, and the gradation value of the input color data is specified. Are included in the color conversion table, interpolation is performed from less than eight grid points close to the gradation value of the input color data in the color conversion table to convert the output color data to gradation values in the second color system. It is characterized by that.

[First Embodiment]
FIG. 1 is a block diagram showing a first embodiment of the present invention, and shows a configuration of a color data conversion apparatus. The color data conversion apparatus stores a color conversion table that stores a color conversion table in which a gradation value in the second color system is associated with each of a plurality of grid points formed in the color space of the first color system. Interpolation is performed on the input color data represented by the gradation values of the table memory 2 and the first color system from a plurality of grid points of the color conversion table close to the gradation values, and the second table. A specific color area detecting unit 1 for detecting whether or not the gradation value of the input color data is included in a predetermined specific color area, as color conversion means for converting the gradation value in the color system into output color data; When the gradation value of the input color data is not included in the specific color area, the cubic interpolation processing unit 3 that performs cubic interpolation with the surrounding eight lattice points, and the gradation value of the input color data is the specific color Tetrahedron to interpolate with fewer than 8 surrounding grid points if included in region And a space between the processing unit 4, an output unit 5 for selecting and outputting the outputs of the tetrahedral interpolation processing unit 4 of the cubic interpolation processing unit 3.

[Description of color conversion table and interpolation]
FIG. 2 shows a simple example of a color conversion table, and FIG. 3 shows one cube including points corresponding to input color data. Here, an example will be described in which the first color system is the RGB color system, the second color system is the Lab color system, and the specific color region is a color near gray. Gray has the same gradation value for the three RGB colors and is located on the diagonal line of the cubic lattice of the color conversion table. When such color data on a diagonal line is obtained by interpolation, in cubic interpolation, Lab color system color data is obtained based on eight lattice points of a cube (see FIG. 3) including the interpolation point. Yes. On the other hand, in tetrahedral interpolation, interpolation is performed based on four grid points when the interpolation point is away from the gray axis, but when the interpolation point is on the gray axis, it is on the diagonal of the cubic grid. Interpolation is performed with reference to only the two points.

  In cubic interpolation, the diagonal of the cubic lattice is also affected by all reference points, so it takes time to process areas that are sensitive to changes in color values, such as the gray axis, and the interpolation accuracy also deteriorates. . On the other hand, tetrahedral interpolation is performed between two points with respect to the diagonal of the cubic lattice, so that the processing time is short and the interpolation accuracy is high. Therefore, high accuracy in the color region can be secured by performing cubic interpolation other than the gray axis. On the other hand, with respect to the gray axis, by performing tetrahedral interpolation, it is possible to limit the reference points to suppress the influence of other reference points, and to reduce variations in values after interpolation.

[Description of operation]
With reference to FIG. 1 again, the operation of the color data converter will be described. Here, it is assumed that the first color system is the RGB color system, the second color system is the Lab color system, and the specific color region is a color near gray. This apparatus receives RGB color image data from a color image processing apparatus such as a computer, converts the color data of each pixel into Lab color system color data, and outputs the data. The specific color area detection unit 1 determines whether or not the input RGB color data represents the vicinity of gray. If it is not near gray, the specific color region detection unit 1 sends the color data to the three-dimensional interpolation processing unit 3. The color data is output to the tetrahedral interpolation unit 4. The three-dimensional interpolation processing unit 3 refers to a color conversion table recorded in advance in the color conversion table memory 2 and performs cube interpolation processing to obtain Lab color system color data. Since the interpolation point is near gray, the tetrahedral interpolation processing unit 4 performs interpolation with reference to only two points on the diagonal of the cubic lattice of the color conversion table to obtain Lab color system color data. The output unit 5 selects one of the output from the cube interpolation processing unit 3 and the output from the tetrahedral interpolation processing unit 4 and outputs Lab color system color data.

[Second Embodiment: Implementation by Computer Program]
FIG. 4 is a block diagram showing a second embodiment of the present invention, and FIG. 5 is a flowchart of an operation for converting RGB color data into Lab color data. Here, an example in which the color data conversion apparatus is implemented as a printer driver program operating in the computer 10 is shown.

  Although the hardware configuration is omitted in FIG. 4, the computer 10 includes a CPU, a RAM, a ROM, a hard disk, various interfaces, and the like, and an application program 11, a video driver program 12, a printer driver program 13, and the like under a predetermined operating system. The program is operational.

  The color image processing of each program will be described. The video driver program 12 displays a color image, a video, a user interface, and the like on the display device 20 according to commands from the application program 11 and the printer driver program 13. The printer driver program 13 converts RGB image data into print data according to an instruction from the application program 11 and outputs the print data to the marking device 30.

  The printer driver program 13 includes a resolution conversion unit 14, a color conversion unit 15, and a print data generation unit 16. The resolution conversion unit 14 converts the resolution of the image processed by the application program 11 into the resolution of the printing apparatus 30. The color conversion unit 15 performs processing equivalent to that of the color data conversion apparatus shown in FIG. 1 and converts RGB image data whose resolution has been converted into Lab image data. The print data generation unit 16 converts the Lab image data into the dot amount of each color ink in the printing apparatus 20, performs halftone processing and literalization processing for each color, and print data in a format that can be printed by the printing apparatus 20 Is output. Data necessary for processing of each unit such as a color conversion table is stored in a memory (not shown), and each unit can access the memory as necessary.

  The operation of the color conversion unit 15 will be described with reference to the flowchart of FIG. When the RGB color data is input (S11), the color conversion unit 15 detects whether the gradation value is included in a predetermined specific color area (S12), and is included in the specific area. If not, eight grid points close to the gradation value of the input color data in the color conversion table in which the gradation value in the Lab color system is associated with each of the plurality of lattice points formed in the RGB color space Is converted into output color data of gradation values in the Lab color system (S13), and when the gradation values of the input RGB color data are included in a specific area, the input RGB of the color conversion table is converted. Tetrahedral interpolation is performed from two grid points close to the gradation value of the color data to convert it to output color data of gradation values in the Lab color system (S14), and the obtained Lab color data is output (S15). ).

The block block diagram which shows the 1st Embodiment of this invention. The figure which shows the simple example of a color conversion table. The figure which shows one cube in a color conversion table. The block block diagram which shows the 2nd Embodiment of this invention. The flowchart of a color conversion operation | movement.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Specific color area detection part 2 Color conversion table memory 3 Cube interpolation process part 4 Tetrahedral interpolation process part 5 Output part 10 Computer 11 Application program 12 Video driver program 13 Printer driver program 14 Resolution conversion part 15 Color conversion part 16 Print data generation Section 20 Display device 30 Printing device

Claims (4)

Color conversion table storage means for storing a color conversion table in which gradation values in the second color system are associated with each of a plurality of grid points formed in the color space of the first color system;
The input color data represented by the gradation values of the first color system is interpolated from a plurality of grid points of the color conversion table close to the gradation value, and the second color system Color conversion means for converting to output color data of gradation values in
The color converting means detects whether the gradation value of the input color data is included in a predetermined specific color area; and the gradation value of the input color data is in the specific color area. When not included, the first interpolation means for performing cubic interpolation with the surrounding eight grid points as the interpolation, and the interpolation when the gradation value of the input color data is included in the specific color area And a second interpolation means for performing interpolation with less than eight surrounding grid points.   The first color system is an RGB color system represented by gradation values of three colors of red, green, and blue, and the specific color region is a gray color having substantially the same gradation values of the three colors. 2. A color data conversion apparatus according to claim 1, wherein said second interpolation means includes means for performing tetrahedron interpolation.   The first color system is an RGB color system represented by gradation values of three colors of red, green, and blue, and the specific color region has one of the gradation values of the three colors set to zero or The color data conversion apparatus according to claim 1, wherein the color area is in the vicinity of the maximum value. Detecting whether or not the gradation value of the input color data represented by the first color system is included in a predetermined specific color region;
When the gradation value of the input color data is not included in the specific area, each of the plurality of grid points formed in the color space of the first color system is in the second color system. Cubic interpolation is performed from eight grid points close to the gradation value of the input color data in the color conversion table in which the gradation value is associated, and converted to output color data of the gradation value in the second color system. ,
When the gradation value of the input color data is included in the specific area, interpolation is performed from less than 8 grid points close to the gradation value of the input color data in the color conversion table, and the second A color data conversion method characterized by converting gradation color output color data in a color system.

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