JP2008312117A - Apparatus and method for creating color conversion look-up table - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that, when a document type is changed in accordance with scanner metamerism in color matching, excellent color matching can not be performed. <P>SOLUTION: A color look-up table is created which outputs colors that are more achromatic than colorimetric values of a target. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a color lookup table creation method and apparatus for converting scanner device colors into a standard color space such as L ^* a ^* b ^* .

In general color matching, RGB data read by a scanner is converted into a standard color space independent of the device. The standard color space is XYZ and L ^* a ^* b ^* values based on the D50 light source as defined by the ICC (International Color Consortium) profile. As a conversion method, a lookup table method is often used.

For the scanner profile, the optimal look-up table for converting RGB to L ^* a ^* b ^* based on the scanner device RGB data read from the profile creation chart and the colorimetric values (L ^* a ^* b ^* ) of each color chart It is created by converting. The colorimetric values of each color chart can be obtained by measuring with a spectrocolorimeter.

When color matching is performed, due to human visual characteristics, the loss of gray balance makes the color difference more sensitive than other hue colors. In order to improve the accuracy of gray balance, there is a method for adjusting a look-up table as disclosed in Patent Document 1.
Japanese Patent Laid-Open No. 2001-320594

When a color chart with different document types is scanned, such as a printed original and a silver halide photograph original, even if the color measurement value of the color chart is the same value, it is scanned as a different device RGB when scanned, so the same L ^* a ^* b ^* may not be converted with a scanner profile. This phenomenon is called scanner metamerism.

As a cause of the occurrence of scanner metamerism, a reduction optical system scanner using a cold cathode fluorescent lamp (CCFL) as a light source will be described as an example. The light source specified by L ^* a ^* b ^* is a D50 light source, but the scanner is a CCFL light source, so the spectral characteristics are different. (Figure 1) Also, the spectral sensitivity characteristics of the scanner sensor through the color filter and the sensitivity characteristics of XYZ are different. (Figure 2)
As described above, in the scanner, even if the color patches have the same colorimetric value, they are scanned as different device RGB due to the scanner metamerism if the document type is different. If color matching is performed on a plurality of types of documents that cause scanner metamerism using a single scanner profile, even color charts with the same colorimetric values are converted as different colors. In particular, in gray, the difference in color difference is sensitive to the human eye.

  An object of the present invention is to solve the above-described problem, and to provide a color look-up table that outputs a good color even for device colors scanned as different colors by scanner metamerism. .

  Create a color lookup table that outputs a color that is achromatic than the target colorimetric value.

  According to the present invention, it is possible to obtain a good gray balance for a plurality of types of documents that cause scanner metamerism.

  Next, details of the present invention will be described in accordance with the description of the embodiments.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows an image processing apparatus according to the present invention. The color conversion unit 101 refers to the lookup table 102 and converts the device RGB into L ^* a ^* b ^* which is a standard color space. In this embodiment, the standard color space is L ^* a ^* b ^* , but it may be XYZ color space or sRGB.

  FIG. 2 is a diagram showing a method of scanning a color chart 301 for creating a scanner profile with a scanner 202 and obtaining a device RGB with a PC 201. The color chart 301 is scanned in a predetermined operation mode, and the device RGB data 302 of the color chart is obtained by the PC 201. A temporary scanner profile is created using the obtained device RGB data 302 and color chart colorimetric values 303. FIG. 3 is a diagram showing an example of the colorimetric values of the color chart 301.

FIG. 4 is a diagram showing obtaining a color chart scanner device RGB.
FIG. 5 is a diagram showing the colorimetric values of the color chart.

  Similarly, a color chart device RGB and a colorimetric value of a color chart created with an original having a spectral reflection characteristic different from that of the scanner profile are obtained. For example, the document is a printed document.

  Next, a method for adjusting the gray area using the created temporary profile will be described.

FIG. 6 is a diagram in which elements constituting the profile are plotted in the L ^* a ^* b ^* space. FIG. 7 shows only the gray region of this profile. The figure shows the relationship between L ^* a ^* b ^* and the colorimetric values when the gray color chart of the printed document is converted with the created temporary profile. An element 401 represented by a black circle represents the coordinates in the L * a * b * space of each element constituting the temporary profile. The colorimetric values are the same values for the silver halide photograph gray and the printed document gray scanned with the scanner 202, and the device RGB obtained by scanning is converted to L ^* a ^* b ^* when converted with a temporary profile. It is represented as silver salt gray 403 and printed original gray 404. The silver salt gray 403 and the printed original gray 404 are scanned as different devices RGB by the scanner metamerism. By becoming different device RGB, the values converted by the scanner profile are also different as shown in the figure. To create a profile so that the grays of these two document types match the colorimetric values, there is a method of eliminating gradation. However, an image having no gradation is generally not preferable. Next, a method for adjusting a profile that can be converted into a preferable gray while maintaining the gradation will be described.

Each element of the gray region profile is moved in the direction of the origin of the a ^* b ^* plane as shown in FIG. The specific method is a method of multiplying the saturation value of the profile element by a coefficient. Hereinafter, a description will be given with reference to the flowchart of FIG. In step S101, the coefficient k is applied to the saturation value of the gray region profile element. For example, if the saturation value of the pre-adjustment element is 5, multiply by 0.8 to obtain a saturation value of 4. The device RGB of the gray color chart is converted with the profile adjusted in step S102. In step S103, the converted value and the colorimetric value of the color chart are compared. If the converted value is smaller than the colorimetric value (Y in step S103), the profile adjustment is terminated. If the converted value is larger than the colorimetric value (Y in step S103), the coefficient value k is decreased in step S104. For example, k = 0.6. Hereinafter, the profile adjustment is performed by repeating from step S101.

The figure showing the spectral characteristics of a D50 light source and a cold cathode tube lamp. The figure which showed the spectral sensitivity characteristic of XYZ and a scanner sensor. The figure showing conversion by profile. The figure showing obtaining the color chart scanner device RGB. The figure showing the colorimetric value of a color chart. The figure which showed the element of the scanner profile in L ^* a ^* b ^* space. The figure which extracted the gray area | region of FIG. The figure showing the adjustment method of a scanner profile. flowchart.

Claims (4)

Look up a lookup table that defines the color conversion from the first color space to the second color space,
Means having a first color space value of a predetermined color chart;
Means having a second color space value of the predetermined color chart;
Adjusting means for adjusting elements of the lookup table so as to output a value smaller than the saturation of the second color space value of the predetermined color chart;
A color conversion look-up table creating apparatus comprising: In the adjusting means,
2. The color conversion lookup table creation device according to claim 1, wherein the adjustment range is gray. Look up a lookup table that defines the color conversion from the first color space to the second color space,
A step having a first color space value of a predetermined color chart;
Having a second color space value of the predetermined color chart;
An adjustment step of adjusting the elements of the lookup table to output a value smaller than the saturation of the second color space value of the predetermined color chart;
A color conversion lookup table creation method characterized by comprising: In the adjustment step,
4. The color conversion lookup table creation method according to claim 3, wherein the adjustment range is gray.

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