JP2005295153A - Color conversion device and its method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable a mapping source to generate a mapping control parameter that controls gamut mapping of expanded color space that is different from sRGB, if a target color is provided with the sRGB color space as a mapping source. <P>SOLUTION: When the color space of a mapping source (for example, AdobeRGB) is designated and a target color data describing correspondence between the sample point of the sRGB color space and L*a*b* value after mapping is inputted (S401 and S402), the target color data representing correspondence between the grid point of color space of the mapping source and L*a*b* value after gamut mapping is generated (S403). Color gamut mapping process is performed using an initial mapping control parameter (S405). The evaluation value of the mapping control parameter is calculated based on the mapping result (S406). The calculated evaluation value is determined to be equal to or less than the preset threshold value or not (S407). If the evaluation value exceeds the threshold value, the mapping control parameter is so updated that the evaluation values becomes lower (S408). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a color conversion apparatus and method, and more particularly to color conversion of a color reproduction editing apparatus for adjusting and controlling color reproduction of an image.

  In recent years, with the spread of personal computers and workstations, desktop publishing (DTP) and computer aided design (CAD) have become widely used. Under such circumstances, color reproduction technology for reproducing colors expressed on a monitor by a computer using a color material has become important. For example, in DTP, a color image is created / edited / processed on a color monitor, and an image is output by a color printer. At this time, it is very important that the color image on the monitor and the output image of the printer coincide with each other. However, it is difficult to achieve perceptual matching between the color image and the output image of the printer for the following reason.

  The color monitor expresses a color image by emitting light of a specific wavelength using a phosphor. On the other hand, a color printer absorbs light of a specific wavelength by using a color material such as ink, and expresses a color image by the remaining reflected light. Thus, due to the different image display forms, the color gamuts of both are greatly different. Therefore, in order to achieve perceptual matching of display color images between display media with different color gamuts, various gamut mapping technologies that correspond one color gamut to another in the uniform color system. Exists.

  As an example, there is a technique as described in Japanese Patent Laid-Open No. 2002-033931 that attempts to perceptually match an output image by a printer without impairing the gradation of the input image. Further, the applicant has proposed a technique in which the user performs color adjustment and evaluation / analysis in a uniform color system while maintaining the gradation of the input image. If these techniques are used in combination, gamut mapping can be controlled and preferable color reproduction can be realized relatively easily. However, according to these techniques, when performing gamut mapping from the monitor color space to the printer color space, the user needs to manually adjust a number of color conversion parameters that need to be controlled. In addition, since the optimum combination of color conversion parameters depends on the model of the monitor or printer, determination of the optimum color conversion parameter for controlling the gamut mapping requires a great time cost. Furthermore, the optimum color conversion parameters are left to the user's subjective judgment, and it is difficult to achieve a uniform image quality.

  Therefore, the applicant has proposed a device for automatically generating color conversion parameters by giving a target color to be output or a look-up table (hereinafter referred to as “LUT”) when converting colors by gamut mapping. . With this technique, color conversion having various characteristics can be automatically set, for example, simply by giving colorimetric data, and color design efficiency can be improved.

  However, even in the above proposal, when a target color after mapping is provided with an RGB color space based on the sRGB standard as a mapping source, for example, the RGB color based on a standard different from sRGB such as AdobeRGB It is impossible to generate a color conversion parameter as a space. For this reason, even if a large amount of time is used to determine a color conversion parameter that controls gamut mapping from the sRGB color space, the color is converted to AdobeRGB or scRGB, which is called an extended color space, which has a wider color gamut than sRGB. It is difficult to apply conversion parameters.

JP 2002-033931 JP

  The present invention solves the above-mentioned problems individually or collectively, and generates mapping control parameters even when a sample point in a color gamut narrower than the color gamut of the mapping source and a target color after mapping are given. The purpose is to do.

  The present invention has the following configuration as one means for achieving the above object.

  According to the present invention, the first and second color reproduction are performed when mapping the color in the first color gamut to the color in the second color gamut different from the first color gamut based on the control parameter. Information indicating multiple sample points in the third color reproduction range different from the range and the color after mapping corresponding to the sample points are input as target color data, and evaluation is performed based on the mapped multiple colors and target color data A value is calculated, and a control parameter is set based on the target color data and the evaluation value.

  According to the present invention, a mapping control parameter can be generated even when a sample point in a color gamut narrower than the color gamut of the mapping source and a target color after mapping are given.

  Hereinafter, an image processing apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.

[Overview]
In the present embodiment, in color conversion by gamut mapping, when target color data to be output using a color space such as sRGB as a mapping source is given, a color space (for example, AdobeRGB or scRGB) different from the color space (for example, sRGB) of the target color data Parameter for controlling mat mapping is generated automatically. At that time, the parameters in the sRGB color gamut are automatically calculated so as to approximate the target color data. This makes it possible to reproduce gamut mapping from an extended color space simply by giving colorimetric data, for example, and it is possible to improve the efficiency of color design and to achieve a uniform color that does not depend on the user's subjective evaluation. Reproducibility can be realized. In general, gamut mapping is devised to suppress the occurrence of pseudo contours as much as possible, so that the occurrence of an image failure can be suppressed as a matter of course. Furthermore, by automatically generating mapping control parameters (or color adjustment parameters, color conversion parameters) that control not only the color conversion LUT but also gamut mapping, for example, in combination with the proposal, based on the generated mapping control parameters Further fine adjustment can be performed.

[Constitution]
FIG. 1 is a block diagram illustrating a configuration example of an image processing apparatus according to an embodiment. A color monitor 10 that displays an image and a printer 20 that prints an image on a recording medium are connected to the image processing apparatus 21.

  The image processing device 21 includes a video signal generation unit 11 that converts image data into a video signal, a memory 12 for storing image data, a color matching processing unit 13 that matches a monitor display color and a print color, and input of target color data Unit 14, a specifying unit 15 for specifying a color space of a mapping source in gamut mapping, a parameter for generating mapping control parameters used for a color gamut mapping process to be described later based on information of the input and specified target color data and mapping source color space Generation unit 16, color gamut mapping unit 17 that maps the color gamut of the monitor 10 to the color gamut of the printer 20, a color correction table 18 that stores the correspondence between the monitor display color and the print color, and the image data to the printer An output image processing unit 19 for converting to a drive signal is provided.

  In the embodiment, the image data to be processed is data digitized by an image input device such as a digital camera or a scanner, or data generated as computer graphics (CG), and is stored in the image memory as a pixel value corresponding to brightness. 12 is stored in advance. Specifically, each pixel of the image data has a red (R), green (G), and blue (B) 8-bit value, and a total of 24-bit value.

  The color correction table 18 is a table for performing color correction processing on the input RGB values in consideration of the output characteristics of the printer 20, and in the embodiment, color coordinate data of grid points regularly arranged in the AdobeRGB color space, and color The correspondence with the color coordinate data after the correction processing is stored. FIG. 2 is a diagram showing an example of data stored in the color correction table 18. At the head of the table, R / G / B value steps are shown, and subsequently color correction data is stored.

  Further, in the present embodiment, the target color data 14 is data representing L * a * b * values to be taken after gamut mapping at each of the regularly arranged grid points in the sRGB color space.

  In this embodiment, the color monitor 10 is a display device such as a CRT or LCD having a color gamut equivalent to AdobeRGB. The printer 20 is based on an ink jet method, and discharges and fixes cyan (C), magenta (M), yellow (Y), and black (K) ink droplets on the output recording paper, and the color depending on the density. It shall express shades. Note that the color monitor 10 and the printer 20 are not limited to such a form. For example, the printer 20 may be based on other methods such as an electrophotographic method or a thermal transfer method.

[Operation]
The image data stored in the image memory 12 of the image processing device 21 is supplied to the color matching processing unit 13. The color matching processing unit 13 performs color matching between an image displayed on the color monitor 10 via the video signal generation unit 11 and an image printed by the printer 20 via the output image processing unit 19. Specifically, image data to be supplied to the output image processing unit 19 is obtained by interpolating output values corresponding to the pixel values of the image data with reference to the color correction table 18. The table of the color correction table 18 is obtained by mapping a plurality of points in the color gamut of the monitor 10 to the color gamut of the printer 20 by referring to the mapping control parameter generated by the parameter generator 16 by the gamut mapping unit 17. As a result.

  Upon receiving the color-corrected image data, the output image processing unit 19 supplies a signal for controlling the ejection of each CMYK ink to the input RGB pixel value to the printer 20, and the desired color is reproduced on the recording medium. The In the case of the printer 20 using toner as a color material as in the electrophotographic method, the output image processing unit 19 supplies the printer 20 with a signal for controlling the photosensitive of the CMYK photosensitive drum with respect to the input RGB pixel value. .

[Mapping control parameter generation]
FIG. 3 is a block diagram illustrating a configuration example of the parameter generation unit 16.

  Based on the mapping result from the monitor color reproduction gamut to the printer color reproduction gamut in the color gamut mapping unit 17, the parameter generation unit 16 optimizes the value of the mapping control parameter, and sets the given mapping control parameter. An evaluation unit 31 that calculates a corresponding evaluation value, a parameter storage unit 32 that stores an optimal mapping control parameter determined based on the evaluation value, and an input from the input unit 14 based on the mapping source color space designated by the designation unit 15 A target color data conversion unit 33 for converting the target color data to be converted, and a target color data storage unit 34 for storing the converted target color data.

  FIG. 4 is a diagram showing a user interface (UI) for designating the mapping source color space of the designation unit 15. The user selects one of the RGB color spaces by using a radio button on the UI. Hereinafter, as shown in FIG. 4, the processing when AdobeRGB is selected as the mapping source color space will be described. However, when other RGB color spaces are selected, or other color spaces are used using the UI. The same is true when is specified.

  FIG. 5 is a diagram showing a format example of target color data. As target color data, R / G / B value steps and L * a * b * values corresponding to each combination of RGB values are described. Hereinafter, a case where target data indicating the sRGB color space is input as target color data will be described.

  FIG. 6 is a flowchart showing parameter generation processing in the parameter generation unit 16.

  An RGB color space (mapping source color space) to be a gamut mapping source is specified by the specifying unit 15 (S401). As described above, it is assumed that AdobeRGB is specified here. Target color data (hereinafter referred to as “sRGB target color data”) that describes the correspondence between the sRGB color space sample points in the input unit 14 and the L * a * b * values after gamut mapping corresponding to the sample points. When a path indicating the storage location is input (S402), the sRGB target data is input, and the correspondence between the grid point in the AdobeRGB color space and the L * a * b * value after gamut mapping is represented by the method described later. AdobeRGB target color data is generated and stored in the target color data storage unit 34 (S403).

  Next, initial mapping control parameters used for the color gamut mapping process described later are generated (S404), and the color gamut mapping unit 17 uses the mapping control parameters to change the color gamut from the monitor color gamut to the printer color gamut. Mapping processing is performed (S405). Details of this color gamut mapping process will also be described later.

  Next, the evaluation unit 31 calculates an evaluation value of the mapping control parameter based on the mapping result (S406), and determines whether the calculated evaluation value is equal to or less than a predetermined threshold value (S407). If the evaluation value exceeds the threshold value, for example, using a known optimization method such as a quasi-Newton method, the mapping control parameter is updated so that the evaluation value becomes smaller (S408), and the processing from step S405 to S407 is repeated. .

  On the other hand, if the evaluation value is equal to or less than the threshold value, the mapping control parameter is stored in the parameter storage unit 32 (S409), and it is determined whether all the mapping control parameters necessary for the color gamut mapping processing have been generated (S410). If there is an ungenerated mapping control parameter, the processes in steps S404 to S409 are repeated until all mapping control parameters necessary for the color gamut mapping process are generated.

Generation of AdobeRGB Target Color Data FIG. 7 is a flowchart showing AdobeRGB target color data generation processing (S403) performed by the target color data conversion unit 33.

  First, one RGB value of grid points evenly arranged in the AdobeRGB color space is converted to an L * a * b * value by calculation (S701), and the converted L * a * b * value (L * a *) It is determined whether or not the grid point (b * value) is within the color gamut of the sRGB color space (hereinafter referred to as “sRGB color gamut”) (S702), and if it is out of the sRGB color gamut, it is out of the sRGB color gamut. The saturation is compressed so that the color (L * a * b * value) is located on the boundary of the sRGB color gamut (S703).

  FIG. 8 is a schematic diagram for explaining saturation compression. Reference numeral 801 denotes an equal hue boundary in the AdobeRGB color space in the L * a * b * color space, reference numeral 802 denotes an equal hue boundary in the sRGB color space, and reference numeral 803 denotes a saturation. Reference numeral 804 denotes a lattice point after saturation compression, and a lattice point before saturation compression. As shown in FIG. 8, saturation compression (S703) compresses colors outside the sRGB color gamut to the boundary surface of the sRGB color gamut having the same hue and lightness. Note that the L * a * b * value before saturation compression and the L * a * b * value indicated by reference numeral 805 on the boundary surface of the color gamut of the AdobeRGB color space (hereinafter referred to as “AdobeRGB color gamut”) are: Save for later processing.

  Subsequently, the L * a * b * value is converted into an sRGB value by calculation (S704), the converted sRGB value is converted into a target L * a * b * value using the sRGB target color data (S705), Based on the relationship between the color before saturation compression and the color after compression, the saturation of the target L * a * b * value is expanded (S706). FIG. 9 is a schematic diagram for explaining saturation expansion. Reference numeral 901 denotes a color gamut boundary indicated by the sRGB target color data, reference numeral 902 denotes a color reproduction area of the printer 20, reference numeral 903 denotes a target color before saturation extension, Reference numeral 904 represents the target color after saturation expansion two-dimensionally. Here, the saturation expansion amount is determined according to the saturation compression amount and the distance between the color 903 before saturation expansion and the boundary between the color reproduction area 902 of the printer 20.

  Also, if the converted L * a * b * value is within the sRGB color gamut, it is converted from the L * a * b * value to the sRGB value by calculation (S707) as in step S704, and as in step S705. The converted sRGB values are converted into target L * a * b * values using the sRGB target color data (S708).

  Then, the correspondence between the grid points of the AdobeRGB color space and the target L * a * b * values obtained by the above processing is stored in the target color data storage unit 34 (S709), and the conversion processing of all grid points is completed. (S710), if not completed, the processing from step S701 to S710 is repeated until the processing of all grid points is completed, that is, until the generation processing of AdobeRGB target color data is completed.

Color Gamut Mapping FIG. 10 is a flowchart showing the color gamut mapping processing (S405) performed by the color gamut mapping unit 17.

  First, the color gamut information of the color monitor 10 having the AdobeRGB color gamut and the color gamut information of the printer 20 are set by specifying the path by the input unit 14 (S1001). Based on the information, a boundary of the mapped color gamut that becomes the final mapping result of the boundary of the color gamut of the color monitor 10 is generated (S1002). The boundary of the mapped color reproduction area is, for example, a boundary line denoted by reference numeral 1302 in FIG.

  Next, the brightness and hue of the color gamut of the monitor 10 are mapped in the uniform color system (S1003). Details of the lightness / hue mapping operation in this embodiment will be described later, but will be briefly described with reference to a schematic diagram illustrating the lightness / hue mapping operation of FIG.

  In FIG. 11, reference numeral 1101 indicates a monitor color gamut in the green hue, reference numeral 1102 indicates a first intermediate mapping color gamut, and reference numeral 1103 indicates a printer color gamut in the same hue. The color gamut mapping unit 17 separates the lightness component and the chromaticity component for colors in the monitor color reproduction region 1101 and maps the lightness component in a non-linear manner. In addition, the hue is adjusted appropriately for the chromaticity component. As a result, the monitor color reproduction area 1101 is mapped to the first intermediate mapping color reproduction area 1102. From this mapping result, the boundary of the first intermediate mapped color reproduction region is generated (S1004).

  Subsequently, the brightness is adjusted with respect to the boundary of the first intermediate mapping color reproduction region 1102, and the boundary of the second intermediate mapping color reproduction region is generated (S1005). Reference numeral 1202 in the schematic diagram for explaining the lightness / hue mapping operation in FIG. 12 indicates the boundary of this color gamut.

  Next, the information of the first intermediate mapping color reproduction area (indicated by reference numeral 1201 in FIG. 12) is mapped to the second intermediate mapping color reproduction area (indicated by reference numeral 1202 in FIG. 12) (S1006). The details of the mapping process will be described later. To explain briefly with reference to the schematic diagram of FIG. 12, the color gamut mapping unit 17 has a lightness component and a chromaticity component for the colors in the first intermediate mapped color reproduction region 1201. Are separated, and only the lightness component is non-linearly mapped while keeping the chromaticity component constant (hereinafter referred to as “lightness adjustment mapping”). The lightness input / output function for realizing the mapping differs depending on the chromaticity. By this brightness adjustment mapping, the first intermediate mapping color reproduction region 1201 is mapped to the second intermediate mapping color reproduction region 1202. Note that reference numeral 1203 in FIG. 12 represents the color gamut of the printer 20 in the hue of green.

  Next, referring to the information on the intermediate mapping color reproduction range and the mapping color reproduction range, the second intermediate mapping color reproduction range 1202 is chromatographed to the mapping color reproduction range (indicated by reference numeral 1302 in FIG. 13). (S1007). Although details of the mapping process will be described later, the color gamut mapping unit 17 converts the colors in the second intermediate mapping color reproduction gamut 1301 into a simple explanation using the schematic diagram explaining the lightness / hue mapping operation of FIG. On the other hand, the lightness component and the chromaticity component are separated, and the saturation component in the chromaticity component is non-linearly mapped while keeping the lightness component constant (hereinafter referred to as “saturation adjustment mapping”). By this saturation adjustment mapping, the second intermediate mapped color reproduction region 1301 is mapped to the mapped color reproduction region 1302, and the color gamut mapping shown in step S405 of FIG. 6 is completed. Note that reference numeral 1203 in FIG. 13 represents the color reproduction range of the printer 20 in the green hue.

● Lightness and Hue Mapping Next, lightness and hue mapping processing (S1003) will be described.

In this embodiment, the mapping of the brightness component is calculated by the following equation using one input / output function that does not depend on chromaticity.
L _mapped = λ (Lin)… (1)

The mapping function λ (·) of Expression (1) is calculated as follows based on the entire brightness control parameter x (one of the mapping control parameters) generated by the parameter generation unit 16.
x ≧ 50: λ (•) = {λ ₅₀ (•) × (100-x) + λ ₁₀₀ (•) × (x-50)} / 50
x <50: λ (•) = {(λ ₀ (•) × (50-x) + λ ₅₀ (•) × x} / 50
Here, λ ₀ (·), λ ₅₀ (·), and λ ₁₀₀ (·) are predetermined functions,
λ ₀ (・) is the mapping function when x = 0,
λ ₅₀ (・) is the mapping function when x = 50,
λ ₁₀₀ (・) is the mapping function when x = 100

  FIG. 14 is a flowchart showing mapping of hue components.

First, based on the hue control parameter (one of the mapping control parameters) generated by the parameter generation unit 16, the hue input / output functions h ₀ (·), h are applied to six brightness values from brightness 0 to 100 in 20 steps. ₂₀ (•), h ₄₀ (•), h ₆₀ (•), h ₈₀ (•) and h ₁₀₀ (•) are generated (S1401). Hue control parameters are red (R), green (G), blue (B), cyan (C), magenta (M), and yellow (Y) hue adjustment amounts corresponding to each of the six brightness values. It is. Here, the hue input / output function is calculated as a function obtained by connecting hue adjustment values in the R / G / B / C / M / Y hue for each lightness with a primary spline, that is, a straight line. For example, when the hue angle is slightly shifted to plus in the two hues R and Y with respect to the lightness value 80, and shifted to minus in the hue of B, the hue input / output function h ₈₀ shown in FIG. (・) Is obtained. In FIG. 15, the hue angle is expressed in radians notation with the b * axis positive direction being the hue angle 0 rad and the counterclockwise rotation being positive in the a * b * chromaticity coordinate system.

Next, the color M to be mapped is acquired (S1402), and the hue input / output function h (•) for the color M is set to h ₀ (•), h ₂₀ (•), h ₄₀ (•), h _{Using 60} (•), h ₈₀ (•) and h ₁₀₀ (•), the calculation is performed as follows (S1403).
h (・) = {h _LW (・) × (up-x) + h _UP (・) × (x-lw)} / 20… (2)
Where x: lightness of color M,
h _UP (•): Input / output function of brightness UP just above brightness x,
h _LW (•): Input / output function of lightness LW directly below lightness x

For example, when the lightness x of the color M is 70, h ₆₀ (•) is used as the lightness input / output function immediately below, and h ₈₀ (•) is used as the lightness input / output function immediately above. Therefore, the hue input / output function h (•) for the color M with the lightness x = 70 is as follows.
h (・) = {h ₆₀ (・) + h ₈₀ (・)} / 2… (3)

Next, hue mapping is performed using equation (4) (S1404).
Hue _M mapped = h (Hue _M )… (4)
Where Hue _M is the hue of color M,
Hue _M mapped is the hue after mapping

  Then, the brightness component is mapped using the brightness control parameter described above (S1405), and then it is determined whether or not the above brightness / hue mapping processing has been performed for all grid points of the LUT (S1406). Steps S1402 to S1406 are repeated until the mapping process is completed for the grid points.

  Through the above processing, the monitor color reproduction region 1101 shown in FIG. 11 is mapped to the first intermediate mapping color reproduction region 1102 shown in FIG.

Brightness Adjustment Map FIG. 16 is a flowchart for explaining the brightness adjustment map (S1006).

  First, the color M to be mapped is designated (S1601). The color M is a color in the first intermediate mapping color reproduction region 1102 (or 1201) and does not represent a color in the monitor color reproduction region 1101.

Next, the upper boundary B _U and the lower boundary B _L of the first intermediate mapped color reproduction region 1201 in the same hue as the color M are calculated (S1602), and the second intermediate mapped color reproduction in the same hue as the color M is calculated. The upper boundary B _U mapped and the lower boundary B _L mapped of the area 1202 are calculated (S1603).

FIG. 17 shows the relationship between the color M, the upper boundary B _U and the lower boundary B _L of the first intermediate mapping color reproduction area, and the upper boundary B _U mapped and the lower boundary B _L mapped of the second intermediate mapping color reproduction area. FIG. In FIG. 17, the color reproduction range indicated by the solid line is the boundary of the first intermediate mapping color reproduction range, and the color reproduction range indicated by the alternate long and short dash line is the boundary of the second intermediate mapping color reproduction range, the color indicated by the dotted line The reproduction area is the color reproduction area of the printer 20.

Subsequently, the input / output function p (•) for performing the brightness adjustment mapping is derived from the value obtained above and the mapping control parameter generated by the parameter generation unit 16 (S1604). In this embodiment, the input / output function p (•) is calculated as a C2 continuous cubic spline function that satisfies the following conditions.
・ The base of p (・) is [LB _L , LB _U ]
・ P (・) monotonically increases on the stand ・ p (LB _L ) = LB _Lm
・ P (LB _U ) = LB _Um
・ P (・) is at least C1 continuous ・ p '(LB _L ) = g0
・ P '(LB _U ) = g1
Where LB _L is the brightness of B _L
LB _Lm is the brightness of B _L mapped,
LB _U is the brightness of B _U ,
LB _Um is the brightness of B _U mapped,
g0> 0, g0 is a variable that controls compression,
g1> 0, g1 is a variable that controls compression

  The values of g0 and g1 are calculated by the parameter generation unit 16 so that the evaluation value calculated by the evaluation unit 31 is minimized. The input / output function p (•) is calculated so as to satisfy the above conditions.

18A and 18B are diagrams showing examples of the input / output function p (•) in the present embodiment. In FIG. 18A, the lightness is almost maintained in the middle part of the table, while it is largely compressed near the low lightness and near the high lightness. Incidentally, LB _L = 40, LB _Lm = 45, LB _U = 68, and LB _Um = 64.

Further, in FIG. 18B, since the change in brightness is large, the function of saving the brightness works in the middle part of the table, but it is not completely saved. In the vicinity of high lightness, it is greatly expanded, and in the vicinity of low lightness, it is greatly compressed. Incidentally, LB _L = 60, LB _Lm = 46, LB _U = 84, and LB _Um = 75.

Subsequently, by using the input / output function p (•) obtained in step S1604, the lightness L _M mapped after the mapping of the lightness L _M of the color M is expressed as L _M mapped = p (L _M ) Then, the brightness adjustment mapping is performed (S1605).

  Then, it is determined whether or not the brightness adjustment mapping has been performed for all grid points of the color correction table 18 (S1606), and steps S1602 to S1606 are repeated until the brightness adjustment mapping is completed for all grid points.

  With the above processing, the first intermediate mapped color reproduction range 1201 shown in FIG. 12 is mapped to the second intermediate mapped color reproduction range 1202 shown in FIG.

Saturation Adjustment Map FIG. 19 is a flowchart for explaining the saturation adjustment map (S1007).

  First, the color M to be mapped is determined (S1901). The color M is a color in the second intermediate mapping color reproduction range 1202.

  Next, the boundary Bp of the mapped color reproduction area in the same brightness and the same hue as the color M is calculated (S1902), and the boundary Bi of the second intermediate mapped color reproduction area in the same brightness and the same hue as the color M is calculated. Is calculated (S1903).

  FIG. 20 is a diagram schematically showing the relationship among the color M, the color Bp, and the color Bi. In FIG. 20, the color gamut indicated by the solid line is the boundary of the second intermediate mapped color gamut, the color gamut indicated by the one-dot chain line is the boundary of the mapped color gamut, and the color gamut indicated by the dotted line is the printer color gamut.

Subsequently, a C2 continuous cubic spline function q (•) for performing saturation adjustment mapping is calculated from the color Bp and color Bi calculated in the above steps and the mapping control parameter generated in the parameter generation unit 16 (S1904). . The input / output function q (•) satisfies the following condition in this embodiment.
・ The base of q (・) is [0, ci]
Q (0) = 0
Q (ci) = cp
Q '(0) = g _LOW
Q '(ci) = g _HIGH
Q '(x) ≠ 0 (0 ≤ x ≤ ci)
Where cp is the saturation of the color Bp,
ci is the saturation of the color Bi,
g _LOW is a value that controls the expansion / compression ratio of saturation from _low to medium saturation.
g _HIGH is a value that controls the expansion / compression rate of saturation correction near the maximum saturation.

The values of g _LOW and g _HIGH are derived by the parameter generation unit 16 so that the evaluation value calculated by the evaluation unit 31 is minimized.

21A and 21B are diagrams illustrating an example of the input / output function q (•) in the present embodiment. In FIG. 21A, while the expansion is performed in the high saturation part, the value of g _LOW is 0.8, so that the function is performed to suppress the saturation in the low saturation to the intermediate saturation part. In FIG. 21B, compression is performed in the high saturation portion, and since the value of g _LOW is 1.2, the saturation is enhanced in the low saturation to medium saturation portions.

  Subsequently, the saturation of the color M is converted using the input / output function q (•) obtained in step S1904 (S1905). Here, if the saturation of the color C is expressed as Corg and the converted saturation is expressed as Cmod, Cmod = q (Corg).

  Finally, it is determined whether or not the saturation adjustment mapping has been performed for all grid points in the color correction table 18 (S1906), and steps S1901 to S1906 are repeated until the saturation adjustment mapping is completed for all grid points.

  Through the above processing, the second intermediate color reproduction region 1301 shown in FIG. 13 is mapped to the mapping color reproduction region 1302 shown in FIG.

[Calculation of evaluation value]
FIG. 22 is a flowchart for explaining the calculation of the evaluation value of the mapping result (S406).

  First, for all grid points of the color correction table 18, the absolute value of the brightness difference between the result mapped by the gamut mapping unit 17 and the converted target color data stored in the target data storage unit 34 | ΔL The average value is calculated (S2201), and for one hue of RGBCMY, the difference between the mapping result of multiple grid points located on the hue plane and the corresponding target color data after conversion is calculated. An average value ΔE is obtained (S2202). Thereafter, it is determined whether or not the calculation of the average value ΔE for the six hues of RGBCMY is completed (S2203), and step S2202 is repeated until the average value ΔE is calculated for the six hues.

When the calculation of the average value ΔE of the six hues is completed, the average value ΔEave of ΔE for the calculated six hues is calculated (S2204), and the final evaluation value eval is calculated by the equation (5) (S2205).
eval = α × | ΔL | + β × ΔEave
Where α and β are weighting factors, α ≧ 0, β ≦ 1

  With the above processing, the evaluation value calculation processing for a set of mapping control parameters is completed.

  As described above, according to this embodiment, the mapping control parameter for controlling the gamut mapping using the extended color space such as AdobeRGB as the mapping source only by giving the target color data after the gamut mapping using sRGB or the like as the mapping source. It can be automatically generated.

[Modification]
In the above, saturation compression is performed for colors outside the sRGB color gamut, mapping to colors within the printer color gamut using the sRGB target data, and then decompressing the saturation to generate AdobeRGB target data. For colors outside the color gamut, other processes may be performed. For example, in the L * a * b * color system, the sRGB color gamut surface may be compressed to the closest point.

  In the above, the evaluation value of the mapping result is calculated using ΔL and ΔE with the target color data, but other evaluation methods may be used.

  In the above description, the brightness difference from the target color in all the color regions and the color difference ΔE in the main six hues are calculated. However, different color regions may be evaluated. For example, in order to place importance on the low saturation portion such as skin color, a method of evaluating the difference from the target color in the low saturation to the intermediate saturation portion may be used.

[Other embodiments]
Note that the present invention can be applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, and a printer), and a device (for example, a copying machine and a facsimile device) including a single device. You may apply to.

  Also, an object of the present invention is to supply a storage medium (or recording medium) on which a program code of software that realizes the functions of the above-described embodiments is recorded to a system or apparatus, and the computer (or CPU or CPU) of the system or apparatus. Needless to say, this can also be achieved by the MPU) reading and executing the program code stored in the storage medium. In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention. Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an operating system (OS) running on the computer based on the instruction of the program code. It goes without saying that a case where the function of the above-described embodiment is realized by performing part or all of the actual processing and the processing is included.

  Furthermore, after the program code read from the storage medium is written into a memory provided in a function expansion card inserted into the computer or a function expansion unit connected to the computer, the function is determined based on the instruction of the program code. Needless to say, the CPU of the expansion card or the function expansion unit performs part or all of the actual processing and the functions of the above-described embodiments are realized by the processing.

  When the present invention is applied to the storage medium, the storage medium stores program codes corresponding to the flowcharts described above.

Block diagram showing a configuration example of an image processing device, The figure which shows an example of the data stored in a color correction table, A block diagram showing a configuration example of a parameter generation unit, The figure which shows the user interface (UI) which designates the mapping original color space of the designated part, The figure which shows the format example of target color data, A flowchart showing parameter generation processing in the parameter generation unit; A flowchart showing processing for generating AdobeRGB target color data performed by the target color data conversion unit, Schematic diagram explaining saturation compression, Schematic diagram explaining saturation expansion, A flowchart showing color gamut mapping processing performed by the color gamut mapping unit; Schematic diagram explaining the lightness / hue mapping operation, Schematic diagram explaining the lightness / hue mapping operation, Schematic diagram explaining the lightness / hue mapping operation, A flowchart showing the mapping of the hue component; Figure showing an example of hue input / output function A flowchart for explaining the brightness adjustment map; The figure showing the relationship between the upper boundary B _U and the lower boundary B _{L of} the color M, the first intermediate mapping color reproduction area, and the upper boundary B _U mapped and the lower boundary B _L mapped of the second intermediate mapping color reproduction area, A diagram showing an example of the input / output function p ( A diagram showing an example of the input / output function p ( A flowchart for explaining the saturation adjustment map; A diagram schematically showing the relationship between color M, color Bp, and color Bi, A diagram showing an example of the input / output function q ( A diagram showing an example of the input / output function q ( It is a flowchart explaining calculation of the evaluation value of a mapping result.

Claims (11)

A color conversion method for mapping a color in a first color gamut to a color in a second color gamut different from the first color gamut based on a control parameter,
A plurality of sample points in a third color gamut different from the first and second color gamuts, and input information indicating the color after mapping corresponding to the sample points as target color data,
An evaluation value is calculated based on the mapped multiple colors and the target color data,
A color conversion method, wherein the control parameter is set based on the target color data and the evaluation value.   2. The color conversion method according to claim 1, wherein the third color gamut is defined by the sRGB standard.   3. The color conversion method according to claim 1, wherein the first color reproduction range is designated as AdobeRGB or scRGB.   2. The evaluation according to claim 1, wherein the evaluation is performed by converting the target color data into target color data of the second color gamut for the first color gamut, and then calculating the evaluation value. Item 4. The color conversion method described in any one of Items 3 above.   The target color conversion is performed by generating a plurality of sample points in the first color gamut and interpolating the converted target color data if the generated sample points are in the third color gamut. The obtained color is determined as a color corresponding to the generated sample point, and if the generated sample point is outside the third color reproduction range, the generated sample point is converted into the third color reproduction range. The color corresponding to the generated sample point is a color obtained by interpolating the color data after conversion based on the converted sample point and changing at least one of the saturation, hue, and brightness of the color. 5. The color conversion method according to claim 4, wherein:   5. The color conversion method according to claim 4, wherein in the evaluation, the evaluation value is calculated based on a distance between the plurality of colors after the mapping and the target color data after the conversion.   7. The color conversion method according to claim 1, wherein the control parameter is a value for controlling mapping conversion of at least one element among brightness, saturation, and hue.   8. The setting according to claim 1, wherein the setting is performed by selecting a parameter that minimizes the evaluation value indicating a difference in distance between the plurality of colors after the mapping and the target color data. Color conversion method.   9. A program that controls an information processing device to realize the color conversion according to claim 1.   10. A recording medium on which the program according to claim 9 is recorded. Mapping means capable of controlling the mapping by a control parameter for mapping a color in the first color gamut to a color in a second color gamut different from the first color gamut;
A plurality of sample points in a third color gamut different from the first and second color gamuts, and input means for inputting information indicating the color after mapping corresponding to the sample points as target color data; ,
Evaluation means for calculating an evaluation value based on the plurality of colors mapped by the mapping means and the target color data;
A color conversion apparatus comprising: setting means for setting the control parameter based on the target color data and the evaluation value.

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