JP4996320B2 - Color processing apparatus and method - Google Patents

Description

  The present invention relates to color processing that performs color gamut mapping.

  Opportunities for printing images acquired with input devices such as digital cameras and scanners have increased. The input device once acquires an image as data in an RGB color space (hereinafter referred to as device RGB color space) depending on the device, and the device RGB color space data is a standard color space such as sRGB or AdobeRGB. Convert to spatial data and save in memory.

  FIG. 1 is an xy chromaticity diagram showing the color reproduction range (color gamut) of the sRGB color space. A triangular area Rs indicated by a dotted line indicates an sRGB color gamut, a horseshoe-shaped area Rv indicated by a solid line indicates a human visible area, and an area Rp indicated by a broken line indicates a color gamut of a general printer. As shown in FIG. 1, the color gamut of the sRGB color space (hereinafter referred to as sRGB color gamut) and the printer color gamut are different in shape and area. In order to absorb this difference in color gamut, when an image in the sRGB color space (hereinafter referred to as sRGB image) is printed by a printer, it is necessary to perform color gamut mapping that associates the color value of the sRGB image with the color value of the printer.

  FIG. 2 is a flowchart showing general color gamut mapping. Hereinafter, an sRGB color space will be described as an example of the color space of the input image (hereinafter referred to as the input color space).

  First, the color value of the color perception space corresponding to the pixel value RGB of the sRGB image is calculated (S41). Examples of the color perception space include CIELAB, CIELUV, and CIECAM02. Hereinafter, CIELAB will be described as the color perception space, and Lab values will be described as color values.

  Next, conversion is performed on the Lab value so that the Lab value falls within the color gamut of an output device such as a printer (hereinafter, output color gamut) in the color perception space (S42). This conversion, in other words, the process of associating the color value of the input device (hereinafter referred to as input color value) with the color value of the output device (hereinafter referred to as output color value) is called color gamut mapping.

  Then, the mapped Lab value is converted into an RGB value in the device color space of the output device (hereinafter, device RGB value) (S43).

  When an image converted to the device RGB value of the output device is printed, the white color value in the input device may not be reproduced as the white color value (paper white) in the output device depending on the color gamut mapping method. In other words, the white of the input image may be reproduced in a printed matter with ink or toner on the recording paper. If the input image is colored white, it is perceived as a color cast, which makes the image look bad. In order to prevent this, generally, processing for reproducing white of an input image with paper white is performed during image processing.

  In order to reproduce the white of the input image with paper white, it is necessary to match the white of the input device and the output device. As a process for that purpose, Patent Document 1 describes a method of translating a gray line for each lightness in order to make the gray line connecting white and black of the device coincide with the lightness axis.

  FIG. 3 is a diagram showing changes in the output color gamut when the gray line is translated for each lightness. Point P indicates the intersection of the gray line before movement and the iso-lightness plane, and point P ′ indicates the intersection of the gray line after movement and the iso-lightness plane. A solid line Gout indicates an output color gamut on the iso-lightness plane before moving the gray line, and a broken line G′out indicates an output color gamut on the iso-lightness plane after moving the gray line. As described above, the output color gamut is translated by the same amount of movement as the point P as the point P moves.

  According to the movement of the gray line, the white of the input device, the white of the output device, and the focal point exist on the same line segment (the gray line after the movement). Therefore, if focus type color gamut mapping is performed to map the color outside the output color gamut toward the focus set in the output color gamut, the white of the input device is mapped toward the focus and matches the white of the output device. The white of the input device can be reproduced with white paper. The focal point (convergence point) is set to a point Lab = (50, 0, 0) on the gray line, for example.

  Patent Document 2 discloses a method of rotating a color gamut in order to match the gray line of the device with the brightness axis as a method of matching the white of the input / output device.

  FIG. 4 is a diagram for explaining a technique for matching the white of the input / output device by rotating the color gamut. The dotted line Gin is the color gamut of the input device in a certain hue, the solid line Gout is the color gamut of the output device in that hue, the broken line G'out is the output color gamut Gout, and the color gamut rotated so that the white of the input / output device matches. Show.

  According to the rotation of the color gamut, as in the technique of Patent Document 1, the white of the input device, the white of the output device, and the focal point exist on the same line segment (the rotated gray line). Therefore, if the color gamut mapping of the focus method is performed, the white of the input device is mapped toward the focus and matches the white of the output device, and the white of the input device can be reproduced with paper white or the like.

  However, the parallel movement of the gray line and the rotation of the color gamut cause the color value of the color perception space to be shifted due to the parallel movement and rotation, and therefore there is a problem that the reproducibility of colors other than white deteriorates when the color gamut mapping is performed.

  FIG. 5 is a diagram for explaining deterioration of color reproducibility due to parallel movement of gray lines. In FIG. 5, the points P, P0, P1 on the iso-lightness plane after translation are P ′, P′0, P′1. As shown in FIG. 5, when the gray line is translated, the achromatic color including white of the input device is reproduced by the achromatic color of the output device, and the reproducibility is improved. However, chromatic colors such as points P0 and P1 are mapped to chromaticity points P′0 and P′1 different from the original chromaticity by translation.

  FIG. 6 is a diagram for explaining deterioration of color reproducibility during rotation of the color gamut. In FIG. 6, points P0 and P1 on the equi-hue surface are points after rotation P′0 and P′1. As shown in FIG. 6, when the color gamut is rotated, the reproducibility of the achromatic color is improved, but the chromatic colors such as the points P0 and P1 have a lightness and saturation different from the original due to the rotation. It will be mapped to 0 or P'1.

Japanese Patent Laid-Open No. 2001-326826 JP 2002-016816 JP

  It is an object of the present invention to improve achromatic color reproducibility in color gamut mapping and prevent deterioration of chromatic color reproducibility.

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

The color processing according to the present invention corresponds to light source data of an observation light source when observing an image output by an image output device , and each lattice point obtained by dividing the range of input data of the image output device into a lattice shape. The color gamut data indicating the color value of the output color gamut of the image output device is input, and based on the white point indicated by the light source data and the white color indicated by the color gamut data, a reference white color for each grid point of the color gamut data And calculating a color value of a color perception space from a color value corresponding to the grid point using a reference white color for each grid point , and color gamut mapping based on the calculated color value of the color perception space It is characterized by performing.

  According to the present invention, it is possible to improve achromatic color reproducibility in color gamut mapping and to prevent deterioration of chromatic color reproducibility.

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

[Device configuration]
FIG. 7 is a block diagram illustrating a configuration example of the color processing apparatus 10 according to the embodiment.

  The input unit 101 inputs image data, data indicating an output color gamut (hereinafter referred to as output color gamut data), and data indicating an observation light source (hereinafter referred to as light source data) from the image input device 11, the storage device 12, or the like. The output unit 106 outputs image data indicating the color value of the device color space to the image output device 13 or the storage device 14. The input unit 101 and the output unit 106 include a serial bus interface such as USB or IEEE1394, and the color processing device 10 is connected to the image input device 11, the image output device 13, and the storage devices 12 and 14 via a serial bus cable. To do.

  The image data holding unit 107 holds the image data input by the input unit 101. The gamut data holding unit 108 holds the gamut data input by the input unit 101. The light source data holding unit 109 holds the light source data input by the input unit 101. The buffer memory 110 is a work memory that temporarily holds various data being processed. The holding units 107 to 109 and the buffer memory 110 are allocated to predetermined areas of a memory such as a RAM or a hard disk (HD).

  The reference white calculation unit 102, which will be described in detail later, is based on the output color gamut data held by the color gamut data holding unit 108 and the light source data held by the light source data holding unit 109, for each grid point of the device RGB value. Is calculated. The color perception space value calculation unit 103 uses the reference white color for each grid point and outputs the color value of the color perception space of each grid point of the device RGB value from the output color gamut data held by the color gamut data holding unit 108 (for example, (Lab value) is calculated.

  The color gamut mapping unit 104 performs color gamut mapping of the color value of the input image data held by the image data holding unit 107 to the output color gamut based on the Lab value of each grid point of the device RGB value. The device color value calculation unit 105 calculates a color value (for example, a device RGB value) of the device color space corresponding to the color value that has been color gamut mapped, and stores it in the image data holding unit 107. The device color value calculation unit 105 includes a lookup table (LUT) that represents the relationship between the device color value and the Lab value, an interpolation operation such as tetrahedral interpolation or cube interpolation, or a conversion matrix that converts the device color value to the Lab value. Is used to calculate the device color value.

  The UI unit 112 provides a user interface to the user through a combination of an operation panel (not shown), a monitor, a keyboard, and a mouse.

  Note that the above-described components are connected to each other via a system bus 111. Further, the input unit 101 and the output unit 106 may include a network interface or a memory card reader / writer. In this case, the color processing device 10 is connected to the image input device 11, the image output device 13, and the storage devices 12 and 14 via a network or a memory card.

[Color processing]
FIG. 8 is a flowchart for explaining the operation of the color processing apparatus 10.

  First, image data necessary for color processing, output color gamut data, and light source data, which are instructed by the UI unit 112 by the input unit 101, are input from the image input device 11 and the storage device 12, and the corresponding holding units 107 are respectively input. To 109 (S11). Specific processing of the input unit 101 will be described later.

  FIG. 9 is a diagram showing a format example of image data stored in the image data holding unit 107. As shown in FIG. The image data holding unit 107 stores pixel data for an image size in a memory in a bitmap format, each set of RGB color component values. For example, when JPEG image data (hereinafter referred to as JPEG data) is input from the image input device 11, the input unit 101 decodes the JPEG data and holds the image data in the bitmap format obtained by YCbCr → RGB conversion. Stored in the unit 107.

  FIG. 10 is a diagram illustrating a format example of output gamut data stored in the gamut data holding unit 108. The output color gamut data has a table format in which pairs of device RGB values and XYZ values are described. In the case of 8-bit data, the XYZ value at R = G = B = 255 corresponds to paper white. For example, printer output color gamut data is generally created as follows.

  First, the range of RGB values (0 to 255) is divided into 9 or 17 slices. Then, RGB values corresponding to the coordinates of each grid point are input to the printer, and a plurality of color charts corresponding to the RGB values are printed on a predetermined recording sheet. Next, each color chart is measured to obtain XYZ values. In this way, the correspondence between the device RGB value and the XYZ value is acquired, and output color gamut data shown in FIG. 10 is created. Accordingly, the paper white of the recording paper on which the color chart is printed is expressed as an XYZ value corresponding to R = G = B = 255. If the input output color gamut data is compressed, the input unit 101 decompresses the output color gamut data and stores the decompressed result table in the color gamut data holding unit 108.

  The light source data stored in the light source data holding unit 109 includes, for example, the XYZ value and intensity of the white point of ambient light including illumination light and ambient light in the image observation environment.

  Next, as described in detail later, the reference white calculation unit 102 calculates the reference white color based on the output color gamut data held by the color gamut data holding unit 108 and the light source data held by the light source data holding unit 109 (S12). . Then, the color perception space value calculation unit 103 calculates the Lab value of each pixel of the input image data held by the image data holding unit 107 using the reference white color (S13).

  Next, as will be described in detail later, the gamut mapping unit 104 refers to the output gamut data held by the gamut data holding unit 108, and gamut maps the Lab value of each pixel of the image data (S14). . Then, the device color value calculation unit 105 calculates a device RGB value corresponding to the Lab value subjected to color gamut mapping and stores it in the image data holding unit 107 (S15).

  When the above processing for the input image data is completed, the output image data (device RGB value) is stored in the image data holding unit 107. Therefore, the output unit 106 outputs the output image data to the image output device 13 or the storage device 14 instructed by the UI unit 112 (S16).

  When there is one input / output device connected to the color processing apparatus 10, image data, output color gamut data, and light source data are input by the input unit 101, and image data is not necessarily output by the output unit 106. Does not require instructions. In this case, the color processing apparatus 10 may input data output from the input device and output the color processed image data to the output device.

Input Section FIG. 11 is a flowchart showing details of the process (S11) of the input section 101.

  The input unit 101 initializes the counter i to 1 (S101), and stores the i-th pixel data of the input image data in a predetermined area of the image data holding unit 107 (S102). Then, the counter i is incremented (S103), and it is determined whether or not all pixel data of the input image data has been input (S104). If there is uninput image data, the process returns to step S102. Note that the pixel order is, for example, raster scan order.

  When all the pixel data of the input image data is input, the input unit 101 initializes the counter i to 1 again (S105), and the RGB value and XYZ of the i-th grid point (grid point number = i) of the output color gamut data The value is stored in a predetermined area of the color gamut data holding unit 108 (S106). Then, the counter i is incremented (S107), and it is determined whether or not the data of all grid points of the output color gamut data have been input (S108). If there is uninput grid point data, the process returns to step S106.

  When the data of all grid points of the output color gamut data is input, the input unit 101 stores the light source data in a predetermined area of the light source data holding unit 109 (S109).

Reference White Calculation Unit FIG. 12 is a flowchart showing details of the process (S12) of the reference white calculation unit 102.

  The reference white calculation unit 102 acquires the XYZ value of paper white (R = G = B = 255) from the output color gamut data held by the color gamut data holding unit 108 (S201). Of course, when the output color gamut data is arranged in the order of RGB values (0, 0, 0) to (255, 255, 255), the XYZ value of paper white can be obtained by obtaining the XYZ value of the bottom row of the table. May be obtained.

Next, the reference white calculation unit 102 calculates the xy value x _wp y _wp from the XYZ value X _wp Y _wp Z _{wp of} the paper white according to the equation (1) (S202).
x _wp = X _wp / (X _wp + Y _wp + Z _wp )
y _wp = Y _wp / (X _wp + Y _wp + Z _wp )… (1)

Next, the reference white calculation unit 102 obtains the XYZ value X _ws Y _ws Z _ws of the white point of the light source from the light source data held by the light source data holding unit 109 (S203), The xy value x _ws y _ws of the point is calculated (S204).

  Next, the reference white calculation unit 102 sets a threshold Yth for the Y value (S205). Although details will be described later, the threshold value Yth is a weight for adjusting the degree of dependence of the reference white color, the white point of the light source, and the white color (for example, paper white) of the output device. The reference white calculation unit 102 inputs the threshold value Yth by the user through the UI unit 112 or sets a default value. Here, for example, Yth = 70 is set. Then, the counter i is initialized to 1 (S206).

Next, the reference white calculation unit 102 uses the xy value of the paper white, the xy value of the white point of the light source, the threshold value Yth, and the Y value of the i-th grid point of the output color gamut data to generate XYZ of the reference white The value is calculated (S207). Reference white of XYZ values X _wr Y _wr Z _wr is calculated by converting the reference white of xy values x _wr y _wr calculated by linear interpolation into XYZ values. Expression (2) is an expression for calculating the xy value x _wr y _wr of the reference white.
x _wr = ( n × x _wp + m × x _ws ) / (m + n)
y _wr = ( n × y _wp + m × y _ws ) / (m + n)… (2)
Where m = Y _wp -Y (m ≧ 0)
n = Y-Yth (n≥ 0)

That is, when the Y value of the lattice point is Yth or less (Y ≦ Yth), n = 0, and the reference white value depends on the XYZ value of the white point of the light source. When the Y value of the grid point exceeds Yth (Y> Yth), n> 0, and the reference white is a value between the white point of the light source and the paper white. That is, the threshold value Yth functions as a weighted average weight that adjusts the degree to which the reference white color depends on the white point of the light source and the white color of the output device (for example, paper white). Also, Equation (3) is used to convert xy values into XYZ values.
X = Y × x / y
Y = y (3)
Z = Y × (1.0-x-y) / y

  Next, the reference white calculation unit 102 stores the reference white (XYZ value) of the i-th lattice point in the buffer memory 110 (S208), and increments the counter i (S209). Then, it is determined whether or not the reference white corresponding to all the grid points has been calculated (S210), and if not completed, the process returns to step S207 until the reference white corresponding to all the grid points is calculated. Repeat ~ S210.

  Thus, the reference white calculation unit 102 calculates the reference white corresponding to the Y value of each grid point of the output color gamut data, the xy value of paper white in the xy chromaticity plane, the xy value of the white point of the light source, and the threshold value Yth. Based on (weight), calculation is performed by linear interpolation (weighted average calculation).

Color Perception Space Value Calculation Unit FIG. 13 is a flowchart showing details of the process (S13) of the color perception space value calculation unit 103.

  The color perception space value calculation unit 103 initializes the counter i to 1 (S301), acquires the reference white of the i-th grid point stored in the buffer memory 110 (S302), and uses the acquired reference white, Lab values corresponding to the XYZ values of the i-th lattice point are calculated (S303).

In calculating the Lab value corresponding to the XYZ value of the grid point, the XYZ value of the grid point is acquired from the color gamut data holding unit 108. Then, the Lab value is calculated by Equation (4).
When Y / Yn> 0.008856
L * = 116 (Y / Yn) ¹ /3-16
When Y / Yn ≤ 0.008856
L * = 903.29 (Y / Yn)
a * = 500 {f (X / Xn)-f (Y / Yn)}… (4)
b * = 200 (f (Y / Yn)-f (Z / Zn)}
Here, when X / Xn> 0.008856, f (X / Xn) = (X / Xn) ^1/3
When X / Xn ≤ 0.008856 f (X / Xn) = 7.78 (X / Xn) + 16/116
When Y / Yn> 0.008856 f (Y / Yn) = (Y / Yn) ^1/3
When Y / Yn ≤ 0.008856 f (Y / Yn) = 7.78 (Y / Yn) + 16/116
When Z / Zn> 0.008856 f (Z / Zn) = (Z / Zn) ^1/3
When Z / Zn ≤ 0.008856 f (Z / zn) = 7.78 (Z / Zn) + 16/116
XnYnZn is the XYZ value of the reference white

  Next, the color perception space value calculation unit 103 stores the calculated Lab values of the grid points in the buffer memory 110 (S304), and increments the counter i (S305). Then, it is determined whether or not Lab values of all grid points have been calculated (S306), and if not completed, the process returns to Step S302, and the processes of Steps S302 to S305 are performed until Lab values of all grid points are calculated. repeat.

  As described above, the color perception space value calculation unit 103 calculates the color perception space value of the lattice point using the reference white color calculated for each lattice point.

Color Gamut Mapping Unit FIG. 14 is a flowchart showing details of the processing (S14) of the color gamut mapping unit 104.

The gamut mapping unit 104 initializes the counter i to 1 (S401), and calculates the Lab value corresponding to the i-th pixel data (RGB value) of the input image data stored in the image data holding unit 107 (S402). . That is, the RGB value of the i-th pixel data is acquired, and the RGB value is converted into an XYZ value. For example, when the image data is expressed in the sRGB color space defined by IEC61966-2-1, the RGB value is converted into an XYZ value by Expression (5).
┌ ┐ ┌ ┐┌ ┐
│X│ │0.4124 0.3576 0.1805││R''│
│Y│ = │0.2126 0.7152 0.0722││G''│… (5)
│Z│ │0.0193 0.1192 0.9505││B''│
└ ┘ └ ┘└ ┘
Where R '= R / 255
G '= G / 255
B '= B / 255
When R ', G', B '≤ 0.04045
R "= R '/ 12.92
G "= G '/ 12.92
B "= B '/ 12.92
When R ', G', B '> 0.04045
R "= {(R '+ 0.055) /1.055} ^2.4
G "= {(G '+ 0.055) /1.055} ^2.4
B "= {(B '+ 0.055) /1.055} ^2.4

  Then, the Lab value is calculated from the XYZ value calculated by Expression (5) using Expression (4). The XYZ value of the reference light source D65 may be used for the white point when calculating the Lab value, or the color temperature described in the header of the input image data or the XYZ value of the white point may be used.

  Next, the color gamut mapping unit 104 determines whether or not the Lab value of the i-th pixel data (hereinafter, input color) exists in the output color gamut (S403). The inside / outside determination of the color gamut is determined by the grid points on the boundary surface surrounding the intersection of the straight line extending from the focal point to the input color and the boundary surface of the output color gamut in the focus type color gamut mapping.

FIG. 15 is a diagram for explaining inside / outside determination of a color gamut. A point A shown in FIG. 15 is a focal point on the gray line of the output color gamut (for example, Lab = (50, 0, 0)). Point P indicates the input color. Points B, C, and D are lattice points on the boundary surface that surround the intersection of the straight line extending from the focal point A to the point P and the boundary surface of the output color gamut. At this time, the vector AP can be expressed by Equation (6).
↑ AP = s ↑ AB + t ↑ AC + u ↑ AD… (6)
Where ↑ AP represents a vector from point A to point P

The coefficients s, t, and u in Equation (6) can be calculated by Equation (7) when the Lab value at an arbitrary point X is expressed as (Lx, ax, bx).
┐ ┐ ┌ ┐-1┌ ┐
│s│ │L _B -L _A L _C -L _A L _P -L _A │ │L _P -L _A │
│t│ = │a _B -a _A a _C -a _A a _P -a _A │ │a _P -a _A │… (7)
│u│ │b _B -b _A b _C -b _A b _P -b _A │ │b _P -b _A │
└ ┘ └ ┘ └ ┘

If the point P exists inside the tetrahedron ABCD, the equation (8) is established.
s + t + u ≤ 1, 0 ≤ s, t, u (8)

  That is, the color gamut mapping unit 104 determines that the input color P exists in the output color gamut if there are grid points B, C, and D that satisfy Equation (8). Further, when there are no grid points B, C, and D that satisfy Expression (8), it is determined that the input color P exists outside the output color gamut. If the input color P exists in the output color gamut, the process proceeds to step S406.

  When the input color P exists outside the output color gamut, the color gamut mapping unit 104 calculates the intersection (point Pi shown in FIG. 15) of the boundary surface between the vector AP and the output color gamut (S404), and the Lab of the input color P The value is replaced with the Lab value of the intersection Pi (S405). That is, the input color P outside the output color gamut is mapped to the intersection Pi of the boundary surface of the output color gamut.

  Next, the color gamut mapping unit 104 increments the counter i (S406), and determines whether all pixel data of the input image data has been processed (S407). If there is unprocessed pixel data, the process returns to step S402, and steps S402 to S407 are repeated until the processing of all pixel data is completed.

  Thus, the color gamut mapping unit 104 maintains the color value of the input color P in the output color gamut, and maps the input color P outside the output color gamut to the intersection Pi of the boundary surface between the vector AP and the output color gamut. As a result, the color value of the input image data is mapped to the output color gamut.

  As described above, the color processing apparatus 10 sets the reference white color for each grid point of the output color gamut data. Then, based on the color values of the color perception space corresponding to the grid points of the output color gamut calculated using the reference white color, the color values of the color perception space of the input image data are color gamut mapped. Therefore, the white of the input device and the white of the output device (paper white) can be matched to improve the achromatic color reproducibility and prevent the deterioration of the chromatic color reproducibility.

  The color processing according to the second embodiment of the present invention will be described below. Note that the same reference numerals in the second embodiment denote the same parts as in the first embodiment, and a detailed description thereof will be omitted.

  FIG. 16 is a flowchart for explaining the operation of the color processing apparatus 10 according to the second embodiment.

  As in the first embodiment, the input unit 101 inputs image data, output color gamut data, and light source data necessary for color processing, which are instructed by the UI unit 112, from the image input device 11 or the storage device 12, respectively. The data are stored in the corresponding holding units 107 to 109 (S11).

  Next, the color perception space value calculation unit 103 converts the XYZ values of the output gamut data held by the gamut data holding unit 108 into Lab values, and acquires the Lab value of the boundary surface of the output gamut (S21). . Then, the device color value calculation unit 105 converts the Lab value on the boundary surface of the output color gamut into the device RGB value of the monitor connected to the UI unit 112 (S22). Then, the UI unit 112 displays a cross section of the output color gamut in a certain hue on the monitor (S23).

  FIG. 17 is a diagram illustrating an example of the user interface 200. The user inputs the file name in the text box of the image data setting unit 201 or sets the input image data by a file selection dialog (not shown) displayed when the browse button is pressed. In addition, when the user inputs a file name in the text box of the output color gamut setting unit 202 or presses a browse button, the user sets output color gamut data by a file selection dialog (not shown).

  The user sets the threshold value Yth by inputting a numerical value in the text box of the Y threshold value setting unit 203 or adjusting the numerical value displayed in the text box with a spin button. In addition, the user inputs a device name, a path name, and an address for specifying the output destination of the image data in the text box of the output destination setting unit 204 or presses a browse button, and the image data is displayed by a file selection dialog (not shown). Set the output device that is the output destination.

  The output color gamut display unit 207 displays a hue section having an output color gamut represented by the output color gamut data.

  The UI unit 112 determines whether or not the “update” button 205 has been pressed (S25), and if pressed, supplies the threshold Yth set in the Y threshold setting unit 203 to the reference white color calculation unit 102 for output. The reference white of each grid point in the color gamut is calculated (S26). Subsequently, the color perception space value calculation unit 103 calculates the Lab value of the boundary surface of the output color gamut using the reference white of the grid point of the output color gamut (S27). Thereafter, the process returns to step S21, and the UI unit 112 updates the display of the output color gamut.

  Further, the UI unit 201 determines whether or not the “OK” button has been pressed (S24). If the button has been pressed, the process proceeds to step S12. Since the process after step S12 is the same as the process of Example 1 shown in FIG. 8, the detailed description is abbreviate | omitted.

  Thus, the user can confirm whether or not a preferable output color gamut can be obtained by the display of the output color gamut display unit 207 by adjusting the threshold Yth by the Y threshold value setting unit 203.

  Hereinafter, color processing according to the third embodiment of the present invention will be described. Note that the same reference numerals in the third embodiment denote the same parts as in the first and second embodiments, and a detailed description thereof will be omitted.

  FIG. 18 is a block diagram illustrating a configuration example of the color processing apparatus 10 according to the third embodiment. In addition to the configuration shown in FIG. 7, a black body radiation locus holding unit 113 that retains the black body radiation locus data input by the input unit 101 is provided. The black body radiation locus holding unit 113 is allocated to a predetermined area of a memory such as a RAM or a hard disk (HD), for example, like the other holding units.

  FIG. 19 is a flowchart for explaining the operation of the color processing apparatus 10. What differs from the operation shown in FIG. 8 is the processing of steps S11 ′ and S12 ′. That is, the input unit 101 inputs data necessary for color processing, the output color gamut, the light source, and the black body radiation locus from the image input device 11 and the storage device 12, which are instructed by the UI unit 112, respectively. The data are stored in the corresponding holding units 107 to 109 and 113 (S11 ′). Note that the blackbody radiation locus holding unit 113 retains the blackbody radiation locus data in a one-dimensional LUT format. Then, the reference white calculation unit 102 calculates the reference white using the black body radiation locus data (S12 ′).

  FIG. 20 is a flowchart showing details of the process (S12 ′) of the reference white calculation unit 102.

After executing the same processing as steps S201 to S204 shown in FIG. 12, the reference white calculation unit 102 calculates the xy value of the point on the black body radiation locus that has the shortest distance from the xy value x _wp y _wp of the paper white. (S211). Further, the xy value of the point on the black body radiation locus that is the shortest distance from the xy value x _ws y _ws of the white point of the light source is calculated (S212). Then, the xy value of the paper white and the xy value of the white point of the light source are replaced with the xy value on the black body radiation locus (S213).

  In step S207, the reference white of each grid point is calculated by calculating two points on the blackbody radiation locus, that is, the xy value of the paper white on the blackbody radiation locus, and the xy value of the white point of the light source on the blackbody radiation locus. Will be used. Accordingly, the reference white calculated by the reference white calculation unit 102 is an Lab value along the black body radiation locus, in other words, a color value in a color perception space that matches human visual characteristics.

[Modification]
In the above embodiment, the example using the CIELab space has been described. However, a color perception space such as CIELuv, CIECAM02, CIECAM97s may be used.

  In the above-described embodiment, the example in which the reference white calculation unit 102 obtains the reference white from a plurality of whites such as paper white and the white point of the light source using linear interpolation has been described. However, nonlinear interpolation may be used. . Moreover, although the example which calculates | requires reference | standard white using xy chromaticity value was demonstrated, you may use uv chromaticity value. Moreover, although the example using paper white or the white point of a light source was demonstrated, you may use partially white adaptation, the absolute white which a human feels pure white, etc.

  In the above embodiment, the color gamut mapping unit 104 maintains the color value of the input color in the output color gamut, and maps the input color outside the output color gamut to the boundary surface of the output color gamut toward the focal point. explained. However, the input color outside the output color gamut may be mapped to the boundary surface of the output color gamut while maintaining the lightness. Further, the input color may be mapped so that the three-dimensional distance in the CIELab space before and after the mapping of the input color is minimized, that is, at the shortest distance point on the boundary surface of the output color gamut. Also, the input color within the output color gamut need not be kept completely, and may be mapped to a preferable color both inside and outside the output color gamut. Further, a masking technique using a conversion matrix that associates color values in the sRGB space with color values in the device RGB space may be used.

  Further, in the above-described embodiment, the example in which the reference white calculation unit 102 calculates the reference white according to the Y value of each grid point of the output color gamut data has been described. However, the same processing may be performed for the input color gamut.

[Other embodiments]
Note that the present invention can be applied to a system composed of a plurality of devices (for example, a computer, an interface device, a reader, a printer, etc.), or a device (for example, a copier, a facsimile machine, a control device) composed of a single device. Etc.).

  Another object of the present invention is to supply a storage medium storing a computer program for realizing the functions of the above-described embodiments to a system or apparatus, and the computer (CPU or MPU) of the system or apparatus executes the computer program. But it is achieved. In this case, the software read from the storage medium itself realizes the functions of the above embodiments, and the computer program and the computer-readable storage medium storing the computer program constitute the present invention. .

  Further, the above functions are not only realized by the execution of the computer program. That is, according to the instruction of the computer program, the operating system (OS) running on the computer and / or the first, second, third,... This includes cases where functions are realized.

  The computer program may be written in a memory of a device such as a function expansion card or unit connected to the computer. That is, the case where the CPU or the like of the first, second, third,... Device performs part or all of the actual processing according to the instruction of the computer program, thereby realizing the above functions.

  When the present invention is applied to the storage medium, the storage medium stores a computer program corresponding to or related to the flowchart described above.

xy chromaticity diagram showing the color reproduction range (color gamut) of the sRGB color space, Flowchart showing general color gamut mapping; The figure which shows the change of the output color gamut when the gray line is moved in parallel for each brightness. The figure explaining the method of matching the white of the input / output device by rotating the color gamut The figure explaining the reproducibility deterioration of the color in the parallel movement of the gray line, The figure explaining the deterioration of the reproducibility of the color in the rotation of the color gamut, A block diagram showing a configuration example of a color processing apparatus of an embodiment, A flowchart for explaining the operation of the color processing apparatus; The figure which shows the example of a format of the image data which an image data holding part stores, The figure which shows the example of a format of the output gamut data which a gamut data holding part stores. A flowchart showing details of processing of the input unit; A flowchart showing details of processing of the reference white calculation unit, A flowchart showing details of processing of a color perception space value calculation unit; A flowchart showing details of processing of the color gamut mapping unit; The figure explaining the inside / outside judgment of a color gamut, Flowchart for explaining the operation of the color processing apparatus of Example 2, The figure which shows an example of the user interface, Block diagram showing a configuration example of a color processing apparatus of Example 3, A flowchart for explaining the operation of the color processing apparatus; It is a flowchart which shows the detail of a process of a reference | standard white calculation part.

Claims (8)

Corresponding to the light source data of the observation light source when observing the image output by the image output device , and each grid point obtained by dividing the range of the input data of the image output device into a grid shape, the output color of the image output device Input means for inputting color gamut data indicating the color value of the color gamut;
Based on the white point indicated by the light source data and the white color indicated by the color gamut data, white calculation means for calculating a reference white for each grid point of the color gamut data;
A color value calculation means for calculating a color value of a color perception space from a color value corresponding to the grid point using a reference white color for each grid point;
A color processing apparatus comprising: mapping means for performing color gamut mapping based on the color value of the color perception space calculated by the color value calculation means. 2. The color processing apparatus according to claim 1, wherein the white color indicated by the color gamut data is paper white.   The white calculation means converts the color value of the white point and the color value of the paper white into a chromaticity value, and calculates a chromaticity value obtained by weighted averaging the chromaticity value of the white point and the chromaticity value of the paper white. 3. The color processing apparatus according to claim 2, wherein the reference white is calculated.   The white calculation means converts the color value of the white point and the color value of the paper white into a chromaticity value, and emits a black body radiation at a shortest distance from the chromaticity value of the white point and the chromaticity value of the paper white. 3. The color processing apparatus according to claim 2, wherein the reference white color is calculated from a chromaticity value obtained by weighted averaging of chromaticity values at two points on the locus.   5. The color processing apparatus according to claim 3, further comprising setting means for setting a weight of the weighted average.   6. The display device according to claim 5, further comprising display means for displaying a color gamut based on a color value of the color perception space calculated by the color value calculation means when the weighted average weight is updated. Color processing device. Corresponding to the light source data of the observation light source when observing the image output by the image output device , and each grid point obtained by dividing the range of the input data of the image output device into a grid shape, the output color of the image output device Enter the color gamut data indicating the color value of the gamut,
Based on the white point indicated by the light source data and the white color indicated by the gamut data, a reference white is calculated for each grid point of the gamut data,
Using the reference white color for each grid point , the color value of the color perception space is calculated from the color value corresponding to the grid point,
A color processing method comprising performing color gamut mapping based on the calculated color value of the color perception space. A program for causing a computer to function as each unit of the color processing device according to any one of claims 1 to 6.

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