JP2007243422A - Color transformation definition data making device and method, and image output apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make color transformation definition data efficiently with high precision, and to perform color transformation with high precision and continuity of color reproduction in a shadow region. <P>SOLUTION: A control unit transforms the output signal value of gray hue on a 3D-LUT into the intermediate chroma of coplementary colors. More specifically, boundary points C11 and C13 of a color gamut at the same lightness of the hue A of blue color and the hue B of its complementary color, i.e. yellow-green, are extracted in a shadow region (0<L*<20), based on the output signal value, and the chroma is calculated at the extracted boundary points. Middle point C12 of the calculated chromas is determined and the output signal value is updated by calculating a* and b* from the chroma at the middle point. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a color conversion definition data creation device and color conversion definition data creation for creating color conversion definition data for color-converting a signal value of input image data into a signal value corresponding to a color reproduction characteristic of an image output device. The present invention relates to a method and an image output apparatus that outputs an image based on image data after color conversion after color conversion based on the color conversion definition data.

  When color matching is performed in an image processing apparatus such as a CRT (Cathode Ray Tube), a liquid crystal, or a color printer, a reference color space PCS (Profile Connection Space) is defined, and the image processing apparatus is set to the color space. A method such as fitting is used. Image processing apparatuses have different characteristics such as brightness and saturation of color spaces that can be reproduced (hereinafter referred to as “color reproduction characteristics”), and have individual color spaces depending on their unique characteristics. For this reason, in order to match the image processing apparatus to the PCS, it is necessary to perform correction according to the specific characteristics for each apparatus.

  However, the intrinsic characteristics of the image processing apparatus are structurally owned by the apparatus itself. For example, in the case of a color printer, the color material, the recording characteristics on the recording paper, the fluctuations inherent in the electric circuit, etc. Therefore, each has its own characteristic. Although these characteristics are corrected as appropriate, since each element is complicated and these elements are intertwined, it is difficult to express the input / output relationship of image data in the image processing apparatus with a simple calculation formula.

  In addition, it is necessary to accurately perform color conversion from the PCS to the color space depending on each device. For example, the input / output function of the image processing device is unknown so that the output characteristics of the color printer have strong nonlinearity. Then, accurate mapping to PCS becomes difficult. Therefore, recently, as a technique for accurate color reproduction of digitized image data and color conversion between devices having different image data, a three-dimensional lookup table (hereinafter referred to as “3D-LUT”) or the like. Color conversion using color profile color conversion definition data has become the mainstream.

  The color conversion definition data is a data table (search table) representing the correspondence between the signal value of the input image data and the signal value of the output image data. FIG. 9B shows a data configuration example of the 3D-LUT 24. . In the color conversion of the input / output signal, as shown in FIG. 9A, the output signal value associated with the input signal value input from the input device is retrieved from the 3D-LUT 24 and output to the output device.

As a method for creating such color conversion definition data, for example, a method is known in which a color measurement result of an output sample is used based on a grid point data group that is uniformly distributed in the color space of an input signal value. (See Patent Document 1). Further, as a color conversion method using the color conversion definition data, it is determined whether the input image data is text image data or natural image data, and the lookup table is switched based on the determination result. A technique for preventing coloring and natural image roughness in a text image is known (see Patent Document 2).
JP 2002-64719 A JP-A-10-145616

  By the way, the color conversion definition data is given as a mapping of input / output signals for N × N × N lattice points when each of the input signal value and the output signal value is N stages (for example, 9 stages). . However, since it is very rare that the color point to be color-coordinated coincides with the grid point, color conversion definition data is created by performing trial and error measurement so that the desired color conversion is performed. Is the reality.

  Specifically, a color patch is prepared, a primary masking parameter is obtained by the method of least squares, and a rough value is predicted by performing the inverse operation. Then, the value is measured to confirm whether it is the calculated value. If the error is larger than the specified value, the value is appropriately changed and measured again. In other words, trial and error measurement is attempted in which the measurement and change of the value are repeated to approach the optimum value. However, in such a creation method, it takes an enormous amount of time to work on color conversion definition data having N × N × N grid points, and it is also efficient due to the influence of calculation accuracy and measurement errors. Absent.

  In addition, as digitalization of photographs and the like progresses, it is possible to output images to various media, and high-precision color reproduction is required. In particular, the depiction of dark areas is becoming important. . However, in the creation method according to the technique of Patent Document 1, the color conversion definition data is created so that the signal values of the grid points are distributed substantially uniformly in the signal space according to simple rules. In other words, the density of the so-called shadow region may be biased.

  In addition, the color conversion by the technique of Patent Document 2 is affected by a low brightness range on an output medium such as recording paper to be image output and a color space (color gamut boundary) in which the color reproduction of the output medium is possible In some cases, the color continuity in the shadow area becomes insufficient. As described above, in color reproduction of a shadow area, the accuracy of color reproduction becomes insufficient due to the density deviation and the influence of the output medium, which may not be practical.

  The present invention has been made in view of the above-described problems. The object of the present invention is to create color conversion definition data efficiently and with high accuracy, as well as the accuracy and continuity of color reproduction in shadow areas. It is to enable high color conversion.

In order to solve the above problems, the invention described in claim 1
In a color conversion definition data creation device for creating color conversion definition data for color conversion of a signal value of input image data into a signal value corresponding to the color reproduction characteristics of the image output device,
Colorimetric means for colorimetrically measuring an image output by the image output device based on the image data;
Based on the result of color measurement by the color measurement means, the signal value of the gray hue in the shadow area of the input image data is color-converted to an intermediate saturation between complementary colors in the output color space of the image output device. Creating means for creating color conversion definition data for
It is characterized by having.

The invention according to claim 2 is the invention according to claim 1,
The colorimetric means is
Measure the color space signal value of the image output by the image output device,
The creating means includes
The signal value of the gray hue in the shadow region of the measured signal value is converted into a signal value of intermediate saturation between complementary colors in the output color space of the image output device, and the converted signal value and the input The color conversion definition data is created by associating with the gray hue signal value in the shadow area of the image data.

The invention according to claim 3 is the invention according to claim 1 or 2,
The colorimetric means is
The color measurement is performed on an image based on a grid point data group in which the color space of the shadow area is evenly distributed.

The invention according to claim 4 is the invention according to any one of claims 1 to 3,
The creating means includes
The color conversion definition data is created so that the brightness reproduction curve of the shadow area in the output color space of the image output apparatus is a linear approximation.

The invention according to claim 5 is the invention according to any one of claims 1 to 4,
The creating means includes
Creating color conversion definition data for converting the signal value of the input image data into a signal value of a luminance extended color space independent of the image output device, and then performing color conversion to intermediate saturation between the complementary colors; It is characterized by.

The invention according to claim 6 is the invention according to any one of claims 1 to 5,
The color conversion definition data includes a color conversion table.

According to a seventh aspect of the present invention, there is provided a color conversion definition data creation method for creating color conversion definition data for color-converting a signal value of input image data into a signal value corresponding to a color reproduction characteristic of an image output device. ,
A colorimetric step of measuring the color of the image output by the image output device based on the image data;
Based on the result of color measurement in the color measurement step, the signal value of the gray hue in the shadow area of the input image data is color-converted to an intermediate saturation between complementary colors in the output color space of the image output device. Creating process to create color conversion definition data for
It is characterized by including.

The invention according to claim 8 is the invention according to claim 7,
The colorimetry step is
Measure the color space signal value of the image output by the image output device,
The creation process includes
The gray hue signal value in the shadow region of the measured signal value is converted into a signal value of intermediate saturation between complementary colors in the output color space of the image output device, and the converted signal value and the input The color conversion definition data is created by associating with the gray hue signal value in the shadow area of the image data.

The invention according to claim 9 is the invention according to claim 7 or 8,
The colorimetry step is
The color measurement is performed on an image based on a grid point data group in which the color space of the shadow area is evenly distributed.

The invention according to claim 10 is the invention according to any one of claims 7 to 9,
The creation process includes
The color conversion definition data is created so that the brightness reproduction curve of the shadow area in the output color space of the image output apparatus is a linear approximation.

The invention according to claim 11 is the invention according to any one of claims 7 to 10, wherein
The creation process includes
Creating color conversion definition data for converting the signal value of the input image data into a signal value of a luminance extended color space independent of the image output device, and then performing color conversion to intermediate saturation between the complementary colors; It is characterized by.

The invention according to claim 12 is the invention according to any one of claims 7 to 11,
The color conversion definition data includes a color conversion table.

The invention according to claim 13
In the image output device that outputs the image based on the color-converted image data after color-converting the signal value of the input image data based on the predetermined color conversion definition data,
A color conversion means for color-converting a gray hue signal value in a shadow area of the input image data to an intermediate saturation between complementary colors in the output color space of the own machine based on the color conversion definition data;
Image output means for outputting an image based on image data having a signal value subjected to color conversion by the color conversion means;
It is characterized by having.

The invention according to claim 14 is the invention according to claim 13,
The image output means includes
Image forming means for forming an image on a recording medium based on the image data having the color-converted signal value;
The lightness of black formed by the image forming means is not more than a predetermined level.

  According to the present invention, based on the result of measuring the color of the image output from the image output device, the signal value of the gray hue in the shadow area is colored to the intermediate saturation between the complementary colors in the output color space of the image output device. As a result of the conversion, color reproducibility in the shadow area and color reproducibility with high continuity are possible. Further, since the color measurement is performed on the image based on the grid point data group in which the color space of the shadow area is evenly distributed, it is possible to efficiently and highly accurately create color conversion definition data.

  In addition, since the brightness reproduction curve of the shadow area becomes a linear approximation, the color reproduction accuracy in the shadow area and high color reproduction in a continuous example are possible. In addition, since the signal value of the input image data is converted into the signal value of the luminance extended color space, the signal value is not compressed, so that the color reproduction accuracy in the shadow area can be improved. In addition, since the blackness of the image forming unit is below a predetermined level, color reproduction with high color reproducibility and high continuity in the shadow area is effectively performed.

Embodiment
Hereinafter, an embodiment in which the color conversion definition data creation device of the present invention is applied to the LUT creation device 1 shown in FIG. 1 and the image output device is applied to the color printer 10 will be described in detail with reference to FIGS. Explained. In the following description, the image output apparatus is described as being applied to the color printer 10, but may be applied to a display apparatus that displays and outputs an image such as a CRT or a liquid crystal display.

  FIG. 1 is a block diagram illustrating an example of functional configurations of the color printer 10 and the LUT creation apparatus 1, and FIG. 2 is a flowchart for explaining specific operations of the color printer 10. First, the functional configuration and specific operation of the color printer 10 will be described with reference to FIGS.

[Functional configuration of color printer]
The color printer 10 is an image forming apparatus that exposes and develops an image based on image data on a photosensitive material by an ink jet method, an electrophotographic method, a thermal method, a sublimation method, and the like to create a photographic print. As shown, the image reading unit 12, the patch image creation unit 13, the color coordinate conversion units 14 and 18, the image processing unit 16, the color conversion unit 20, the 3D-LUT 24, and the print creation unit 22 are configured.

  The image reading unit 12 is a functional unit that reads an image stored in various digital media and an image obtained by subjecting a manuscript to split photometry. The image reading unit 12 includes a reflective manuscript reading device having a scanner, a PC card adapter, and a floppy (registered trademark) disk. It is configured with an adapter. The image reading unit 12 converts an image read from a digital medium or an image read from a document into color image data and inputs the color image data to the color coordinate conversion unit 14 (step S1).

  The patch image creation unit 13, the color coordinate conversion units 14 and 18, the image processing unit 16, and the color conversion unit 20 are a central processing unit (CPU), a digital signal processor (DSP), a read only memory (ROM), A RAM (Random Access Memory) is provided. In the present embodiment, the color coordinate conversion units 14 and 18, the image processing unit 16, and the color conversion unit 20 are described as separate functional units, but may be realized by appropriately combining them. You may implement | achieve as one function part.

The patch image creation unit 13 is a functional unit that creates color patch image data (hereinafter referred to as “patch image data”). The patch image creating unit 13 is an N × N × N combination of N stages of input signal values (r _i , g _i , b _i , (i = 1 to N)) for each color of sRGB having a primary independent hue. Patch image data is created and input to the color coordinate conversion unit 14 and the LUT creation device 1. The color coordinate conversion unit 14 converts the color coordinates in the sRGB color space of the input image data into color coordinates in the device-specific YIQ color space, and inputs them to the image processing unit 16 (step S3).

  The image data read and generated by the image reading unit 12 and the patch image data generated by the patch image creation unit 13 include an sRGB color space signal, an RGB space signal to be output to the print creation unit 22, and CMYK. It is desirable that the signal is a luminance extended color space such as a spatial signal. The luminance extended color space is preferably an extended color space that is larger than the sRGB color space, close to the CIE color space, and further includes the CIE color space.

  Further, the range of the sYCC color space derived from the luminance color difference separation space using the international standard of the conversion matrix from the sRGB color space to the YCC color space and the sRGB color space of the output device such as a printer is expanded to satisfy γ = 2.2. An example is a bg-sRGB color space in which the curve is extended to negative (minus).

  By inputting image data in such a luminance extended color space, compression of signal values is eliminated, so that it is possible to improve the accuracy of color reproduction in a shadow area of a low brightness area where the brightness L * is about 0 to 20. it can. In particular, the white point, primary color chromaticity, observation environment, etc. are the same as in the sRGB color space, but a range of −0.5 to 7.5 for each of sRGB is allowed, and a wider color gamut can be achieved with 16 bits for each of sRGB. By using scRGB that can be covered and expressed, there is no compression of signal values in the shadow area, and color reproduction in the shadow area can be improved.

  The image processing unit 16 performs noise reduction (Y) processing on the image data using a filter and a mask operation as necessary, and performs enlargement / reduction / rotation (YIQ) processing (step S5). Then, after sharpness enhancement (Y) processing (step S7), level adjustment such as key correction, offset adjustment and contrast adjustment is performed (step S9), color enhancement is further performed, and image data is transferred to the color coordinate converter 18. Input (step S11). The color coordinate conversion unit 18 obtains image data obtained by converting the color coordinates of the input image data in the YIQ color space into color coordinates in the RGB color space (step S13).

  The color conversion unit 20 reads the output signal value associated with the input signal value of the image data coordinate-converted by the color coordinate conversion unit 18 from the 3D-LUT 24 and outputs the output signal value to the print creation unit 22 to perform color conversion. This is performed (step S15). The 3D-LUT 24 is stored in a readable / writable nonvolatile storage medium such as an HDD (Hard Disk Drive) or a flash memory. The color conversion unit 20 performs gradation conversion (RGB) using a one-dimensional lookup table defining density gradation conversion and inter-pixel correction on the image data after color conversion, and inputs them to the print creation unit 22. (Step S17).

  The print creating unit 22 has a magazine loading unit for loading a photosensitive material (color paper) as a recording medium, an exposure processing unit for exposing an image to the photosensitive material, and develops and exposes the exposed photosensitive material. And a drying processing unit. The color printer 10 will be described as an example in which a photographic print P1 is created by exposing and developing a photosensitive material. However, the present invention is not limited to this, as long as it can create a print based on image data. For example, the color printer 10 of an inkjet system, an electrophotographic system, a thermal system, and a sublimation system may be used. The coordinate system on which the image processing unit 16 performs various image processing is YCbCr coordinate system, L * a * b * coordinate system, L * u * v * coordinate system, extended color space, or linear transformation on them. In this case, the color coordinate conversion units 14 and 18 perform conversion to a corresponding coordinate system.

[Functional configuration of LUT creation device]
Next, the functional configuration and specific operation of the LUT creation apparatus 1 will be described with reference to FIGS. 1 and 3 to 8. The LUT creation device 1 is a device that creates a 3D-LUT 24 that the color conversion unit 20 of the color printer 10 refers to when performing color conversion, and includes a control unit 3 and a color measurement unit 5 as shown in FIG. Configured.

  The control unit 3 includes a CPU, a ROM, a RAM, and the like. The control unit 3 is a functional unit that performs overall control of each functional unit constituting the LUT creation device 1 and performs various information processing. The color measurement unit 5 includes a CCD camera, a spectrophotometer, and the like, performs color measurement of an image, and outputs a signal value of the measured color to the control unit 3.

  FIG. 3 is a flowchart for explaining a specific operation of the LUT creation apparatus 1. The CPU of the control unit 3 performs the process of the flowchart of FIG. 3 according to a program stored in the ROM.

  When the LUT creation apparatus 1 starts the processing of the flowchart of FIG. 3, first, the color printer 10 prints the color patch P <b> 2 from which the 3D-LUT 24 is created. Specifically, the patch image creation unit 13 creates patch image data (sRGB) having 729 points of lattice point data composed of RGB in units of 32 bits divided into 9 × 9 × 9 as shown in FIG. To do.

  After the color coordinate conversion unit 14 performs color coordinate conversion (RGB → YIQ), the image processing unit 16 performs image processing such as level adjustment. Next, the color coordinate conversion unit 18 performs inter-pixel correction on the image data obtained by performing color coordinate conversion (YIQ → RGB), converts the image data into a printer head signal, and prints out the color patch P2.

  The color measurement unit 5 of the LUT creation apparatus 1 performs color measurement of the color patch P2 printed by the color printer 10. In this way, the color patch P2 based on the equally divided grid point data is color-measured, and the 3D-LUT 24 is created based on the color measurement result. Therefore, the number of grid points of the color to be color-converted can be increased. The 3D-LUT 24 can be created efficiently and with high accuracy without performing trial and error measurement as in the prior art.

  For the color measurement of the color patch P2, for example, an XYZ value of an image can be measured with a signal in increments of 16 bits by a spectrophotometer or the like. Needless to say, a highly accurate color measurement result can be obtained when the signal unit of the color measurement image is in units of 8 bits and further 8 bits or less. In addition, when measuring colors in 8-bit increments, the color hues of gray hues are measured in finer 4-bit increments so that the color gamut of gray hues can be focused on to obtain neutral (achromatic) color reproduction. Accuracy can be improved.

The control unit 3 generates input signal values 240 (R _j , G _j , B _j , (j = 1 to N)) for three primary independent colors corresponding to the patch image data input from the patch image creation unit 13. Obtain (step S20). Then, the color measurement unit 5 is controlled to perform color measurement of the color patch P2 output from the color printer 10 (step S21), and the color measurement result is acquired as an output signal value 242 as shown in FIG. 6 (step S21). S23).

Then, the control unit 3 uses the input signal values (r _i , g _i , b _i , (i = 1 to N)) of the patch image data input from the patch image creation unit 13 and the color measurement unit 5 to perform color measurement. The output signal values (R _j , G _j , B _j , (j = 1 to N)) thus obtained are associated with each other to create the 3D-LUT 24 (step S25). Note that the 3D-LUT 24 being created may be stored in the RAM of the control unit 3 until the creation of the 3D-LUT 24 in step S29 is completed.

  Subsequently, the control unit 3 interpolates the 3D-LUT 24 of N × N × N stages by three-dimensional interpolation such as volume interpolation or spline interpolation, and changes the number of division levels for each hue from N stages to M stages (M> N). The M stages are obtained by M = (N−1) × L + 1. For example, the 3D-LUT 24 is created by increasing the number of division levels from 9 stages to 33 stages.

  Here, as a complementing method of the 3D-LUT 24, three-dimensional interpolation, linear interpolation, area interpolation, volume interpolation, or the like of a known technique can be applied, and volume interpolation is more preferable. As volume interpolation, for example, tetrahedral interpolation, triangular prism (prism) interpolation, hexahedron (cube) interpolation, pyramid interpolation, spline interpolation, and the like can be adopted, and spline interpolation is more preferable in terms of accuracy of color reproducibility.

  After complementing the 3D-LUT 24, the control unit 3 converts the 3D-LUT 24 so that the saturation of the gray hue in the shadow area becomes an intermediate saturation between the complementary colors (step S29). Here, in the RGB color solid shown in FIG. 5, the gray hue is a grid point where RGB values are evenly distributed, that is, a white grid point W (255, 255, 255) and a black grid point BL (0, 0). , 0,) is a color based on the data of diagonal lattice points (R = G = B).

  When converting the saturation of the gray hue, the input signal values of the patch image data to be printed as the color patch P2 are evenly distributed in units of bits, for example, 0, 32, 64,. By dividing substantially equally with 255 etc., an error is reduced in the calculation of intermediate saturation, which will be described later.

  The intermediate saturation between complementary colors is the intermediate saturation between the highest saturations in the color gamut between complementary colors of a certain lightness. The complementary color is an opposite color to a color in a certain color gamut. For example, the complementary color of red is cyan, the complementary color of green is magenta, and the complementary color of blue is yellow.

  The complementary color may be selected from the Munsell color chart, and in that case, it may be a color in the vicinity of 180 degrees opposite to a * b *. In addition, when selecting a complementary color of blue (5PB), up to yellow green (5GY) in the vicinity of yellow (5Y) opposite to 180 degrees may be selected as a complementary color, and the selection range is about ± 30 degrees. The relationship is preferable from the continuity of intermediate saturation in the color gamut.

  The intermediate saturation is set based on a pair of colors and complementary colors, but is preferably set based on a combination of two or more pairs of colors and complementary colors. Further, in consideration of the complementary color hue range of ± 30 degrees and the three primary colors, three or more pairs are more preferable.

  The conversion of the gray hue into the intermediate saturation will be specifically described with reference to FIGS. 6 and 7 by taking as an example the case where the hue A to be adjusted is set to blue. In FIG. 7, the vertical axis represents lightness (L *), and the horizontal axis represents saturation (C *) of different hues with the L * axis as a boundary.

  First, the control unit 3 converts the output signal value 242 (XYZ) of the XYZ color space, which is a color result for the blue input signal value 240 of the 3D-LUT 24 complemented to M × M × M stages, to a device independent L *. The output value 244 (L * a * b *) of the a * b * color space is converted. Next, the control unit 3 obtains an output value 246 by converting only the output value 244 corresponding to the gray hue of the input signal value 240 into an intermediate saturation between complementary colors.

  Specifically, based on the output signal value 244, the boundary point of the color gamut at the same brightness between the hue A of blue (5BP) and the hue B of its complementary yellow-green (5GY) (for example, the boundary of FIG. 7). Points C11 and C13) are extracted in the shadow area (0 <L * <20), and the saturation of the extracted boundary points is calculated. Then, an intermediate point (for example, intermediate point C12 in FIG. 7) that is the middle of the calculated saturation is obtained, a * and b * are calculated from the saturation of this intermediate point, and the output signal value 244 is updated.

  The color reproduction characteristic C1 (broken line) in FIG. 7 indicates the brightness and saturation characteristics of the color gamut that the input signal value of the 3D-LUT 24 can reproduce, and the color reproduction characteristic C2 (solid line) indicates the output signal value. Indicates the lightness and saturation characteristics of the color gamut in which color reproduction is possible. As shown in FIG. 7, the color reproduction characteristic C2 of the color patch P2 printed based on the patch image data varies from the color reproduction characteristic C1 of the input signal value depending on the characteristic inherent to the color printer 10.

  In the blue and yellow-green image output in the prior art, the image is output as a gray hue having the color characteristic G1 of FIG. However, since the color characteristic G1 in the color reproduction characteristic C2 that the color printer 10 actually reproduces is biased to the blue side in the color reproduction characteristic C2, the continuity of image output in the blue to gray range is inferior. At the same time, the color rendering in the range of yellowish green to gray is output at an insufficiently dense density.

For this reason, in this embodiment, the color characteristic G1 is set to the color characteristic so that the saturation of the gray hue becomes intermediate saturation between the saturation of the hue A of the color reproduction characteristic C2 and the saturation of the complementary hue B. Convert and adjust to G2. The intermediate saturation C * (0) between the saturation C * (A) of the hue A and the saturation C * (B) of the complementary hue B is, for example,
0.25 <C * (0) / (C * (A) + C * (B)) <0.75
It is preferable to adjust so as to satisfy the relationship.

Also,
0.35 <C * (0) / (C * (A) + C * (B)) <0.65
It is particularly preferable to satisfy the relationship:
0.45 <C * (0) / (C * (A) + C * (B)) <0.55
Is preferable.

  In this way, the 3D-LUT 24 is converted into the output signal value 246 shown in FIG. 6 by converting the gray hue of the output signal value 244 to the intermediate saturation as shown in FIG. Then, the output signal value 246 is converted into an xyz signal value and stored and updated as an output signal value of the 3D-LUT 24.

  As described above, according to the present embodiment, the 3D-LUT 24 is created by converting the saturation of the gray hue in the shadow area into the intermediate saturation between the complementary colors, thereby producing excellent color and gradation description, particularly color reproducibility. Color conversion with good accuracy and continuity (connection) becomes possible.

  When converting the 3D-LUT 24 to intermediate saturation in the gray hue region, the 3D-LUT 24 is created so that the lightness reproduction curves of the input signal value and the output signal value of the gray hue in the shadow region are linearly approximated. It is desirable.

  FIG. 8 shows a lightness reproduction curve of the gray hue in the shadow area of the 3D-LUT 24 created by the LUT creation device 1. In FIG. 8, the horizontal axis represents the brightness of the input signal value of the 3D-LUT 24, and the vertical axis represents the brightness of the output signal value. By plotting the brightness of the input signal value and the output signal value, a brightness reproduction curve indicating the degree of change of the output signal value with respect to the input signal value is obtained.

  This lightness reproduction curve is approximated to a straight line, so that color continuity is maintained. In particular, it is preferable that the lightness reproduction curve is linearly approximated when the lightness L * of the shadow region is 15 or less. In order to approximate the lightness reproduction curve to a linear approximation, the boundary points (for example, boundary points C11 and C13, boundary points C14 and C16) extracted from the color gamut of the color reproduction characteristic C2 are changed in the conversion of intermediate saturation between complementary colors. Is possible.

  In addition, the linear approximation is mathematically preferably 0.95 or more as the square of the correlation coefficient, and plot points (points D1 to D4 in FIG. 8) of coordinates corresponding to the lightness of the output signal value of 0 and 15. The difference from the connected straight lines is preferably 3 or less, more preferably 2 or less.

  Three patterns of 3D-LUTs 24 for obtaining the lightness reproduction curves CV1 to CV3 shown in FIG. 8 are created and compared. In FIG. 8, primary approximate straight lines ST1 to ST3 are shown in order to clearly indicate whether or not the brightness reproduction curve is a linear approximation.

  Since the lightness reproduction curve CV1 shown in FIG. 8A is linearly approximated when the lightness L * is 15 or less, by using the 3D-LUT 24 having the lightness reproduction curve CV1, the description accuracy in the shadow region and the continuity are illustrated. Highly efficient color conversion is possible.

  Further, the lightness reproduction curve CV2 shown in FIG. 8B is a little linear approximation when the lightness L * is 15 or less. Therefore, by using the 3D-LUT 24 having the lightness reproduction curve CV2, the lightness reproduction curve CV2 shown in FIG. Although slightly inferior to 3D-LUT 24, color conversion with high accuracy and continuity is possible.

  The conversion to the intermediate saturation of the 3D-LUT 24 can adjust the output signal value near the gray hue so that a tone jump does not occur at the boundary between the gray hue gamut and the neighboring gamut. Desirably, there is a method of calculating and adjusting a shift amount, a vector distance, and the like with attenuation such that the influence of the adjustment on the intermediate saturation is reduced in the signal value adjacent to the output signal value corresponding to the gray hue.

  Further, the adjustment of the gray hue to the intermediate saturation is preferably performed in the vicinity of black to shadow area where the brightness range of L * is 30 or less, and more preferably in the vicinity of black to shadow area of 20 or less or 15 or less. Moreover, the adjustment to the intermediate saturation of the gray hue is not limited to the vicinity of black to the shadow area, and may be performed in an area having a lightness of 15 to 30, for example.

  Further, in an output medium such as recording paper or photosensitive material, the color corresponding to the maximum visual density and the minimum brightness is set as the vicinity of black, and the intermediate saturation is applied in the shadow area from the vicinity of black to the brightness of 20 or less or 15 or less. It is also possible to make adjustments. This makes it possible to adjust the gray hue according to the output medium. Also, if the input signal value produces an achromatic color equivalent to black in the output medium, the same effect can be obtained by setting it as near black and adjusting to the intermediate saturation. Of course.

  Further, as the color space to be converted in the creation of the 3D-LUT 24, a luminance extended color space that does not depend on a device such as an L * a * b * color space or a Luv color space is preferable. An area close to the color space, or an extended color space including the CIE color space is preferable. Also, by using the international standard of RGB to YCC conversion matrix, the range of sYCC derived from the luminance color difference separation space and the sRGB range of the printer is expanded, and the curve of γ = 2.2 is expanded to a negative value bg -SRGB is cited as the luminance extended color space.

  In particular, the white point, primary color chromaticity, observation environment, etc. are the same as sRGB, but allow a range of -0.5 to 7.5 for each RGB, and cover a wider color gamut with 16 bits for each RGB. By using scRGB that can be expressed as follows, there is no compression of data on the shadow side, and shadow color reproduction can be improved, which is preferable.

  In the above-described embodiment, the LUT creation apparatus 1 is configured as a separate apparatus from the color printer 10. However, the LUT creation apparatus 1 may be configured inside the color printer 10. Can be appropriately changed.

FIG. 2 is a block diagram illustrating an example of a functional configuration of a color printer and an LUT creation apparatus. 6 is a flowchart for explaining a specific operation of the color printer. The flowchart for demonstrating the specific operation | movement of a LUT production apparatus. The schematic diagram of patch image data. The schematic diagram of the RGB signal value of the gray hue in a RGB color solid. The figure for demonstrating the conversion process to the gray hue of 3D-LUT gray hue. The figure for demonstrating the conversion process to the intermediate | middle saturation of the gray hue in L * a * b * color space. The figure which shows the brightness reproduction curve of 3D-LUT. (A) is a figure explaining the outline | summary of color conversion, (b) is a figure which shows an example of a data structure of 3D-LUT.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 LUT creation apparatus 3 Control part 5 Color measurement part 10 Color printer 12 Image reading part 13 Patch image creation part 14 Color coordinate conversion part 16 Image processing part 18 Color coordinate conversion part 20 Color conversion part 22 Print creation part 24 Three-dimensional lookup Table (3D-LUT)
240 Input signal value 242 to 248 Output value A Hue B Complementary hue C1 Color reproduction characteristic C2 Color reproduction characteristic P2 Color patch

Claims (14)

In a color conversion definition data creation device for creating color conversion definition data for color conversion of a signal value of input image data into a signal value corresponding to the color reproduction characteristics of the image output device,
Colorimetric means for colorimetrically measuring an image output by the image output device based on the image data;
Based on the result of color measurement by the color measurement means, the signal value of the gray hue in the shadow area of the input image data is color-converted to an intermediate saturation between complementary colors in the output color space of the image output device. Creating means for creating color conversion definition data for
A color conversion definition data creation device comprising: The colorimetric means is
Measure the color space signal value of the image output by the image output device,
The creating means includes
The gray hue signal value in the shadow region of the measured signal value is converted into a signal value of intermediate saturation between complementary colors in the output color space of the image output device, and the converted signal value and the input The color conversion definition data creating apparatus according to claim 1, wherein the color conversion definition data is created by associating a gray hue signal value in a shadow area of the image data. The colorimetric means is
The color conversion definition data creation device according to claim 1 or 2, wherein an image based on a grid point data group in which a color space of the shadow area is uniformly distributed is measured. The creating means includes
The color conversion according to any one of claims 1 to 3, wherein the color conversion definition data is created so that a brightness reproduction curve of a shadow area in an output color space of the image output device is a linear approximation. Definition data creation device. The creating means includes
Creating color conversion definition data for converting the signal value of the input image data into a signal value of a luminance extended color space independent of the image output device, and then performing color conversion to intermediate saturation between the complementary colors; The color conversion definition data creation device according to any one of claims 1 to 4.   The color conversion definition data creation device according to claim 1, wherein the color conversion definition data includes a color conversion table. In a color conversion definition data creating method for creating color conversion definition data for color converting a signal value of input image data into a signal value corresponding to a color reproduction characteristic of an image output device,
A colorimetric step of measuring the color of the image output by the image output device based on the image data;
Based on the result of color measurement in the color measurement step, the signal value of the gray hue in the shadow area of the input image data is color-converted to an intermediate saturation between complementary colors in the output color space of the image output device. Creating process to create color conversion definition data for
A method for creating color conversion definition data, comprising: The colorimetry step is
Measure the color space signal value of the image output by the image output device,
The creation process includes
The gray hue signal value in the shadow region of the measured signal value is converted into a signal value of intermediate saturation between complementary colors in the output color space of the image output device, and the converted signal value and the input The color conversion definition data creating method according to claim 7, wherein the color conversion definition data is created by associating with a gray hue signal value in a shadow area of the image data. The colorimetry step is
The color conversion definition data creation method according to claim 7 or 8, wherein an image based on a grid point data group in which a color space of the shadow area is uniformly distributed is measured. The creation process includes
The color conversion according to any one of claims 7 to 9, wherein the color conversion definition data is created so that a brightness reproduction curve of a shadow area in an output color space of the image output device becomes a linear approximation. Definition data creation method. The creation process includes
Creating color conversion definition data for converting the signal value of the input image data into a signal value of a luminance extended color space independent of the image output device, and then performing color conversion to intermediate saturation between the complementary colors; The color conversion definition data creation method according to any one of claims 7 to 10.   The color conversion definition data creation method according to any one of claims 7 to 11, wherein the color conversion definition data includes a color conversion table. In the image output device that outputs the image based on the color-converted image data after color-converting the signal value of the input image data based on the predetermined color conversion definition data,
A color conversion means for color-converting a gray hue signal value in a shadow area of the input image data to an intermediate saturation between complementary colors in the output color space of the own machine based on the color conversion definition data;
Image output means for outputting an image based on image data having a signal value subjected to color conversion by the color conversion means;
An image output apparatus comprising: The image output means includes
Image forming means for forming an image on a recording medium based on the image data having the color-converted signal value;
14. The image output apparatus according to claim 13, wherein the brightness of black formed by the image forming unit is not more than a predetermined level.