JP2010093477A - Color processing device and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve color reproduction accuracy and gradation preservability in color gamut conversion to a narrower color gamut. <P>SOLUTION: In generating a color gamut conversion condition for converting an input color gamut into an output color gamut, minimum color-difference mapping is applied to a specific color value (for example, LC<SB>10</SB>) outside the output color gamut, fixed point mapping is applied to the mapped color value (for example, LC<SB>2</SB>) to make it close to a target point (for example LC<SB>anc</SB>) (the mapped color value is, for example, LC<SB>3</SB>), and a color value of a position separated only by a distance obtained by multiplying the ratio of a distance between the specific color value LC<SB>10</SB>and the color value LC<SB>2</SB>to a distance between the color value LC<SB>3</SB>and the color value LC<SB>2</SB>by the distance between the color value LC<SB>3</SB>and the color value LC<SB>2</SB>is operated to the color value LC<SB>3</SB>after the fixed point mapping as an output color value after the color gamut conversion of the specific color value LC<SB>10</SB>in a direction toward the color value LC<SB>2</SB>after the minimum color-difference mapping. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a color processing apparatus, a color processing method, and a color processing program.

  Image input / output devices such as printers, color printers, displays, and scanners have different color reproduction characteristics such as a color reproduction gamut (referred to as a color gamut). For this reason, when colors are transferred between devices, color conversion (color gamut conversion) is required to correct the difference in color reproduction characteristics of each device and to match the color reproduction.

  As a color gamut mapping method applied to the generation of conversion conditions in color gamut conversion, for example, Patent Document 1 discloses a color value that cannot be reproduced by an output device in an input color signal in the color gamut of the output device. A color gamut conversion technique is described by mapping weights of differences, saturation differences, and hue differences to colors with the smallest color difference calculated.

  Japanese Patent Application Laid-Open No. 2004-228561 discloses another gamut mapping method in which a projection target point is set on an achromatic color axis, and a color outside the gamut of the output device is mapped into the gamut of the output device with a constant hue. It is disclosed.

In Patent Document 3, as another color gamut mapping method, the hue and brightness of the input image signal are adjusted and converted to an intermediate image signal in an intermediate color gamut based on the color gamut of the input device. Disclosed is a technique for mapping within the color gamut of the output device by preserving lightness and compressing saturation in the high lightness region of the intermediate color gamut, and compressing lightness and saturation toward the target point in the low lightness region. Yes.
JP-A-10-84487 Japanese Patent Laid-Open No. 9-168097 JP 2000-184222 A

  An object of the present invention is to provide a color processing apparatus capable of realizing improvement in color reproduction accuracy and gradation preservation even in a color gamut conversion to a narrower color gamut.

  In order to achieve the above object, a color processing apparatus according to a first aspect of the present invention relates to a calculation target color value located within a first color gamut and outside a second color gamut in a predetermined color space. First calculation means for calculating a first color value when the first mapping is performed to move the outline of two color gamuts, and the second color for the first color value calculated by the first calculation means. A second computing means for computing a second color value in the case of performing a second mapping for approaching a target point set in the color gamut, and an output color for color gamut conversion for moving the color value to be computed into the second color gamut As a value, an object to be calculated for a first distance between the first color value and the second color value in the predetermined color space and corresponding to a distance between the first color value and the second color value. The ratio of the second distance corresponding to the distance between the color value and the first color value, or the first color value and the second color A color value at a position corresponding to the ratio of the second distance to the third distance corresponding to the distance between the specific color value corresponding to the intersection of the straight line passing through the first color gamut and the first color value; And an arithmetic means.

  According to a second aspect of the present invention, in the first aspect of the present invention, the third calculation means uses a predetermined color space as an output color value for color gamut conversion that moves the color value to be calculated into the second color gamut. A color value at a position separated from the position of the second color value above by a distance obtained by multiplying the first distance by a ratio of the second distance to the first distance in a direction toward the first color value. Calculate.

  According to a third aspect of the present invention, in the second aspect of the present invention, when the second distance is greater than the first distance, the third calculation means sets the color value to be calculated within the second color gamut. The first color value is set as an output color value for color gamut conversion to be moved to.

  According to a fourth aspect of the present invention, in the first aspect of the present invention, the third calculation means uses a predetermined color space as an output color value for color gamut conversion for moving the color value to be calculated into the second color gamut. A color value at a position separated from the position of the second color value above by a distance obtained by multiplying the first distance by a ratio of the second distance to the third distance in a direction toward the first color value. Calculate.

  According to a fifth aspect of the invention, in the fourth aspect of the invention, when the second distance is greater than the third distance, the third calculation means sets the color value to be calculated within the second color gamut. The first color value is set as an output color value for color gamut conversion to be moved to.

  The invention according to claim 6 is the invention according to any one of claims 1 to 5, wherein the first calculation means sets the color value of the calculation target as the first mapping in the second color gamut. The color value when mapping is performed to move to a position where the color difference before and after the mapping is minimized is calculated as the first color value.

  According to a seventh aspect of the invention, in the sixth aspect of the invention, the first calculation means is configured such that when the color value to be calculated has a lightness lower than the maximum saturation point of the first color gamut, As the distance of the color value to be calculated with respect to the maximum saturation point of one color gamut increases, the weight for the brightness difference before and after mapping is set to be relatively smaller than the weight for the hue difference and the saturation difference. The color difference before and after the mapping is evaluated using the calculated color difference formula.

  According to an eighth aspect of the present invention, in the invention according to any one of the first to seventh aspects, the second calculation means includes a minimum lightness point of the first color gamut and a minimum lightness point of the second color gamut. The maximum compression ratio is set so that the maximum compression ratio increases as the brightness difference increases, the compression ratio for the first color value is calculated based on the set maximum compression ratio, and the second mapping is used as the second mapping. The color value when mapping is performed to compress at the compression ratio obtained by calculating the distance between the one color value and the target point is calculated as the second color value.

  The invention according to claim 9 is the invention according to any one of claims 1 to 8, wherein the second calculation means includes the maximum saturation point of the second color gamut and the target point for each hue. Based on the ratio of the angle between the straight line connecting the first color value and the target point to the reference angle formed by the lightness axis of the predetermined color space and the reference angle formed by the lightness axis of the predetermined color space. A compression rate for one color value is calculated, and a color value when mapping is performed with the compression rate calculated by calculating the distance between the first color value and the target point as the second mapping is the second color value. Calculate as

  According to a tenth aspect of the present invention, in the invention according to any one of the first to ninth aspects, the calculation target color is located in the first color gamut and in the second color gamut in a predetermined color space. A fourth color value is calculated when mapping is performed in which the color value to be moved is moved closer to the target point as the output color value of the color gamut conversion for moving the color value to be calculated into the second color gamut. An arithmetic means is further provided.

  According to an eleventh aspect of the present invention, there is provided a color processing program for causing a computer to calculate a color value of the second color gamut with respect to a calculation target color value located within a first color gamut and outside a second color gamut in a predetermined color space. First calculation means for calculating a first color value when the first mapping is performed to move the outline, and the first color value calculated by the first calculation means is set in the second color gamut. As the output color value of the color gamut conversion for moving the color value to be calculated into the second color gamut, the second calculation means for calculating the second color value when performing the second mapping approaching the target point A color value to be calculated with respect to a first distance between the first color value and the second color value in a predetermined color space and corresponding to a distance between the first color value and the second color value; The ratio of the second distance corresponding to the distance of the first color value, or the first color value and the second color A color value at a position corresponding to the ratio of the second distance to the third distance corresponding to the distance between the specific color value corresponding to the intersection of the straight line passing through the value and the first color gamut and the first color value is calculated. It is made to function as 3 calculating means.

  According to the first and eleventh aspects of the present invention, the color reproduction accuracy and the gradation preservation can be improved even in the color gamut conversion to a narrower color gamut compared to the case where the present configuration is not provided. Has an effect.

  The inventions according to claims 2 and 3 have an effect that the output color value can be obtained by a simple calculation as compared with the case where the present configuration is not provided.

  According to the fourth and fifth aspects of the present invention, the gradation can be preserved even in the range where the second distance is larger than the first distance and equal to or smaller than the third distance, compared with the case where the present configuration is not provided. Have.

  The invention according to claim 6 has an effect that the color reproduction accuracy can be improved as compared with the case where the present configuration is not provided.

  According to the seventh aspect of the present invention, compared to the case where the present configuration is not provided, the gradation preservability is deteriorated when an outwardly convex region exists in a part of the outline of the second color gamut. It has the effect that it can suppress.

  The invention according to claim 8 has an effect that the collapse of gradation can be suppressed as compared with the case where this configuration is not provided.

  The invention according to claim 9 has an effect that the collapse of gradation can be suppressed without adversely affecting the color value having a relatively high saturation as compared with the case without this configuration.

  According to the tenth aspect of the present invention, the output color value of the color gamut conversion can be obtained even for the color value located in the first color gamut and in the second color gamut, compared with the case where this configuration is not provided. Have

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

[First Embodiment]
FIG. 1 shows a schematic configuration of a printing system 10 according to the present embodiment. The printing system 10 includes a plurality of client terminals 14 including PCs (Personal Computers) and the like and an output device that outputs an image represented by image data input from the printing system 10 to a network 12 including a LAN. The printer 16 and the printing plate printing system 18 for recording the image represented by the image data on the recording paper are connected to each other. The printer 16 uses toners of a plurality of colors (for example, C, M, Y, and K) as color materials, forms a full-color toner image by an electrophotographic process based on the input image data, and transfers it to a predetermined recording sheet. Thus, the color image represented by the input image data may be recorded on a recording sheet, or input using a plurality of colors (for example, C, M, Y, K) as color materials. A configuration may be adopted in which the color image represented by the input image data is recorded on the recording paper by ejecting ink of each color from the nozzle based on the image data and adhering the ink to the recording paper. Note that the printer 16 may be a multifunction machine in which a function as a copying machine or a facsimile machine is added to the printer.

  On the other hand, the printing plate printing system 18 includes a plate making apparatus (CTP: Computer To Plate) that creates printing plates (printing plates) of a plurality of colors (for example, C, M, Y, K) based on input image data, A printing plate created by a plate making apparatus is set and includes a printing machine (press device) that prints an image on recording paper using a plurality of colors (for example, C, M, Y, K) as color materials. ing. As the plate making apparatus, for example, a sheet-shaped printing plate is wound around and fixed to the outer peripheral surface of a cylindrical rotating drum, the rotating drum is rotated, and light modulated according to the input print data of a specific color component is used. Employs a configuration that creates a printing plate of a specific color by developing the printing plate on which the printing image is recorded after the printing image is recorded on the printing plate by irradiating the printing plate rotating integrally with the rotating drum. be able to. In this case, the printing plate of each color can be obtained by repeating the above process for each color of C, M, Y, and K.

  Each client terminal 14 connected to the network 12 includes a CPU 14A, a memory 14B composed of ROM, RAM, etc., a nonvolatile storage unit 14C composed of HDD (Hard Disk Drive), flash memory, etc., a network interface (I / I). F) unit 14D, and is connected to the network 12 via the network I / F unit 14D. The client terminal 14 is connected to a display 20, a keyboard 22 and a mouse 24 as input means. Note that an image input device such as a scanner or the printer 16 may be directly connected to the client terminal 14 in the same manner as the display 20. For example, as an image input device, there is a digital still camera or the like in addition to the scanner, but the digital still camera or the like is directly connected to the client terminal 14.

  The storage unit 14C of the client terminal 14 includes an OS (Operating System) program, various application programs that operate on the OS and use the printer 16 and the printing plate printing system 18, and the colors described below on the client terminal 14. A color conversion program (for causing the client terminal 14 to function as a color conversion processing section described below) and a color prediction model program for performing conversion processing are installed in advance, and profiles used for the color conversion processing, etc. A color conversion condition DB (database) capable of registering color conversion conditions is also stored, and further, raw data (base data) of the printer 16 and the printing plate printing system 18 input to the client terminal 14 for use in color conversion processing. ) Is also memorized. In the present embodiment, the client terminal 14 functions as a color processing apparatus in the present embodiment when the CPU 14A executes a color conversion program.

  Next, the operation of the first embodiment will be described. In the client terminal 14 according to the first embodiment, image data used for image recording by the printing plate printing system 18 is recorded on a recording sheet by the printing plate printing system 18 and the printer 16 when used for image recording by the printer 16. In order to correct the difference in color appearance of the image to be displayed, a color conversion processing unit 30 that performs the color conversion processing shown in FIG. 2 is provided. Prior to the description of the color conversion processing unit 30, the raw data (base data) used for the color conversion processing will be described below.

  The elementary data (base data) is data representing the relationship between the input color value and the output color value in a certain device. For example, the elementary data of the printer 16 is the input color value to the printer 16 (in this embodiment, the device data). A patch (color chart) of a plurality of colors with known color values C ′, M ′, Y ′, K ′) in the CMYK color space as a dependent color space is created by the printer 16 (recorded on a recording sheet) and created The output color values of the individual color charts (in this embodiment, the color values L * ′, a * ′, b * ′ in the CIEL * a * b * color space as a device-independent color space) are measured by a colorimeter or the like Are obtained by associating the input color values C ′, M ′, Y ′, K ′ and the output color values L * ′, a * ′, b * ′ as a set of data for each color chart. be able to.

  As for the raw data of the printing plate printing system 18 as well, first, the input color values to the printing plate printing system 18 (in this embodiment, the color values C, C, K in the CMYK color space as the device-dependent color space) are used. Multiple color patches (color charts) with known M, Y, K) are created by the printing plate printing system 18 (recorded on recording paper), and output color values of the created individual color charts (devices in this embodiment) The color values L *, a *, b *) in the CIE L * a * b * color space as an independent color space are measured by a colorimeter or the like, and the input color values C, M, Y, K and output color values L *, a *, b * can be obtained by associating them as a set of data.

  Next, color conversion processing performed by the color conversion processing unit 30 using the raw data of the plate printing system 18 and the printer 16 described above will be described. As shown in FIG. 2, the color conversion processing unit 30 includes a color conversion profile generation unit 32, and the color conversion profile generation unit 32 includes a first color conversion condition generation unit 34 and a color gamut conversion condition generation unit 36. And a second color conversion condition generation unit 46 for converting the input color values C, M, Y, K to the printing plate printing system 18 into input color values C ′, M ′, Y ′, K ′ to the printer 16. Generate conversion conditions for color conversion.

  That is, the first color conversion condition generation unit 34 uses the input color values C, M, Y, and K to the printing plate printing system 18 as color values L in the CIEL * a * b * color space as a device-independent color space. A conversion condition for the first color conversion to be converted into *, a *, b * is generated. Specifically, the conversion condition of the first color conversion is, for example, by setting the raw data of the printing plate printing system 18 input to the color conversion processing unit 30 in the color prediction model, and in the color prediction model in which the raw data is set, Color conversion coefficients (color values C, M, Y, K at each grid point on the CMYK color space as a device-dependent color space) are sequentially input, and color values L *, a *, It can be generated by storing b * in the memory 14B as a new color conversion coefficient (conversion condition for the first color conversion).

  Note that the color prediction model predicts and estimates the color conversion characteristics (color conversion conditions) between different color spaces based on raw data representing the correspondence between a small number of input color values and output color values. It has higher interpolation performance and smoothing performance than linear calculation (linear interpolation) (strong against measurement errors, device noise, and in-plane unevenness). For the color prediction model, a method using a statistical method (Makoto Sasaki and Hiroaki Ikegami, Proc. Of International Congress of Imaging Science 2002 (2002) p.413-141), a method using a neural network, Neugebauer and Kubelkamunk There is a method based on a physical model such as, and any of them may be used.

  The color gamut conversion condition generation unit 36 includes a color gamut outline calculation unit 38, a first mapping unit 40, a second mapping unit 42, and a color value correction unit 44, and CIEL * a * as a device-independent color space. Conversion conditions for color gamut conversion (also referred to as gamut mapping) for converting the color gamut (color reproduction gamut) of the printing plate printing system 18 into the color gamut (color reproduction gamut) of the printer 16 in the b * color space are generated. Although the color gamut conversion condition generating unit 36 will be described in detail later, the first mapping unit 40 is an example of a first computing unit according to the present invention, an example of the second mapping unit 42, and a color value correcting unit. Reference numeral 44 is an example of third computing means according to the present invention.

  Further, the device-independent color space for performing color gamut conversion is not limited to the CIE L * a * b * color space, but is a color space that represents the appearance of a color excluding the influence of viewing conditions, for example, a color appearance model. It may be a color space JCh defined by CIECAM02 or a color space Jab obtained from the color space JCh. Note that the color attribute values a and b of the color space Jab correspond to ac and bc generated from the color attribute values C and h of the color space JCh, and have characteristics that can be converted into hue and saturation. . Further, the color appearance model CIECAM97s may be used instead of the color appearance model CIECAM02.

  The second color conversion condition generation unit 46 includes a black amount calculation unit 48 and a reverse color prediction unit 50. The black amount calculation unit 48 and the reverse color prediction unit 50 perform processing by the color gamut conversion condition generation unit 36. The color values L * ′, a * ′, b * ′ (color conversion coefficients) stored in the memory 14B via the above are converted into input color values C ′, M ′, Y ′, K ′ to the printer 16. A conversion condition for two-color conversion is generated. Specifically, in the second color conversion, variables whose input values are unknown (input color values C ′, M ′, Y ′, K ′) are variables whose values are known (color values L * ′, a * ′, b * ′), the black amount calculation unit 48 determines the input black amount K ′, and the reverse color prediction unit 50 determines the color values L * ′, a * ′, b * ′ and the input black amount K ′. The input color values C ′, M ′, Y ′ to the printer 16 are determined.

  As the value of the input black amount K ′ to the printer 16, first, the black amount of lightness equivalent to the input black amount K in the input color values C, M, Y, K to the printing plate printing system 18 is calculated. Specifically, the lightness is calculated from the input black amount K based on the raw data of the printing plate printing system 18, and the input black amount K ′ to the printer 16 having the same lightness as the lightness is calculated based on the raw data of the printer 16. To calculate. Next, correction is performed so that the black amount K ′ input to the printer 16 decreases as the saturation increases. As a result, an excessive amount of black can be prevented and vivid color reproduction can be maintained in a region with high saturation. Then, any one of the color values L * ′, a * ′, b * ′ (color conversion coefficients) stored in the memory 14B through the processing by the color gamut conversion condition generation unit 36, The printer 16 corresponding to the color values L * ′, a * ′, b * ′ to be processed when the input black amount K ′ to the printer 16 is the initial value set as the processing target a * ′, b * ′. The input color values C ′, M ′, Y ′ to be calculated based on the raw data of the printer 16 input to the color conversion processing unit 30, and the calculated input color values C ′, M ′, It is determined whether Y ′ is within the range of color values that can be input to the printer 16 (for example, C ′, M ′, Y ′ are each in the range of 0 to 100). When the calculated input color values C ′, M ′, Y ′ to the printer 16 deviate from the range of color values that can be input to the printer 16, the input black amount K ′ to the printer 16 is appropriately changed. The above process is repeated to search for the value of the black amount K ′ input to the printer 16. Thereby, the value of the input black amount K ′ to the printer 16 can be determined.

  The inverse color prediction unit 50 calculates the color values L * ′, a * ′, b * ′ (color conversion coefficients) and the black amount of the processing target based on the raw data of the printer 16 input to the color conversion processing unit 30. The input color values C ′, M ′, Y ′ to the printer 16 corresponding to the input black amount K ′ to the printer 16 determined by the unit 48 are calculated (predicted), and the calculated input color values C ′, M ′ are calculated. , Y ′ are stored in the memory 14B as new color conversion coefficients (conversion conditions including first color conversion, color gamut conversion, and second color conversion) together with the input black amount K ′. The above process is performed on all color values L * ′, a * ′, b * ′ (color conversion coefficients) stored in the memory 14B through the process by the color gamut conversion condition generation unit 36, thereby Conversion conditions for two-color conversion (more specifically, conversion conditions including first color conversion and color gamut conversion in addition to the second color conversion) are generated.

  In addition, the color conversion processing unit 30 includes a color conversion profile storage unit 52 and a color conversion unit 54, and conversion conditions for color conversion obtained through the above-described processing by the color conversion profile generation unit 32 (through the above-described processing). The color conversion coefficient stored in the memory 14B) is stored in the color conversion profile storage unit 52 (color conversion condition DB).

  The color conversion unit 54 includes a CLUT (color lookup table) such as a four-dimensional lookup table, and the input color values C, M, Y, and K to the printing plate printing system 18 are input to the printer 16 as input color values C ′, When converting to M ′, Y ′, K ′, the original color values C, M of the respective grid points on the CMYK color space are used as input values for defining the color conversion conversion conditions by the CLUT of the color conversion unit 54. , Y, K are respectively set as color conversion coefficients stored in the color conversion profile storage unit 52 as output values defining the conversion conditions. The color conversion unit 54 converts the input color values C, M, Y, K to the printing plate printing system 18 into input color values C ′, M ′, Y ′, K ′ to the printer 16 according to the CLUT. To do. Thus, the first color conversion, the color gamut conversion, and the second color conversion are realized by one color conversion by the CLUT, and the color values C ′, M ′, Y ′, and K ′ obtained by the conversion are transmitted to the printer 16. By inputting the image and recording the image on the recording paper in the printer 16, the appearance of the color of the image recorded on the recording paper by the printer 16 is changed to the color of the image recorded on the recording paper by the plate printing system 18. Can roughly match the appearance.

  Next, the color gamut conversion condition generation processing performed by the color gamut conversion condition generation unit 36 will be described with reference to FIG. In this color gamut conversion condition generation process, first, in step 100, based on the raw data of the printing plate printing system 18 input to the color conversion profile generation unit 32, the plate printing in the CIEL * a * b * color space is performed. The outline of the color gamut (input color gamut) of the system 18 is calculated. Specifically, first, in the CMYK color space, the input color values C, M, Y, K to the printing plate printing system 18 included in the raw data of the printing plate printing system 18 input to the color conversion profile generation unit 32 are used. A color value group corresponding to the color gamut outline of the printing plate printing system 18 (for example, a color value group in which at least one of the color values of each color of C, M, Y, and K shows a maximum value or a minimum value) is extracted. Next, the individual color values of the extracted color value group are input color values C, M, Y to the printing plate printing system 18 represented by the raw data of the printing plate printing system 18 input to the color conversion profile generation unit 32. , K and the output color values L *, a *, b *, respectively, are converted into color values L * a * b * in the CIE L * a * b * color space. Then, a straight line connecting the converted color values L * a * b * is set, and a plane having the set individual straight lines as sides is set, so that the color gamut outline (input color gamut of the input color gamut) is set. The outline can be calculated.

  In step 102, based on the raw data of the printer 16 input to the color conversion profile generation unit 32, the color gamut (output color gamut) of the printer 16 in the CIEL * a * b * color space is obtained in the same manner as in step 100. ) Is calculated. Steps 100 and 102 correspond to the color gamut outline calculation unit 38.

  By the way, in the color gamut conversion condition generation processing by the color gamut conversion condition generation unit 36, the color gamut (input color gamut) of the printing plate printing system 18 whose outline has been calculated in step 100, and the printer 16 whose outline has been calculated in step 102. A conversion condition for converting to a color gamut (output color gamut) is generated. If the printer 16 is configured to use toner as a color material, various conditions such as image recording speed, toner characteristics, and fixing device performance are set. The amount of color material used is often limited due to restrictions, and if the printer 16 is configured to use ink as the color material, the amount of ink used may be limited by various conditions such as image recording speed, ink characteristics, and paper water absorption characteristics. The amount of use is often limited, and the color gamut of the printer 16 becomes narrower when the amount of color material that can be used during image recording is limited. When the output color gamut in the color gamut conversion is significantly narrower than the input color gamut, the gradation is crushed or the color reproduction accuracy is deteriorated by the color gamut conversion. In the next step 104 and subsequent steps, a conversion condition for color gamut conversion is generated in consideration of this point.

That is, first, in step 104, CIE L * a * b * color among the color values L *, a *, b * (color conversion coefficients) stored in the memory 14B after being processed by the first color conversion condition generation unit 34. One color value in the input color gamut in space is extracted from the memory 14B, and in the next step 106, whether or not the color value (input color value) extracted in step 104 is outside the output color gamut calculated in the previous step 102. To determine. If the input color value is out of the output color gamut, the determination in step 106 is affirmed and the process proceeds to step 108. As the output color value corresponding to the input color value, the color difference ΔE within the output color gamut and with the input color value is obtained. A first mapping for searching and extracting the minimum color value is performed. This first mapping is processing corresponding to the first mapping unit 40, and the input color value is L * _in , a * _in , b * _in , and the output color value is L * _out , a * _out , b * _out . Sometimes, a color value having the smallest color difference ΔE defined by the following equation (1) is searched for and extracted.
ΔE = {((L * _out− L * _in ) × WL) ² + ((a * _out− a * _in ) × Wa) ² + ((b * _out− b * _in ) × Wb) ² } ^{1 / 2}
... (1)
However, WL, Wa, Wb are weighting factors. In this embodiment, when the lightness of the input color value is equal to or higher than the lightness of the maximum saturation point of the input color gamut in the same hue as the input color value, the weighting factor WL = Wa = Wb = 1.0, and when the lightness of the input color value is lower than the lightness of the maximum saturation point of the input color gamut in the same hue as the input color value, the weighting factor Wa = Wb = 1.0, As the weighting coefficient WL, as shown in FIG. 4 as an example, a value that decreases from 1.0 according to the distance from the maximum saturation point in the CIEL * a * b * color space is used. Instead of decreasing the value of the weighting coefficient WL for the lightness difference, the values of the weighting coefficient WL for the lightness difference are relatively increased by increasing the values of the weighting coefficients Wa and Wb for the hue difference and the saturation difference. It may be made smaller.

A first calculation of output color values L * _out , a * _out , and b * _out that minimize the color difference ΔE calculated by equation (1) using the weighting factors WL, Wa, and Wb set as described above. By mapping, if the input color value is located in a high lightness area that is greater than or equal to the lightness of the maximum saturation point, the color on the outline of the output color gamut and at the minimum distance from the input color value When the value is extracted as an output color value and the input color value is located in a low lightness area less than the lightness of the maximum saturation point, the weight for the lightness L * increases as the lightness difference from the maximum saturation point increases. By decreasing the value of the coefficient WL, the color value at the position where the saturation or hue of the input color value is preserved as much as possible and the amount of movement in the lightness direction (L * direction) is increased is extracted as the output color value. Accordingly, as shown in FIG. 5 as an example, in the high lightness region, the input maximum saturation point is mapped to the output maximum saturation point, and the output color has good pure color reproducibility (in which color turbidity is suppressed). A value is obtained. Further, in the low brightness area, even when an outwardly convex area exists in a part of the outline of the output color gamut, such as the point P shown in FIG. Mapping with a large amount of movement prevents the output color value after mapping from concentrating on the point P, and the output color value is determined so that good gradation reproducibility can be obtained.

  In FIG. 5, the uniform hue plane (cross section of the color gamut) is shown as the color gamut, and the color values appear to move within the uniform hue plane by the first mapping. The color value also moves in the direction intersecting the hue plane (hue direction). Further, the first mapping is not limited to the expression (1) and is performed by changing the value of the weighting coefficient WL with respect to the brightness difference as shown in FIG. In order to perform mapping with a large amount of movement in the brightness direction (L * direction), it is desirable that the first mapping is a mapping in which changes in saturation and hue are suppressed as much as possible.

  In the next step 110, the output color value of the first mapping is set as the input color value for the purpose of improving the collapse of gradation associated with the first mapping in step 108. As shown in FIG. A target point is set on the lightness axis (L * axis) in the output color gamut, and the input color value is moved toward the target point based on the compression rate set for each hue (from the target point to the input color value). The second mapping is performed to calculate the color value when the distance to be reached is moved so as to be compressed (decreased). This second mapping is a process corresponding to the second mapping unit 42, and the compression rate CR in the second mapping is the maximum saturation point and target point of the output gamut for each hue as shown in FIGS. The angle formed by the straight line connecting L * axis and the L * axis is taken as the reference angle, and the compression ratio CR increases as the ratio of the angle θ formed by the straight line connecting the input color value and the target point to the reference point relative to the reference angle becomes L * axis. Thus, when the ratio is 1 (angle θ = reference angle), the compression ratio CR = 0 is set. The compression rate on the lightness axis (L * axis) (the maximum compression rate described in claim 8) is the minimum lightness point on the L * axis of the input color gamut and the minimum lightness on the L * axis of the output color gamut. Based on the brightness difference from the point, the compression rate CR on the L * axis is set to increase as the brightness difference increases. This is because the amount of change in color value (movement distance in the color space) in the lightness direction (L * direction) by the first mapping becomes longer as the minimum lightness point is approached, and the collapse of gradation becomes conspicuous.

  By setting the compression rate CR in the second mapping as described above, the output color value is calculated so as to improve the collapse of the gradation due to the first mapping. The setting of the compression rate CR is not limited to the above, and the compression rate CR may be set to be constant without changing according to the angle θ, or according to the difference between the input color gamut and the output color gamut. The compression rate CR may be set. In FIG. 7, the target point is set in the range of lightness higher than the maximum saturation point in the output color gamut on the lightness axis (L * axis) in the output color gamut. In order to make the change in lightness dominant over the change in saturation, especially for color values in the range where the angle θ is small, the color value change due to is not limited to this, The target point may have the same brightness as the maximum saturation point, or may have a lower brightness than the maximum saturation point.

Subsequently, in step 112, the collapse of gradation by the first mapping is improved based on the output color value obtained by performing the first mapping and the output color value obtained by further performing the second mapping. As described above, the final output color value (output color value of color gamut conversion) corresponding to the input color value extracted in the previous step 104 is calculated. That is, FIG. 8 shows changes in color values (movement of positions in the color space) when the first mapping and the second mapping are sequentially performed on an arbitrary input color value. For example, if the input color value LC _obj is the color value LC ₁₀ located on the outline of the input color gamut, it is converted to the color value LC ₂ located on the outline of the output color gamut by the first mapping, and is converted by the second mapping. It is further converted into a color value LC ₃ located inside the outline of the output color gamut. Similarly, when the input color value LC _obj is the color value LC ₁₁ located between the color value LC ₁₀ and the color value LC ₂ , it is similarly converted to the color value LC ₂ by the first mapping, Since the color value LC ₃ is converted by the mapping, the gradation between the color value LC ₁₀ and the color value LC ₁₁ is destroyed by the first mapping.

Therefore, in step 112, the distance from the target point (LC _anc ) on the L * C * cross section (equal hue plane) shown in FIG. 9A (the distance r _obj from the target point to the original input color value, the target Based on the distance r ₂ from the point to the color value after the first mapping and the distance r ₃ from the target point to the color value after the second mapping), the correction coefficient CK is set by the following equations (2) and (3) Then, based on the set correction coefficient CK, a color value that reproduces a gradation that is crushed by the first mapping is calculated as the final output color value LC ₄ by the following equations (4) to (6).
CK = d ₂ / d ₁ (when 0 ≦ d ₂ ≦ d ₁ ) (2)
CK = 1.0 (when d ₂ > d ₁ ) (3)
L * (LC ₄ ) = L * (LC ₃ ) + CK × (L * (LC ₂ ) −L * (LC ₃ )) (4)
a * (LC ₄ ) = a * (LC ₃ ) + CK × (a * (LC ₂ ) −a * (LC ₃ )) (5)
b * (LC ₄ ) = b * (LC ₃ ) + CK × (b * (LC ₂ ) −b * (LC ₃ )) (6)
However, the distances d ₂ = r _obj −r ₂ and d ₁ = r ₂ −r ₃ , the distance d ₂ is an example of the second distance according to the present invention, and the distance d ₁ is the first distance according to the present invention. The correction coefficient CK is an example of the “ratio of the second distance to the first distance” according to the present invention. Then, the final output color value LC ₄ obtained by the above calculation is stored in the memory 14B as a color conversion coefficient, and the process proceeds to step 116. The calculation in step 112 described above is a process corresponding to the color value correction unit 44.

FIG. 9B shows the relationship between the initial input color value (before mapping) and the final output color value (after mapping) obtained by the above calculation. In the first embodiment, for the input color values in the range where d ₂ > d ₁ , the output color value “◆” corresponding to the input color value “◇” before mapping in FIG. As is clear from the fact that the output color value “■” corresponding to the input color value “□” before mapping is the same (= LC ₂ ), the gradation is crushed along with the color gamut conversion. However, for the input color values in the range where d ₂ ≦ d ₁ , the output color value “▲” corresponding to the input color value “Δ” before mapping and the input color value before mapping in FIG. As is clear from the fact that the output color value “●” corresponding to the input color value “◯” is different, the gradation before mapping is maintained. Therefore, by appropriately setting the compression ratio CR in the second mapping and adjusting the distance d ₁ , it is possible to improve the collapse of gradation due to the first mapping relatively easily.

  On the other hand, if the input color value extracted in step 104 is within the output color gamut, the determination in step 106 is denied and the process proceeds to step 114, and the input color value extracted in step 104 is described above. Similar to step 110, based on the compression rate set for each hue, the second mapping that calculates the color value when moved toward the target point set on the lightness axis (L * axis) in the output color gamut The color value obtained by the calculation is stored in the memory 14B as the final output color value (color conversion coefficient), and the process proceeds to step 116. The above step 114 corresponds to the fourth calculation means described in claim 10.

  In step 116, all of the color values in the input color gamut from the memory 14B out of the color values L *, a *, b * (color conversion coefficients) stored in the memory 14B through the processing by the first color conversion condition generation unit 34 are stored. It is determined whether it has been taken out. If the determination is negative, the process returns to step 104, and steps 104 to 116 are repeated until the determination of step 116 is affirmed. As a result, for all color values in the input color gamut, the processing in steps 108 to 112 is performed for the color values outside the output color gamut, and the processing in step 114 is performed for the color values in the output color gamut. Thus, a conversion condition for color gamut conversion for converting the input color gamut (internal color value) to the output color gamut (internal color value) is generated.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. In addition, the same code | symbol is attached | subjected to the part same as 1st Embodiment, and description is abbreviate | omitted. The second embodiment is different from the first embodiment only in the calculation method in the calculation of the output color value in step 112 in the color gamut conversion condition generation processing (FIG. 3) described in the first embodiment. Note that the color value correcting unit 44 that performs the arithmetic processing described below corresponds to the third arithmetic means according to claims 4 and 5 in more detail.

That is, in the second embodiment, in the calculation of the output color value in step 112, as shown in FIG. 10 as an example, the first input color value (color value LC _{obj in} the example of FIG. 10) is first changed. The color value obtained by performing one mapping (color value LC _{2 in} the example of FIG. 10) and the target point set on the lightness axis (L * axis) in the output color gamut (color value in the example of FIG. 10) The color value (the color value LC _{20 in} the example of FIG. 10) at the point where the straight line passing through the LC _anc point intersects the outline of the input color gamut is calculated. Then, the distance from the target point (LC _anc ) on the L * C * cross section (equal hue plane) shown in FIG. 11A (distance r _obj from the target point to the original input color value, Based on the distance r ₁ to the obtained color value, the distance r ₂ from the target point to the color value after the first mapping, and the distance r ₃ from the target point to the color value after the second mapping) (7) , (8) is used to set the correction coefficient CK, and based on the set correction coefficient CK, the final output color value LC ₄ is collapsed by the first mapping according to the previous expressions (4) to (6). The color value that reproduces the key is calculated.
CK = d ₂ / d ₃ (when 0 ≦ d ₂ ≦ d ₃ ) (7)
CK = 1.0 (when d ₂ > d ₃ ) (8)
However, the distances d ₂ = r _obj −r ₂ , d ₃ = r ₁ −r ₂ , the distance d ₃ is an example of the third distance according to the present invention, and the correction coefficient CK is the “third value” according to the present invention. It is an example of "ratio of 2nd distance to distance". Then, the final output color value LC ₄ obtained by the above calculation is stored in the memory 14B as a color conversion coefficient.

The distance d ₃ in the above process corresponds to the difference between the color gamut of the input device and the output device. In the above process, the color value (1) obtained by performing the first mapping for each input color value ( LC ₂₎ and that the straight line passing through the target point (the point color value LC _anc) is for calculating the color value of a point which intersects the contour of the input gamut (color value LC _20), respectively necessary to calculate the distance d ₃ In FIG. 11B, the output color value “◆” corresponding to the input color value “◇” before mapping, the output color value “■” corresponding to the input color value “□” before mapping, and the mapping in FIG. As is clear from the difference between the output color value “▲” corresponding to the previous input color value “△” and the output color value “●” corresponding to the input color value “○” before mapping, Except for the rare case where the input color value is d ₂ > d ₃ , the gradation before mapping can be substantially retained.

  In this embodiment, the CMYK color space of the printing plate printing system 18 is used as input image data. However, the input image data is not limited to this, and may be an RGB color space such as sRGB (standardRGB) or AdobeRGB.

  In the above description, the mode executed by the client terminal 14 which is an example of the color conversion processing according to the present invention has been described. However, the present invention is not limited to this. For example, the above processing is performed by the client terminal 14. In accordance with the instruction, it may be executed by a server computer or the like connected to the same network 12, and the above process may be executed by the printer 16 or the plate printing system 18. When the above process is executed by the printer 16 or the printing plate printing system 18, the client terminal 14 can be omitted.

  In the above description, the aspect in which the present invention is applied to the conversion from the color gamut of the printing plate printing system 18 to the color gamut of the printer 16 has been described. However, the present invention is not limited to this. To the color gamut of the printing plate printing system 18 or the color gamut conversion between other devices (for example, a display).

  In the above description, the color conversion program, which is an example of the color processing program according to the present invention, has been described in advance as being stored (installed) in the storage unit 14C of the client terminal 14, but the color processing program according to the present invention is It is also possible to provide the information recorded in a recording medium such as a CD-ROM or a DVD-ROM.

1 is a block diagram illustrating a schematic configuration of a printing system according to an embodiment. It is a functional block diagram of a color conversion processing unit showing the flow of color conversion processing. It is a flowchart which shows a color gamut conversion condition production | generation process. In a 1st mapping, it is a diagram which shows the relationship of the weighting coefficient with respect to the distance of an input color value and the maximum saturation point, and the brightness used for a 1st mapping. It is a conceptual diagram which shows an example of the conversion (mapping) of the color value by 1st mapping. In a 2nd mapping, it is a diagram which shows the relationship between angle (theta) and compression rate CR. It is a conceptual diagram which shows an example of conversion (mapping) of the color value by 2nd mapping. It is a conceptual diagram for demonstrating the color value correction | amendment which concerns on 1st Embodiment. It is a conceptual diagram for demonstrating the color value correction | amendment which concerns on 1st Embodiment. It is a conceptual diagram for demonstrating the color value correction | amendment which concerns on 2nd Embodiment. It is a conceptual diagram for demonstrating the color value correction | amendment which concerns on 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Printing system 14 Client terminal 30 Color conversion process part 32 Color conversion profile generation part 36 Color gamut conversion condition generation part 40 1st mapping part 42 2nd mapping part 44 Color value correction | amendment part

Claims (11)

The first color when the first mapping is performed for moving the color value to be calculated located within the first color gamut and outside the second color gamut in a predetermined color space onto the outline of the second color gamut. First calculating means for calculating a value;
Second computing means for computing a second color value when performing a second mapping for bringing the first color value computed by the first computing means closer to a target point set in the second color gamut;
As the output color value of the color gamut conversion for moving the color value to be calculated into the second color gamut, between the first color value and the second color value in the predetermined color space and the first color value. The ratio of the color value to be calculated to the first distance corresponding to the distance between the color value and the second color value and the second distance corresponding to the distance between the first color value or the first color value and the second color value A color value at a position corresponding to a ratio of the second distance to the third distance corresponding to the distance between the specific color value corresponding to the intersection of the straight line passing the color value and the first color gamut and the first color value is calculated. Third computing means;
Including color processing device.   The third calculation means is configured to output the first color with respect to a position of the second color value in a predetermined color space as an output color value of color gamut conversion for moving the color value to be calculated into the second color gamut. The color processing apparatus according to claim 1, wherein color values at positions separated by a distance obtained by multiplying the first distance by a ratio of the second distance to the first distance in a direction toward the value are calculated.   When the second distance is greater than the first distance, the third calculation means is configured to output the first color value as an output color value for color gamut conversion for moving the color value to be calculated into the second color gamut. The color processing apparatus according to claim 2, wherein:   The third calculation means is configured to output the first color with respect to a position of the second color value in a predetermined color space as an output color value of color gamut conversion for moving the color value to be calculated into the second color gamut. The color processing apparatus according to claim 1, wherein a color value at a position separated by a distance obtained by multiplying the first distance by a ratio of the second distance to the third distance in a direction toward the value is calculated.   When the second distance is greater than the third distance, the third calculation means is configured to output the first color value as an output color value for color gamut conversion for moving the color value to be calculated into the second color gamut. The color processing apparatus according to claim 4, wherein:   The first calculating means performs color mapping when the color value to be calculated is moved to a position where the color difference before and after mapping is minimized on the outline of the second color gamut as the first mapping. The color processing apparatus according to claim 1, wherein a value is calculated as the first color value.   When the color value to be calculated is lower in brightness than the maximum saturation point of the first color gamut, the first calculation means determines the color value of the calculation target with respect to the maximum saturation point of the first color gamut. As the distance increases, the color difference before and after the mapping is evaluated using a color difference calculation formula determined so that the weight for the lightness difference before and after the mapping is relatively smaller than the weight for the hue difference and the saturation difference. The color processing apparatus according to claim 6.   The second calculation means sets and sets the maximum compression ratio so that the maximum compression ratio increases as the brightness difference between the minimum brightness point of the first color gamut and the minimum brightness point of the second color gamut increases. A color when a compression ratio for the first color value is calculated based on a maximum compression ratio, and mapping is performed by compressing at a compression ratio obtained by calculating a distance between the first color value and a target point as the second mapping. The color processing apparatus according to claim 1, wherein a value is calculated as the second color value.   The second calculation means includes the first color value with respect to a reference angle formed by a straight line connecting the maximum saturation point of the second color gamut for each hue and the target point and the brightness axis of the predetermined color space. A compression rate for the first color value is calculated based on a ratio of an angle formed by a straight line connecting the target point and a brightness axis of the predetermined color space, and the first color value and the target point are calculated as the second mapping. The color processing apparatus according to claim 1, wherein a color value when mapping is performed to compress at a compression ratio obtained by calculating a distance between the first color value and the second color value is calculated as the second color value.   Output of color gamut conversion for moving the color value of the calculation target into the second color gamut with respect to the color value of the calculation target located in the first color gamut and the second color gamut in a predetermined color space 10. The color processing apparatus according to claim 1, further comprising a fourth calculation unit that calculates a color value when mapping is performed in which the color value is moved in a direction approaching the target point. 11. Computer
The first color when the first mapping is performed for moving the color value to be calculated located within the first color gamut and outside the second color gamut in a predetermined color space onto the outline of the second color gamut. First calculating means for calculating a value;
Second computing means for computing a second color value when performing a second mapping for bringing the first color value computed by the first computing means closer to a target point set in the second color gamut;
And, as the output color value of the color gamut conversion for moving the color value to be calculated into the second color gamut, between the first color value and the second color value on the predetermined color space, and The ratio of the color value to be calculated to the first distance corresponding to the distance between the first color value and the second color value and the second distance corresponding to the distance between the first color value, or the first color value and the first color value A color value at a position corresponding to a ratio of the second distance to the third distance corresponding to the distance between the specific color value corresponding to the intersection of the straight line passing through the second color value and the first color gamut and the first color value. A color processing program for functioning as third computing means for computing.

Images