JP2004120483A - Method and device for preparing color chart for profile preparation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device for preparing a profile preparation chart for obtaining a profile which has reduced the errors generated by interpolation as much as possible while considering the structure of the profile in the case of preparing a device profile to be used for color management. <P>SOLUTION: A color chart is prepared by successively executing a step for finding out the grating point coordinates of a multi-dimensional LUT for converting color space specific to a device into standard color space on the basis of a temporarily prepared device profile, a step for finding out the combination of specific device colors for providing these grating coordinates and a step for preparing a color chart including the found combination of specific device colors. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to color management that manages colors handled between a plurality of color devices by referring to a standard color space independent of devices, and is used when creating a device profile required for color management. The present invention relates to a method and an apparatus for creating a color chart.
[0002]
[Prior art]
A device profile (hereinafter referred to as an ICC profile) defined by an ICC (International Color Consortium) is used to enable appropriate conversion of color data between various color devices of various manufacturers such as a printer and a flat scanner for image input. Performing color management has become widespread.
[0003]
The ICC profile includes several tables that describe the correspondence between a unique color space handled by the device and a standard color space used for data conversion with another device.
[0004]
The standard color space (Profile Connection Space) used for the conversion of the color data includes a CIEXYZ space defined by the International Commission on Illumination (CIE) and a CIELAB (CIE1976L) ^* a ^* b ^* ) Space or the like is used. In the following description, a CIELAB space is used as a standard color space.
[0005]
In order to perform appropriate color management, an ICC profile for a target color device must first be created. At that time, a color chart for creating a profile is used.
[0006]
In the case of an output device such as a color printer, the profile creation chart is a color chart in which color patches printed in various combinations with color materials used by the printer are collected.
[0007]
Then, L on the CIELAB space obtained by measuring each printed patch with a colorimeter ^* a ^* b ^* A device profile is associated with a combination of device colors used to create each color patch.
[0008]
Here, as an aid for understanding the present invention described later, first, an outline of color management proposed by the ICC will be described. The ICC profile includes a standard color space, LMY, from the device color space, CMYK. ^* a ^* b ^* (Hereinafter, referred to as A2B tag) used for performing conversion to A (hereinafter referred to as A2B conversion) and L2 which is a standard color space ^* a ^* b ^* There are data (hereinafter, referred to as B2A tags) used for performing conversion into CMYK, which is the color space of the device (hereinafter, referred to as B2A conversion), each of which includes several look-up tables (hereinafter, LUTs and LUTs). Note)) and recorded.
[0009]
FIG. 7 illustrates how the color management module (Color Management Module: hereinafter, referred to as CMM) performs color conversion with reference to an ICC profile for A2B conversion. For the A2B conversion, the ICC profile includes four input one-dimensional LUTs for converting respective values of CMYK input from four input channels, a multidimensional LUT 100 for performing the A2B conversion itself, And three output channels L ^* a ^* b ^* There are prepared three output one-dimensional LUTs (not shown in FIG. 7) for performing the conversion for each value of. The four input one-dimensional LUTs and the three output one-dimensional LUTs are used for various purposes such as gradation correction, as is well known.
[0010]
The multidimensional LUT 100 stores a corresponding L for each of a predetermined number of grid points specified by coordinates in the CMYK color space. ^* a ^* b ^* It is a table in which values are recorded. Note that the multidimensional LUT 100 shown in FIG. 7 is a CMY three-dimensional LUT at K = 0.
[0011]
When performing the A2B conversion, the CMM first supplies the respective values of CMYK to four input-side one-dimensional LUTs to obtain one-dimensionally converted values C′M′Y′K ′. In FIG. 7, since the conversion function is in the form of y = x, the value directly input to the one-dimensional LUT is output.
[0012]
Next, the CMM refers to the value C′M′Y′K ′ of each channel after the one-dimensional conversion and the multidimensional LUT 100 to output L ^* a ^* b ^* Get the value of At this time, if the combination of C′M′Y′K ′ values input to the multidimensional LUT 100 corresponds to one of the grid points defined in the multidimensional LUT 100, the CMM records at the grid point. Done L ^* a ^* b ^* Although the values are output as they are, if the C′M′Y′K ′ values do not correspond to the grid points defined in the multidimensional LUT, the CMM outputs the multidimensional LUT 100 close to the C′M′Y′K ′ values. L recorded at some grid points of ^* a ^* b ^* By performing an interpolation operation based on the values, the L ′ corresponding to the C′M′Y′K ′ value is calculated. ^* a ^* b ^* Output the value.
[0013]
In the example shown in FIG. 7, C′M′Y′K ′ = (102, 102, 153, 0), which is assumed to correspond to one of the lattice points of the multidimensional LUT 100. Therefore, in this case, no interpolation operation is necessary, and the value L recorded at the corresponding grid point of the multi-dimensional LUT 100 is used. ^* a ^* b ^* = (63, -5, 20) is output as it is.
[0014]
As described above, it can be understood that the accuracy of the A2B conversion of the CMM, that is, the accuracy of the A2B conversion of the color management system generally depends on the number of grid points of the multidimensional LUT 100 and the accuracy of the interpolation operation.
[0015]
The above is the outline of the A2B conversion. Next, the outline of the B2A conversion will be described. FIG. 8 illustrates how the CMM proceeds with the color conversion with reference to the ICC profile for the B2A conversion, similarly to FIG.
[0016]
For the B2A conversion, the ICC profile includes L input from three input channels. ^* a ^* b ^* , Three-dimensional one-dimensional LUTs for converting the respective values, a multidimensional LUT 200 for performing the B2A conversion itself, and four outputs for performing the conversion on the respective values of the four output channels CMYK. A one-dimensional LUT is prepared. The three input one-dimensional LUTs and the four output one-dimensional LUTs are used for various purposes such as gradation correction, as is well known.
[0017]
The multidimensional LUT 200 is L ^* a ^* b ^* Is a table in which corresponding CMYK values are recorded for each of a predetermined number of grid points specified by the coordinates of the color space.
[0018]
When performing B2A conversion, the CMM first ^* a ^* b ^* Each value is given to three input-side one-dimensional LUTs, and the one-dimensionally converted value L ^* 'a ^* 'b ^* Get ' In FIG. 8, since the conversion function of the input one-dimensional LUT is in the form of y = x, the value input to the one-dimensional LUT is output as it is.
[0019]
Next, the CMM calculates the value L of each channel after the one-dimensional conversion. ^* 'a ^* 'b ^* And the multidimensional LUT 200 to obtain the value of CMYK to be output. At this time, L input to the multidimensional LUT 200 ^* 'a ^* 'b ^* 'If the combination of values corresponds to one of the grid points defined in the multidimensional LUT 200, the CMM outputs the CMYK values corresponding to the grid point as they are, ^* 'a ^* 'b ^* 'If the value does not correspond to a grid point defined in the multi-dimensional LUT, ^* 'a ^* 'b ^* 'Interpolation is performed based on CMYK values corresponding to some lattice points of the multidimensional LUT 200 close to the value, and the L input to the multidimensional LUT 200 ^* 'a ^* 'b ^* 'Output the CMYK value corresponding to the value.
[0020]
Finally, the CMM gives this CMYK value to each of the four output one-dimensional LUTs. Thus, the one-dimensionally converted C′M′Y′K ′ values are output. In FIG. 8, since the conversion functions of the output one-dimensional LUT are all in the form of y = x, the CMYK values input to the output one-dimensional LUT are directly output as C′M′Y′K ′ values. .
[0021]
As described above, it can be understood that the accuracy of the B2A conversion of the CMM, that is, the accuracy of the B2A conversion of the color management system generally depends on the number of grid points of the multidimensional LUT 200 and the accuracy of the interpolation operation.
[0022]
By the way, an ICC profile includes at least two tags, an A2B tag and a B2A tag, and both are usually created using profile creation software.
[0023]
In the case of an ICC profile of an output device such as a color printer, the A2B tag prints a color chart on an output device for which an ICC profile is created, measures each color patch of the color chart, , CMYK values used for printing, and L obtained by color measurement ^* a ^* b ^* It is created by associating the value with the profile creation software. The same applies to the creation of a B2A tag.
[0024]
Hereinafter, the processing of the profile creation software will be briefly described.
FIG. 9 is a block diagram illustrating a process for creating a multidimensional LUT of A2B tags of the profile creation software. Here, each of CMYK has 8 bits and 256 gradations, and the CMYK values are indicated by the gradation values in FIG.
[0025]
In FIG. 9, reference numeral 10 denotes a CMYK value used for printing each color patch of the color chart, and reference numeral 11 denotes L obtained by measuring the color of the corresponding color patch. ^* a ^* b ^* Value. In FIG. 9, L of the color patch of CMYK = (63, 95, 63, 0) ^* a ^* b ^* The value indicates that it is (L1, a1, b1). The same applies to other cases.
[0026]
The CMYK value 10 of the color chart and the colorimetric value 11 (L ^* a ^* b ^* ) Is input, the software performs a one-dimensional conversion process (step S1) on the CMYK value 10, and converts the CMYK value (this is referred to as C'M'Y'K 'for convenience). Get 12. The one-dimensional conversion in step S1 in this case is the same as the conversion by the input one-dimensional LUT applied to the input CMYK in the A2B conversion.
[0027]
Further, the software determines the grid points 101 on the CMYK color space. Here, it is assumed that grid points are arranged at the positions where the gradation values are 0, 51, 102, 153, 204, and 255 for each of the CMYK colors.
[0028]
The coordinates of the grid point are determined in advance by the profile creation software. In other words, according to the ICC rules, the grid point has a minimum value (minimum gradation value, that is, gradation value = 0 in this case) and a maximum value (maximum gradation value, in this case, (Gray scale value = 255), and it is only determined that the grid points are arranged at equal intervals between them. The number of grid points is not specified by the ICC, but is left to the profile creation software. . The number of CMYK gradations is also left to the profile creation software.
[0029]
Then, the software executes the C′M′Y′K ′ value 12 and the L, which is the colorimetric value 11 of the color chart. ^* a ^* b ^* Based on the correspondence with the values, an interpolation operation is performed to calculate L at each grid point. ^* a ^* b ^* The value 102 is obtained (step S2), and L obtained by the interpolation operation is obtained. ^* a ^* b ^* Record the value for each grid point.
[0030]
Therefore, if the C′M′Y′K ′ value obtained by one-dimensionally converting the CMYK value of a certain color patch in the color chart corresponds to the grid point defined in the multidimensional LUT 100, the grid point Has L obtained by measuring the color patch ^* a ^* b ^* A value based on the value can be recorded without performing an interpolation operation as described later.
Specifically, the profile creation software uses the colorimetric L ^* a ^* b ^* The value obtained by converting the value by the inverse function of the output one-dimensional LUT is recorded for the grid point. In any case, C′M′Y′K ′ obtained by one-dimensionally converting the CMYK values of the color patch is used. If the value just corresponds to a grid point defined in the multidimensional LUT 100, the L to be recorded at the grid point ^* a ^* b ^* The value can be obtained without interpolation.
[0031]
However, in most cases, such a thing cannot be expected. Therefore, for each grid point, the C'M'Y'K 'value 12 is used to determine the combination in the vicinity of the target grid point. A point is selected, an interpolation calculation process (step S2) is performed, and the result is stored in the L corresponding to the grid point. ^* a ^* b ^* The value will be 102. At this time, what kind of interpolation calculation is performed depends on the algorithm adopted by the profile creation software used.
Also at this time, the profile creation software uses the L ^* a ^* b ^* The value obtained by converting the value by the inverse function of the output one-dimensional LUT is recorded for the grid point.
[0032]
Thus, the multidimensional LUT 100 of the A2B tag is created. From the above, the interpolation calculation processing in step S2 in FIG. 9 is one of the important parts for determining the quality of the profile creation software. You can see that there is.
Given the same grid point arrangement, profile creation software is required to create a multidimensional LUT so as to most closely approximate the true conversion characteristics of the color device.
[0033]
In FIG. 9, what is indicated by 101 is “CMY grid point” because the value of K is 0, 51, 102, 153 as shown in the multidimensional LUT 100 of FIG. 7. , 204, and 255, a CMY three-dimensional LUT is created.
[0034]
The above is the processing of creating the multi-dimensional LUT 100 of the A2B tag. The creation of the multi-dimensional LUT 200 of the B2A tag is also performed in the same manner. The difference from creating A2B tags is that when creating a multidimensional LUT for B2A tags, the profile creation software ^* a ^* b ^* This is a point determined on a color space, and CMYK values corresponding to those grid points are obtained by interpolation from CMYK values of each color patch of the color chart.
[0035]
By the way, conventionally, as a color chart for profile creation, a chart recommended by a manufacturer of software used for profile creation is often used, but such a color chart is almost always “device dependent. The color patches in which the values of the basic colors of the “combination of color data to be performed” are sequentially changed at an appropriate ratio are arranged.
[0036]
For example, when the basic colors of the "device-dependent color data combination" are CMYK, each of these basic colors is set to an appropriate halftone step, for example, C, M, and Y are set at 20 halftone steps, and K is 25. A color chart in which color patches that are sequentially changed at halftone% steps are arranged, or a plurality of types are created by changing the percentage of halftone dots to be changed in some cases, and are combined as one profile creation color chart. is there.
[0037]
[Patent Document 1]
JP-A-2002-27272
[Patent Document 2]
JP-A-2002-94821
[0038]
[Problems to be solved by the invention]
However, such a conventionally used color chart has a structure of an ICC profile created from its colorimetric values, more specifically, a structure of a multidimensional LUT of an A2B tag and a structure of a multidimensional LUT of a B2A tag. Is not taken into account.
[0039]
Therefore, as described above, interpolation calculation is required to obtain the output color value recorded at the lattice point of the multi-dimensional LUT of the A2B tag and the output color value recorded at the lattice point of the multi-dimensional LUT of the B2A tag. However, it is unavoidable that an interpolation error occurs during the interpolation operation.
[0040]
Due to this interpolation error, if A2B conversion or B2A conversion is performed using an ICC profile created by measuring the color of a conventional color chart, there is a problem that an error occurs between the actual value and the actual value.
[0041]
The present invention solves the above-mentioned problem, and takes into account the structure of an ICC profile to be created, and converts the color of a device color space to a color of a standard color space or vice versa. An output color value to be recorded at a grid point of a multidimensional LUT for performing color conversion can be obtained as much as possible without an interpolation operation, so that a color conversion error at the time of color conversion can be minimized. It is an object of the present invention to provide a method and apparatus for creating a profile creation chart for obtaining a possible ICC profile.
[0042]
[Means for Solving the Problems]
In order to achieve the above object, a color chart creation device for creating a profile according to claim 1 is provided.
A color chart creation device for creating a profile for creating a color chart for creating a device profile for color conversion used for performing color conversion between devices that handle color images,
Processing for obtaining the number of grid points of a multidimensional LUT for performing conversion from a device-specific color space to a standard color space from a preliminarily created temporary device profile;
Processing for obtaining the characteristics of the one-dimensional LUT from the temporary device profile;
A process of obtaining a combination of device-specific colors giving coordinates of the grid points of the multi-dimensional LUT from the obtained number of grid points and the characteristics of the one-dimensional LUT
A process of arranging a color patch having a color of a selected combination of device-specific colors from among the obtained combinations of device-specific colors to create image data of a color chart
Is performed.
The color chart creation method for creating a profile according to claim 2,
A method for creating a color chart for creating a device profile for color conversion used to perform color data conversion between devices that handle color images,
A first step of creating a temporary device profile;
A second step of obtaining a device-specific color recorded at a lattice point of a multidimensional LUT for performing conversion from a standard color space to a device-specific color space;
A third step of obtaining a function of the output one-dimensional LUT of the temporary profile;
A fourth step of converting the value of the device-specific color recorded at the grid point obtained in the second step with a function of the output one-dimensional LUT obtained in the third step;
A fifth step of changing the value of the device-specific color combination obtained in the fourth step within a predetermined range to create a new device-specific color combination;
A sixth step of creating a color chart including the combination of device-specific colors determined in the fifth step;
Are sequentially performed.
The color chart creation apparatus for creating a profile according to claim 3,
A color chart creation device for creating a profile for creating a color chart for creating a device profile for color conversion used for performing color conversion between devices that handle color images,
A first process of obtaining a device-specific color recorded at a lattice point of a multidimensional LUT for performing conversion from a standard color space to a device-specific color space from a temporary device profile created in advance;
A second process for obtaining a function of the output one-dimensional LUT of the temporary profile;
A third process of converting a device-specific color recorded at a lattice point of the multidimensional LUT obtained in the first process by a function of the output one-dimensional LUT obtained in the second process;
A fourth process of changing the value of the device-specific color combination obtained in the third process within a predetermined range to create a new device-specific color combination;
A color chart in which color patches having colors of a combination of device-specific colors selected from the combination of device-specific colors obtained in the third process and the combination of device-specific colors obtained in the fourth process are arranged. Processing for Creating Image Data of Image
Is performed.
[0043]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the invention will be described with reference to the drawings.
First, before describing an embodiment of a color chart for profile creation according to the present invention, a preferred embodiment as a color chart for profile creation will be described. Here, each of CMYK has 8 bits and 256 gradations, and the CMYK values are indicated by the gradation values. Regarding a method of creating a color chart for profile creation described below, the present applicant has previously filed Japanese Patent Application Nos. 2001-285157 and 2001-285115.
[0044]
[1] Desirable form of color chart for A2B tag creation
As described above with reference to FIG. 9, if the C'M'Y'K 'values obtained by one-dimensionally converting the CMYK values of a certain color patch in the color chart exactly correspond to the grid points, the color patch L obtained by measuring ^* a ^* b ^* The value obtained by converting the value by the inverse function of the output one-dimensional LUT is recorded as it is for the lattice point. That is, in this case, no interpolation calculation is necessary.
[0045]
In other words, conversely, as the CMYK values of all the color patches constituting the color chart, the values obtained by converting the CMYK values with the input one-dimensional LUT are exactly the grid points defined in the multidimensional LUT. Means that any value can be used as the value of the coordinates.
[0046]
For example, as shown in FIG. 9, when grid points defined on the CMYK color space are arranged at the positions of gradation values 0, 51, 102, 153, 204, and 255 for each color of CMYK. In the table, the CMYK values of each color patch constituting the color chart, the C′M′Y′K ′ values after conversion by the input one-dimensional LUT, and the gradation values 0, 51, 102, 153, 204 , 255 may be used.
[0047]
A procedure of a method for creating a color chart for creating such an A2B tag will be described with reference to a flowchart shown in FIG.
First, an ICC profile is once created using a predetermined profile creation program (step S11). This will be referred to as a temporary profile. This temporary profile may be created by a conventionally used method. Therefore, the color chart used at this time may be an appropriate one, for example, the same as the one conventionally used.
[0048]
The provisional profile is once created in order to know the structure of the ICC profile, more specifically, in this case, the structure of the multidimensional LUT of the A2B tag. That is, as described above, the layout positions of the grid points, the number of CMYK gradations, and the like vary depending on the profile creation program, and therefore, what kind of structure of the ICC profile is created by the profile creation program to be used. You need to know.
Therefore, when an ICC profile has already been created, for example, when the ICC profile used so far is to be made more accurate, step S11 can be omitted.
[0049]
Next, the structure of the tentative profile created in step S11 is checked to obtain grid point information of the multidimensional LUT (step S12). Since the ICC profile is described in a binary format, it can be converted into text data using commercially available editor software and displayed on a display device.
[0050]
FIG. 2 shows an example of a screen on which the information of the multidimensional LUT is converted into text data and displayed. In FIG. 2, “A2B0” in the first line indicates that this is the structure of the multidimensional LUT of the A2B0 tag, and “lut8Type” indicates that each color is represented by 8 bits. . The A2B tag has three types of tags, A2B0, A2B1, and A2B2, and the A2B tag is a generic term for these tags. In the present invention, the numbers 0, 1, and 2 following "A2B" are essentially The procedure for creating a color chart is the same for any of these tags, and the description is omitted. This is the same for the B2A tag.
[0051]
In FIG. 2, “4 input channels” in the second row indicates that the input is four channels, that is, four pieces of color data are input, and “3 output channels” in the third row is “3 input channels”. It indicates that the output is three channels, that is, that there are three pieces of color data to be output, and “6 grid points in each CLUT dimension” on the fourth line indicates that the grid points are axes of each color in the multidimensional LUT. Is six. Therefore, in this case, the multidimensional LUT has a structure indicated by 100 in FIGS.
[0052]
In FIG. 2, items below the matrix are not essential in the present invention, and therefore, description thereof is omitted.
[0053]
By this processing, the positions of the lattice points on each color axis of CMYK of the multidimensional LUT can be known. That is, in the ICC profile, as described above, in the case of FIG. 2, there is a rule that grid points are set at 0 and 255, and grid points are placed at positions on the color axis that divide the grid points equally. , The positions of the grid points on each color axis can be known.
[0054]
Next, the input-side one-dimensional LUT of the temporary profile is checked, and an inverse function of the input-side one-dimensional LUT is obtained (step S13).
[0055]
Next, the value of the position of the grid point on the color axis of the color space formed by the multi-dimensional LUT obtained in step S12 is converted by the inverse function of the input-side one-dimensional LUT obtained in step S13. CMYK values to be input to the input-side one-dimensional LUT are obtained (step S14). As a result, the CMYK values on the input side of the input one-dimensional LUT that give the grid point positions on the color axis of the multidimensional LUT are obtained.
[0056]
Next, a combination of the CMYK values obtained in step S14 is obtained, all or some of the combinations are used as the color of the color patch, and the color patches are arranged in a predetermined form to create a color chart ( Step S15).
[0057]
When determining the colors of the color patches from the combinations of the CMYK values obtained in step S15, it is apparently desirable that the color patches of the colors of all the combinations of these CMYK values can be arranged on the color chart. In this way, all grid points of the multi-dimensional LUT have L ^* a ^* b ^* This is because the value obtained by converting the value using the inverse function of the output one-dimensional LUT is recorded as it is without interpolation.
[0058]
However, when the number of color patches constituting the color chart increases, not only does the proportion of the number of output papers used to output the color chart increase in proportion to the increase, but also the output time of the color chart and the color measurement time of the color patch Therefore, it is practical to limit the number of color patches to be arranged on the color chart to about 1,000.
[0059]
Therefore, when the number of combinations of CMYK values obtained in step S15 greatly exceeds about 1,000, it is preferable to select a part of those combinations and determine the color of the color patch. In the case of the multidimensional LUT shown in FIG. 2, the number of grid points, that is, the number of combinations of CMYK values is six. ⁴ = 1296, but at this level, the colors of all combinations of CMYK values may be made into a color chart as color patches.
[0060]
Of course, when a part of the combination of the CMYK values is selected to be the color of the color patch, the L recorded at the grid point corresponding to the color of the combination of the CMYK values that are not selected. ^* a ^* b ^* The value is determined by interpolation as in the conventional case.
[0061]
However, when selecting some of the combinations of CMYK values, it is desirable that the colors of the color patches are not biased. If there is a bias in the color of the color patch, L recorded at a grid point in an area where many colors are adopted as the color of the color patch ^* a ^* b ^* The value becomes highly accurate, and L is recorded at a grid point in an area not adopted as a color of a color patch. ^* a ^* b ^* This is because the value has low accuracy, and as a result, when A2B conversion is performed, the color conversion accuracy is biased.
[0062]
When an ICC profile is created from a color chart for profile creation, an LC independent of a device converted by the ICC profile is used. ^* a ^* b ^* The value area is largely related to gamut compression during color conversion. It is known that the information characterizing this region can be obtained by measuring the most vivid color in each hue, for example, a solid color of CMY among color data of CMYK values depending on the device. . Therefore, it is desirable to add a color patch of a color that gives the information of the outermost shell to a color chart, with the most vivid color set in each hue as the outermost information.
[0063]
After the color chart is created as described above, the color patches of the color chart are measured, and the CMYK values of the color chart and the color measurement values L of the color chart are measured in the same manner as described with reference to FIG. ^* a ^* b ^* Is given to the profile creation software, and the A2B tag is created again.
[0064]
As described above, in this color chart creation method, a step of creating a provisional device profile, a step of obtaining the number of grid points of a multidimensional LUT for performing conversion from a device-specific color space to a standard color space, Obtaining a characteristic of the input one-dimensional LUT from the profile; and obtaining a combination of device-specific colors giving coordinates of the grid points of the multidimensional LUT from the obtained number of grid points and the characteristics of the input one-dimensional LUT. Since a step of creating a color chart including a combination of device-specific colors is performed in order, an ICC profile created using the color chart created by this method, more specifically, a multidimensional LUT of an A2B tag, Output color values recorded at the grid points of the multidimensional LUT are obtained by the interpolation calculation processing S2 shown in FIG. Value is less minute, and be accurate.
[0065]
Therefore, the accuracy of color conversion in a system that handles color data using the ICC profile is improved. In other words, in the color management system proposed by the ICC, an error in modeling a characteristic unique to a color device when a profile is created, that is, an error generated by the interpolation calculation S2 in FIG. And two types of errors that occur when color conversion calculation is actually performed with reference to the respective profiles of the output device and the output device. However, if the color chart created by the above method is used, the former method can be used. The error can be minimized.
[0066]
[2] Desirable form of color chart for B2A tag creation
Next, a description will be given of a form of a desirable color chart for creating a profile for improving the color conversion accuracy of the B2A conversion.
[0067]
Now consider a device, and with this device, L ^* a ^* b ^* It is assumed that CMYK = (c1, m1, y1, k1) that truly corresponds to = (L1, a1, b1). That is, in the B2A conversion, when (L1, a1, b1) is input to the input one-dimensional LUT, (c1, m1, y1, k1) should be output from the output one-dimensional LUT.
[0068]
At this time, in the color chart, there is a color patch of a color composed of the combination of (c1, m1, y1, k1), the colorimetric value of which is (L1, a1, b1). If the value converted by the one-dimensional LUT corresponds to the lattice point of the multi-dimensional LUT of the B2A tag, (c1, m1, y1, k1) is added to this lattice point by the inverse of the one-dimensional LUT on the output side. The value converted by the function will be recorded. Therefore, in this case, it is not necessary to perform an interpolation operation when recording the CMYK values at the grid points.
[0069]
That is, the CMYK value that gives the color of the color patch is the colorimetric L ^* a ^* b ^* The value is L which corresponds to the CMYK value of the device. ^* a ^* b ^* Value, and its L ^* a ^* b ^* When a value is converted by an input-side one-dimensional LUT, it is desired that the value corresponds to a lattice point of a multidimensional LUT.
In such a case, a value obtained by converting the CMYK value that gives the color of the color patch by the inverse function of the output one-dimensional LUT is recorded at the corresponding grid point without interpolation. The accuracy of the multidimensional LUT can be improved.
At the time of B2A, the input side one-dimensional LUT is ^* a ^* b ^* When a value is input, the corresponding one-dimensional LUT is ^* a ^* b ^* A CMYK value that truly corresponds to the value will be output.
[0070]
However, considering a device, L ^* a ^* b ^* The true correspondence between values and CMYK values is generally unknown.
Therefore, here, the CMYK values that determine the colors of the color patches as described above are determined in consideration of the structure of the multidimensional LUT of the B2A tag by the method described below.
[0071]
A method of determining CMYK values that determine the color of a color patch is roughly as follows.
First, a temporary device profile is temporarily created for the target device, a combination of CMYK values recorded at the lattice points is known by focusing on one lattice point of the multidimensional LUT, and the CMYK values are output to the output side 1. Conversion is performed using a function of the dimensional LUT.
Since the CMYK values obtained in this manner are determined by the provisional profile, even if the color given by the CMYK values is measured, the color measurement L ^* a ^* b ^* The value is L, which truly corresponds to this CMYK value. ^* a ^* b ^* It is likely that this will not be the value. This is because the provisional profile is created from a conventionally used color chart, and thus has a problem in accuracy as described above.
[0072]
However, even if the temporary profile has a problem in accuracy, the CMYK values read from the grid points of the multidimensional LUT of the temporary profile and converted by the output one-dimensional LUT are L ^* a ^* b ^* It can be expected that the value does not deviate significantly from the CMYK value that truly corresponds to the value, and will be close to the CMYK value that truly corresponds to the value.
[0073]
Therefore, several combinations of CMYK values having values close to the CMYK values which are read from the grid points and converted by the one-dimensional LUT on the output side are created, and the color composed of the combinations of the CMYK values is selected as a color patch candidate color. That is.
Then, among the combinations of those CMYK values, the colorimetry L of the color is included. ^* a ^* b ^* Value truly corresponds to the color of the combination of CMYK values, and the colorimetric L ^* a ^* b ^* When the values are converted by the input-side one-dimensional LUT, it is expected that a value corresponding to a lattice point of the multidimensional LUT is included.
[0074]
Then, the above operation is performed for the CMYK values recorded at all the grid points of the multidimensional LUT, candidate colors of all the color patches are obtained, and all or a part of the candidate colors are selected to select the color patches. The CMYK values that determine the colors are determined, and they are arranged in a predetermined manner to create a color chart.
[0075]
Hereinafter, a procedure of a method for creating a color chart for creating such an A2B tag will be described with reference to a flowchart shown in FIG.
First, an ICC profile is once created using a predetermined profile creation program (step S21). This will be referred to as a temporary profile. This temporary profile may be created by a conventionally used method. Therefore, the color chart used at this time may be an appropriate one, for example, the same as the one conventionally used.
[0076]
Next, the structure of the tentative profile created in step S21 is checked to obtain grid point information of the multidimensional LUT 200 (step S22). As described above, since the ICC profile is described in a binary format, it can be converted into text data using commercially available editor software and displayed on a display device.
[0077]
FIG. 4 shows an example of a screen on which the information of the multidimensional LUT 200 is converted into text data and displayed. In FIG. 4, "B2A0" in the first line indicates that this is the structure of the multidimensional LUT of the B2A0 tag, and "lut8Type" indicates that each color is represented by 8 bits. .
[0078]
In FIG. 4, “3 input channels” in the second row indicates that there are three input channels, that is, three pieces of color data are input, and “4 output channels” in the third row is “4 input channels”. This indicates that the output is four channels, that is, that there are four pieces of color data to be output. In the fourth row, "17 grid points in each CLUT dimension" indicates that in the multidimensional LUT, the grid points are the axes of each color. Indicates that the number is 17. In FIG. 4, items below the matrix are not essential in the present invention, and a description thereof will be omitted.
By the processing of this step, the L of the multidimensional LUT is ^* a ^* b ^* The position of the lattice point on each color axis can be found.
[0079]
Next, the function of the output side one-dimensional LUT of the temporary profile is obtained (step S23).
[0080]
Next, the CMYK values recorded at the grid points are obtained for all the grid points of the multidimensional LUT (step S24).
The B2A tag has L for all grid points of the multidimensional LUT. ^* a ^* b ^* Since the correspondence between the values and the CMYK values is described, the CMYK values recorded at each grid point can be known by referring to the correspondence.
[0081]
Next, the CMYK values of the respective grid points obtained in step S24 are converted by the function of the output one-dimensional LUT obtained in step S23 to obtain CMYK values to be output of the output one-dimensional LUT (step S25). ).
[0082]
Next, for each of the combinations of CMYK values obtained in step S25, the CMYK values are changed within a predetermined range, and further several combinations of CMYK values are created (step S26).
For example, assuming that one combination of CMYK values obtained in step S25 is (c, m, y, k), the value of c in this combination is changed within a range of ± 2 in tone value, for example. And (c-2, m, y, k), (c-1, m, y, k), (c + 1, m, y, k), and (c + 2, m, y, k) You can create a combination of the two. Similarly, a new combination can be created by changing the values of M, Y, and K within a predetermined range. Of course, instead of changing only one color value in CMYK, two or more color values may be changed simultaneously.
[0083]
In this way, several new combinations of CMYK values are created from one combination of CMYK values obtained by converting the CMYK values recorded at the lattice points of the multidimensional LUT by the output one-dimensional LUT, This process is performed for all combinations of CMYK values obtained in step S25.
[0084]
Then, a combination of the CMYK value obtained by converting the CMYK value recorded at the grid point obtained in step S25 by the output one-dimensional LUT and the CMYK value newly created in step S26 is used as the CMYK value for giving the color of the color patch. Is a candidate color.
[0085]
As a result, the candidate colors of the color patches are determined, and all or a part of the color patches of the determined candidate colors are arranged in a predetermined form, and a color chart for profile creation is created (step S27).
[0086]
Create a color chart in which color patches of the above colors are arranged, measure each color patch, give it to profile creation software, and create a B2A tag again. Can obtain an accurate multidimensional LUT.
[0087]
That is, as described above, among the combinations of the CMYK values of the color patches of the color chart as described above, the colorimetric L of that color is included. ^* a ^* b ^* Value truly corresponds to the color of the combination of CMYK values, and the colorimetric L ^* a ^* b ^* When the value is converted by the input-side one-dimensional LUT, it can be expected that a value corresponding to the lattice point of the multi-dimensional LUT is included. Even if an interpolation operation is required to obtain the CMYK values to be recorded, since the interpolation error due to the interpolation operation can be expected to be smaller than before, the accuracy of the B2A tag, specifically, the accuracy of the multi-dimensional LUT, is high. Therefore, it can be improved as compared with the conventional case.
[0088]
In addition, a multi-dimensional LUT with higher accuracy can be obtained by repeating the process of creating a color chart as described above and re-creating a device profile several times.
[0089]
In the above method, it is naturally desirable that all the color patches of the combination of CMYK values obtained in step S25 and the color patches of the combination of CMYK values newly created in step S26 can be arranged on the color chart. However, as described above, it is realistic to limit the number of color patches to about 1,000, and according to the above-described method, a plurality of CMYK value combinations obtained in step S25 is Since a color patch of a color of the combination of CMYK values is created, as shown in FIG. ³ If there are 4913, it is impossible to arrange color patches corresponding to all grid points on the color chart.
[0090]
Therefore, in such a case, a part of the candidate colors is selected to determine the color of the color patch. In this case, the color of the color patch is not biased. Desirably, as described above.
[0091]
After the color chart is created as described above, the color patches of the color chart are measured, and the CMYK values of the color chart and the color measurement values L of the color chart are measured. ^* a ^* b ^* Is given to the profile creation software, and the B2A tag is created again.
[0092]
As described above, in this color chart creation method, the first step of creating a temporary device profile and the conversion to the device-specific color space from the standard color space are recorded at the lattice points of the multidimensional LUT. A second step of obtaining a device-specific color, a third step of obtaining a function of an output-side one-dimensional LUT of a temporary profile, and a device-specific color value recorded at a grid point obtained in the second step. A fourth step of converting with a function of the output one-dimensional LUT obtained in the third step, and changing a value of a combination of device-specific colors obtained in the fourth step within a predetermined range, to obtain a new device-specific color. The fifth step of creating a color combination and the sixth step of creating a color chart including the combination of device-specific colors determined in the fifth step are performed in order. ICC profile created using the color chart created by this method, more multidimensional LUT specific to B2A tag, the error caused by the interpolation calculation process only small amount, a precise profile. Therefore, the accuracy of color conversion in a system that handles color data using this profile is improved.
[0093]
The method of creating a color chart for creating an A2B tag and the method of creating a color chart for creating a B2A tag have been described above. However, these color charts may be created separately, or a color chart for creating an A2B tag and a color chart for creating a B2A tag. A color chart including both of the tag creation color charts may be created and used as a device profile creation chart.
[0094]
Next, an embodiment of a profile creation color chart creation apparatus (hereinafter, simply referred to as a creation apparatus) according to the present invention will be described. This creating apparatus creates a color chart by performing the processing of the above-described profile creating color chart creating method.
[0095]
FIG. 5 is a diagram showing an embodiment of the creation apparatus according to the present invention, in which 1 is a control device, 2 is a display device, 3 is an input device, 4 is a storage device, and 5 is an output interface (hereinafter simply referred to as an output interface). I / F).
[0096]
The control device 1 is composed of a CPU and its peripheral circuits. Editor software, spreadsheet software for reading the internal information of the ICC profile, specifically, the information of the A2B tag and the information of the B2A tag, the color of the determined color Software such as image generation software for creating color chart image data by arranging patches in a predetermined form is installed. Note that commercially available image generation software can be used.
[0097]
The display device 2 functions as a monitor, and is constituted by appropriate display means such as a CRT color display device. The input device 3 includes a keyboard, a mouse, and the like. The control device 1, the display device 2, and the input device 3 constitute an interactive graphical user interface (GUI).
[0098]
The storage device 4 is configured by storage means such as a hard disk device. The I / F 5 provides a target device such as a printer or a printing machine to which the multi-dimensional LUT of the A2B tag and the multi-dimensional LUT of the B2A tag are to be re-created with higher accuracy. An interface for outputting image data.
[0099]
The creation device as shown in FIG. 5 can be constituted by a personal computer system or the like.
[0100]
Hereinafter, the operation will be described. Here, it is assumed that the temporary profiles created in step S11 of FIG. 1 and step S21 of FIG. 3 are stored in the storage device 4 in advance.
[0101]
First, an operation for creating a color chart for A2B tag creation will be described.
The operator selects a menu for creating an A2B tag creation color chart using the GUI, and specifies the A2B tag of the target device as a file name to be read. Thereby, the control device 1 activates the editor software and displays the information of the A2B tag on the display device 2. Thereby, the information of the A2B tag as shown in FIG. 2 is displayed on the screen of the display device 2.
[0102]
Then, at this time, the control device 1 automatically performs the following processing.
(I) Recognize the contents of the multidimensional LUT of the A2B tag. That is, the number of color gradations, the number of input channels, the number of output channels, and the number of grid points on each color axis of the multidimensional LUT are recognized.
(Ii) Find all combinations of CMYK values that give grid points defined in the multidimensional LUT.
(Iii) Recognize the structure of the one-dimensional LUT on the input side of the multi-dimensional LUT from the information of the A2B tag and obtain the inverse function.
(Iv) The CMYK values of all the grid points obtained in (ii) are converted by the inverse function of the input one-dimensional LUT obtained in (iii).
(V) All combinations of CMYK values obtained by the conversion in (iv) are displayed in a list on the display device 2 by predetermined software, for example, spreadsheet software. Each combination of the CMYK values displayed in the list is a candidate for a color of a color patch.
[0103]
FIG. 6 shows an example of a list of combinations of CMYK values displayed on the display device 2 at this time. In FIG. 6, the A2B tag is as shown in FIG. 2, and the one-dimensional LUT on the input side of the multi-dimensional LUT has the form of y = x. Each row in FIG. 6 shows each combination of CMYK values obtained by the conversion in the above process (iv), that is, a combination of CMYK values that are color patch color candidates. , A column shows the value of C, B column shows the value of M, C column shows the value of Y, and D column shows the value of K.
[0104]
When the list of combinations of CMYK values is displayed in this manner, the operator selects all or some of the combinations of CMYK values listed as colors of the color patches to be arranged on the color chart. As described above, it is realistic to set the number of color patches to about 1,000, so if the number of combinations of CMYK values displayed as described above is about 1,000, select all combinations. However, when the number of combinations of CMYK values displayed in the list greatly exceeds about 1000, an appropriate combination of CMYK values may be selected. At this time, it is desirable that the colors of the color patches are not biased, as described above. Further, as described above, it is desirable that a set of the most vivid colors in each hue be set as the outermost contour information, and a color patch of a color giving the outermost shell information be added to the color chart. It is known that the CMYK value of the most vivid color in each hue serving as the outermost information is a unique value, so the operator determines the most vivid color in each hue from the CMYK values displayed in the list. You can choose.
[0105]
Then, when the operator selects a combination of CMYK values to be used as the color of the color patch, the control device 1 stores all data of the selected combination of CMYK values in the storage device 4, and further stores the image generation software. Is started to generate color patch images based on the selected combination of CMYK values, and arrange them in a predetermined order to create color chart image data. Then, the control device 1 displays the created color chart image on the display device 2 and outputs the created color chart image data to the target device via the I / F 5. Thus, when the target device is a printer, the printer outputs a color chart based on the image data received via the I / F 5. If the target device is a printing machine, a printing plate of a color chart is created based on the image data received via the I / F 5.
[0106]
The size of each color patch is set in the control device 1 in advance. Also, in what order the color patches are arranged, for example, the color patches are arranged vertically in ascending order of the number of the line displayed in the list, and when the predetermined number is arranged vertically, the color patches are arranged vertically in the next column. It is set in the control device 1 in advance.
[0107]
As described above, the control device 1 recognizes the contents of the A2B tag of the temporary profile, automatically performs the processing from step S12 to step S14 in FIG. 1, and obtains the grid points of the multidimensional LUT obtained in step S14. Are displayed in a list, and when a combination of CMYK values used as a color of a color patch is selected, the selected CMYK value is selected. Based on the combination of the CMYK values, the image of the color patch is generated and arranged in a predetermined order to generate the image data of the color chart.
[0108]
As described above, this creating apparatus can automatically create a color chart that can create an ICC profile with high color conversion accuracy of A2B conversion.
[0109]
Next, an operation for creating a color chart for creating a B2A tag will be described.
The operator selects a menu for creating a color chart for creating a B2A tag using the GUI, and designates the B2A tag of the target device as a file name to be read. Thereby, the control device 1 activates the editor software and displays the information of the B2A tag on the display device 2. Thereby, the information of the B2A tag as shown in FIG. 4 is displayed on the screen of the display device 2.
[0110]
Then, at this time, the control device 1 automatically performs the following processing.
(A) Recognize the contents of the multidimensional LUT of the B2A tag. That is, the number of color gradations, the number of input channels, the number of output channels, and the number of grid points on each color axis of the multidimensional LUT are recognized.
(B) The function of the output one-dimensional LUT of the B2A tag is obtained.
(C) With respect to all grid points defined in the multidimensional LUT, CMYK values recorded at the grid points are obtained.
As described above, since the CMYK values recorded at each grid point are described in the B2A tag, the control device 1 interprets the description to obtain the CMYK values recorded at each grid point. Can be recognized.
(D) The CMYK values recorded at the grid points obtained in (c) are converted by the output one-dimensional LUT function obtained in (b).
(E) Paying attention to one combination of CMYK values converted by the function of the output one-dimensional LUT obtained in (D), changing the CMYK value of the combination of the CMYK values within a predetermined range, and creating a new CMYK value. Create a value combination. Here, the range in which the CMYK values are changed is set in the control device 1 in advance. This processing is as described above.
(F) The processing of (e) is performed for all combinations of CMYK values obtained in (d).
(G) The combination of the CMYK values obtained in (d) and the new combination of CMYK values created in (f) are displayed in a list on the display device 2 using predetermined software, for example, spreadsheet software. This list display is performed in the same manner as shown in FIG. Then, each combination of the CMYK values displayed in the list is a candidate for a color of a color patch.
[0111]
When the list of combinations of CMYK values is displayed in this manner, the operator selects all or some of the combinations of CMYK values listed as colors of the color patches to be arranged on the color chart. At this time, when only a part is selected, it is desirable that the colors of the color patches are not biased, as described above.
[0112]
Then, when the operator selects a combination of CMYK values to be used as the color of the color patch, the control device 1 stores all data of the selected combination of CMYK values in the storage device 4, and further stores the image generation software. Is started to generate color patch images based on the selected combination of CMYK values, and arrange them in a predetermined order to create color chart image data.
[0113]
Then, the control device 1 displays the created color chart image on the display device 2 and outputs the created color chart image data to the target device via the I / F 5. Accordingly, when the target device is a printer, the printer outputs a color chart based on the image data received via the I / F 5. If the target device is a printing machine, a printing plate of a color chart is created based on the image data received via the I / F 5.
[0114]
As described above, the size of each color patch and the arrangement order of the color patches are set in the control device 1 in advance.
[0115]
As described above, the control device 1 recognizes the content of the B2A tag of the provisional profile, automatically performs the processing from step S22 to step S26 in FIG. 3, and combines the CMYK values as the color patch color candidates. Are displayed in a list, and when a combination of CMYK values to be used as the color of the color patch is selected, a color patch image is generated based on the selected combination of CMYK values, and the images are arranged in a predetermined order and It creates the image data for the chart.
[0116]
As described above, this creating apparatus can automatically create a color chart that can create an ICC profile with high color conversion accuracy of B2A conversion.
[Brief description of the drawings]
FIG. 1 is a flowchart for explaining a procedure of a method of creating a color chart for profile creation for improving the accuracy of A2B conversion.
FIG. 2 is a diagram showing an example of a screen on which information of a multidimensional LUT of an A2B tag is converted into text data and displayed.
FIG. 3 is a flowchart illustrating a procedure of a method of creating a color chart for profile creation for improving the accuracy of B2A conversion.
FIG. 4 is a diagram showing an example of a screen on which information of a multidimensional LUT of a B2A tag is converted into text data and displayed.
FIG. 5 is a diagram showing an embodiment of a profile creation color chart creation apparatus according to the present invention.
FIG. 6 is a diagram showing an example of a list display of combinations of CMYK values that are color candidate colors of a color patch displayed on the display device 2.
FIG. 7 is a diagram for explaining A2B conversion.
FIG. 8 is a diagram for explaining B2A conversion.
FIG. 9 is a diagram for explaining the operation of profile creation software.
[Explanation of symbols]
1 ... Control device
2 ... Display device
3. Input device
4. Storage device
5 Output interface (I / F)

Claims (3)

A color chart creation device for creating a profile for creating a color chart for creating a device profile for color conversion used for performing color conversion between devices that handle color images,
Processing for obtaining the number of grid points of a multidimensional LUT for performing conversion from a device-specific color space to a standard color space from a preliminarily created temporary device profile;
Processing for obtaining the characteristics of the one-dimensional LUT from the temporary device profile;
A process of obtaining a combination of device-specific colors giving coordinates of the grid points of the multi-dimensional LUT from the obtained number of grid points and the characteristics of the one-dimensional LUT
A profile creation process characterized by performing a process of arranging a color patch having a color of a selected device unique color combination from among the obtained device unique color combinations and creating image data of a color chart. Color chart creation device. A method for creating a color chart for creating a device profile for color conversion used to perform color data conversion between devices that handle color images,
A first step of creating a temporary device profile;
A second step of obtaining a device-specific color recorded at a lattice point of a multidimensional LUT for performing conversion from a standard color space to a device-specific color space;
A third step of obtaining a function of the output one-dimensional LUT of the temporary profile;
A fourth step of converting the value of the device-specific color recorded at the grid point obtained in the second step with a function of the output one-dimensional LUT obtained in the third step;
A fifth step of changing the value of the device-specific color combination obtained in the fourth step within a predetermined range to create a new device-specific color combination;
A color chart creating method for creating a color chart including a combination of device-specific colors obtained in the fifth step. A color chart creation device for creating a profile for creating a color chart for creating a device profile for color conversion used for performing color conversion between devices that handle color images,
A first process of obtaining a device-specific color recorded at a lattice point of a multidimensional LUT for performing conversion from a standard color space to a device-specific color space from a temporary device profile created in advance;
A second process for obtaining a function of the output one-dimensional LUT of the temporary profile;
A third process of converting a device-specific color recorded at a lattice point of the multidimensional LUT obtained in the first process by a function of the output one-dimensional LUT obtained in the second process;
A fourth process of changing the value of the device-specific color combination obtained in the third process within a predetermined range to create a new device-specific color combination;
A color chart in which color patches having colors of a combination of device-specific colors selected from the combination of device-specific colors obtained in the third process and the combination of device-specific colors obtained in the fourth process are arranged. A profile creation color chart creating apparatus for performing a fifth process of creating the image data.

Images