JP2003101802A - Method for preparing profile preparation chart - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for preparing a profile preparation chart for obtaining a profile which has reduced the errors generated by interpolation as much as possible while considering he structure of the profile in the case of preparing a device profile to be used for color management. 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 standard color space into color space specific to a device, 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.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to color management that manages colors handled between a plurality of color devices by comparing them with a standard color space that does not depend on devices, and a device profile required for color management. The present invention relates to a method of creating a color chart used when creating a.

[0002]

2. Description of the Related Art ICC (Internat) has been developed to enable appropriate exchange of color data between various color devices of various manufacturers such as printers and flat panel scanners for image input.
It is widely used to perform color management using a device profile (hereinafter referred to as ICC profile) defined by the ional Color Consortium. The ICC profile contains several tables that describe the correspondence between the native color space handled by the device and the standard color space used for data exchange with other devices. In order to perform appropriate color management, first, an ICC profile of a target color device must be created for that device. A color chart for profile creation is used at that time.

As a standard color space (Profile Connection Space) used for exchanging color data, there are CIEXYZ space and CIELAB (CIELAB (C) defined by the International Commission on Illumination (CIE).
IE1976L * a * b *) space is used.

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 by various combinations of color materials used by the printer are collected. The device profile is a correspondence between the L * a * b * values in the CIELAB space measured by the colorimeter for the patch and the product of the device colors used to create the color patch.

[0005]

The chart created in this way is not created in consideration of the structure of the profile created from the colorimetric data. Therefore, the value interpolated from the colorimetric data of the chart is applied to the information for color conversion described in the profile, and when color conversion is performed using the profile, the error from the actual value Will occur.

The present invention has been made in consideration of such a problem, and in order to obtain a profile in which an error caused by interpolation is made as small as possible in consideration of the structure of a profile to be created. It is an object to provide a method for creating a profile creation chart.

[0007]

A first invention for solving the problem is to create a color chart for creating a device profile for color conversion used for exchanging color data between devices that handle color images. a method, target
A step of obtaining grid point coordinates of a multi-dimensional LUT for converting from a quasi-color space to a device-specific color space, a step of obtaining a combination of device-specific colors giving these grid coordinates, and a combination of the obtained device-specific colors. It is a method of creating a color chart in which the steps of creating the included color chart are sequentially performed.

Since this method reduces the error caused by the interpolation calculation performed when the profile creating software constructs the multi-dimensional LUT, color management with higher color conversion accuracy than a profile created using a normal color chart is performed. To be realized.

Another aspect of the first invention for solving the problem is a method for creating a color chart for creating a device profile for color conversion used for exchanging color data between devices that handle color images. Yes, the step of creating a temporary device profile, is it a standard color space?
Multi-dimensional LUT for conversion from device to device-specific color space
, The step of obtaining the characteristic of the one-dimensional LUT from the temporary device profile, the step of obtaining the characteristic of the one-dimensional LUT from the provisional device profile, and the combination of the device-specific color that gives the coordinates of the lattice point of the multi-dimensional LUT from the obtained number of the lattice points and the characteristic of the one-dimensional LUT It is a color chart creating method in which the steps and the steps of creating a color chart including a combination of the obtained device-specific colors are sequentially performed.

This method is used when necessary information cannot be obtained unless a temporary profile is created. This method also reduces the error caused by the profile creation software performing the interpolation calculation, so that color management with higher color conversion accuracy than a profile created using an ordinary color chart is realized.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the embodiments, IC
The outline of the color management advocated by C will be described. I
For CC profile, conversion from CMYK or device-specific color space to L * a * b * or standard color space (A below)
Data used to perform 2B conversion) and L * a * b *
There is data used to convert (standard color space) to CMYK (device-specific color space) (hereinafter referred to as B2A conversion), each of which is composed and recorded in the form of several LUTs. There is.

FIG. 3 shows CMM (Colo) for A2B conversion.
r Management Module) refers to the ICC profile and illustrates how to proceed with color conversion.
For A2B conversion, the ICC profile contains four 1's for performing linear conversion of each input channel CMYK.
A dimension LUT 100, a multidimensional LUT for performing A2B conversion itself, and three one-dimensional LUTs for performing linear conversion of each of the output channels L * a * b * are prepared. The multi-dimensional LUT is a table in which corresponding L * a * b * values are recorded for each of a predetermined number of grid points specified by CMYK coordinates.

FIG. 3 shows three one-dimensional L of output channels.
The UT is omitted in the drawing. When performing A2B conversion, C
The MM first inputs the CMYK values into the four Ls on the input side.
It is given to the UT to obtain a one-dimensional linearly transformed value C'M'Y'K '. In FIG. 3, since the linear conversion function has the form of y = x, the value is output as it is. Next, referring to the value C'M'Y'K 'after the one-dimensional linear conversion of each channel and the multi-dimensional LUT,
Get the value of L * a * b to be output. At this time, CMM is C'M'Y '
L * a * b * is obtained by performing an interpolation operation based on the data of some grid points of the multidimensional LUT close to the K'value. Therefore CMM
The accuracy of the color conversion of (and thus of its color management system) generally depends on the number of grid points of the multi-dimensional LUT and the accuracy of the interpolation operation. Figure 3 shows that C'M'Y'K '=
In the case of (102,102,153,0), since it is just a grid point at this time, interpolation calculation is not necessary, that is, L * a * b * = (63, -5,20) is recorded in the multidimensional LUT. It is output as is.

An ICC profile of an output device such as a color printer is obtained by associating the CMYK value of each printer that gives each color patch when a color chart is output with the L * a * b * value obtained by measuring the color of the patch. Create by giving the data of many combinations of them to the profile creation software. FIG. 4 is a block diagram for explaining the processing of the profile creation software.

Reference numeral 10 is a CMYK value of the color chart. 1
1 is the L * a * b * value obtained by measuring the color of the corresponding color patch. The CMYK value 10 is subjected to an appropriately determined one-dimensional linear conversion process (S1), and C ′ after the one-dimensional linear conversion shown in 12 is obtained.
It becomes M'Y'K 'value. Reference numeral 100 denotes a multidimensional LUT table. The CMYK lattice points 101 of the multidimensional LUT are predetermined by the profile creation software. Therefore, there is
If the CMYK grid points are included in the C'M'Y'K 'value 12, the L * a * b * value corresponding to the combination is extracted from 11 and the corresponding L * a * of the multidimensional LUT is obtained. The b * value may be 102. However, in most cases, such coincidence does not occur, and for each grid point, from the C'M'Y'K 'value of 12, how many combinations in the neighborhood of the target grid point Is selected, interpolation calculation processing (S2) is performed, and the result is used as the corresponding L * a * b * value 102
And At this time, how to actually perform the interpolation calculation depends on the algorithm adopted by the profile creation software.

This interpolation calculation process (S2) is one of the important parts for determining the quality of the profile creation software. When the same grid point arrangement is given, the multidimensional LUT is best approximated to the true conversion characteristic of the color device.
It is required for profile creation software to create.

FIG. 5 shows a CMM (Colo) for B2A conversion.
FIG. 4 is a diagram similar to FIG. 3 showing how an r Management Module) refers to an ICC profile to proceed with color conversion. For B2A conversion, the ICC profile has three one-dimensional LUTs for performing linear conversion of each input channel L * a * b *, a multi-dimensional LUT 200 for performing B2A conversion itself, and an output channel CM.
There are four one-dimensional LUTs that perform YK linear transformation. The multi-dimensional LUT is a table in which corresponding CMYK values are recorded for each of a predetermined number of grid points specified by L * a * b * coordinates.

FIG. 5 shows four one-dimensional L of output channels.
The UT is omitted in the drawing. When performing B2A conversion, C
The MM first sets the L * a * b * values to the three LUs on the input side.
It is given to T to obtain a one-dimensional linearly transformed value L * 'a *' b * '. In FIG. 5, since the linear conversion function has the form of y = x, the value is output as it is. Next, the value L * 'a *' b * 'after the one-dimensional linear conversion of each channel and the multi-dimensional LUT 200 are referenced to obtain the CMYK value to be output. At this time, the CMM obtains the CMYK values by performing the same interpolation calculation as in the A2B conversion.

Further, a multidimensional LUT2 used for B2A conversion
00 is also created by the profile creation software.
FIG. 4 illustrates a process in which the profile creation software creates the multidimensional LUT 100 used for A2B conversion. In the multidimensional LUT 100, grid point coordinates are defined in the device eigenspace, and the value of the corresponding standard color space is obtained by interpolation calculation. Multidimensional LUT 20 used for B2A conversion
At 0, the grid point coordinates are defined on the standard color space, and the profile creation software obtains the value of the device specific color space corresponding to the grid point coordinates by interpolation calculation.

The color chart producing method of the present invention will be described below with reference to the drawings. FIG. 1 is a flow chart showing the procedure of the method of the present invention. A description will be given below with reference to FIG. First, a temporary profile is created in order to know the structure of the profile (S01). Next, the provisional profile is examined to obtain lattice point information of the multidimensional LUT 100. The contents of the device profile can be checked by using commercially available profile analysis software and using it. If the number of grid points arranged on one color axis is known, I
In the CC profile, grid points are placed at 0 and 255, and on the color axis, there is a rule to put grid points at positions that divide them evenly. Therefore, know the positions of all grid points in CMYK space. Can be done (S02).

Next, the one-dimensional LUT of the provisional profile is examined to find the inverse function of the one-dimensional LUT (S03). Each value of CMYK that gives the grid point position on the color axis is obtained from the one grid point position on the color axis obtained in step S02 and the inverse function of the one-dimensional LUT obtained in step S03 (S04). A color chart composed of color patches is created by combining the obtained CMYK values (S0
5).

In the profile created using the color chart created by the above method, for the A2B conversion, the value included in the multidimensional LUT 100 has a small value obtained by the interpolation calculation process S2 shown in FIG. , Will be an accurate profile. Therefore, the accuracy of color conversion in the system that handles color data using this profile is improved. In other words, in the color management system proposed by ICC, an error in modeling the characteristic peculiar to the color device when the profile is created, that is, an error caused by the interpolation calculation S2,
Although there are always two kinds of errors, which occur when the CMM actually refers to the profiles of the input device and the output device and performs the color conversion calculation, the color chart created by the method of the present invention is used. By doing so, the former error can be minimized.

The method of creating the color chart for profile creation for improving the color conversion accuracy related to A2B conversion has been described above. Next, a method of creating a color chart for profile creation for improving the color conversion accuracy related to B2A conversion will be described with reference to the flowchart of FIG.

First, in order to know the structure of the profile,
A temporary profile is created (S21). Next, the provisional profile is examined to obtain lattice point information of the multidimensional LUT 200. If you know the number of grid points arranged on one color axis,
In the ICC profile, put grid points at 0 and 255,
Since there is a rule that grid points are placed at positions where the grid points are equally divided, the positions of all the grid points in the standard color space can be known (S22).

Next, the one-dimensional LUT of the provisional profile is examined to find the inverse function of the one-dimensional LUT (S23). Each value of L * a * b * giving the grid point position on the color axis is calculated from the grid point position on one color axis obtained in step S22 and the inverse function of the one-dimensional LUT obtained in step S23. Ask (S24).

Next, give L * a * b * which is the grid point position,
The combination of CMYK of the target color device (printer or the like) is obtained (S25). As an example, the following procedure can be considered. The combination of L * a * b * to be obtained is called “combination A”. 1) CM that has a colorimetric value close to Combination A from the colorimetric values of the color chart when the temporary profile was created
Select multiple "combination B" combinations of YK values. 2) "Combination B" is a combination of B1, B2, ...
It is a combination of CMYK values.
Configure some combinations with slightly changed values, and add combinations with slightly changed CMYK values to “combination B” to compose a color patch again. 3) Output the color patch on the target color device and measure the color. If there is a combination of CMYK values of the color patch that gives a colorimetric value that is close enough to the target "combination A" at this time, profile it. Incorporate in the color chart for making. 4) If there is no such color patch, CMYK of the color patch that gives the colorimetric value closest to "combination A".
Create a color patch again by adding a combination with slightly changed values. Repeat 3). In this way
Find the combination of CMYK values that gives all or as much as possible grid points in L * a * b * space.

Color chart data incorporating the combination of CMYK values obtained in step S25 is constructed, and the color chart output by the target color device is used as a profile creation color chart (S26).

For the profile created using the color chart created by the above method, for the B2A conversion, the value included in the multidimensional LUT 200 is small due to the interpolation calculation processing performed by the profile creation software, or Since the values are close to the lattice points, the error is reduced due to the interpolation calculation process, and the profile becomes accurate. Therefore, the accuracy of color conversion in the system that handles color data using this profile is improved.

Alternatively, a color chart containing both the CMYK values obtained by the method of FIG. 1 and the CMYK values obtained by the method of FIG. 2 may be created and used as a chart for creating a device profile.

[0030]

As described above in detail, according to the method of the present invention, the profile for obtaining the profile in which the error caused by the interpolation is minimized in consideration of the structure of the profile to be created. The remarkable effect of being able to create a chart for creation is achieved.

[Brief description of drawings]

FIG. 1 is a flowchart illustrating a procedure of a profile creation color chart creation method for improving the accuracy of A2B conversion.

FIG. 2 is a flowchart illustrating a procedure of a color chart creation method for profile creation for improving the accuracy of B2A conversion.

FIG. 3 is a diagram illustrating A2B conversion.

FIG. 4 is a diagram illustrating the operation of profile creation software.

FIG. 5 is a diagram illustrating B2A conversion.

[Explanation of symbols]

10 CMYK value of color chart 11 Colorimetric value of color chart 12 CMYK value after passing through one-dimensional LUT 100 Multi-dimensional LUT (for A2B conversion) 101 Multi-dimensional LUT (for A2B conversion) device-specific color space grid point 102 Many Value of standard color space corresponding to grid points of device-specific color space of dimensional LUT (for A2B conversion) 200 Multidimensional LUT (for B2A conversion)

Claims (2)

[Claims] 1. A method of creating a color chart for creating a device profile for color conversion used for exchanging color data between devices that handle color images, the method comprising:
A step of obtaining grid point coordinates of a multi-dimensional LUT for performing conversion from a standard color space to a device-specific color space, a step of obtaining a combination of device-specific colors corresponding to these grid coordinates, a combination of the obtained device-specific colors A method for creating a color chart in which steps for creating a color chart including are sequentially performed. 2. A method of creating a color chart for creating a color conversion device profile used for exchanging color data between devices that handle color images.
Steps for creating a temporary device profile, standard
A step of obtaining the number of grid points of a multi-dimensional LUT that performs conversion from a color space to a device-specific color space, a step of obtaining the characteristics of a one-dimensional LUT from a temporary device profile, a multi-dimensional LUT from the obtained number of grid points and the characteristics of a one-dimensional LUT A method of creating a color chart in which the steps of obtaining the coordinates of the lattice points, the step of obtaining the combination of the device unique colors corresponding to these lattice coordinates, and the step of producing the color chart including the obtained combination of the device unique colors are sequentially performed.