JP2002094821A - Color chart creation method and color chart - Google Patents

Abstract

(57) [Problem] To determine a color of a color patch so as to be uniform in a device color space and a standard color space. SOLUTION: A step of creating a color chart composed of color patches composed of a combination of a plurality of color materials used for an output device in a device color space, and plotting each color patch of the color chart in a standard color space. Creating a color region by using a color chart, and selecting the color patches so that the color patches are uniformly distributed in the color region.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color chart used in color management, and more particularly to a color chart creation method and a color chart in consideration of the color reproduction characteristics of an output device.

[0002]

2. Description of the Related Art Conventionally, when matching colors between output devices,
For example, when approximating the colors of a printing machine and a color printer, color conversion using a color look-up table (hereinafter, referred to as CLUT) such as ICC Profile proposed by the International Color Commission, because the color material characteristics and output methods are different. The scheme is commonly used. The CLUT
Is output from the output device, a color value of each color patch of the color chart is measured by a spectrophotometer, and is created from the obtained color values.

In general, color patches for creating a CLUT are completely uniformly distributed in the device color space in order to convert from the device color space to the standard color space, and in order to convert from the standard color space to the device color space. It is known that using a color chart configured to be completely uniformly distributed in a standard color space has better conversion accuracy. However,
Even for a color chart consisting of color patches that have a completely uniform distribution in the device color space, the distribution of points measured in each color patch and plotted in the standard color space is not uniform, while the standard color space The distribution of points obtained by plotting points having a completely uniform distribution on the device color space is also non-uniform.

For example, Japanese Patent Application Laid-Open No. H10-173940 has devised the generation of a color patch having a completely uniform color distribution in a standard color space. However, a color chart used for generating the color patch will be described. No mention is made of the uniformity of the color distribution in the device color space. In JP-A-11-187258, uniformity in a standard color space is adjusted by shifting lattice points for each color of a uniform color chart in a device color space. It is not mentioned how much uniformity will be achieved.

Further, as a result of investigating the color reproduction regions of various output devices, the region where the device color space is projected onto the standard color space has a curved and distorted shape. In order to reproduce this distorted shape on the CLUT, a combination of the maximum and minimum densities of each color material in the device color space and a color patch having an appropriate density near the middle (hereinafter, referred to as a color patch).
It is necessary to include a “feature point”), but the color patch of the color corresponding to this feature point is insufficient in the CLUT creation chart generally used at present.

A commonly used color chart as shown in FIG. 1 shows that in the distribution of color patches in the device color space, the halftone density is less than 50% and the number of combinations of CMY for the value of K lower than 50%. The number of combinations of CMY with respect to the value of K having a halftone dot density higher than 50% is small, and the distribution is biased even in the standard color space as shown in FIG.
The color distribution of the color patches is not uniform in both the device color space and the standard color space. Further, with respect to the color in combination with the halftone dot density of 100%, which is the maximum value of K, the color patch of the color corresponding to the above-mentioned characteristic point is insufficient.

Further, looking at the arrangement of the color patches in FIG. 1 for each color material, the color material consumption in the x-axis direction and the y-axis direction on the paper is not uniform as shown in FIG. In the case of a necessary printing machine, it is difficult to distribute ink of a uniform density on the paper of the color chart.
For example, in an output device such as a printing machine that requires adjustment of the ink supply mechanism, if a row consuming a large amount of ink is adjacent to a row consuming a small amount of ink, adjusting the density to the row consuming a large amount of ink will adjust the ink consumption. In a row having an extremely small amount of ink, ink is excessively supplied, and density unevenness (hereinafter, referred to as in-plane density fluctuation) such as local increase in solid density and thickening of halftone dots (dot gain) caused by the increase. On the other hand, in the case of an electrophotographic output device, a ghost phenomenon that the density is reduced due to a shortage of the color material in only the row that consumes a large amount of the color material easily occurs. As described above, each output device has a color reproduction characteristic, and the measured value of a color chart having a bias in color material consumption includes an error due to the in-plane density fluctuation.
A CLUT with good conversion accuracy cannot be created.

[0008]

However, in the case of a color chart taking into account that color patches are uniformly distributed in the device color space, a CLUT used for conversion from the device color space to the standard color space can be created with high accuracy. However, since the distribution of points where color patches uniformly distributed in the device color space are plotted in the standard color space is non-uniform,
C used for conversion from standard color space to device color space
The conversion accuracy of the LUT is not good.

On the other hand, in the case of a color chart in which the points plotted in the standard color space are uniformly distributed, the distribution of the color patches in the device color space becomes non-uniform, and the conversion from the device color space to the standard color space is performed. CL used for
The conversion accuracy of the UT is not good.

An object of the present invention is to determine a color of a color patch so as to be uniform in a device color space and a standard color space. Furthermore, by using the feature points necessary to reproduce the distorted shape of the device color space projected on the standard color space,
It is an object to improve the conversion accuracy of a CLUT for converting from a device color space to a standard color space or from a standard color space to a device color space.

Another object of the present invention is to reduce the bias of the color material consumption in the color chart, improve the color reproducibility of the output color chart, and improve the reliability of the measured values of the color patches. I do.

[0012]

SUMMARY OF THE INVENTION The present invention comprises the steps of: preparing a color chart composed of color patches composed of a combination of a plurality of color materials used for an output device in a device color space; Plotting the color patches in a standard color space to create a color gamut; and selecting the color patches such that the color patches are evenly distributed within the color gamut. I do.

In the present invention, preferably, the device color space is equally divided into 2 ^M (M is the number of color materials of the output device) region, and the color patches are uniformly distributed in the device color space division region. The method further comprises the step of selecting the color patch.

The present invention is preferably characterized by further comprising the step of including a combination of a color patch having an appropriate density existing near the maximum value and the minimum value of the density for each color material and an intermediate value.

Further, according to the present invention, there is provided a step of obtaining the density values of the respective color patches in the vertical and horizontal directions of the color chart, and a step of rearranging the respective color patches so that the obtained density values are uniform in both directions. And characterized in that:

Further, the present invention is characterized by a color chart in which a combination ratio of a plurality of color materials used in an output device for determining a color of each color patch constituting the color chart is uniform.

Also, the present invention provides a color chart which uses a combination of a maximum value, a minimum value, and a combination near a middle value of a plurality of color materials used in an output device for determining a color of each color patch constituting a color chart. Color chart including patches.

[0018]

Therefore, according to the present invention, it is possible to determine the color of a color patch so as to be uniform in the device color space and the standard color space, and furthermore, the distorted shape of the device color space projected on the standard color space. CL for converting from the device color space to the standard color space or from the standard color space to the device color space by using the feature points necessary to reproduce
The conversion accuracy of the UT can be improved.

Further, according to the present invention, it is possible to reduce the bias of the color material consumption in the color chart, improve the color reproducibility of the output color chart, and improve the reliability of the measured values of the color patches.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. FIG. 4 shows a flowchart of the embodiment of the present invention.

First, the color materials of the output device (in the case of an output device composed of four colors of CMYK) so as to cover the device color space.
Each time (0%, 12.5%, 25.0%,
37.5%, 50.0%, 62.5%, 75.0%, 8
A color chart of 6561 colors composed of a combination of 7.5% and 100.0% (changed to 9 gradations) is output, and the color value of each color patch of the color chart is measured (step 1).

Next, the standard color space is set to L * = 50 so that the entire R of the device color space projected onto the standard color space is included.
Eight areas Ai (i is 1) centering on a * = 0 and b * = 0
(The variables from to 8) are spatially equally divided (step 2). The number Pi of each of the points plotted in the equally divided standard color space Ai (when the plotted points are on the boundary of the equally divided standard color space area, the calculation is repeated) is obtained. Each of the equally divided regions Ai, where ri is the ratio of the device color space to the equally divided standard color space, respectively. Note that R creates a profile once with software for creating an ICC profile,
With reference to the color reproduction area information described in the file, the number of 0s indicating that the value is within the color gamut is obtained for each L * a * b * value. It can be approximated by the product of the sampling interval and the size of a small cube whose length is one side. Also, ri, for example, creates a profile once with software for creating an ICC profile, and refers to the color reproduction information described in the profile, and for each L * a * b * value included in Ai, , The number in which 0 indicating that it is within the color gamut is used for all L * a * b * included in Ai.
Can be approximated by dividing by the number of. The number Pi of points plotted in the equally divided space Ai is
From the range defined in equation (1)

As described above, by adjusting the number of color patches Pi plotted in the equally divided standard color space Ai to be within the range of the above equation (1), the color of the color patches uniform in the standard color space is obtained. A combination of materials is selected.

In the present invention, preferably, after performing the above-mentioned operation, the device color space is equally divided into 2 ^M colors to adjust the color bias of the color chart in the device color space, and the device color space is equally divided. The number of color patches in the region is adjusted to the ideal number of color patches, and a combination of color materials of uniform color patches in the device color space is selected. In the present embodiment, a case of an output device including four colors of CMYK is shown.

The number Qj of color patches existing in the equally divided space Dj is within a range defined by the equation (2). In this way, by adjusting the number Qj of color patches plotted in the equally divided device space Dj to be within the range of the above equation (2), a uniform combination of color materials of color patches in the device color space can be obtained. Selected.

In the present invention, more preferably, it is confirmed that the color chart constituted by the above operation includes the maximum value and the minimum value of the density for each color material, and if it is not included, it is deleted. Add a color patch containing the maximum or minimum density from the color patches.

By repeating the above operations, the colors of the color patches that are uniform in the device color space and the standard color space are determined, and the distorted shape of the device color space projected on the standard color space is reproduced. A color chart including the characteristic points necessary for the above can be created.

The method will be described below. First, the color patch selected from the above method is mxn
(M and n are each an integer). Arrangements are arranged in such a way that the gradation changes are aligned in the row direction or column direction of the matrix. At this time, if a matrix cannot be formed due to lack of color patches, a color patch or the like in which the density of all color materials is 0% is added. The layout of the color chart at this time

If the color patches are not included in the ranges defined by the equations (3) and (4), color patches are selected every other row as shown in FIG. This operation is called a left-right inversion method). Arrange the color chart at this time

If not included in the range defined by the equations (3) and (4), color patches are further selected every other row as shown in FIG. 6, and the order of the color patches in the row is inverted upside down. (Hereinafter, this operation is referred to as upside down method).
Color chart layout at this time

When the color patches are not included in the ranges defined by the equations (3) and (4), color patches are selected in a zigzag pattern as shown in FIG. Rotate by 180 degrees (hereinafter, this operation is performed by 180
Degree rotation method). Mosquito at this time Then, it is checked whether the value falls within the range defined by the expressions (3) and (4). Left-right flip method, upside-down flip method, 18
By arranging the color patches by combining the operations based on the 0-degree rotation method, it is possible to create a color chart with less bias in the color material consumption.

FIG. 8 shows an example of a color chart created according to this embodiment. The distribution in which the color patches of the color chart shown in FIG. 8 are measured with a spectrophotometer and the measured color values are plotted in a standard color space is almost uniformly distributed as shown in FIG. Further, the number of plotted color values included in each area of the equally divided standard color space is as shown in FIG. 10 and is included in a specified range.

When the number and average value of the color patches in the device color space of the color chart shown in FIG. 8 in the device color space are calculated, the average value of the number of color patches entering the unit division area is 69. As shown in FIG. 11, the number of color patches in each divided region is all within the allowable range defined by the equation (2), that is, in the range of 20.799 to 118.881.

Further, the color created by this embodiment is
The distribution of the color material consumption of the chart is as shown in FIG.
The color material consumption is substantially uniformly distributed on both the x-axis and the y-axis.
When created with the measurement data of a general color chart,
L from CMYK^*a^*b ^* The conversion accuracy to the average color difference ΔE
= 5.06 and L^*a^*b^*Conversion accuracy from CMYK to CMYK
Is an average color difference ΔE ≒ 2.0. However, this implementation
In the case of a color chart created by the example, CMYK
L^*a^*b^* The conversion accuracy to the average color difference ΔE = 1.5
2, again L^*a^*b^*Conversion accuracy from CMYK to average color
Good results can be obtained with a difference ΔE ≒ 2.0.

[0035]

As described above, according to the present invention, it is possible to reduce the number of colors by creating a color chart which is substantially uniformly distributed in the device color space and the standard color space and which can reproduce a curved color gamut. CL for color conversion using color chart
The UT can be created with high accuracy.

Further, by forming the color patches in the form of a color chart in which the deviation of the color material consumption in the x-axis direction and the y-axis direction on the paper surface is reduced, ink adjustment can be performed in an output device such as a printing press. A color chart can be output with a simple and uniform density.

The color chart can be used for color management to improve the similarity of colors.

[Brief description of the drawings]

FIG. 1: General color chart (IT8.7)

FIG. 2 Distribution of a general color chart in a standard color space

FIG. 3 shows a distribution of color material consumption of a general color chart.

FIG. 4 is a flowchart of an embodiment.

FIG. 5 is an explanatory diagram of a left-right inversion method.

FIG. 6 is an explanatory view of the upside down method.

FIG. 7 is an explanatory diagram of a 180-degree rotation method.

FIG. 8 is a color chart according to the embodiment.

FIG. 9 is a distribution of a color chart in a standard color space according to the embodiment.

FIG. 10 shows a distribution of a color chart in each equally divided standard color space according to the embodiment.

FIG. 11 shows a distribution of a color chart in each equally divided device color space according to the embodiment.

FIG. 12 shows a color material consumption distribution of a color chart according to the embodiment.

Continued on the front page F-term (reference) 5B057 AA11 BA02 BA23 CA01 CA08 CA12 CB01 CB08 CB12 CB16 CC01 CE17 CE18 CH07 CH08 CH18 DA17 5C077 MM27 MP08 PP33 PP36 PP37 PP38 PP43 PP46 PP48 PQ08 PQ23 5C079 HB03 MAB11 MAB11MA11 NA03 NA27 NA29

Claims (10)

[Claims] 1. A step of creating a color chart composed of color patches composed of a combination of a plurality of color materials used for an output device in a device color space, and plotting each color patch of the color chart in a standard color space. Creating a color area by selecting the color patches so that the color patches are evenly distributed in the color area. 2. The color chart creation method according to claim 1, wherein the device color space is equally divided into 2 ^M (M is the number of color materials of the output device) region, and the color is divided into the device color space divided region. A method for creating a color chart, further comprising the step of selecting the color patches so that the patches are evenly distributed. 3. A method for producing a color chart according to claim 1, wherein a combination of a maximum value and a minimum value of the density for each color material and a color patch having an appropriate density near the middle thereof is included. A method for creating a color chart further comprising: 4. A step of obtaining a density value of each color patch in the vertical direction and the horizontal direction of the color chart, and a step of rearranging the color patches so that the obtained density value is uniform in both directions. How to create a color chart. 5. In a device color space (a color reproduction area unique to an output device), a density or a halftone dot density (hereinafter referred to as a density) for each color material (CMYK or the like) used for the output device is changed in a gradation manner. Outputting a color chart composed of the combined combinations; and a color value (CI) of each color patch of the color chart.
EL * a * b * values) and plotting the color values to the corresponding coordinates in a standard color space (such as the CIE L * a * b * space defined by the International Commission on Illumination) , The whole of the device color space projected onto the standard color space is spatially equally divided into 2 ^N (N is the number of axes constituting the standard color space) so that R and R are included. Where Pi is the number of points plotted in each of the equally divided standard color spaces, where i is a variable from 1 to ^2N , and the plotted points are on the boundary of the equally divided standard color space region. , The ratio of the device color space to each of the equally divided standard color spaces is ri, and each P
selecting points plotted in the standard color space such that i falls within the range of equation (1); How to create a color chart that includes. 6. A method for producing a color chart according to claim 1, wherein the device color space is spatially equally divided into 2 ^M (M is the number of color materials of the output device), and each of the equally divided device colors is divided. The number Qj of color patches included in the space area (j is 1 to 2
( ^M ) is within the range of the expression (2), and further comprises the step of selecting a color chart in the device color space. How to create a color chart. 7. A method for producing a color chart according to claim 1 or 2, wherein a combination of a maximum value and a minimum value of the density for each color material and a color patch having an appropriate density near the middle thereof is included. A method for creating a color chart further comprising: 8. A color chart creating method according to claim 1, wherein the color patches are arranged in a matrix of m × n (m and n are integers), and each color material is arranged in the i-th row and the j-th column. The density of the color patch is set to Cij (where i is 1).
To m, j is an integer from 1 to n) Sum of the density of the color patches in the j-th column from the 1st to the m-th row 9. A color chart having a uniform combination ratio of a plurality of color materials used in an output device for determining a color of each color patch constituting the color chart. 10. A color including a color patch using a combination of a maximum value, a minimum value, and an intermediate value of a plurality of color materials used in an output device for determining a color of each color patch constituting a color chart. chart.