JP2008054245A - Method and system for creating conversion table for color matching profile - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide technologies for reducing color inconstancy regarding an image formed by an image forming apparatus with much less workload. <P>SOLUTION: L*a*b* data of a patch image are calculated under a plurality of light sources including a reference light source. In a case where a plurality of patch images from which approximately identical L*a*b* data are obtained under the reference light source exist, the total of differences between the L*a*b* data under the reference light source and L*a*b* data under the other light sources is calculated. Color component data (e.g., CMYKR data) for printing corresponding to a patch image that minimizes the total of differences are registered on an LUT 106. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for reducing a phenomenon in which the color of an image formed by an image forming apparatus such as a color copying machine or a color printer looks different depending on an observation light source.

  The color of an image formed by an image forming apparatus such as a color copying machine or a color printer may look different depending on the observation light source, and this phenomenon is called color inconstancy.

As a technique for reducing color inconstancy, there is a technique for performing color matching according to ambient light (for example, Patent Document 1). According to Patent Document 1, the spectral distribution of the observation light source is acquired, and the tristimulus value of the color chart under the corresponding light source is calculated from the spectral distribution of each light source and the spectral reflection data of the color chart, and profile data To do. Then, using this profile data, color matching processing in consideration of chromatic adaptation by ambient light is performed.
JP 2002-218266 A

  However, since the technique of Patent Document 1 creates an optimal color matching profile for each ambient light, the amount of work required is relatively large.

  The present invention has been made in view of such a situation, and an object thereof is to provide a technique capable of reducing color inconstancy related to an image formed by an image forming apparatus with a smaller work amount. To do.

  In order to solve the above problems, the method of the present invention is a method of creating a conversion table for a color matching profile corresponding to a plurality of light sources, and forms an image using four or more color materials. A forming step of forming a plurality of patch images on a recording medium based on a plurality of sets of color component data determined in advance in a first color space used by the image forming apparatus; and a spectrocolorimeter Using the plurality of light sources including the first light source to measure each of the plurality of patch images to obtain a plurality of color measurement data, and based on the plurality of color measurement data, A first calculation step of calculating a plurality of sets of color component data corresponding to each of the plurality of patch images and expressed by a second color space independent of an apparatus for each of the light sources; Of the plurality of sets of color component data expressed by space, the first light source having substantially the same value as the color component data corresponding to the color measurement data obtained by measuring the color with the first light source. A first selection step of selecting all sets of color component data corresponding to the colorimetric data obtained by colorimetry using all light sources except for each of the sets of all color component data selected in the first selection step A second calculation step of calculating a color difference with color component data corresponding to the color measurement data obtained by the color measurement by the first light source, and the smallest of the color differences calculated in the second calculation step A second selection step of selecting color component data in the first color space expressing a patch image corresponding to color component data having a color difference; and the color component data selected in the second selection step in the conversion table. Set as value A setting step that, characterized in that it comprises a.

  In addition, the system of the present invention includes an image forming apparatus that performs image formation using four or more color materials and a spectrocolorimeter, and creates a conversion table for a color matching profile corresponding to a plurality of light sources. A forming unit for forming a plurality of patch images on a recording medium based on a plurality of sets of color component data determined in advance in a first color space used by the image forming apparatus; Using each of a plurality of light sources including a light source, color measuring means for measuring each of the plurality of patch images to obtain a plurality of color measurement data, and each of the plurality of light sources based on the plurality of color measurement data First calculation means for calculating a plurality of sets of color component data corresponding to each of the plurality of patch images and expressed by a second color space independent of the device, and the second color space. Of the plurality of sets of color component data that are expressed, all except the first light source, which have substantially the same value as the color component data corresponding to the colorimetric data obtained by measuring the color with the first light source A first selection unit that selects all sets of color component data corresponding to colorimetric data obtained by color measurement using a light source, and for each set of all color component data selected by the first selection unit, A second calculating unit that calculates a color difference with color component data corresponding to the colorimetric data obtained by the color measurement using the first light source; and a minimum color difference among the color differences calculated by the second calculating unit. Second selection means for selecting color component data in the first color space expressing a patch image corresponding to the color component data possessed, and color component data selected by the second selection means as values of the conversion table Setting means for setting And wherein the door.

  Other features of the present invention will become more apparent from the accompanying drawings and the following description of the best mode for carrying out the invention.

  With the above configuration, according to the present invention, it is possible to reduce color inconstancy related to an image formed by the image forming apparatus with a relatively small work amount.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. The individual embodiments described below will help to understand various concepts, such as the superordinate concept, intermediate concept and subordinate concept of the present invention.

  The technical scope of the present invention is determined by the claims, and is not limited by the following individual embodiments. In addition, not all combinations of features described in the embodiments are essential to the present invention.

[First Embodiment]
The color of image data to be output to a display by a personal computer (PC) or the like is generally managed by color component data (hereinafter referred to as RGB data) in the RGB color space. On the other hand, the color of image data for a PC or the like to output to an image forming apparatus such as a printer is generally color data in a color space using four or more color materials such as CMYK color space (hereinafter referred to as CMYK data). Are managed). Therefore, when the PC outputs image data to the image forming apparatus, it is often necessary to convert RGB data into CMYK data.

  FIG. 1 is a diagram showing functional blocks for converting color component data of image data from RGB data to CMYK data and generating rasterized data used by a printer for printing.

  The image data is subjected to γ conversion processing (brightness adjustment processing) designated by the user in the input γ conversion processing unit 101. Next, the color matching processing unit 102 performs color matching processing. Ink color separation processing unit 103 performs ink color separation processing, and converts color component data from RGB data to CMYK data.

  Next, the image data is subjected to γ conversion processing in the output γ conversion processing unit 104 so that the dot gain is absorbed and each ink color density becomes linear. Finally, the halftone processing unit 105 performs processing for converting multi-value data of each color (here, CMYK) into binary data. Here, this binary data is rasterized data.

  The rasterized data is output to the printer.

  Here, the look-up table (LUT) 106 is a table for converting color component data from RGB data to CMYK data. The LUT 106 holds correspondence information that associates predetermined RGB data and predetermined CMYK data on a one-to-one basis. The ink color separation processing unit 103 refers to the LUT 106 and converts color component data from RGB data to CMYK data.

  By the way, different CMYK data may theoretically express the same color. For example, when the range that each component of CMYK can take is 0 to 255, (C, M, Y, K) = (0, 0, 0, 255) represents black. On the other hand, (C, M, Y, K) = (255, 255, 255, 0) also represents black. Therefore, when (R, G, B) = (0, 0, 0), which is RGB data representing black, is converted into CMYK data, (C, M, Y, K) = (0, 0, 0, 255). ) Or (255, 255, 255, 0).

  At this time, even if both color component data in the CMYK color space theoretically represent black, the color may appear different to the human eye. Furthermore, because of color inconstancy, it may appear the same color under one light source but appear different under other light sources. Therefore, in this embodiment, color inconstancy is reduced by associating RGB data with CMYK data that appears stably in the same or substantially the same color under various observation light sources.

  Hereinafter, a method of generating the LUT 106 that reduces color inconstancy will be described with reference to FIGS. 2 and 3. For the sake of explanation, it is assumed that the printer according to the present embodiment uses five types of color materials of CMYKR (Cyan, Magenta, Yellow, Black, Red). Therefore, the color of the image data that the PC outputs to the printer is managed by the CMYKR data.

  FIG. 2 is a flowchart showing a flow of processing for registering correspondence information between RGB data and CMYKR data in the LUT 106.

  In step S201, the printer prints a plurality of patch images on a recording medium such as paper. The patch image is generated by slicing each color component of CMYKR data at a predetermined interval to generate CMYKR data as shown in FIG. 3 and performing printing based on each CMYKR data. In FIG. 3, each color component is sliced at intervals of 64 (or 63), but this interval can be arbitrarily determined and does not need to be equal.

In step S202, the spectral colorimeter, in the plurality of light sources, including a D ₅₀ light source, measuring the spectral reflectance of the patch image to obtain the spectral reflectance data. The D ₅₀ light source is a reference light source for generating the LUT 106, but other than the D ₅₀ light source, for example, a D ₆₅ light source may be used as a reference.

In step S203, the information processing apparatus such as a PC, based on the measured spectral _reflectance, calculates the ^L * ^a * ^{b *} data in a plurality of light sources, including a _{D 50} light source. Unlike RGB data and CMYKR data, L ^* a ^* b ^* data is color component data in a color space that does not depend on a device such as a PC or a printer. Details of the method of calculating L ^* a ^* b ^* data will be described later.

In step S204, the information processing apparatus such as a _PC, selects the group consisting of ^L * ^a * ^{b *} of the plurality of data is approximately the same CMYKR data all at _{D 50} light source. For example, the red color (L ^* , a ^* , b ^* ) = (44.35, 74.27, 68.60) in the D ₅₀ light source can be obtained from the following three types of image patches based on CMYKR data. Yes (each color component is 8 bits). Strictly speaking, if the slice interval of the color component data is as shown in FIG. 3, the following patch images (1) and (3) cannot be obtained. Please understand that it is for. That is, some specific color is obtained from a plurality of types of CMYKR data.

(1) (C, M, Y, K, R) = (0,253,240,0,0)
(2) (C, M, Y, K, R) = (0, 0, 0, 0, 255)
(3) (C, M, Y, K, R) = (0, 100, 111, 0, 128)
Then, the difference in step S205, the information processing apparatus such as a PC, set for selected in step _S204, the ^L * ^a * ^{b *} data in ^{the L} * ^a * ^{b *} data and each of the other light sources in the _{D 50} light source Is calculated (or the average of the differences). Details of the calculation method will be described later. Then, the CMYKR data with the smallest difference is registered in the LUT 106 in association with the RGB data. Details of a method for calculating RGB data corresponding to CMYKR data will be described later.

Although not shown, the information processing apparatus such as a PC _performs the processing of steps S204 and S205 for the set of all CMYKR ^{data L} * ^a * ^{b *} data in _{D 50} light source is substantially the same. When the L ^* a ^* b ^* data in the D ₅₀ light source corresponds to only a single CMYKR data, the CMYKR data is registered in the LUT 106 in association with the RGB data.

  As described above, correspondence information between RGB data and CMYKR data is registered in the LUT 106.

<Summary>
What is particularly important in the present embodiment is the processing in steps S204 and S205. As (1) to (3) below, a plurality of patch images based on CMYKR data is red in _{D 50} light source color ^{(L *, a *, b} *) = (44.35,74.27, 68.60).

(1) (C, M, Y, K, R) = (0,253,240,0,0)
(2) (C, M, Y, K, R) = (0, 0, 0, 0, 255)
(3) (C, M, Y, K, R) = (0, 100, 111, 0, 128)
Patch images based on these CMYK data generally look different colors under other light sources. In this embodiment, it registers the ^L * ^a * ^{b *} data and the _{D 50} ^L in the light source * ^a * ^{b *} color component data sum of the differences is the minimum data to LUT106 in each other light sources. Therefore, CMYKR data that can represent relatively similar colors under different light sources is registered in the LUT 106, and color inconstancy is reduced.

  Further, the concept of the present embodiment is applicable when there are two or more correspondences when converting color component data in the first color space into color component data in the second color space. Therefore, the source color component data is not limited to RGB data, and the destination color component data is not limited to CMYK data, CMYKR data, or the like.

As light sources other than D ₅₀ light source, A light source, C light source, D ₆₅ light source, F1 light source, F2 light source, F3 light source, F4 light source, F5 light source, F6 light source, F7 light source, F8 light source, F9 light source, F10 light source , F11 light source, F12 light source, and the like can be used, but are not limited thereto.

<Method of calculating L ^* a ^* b ^* data>
First, tristimulus values (X _i , Y _i , Z _i ) of the patch image under the light source i are calculated according to the equation (1).

ここで、

here,

Next, the tristimulus values (X ′ _i , Y ′ _i , Z ′ _i ) of the complete diffuse reflection surface under the light source i are calculated according to the equation (2).

Then, L ^* a ^* b ^* data of the patch image under the light source i is calculated according to the equations (3) to (5).

ここで、

here,

<Method of calculating the difference (color difference) Delta] E _{94 (i)} of the ^L * ^a * ^{b *} data in ^{the L} * ^a * ^{b *} data and the light source i in _{D 50} light source>
ΔE _{94 (i)} is calculated according to equation (6).

ここで、

here,

<Calculation method of RGB data corresponding to CMYKR data>
RGB data corresponding to the CMYKR data is calculated according to a known conversion formula. Here, for the sake of simplicity, an expression for converting CMYK data into RGB data is shown.

R = 1-min (1, C × (1-K) + K)
G = 1-min (1, M × (1-K) + K)
B = 1-min (1, Y × (1-K) + K)

  However, it is assumed that the RGB data is normalized as (0 to 1, 0 to 1, 0 to 1).

[Second Embodiment]
In the first embodiment, the printer uses CMYKR five color materials. In the second embodiment, a case where a printer uses a Gr (Gray) color material (that is, CMYKRGr) instead of R will be described.

  In the present embodiment, the description of parts common to the first embodiment (particularly, FIGS. 1 and 2) is omitted.

  FIG. 4 is a diagram illustrating color component data for generating a plurality of patch images, as in FIG. 3. In FIG. 4, unlike FIG. 3, the color component data has five types of color components CMYKGr.

Here, in order to express the halftone gray color (L ^* , a ^* , b ^* ) = ( ₅₀ , 0, 0) in the D ₅₀ light source, there are a plurality of CMYKGr data as follows (each color The component is 8 bits).

(1) (C, M, Y, K, Gr) = (107, 123, 82, 0, 0)
(2) (C, M, Y, K, Gr) = (10, 14, 0, 0, 200)
(3) (C, M, Y, K, Gr) = (51, 64, 34, 0, 128)
Therefore, as in the first embodiment, and also calculates the ^L * ^a * ^{b *} data under other light _sources, to calculate the sum of the difference between ^L * ^a * ^{b *} data under _{D 50} light source. Then, color inconstancy can be reduced by registering the color component data (CMYKGr:-) having the smallest difference in association with the RGB data in the LUT 106.

It is a figure which shows the functional block for converting the color component data of image data from RGB data to CMYK data, and producing | generating the rasterization data which a printer uses for printing. 10 is a flowchart showing a flow of processing for registering correspondence information between RGB data and CMYKR data in an LUT. It is a figure which shows several CMYKR data for printing a patch image. It is a figure which shows the several CMYKGr data for printing a patch image.

Explanation of symbols

101 Input γ conversion processing unit 102 Color matching processing unit 103 Ink color separation processing unit 104 Output γ conversion processing unit 105 Halftone processing unit 106 Look-up table (LUT)

Claims (5)

A method for creating a conversion table for a color matching profile corresponding to a plurality of light sources,
A plurality of patch images are recorded by an image forming apparatus that forms an image using color materials of four or more colors based on a plurality of sets of color component data predetermined in a first color space used by the image forming apparatus. Forming process on the medium;
A colorimetric step of measuring each of the plurality of patch images using a plurality of light sources including a first light source by a spectrocolorimeter to obtain a plurality of colorimetric data;
Based on the plurality of colorimetric data, a plurality of sets of color component data expressed by a second device-independent color space corresponding to each of the plurality of patch images in each of the plurality of light sources is calculated. 1 calculation step;
Of the plurality of sets of color component data expressed by the second color space, the color component data corresponding to the color measurement data obtained by measuring the color with the first light source has substantially the same value. A first selection step of selecting all sets of color component data corresponding to colorimetric data obtained by colorimetry using all light sources except the first light source;
A second calculation step of calculating a color difference from the color component data corresponding to the color measurement data obtained by measuring the color with the first light source for each set of all the color component data selected in the first selection step; When,
A second selection step of selecting color component data in the first color space representing a patch image corresponding to the color component data having the smallest color difference among the color differences calculated in the second calculation step;
A setting step of setting the color component data selected in the second selection step as a value of the conversion table;
A method comprising the steps of:   2. The method according to claim 1, wherein among the four or more color materials, three colors are cyan, magenta, and yellow, and the other colors are achromatic colors.   2. The color material of four or more colors, wherein three colors are cyan, magenta, and yellow, and the other colors are chromatic colors different from cyan, magenta, and yellow. the method of. The first light source A method according to any one of claims 1 to 3, characterized in that the D ₅₀ light source. A system that includes an image forming apparatus that forms an image using color materials of four or more colors and a spectrocolorimeter, and that creates a conversion table for a color matching profile corresponding to a plurality of light sources,
Forming means for forming a plurality of patch images on a recording medium based on a plurality of sets of color component data determined in advance in a first color space used by the image forming apparatus;
A colorimetric unit that obtains a plurality of colorimetric data by measuring each of the plurality of patch images using each of a plurality of light sources including a first light source;
Based on the plurality of colorimetric data, a plurality of sets of color component data expressed by a second device-independent color space corresponding to each of the plurality of patch images in each of the plurality of light sources is calculated. 1 calculating means;
Of the plurality of sets of color component data expressed by the second color space, the color component data corresponding to the color measurement data obtained by measuring the color with the first light source has substantially the same value. First selection means for selecting a set of all color component data corresponding to colorimetric data obtained by colorimetry with all light sources except the first light source;
Second calculation means for calculating the color difference between the color component data corresponding to the color measurement data obtained by measuring the color with the first light source for each set of all color component data selected by the first selection means. When,
Second selection means for selecting color component data in the first color space representing a patch image corresponding to color component data having the smallest color difference among the color differences calculated by the second calculation means;
Setting means for setting the color component data selected by the second selection means as a value of the conversion table;
A system comprising:

Images