JP2002247394A - Method of preparing color converting rule, method of converting color, and colored image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of preparing a color converting rule that does not cause errors even when the rule is applied to various combinations of a coloring agent and a medium. SOLUTION: At the time of preparing the 3×3 matrix or three-dimensional lookup table for color conversion of a digital color copying machine, converted data which are obtained by covering read measured data in color and reference data measured by means of a spectrophotometer are prepared with respect to the different combinations of the coloring agent and medium (S11 and S12). Then the color converting rule is decided so as to minimize the color error between both data with respect to all combinations of the coloring agent and medium (S14). The minimized color error is weighted in accordance with (1) the importance (frequency in use) of the coloring agent/medium of a read original, (2) the importance of divided color spaces, and (3) the color gamut of the coloring agent/medium (S13).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preparing a color conversion rule, a color conversion method, and a color image forming apparatus, and more particularly, to a method for processing color image data in a digital color copying machine. A colorimetric color space (calibrat) in which unprocessed RGB data is adjusted
The present invention relates to a method for creating a color conversion rule for converting image data into an ed colorimetric space), a method for performing color conversion according to the created color conversion rule, and an apparatus for forming a color image using this color conversion method.

[0002]

2. Description of the Related Art A state-of-the-art color original scanner (color scanner) includes an illumination light source, three primary color filters (RGB filters), and a CCD (charge coupled device) detector. In the above color original scanner, after the reflected light radiated from the light source and reflected by the illumination target (target) is filtered by an RGB filter,
RGB signals are generated by the detector. The light source is a broadband artificial illumination light source, which is different from a viewing illuminant.

Here, the combined spectral response of the light source, RGB filter and CCD detector is represented by CIE (Commissi
on Internationale de l'Eclairage)
If color matching functions (CMFs) are linearly combined, the color scanner can recognize colors as seen by the human visual system (HVS) by performing appropriate processing. . However, this is generally not the case because of the need to compromise light source efficiency with the cost of designing and manufacturing color filters.

[0004] Thus, while a color scanner may perceive two different colors as the same color, the human visual system may perceive them as different colors, and vice versa. This is the condition-matching problem of the scanner.

[0005] Because of this problem, the color characterization of a color scanner is highly dependent on the media (recording material) and the colorant (coloring material). That is, a color scanner having color characteristics suitable for a certain combination of a medium and a colorant always uses spectral characteristics specialized for the combination, and is therefore suitable for another combination of a medium and a colorant. Absent.

A colorimetric value (co
In order to obtain lorimetric values), color characterization must be performed for each color scanner.

[0007] As a specific procedure of the characterization, first,
A color patch based on a combination of a certain colorant and a medium is read to obtain a conversion source RGB value. The color patch is also measured by a spectrophotometer such as Gretag Spectrolino to obtain the CIE XYZ value or Lab value. Next, these two sets of data, namely, RGB values and XY
Regression or interpolation processing is performed on the Z value or the Lab value, and the read RGB values are converted into the corresponding XYZ values.
Create a mathematical model to convert to CIE numbers, such as values, Lab values, or other output matrices.

Here, as the mathematical model, a model using a matrix and a model using a three-dimensional lookup table (LUT) are generally used. The profile format of the color scanner based on these two models is based on the recently announced “ICC Profile Format Standard” (International Color Consortium “IC
C Profile Format Standard ", 3.4 Edition, 1997
August 15, 2008).

In general, these techniques produce satisfactory results for certain colorant and media combinations. However, because of the color matching of the scanner, the conversion created for one combination of colorant and media does not work well for the other combination.

For reading in the field of graphic arts, reflection photo prints are often used.
graphic prints), color transparen
cies), a color negative film, etc. to produce an image with a limited combination of colorants and media. for that reason,
For each combination of colorant and medium, a specific color conversion can be used, resulting in a very accurate color conversion. That is, in such applications, the color matching of the scanner does not matter.

However, manuscripts read by digital color copiers come from a variety of sources and contain many different combinations of colorants and media. These combinations include:
Examples include electronic printing, graphic arts materials, inks on various inkjet papers, and laser printer output.

Thus, it is impractical to require a standard user of a digital color copier to identify the colorant and medium used in the document being read. Therefore, only one color conversion may be prepared for different combinations of the colorant and the medium. Then, as one of the general methods, there is a method of combining conversions by calculating an average of conversions created individually. But,
This approach suffers from the problem that, although each transformation is suitable for the colorant and medium combination it is intended for, the combined transformations are prone to errors when used in various combinations of colorant and medium. is there.

[0013] Attempts to produce colorimetric digital input devices have been described in PG Engeldrum, "Colorimetric Design Requirements for Color Scanners" (SPIE, Vol. 1909, 1993).
Year) and REBurger "Reading device-independent colors"
(SPIE, Vol. 1909, 1993). However, these devices have not been successful. This is because color scanners manufactured according to the proposals of Engeldrum and Burger require that the combined spectral response of the digital filter, light source, and CCD detector be well matched to the color matching function. Because it is very expensive. Therefore, even the latest color scanner is only required to be approximately colorimetric, and the combination of a colorant and a medium that can accurately perform color conversion is limited.

The following patent describes a method of adjusting an RGB scanner to generate a color value in a colorimetric color space.

US Pat. No. 4,608,839 (Sakamoto
"Color Correction Method" registered on November 29, 1977)
A method of correcting an RGB signal in a YK color space is described.

US Pat. No. 4,432,047 (Niemczyk et al. “Accuracy of Color Adjustment in Color Raster Scanners”).
(Registered Jul. 27, 982) describes a scanner that analyzes a small area of a single color image by moving a scan head within the small area.

US Pat. No. 5,149,960 (Dunne et al., “Method of Converting Scanner Signals to Colorimetric Signals”, registered on Sep. 22, 1992) detects an input image, generates an input signal, and converts the input signal. Processing in an iterative correction loop is described.

US Pat. No. 5,2008,817 (Birnbaum)
"Conversion of RGB color scanner to colorimetric scanner" 1
(Registered on April 6, 993), the RGB input is equivalent neutral density (END) RGB
It describes converting to a signal and then to an XYZ signal that is a tristimulus value.

No. 5,452,112 (Wan et al., Field Calibration for Color Image Reproduction Systems).
ation) Method and Apparatus ”, registered September 19, 1995)
Describes that a printer adjustment table is generated using an object to be read, a reference file, and an image.

US Pat. No. 5,491,568 (Wan, "Method and Apparatus for Adjusting Digital Color Image Reproduction Apparatus")
(Registered Feb. 13, 1996) describes that a three-linear interpolation method based on a three-dimensional LUT is used to execute color conversion.

No. 5,543,940 (Sherman
"Method and Apparatus for Converting Color Scanner Signals to Colorimetric Values" (registered Aug. 6, 1996) describes specifying a specific medium for constructing an adjustment model of a scanner.

Further, the following patent describes color adjustment of a scanner.

US Pat. No. 4,346,402 (Pugsly "Image Reproduction Apparatus", registered Aug. 24, 1982) describes the use of a three-dimensional LUT or ROM to supply a color correction signal.

US Pat. No. 4,929,978 (Kanamori et al., “Using a Correction Table Based on Printed Color Samples,
Color Correction Method for Color Copier "May 29, 1990
Day Registration) describes using a sample color that is compared to the read value to determine an appropriate output.

US Pat. No. 5,185,661 (Ng, "Input scanner color mapping and color gamut input / output conversion")
(Registered Feb. 9, 993) describes color adjustment of a scanner using a 3 × 3 conversion matrix and a color gamut compression list.

Further, the following references describe a method applied to an RGB scanner to make the output signal of the RGB scanner more colorimetric.

No. 5,285,271 (Gennetten
"Digital Color Matrix Circuit", February 8, 1994
Day registration) describes converting the density of each coded color.

No. 5,665,963 (Campbell)
In "Reflection Color Filter for Color Correction of Photodetector Filter" (registered September 9, 1997), a filter having a limited wavelength is used to suppress infrared rays and visible red light having a relatively long wavelength. It is described.

US Pat. No. 5,773,814 (Phillips et al., Collective Sensor for Grayscale and Color Detection, Used in Optical Image Scanners, Registered Jun. 30, 1998).
Has three sub-arrays, one white (no filter) sub-array and two color sub-arrays,
A sensor array is described in which only white subarrays are used for grayscale detection and all three subarrays are used for color detection.

The following references describe various scanner processes and their features. KDGennette
n, MJSteinle "Designing Color Scanners"("Designi
ng ascanner with color vision ", Hewlett-Packard J
ournal, August 1993), Lee et al., "Linear model of surface and method of scanner characterization"("Linear model of surveillance").
face and scanner characterization method ", SPI
E, 2414 (1995), Hung, "Colorimetric calibration for scanners and media"
scanners and media ", SPIE, 1448, 199
1 year), Holub et al. "Color system adjustment for graphic arts: I. Input device "(" Colorsystems calibr
ation for graphic arts: I. Input devices ", Journal o
f Imaging Technology, vol. 14, no. 2,1988
April, 1997), International Color Consortium "ICC Profile Format Standards", Edition 3.4, August 15, 1997
Day).

[0031] Also, a published patent publication "Japanese Patent Laid-Open No. 11-127"
No. 359 (publication date: 1999, May 1, 1999)
1)), a selection color conversion table for each color region and an entire color conversion table for all color regions are created, and these are combined based on the contribution ratio to switch the color conversion table for each color region. However, there is described a “color conversion table creating method and a color conversion method” that can perform color conversion so that colors are continuous in an image area that spans a color area.

In addition, a published patent publication “Japanese Patent Laid-Open No. 10-136”
No. 20 (publication date: January 16, 1998 (1998))
The “image processing apparatus” determines whether the original is photographic paper based on the image data in the effective area of the original at the time of pre-scanning, and switches the filter and color correction coefficients according to the determination result. Has been described.

[0033]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and its object is to produce an error even when applied to various colorant and medium combinations. An object of the present invention is to provide a color conversion rule creation method capable of creating a color conversion rule without any color conversion method, a color conversion method according to the created color conversion rule, and a color image forming apparatus using this color conversion method.

[0034]

In order to solve the above-mentioned problems, the color conversion rule creating method according to the present invention is to convert the image data of the first color system generated by reading by the image reading means.
A color conversion rule creating method for creating a color conversion rule for converting into image data of a second color system, wherein the reference original prepared for each combination of a colorant and a medium carrying the colorant is referred to as the above. A conversion data generation process for converting the first color system conversion source data read and generated by the image reading means in accordance with a color conversion rule to generate a second color system conversion data; For each combination of the colorant and the medium, in the reference data generation process of reading the reference document by the reference data measurement unit and generating the reference data of the second color system, and in the entire combination of the colorant and the medium, A color error minimizing process for determining the color conversion rule so as to minimize the color error between the converted data and the reference data as a whole;
It is characterized by including.

According to the above-described method, conversion data obtained by reading a reference original by an image reading unit and performing color conversion in accordance with a color conversion rule and reference data read by a reference data measuring unit are generated for a combination of a plurality of colorants and a medium. Then, a color error between the conversion data and the reference data for each combination of the colorant and the medium is determined, and a color conversion rule is determined so as to minimize the color error in the entire combination of the colorant and the medium.

Therefore, one color conversion rule with a minimum color error can be created for a combination of a plurality of colorants and a medium. Therefore, even when applied to a combination of a plurality of colorants and a medium, one color conversion rule that does not cause a color error can be created.

The operation in the color error minimizing process is as follows.
Calculation methods such as regression and linear interpolation can be used. Also,
The color error between the converted data to be minimized and the reference data is 2
In addition to the average value of the squared error, various statistics such as the average value of the error and the maximum value of the error can be used. The color conversion rule is 3 ×
It may be a three-matrix or three-dimensional look-up table, or may be another format.

In order to solve the above-mentioned problems, the color conversion rule creating method of the present invention further includes a material-specific weighting process for weighting the color error for each combination of the colorant and the medium. And the above color error minimization processing is
The present invention is characterized in that the color conversion rule is determined so as to minimize the weighted color error to which the weight is added in the material-specific weighting process as a whole in the entire combination of the colorant and the medium.

According to the above method, a color error between the conversion data and the reference data is further determined for each combination of the colorant and the medium, and a weight assigned to each combination of the colorant and the medium is assigned to the color error. The color conversion rule is determined so as to minimize the obtained weighted color error in the entire combination of the colorant and the medium.

Therefore, the combination of a plurality of colorants and a medium is weighted according to the importance of the colorant and the medium (for example, the frequency of use) of the original to be read, and the weighted color error is minimized. One color conversion rule can be created.

Therefore, it is possible to create one color conversion rule that does not cause a color error even when applied to a combination of a plurality of colorants and a medium. In addition, color conversion can be performed with higher accuracy for an important document.

In order to solve the above-mentioned problems, the color conversion rule creating method of the present invention further comprises a display area of each combination of the above-mentioned colorant and medium in the second color system color space. A color gamut division process for dividing each color gamut according to the overlapping relationship between the color gamuts, and a color gamut division process for each combination of the colorant and the medium, and for each region divided by the color gamut division process. Further comprising a color gamut weighting process for weighting the color error corresponding to the conversion data included in the area, and, the color error minimization process, in the entire combination of the colorant and the medium, The present invention is characterized in that the color conversion rule is determined so as to minimize the weighted color error to which the weight has been added in the color gamut weighting process as a whole.

According to the above-described method, the color error between the conversion data and the reference data is further determined for each combination of the colorant and the medium, and the color gamut of each combination of the colorant and the medium is compared with each other. Divided according to the overlapping relationship of
A color conversion rule is set such that the weight assigned to each region of each color gamut is assigned to the color error included in the region, and the obtained weighted color error is minimized for the entire combination of the colorant and the medium. decide.

Therefore, for a combination of a plurality of colorants and a medium, the overlapping relationship between the colorant of the original to be read and the color gamut of the medium is taken into consideration (for example, weighting of an area where many color gamuts overlap). Is weighted), and one color conversion rule in which the weighted color error is minimized can be created.

Therefore, it is possible to create one color conversion rule that does not cause a color error even when applied to a combination of a plurality of colorants and a medium. In addition, it is possible to control the color conversion accuracy in consideration of the overlapping relationship between the colorant of the document to be read and the color gamut of the medium.

In order to solve the above-mentioned problems, the color conversion rule creating method of the present invention further comprises a color space dividing process for dividing the color space of the second color system into a plurality of non-overlapping divided spaces; For each of the divided spaces divided by the color space dividing process, a weighting process for each divided space for weighting the color error corresponding to the reference data included in the divided space, and the color error minimization The process determines the color conversion rule so as to minimize the weighted color error to which the weight is added in the divided space weighting process as a whole in the entire combination of the colorant and the medium. Features.

According to the above method, the color error between the converted data and the reference data is determined for each combination of the colorant and the medium, and the color space of the second color system is divided into a plurality of non-overlapping divided spaces. Then, a weight assigned to each divided space is assigned to a color error included in the divided space, and the obtained weighted color error is minimized in the entire combination of the colorant and the medium. Determine rules.

Therefore, a combination of a plurality of colorants and a medium is weighted in consideration of the importance of each divided space (for example, by increasing the weight of a color region sensitive to a human).
One color conversion rule in which the weighted color error is minimized can be created.

Therefore, it is possible to create one color conversion rule which does not cause a color error even when applied to a combination of a plurality of colorants and a medium. In addition, it is possible to control the accuracy of color conversion in consideration of the importance of the divided color space.

In order to solve the above-mentioned problem, the color conversion rule creating method of the present invention is further characterized in that the second color system is a uniform color space.

According to the above configuration, the conversion source data of the first color system generated by reading by the image reading means can be converted into conversion data of a uniform color space (such as CIE Lab color space). That is, since the image data is in a uniform color space, image data having no color error with respect to the human visual system can be obtained. In addition, weighting can be made to optimize for the human visual system.

Therefore, it is possible to create a color conversion rule capable of converting the first color system conversion source data read and generated by the image reading means into image data of a color recognized by the human visual system.

Further, in order to solve the above-mentioned problems, the color conversion method of the present invention converts the first color system image data according to the color conversion rules created by the above-described color conversion rule creation method. It is characterized in that the image data is converted into image data of the second color system.

According to the above-described method, the color error is minimized as a whole in the entire combination of the colorant and the medium.
The image data of the first color system can be converted into image data of the second color system according to one color conversion rule. In addition, the color error of the original to be read / importance of the medium (frequency of use), the importance of the divided color space, the colorant /
A more optimized color conversion rule created by performing weighting according to the color gamut of the medium can also be used. In addition, color conversion rules optimized for the human visual system can be used.

Therefore, it is possible to perform color conversion of image data obtained by reading a document of a combination of a plurality of colorants and a medium using only one color conversion rule so that no color error occurs.

Further, in order to solve the above-mentioned problem, the color image forming apparatus of the present invention converts the first color system image data according to the color conversion rule created by the above-described color conversion rule creation method. Image reading means for converting the image data into image data of the second color system, image reading means for reading the original image to generate the image data of the first color system, Image forming means for forming a color image on a medium using a colorant of a second color system based on image data.

According to the above configuration, the image reading means reads the image data obtained by reading the original of the combination of a plurality of colorants and a medium, and the image processing means optimizes the image data so that no color error occurs in the human visual system. The color conversion is performed according to only one color conversion rule, and the obtained image data can be formed into a color image by the image forming means.

Therefore, according to the color image forming apparatus, it is possible to form a high quality image in which the original image is faithfully reproduced. Moreover, since there is no need to provide a plurality of color conversion rules and switch according to the colorant and medium of the document to be read, and thus it is not necessary to specify the colorant and medium of the document, the configuration is simple and easy to use.

[0059]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS.

In the present embodiment, a digital color copying machine will be exemplified as an example of the color image forming apparatus according to the present invention, and its color conversion processing and a method of creating a color conversion rule used in the color conversion processing will be described.

As shown in FIG. 5, a digital color copying machine 50 according to the present embodiment includes a color image input unit 52 as an input color scanner, a color image forming unit 53 as an output color printer, and a scanner / printer. The image processing apparatus includes a color image processing unit 51 which is a color image processing pipeline. Then, the color image processing unit 51
Performs a process of converting the input image acquired by the color image input unit 52 into an output image suitable for the color image forming unit 53. In particular, the color image processing unit 51 includes a color conversion unit 64. In the color conversion unit 64, the original RGB signals from the color image input unit 52 are adjusted in a colorimetric color space for processing at a later stage. Convert to The details of the digital color copying machine 50 will be described later.

Here, the CIE colorimetric color space (CIE colo
In rimetric color spaces), the color is usually XY
It is specified by Z value or Lab value. The XYZ color space is
It is a tristimulus value color space based on the CIE1931 standard observer. It is device independent and is based on human color matching experiments by CIE. However, this color space is not visually uniform. That is, equal changes in the XYZ color space are not necessarily perceived as equal differences by the human visual system. Therefore, the CIE aims to minimize the perceptual differences of the human visual system,
A mathematical transformation has been developed that transforms the XYZ color space into a perceptually approximately uniform Lab color space. And since Lab values are perceptually roughly equivalent to the human visual system, they are commonly used as a calibrated colorimetric color space, and color errors are measured in this color space. An appropriate color measuring device is used for measuring the XYZ values or Lab values of each color.

FIG. 6 shows the RG read by the color scanner.
FIG. 9 is an explanatory diagram illustrating an outline of a process of converting B data into CIE Lab data.

Normally, the hard copy image 101 is read by a color scanner 102 (corresponding to the color image input unit 52), and is converted into pixel data in which the hard copy image 101 is arranged in a rectangular shape. Here, each pixel data includes the RGB signal 103 respectively. Next, these RGB signals 103 are converted into a colorimetric color space signal 105 such as a CIE Lab color space by a color conversion block 104 (corresponding to the color conversion unit 64). Here, if the color characteristics are correctly set in the color scanner 102, the output CIE Lab value image is an accurate representation of the input image in the Lab color space.

As a specific method of the color conversion processing in the color conversion block 104, for example, (a) a method using a 3 × 3 matrix multiplier 111 (FIG. 7 (a)), and (b) a three-dimensional look There is a method of interpolating using an up table (three-dimensional LUT) 112 (FIG. 7B). These techniques are well known in color conversion processing.
However, in order to obtain a good color conversion result, a good matrix or a good three-dimensional LUT is required.

Next, a method of generating a matrix or a three-dimensional LUT for color conversion of a color scanner will be described with reference to FIG.

First, a scanner target (a standard color original as a scanner calibration target) including a color patch indicating a specific photographic processing color gamut, such as an IT-8 target 121, is assigned to GretagMacbeth.
The reference Lab value (L ₀ , a ₀ , b ₀ ) is obtained by measuring with a spectrophotometer 122 such as Spectrolino of the company. Note that this reference Lab value is the target 121 of IT-8.
May be obtained from the manufacturer.

Second, by reading and adjusting the target 121 with the color scanner 102, the RGB values obtained by reading the color patches are extracted.

Third, the regression / interpolation processing section 123 performs regression processing and / or linear interpolation processing on the reference Lab value and the read RGB values to obtain an optimal 3
A 3 × 3 matrix or a 3D LUT is obtained and incorporated into the color conversion block 104 as a 3 × 3 matrix multiplier 111 or 3D LUT 112. Note that a regression process is used to obtain a 3 × 3 matrix. To obtain an optimal three-dimensional LUT, linear interpolation processing and regression processing are used.

Here, the purpose of performing the regression processing and / or the linear interpolation processing in the regression / interpolation processing section 123 is to average the square error (MSE (me
an squared error)). In other words, the regression / interpolation processing unit 123 calculates the L-value from the RGB value such that the average of the square error in the Lab color space becomes minimum.
3x3 matrix or three-dimensional L for color conversion to ab values
Seeking UT.

The Lab value of each color patch obtained in the color conversion block 104 (FIG. 6) is calculated as (L ₁ , a ₁ , b ₁ ).
Then, the average of the above square errors can be expressed by the following equation (1).

[0072]

(Equation 1)

In the above equation (1), the average of the square errors of the N color patches included in a certain target (reference document) is obtained. Instead of minimizing the average value of the square error, other error metrics such as the average value of the error (average error) and the maximum value of the error (maximum error) may be minimized. In addition, regression and linear interpolation processes are performed using software for technical calculation (for example, The MathWorks).
It may be executed using the MATLAB Optimization Toolbox.

The above formula (1) can be similarly applied to a scanner target for a combination of another medium and a colorant, and the color conversion for minimizing the average of the square error for the combination. 3 × 3 matrix or 3D L
UT can be created.

However, in general, for a combination of a certain medium and a colorant (for example, a photographic print), the above procedure,
That is, a 3 × 3 matrix or a three-dimensional LUT obtained by minimizing the average of the square error is used in combination with another medium and a colorant (for example, ink and paper for graphic art, (Such as printing on different types of paper), the mean of the square errors is unacceptably large.

In one color copying machine environment,
For example, an original is read using a combination of various colorants and media, such as photographic prints, graphic arts, color laser prints, and ink jet prints on various types of paper. In this regard, different colorant and media combinations have different color gamuts. Also, the probability of reading each colorant and media combination in a typical office environment can be estimated. In addition, the human visual system is sensitive to certain regions of the color space, such as neutral gray and skin color.

Therefore, if these information are used, the region of the different color gamut of the combination of the different colorant and the medium can be used.
Each weighting factor can be assigned. Using the regression and interpolation techniques based on these weighting factors, one scanner color conversion rule can be used to minimize the weighted color error for various colorant and media combinations. Can be created by According to this method, the overall color conversion error can be controlled for various combinations of the colorant and the medium.

In the color conversion rule creating method according to the present embodiment, it is considered that a combination of a plurality of colorants and a medium simultaneously obtains the optimum coefficient of the model. In the above-described color conversion rule creation method, different weighting factors are assigned to regions of different color gamuts in which different colorants and media are combined.

Specifically, the above-mentioned color conversion rule creation method is as follows.
When creating color conversion rules (matrix or three-dimensional LUT), the following three points are used to assign weights specified for each condition to data measured under different conditions, and combinations of colorants and media used. Weighting according to the importance of the regions of the color space Considering the weighting according to the position of the color gamut based on the differences in the color gamuts of the colorants and the medium.

Hereinafter, the above three points will be described in order with reference to FIGS.

(1) Importance of Colorant / Medium of Original to be Read (Frequency of Use) FIG. 1 is a flowchart showing an outline of a process for creating a color conversion rule. FIG. 2 is an explanatory diagram showing a process of creating a color conversion rule by assigning a weighting factor to a combination of a coloring material and a medium to a color error.

A scanner target (reference original) referred to as a fragment of the input color space is created for each combination of a plurality of colorants and a medium (hereinafter, sometimes referred to as a “material”). Each target then contains a number of color patches that are evenly extracted from the entire color gamut of the colorant and medium combination. Here, the number of combinations of the colorant and the ink medium, that is, the number of targets is L.

First, each target is read by the color image input unit 52 (FIG. 5), which is a color scanner, and the RGB values (RGB set # 1, RGB set # 1) of the color patches on each target are read.
.., #L) (conversion source data) (S11: conversion data generation process). Note that the process of performing color conversion from RGB values to Lab values (conversion data) according to the provisional color conversion rules is performed by the weighting regression / interpolation processing unit 1 in the process of repeatedly calculating while correcting the color conversion rules. Done at the same time.

Further, each target was designated as Gretag Spectrolino.
The CIELab value (reference Lab value: reference Lab set # 1,..., #L) (reference data) is generated by measuring with a spectrophotometer (reference data measuring means) such as (S12:
Reference data generation processing).

Next, the weighted regression / interpolation processing unit 1
Material-specific weighting factors W _1i (i = 1,..., L) preset for each colorant and medium combination are
Each is assigned to a corresponding target (S13: weighting process).

Then, the weighted regression / interpolation processing unit 1
An RGB value (first color system) based on the RGB value of each target, the reference Lab value, and the material-specific weighting factor W _1i so that the average of the weighted square errors in the Lab color space is minimized. 3 × 3 matrix 11 or three-dimensional LUT 12 for color conversion from image data to Lab values (second color system)
(S14: color error minimizing process).

Specifically, the color conversion rules for the combination of L colorants and the ink medium (that is, 3 × color conversion)
When creating the three-matrix 11 or the three-dimensional LUT 12), it is necessary for the weighted regression / interpolation processing unit 1 to minimize the weighted mean square error (WMSE) represented by the following equation (2). Be the purpose. For this purpose, the weighted regression / interpolation processing unit 1 can execute an operation by regression and linear interpolation.

[0088]

(Equation 2)

Each (MSE) _i in the equation (2) is calculated by the equation (1). (L ₀ , a ₀ , b ₀ ) in the equation (1) is set to (L ₁ ,
a ₁ , b ₁ ) Lab obtained by color-converting the read RGB values
Corresponds to the value. The sum of the material-specific weighting factors W _1i is 1 (Equation (3)).

[0090]

(Equation 3)

Here, the combination of the colorant and the medium to be read
Selection and weighting factor W for each material _1iIs the weight of the combination
It can be set according to the necessity. For example, Colors
Probability of actually reading in the canna, ie, color copier
May be set according to the market share of the target market.
As a specific example, the following 6 is used as a combination of a coloring agent and a medium.
Consider the case where one is selected. Kodak Photo
Prints, graphic art materials, Heuret
Printing by To Packard's inkjet printer
Printing on photo paper, Tektronix
Laser printer output, Canon laser printer
Output from linters to Xerox laser printer
Output.

In this case, statistically, most of the materials copied by the color copying machine are graphic art materials. So, first of all,
0.6 is assigned as a material-specific weighting factor. next,
The second most frequently copied material is photographic prints. Therefore, 0.2 is assigned as a material-specific weighting factor to the target of a Kodak photo print. Then there are many occasions where prints from inkjet and laser printers are copied. Therefore, 0.1 is assigned as each material-specific weighting factor. Here, Hewlett-Packard's inkjet targets are only materials in this category, so the material-specific weighting factor is 0.1. In contrast, laser printer targets include Tektronix, Canon,
Since Xerox laser printer targets are included, each has a material-specific weighting factor of about 0.1.
033 each.

(2) Importance of Each Color Space Region FIG. 3 shows a process of creating a color conversion rule by assigning a weighting factor to each color region (division space) obtained by dividing a CIE Lab color space into color errors. FIG.

First, the CIE Lab color space Ω has a plurality of
(M) non-overlapping regions Ω₁, Ω_Two,…, Ω_M Can be divided into

[0095]

(Equation 4)

Here,

[0097]

(Equation 5)

Therefore, the weighted regression / interpolation processing unit 1 shown in FIG. 3 uses a plurality of colorant / medium combination targets and _{assigns a} material-specific weighting factor W _1i for each combination.
And assigning a weighting factor W _2j for each color region in each divided color region of the Lab color space (S
13) A color conversion rule can be determined.

That is, the weighted regression / interpolation processing unit 1 sets the weighting factor for each color region W _2j (for each color region having a different color gamut) according to the relative importance of each color region. j = 1,..., M) to the corresponding color patches of all targets. Then, the weighted regression / interpolation processing unit 1 calculates a weighted square in the Lab color space based on the RGB value, the reference Lab value, the material-specific weighting factor W _1i , and the color-region-specific weighting factor W _2j of each target. A 3 × 3 matrix 11 or a three-dimensional LUT 12 for color conversion from RGB values to Lab values is obtained so that the average of the errors is minimized.

Specifically, the color conversion rule for the combination of the L colorants and the ink medium (that is, 3 × color conversion)
3 matrix 11 or 3D LUT 12)
When the b color space Ω is divided into _M non-overlapping regions Ω ₁ , Ω ₂ ,..., Ω _M , the average (WMSE) of the weighted square errors represented by the following equation (6) ) Is the purpose of the weighted regression / interpolation processing unit 1.

[0101]

(Equation 6)

Here,

[0103]

(Equation 7)

As in the case of the equation (1), (L ₀ , a ₀ , b ₀ ) in the equation (7) becomes (L ₁ ,
a ₁ , b ₁ ) Lab obtained by color-converting the read RGB values
Corresponds to the value. Also, the weighting factor W _2j (j =
, M) is 1 (Equation (8)).

[0105]

(Equation 8)

The sum inside the above equation (7) is the i-th
Error for color patches of combinations of different colorants and media
Belongs to the j-th color area in the CIE Lab color space
N _ijThis is the sum of the individual color patches. In addition, coloring
Target and Lab for Combination of Agent and Medium
The color gamut of the color space means that all color gamut
Are designed to include one or more color patches. Yo
And N_ijDoes not become zero.

Here, a plurality of color regions in the Lab color space are
Division and weighting factor W for each color region _2jIs the weight of the color area
It can be set according to the necessity. For example, the human eye
Three color regions: almost halftone, depending on the characteristics of the sense system
Gray (near-neutral gray) color area, skin color area
Gamut and CIEs not included in these two color regions
The color region may be divided into color regions including the Lab color space. This
In the case of
Due to its sensitivity, this color area is
And assign 0.5. Next, the human observation organ is the second
The color area to pay attention to is the color area of skin tone.
Therefore, 0.3 is added to this color area as a weighting factor for each color area.
assign. The remaining color areas in the color space
0.2 is assigned as a regional weighting factor.

As described above, in the above-described color conversion rule creating method, the selection of the color region and the weighting factor for each color region are determined depending on the usage in the color copying machine. Note that Lab
The number of divisions of the color space can be appropriately selected.

(3) Color gamut of colorant / medium FIG. 4 is a diagram showing a-color data at an L level in the CIE Lab color space.
It is explanatory drawing which shows two typical color gamuts on a b plane. Here, the gamut volume A is the first gamut volume including the color gamut of an ink jet print or a photographic print.
Represents a set of media types. on the other hand,
The color gamut amount B represents a set of second materials including a colorant of a graphic art and a color gamut of a medium.

As shown in FIG. 4, the entire color gamut including these two color gamuts belongs to three color regions: color region A 、 B common to both color gamut A and B, color gamut A It can be divided into a color area AB that does not belong to the color gamut B and a color area BA that belongs to the color gamut B but does not belong to the color gamut A.

The weighted regression / interpolation processing unit 1
Utilizes the difference of each color gamut for a combination of a plurality of colorants and a medium. That is, the weighted regression / interpolation processing unit 1 can calculate by assigning a color gamut difference weighting factor according to the overlapping relationship between the color gamuts of the materials using the targets of the combination of a plurality of colorants and the medium.

For example, in the case of FIG.
Two color gamut difference weighting factors W _A1 and W _A2 are respectively assigned to the overlapping region and the non-overlapping region. Similarly, in the color gamut B, two color gamut difference weighting factors _WB1 and _WB2 are assigned to an overlapping area and a non-overlapping area with the color gamut A, respectively.

The weighted regression / interpolation processing unit 1
RGB values, reference Lab values, material-specific weighting factors W _1i , and color gamut difference weighting factors W _A1 , W _A2 , W
Based on _B1 and W _B2 , La values are calculated from the RGB values so that the average of the weighted square errors in the Lab color space is minimized.
A 3 × 3 matrix 11 or a three-dimensional LUT 12 for color conversion to b value is obtained. Specifically, in the case of FIG. 4, the weighted regression / interpolation processing unit 1 performs an operation for minimizing the average (WMSE) of the weighted square errors represented by the following equation (9).

[0114]

(Equation 9)

Here, the subscript of the MSE in the equation (9) indicates the color gamut for calculating the average of the square error according to the equation (1), and the superscript indicates the reference Lab value and the read RGB value. Indicates the target from which was obtained.

Note that the number of combinations of the colorant and the medium, that is, the number of color gamuts is not limited to two, but can be set arbitrarily.
The weighted regression / interpolation processing unit 1 can also weight the color gamut difference weighting factors in combination with the above-described material-specific weighting factors W _1i and color-region-specific weighting factors W _2j . The specification can be controlled more precisely.

As described above, the above-described method for creating a color conversion rule is suitable for creating a color conversion rule for a scanner mounted on a document copying machine that reads a document using a combination of a plurality of colorants and a medium. However, the present invention is not limited to this. In a process of generating a color conversion rule, a basic technique of assigning a weighting factor to each of a combination of a colorant and a medium and a region of a color space has a problem of conditional color matching. Applicable to all color image input devices.

A color image forming apparatus provided with a color image processing section for performing color conversion according to the color conversion rules created by the above-described color conversion rule creation method will be described below with reference to FIG.

FIG. 5 is a block diagram schematically showing the configuration of a digital color copying machine 50 which is an example of the color image forming apparatus.

As shown in FIG. 5, the color image input unit 52
, A color image processing unit 51 and a color image forming unit 53 are connected in this order, and a digital color copying machine 50 is configured as a whole.

The color image processing unit 51
A D / D conversion unit 60, a shading correction unit 61, an input gradation correction unit 62, an area separation processing unit 63, a color conversion unit 64, a spatial filter processing unit 65, an output gradation correction unit 66, and a gradation reproduction processing unit 67. It is configured.

The color image input section (image reading means) 2 is constituted by a scanner section provided with, for example, a CCD (charge coupled device), and reads a reflected light image from a document as RGB analog signals by the CCD. , Input to the color image processing unit 51.

The analog signal read by the color image input unit 52 is passed through the A / D conversion unit 60, the shading correction unit 61, the input gradation correction unit 62, the area separation processing in the color image processing unit 51. The signal is sent to the color image forming unit 53 as a CMYK digital color signal in the order of the unit 63, the color conversion unit 64, the spatial filter processing unit 65, the output gradation correction unit 66, and the gradation reproduction processing unit 67.

The A / D (analog / digital) converter 60 converts an RGB analog signal into a digital signal. The shading correction unit 61
Digital RGB sent from A / D converter 60
The signal is subjected to processing for removing various types of distortion generated in the illumination system, the imaging system, and the imaging system of the color image input unit 52.

The input tone correction section 62 adjusts the color balance of the RGB signals (RGB reflectance signals) from which various types of distortion have been removed by the shading correction section 61 and simultaneously performs image processing such as density signals. A process for converting the signal into a signal which can be easily handled by the image processing system employed in the section 1 is performed.

The area separation processing section 63 converts each pixel or a block composed of a plurality of pixels in the input image from the RGB signal sent from the input gradation correction section 62 into a character area, a halftone area, a photograph area, and the like. Are separated into regions. Then, based on the result of the separation, the region separation processing unit 63 converts the region identification signal indicating to which region the pixel or the block belongs to the color conversion unit 64, the spatial filter processing unit 65, the gradation reproduction processing unit 67. And the RGB signals input from the input tone correction unit 62 are output to the subsequent color conversion unit 64 as they are.

The separation method in the area separation processing section 63 is described in, for example, the document “Method of identifying halftone pictures” (Transactions of the Institute of Electronics, Information and Communication Engineers, 1987/2 Vol.2 J70-B No.2).
"Block Separate Transformation Method: BST" published in p.222-p.232.)
M method) can be used. In this method, image data to be processed is divided into blocks, and non-halftone areas (characters / photographs) and halftone areas are separated based on density changes in the blocks. This method has a feature that pixels having a high density are spatially dispersed because halftone dots are periodically arranged in a halftone dot region.

The color conversion section 64 converts the RGB signals (image data of the first color system) into CMY signals (image data of the second color system: color correction signals). (Color correction processing). Further, in order to realize faithful color reproduction, a process for removing color turbidity based on the spectral characteristics of the CMY color materials including unnecessary absorption components is performed. Further, a black generation process for generating a black (K) signal from the corrected CMY signal and an undercolor removal process for generating a new CMY signal by subtracting the K signal obtained by the black generation from the CMY signal are performed. , CMY signals are converted to CMYK signals. The details of the color conversion unit 64 will be described later.

The spatial filter processing section 65 performs a spatial filter processing by a digital filter on the image data of the CMYK signal input from the color conversion section 64 based on the area identification signal generated by the area separation processing section 63. Do
The processing is performed to correct the spatial frequency characteristic so as to prevent the output image from being blurred or degraded in graininess.

For example, the region separated into characters by the region separation processing section 63 is subjected to a sharp emphasis processing of the spatial filter processing by the spatial filter processing section 65 in order to enhance the reproducibility of a black character or a color character. The emphasis amount is increased. At the same time, the tone reproduction processing section 67 selects binarization or multi-value processing on a high-resolution screen suitable for reproduction of a high frequency band.

On the other hand, with respect to the area separated into halftone dots by the area separation processing section 63, an appropriate processing such as a low-pass filter processing for removing an input halftone dot component is performed in the spatial filter processing section 65. . The output tone correction unit 66 outputs a signal such as a density signal to the color image forming unit 53.
After performing an output tone correction process for converting to a dot area ratio which is a characteristic value of the above, a tone reproduction processing section 67 performs processing so that an image is finally separated into pixels and each tone can be reproduced. Is performed (halftone generation). With respect to the regions separated into photographs by the region separation processing unit 63, binarization or multi-value processing is performed on a screen that emphasizes tone reproducibility.

The image data of the CMYK signal subjected to each of the above-described processes is temporarily stored in storage means (not shown).
The color image forming unit 53 is read out at a predetermined timing and is read out.
Is input to This color image forming section (image forming means)
53 is for outputting image data on a recording material (for example, paper, etc.), and is not particularly limited.
An image forming unit using an electrophotographic system or an inkjet system can be used.

Next, the details of the color conversion section 64 will be described. As shown in FIG. 5, the color conversion unit 64
Color coordinate converter 64A and second color coordinate converter 64B
It is provided with.

The first color coordinate conversion unit 64A converts the RGB signal image data input from the input gradation correction unit 62 into CIE (Commission Internationale de l'Eclaira).
ge: CI which is a uniform color space defined by the International Commission on Illumination)
The data is converted to E 1976 Lab color space (CIELAB color space) data. In the above Lab data, L represents lightness, and a and b represent chromaticity.

In the present embodiment, the method described with reference to FIG. 1 is used as a conversion method for converting RGB data into Lab data in the first color coordinate conversion section 64A. That is, each color patch of the color chart document (reference document) is measured with a colorimeter to obtain a Lab value (Lab color system data), and the color chart document used for the color measurement is input from the color image input unit 52 to the color image input unit 52. The RGB data is obtained by reading, and based on the Lab value and the RGB value thus obtained,
A coefficient for associating the two is obtained, and color coordinate conversion is performed using the obtained coefficient. The above coefficient can also be obtained by using a neural network or a masking operation coefficient determination method.

The first color coordinate conversion unit 64A is configured to perform a masking operation using a matrix or a look-up table (LU) for the color coordinate conversion.
T: Lookup table) may be used to perform the interpolation operation.

The second color coordinate conversion unit 64B converts the Lab data converted by the first color coordinate conversion unit 64A into C (cyan), M (magenta), and Y (yellow), and generates black. The amount K is calculated, and the black generation amount K is subtracted from the converted CMY value to obtain an output value C'M'Y '(under color removal), which is output as a CMY signal to the spatial filter processing unit 65 together with the K signal. I do.

In the second color coordinate conversion section 64B,
As a conversion method for converting Lab data into CMY data, for example, the following method can be used. That is, color image data (CMY) having color coordinates that can be output by the color image forming unit 53 is output, and the color image data is measured by the above-described colorimeter to obtain data of the Lab color system, and the colorimetric La is output.
In order to associate the b value with the image data (CMYK) of the color coordinates, a coefficient between the Lab value and the CMY value is obtained by a neural network or a masking operation coefficient determination method, and color coordinates conversion is performed using the obtained coefficient.

Therefore, the second color coordinate conversion section 64B is also configured to perform the above-described matrix-based masking operation for the color coordinate conversion, or to perform the interpolation operation using the LUT. You may.

The present embodiment does not limit the scope of the present invention, and various changes can be made within the scope of the present invention. For example, the present invention can be configured as follows.

In the present embodiment, the color conversion rule for a combination of a plurality of colorants and a medium is created as a 3 × 3 matrix or a three-dimensional LUT for color conversion. However, the present invention is not limited to this. Any other format may be used as long as it performs coordinate conversion from to the second color space.

In this embodiment, a digital color copying machine has been described as an example of a color conversion rule creating method and a color image forming apparatus using the color conversion method according to the present invention. However, the present invention is not limited to this. That is, the color conversion rule creating method, the color conversion method, and the color image forming apparatus according to the present invention include, for example, a color scanner combining a color image input unit 52 and a color image processing unit 51, a color image processing unit 51 and a color image processing unit. The present invention can also be applied to a color printer combined with the forming unit 53, a color facsimile having a facsimile function, and the like.

Further, the color conversion method according to the present invention is a method of converting the read RGB values into a colorimetric color space, and the type of a plurality of input materials (combinations of a colorant and a medium) to be read is determined. Identifying process, assigning a weighting factor to each type of input material, dividing input color space into partial regions, assigning weighting factors to partial regions of each input color space, regression and interpolation techniques The method may include a process of creating a color conversion rule by minimizing a color error by using the above, and a process of generating a colorimetric output according to the color conversion rule.

Further, in the above-mentioned color conversion method, the processing for specifying the types of the plurality of input materials may be carried out by printing a Kodak photo print medium, a graphic art medium, or a Hewlett-Packard ink jet print on a photographic paper. , Output from Tektronix laser printers,
The process may specify at least one type of input material selected from a group consisting of an output from a Canon laser printer and an output from a Xerox laser printer.

The color conversion method may further include a process of dividing the colorimetric color space into non-overlapping regions and a process of assigning a weighting factor to each non-overlapping region.

Further, in the color conversion method, the process of dividing the colorimetric color space into non-overlapping regions may be performed by dividing the colorimetric color space into a substantially halftone gray region and a skin color region. And a process of dividing the image into other regions.

Further, in the color conversion method, the process of dividing the input color space into partial regions may be performed by converting the input color space into a color gamut amount indicating a first set of input material types and a second
And a process of dividing the color gamut into a color gamut amount indicating the set of color gamut.

Further, the color conversion method further comprises: combining the color gamut of the first and second sets of input material types into a region common to both input material sets and a first set. A process of dividing into areas that belong but do not belong to the second set, and areas that belong to the second set but do not belong to the first set; Are assigned weighting factors,
Assigning a weighting factor to each of the overlapping region and the non-overlapping region for the second set.

In the above color conversion method, the process of creating a color conversion rule by minimizing a color error by using regression and interpolation techniques is performed in each area of a colorimetric color space divided non-overlapping. According to the weighting factor assigned to
The weighted average of the squared error may be minimized. Note that the sum of the weighting factors is 1.

In the above color conversion method, the processing of dividing the colorimetric color space into non-overlapping ranges is performed by dividing the colorimetric color space into a substantially halftone gray area, a flesh color area, and other areas. May be divided.

In the color conversion method, the process of dividing the input color space into partial regions may include the step of dividing the input color space into a color gamut amount indicating a first set of input medium types and a second set. It may be divided into the indicated color gamut amounts.

[0152]

According to the color conversion rule creating method of the present invention, as described above, image data of the first color system generated by reading by the image reading means is converted into image data of the second color system. A color conversion rule creating method for creating a color conversion rule for creating a color conversion rule, wherein a first document prepared by reading a reference original prepared for each combination of a colorant and a medium carrying the colorant by the image reading means is generated. A conversion data generating process for converting color conversion source data into color conversion rules in accordance with a color conversion rule to generate second color conversion data; Is read by the reference data measuring means to generate reference data of the second color system, and the color error between the conversion data and the reference data in the entire combination of the colorant and the medium. Minimize as a whole So that the, the method comprising the color error minimization process for determining the color conversion rules, and.

Therefore, it is possible to create one color conversion rule in which the color error is minimized for a combination of a plurality of colorants and a medium. Therefore, even when applied to a combination of a plurality of colorants and a medium, one color conversion rule that does not cause a color error can be created.

As described above, the color conversion rule creating method of the present invention further includes a material-specific weighting process for weighting the color error for each combination of the colorant and the medium. The color conversion rule is determined so that the error minimizing process minimizes the weighted color error to which the weight is added in the material-specific weighting process as a whole in the entire combination of the colorant and the medium. Is the way.

Therefore, a combination of a plurality of colorants and a medium is weighted according to the importance of the colorant of the document to be read and the medium (for example, frequency of use).
One color conversion rule in which the weighted color error is minimized can be created. Therefore, there is an effect that one color conversion rule that does not cause a color error even when applied to a combination of a plurality of colorants and a medium can be created. In addition, the color conversion can be performed with higher accuracy for an important document.

As described above, the color conversion rule creating method according to the present invention further includes the step of further setting each color gamut, which is the displayable area of each combination of the colorant and the medium, in the color space of the second color system. A color gamut dividing process for dividing the color gamut according to the overlapping relationship with each other, and a combination of the colorant and the medium, and a region divided by the color gamut dividing process included in the region. A color gamut weighting process for weighting the color error corresponding to the conversion data,
In addition, the color error minimizing process minimizes the weight of the color error weighted by the color gamut-specific weighting process as a whole in the entire combination of the colorant and the medium. This is a method for determining a conversion rule.

Therefore, for the combination of a plurality of colorants and a medium, the overlapping relationship between the colorant of the document to be read and the color gamut of the medium is taken into consideration (for example, many color gamuts overlap). Weighting can be performed (by increasing the weight of the region) to create a single color conversion rule that minimizes weighted color errors. Therefore, there is an effect that one color conversion rule that does not cause a color error even when applied to a combination of a plurality of colorants and a medium can be created. In addition, the color conversion accuracy can be controlled in consideration of the overlapping relationship between the colorant of the document to be read and the color gamut of the medium.

As described above, the color conversion rule creating method according to the present invention further comprises a color space dividing process for dividing the color space of the second color system into a plurality of non-overlapping divided spaces. For each divided space divided by the process, further includes a divided space weighting process that weights the color error corresponding to the reference data included in the divided space, and the color error minimizing process, In this method, the color conversion rule is determined so as to minimize the weighted color error to which the weight is added in the divided space weighting process as a whole in the entire combination of the colorant and the medium.

Therefore, a combination of a plurality of colorants and a medium is weighted in consideration of the importance of each divided space (for example, by increasing the weight of a sensitive color region by a human). , One color conversion rule in which the weighted color error is minimized. Therefore, there is an effect that one color conversion rule that does not cause a color error even when applied to a combination of a plurality of colorants and a medium can be created. Moreover,
This brings about an effect that the precision of color conversion can be controlled in consideration of the importance of the divided color space.

As described above, according to the color conversion rule creating method of the present invention, the second color system is a uniform color space.

Therefore, a color conversion rule capable of converting the conversion source data of the first color system generated by reading by the image reading means into image data of a color recognized by the human visual system is created. This has the effect that it can be performed.

Further, as described above, according to the color conversion method of the present invention, the image data of the first color system is converted into the second image data in accordance with the color conversion rule created by the above-described color conversion rule creation method. This is a method of converting to color system image data.

Therefore, it is possible to perform color conversion of image data obtained by reading an original of a combination of a plurality of colorants and a medium by using only one color conversion rule so that no color error occurs. This has the effect.

Further, as described above, the color image forming apparatus of the present invention converts the image data of the first color system into the second table in accordance with the color conversion rules created by the above-described color conversion rule creation method. Image processing means for converting the image data into color image data; image reading means for reading the original image to generate the first color system image data; and image processing means for converting the image data based on the second color system image data. And an image forming means for forming a color image on a medium using a colorant of the second color system.

Therefore, according to the color image forming apparatus,
This has the effect of forming a high-quality image faithfully reproducing the original image. Moreover, by providing a plurality of color conversion rules,
There is no need to switch according to the colorant and medium of the document to be read, and hence there is no need to specify the colorant and medium of the document, so that there is an effect that the configuration is simple and easy to use.

[Brief description of the drawings]

FIG. 1 is a flowchart showing an outline of a process for creating a color conversion rule according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a process of creating a color conversion rule by assigning a weighting factor for each material to a combination of a coloring material and a medium to a color error in the process shown in FIG. 1;

FIG. 3 shows an example of the processing shown in FIG.
FIG. 9 is an explanatory diagram showing a process of creating a color conversion rule by assigning a weighting factor for each color region to each color region obtained by dividing the ab color space.

FIG. 4 is a diagram showing a- at a certain L level in a CIE Lab color space.
It is explanatory drawing which shows two typical color gamuts on a b plane.

FIG. 5 is a block diagram schematically showing the configuration of a digital color copying machine that performs color conversion using the color conversion rules created by the processing shown in FIG.

FIG. 6 shows the RGB data read by the color scanner as C
FIG. 4 is an explanatory diagram illustrating an outline of a process of converting into IE Lab data.

FIG. 7 is an explanatory diagram showing a specific method of a color conversion process;
FIG. 7A shows a method using a 3 × 3 matrix multiplier,
FIG. 7B shows a method using a three-dimensional lookup table.

FIG. 8 is a diagram illustrating a method of generating a matrix or a three-dimensional LUT for color conversion of a color scanner.

[Explanation of symbols]

11 3 × 3 matrix (color conversion rule) 12 3D LUT (color conversion rule) 50 digital color copier (color image forming apparatus) 51 color image processing unit (image processing unit) 52 color image input unit (image reading unit) 53 Color image forming unit (image forming means) S11 (conversion data generation processing) S12 (reference data generation processing) S13 (weighting processing by material, color gamut division processing, weighting processing by color gamut, color space division processing, weighting by division space Processing) S14 (Color error minimization processing)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C262 AA26 AC02 AC04 BA01 BC13 BC19 EA11 5B057 AA11 BA02 CA01 CA08 CA12 CA16 CB01 CB08 CB12 CB16 CC01 CE17 CE18 CH07 CH08 5C077 LL19 MM27 MP08 PP32 PP33 PP36 PP37 PP48 PQ12 P02 TT06 5C079 HB01 HB03 HB05 HB08 HB12 KA15 LB02 MA04 MA11 NA03 NA27 PA02

Claims (7)

[Claims] 1. A color conversion rule creating method for creating a color conversion rule for converting image data of a first color system generated by reading by image reading means into image data of a second color system. The conversion source data of the first color system generated by reading the reference original prepared for each combination of the colorant and the medium carrying the colorant by the image reading means is subjected to color conversion according to a color conversion rule. Conversion data generation processing for generating conversion data of the second color system, and reading the reference document with reference data measurement means for each combination of the colorant and the medium, and reading the conversion data of the second color system. A reference data generation process for generating reference data; and determining the color conversion rule so as to minimize the color error between the conversion data and the reference data as a whole in the entire combination of the colorant and the medium. Color error The color-conversion-rule making method characterized by comprising a process, a. 2. The method according to claim 2, further comprising a weighting process for each material for weighting the color error for each combination of the colorant and the medium, and wherein the color error minimization process includes the combination of the colorant and the medium. 2. The color conversion rule creation method according to claim 1, wherein the color conversion rule is determined so as to minimize the weighted color error to which the weight is added in the material weighting process as a whole. Method. 3. A color gamut, which is a displayable area of each combination of the colorant and the medium in the color space of the second color system, is divided according to an overlapping relationship between the color gamuts. Color gamut dividing processing, and for each combination of the colorant and the medium, for each region divided by the color gamut dividing process, a color gamut for weighting the color error corresponding to the conversion data included in the region And a color error minimizing process, and the color error minimizing process collectively includes the weighted color errors weighted by the color gamut weighting process as a whole in the entire combination of the colorant and the medium. 3. The method according to claim 2, wherein the color conversion rule is determined so as to minimize the color conversion rule. 4. A color space dividing process for dividing the color space of the second color system into a plurality of non-overlapping divided spaces; and a divided space divided by the color space dividing process is included in the divided space. A weighting process for each divided space that weights the color error corresponding to the reference data, and wherein the color error minimizing process is performed for all combinations of the colorant and the medium. The color conversion rule creating method according to claim 1, wherein the color conversion rule is determined so as to minimize the weighted color error to which the weight is added in the divided space weighting process as a whole. 5. The method according to claim 1, wherein the second color system is a uniform color space. 6. The image data of the first color system according to the color conversion rules created by the color conversion rule creating method according to claim 1. A color conversion method characterized by converting into color system image data. 7. Image processing for converting image data of a first color system into image data of a second color system according to a color conversion rule created by the color conversion rule creating method according to claim 5. Means for reading an original image to generate image data of the first color system; and coloring of a second color system based on the image data of the second color system. An image forming means for forming a color image on a medium using an agent.