JP2002262120A - Image processing apparatus, image processing method, color conversion table creation method, and recording medium - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To perform mapping so that an input color signal of the same hue with a small color change becomes an output color signal of a hue with a minimum color difference. A gamut conversion processing unit includes a reference color signal calculation unit, a corresponding color determination unit, an output hue determination unit,
The mapping unit 24 includes gamut data 25 of an input device and gamut data 26 of an output device. When the input color signal Pi is input, the reference color signal calculation unit 21 refers to the gamut data 25 of the input device, and determines the maximum chroma color on the hue whose hue matches the input color signal Pi as the reference color signal. The corresponding color determination unit 22 refers to the gamut data 26 of the output device, and calculates a corresponding color signal M that minimizes the color difference within the color reproduction range of the output device corresponding to the reference color signal T. The output hue determination unit 23 determines the hue h ′ of the output color signal Po based on the corresponding color signal M.
Is determined, and the brightness and saturation components are calculated by the mapping processing unit 24 and output as the output color signal Po.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus, an image processing method, a method for creating a color conversion table, and a recording medium. A color reproduction processing apparatus for converting input color image information into color image information within a color reproduction range of an output system, for example, a color image output apparatus such as a color facsimile, a color printer, and a color copying machine, and a color reproduction apparatus used in the apparatus. The present invention relates to software for generating color conversion parameters.

[0002]

2. Description of the Related Art Conventionally, when a system that handles image data such as a printing system using image input / output devices having different characteristics, such as a CRT display, a scanner, and a printer, is constructed, the color reproduction range between each device ( This is a problem when performing color conversion processing between different devices.

In order to solve the above-mentioned problem, a technique of mapping colors that cannot be reproduced by an output device to reproducible colors in a three-dimensional space having lightness, saturation, and hue as axes (hereinafter referred to as gamut compression) has been proposed. Several schemes are known and have been proposed.

With respect to the conventional color reproduction processing technology, for example, for a color which cannot be reproduced by an output device among input color signals described in JP-A-10-84487 (image processing apparatus and method), 2. Description of the Related Art There is known a technique in which, among colors that can be reproduced by an output device, the color difference calculated by changing the weights of the brightness difference, the saturation difference, and the hue difference is a color that minimizes the color difference. Further, a saturation axis having the same hue as an achromatic axis or an input color signal described in JP-A-9-168097 (color signal conversion device) and JP-A-9-18727 (color reproduction device). A technique is known in which a projection target point is set above, and a color outside the gamut of the output device is set to a constant hue and compression-mapped in the gamut of the output device.

[0005]

The above-mentioned JP-A-10-8
The method proposed in Japanese Patent No. 4487 has a feature that the mapping direction is switched to the direction having the least color difference according to the shape of the color reproduction range, and the color change before and after color reproduction processing is small. However, in this method, the mapping direction depends on the shape of the color gamut of the output system, and the continuity of the mapping direction may not be maintained. In such a case,
An input color that changes continuously is mapped to a color ahead in a discontinuous mapping direction by a color reproduction process, so that the image quality is degraded such as crushed gradation or skipping.

In the methods proposed in Japanese Patent Application Laid-Open Nos. 9-168097 and 9-18727, gamut compression with excellent continuity is performed by performing conversion while maintaining hue. However, while trying to maintain the hue, there is a case where a color may be mapped to a color having a large color difference even though a color having a small color difference is originally present, and insufficient color saturation or the like is conspicuous.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and the input color signal of the same hue is inconspicuous regardless of the hue of the input color signal and the shape of the color reproduction range of the image output device. High-precision color gamut mapping so that output color signals of the same hue are obtained; control of the hue of the output color signal so that the output color signal does not concentrate on a specific hue; High-speed determination of a reference color signal used in determining hue, high-speed determination of a color corresponding to a reference color signal, inconspicuous and the same color change without calculating a reference color signal for each input color signal The purpose is to perform gamut mapping so that an input color signal of hue always becomes an output color signal of the same hue, and to provide a color conversion table incorporating gamut mapping. That.

[0008]

According to a first aspect of the present invention, there is provided an image processing apparatus for mapping an input color signal to a color signal within a color reproduction range of an image output device, the reference color having a hue coincident with the input color signal. Means for obtaining a signal, and means for obtaining a corresponding color within the color reproduction range with respect to the reference color signal, determining a hue of an output color signal based on the corresponding color, and It is characterized by mapping to a color signal on the hue of the output color signal.

A second aspect of the present invention is characterized in that, in the first aspect of the present invention, a color having a minimum color difference from the reference color signal among the colors within the color reproduction range is the corresponding color. It is.

A third aspect of the present invention is the invention according to the first or second aspect, wherein the hue of the output color signal is made to coincide with the hue of the corresponding color.

According to a fourth aspect of the present invention, in the first or second aspect, the hue of the output color signal is obtained by linearly interpolating the hue of the corresponding color and the hue of the reference color signal based on a predetermined conversion formula. It is characterized by having means.

According to a fifth aspect of the present invention, in the first or the second aspect of the present invention, there is provided a means for obtaining a color signal having the same hue as the input color signal and the highest chroma in the input color reproduction range as the maximum chroma color. , Wherein the maximum chroma color is used as the reference color signal.

According to a sixth aspect of the present invention, in accordance with the fifth aspect of the present invention, the maximum chroma color is obtained at a predetermined hue interval in the input color reproduction range, and the correspondence between the hue and the maximum chroma color is described in a table. And means for reading the table to determine a plurality of maximum chroma colors corresponding to hues adjacent to the hue of the input color signal, and obtaining the reference color signal by interpolating the plurality of maximum chroma colors. It is characterized by the following.

According to a seventh aspect of the present invention, in the first or second aspect, the mapping to the color reproduction range is performed in a color system based on a color appearance model.

According to an eighth aspect of the present invention, in the second aspect, means for comparing the hue of the reference color signal with the six hues represented by the primary color and the secondary color of the image output device. And means for determining a search range of the corresponding color based on the comparison result of the magnitude comparison.

According to a ninth aspect of the present invention, in an image processing apparatus for mapping an input color signal to a color signal within a color reproduction range of an image output device, the correspondence between the hue of the input color signal and the hue of the output color signal is described. And mapping the input color signal to a color signal on the hue of the output color signal corresponding to the hue of the input color signal when mapping the input color signal. .

According to a tenth aspect of the present invention, in accordance with the ninth aspect of the present invention, the means for obtaining a reference color signal at a predetermined hue interval in the input color reproduction range, Means for determining the hue of the output color signal based on the corresponding color, and means for describing the correspondence between the hue of the input color signal and the hue of the output color signal in a table. It is characterized by.

According to an eleventh aspect of the present invention, in the image processing method for mapping an input color signal to a color signal within a color reproduction range of an image output device, a reference color signal whose hue matches the input color signal is obtained. Obtaining a corresponding color within the color reproduction range for the color signal, determining a hue of an output color signal based on the corresponding color, and mapping the input color signal to a color signal on the hue of the output color signal. It is characterized by.

According to a twelfth aspect of the present invention, in the eleventh aspect, a color having a minimum color difference from the reference color signal among the colors within the color reproduction range is set as the corresponding color. It is.

The invention of claim 13 is the invention of claim 11 or 1
A second aspect of the present invention is that the hue of the output color signal is made to match the hue of the corresponding color.

The invention of claim 14 is the invention of claim 11 or 1
The invention according to the second aspect is characterized in that the hue of the output color signal is obtained by linearly interpolating the hue of the corresponding color and the hue of the reference color signal based on a predetermined conversion formula.

The invention of claim 15 is the invention of claim 11 or 1.
The invention of claim 2 is characterized in that a color signal having the same hue as the input color signal in the input color reproduction range and having the highest saturation is obtained as the maximum saturation color, and the maximum saturation color is used as the reference color signal. Things.

According to a sixteenth aspect of the present invention, in the image processing method for mapping an input color signal to a color signal within a color reproduction range of an image output device, a correspondence between a hue of the input color signal and a hue of the output color signal is described. When mapping the input color signal using the table, the hue of the output color signal corresponding to the hue of the input color signal is read from the table, and the color on the hue of the output color signal from which the input color signal is read is read. It is characterized by mapping to a signal.

According to a seventeenth aspect of the present invention, in accordance with the sixteenth aspect, a reference color signal is obtained at a predetermined hue interval in the input color reproduction range, and a correspondence between the reference color signal and the color reproduction range of the image output device is obtained. A color is obtained, a hue of an output color signal is determined based on the corresponding color, and a correspondence between the hue of the input color signal and the hue of the output color signal is described in a table.

The invention according to claim 18 is a color conversion table creating method for converting an input color signal into a color signal within a color reproduction range of an image output device, wherein a reference color signal whose hue matches the input color signal is obtained. A corresponding color within the color reproduction range is obtained for the reference color signal, a hue of an output color signal is determined based on the corresponding color, and the input color signal is mapped to a color signal on the hue of the output color signal. The output color signal is converted to an output color signal, and a three-dimensional lookup table for color conversion is created using the conversion result.

According to a nineteenth aspect of the present invention, there is provided a color conversion table creating method for converting an input color signal into a color signal within a color reproduction range of an image output device, wherein a correspondence relationship between a hue of the input color signal and a hue of the output color signal. When mapping the input color signal using the table describing the hue of the input color signal, the hue of the output color signal corresponding to the hue of the input color signal is read from the table describing the correspondence relationship of the hue, and the input color signal is read out. The output color signal is mapped to a color signal on the hue of the output color signal, converted into an output color signal, and a three-dimensional lookup table for color conversion is created using the conversion result.

[0027] The invention of claim 20 is the invention of claim 18 or 1.
9 is a computer-readable recording medium on which a color conversion table created by the color conversion table creation method described in 9 is recorded.

According to a twenty-first aspect of the present invention, there is provided a recording medium storing a program for converting an input color signal into a color signal within a color reproduction range of an image output device, wherein the reference color and the hue coincide with each other. A step of obtaining a color signal, a step of obtaining a corresponding color within the color reproduction range with respect to the reference color signal, and a step of determining a hue of an output color signal based on the corresponding color;
And a step of mapping the input color signal to a color signal on the hue of the output color signal.

According to a twenty-second aspect of the present invention, there is provided a recording medium storing a program for converting an input color signal into a color signal within a color reproduction range of an image output device, wherein a hue of the input color signal and an output color signal are recorded. Generating a table describing the correspondence between the hues of the input color signal, reading the hue of the output color signal corresponding to the hue of the input color signal from the table, And a computer-readable recording medium on which a program for causing a computer to execute a procedure for mapping to a color signal is recorded.

According to a twenty-third aspect of the present invention, there is provided a recording medium storing a program for creating a color conversion table for converting an input color signal into a color signal within a color reproduction range of an image output device, And a procedure for obtaining a reference color signal having the same hue, a procedure for obtaining a corresponding color within the color reproduction range for the reference color signal, and a procedure for determining the hue of the output color signal based on the corresponding color. A step of mapping the input color signal to a color signal on the hue of the output color signal and converting it to an output color signal; and a procedure of creating a three-dimensional lookup table for color conversion using the conversion result. Is a computer-readable recording medium on which a program to be executed by a computer is recorded.

According to a twenty-fourth aspect of the present invention, there is provided a recording medium on which is recorded a program for creating a color conversion table for converting an input color signal into a color signal within a color reproduction range of an image output device. A step of generating a table describing the correspondence between the hues of the output color signal and the hue of the output color signal, and a step of reading the hue of the output color signal corresponding to the hue of the input color signal from the table describing the correspondence of the hue, A procedure for mapping the input color signal to a color signal on the hue of the read output color signal and converting the input color signal into an output color signal, and a procedure for creating a three-dimensional lookup table for color conversion using the conversion result. It is a computer-readable recording medium that records a program to be executed by a computer.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (Embodiment 1) A basic method for performing gamut mapping by distorting hue. FIG. 1 is a block diagram showing a configuration example of an image processing system according to the present invention. In FIG.
Display, 12 is an image output device, 13, 14 is
The color space conversion unit 20 is a gamut conversion processing unit. The image processing system shown in FIG. 1 includes a computer 10, a display 11 connected to the computer, an image output device 12, and a device that adopts a device-specific color signal (RGB signal) supplied from the computer in a gamut conversion processing unit 20. The output results of the color space conversion unit 13 and the gamut conversion processing unit 20 for converting into independent color signals are converted into color signals (CMY signals and CMs) unique to the image output device 12.
A color space conversion unit 14 for converting the color space into a YK signal and the like, and a gamut conversion processing unit 20. Image output device 12
Is an output device for printing out image data. For example, an image forming apparatus such as a color printer or a color facsimile can be used.

2. Operation of Image Processing System First, the computer 10 outputs image data in order to print out image data inside the computer using the image output device 12. This image data is usually represented by R (red), G (green), B
This is a color signal composed of (blue) color components. Computer 1
The RGB signals transmitted by 0 are transmitted to the color space conversion unit 13 and converted into color signals adopted by the gamut conversion processing unit 20. The color signal adopted by the gamut conversion processing unit 20 is C
Any color signal having color components corresponding to lightness, saturation, and hue, such as the LCH signal standardized by the IE, may be used. It is desirable to use Therefore, in the color space conversion unit 13,
The input RGB signal is converted into a color signal Pi (j, c, h) corresponding to the lightness J, chroma C, and hue H of CIECAM97s and output.

The above Pi (j, c, h) signal is RGB
Since the color signal is a color signal generated from the color signal, the color signal includes a color signal that cannot be reproduced by the image output device 12 as it is. Therefore, the gamut conversion processing unit 20 outputs Pi (j, c, h) to the image output device 12 using the means described later.
Is converted into a reproducible color signal Po (j ′, c ′, h ′). The color space conversion unit 14 converts Po (j ′, c ′, h ′) output from the gamut conversion processing unit 20 into a color signal such as a CMY signal or a CMYK signal that can be processed by the image output device 12, and converts the signal into a computer. Send to 10. By transmitting the color signal converted by the above processing to the image output device 12, print output is performed.

In the configuration example shown in FIG. 1, the color space conversion process and the gamut conversion process are provided by a device separate from the computer 10 and the image output device 12, but are implemented in the computer 10. It may be done,
Alternatively, it may be implemented in the image output device 12.

The above-described processing can be realized by software. For example, the functions can be realized by a printer driver existing as a program in the computer 10.

3. FIG. 2 is a block diagram showing an example of the configuration of the gamut conversion processing unit 20 to which the present invention is applied. In the figure, the gamut conversion processing unit 20 corresponds to the reference color signal calculation unit 21. Color determination unit 22, output hue determination unit 23, gamut data 2 of input device
5, an output device gamut data 26 and a mapping processing unit 24.

4. Operation of the gamut conversion processing unit 20 First, the input color signal Pi transmitted from the color space conversion unit 13
(J, c, h) is input to the reference color signal calculation unit 21.
The reference color signal calculation unit 21 refers to the gamut data 25 of the input device while input color signals Pi (j, c, h).
Is calculated for the reference color signal T (jt, ct, ht). In the present invention, since the color signal whose hue matches the input color signal Pi is used as the reference color signal T, ht = h. Next, when the reference color signal T is output by the reference color signal calculation unit 21, the corresponding color determination unit 22 refers to the gamut data 26 of the output device and determines the color reproduction range of the output device corresponding to the reference color signal T. Color signal M (jm, cm, h
m) is calculated. Then, the output hue determination unit 23 determines the hue h ′ of the output color signal Po (j ′, c ′, h ′) based on the corresponding color signal M obtained by the corresponding color determination unit 22. Finally, when h 'is determined, the mapping processing unit 24
, The j ′ and c ′ components of the output color signal Po are calculated with reference to the gamut data 26 of the output device,
Output to the color space conversion unit 14.

5. (1) Reference Color Signal Calculator 21 In the reference color signal calculator 21, information on the gamut data of the input device is stored in the gamut data 25 of the input device.
While reading, the reference color signal T is calculated. FIG.
FIG. 3 is a diagram illustrating an example of a reference color signal T. In the figure, reference numeral 30 denotes a gamut of an input device, and reference numeral 40 denotes a gamut of an output device. FIG. 3 illustrates the relationship between the gamut 30 of the input device and the gamut 40 of the output device on the hue plane to which the input color signal Pi belongs. The gamut 30 of the input device does not match the gamut 40 of the output device. There are many colors in the input color signal that cannot be reproduced by the output device. When such a color is mapped to the gamut 40 of the output device with a constant hue, the color change before and after the mapping at the point where the saturation becomes maximum among the colors included in the gamut 30 of the input device (= maximum saturation color). (Point T → point To in the figure) tends to be conspicuous. Therefore, by mapping such a maximum chroma color (equivalent to the point T in the figure) in which the color change is conspicuous, it is possible to reproduce a color in which the color change is not conspicuous as a whole image by mapping the color as little as possible.

Therefore, in the present invention, when the input color signal Pi is given, it is considered that the gamut conversion processing is performed so that the color change of the maximum chroma color T on the same hue becomes small.
The maximum chroma color T is set as a reference color signal of the input color signal Pi. As a specific means, a reference color signal for each hue is described in advance in the gamut data 25 of the input device, and when calculating the reference color signal, the reference color signal T corresponding to the hue h of the input color signal Pi is calculated. Should be read. Hue
Is not included in the gamut data 25 of the input device, the data can be approximately calculated by reading data near the hue h and performing an interpolation operation.

(2) Gamut Data 25 of Input Device FIG. 4 is a diagram showing an example of a data table describing a reference color signal T for each hue H. The gamut data 25 of the input device is a data table describing a reference color signal T for each hue H as shown in FIG. 4, and data is created in advance and stored in a memory such as a RAM. FIG.
In the example of the data table shown in (1), the reference color signal data for each hue angle of 5 ° is shown, but the step width may be finer or coarser. Further, the step width does not need to be uniform, and may be varied.

For example, when the hue h of the input color signal is h = 5 °, the reference color signal T (jt, ct, ht) = (92,8)
3, 5). When the hue h = 8 °, the reference hue is not found in the data table, so that the reference color signal T is obtained by performing an interpolation operation. The calculation formula of the interpolation calculation is shown in the following formula (1). T (jt, ct, ht) = [(94-92) * 3/5 + 92, (85-83) * 3/5 + 83,8] = [93.2,84.2,8] Equation (1) Thus, when the hue h = 8 °, the reference color signal T (jt,
ct, ht) = (93.2, 84.2, 8).

FIG. 5 is a diagram showing an example of the locus of the highest chroma color in the RGB space. The above-described gamut data creation method will be described with an example in which the input color signal RGB is an sRGB signal. The maximum chroma color T described above is shown in FIG.
Exists on a locus connecting red-magenta-blue-cyan-green-yellow-red as indicated by a thick line shown in FIG. For example, point S
Are given by (255, 0, 128).
In this way, the points on the trajectory are such that one of the R component, the G component, and the B component is 0, the other is 255,
The remaining components are color signals represented by 0 to 255. A table is constructed by sequentially calculating JCH for each of the RGB signals on this locus. The conversion from the RGB signal to the JCH signal is performed by performing RGB → XYZ conversion according to a conversion formula standardized by IEC 61966-2-1,
XYZ → JCH standardized by IE TC8-01
This can be performed by using a conversion formula. When the RGB signal characteristics of the input device are other than the sRGB signal,
Color conversion is performed according to each color characteristic. By performing the above calculations, the JCH value of the maximum chroma color is obtained, and this is described in the gamut data 25 of the input device.

(3) Corresponding Color Determining Unit 22 FIG. 6 is a diagram for explaining an example of the correspondence between the reference color signal T and the corresponding color M. In FIG. 6, reference numeral 41 denotes the hue ho of the output device.
The gamut above. The corresponding color determination unit 22 calculates a corresponding color M of the reference color signal T obtained by the reference color signal calculation unit 21. Since the relationship between the corresponding color M and the reference color signal T is a three-dimensional positional relationship and cannot be shown in one diagram, FIG. 6A shows a relationship diagram between saturation and hue, and FIG. FIG. 4 shows a relationship diagram between lightness and saturation. However, in FIG. 6B, the gamut 40 in the hue hi of the output device and the gamut 41 in the hue ho are shown in the same two-dimensional manner. Also,
For comparison, a mapping color To when the reference color signal T is mapped with a constant hue is also shown.

As described above, the present invention aims to map the reference color signal T in which the color change is conspicuous so that the color change is as small as possible. Therefore, among the color signals included in the gamut of the output device as colors corresponding to the reference color signal T, a color having the smallest color difference from the reference color signal T is defined as a corresponding color M. When the corresponding colors are determined in this way, the hues of T and M do not match as a result.

For example, as shown in FIGS. 6 (A) and 6 (B), To and the corresponding colors M on the hue hi of the reference color signal T clearly show the corresponding colors. M is a color closer to the reference color signal T. This phenomenon is also caused by the fact that the shape of the gamut of the image output device is very complicated, and the shape greatly differs depending on the hue. As described above, when mapping a reference color signal, color reproduction with less color change is possible by shifting the hue than by mapping on the same hue. In the present invention, the corresponding point M is determined by calculating the minimum color difference point for the reference color signal T based on the above principle.

Next, the minimum color difference point can be obtained by calculating the color difference for all output colors reproducible by the image output device and calculating the minimum value. However, the number of output color combinations is so large that the calculation time becomes enormous. For example, if the output color signal is 8 for each of CMY
When given as a bit color signal, the output color pattern is as large as 256 × 256 × 256 = 16.77 million colors. Therefore, in order to reduce the processing time, the output colors for calculating the color difference are narrowed down.

FIG. 7 is a flowchart for explaining the flow of processing for searching for a color difference minimum point. First, the reference color signal calculation unit 21 calculates a reference color signal T (step S1),
It is determined which of the categorized hue regions (the hue region is referred to as REG in the drawing) the reference color signal T belongs to (Step S2). As a categorization pattern,
For example, it is divided into six hue regions of red to magenta, magenta to blue, blue to cyan, cyan to green, green to yellow, and yellow to red. However, the hues of each color of red, magenta, blue, cyan, green, and yellow use the basic six colors represented by the primary and secondary colors of the output device, and the determination process is performed in the magnitude of the hues of the basic six colors. Performed by comparison. Next, a gamut data list belonging to the hue gamut determined in step S2 is read from the gamut data 26 of the output device (step S3). For example, when the reference color signal T is located in the magenta to blue hue, all gamut data included in the magenta to blue hue range is read. However, this gamut data includes data of only the gamut boundary surface, as described later, and does not include data inside the gamut. Then, the color difference is calculated for each of the read gamut data, and the point at which the color difference is minimized is determined (step S4). The minimum color difference point obtained by the above procedure is finally output as the corresponding point M (jm, cm, hm) (step S5).

In the above-described method, the color difference is calculated for all the gamut data belonging to the corresponding hue range. However, the color difference may be calculated for only a part of the gamut data. For example, if the color difference is calculated only for the gamut data whose lightness is substantially equal to that of the reference color signal T, the processing speed can be greatly improved although the accuracy is slightly inferior.

The Euclidean distance in the JCH space is usually used for the color difference. However, the present invention is not limited to this. For example, a lightness difference, a saturation difference, and a hue difference described in JP-A-10-84487. May be used.

(4) Gamut Data 26 of Output Device FIG. 8 is a view for explaining an example of the gamut data 26 of the output device. Output device gamut data 26
FIG. 8 shows the relationship between the hue Hi determined in a predetermined step and FIG.
Is a table describing a data list GMP of color signal values on a gamut boundary as shown in FIG. 1, in which data is created in advance and stored in a memory such as a RAM. Here, GMP refers to gamut data representing the maximum chroma Cmax with respect to hue Hi and lightness Jj. FIG.
In the example shown in (1), the maximum chroma Cmax for the hue Hi and the lightness Jj is recorded as the color signal value on the gamut boundary.
Therefore, the signal value of the gamut data is [Jj, Cma
x, Hi].

The gamut data includes a predetermined hue step,
For example, it may be obtained for each hue angle of 5 °, or a non-uniform step width may be used. The brightness Jj for obtaining the gamut data may be obtained by equally dividing the maximum brightness Jmax and the minimum brightness Jmin of the output device. However, as shown in FIG. 8, the highest saturation color in the hue Hi of the output device is high brightness. In the case where the brightness is biased toward either the lightness side or the low lightness side, if the lightness Ji is equalized, the density of the gamut data is significantly different between the high lightness side and the low lightness side. Therefore,
The step of Jj defining the gamut data (Δ in FIG. 8)
It is more preferable that J_high and ΔJ_low) be different from each other on the basis of the maximum chroma color of the output device.

FIG. 9 shows the gamut data 2 of the output device.
6 is a flowchart illustrating an example of a process of creating No. 6. First, a color prediction formula is constructed to obtain the color reproduction characteristics of the image output device (step S11). Here, the color prediction formula is a model in which an output characteristic of the image forming apparatus is modeled by a mathematical expression, and a calorimetric value can be obtained from the output color signal by calculation. For example, the output signal value (c, m,
y) can be converted into (j, c, h) by calculation. A common way to construct a color prediction formula is
The color patch is output by the image forming apparatus, and the output patch is measured with a spectrophotometer. Next, a color prediction formula is constructed by approximating the relationship between the colorimetric data of the color patch and the output color signal.

After constructing the color prediction formula in step S11,
Using this, modeling of the gamut of the image output device is started. First, GMP is initialized (step S1).
2). GMP is gamut data representing the maximum chroma Cmax for the hue Hi and the lightness Jj, as described above. In the initialization operation, the CMP for all Hi and Jj is used.
Is set to 0. When the initialization is completed, step S1
In steps 3 to S17, all C,
GMP is constructed using the M and Y values. Since the output signal value used for the GMP construction is sufficient only with the data of the gamut boundary surface, it is necessary to use a combination of output values such that two of the three components C, M, and Y are 0 or 255. I do.

The output signal value (ci, m) located at the boundary surface
A JCH value is calculated for i, yi) using a color prediction formula (step S13). Next, the value of GMP corresponding to the color gamut including this output signal value is read, and the variable Cma is read.
It is set to x (step S14). The GMP value including the output signal value is calculated using appropriate functions f1 and f2.
An address for accessing the GMP is generated from the lightness j and the hue h obtained in step S13, and the maximum saturation value stored in the GMP is read. For example, GMP
Can be easily calculated using the following equation (2) when hue is equally divided by ΔH and brightness is ΔJ. f1 (j) = int (j / ΔJ), f2 (h) = int (h / ΔH) Equation (2) When the non-uniform division is performed as described above, calculation is performed in step S13. A look-up table for converting j and h into GMP addresses may be created, and the table may be referred to.

According to the above-described procedure, the maximum saturation value Cmax in the color gamut including the output signal value stored in the GMP.
Is read out, a comparison is made with the saturation value c of the output color signal calculated in step S13 (step S15). If c is greater than Cmax, the contents of the GMP are updated (step S16). The above processing is calculated for the output signals cmy of all the gamut boundaries (step S1).
7) The finally constructed GMP can be used as gamut data (step S18).

(5) Output Hue Determination Unit 23 The output hue determination unit 23 determines a hue plane on which mapping processing is actually performed. At this time, for example, in the case where the input image is a character image and the aim is to reduce the color change as much as possible without significant problems in gradation, the mapping is performed using the hue of the corresponding color determined by the corresponding color determination unit 22. To do it. In this case, the hue hm of the corresponding color is sent to the mapping processing unit 24 as the hue h 'without further processing.

On the other hand, when gradation is considered to be a problem as in the case of a gradation image, if the hues to be mapped are determined as described above, the mapping is performed on the basic six hues including the primary colors and the secondary colors of the output device. It is easy to concentrate and there is a high possibility that the gradation will be destroyed. Therefore, when importance is placed on the gradation, the hue hm of the corresponding point and the hue ht of the reference color signal are linearly interpolated to obtain h ', and the mapping process is performed using the hue h'. Formula (3) below shows a calculation formula used for linear interpolation. h ′ = α · ht + (1−α) · hm (3) where α is a constant of 1 or less. FIG. 10 is a diagram for explaining an example of the relationship between the hues depending on the difference in the method of determining the mapping hue. FIG. 10A shows a case where mapping is performed with a hue in which the hue of the corresponding color matches the hue of the reference color. FIG.
(B) shows a case where the hue of the corresponding color and the hue of the reference color are linearly interpolated and mapped.

(6) Mapping Processing Unit 24 The mapping processing unit 24 outputs the gamut data 26 of the output device on the hue plane determined by the output hue determining unit 23.
, The output color signal Po (j ′, c ′, h ′) for the input color signal Pi (j, c, h) is calculated.
Since h 'has already been obtained by the output hue determination unit 23, the mapping processing unit 24 may obtain the remaining j' and h '. At this time, if it is virtually considered that Pi and Po are on the same hue, the mapping process is replaced with a process for performing a two-dimensional conversion from Pi (j, c) to Po (j ', c'). For example, Japanese Patent Application Laid-Open Nos. 9-168097 and 9-187.
The gamut processing on the same hue surface, which has been conventionally proposed in JP-A-27-27, can be applied as it is.

In addition to the method of mapping only the colors outside the gamut on the gamut boundary surface, a parser that uniformly compresses and maps the entire color space so as to preserve the gradation of the input colors outside the gamut of the output device. There is no problem using a method called virtual matching.

When the output color signals (j ', c', h ') are obtained by the gamut conversion processing described above, the color space conversion processing unit 14 converts them into color signals for an output device.
By performing print output, color reproduction with little color change and excellent gradation continuity can be performed.

(Embodiment 2) Embodiment Considering High Speed In Embodiment 1 described above, a reference color signal and a corresponding point are calculated for each input color signal. However, since input color signals on the same hue are converted into output hue signals of the same hue, the hue of the input hue signal and the hue of the output hue signal have a one-to-one relationship.
By calculating the correspondence in advance, the processing can be speeded up.

FIG. 11 is a block diagram showing another example of the configuration of the image processing system according to the present invention. The gamut conversion processing unit 20 according to the present embodiment includes an address generation unit 27,
It comprises a hue conversion table 28, gamut data 26 of an output device, and a mapping processing unit 24.

The address generator 27 receives the hue value h of the input color signal Pi as input and generates an address for accessing the hue conversion table 28. Hue conversion table 2
8 outputs the hue h 'of the output color signal using the address output from the address generation unit 27. Output device gamut data 26 and mapping processing unit 24
And the mapping processing unit 24 outputs the input color signal P
i (j, c, h) is converted to a point Po (j ', c',
h ′) and outputs the result to the color space conversion unit 14.

In the above, the hue conversion table 28 executes the processing of the reference color signal calculation unit 21, the corresponding point determination unit 22, and the output hue determination unit 23 of the first embodiment for the hue value having an appropriate step width. The result is replaced with a single table.

FIG. 12 is a flowchart illustrating an example of the gamut conversion processing to which the present invention is applied. First,
An input color signal Pi (j, c, h) is input (step S21), and a reference color signal T for the input color signal Pi (j, c, h) is calculated (step S22). As described above, the maximum chroma color having the same hue as the input color signal Pi is used as the reference color signal T. Next, a corresponding color M for the reference color signal T is obtained (step S23). The corresponding color M is a color signal at the minimum color difference point as described above. Then, based on the corresponding color M, a hue plane h 'for which gamut mapping is performed is determined (step S24). As the hue plane h ′, the hue of the corresponding point M or an intermediate hue between the hue of the corresponding point M and the hue of the reference color signal T can be used. When the mapping hue is determined, a mapping process is performed in step S25 to output an output color signal Po (j ', c', h ') (step S25).
26).

In the gamut conversion processing method shown in FIG.
Although the gamut conversion processing is performed at the time of color conversion of input image data, output CMY (K) signal values corresponding to predetermined RGB values in the RGB space are stored in a three-dimensional lookup table, and color conversion is performed. At the time of processing, it is often used to read a plurality of output values from a three-dimensional lookup table and perform an interpolation operation. In such a color conversion system, the result of the gamut conversion processing according to the present invention can be used as a value of a three-dimensional lookup table.

FIG. 13 is a flowchart illustrating an example of the flow of a process for creating a three-dimensional lookup table. Here, the processing of steps S31 to S36 is the same as that of FIG.
Steps S21 to S26 of the gamut conversion processing method shown in FIG.
Is similar to the above, but creates a three-dimensional lookup table using the conversion result of the output color signal Po (j ', c', h ') obtained in step S36 (step S37). As described above, the output CMY (K) signal values corresponding to the predetermined input RGB signal values in the RGB space are stored in the three-dimensional lookup table.

FIG. 14 is a block diagram showing a configuration example of an image processing system using a three-dimensional lookup table.
In the figure, 50 is an input color signal composed of RGB signals, 51
Is an interpolation operation unit that performs matrix conversion, and 52 is CMY
(K) An output color signal composed of a signal, 53 is an address generation unit, and 54 is a three-dimensional lookup table. Figure 1
In the configuration example shown in FIG. 4, the address generation unit 53 performs a three-dimensional lookup table 54 based on the input color signal 50.
And outputs a plurality of output values stored in advance in the three-dimensional look-up table 54 based on the generated address to obtain an interpolation operation unit 51.
By performing the interpolation operation in, the output color signal 52 becomes C
The MY (K) signal can be obtained.

FIG. 15 is a diagram showing an example of a hardware configuration of the image processing system shown in FIG.
0 is a program reader 10a, a CP that controls the whole
U10b, a RAM 10c used as a work area of the CPU 10b, a ROM 10d storing a control program of the CPU 10b, a hard disk 10e,
It includes an NIC 10f, a mouse 10g, a keyboard 10h, a display 11 for displaying image data, and an image forming apparatus 12 such as a color printer. This image processing system can be realized by, for example, a workstation or a personal computer.

Here, the CPU 10b, ROM 10d, R
The AM 10c and the hard disk 10e have the same functions as the computer 10 shown in FIG. In this case, the functions of the color space conversion units 13 and 14 and the gamut conversion processing unit 20 shown in FIG. 1 can also be provided to the CPU 10b. That is, the function as the image processing apparatus of the present invention can be provided to the CPU 10b.

The function as an image processing device such as the CPU 10b can be provided in the form of a software package, specifically, an information recording medium such as a CD-ROM. In the example shown, when an information recording medium is set, a medium driving device for driving the information recording medium is provided (not shown).

As described above, the image processing apparatus and the image processing method according to the present invention allow a general-purpose computer system having a display or the like to read a program recorded on an information recording medium such as a CD-ROM, and It is also possible to implement the present invention in an apparatus configuration in which a microprocessor performs color space conversion processing and gamut conversion processing. In this case, the program for executing the color space conversion process and the gamut conversion process of the present invention, that is, the program used in the hardware system is:
It is provided in a state recorded on a recording medium. The information recording medium on which the program and the like are recorded is not limited to the CD-ROM, but may be, for example, a ROM, a RAM, a flexible disk, or a memory card. The program recorded on the recording medium is installed in a storage device incorporated in the hardware system, for example, the hard disk 10e, and executes the program to realize the color conversion function and the color conversion profile generation function. be able to.

A program for realizing the color conversion function and the color conversion profile generation function of the present invention is provided not only in the form of a recording medium but also from a server by communication via a network, for example. It may be something.

[0075]

According to the invention of claims 1, 11, and 21,
Regardless of the hue of the input color signal or the shape of the color reproduction range of the image output device, it is possible to perform color gamut mapping so that the color change is inconspicuous and the input color signal of the same hue is always the output color signal of the same hue. it can.

According to the second and twelfth aspects of the invention, even if the input color signal is a color that cannot be reproduced by the output device,
It can be converted to a color with very little color change.

According to the third and thirteenth aspects of the present invention, the input color signals having the same hue can be color-converted so that the hue of the output color signal always coincides, and the color change can be performed without impairing the gradation. Inconspicuous color reproduction can be performed.

According to the present invention, it is possible to prevent the output color signals from being concentrated on a specific hue.

According to the fifth and fifteenth aspects of the present invention, it is possible to perform the color conversion processing for reducing the color change of the color signal in which the color change is most noticeable.

According to the present invention, the gamut conversion processing can be performed at high speed.

According to the seventh aspect of the present invention, the gamut conversion processing can be performed with high accuracy, and more preferable color reproduction results can be obtained.

According to the eighth aspect of the present invention, the color difference minimum point can be calculated efficiently.

According to the present invention, the gamut conversion processing can be executed at high speed.

According to the tenth and seventeenth aspects of the present invention, regardless of the hue of the input color signal and the shape of the color reproduction range of the image output device, the color change is inconspicuous and the input hue of the same hue is always the same hue output. A color gamut mapping that becomes a color signal can be executed at a high speed.

According to the eighteenth, nineteenth, twenty-third, and twenty-fourth aspects of the present invention, the color change is inconspicuous and the input hue of the same hue is independent of the hue of the input hue signal and the shape of the color reproduction range of the image output device. Can be generated such that the output color signal always has the same hue.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration example of an image processing system according to the present invention.

FIG. 2 is a block diagram illustrating a configuration example of a gamut conversion processing unit to which the present invention is applied;

FIG. 3 is a diagram illustrating an example of a reference color signal.

FIG. 4 is a diagram showing an example of a data table describing reference color signals for each hue.

FIG. 5 is a diagram illustrating an example of a locus of the highest chroma color in an RGB space.

FIG. 6 is a diagram illustrating an example of a correspondence relationship between a reference color signal and a corresponding color.

FIG. 7 is a flowchart illustrating a flow of a process of searching for a color difference minimum point.

FIG. 8 is a diagram illustrating an example of gamut data of an output device.

FIG. 9 is a flowchart illustrating an example of a process for creating gamut data of an output device.

FIG. 10 is a diagram illustrating an example of a relationship between hues depending on a difference in a method of determining a mapping hue.

FIG. 11 is a block diagram showing another configuration example of the image processing system according to the present invention.

FIG. 12 is a flowchart illustrating an example of a gamut conversion process to which the present invention is applied.

FIG. 13 is a flowchart illustrating an example of the flow of processing for creating a three-dimensional lookup table.

FIG. 14 is a block diagram illustrating a configuration example of an image processing system using a three-dimensional lookup table.

FIG. 15 is a diagram illustrating an example of a hardware configuration of the image processing system illustrated in FIG. 1;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Computer, 11 ... Display, 12 ... Image output device, 13, 14 ... Color space conversion part, 20 ... Gamut conversion processing part, 21 ... Reference color signal calculation part, 22 ... Corresponding color determination part, 23 ... Output hue determination Unit, 24 mapping processing unit, 25 gamut data of input device, 26 gamut data of output device, 27 address generation unit, 2
8: hue conversion table, 30: gamut of input device, 40: gamut of hue hi of output device, 41
.. Gamut on the hue ho of the output device, 50... Input color signal, 51... Interpolation calculation unit, 52... Output color signal, 53... Address generation unit (for three-dimensional lookup table), 54.
... 3D lookup table.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C262 AA24 AA26 AB11 BA17 BA18 BA19 BC01 BC03 BC09 BC11 BC19 EA13 5B057 AA11 CA01 CA08 CA12 CA16 CB01 CB08 CB12 CB16 CC01 CE17 CH07 5C077 LL02 LL19 MP01 MP08 PP35 PQ23H03 HB06 HB12 LA02 LB02 MA04 MA11 NA03 NA11 NA29 PA01 PA02 PA03 PA05

Claims (24)

[Claims] 1. An image processing apparatus for mapping an input color signal to a color signal within a color reproduction range of an image output device, means for obtaining a reference color signal having a hue that matches the input color signal, and Means for determining a corresponding color within the color gamut, determining a hue of an output color signal based on the corresponding color, and mapping the input color signal to a color signal on the hue of the output color signal. An image processing apparatus, comprising: 2. The image processing apparatus according to claim 1, wherein a color having a minimum color difference from the reference color signal among the colors within the color reproduction range is set as the corresponding color. 3. The image processing apparatus according to claim 1, wherein the hue of the output color signal is made to coincide with the hue of the corresponding color. 4. The apparatus according to claim 1, further comprising means for linearly interpolating the hue of the corresponding color and the hue of the reference color signal based on a predetermined conversion formula to obtain the hue of the output color signal. Image processing device. 5. A means for obtaining a color signal having the same hue as the input color signal and the highest saturation in the input color reproduction range as the maximum saturation color, and using the maximum saturation color as the reference color signal. The image processing apparatus according to claim 1, wherein: 6. A means for obtaining the maximum chroma color at a predetermined hue interval in an input color reproduction range, and for describing a correspondence relationship between the hue and the maximum chroma color in a table, reading the table to obtain a hue of an input color signal. 6. The image processing apparatus according to claim 5, further comprising: means for determining a plurality of maximum chroma colors corresponding to the hue adjacent to the reference color signal, and obtaining the reference color signal by interpolating the plurality of maximum chroma colors. apparatus. 7. The image processing apparatus according to claim 1, wherein the mapping to the color reproduction range is performed in a color system based on a color appearance model. 8. A means for performing a magnitude comparison between the hue of the reference color signal and the hues of six colors represented by primary and secondary colors of the image output device, based on a result of the magnitude comparison. 3. The image processing apparatus according to claim 2, further comprising means for determining a color search range. 9. An image processing apparatus for mapping an input color signal to a color signal within a color reproduction range of an image output device, comprising a table describing a correspondence between a hue of the input color signal and a hue of an output color signal. An image processing apparatus for mapping the input color signal, wherein the table is read out and mapped to a color signal on a hue of an output color signal corresponding to a hue of the input color signal. 10. A means for obtaining a reference color signal at a predetermined hue interval in an input color reproduction range, a means for obtaining a corresponding color in the color reproduction range of an image output device for the reference color signal,
10. The apparatus according to claim 9, further comprising: means for determining a hue of an output color signal based on the corresponding color; and means for describing in a table a correspondence relationship between a hue of the input color signal and a hue of the output color signal. Image processing device. 11. An image processing method for mapping an input color signal to a color signal within a color reproduction range of an image output device.
Determine a reference color signal whose hue matches the input color signal, determine a corresponding color within the color reproduction range for the reference color signal,
An image processing method comprising: determining a hue of an output color signal based on the corresponding color; and mapping the input color signal to a color signal on the hue of the output color signal. 12. The image processing method according to claim 11, wherein a color having a minimum color difference from the reference color signal among the colors within the color reproduction range is set as the corresponding color. 13. The apparatus according to claim 11, wherein a hue of the output color signal is made to coincide with a hue of the corresponding color.
2. The image processing method according to 2. 14. A hue of an output color signal is obtained by linearly interpolating a hue of the corresponding color and a hue of the reference color signal based on a predetermined conversion equation.
The image processing method described in the above. 15. A color signal having the same hue as the input color signal and the highest saturation in the input color reproduction range is determined as a maximum saturation color, and the maximum saturation color is used as the reference color signal. The image processing method according to claim 11. 16. An image processing method for mapping an input color signal to a color signal within a color reproduction range of an image output device,
When mapping the input color signal using a table describing the correspondence between the hue of the input color signal and the hue of the output color signal, the hue of the output color signal corresponding to the hue of the input color signal is obtained from the table. And reading the input color signal and mapping the input color signal to a color signal on the hue of the read output color signal. 17. A reference color signal is determined at a predetermined hue interval in an input color reproduction range, a corresponding color within a color reproduction range of an image output device is determined for the reference color signal, and an output color is determined based on the corresponding color. 17. The image processing method according to claim 16, wherein the hue of the signal is determined, and the correspondence between the hue of the input color signal and the hue of the output color signal is described in a table. 18. A color conversion table creating method for converting an input color signal into a color signal within a color reproduction range of an image output device, wherein a reference color signal whose hue coincides with the input color signal is obtained, and On the other hand, a corresponding color within the color reproduction range is obtained, and a hue of an output color signal is determined based on the corresponding color,
The input color signal is mapped to a color signal on the hue of the output color signal and converted into an output color signal, and a three-dimensional lookup table for color conversion is created using the conversion result. Conversion table creation method. 19. A color conversion table creating method for converting an input color signal into a color signal within a color reproduction range of an image output device, wherein a table describing the correspondence between the hue of the input color signal and the hue of the output color signal. When mapping the input color signal using, the hue of the output color signal corresponding to the hue of the input color signal is read from a table describing the correspondence relationship of the hue, and the output color signal of the read output color signal is read. A color conversion table creation method, which comprises mapping a color signal on a hue and converting it into an output color signal, and using the conversion result to create a three-dimensional lookup table for color conversion. 20. A computer-readable recording medium recording a color conversion table created by the color conversion table creation method according to claim 18. 21. A recording medium on which a program for converting an input color signal into a color signal within a color reproduction range of an image output device is recorded, wherein a reference color signal whose hue matches the input color signal is obtained. Obtaining a corresponding color within the color reproduction range for the reference color signal; determining a hue of the output color signal based on the corresponding color; and converting the input color signal into a hue of the output color signal. And a computer-readable recording medium storing a program for causing a computer to execute the above-described procedure for mapping to a color signal. 22. A recording medium storing a program for converting an input color signal into a color signal within a color reproduction range of an image output device, wherein a correspondence relationship between a hue of the input color signal and a hue of the output color signal. To generate a table describing the
A program for causing a computer to execute a procedure of reading a hue of an output color signal corresponding to the hue of the input color signal from the table and a step of mapping the input color signal to a color signal on a hue of the read output color signal. A computer-readable recording medium that has been recorded. 23. A recording medium recording a program for creating a color conversion table for converting an input color signal into a color signal within a color reproduction range of an image output device, wherein the input color signal and the hue match. Obtaining a reference color signal, obtaining a corresponding color within the color gamut with respect to the reference color signal, determining a hue of an output color signal based on the corresponding color, A program for causing a computer to execute a procedure of mapping to a color signal on a hue of the output color signal and converting the output color signal into an output color signal, and a procedure of creating a three-dimensional lookup table for color conversion using the conversion result. A computer-readable recording medium that has been recorded. 24. A recording medium storing a program for creating a color conversion table for converting an input color signal into a color signal within a color reproduction range of an image output device, wherein the hue of the input color signal and the output color are output. A procedure for generating a table describing the correspondence between the hues of the signals, a procedure for reading the hue of the output color signal corresponding to the hue of the input color signal from the table describing the correspondence between the hues, A program for causing a computer to execute a procedure of mapping a read output color signal on a hue color signal and converting the output color signal into an output color signal, and a procedure of creating a three-dimensional lookup table for color conversion using the conversion result. A computer-readable recording medium on which is recorded.