JPH10290375A - Color reproduction processing method and color image processing apparatus - Google Patents

Abstract

(57) [Summary] [Problem] Based on the results of subjective evaluation using color patches, reflect the color taste, that is, the color reproduction tendency based on the impression given to the human eye, to increase the taste of human color reproduction. Reflect faithfully and obtain the reproduction color desired by the subject. SOLUTION: A step S301 of calculating a distance of a given color from a white point, and a suitable compression destination color on a gamut boundary surface formed of a triangle connecting a maximum saturation point, a white point and a black point on a certain hue plane. Step S302 of calculating the distance from the white point of the target color, and the intersection of the straight line connecting the coordinate value of the given color and the coordinate of the color given in Step S302 with the actual gamut boundary is obtained. And step S303 for setting the color of

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color reproduction processing method and apparatus for transferring color image information between devices having different color reproduction ranges, such as a CRT, a color printer, and a color copy. Is based on the relationship between the results of subjective evaluation using color patches, and reflects the color reproduction tendency based on the color, that is, the impression given to the human eye, to faithfully reflect the taste of human color reproduction. The present invention relates to a color reproduction processing method and a color image processing apparatus for obtaining a reproduction color desired by a subject.

[0002]

2. Description of the Related Art In recent years, media handling color images have been developed in various forms. For example, there is well known a system in which a character or an image is read by a color scanner, edited and processed on a display of a computer, and the result is printed out by a color printer or the like.
However, in this case, the color reproduced on the display and the color printed out are determined by a color reproduction method or a mixed color system (RG
(B, YMC), and the fact that colors differ from each other due to the mismatch of color reproduction ranges.

As described above, since the gamut of colors that can be represented by a device is usually different, colors that cannot be output by a color image output device among input colors are converted into reproducible colors. A so-called color gamut compression process is performed.

However, in general, when a non-reproduced color is included in an output image, the above-described compression processing is performed regardless of colors around the non-reproduced color. Therefore, the color may be reproduced in the same color as the periphery of the non-reproduced color, and as a result, the gradation may be lost.

Therefore, conventionally, for example, Japanese Patent Publication No. 6-365
As disclosed in “Color Image Signal Processing Method” of JP-A-48-48, a compression ratio suitable for the characteristics of an image represented by an input color image signal is determined for both lightness and saturation, and the lightness and saturation are determined. Compression and mapping within the color reproduction range of the output system so as to preserve the gradation of That is, in a general full-color image compression method (gamut processing method), in the color conversion processing, compression in the lightness direction is performed first, and then compression in the saturation direction is performed. The same processing is performed for all hues, and the compression in the saturation direction is performed with constant brightness. In other words, the same process is executed regardless of the distance from the gamut boundary.

However, as shown above,
In the "Color Image Signal Processing Method" of JP-A-36548, since the compression processing is performed by simple proportional calculation in both the brightness and the saturation, there is a problem that the reproduced color becomes unnatural. Was.

Therefore, in "Color Correction Method for Color Image Output Apparatus" of JP-A-4-334267, the maximum chroma portion of the color reproduction range of the monitor is determined for each color by the maximum color reproduction range of the output destination printer. By compressing to the saturation part,
Trying to achieve high saturation and natural compression.

[0008]

However, in the conventional "Color Correction Method for Color Image Output Apparatus" of JP-A-4-334267 as described above, a subjective evaluation experiment using color patches is performed. When reproduction is performed only from the background, there is a problem in that the color reproduction tendency, which is based on the so-called impression given to the human eye, is not reflected, so that the reproduction color intended by the subject may be reproduced. Was.

The present invention has been made in view of the above, and it is an object of the present invention to reflect a color tone, that is, a color reproduction tendency based on an impression given to human eyes, based on a result of a subjective evaluation using a color patch. Accordingly, an object of the present invention is to faithfully reflect the taste of a person regarding color reproduction and obtain a reproduction color desired by a subject.

[0010]

In order to achieve the above object, in the color reproduction processing method according to the first aspect, at least a part of the color image information is out of the color reproduction range of the color image output system. In the color reproduction processing method for converting the non-reproducible color into a reproducible color of a color image output system when the color is a non-reproducible color that is not reproduced, the color is given to the human eye using the result of the subjective evaluation using color patches. A color reproduction processing method for obtaining a reproduction color desired by a subject by reflecting a color reproduction tendency based on an impression, comprising: a first step of calculating a distance from a white point of a given color; A second step of calculating a distance from the white point of a suitable compression destination color on the gamut boundary surface consisting of a triangle connecting the maximum saturation point with the white point and the black point on the surface, and coordinates of the given color When seeking the intersection of the actual gamut boundary a straight line connecting the color coordinates given in the second step, those containing a third step of it with compressed destination color, the.

That is, when the color space of color image information is converted, color reproduction processing is performed using a distance from a white point in a uniform color space based on the relationship obtained from the result of the subjective evaluation experiment. Accordingly, it is possible to reflect the user's preference regarding color matching.

Further, in the color image processing apparatus according to the present invention, when at least a part of the color image information is a non-reproducible color that cannot be reproduced in the color reproduction range of the color image output system, A color image processing apparatus for converting a color into a reproducible color of a color image output system, comprising color reproduction processing means for converting a color space of color image information using the color reproduction processing method according to claim 1. is there.

That is, when converting the color space of the color image information, the color reproduction processing means uses the distance from the white point on the uniform color space based on the relationship obtained from the result of the subjective evaluation experiment. By performing the color reproduction process, it is possible to reflect the user's preference regarding color matching.

In the color reproduction processing method according to the third aspect, when at least a part of the color image information is a non-reproduction color that cannot be reproduced in the color reproduction range of the color image output system, the non-reproduction color In a color reproduction processing method that converts colors into colors that can be reproduced by a color image output system, the results of subjective evaluation using color patches reflect the color reproduction tendency based on the impression given to human eyes, A color reproduction processing method for obtaining a desired reproduction color, comprising: a first step of calculating a distance from a white point of a given color; and a step of calculating a maximum saturation point, a white point, and a black point on a certain hue plane. Calculates the angle between the original color coordinate value and the color coordinate of the gamut compression destination when the center is the white point on the gamut boundary surface consisting of connecting triangles, and calculates the distance from the white point of the appropriate compression destination color A second step, and a crossing point between a straight line connecting the coordinate values of the given color and the coordinates of the color given in the second step and the actual gamut boundary is determined as the compression destination color. And a third step.

That is, when converting the color space of the color image information, the original color coordinate values centered on the white point in the color matching space are determined based on the relationship obtained from the results of the subjective evaluation experiment. By calculating the distance from the white point using the angle between the color coordinates of the gamut compression destination and performing color reproduction processing using the calculated value, it is possible to reflect the user's preference regarding color matching. Become.

According to a fourth aspect of the present invention, in the color image processing apparatus, when at least a part of the color image information is a non-reproducible color that cannot be reproduced in the color reproduction range of the color image output system, A color image processing apparatus for converting a color into a reproducible color of a color image output system, comprising a color reproduction processing means for converting a color space of color image information using the color reproduction processing method according to claim 3. is there.

That is, when the color space of the color image information is converted, the color reproduction processing means performs processing when the white point in the uniform color space is centered on the basis of the relationship obtained from the result of the subjective evaluation experiment. By calculating the distance from the white point using the angle between the original color coordinate value and the color coordinate of the gamut compression destination, and performing color reproduction processing using the calculated value, the user's preference for the desired color matching can be improved. It can be reflected.

Further, in the color reproduction processing method according to the present invention, when at least a part of the color image information is a non-reproduction color that cannot be reproduced in the color reproduction range of the color image output system, In a color reproduction processing method that converts colors into colors that can be reproduced by a color image output system, the results of subjective evaluation using color patches reflect the color reproduction tendency based on the impression given to human eyes, A color reproduction processing method for obtaining a desired reproduction color, comprising: a first step of calculating a distance from a white point of a given color; A second step of calculating a distance from the gamut boundary when the triangle is regarded as a triangle connecting the white point and the black point; and a third step of connecting the maximum saturation point to the white point and the black point on a certain iso-hue surface. A third step of calculating a distance from a white point of a suitable compression destination color on a gamut boundary surface comprising a shape, and a straight line connecting coordinate values of the given color and coordinates of the color given in the third step And an intersection with the actual gamut boundary, and using it as the compression destination color.

That is, when the color space of the color image information is converted, the color reproduction processing is performed by using the distance from the gamut boundary based on the relationship obtained from the result of the subjective evaluation experiment, so that the user can perform the color reproduction. It is possible to reflect desired tastes of color matching.

In the color image processing apparatus according to the present invention, when at least a part of the color image information is a non-reproducible color that cannot be reproduced in the color reproduction range of the color image output system, A color image processing apparatus for converting a color into a reproducible color of a color image output system, comprising a color reproduction processing means for converting a color space of color image information using the color reproduction processing method according to claim 5. is there.

That is, when the color space of the color image information is converted, the color reproduction processing means performs the color reproduction processing using the distance from the gamut boundary based on the relationship obtained from the result of the subjective evaluation experiment. This makes it possible to reflect the user's preference regarding color matching.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a color reproduction processing method and a color image processing apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

[Subjective Evaluation Experiment] First, a subjective evaluation experiment regarding color matching will be described with reference to FIGS. According to this subjective evaluation experiment on color matching, there is the following tendency for the color matching desired by the user.

FIGS. 6 to 8 show what colors the subject pointed out to be suitable as the compression destination color when conducting a subjective evaluation experiment of color matching between a color printer and a color monitor on the same hue plane. It shows whether there was.
In this experiment, the subject displayed a single color on a color monitor and selected the color closest to the displayed color from several sample patches on the gamut boundary of the same hue, which was printed out in advance.

In this example, eight hues of the monitor RGBCMY and the printer GC are taken, and a desirable compression direction is analyzed from data obtained therefrom, and another formulation of a gamut processing method is attempted.

FIG. 6 is a graph showing the desirable properties of the compression direction in the statistical data example (1). Here, the Munsell color solid, that is, the HVC space (Hu
e: hue, Value: lightness, Chroma: saturation 3
The horizontal axis is the distance from the white point of the test patch, and the vertical axis is the color of that patch as the compression destination for the Euclidean distance from the white point in the three-dimensional space defined by the attributes. It is the distance from the white point that was pointed out to be appropriate.

FIG. 6 shows that "the distance (ΔLw1) from the white point of the color coordinate value is proportional to the distance (ΔLw2) from the white point at the gamut compression destination".
FIG. 2 shows an arrow indicating a desirable compression tendency in a certain hue plane.

FIG. 7 is a graph showing a desirable property of the compression direction in a statistical data example (2). Here, ΔL
w2 = (1−p1 ^* Θ) ^* ΔLw1 is an estimated value of the distance from the white point when the compression destination is determined by a certain calculation formula on the horizontal axis, and the vertical axis is its actual value. .
Here, Θ represents the angle between the original color coordinate value and the color coordinate of the gamut compression destination when the white point on the equal hue space is the center.

FIG. 8 is a graph showing a desirable property of the compression direction in a statistical data example (3). Here, when the gamut boundary surface is regarded as a triangle connecting the maximum saturation point, white point and black point on a certain hue plane,
The feature value of the distance from the gamut boundary is an estimated value when the compression destination is determined by a certain calculation formula on the horizontal axis, and the vertical axis is the actual value.

In FIG. 8, in appropriate color matching, as the distance (hGvc) from the gamut boundary increases, the difference in distance from the white point (dLen = ΔLw1-Δ)
Lw2) also increases.

[Configuration and Operation of Color Reproduction Processing Apparatus] FIG.
FIG. 1 is a block diagram illustrating a configuration of a color reproduction processing device according to an embodiment. In the figure, this device is an image input device 101 for inputting a color image and outputting the input image signal.
A color reproduction processing device 102 as color reproduction processing means for executing color reproduction processing to be described later on the input image signal, and reproduce / output a color image based on an output image signal given from the color reproduction processing device 102 And an image output device 103 that performs the processing.

Further, the color reproduction processing unit 102 includes a pre-compression processing unit 104 for executing a pre-compression process for converting an input image signal into a signal value used in the compression process, and a compression processing unit 105 for executing a compression process described later. And a post-compression processing unit 106 for executing post-compression processing for converting an output image signal into a signal value used in the compression processing. Hereinafter, the processing operation of each unit will be described.

(Operation of the pre-compression processing unit 104) After the RGB signals, which are input image signals, are converted into XYZ values in the XYZ space, normalization processing for matching the display and the hard copy reference white and black points is executed. The signal is converted into a signal value in a uniform color space.

The processing for matching the display with the reference white color of the hard copy is, for example, (X1, Y1, Z
1) = Ypaper ÷ 100 × (Xin, Yin, Zin) based on the relational expression. Note that Ypaper is the white stimulus value of the recording paper when the white Y stimulus value of the monitor is 100.

The process of matching the black point is a process of converting the hue and the saturation into linear luminance while keeping the same. For example, Y '= (Yw-Y
b) (X1, Y1 ', Z1) converted based on the relational expression of ÷ Yw × Y1 + Yb is used.

(Operation of Post-Compression Processing Unit 106) The post-compression processing unit 106 performs a process of converting an output image signal processed and output by the image processing unit 105 into a CMYK signal for printing.

(Operation of Compression Processing Unit 105) The compression processing unit 105 determines whether the gamut area of the printer is inside or outside. If it is outside the gamut area, it is not converted. On the other hand, if it is outside the gamut area, it is converted as follows for a color with a lightness higher than the lightness of the maximum saturation point on a certain hue surface, and For example, a color on a gamut boundary surface having the same value of brightness is set as a reproduction destination color.

In order to realize the above-described compression processing, for example, a three-dimensional lookup table may be created in advance for a four-color printer, and color conversion processing may be performed using the lookup table. .

Next, a description will be given of an example of the color reproduction processing of the color reproduction processing apparatus based on the relationship obtained in the subjective evaluation experiment shown in FIGS.

(Example 1 of Color Reproduction Process) Here, an example in which, when converting the color space of color image information, the color reproduction process is executed using a distance from a white point on a uniform color space will be described.

The first color reproduction processing example is based on the relationship of the subjective evaluation experiment shown in FIG. FIG. 3 is a flowchart illustrating a processing operation according to the first example of the color reproduction processing. First, the distance from a white point of a given color is calculated using, for example, Equation 1 (S301).

[0042]

(Equation 1)

Subsequently, the distance from the white point of the appropriate compression destination color on the gamut boundary surface (a triangle connecting the maximum chroma point with the white point and the black point on a certain hue plane) is calculated by ΔLw2 = a ^* ΔLw1 + b. (S302).

Thereafter, the intersection of the straight line connecting the coordinate values of the given color and the coordinates of the color given in step S302 with the actual gamut boundary is determined, and is set as the compression destination color (S303). . For example, a boundary point between a non-reproducible color and a non-reproducible color is searched while sequentially examining whether the color is reproducible.

(Color Reproduction Processing Example 2) Here, in the color reproduction processing example 1, the angle between the original color coordinate value and the color coordinate value of the gamut compression destination when the white point is the center in the uniform color space. The following describes an example in which the distance from the white point is calculated using, and the color reproduction process is executed using the calculated value.

The second color reproduction processing example is based on the relationship of the subjective evaluation experiment shown in FIG. FIG. 4 is a flowchart illustrating a processing operation according to the color reproduction processing example 2. First, a distance from a white point of a given color is calculated using, for example, the above-described Equation 1 (S401).

Subsequently, when the white point on the gamut boundary plane (a triangle connecting the maximum saturation point and the white point and the black point on a certain hue plane) is centered, the original color coordinate values and the color coordinates of the gamut compression destination are obtained. Is calculated, and the distance from the white point of the appropriate compression destination color is calculated as ΔLw2 =
(1-p1 ^* Θ) ^* Calculated by ΔLw1 (S40)
2).

Thereafter, an intersection between a straight line connecting the coordinate values of the given color and the coordinates of the color given in step S402 and the actual gamut boundary is obtained, and is set as the compression destination color (S403). . For example, a boundary point between a non-reproducible color and a non-reproducible color is searched while sequentially examining whether the color is reproducible.

(Example 3 of Color Reproduction Processing) Here, an example of executing color reproduction processing using the distance from the gamut boundary will be described.

The third example of the color reproduction processing is based on the relationship of the subjective evaluation experiment shown in FIG. FIG. 5 is a flowchart illustrating a processing operation according to the color reproduction processing example 3. First, a distance from a white point of a given color is calculated using, for example, Equation 1 (S501).

Subsequently, a distance (hGvc) from a gamut boundary surface of a given color (a gamut boundary surface when a triangle connecting the maximum saturation point with a white point and a black point in a certain hue plane) is calculated. Yes (S50
2). Further, the distance from the white point of the appropriate compression destination color on the gamut boundary surface (a triangle connecting the maximum saturation point and the white point and the black point in a certain hue plane) is ΔLw2 = ΔLw1 when hGvc <b,
Otherwise, ΔLw2 = ΔLw−a ^* (hGvc
-B) (S503).

Thereafter, an intersection point between a straight line connecting the coordinate values of the given color and the coordinates of the color given in step S503 and the actual gamut boundary is obtained, and is set as the compression destination color (S504). . For example, a boundary point between a non-reproducible color and a non-reproducible color is searched while sequentially examining whether the color is reproducible.

[Effects of the Embodiment] Therefore, in each of the above color reproduction processing examples, since the user's preference regarding color matching is reflected, the present invention can also be used in a color image processing apparatus for converting the color space range of color image information. Moreover, faithful color reproduction is realized.

Further, in the color reproduction of a single color such as a computer graphic image or a graph, it is possible to reproduce the color that meets the human subjectivity. Furthermore, since the processing is extremely simple, the realization is very easy.

[0055]

As described above, according to the color reproduction processing method (claim 1) / color image processing device (claim 2) of the present invention, when the color space of color image information is converted, Based on the relationship obtained from the results of the subjective evaluation experiment, it is possible to reflect the user's preference for color matching by performing color reproduction using the distance from the white point in the color matching space. Therefore, it can also be used in a color image processing device that converts the color space range of color image information, and achieves faithful color reproduction. In addition, it is human subjective in color reproduction of single colors such as computer graphic images and graphs. Color reproduction becomes possible,
Since the processing is extremely simple, it can be realized very easily.

According to the color reproduction processing method (claim 3) and the color image processing device (claim 4) of the present invention, when converting the color space of color image information, the results of the subjective evaluation experiment are used. Based on the obtained relationship, the distance from the white point is calculated using the angle between the original color coordinate value and the color coordinate of the gamut compression destination centered on the white point in the color matching space,
By performing color reproduction processing using the calculated values, it is possible to reflect the user's preference regarding color matching, so that the present invention can also be used in a color image processing apparatus that converts the color space range of color image information. In addition, faithful color reproduction is realized, and color reproduction suitable for human subjectivity in single-color reproduction of computer graphic images and graphs is also realized. In addition, since the processing is extremely simple, it can be realized very easily. be able to.

According to the color reproduction processing method (claim 5) / color image processing apparatus (claim 6) of the present invention, when converting the color space of color image information, the results of subjective evaluation experiments Based on the obtained relationship, by performing the color reproduction process using the distance from the gamut boundary, it is possible to reflect the user's preference regarding color matching, so that the color space range of the color image information can be reflected. It can also be used for color image processing equipment for conversion, and realizes faithful color reproduction. In addition, it enables color reproduction suitable for human subjects in color reproduction of single colors such as computer graphic images and graphs. Processing can be implemented very easily.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a color reproduction processing device according to an embodiment.

FIG. 2 shows a desirable compression tendency on an iso-hue surface with an arrow.

FIG. 3 is a flowchart illustrating a processing operation according to a color reproduction processing example 1;

FIG. 4 is a flowchart illustrating a processing operation according to a color reproduction processing example 2;

FIG. 5 is a flowchart illustrating a processing operation according to a color reproduction processing example 3;

FIG. 6 is a graph showing, in a first example of statistical data, desirable properties in the compression direction according to the first example of the color reproduction process.

FIG. 7 is a graph showing a desirable compression direction property according to a second example of statistical data according to the second example of the color reproduction processing.

FIG. 8 is a graph showing a desirable property of a compression direction in a third example of statistical data according to the third example of the color reproduction processing.

[Explanation of symbols]

 Reference Signs List 101 Image input device 102 Color reproduction processing device 103 Image output device 104 Image preprocessing unit 105 Compression processing unit 106 Post-compression processing unit

Claims (6)

[Claims] If at least a part of the color image information is a non-reproducible color that cannot be reproduced in a color reproduction range of a color image output system, the non-reproduction color is converted into a color reproducible by the color image output system. In the color reproduction processing method, the color reproduction tendency based on the impression given to the human eye is reflected by using the result of the subjective evaluation using the color patch, and the reproduction color desired by the subject is obtained. , The first step of calculating the distance of a given color from the white point, and the appropriate compression destination color at the gamut boundary surface consisting of a triangle connecting the maximum saturation point, the white point and the black point on a certain hue plane. A second step of calculating the distance from the white point, and finding an intersection of a straight line connecting the coordinate values of the given color and the coordinates of the color given in the second step with the actual gamut boundary. A third step of using the color as a compression destination color. 2. When at least a part of the color image information is a non-reproducible color that cannot be reproduced in the color reproduction range of the color image output system, the non-reproduction color is converted into a color reproducible by the color image output system. 2. A color image processing apparatus, comprising: a color reproduction processing means for converting a color space of color image information using the color reproduction processing method according to claim 1. 3. When at least a part of the color image information is a non-reproducible color that cannot be reproduced in the color reproduction range of the color image output system, the non-reproduction color is converted into a color reproducible by the color image output system. In the color reproduction processing method, the color reproduction tendency based on the impression given to the human eye is reflected by using the result of the subjective evaluation using the color patch, and the reproduction color desired by the subject is obtained. , The first step of calculating the distance of a given color from the white point, and centering on the white point on the gamut boundary surface consisting of a triangle connecting the maximum saturation point, the white point and the black point on a certain hue plane A second step of calculating the angle between the original color coordinate values and the color coordinates of the gamut compression destination and calculating the distance from the white point of the appropriate compression destination color; Find the intersection between the line connecting the coordinates of the color given in step 2 and the actual gamut boundary,
A third step of using the color as a compression destination color. 4. When at least a part of the color image information is a non-reproducible color that cannot be reproduced in the color reproduction range of the color image output system, the non-reproduction color is converted into a color reproducible by the color image output system. 4. A color image processing apparatus comprising: a color reproduction processing means for converting a color space of color image information using the color reproduction processing method according to claim 3. 5. When at least a part of the color image information is a non-reproducible color that cannot be reproduced in the color reproduction range of the color image output system, the non-reproduction color is converted into a color reproducible by the color image output system. In the color reproduction processing method, the color reproduction tendency based on the impression given to the human eye is reflected by using the result of the subjective evaluation using the color patch, and the reproduction color desired by the subject is obtained. The first step of calculating the distance of a given color from a white point, and the gamut of the given color as a triangle connecting the maximum saturation point, the white point and the black point on a given iso-hue surface A second step of calculating the distance from the boundary, and a suitable pressure at a gamut boundary consisting of a triangle connecting the maximum saturation point, the white point and the black point on a certain hue plane. A third step of calculating the distance from the white point of the destination color, and the intersection of the actual gamut boundary with a straight line connecting the coordinate values of the given color and the coordinates of the color given in the third step And a fourth step of determining the compression target color as the compression destination color. 6. When at least a part of the color image information is a non-reproducible color that cannot be reproduced in the color reproduction range of the color image output system, the non-reproduction color is converted into a color reproducible by the color image output system. 6. A color image processing apparatus comprising: a color reproduction processing means for converting a color space of color image information using the color reproduction processing method according to claim 5.