JP3205054B2 - Color image processing apparatus and method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image processing apparatus and method for mapping color image data to a color reproduction range of an output device.

[0002]

2. Description of the Related Art At present, as a general device for visualizing and outputting a color image, there are a color CRT monitor and a color hard copy device. The former forms the visible image by additive color mixing by modulating the light emission levels of the phosphors of R, G and B colors on the tube surface. On the other hand, the latter forms a color image on paper by subtractive color mixture using Y (yellow), M (magenta), C (cyan), and K (black) color materials.

[0003]

The two image display devices described above have different color reproduction capabilities in principle. As shown in FIG. 3, a CRT generally has a wider color reproduction range than a hard copy. Therefore, when two colors of point A and point B exist in a certain image, when this image is displayed on a CRT, it is reproduced as a different color, but when a hard copy is output, both A and B are reproduced as point C. As a result, the two colors cannot be distinguished, and as a result, information originally possessed by the image is lost.

Therefore, in the case of outputting a hard copy of an input color image, a method of converting a color signal in the image so that the color signal falls within the color reproduction range of the hard copy and then outputting the converted image can be considered. That is, if the conversion is performed such that point A becomes point D and point B becomes point E in FIG. 3, the colors can be distinguished even on the hard copy. However, since such a conversion method is not unique, depending on the method, the output image may be entirely unnatural.

In order to solve such a problem, the following proposal has been made as a conversion method of an input color image. That is, it is possible to map the color signal information included in the input image to the color reproduction range of the hard copy while preserving the basic primary colors (red, green, blue, cyan, magenta, and yellow) of the color image. This proposal refers to the distribution of color signals contained in an input image, detects the color with the highest saturation, and maps that color to the color reproduction range of a hard copy device. In the detection of, the input color image is evaluated in the R, G, B signal space. However, since the R, G, and B spaces do not include the non-linearity of human visual characteristics, even if the most saturated color is detected, even if it is actually seen by the human eye, the most saturated color is obtained. There was a problem that there was no guarantee that the color was high.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and provides six primary color values indicating the color reproduction range of an input image.
To be converted to six primary color values that indicate the color gamut of the power device
Using the calculated matrix coefficients, a color indicating the input image
An object of the present invention is to provide a color image processing apparatus and method for mapping image data within a color reproduction range of an output device.

[0007]

According to the present invention, there is provided a color image processing apparatus for mapping color image data representing an input image to a color reproduction range of an output device. Calculating the component values of the color image data in the respective directions of the six primary colors with reference to the respective directions of the six primary colors of red, green, blue, cyan, magenta, and yellow indicating the color reproduction range of the output device. a calculation unit, the component values the calculated red indicating the color gamut of the input image, green, blue, cyan, and detecting means for detecting magenta, six primary values of the yellow, the detected input image The color image data is converted to the color reproduction range of the output device by using a matrix coefficient determined so that the six primary color values indicating the color reproduction range of the output device are converted into the six primary color values indicating the color reproduction range of the output device. And having a mapping means for mapping the.

According to another aspect of the present invention, there is provided a color image processing method for mapping color image data representing an input image to a color reproduction range of an output device.
In a uniform color space, six primary colors of red, green, blue, cyan, magenta, and yellow indicating the color reproduction range of the output device are provided.
Based on each direction, a calculation step of calculating component values in each direction of the six primary colors of the color image data, and from each of the calculated component values, red, green, A detection step of detecting six primary color values of blue, cyan, magenta, and yellow; and the six primary color values indicating the color reproduction range of the detected input image are converted into six primary color values indicating the color reproduction range of the output device. A mapping step of mapping the color image data to a color reproduction range of the output device using the matrix coefficient obtained as described above.

[0009]

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. <First Embodiment> FIG. 1 is a block diagram showing a configuration of a color image processing apparatus according to a first embodiment of the present invention. In the figure, 101 is a selector, 102 and 104 are buses,
103 is a matrix conversion circuit, 105 is a sampling circuit, 106 is a buffer memory, 107 is a CPU,
Reference numeral 8 denotes a ROM that stores programs for operating the CPU 107, and 109 denotes a RAM that stores various parameters and various programs.

Next, the operation of the above configuration will be described with reference to FIG. The input R, G and B signals are supplied to the selector 10
1, when the image signal compression is performed, the image signal is output to the bus 102 and output to the matrix conversion circuit 103 to be output signals R ′, G ′, and B ′. When a color signal distribution is detected in an input image, the signal is output to a bus 104, sampled at a predetermined sampling pitch by a sampling circuit 105, and written to a buffer memory 106. CPU
107 sequentially reads out the image signals from the buffer memory 106 and detects the color signal having the highest saturation in the image by the method described later. From the detection result, the CPU 107 calculates a matrix conversion coefficient by a method described later, and sets the calculated coefficient in the matrix conversion circuit 103.

FIG. 2 is a circuit block diagram for realizing the matrix conversion circuit 103 according to the first embodiment. In the figure, an input color image signal is input from above the figure. The image signal is sequentially transferred to the lower part of the figure together with a drive clock, a reset signal, and the like (not shown). 201
Is a minimum extraction circuit for extracting the minimum value among R, G, and B, and outputs a minimum value signal X (= min (R, G, B)). Reference numerals 202, 203, and 204 denote subtraction circuits that take the difference between the input signal and the minimum value signal. The subtraction circuit 202 outputs RX, the subtraction circuit 203 outputs GX, and the subtraction circuit 204 outputs BX. 205, 206 and 207 are 202, 203 and 20
4 is a latch circuit for temporarily latching the output of the latch circuit 4. 20
8, 209 and 210 are multiplication circuits for calculating quadratic terms of RX, GX and BX. The multiplication circuit 208 outputs (R−
X) × (G−X), and the multiplication circuit 209 calculates (G−X) × (B
−X), the multiplying circuit 210 outputs (B−X) × (R−X). 211 is 6 obtained by the multiplication circuits 205 to 210
This is a matrix conversion circuit for performing a matrix conversion of at least signal values, and specifically, performs an operation of the following equation (1),
R, dG and db are output. That is,

[0012]

(Equation 1)

[0013] The matrix conversion coefficient a _ij used here is obtained by a method described later. 212, 21
3,214 are dR, dG, dB obtained by the above equation (1).
And an adder for adding the original R, G, B signals, and outputs the converted color separation signals R ′, G ′, B ′. That is, the following equation (2) is executed.

[0014]

(Equation 2)

Next, the matrix conversion coefficient a in equation (1)
A method for _{obtaining ij} will be described. The purpose of the matrix conversion is to map a wide color range of colors included in the input image to the color reproduction range of the hard copy. Here, the reproduction of red will be considered first. Assuming that the image signal is represented by 8 bits for each of R, G, and B, the highest saturation red included in the input image is R = 200, G = 1.
It is assumed that the color signal is 5, B = 0. This color is Figure 3
Is at the point R _i on the chromaticity diagram of FIG. But most saturation red can reproduce a hard copy that would be hard copy output as the color is downy FIG R _H point located between the R _H point and R _C point by conventional methods because it is R _H point become.

Conversely, when the color signal value at the point R _H is represented by the R, G, and B signals, usually, R = 160, G = 20, and B = 10. Therefore, the input color signals R = 200, G =
By converting so that 15, B = 10, R _i
The points are converted to points R _H , the colors between them are mapped inside the points R _H , and the color reproduction area included in the input image is mapped to the color reproduction area of the hard copy while preserving the primary colors of the input signal. I can do it.

When such correspondences are set for all six primary colors, 18 simultaneous linear equations can be obtained from the above equations (1) and (2). _Aij is also 18 as an unknown
Since there are a number of them, they can be uniquely solved and the matrix variable coefficients can be determined. An example of the correspondence relationship is shown below.

[0018]

(Equation 3) (3) By the above procedure, a desired color signal conversion shown in FIG. 1 is obtained. Here, the six primary color signal values having the highest saturation included in the input image signal (R, G, B on the left side of the equation (3))
Value) must be detected. The method for that will be described in detail below with reference to the drawings.

FIG. 4 is a diagram in which the color reproduction range of a CRT and the color reproduction range of a hard copy are schematically plotted on L ^* , u ^* , v ^* and a uniform color space. The L ^* , u ^* , and v ^* spaces are uniform color spaces defined by the CIE (International Commission on Illumination) and are said to correspond almost linearly to human visual characteristics. The following equations (4) to (6) may be used to convert the R, G, B signals into L ^* , u ^* , v ^* signals.

L ^* = 116 (Y / Y ₀ ) ^1/3 −16 u ^* = 13 L ^* (u−u ₀ ) v ^* = 13 L ^* (v−v ₀ ) (4) where u = 4X / (X + 15Y + 3Z) v = 9Y / (X + 15Y + 3Z) (5) X = 0.6767R + 0.1736G + 0.2001B Y = 0.2988R + 0.5868G + 0.1144B Z = 0.0000R + 0.0661G + 1.1150B (6) The hexahedron 401 is a CRT. A hexahedron 402 of the color reproduction range 402 represents the color reproduction range of the hard copy. Here, the color signals contained in the input image are sampled first,
The configuration is such that the color with the highest saturation in the L ^* , u ^* , v ^* color space is detected, and the matrix coefficient of equation (1) is determined from this value. It is assumed that the signal values of certain pixels in the input image are R _S , G _S , and B _S. This value is L ^* , u
^When converted into ^* and v ^* values, a point S indicated by 403 in FIG. 4 is obtained. What needs to be obtained is the RGB signal value of the color with the highest saturation among the R, G, B, Y, M, and C6 primary colors among the signal values included in the input image. Then L ^* , u
Consider a vector as shown in FIG. 5 (an enlarged view of the range indicated by 400 in FIG. 4) in the ^* and v ^* spaces. FIG. 3 shows the case of the M (magenta) direction. M _H is the hard copy of magenta L ^*, u ^*, v ^* coordinates in space, M _C is CRT
In L ^* , u ^* , v ^* space, and is the same as FIG. Vector M _H S from M _H to the input color S, and M _H
M _H M _C of taking the vector M _H M _C to M _C from M _H S
Find the direction cosine r _M of the direction. The input color S increases as r _M increases.
It can be seen that the saturation in the magenta direction is high. r _M
Can be obtained by the inner product operation of the following equation (3). That is,

[0021]

(Equation 4)

Similarly, for the other primary colors, the direction cosines r _R , r _G , r _B , r _Y , and r _C in that direction are obtained. If the largest one is selected from these, the input color can be determined in any direction (hue). )
You can see if the color is If the maximum value of the direction cosine is obtained for each color for all the pixels in the input image or the pixels sampled at a constant interval in this manner, the L ^* , u ^* , v ^* values at the time of giving the maximum values are obtained. Inverted R
The GB signal becomes the color signal value to be obtained.

FIG. 6 is a flowchart showing the above procedure. Note that the overall control of this process is performed by the CPU 107. In the figure, R _max , G _max , B _max , Y _max , M _max ,
C _max is the maximum value of the direction cosine in each primary color direction. These are set to 0 (step S601). Next, the R, G, and B values of a specific pixel are read from the input image (step 602). The obtained RGB signal values are treated as data on the left side of the correspondence between the primary colors in the equation (3). The R, G, B values are converted into L ^* , u ^* , v ^* (step S603). And cosines r _R , r in the six primary color directions
_{G, r B, r Y,} r M, seek r _C (step S60
4) Find the maximum value from among them (step S60)
5).

The maximum value is r_X (X is R, G, B, Y, M,
C has reached the maximum value (step S60).
6), r_X > X_max X_max To r_X Replace with
(Step S607). At this time, L^* , U^* , V^* Faith
Signal is converted back to R, G, B signals, and the maximum saturation signal in the X direction
(Step S608). Sampling vs
When all the elephant pixels are completed, the process proceeds to step S610,
Upon completion, the flow advances to step S602 to repeat the above processing. S
In step S610, the maximum chroma signal is transformed into a matrix.
Find the permutation coefficient. In this way, the optimal transformation for the input image
A permutation coefficient is obtained. <Second Embodiment> In the above embodiment, the input image signal
From the color that maximizes the direction cosine in each primary color direction (input image signal
No. 403) as the maximum chroma color signal,
The point 403) connects the primary color of the hard copy with the primary color of the monitor.
(In FIG. 5, the vector M_H M _C Far away from
To convert this maximum saturation color to a hard copy primary
Will cause a change in hue.
You.

Therefore, as a second embodiment, the colors detected as the maximum chroma color signals are limited to linear colors in the hard copy primary colors and the monitor primary colors on a uniform color space.
FIG. 7 shows this state. FIG. 7 shows a case where the maximum chroma color in the magenta direction is detected, as in FIG. 5, and the meanings of the symbols are the same as in FIG. Here, as in the first embodiment, the color signal S that maximizes the direction cosine in the direction of the vector M _H M _C is obtained. Is the point Q indicated by 601. Q
Point has a point obtained by projecting the point at a distance r _M from M _H on the vector M _H M _C, i.e. the maximum saturation color signals obtained in the first embodiment on the vector M _H M _C. This point 6
The L ^* , u ^* , and v ^* values of 01 can be obtained by a simple vector operation. The signal value can be obtained by inverting this value again into R, G, B signals. <Third Embodiment> In the above two embodiments, the CIE L ^* , u ^* , and v ^* are used as the uniform color space, but the same effect can be obtained by using another uniform color space.

For example, C ^* L ^* , a ^* , b ^*
Can also be used. Also, in order to simplify the conversion operation to the uniform color space, the R, G, and B signals are converted to 1 /
R3, G ', B' are converted to the third power, and this is expressed by the following equation (4)
, A signal value close to a substantially uniform color space (here, L, c1,
c2). R ′ = (R / 255) ^1/3 G ′ = (G / 255) ^1/3 B ′ = (B / 255) ^1/3 L = 100 (R ′ + G ′ + B ′) / 3 c1 = 100 ( R′−G ′) c2 = 100 (R ′ / 2 + G ′ / 2−B ′) (4) L, c1 and c2 obtained here are slightly inferior in the uniformity of the color space, but the operation is simple. Therefore, there is an advantage that the load on the CPU is reduced.

The present invention may be applied to a system composed of a plurality of devices or an apparatus composed of one device. Needless to say, the present invention can be applied to a case where the present invention is achieved by supplying a program to a system or an apparatus.

[0028]

As described above, according to the present invention,
The six primary color values indicating the color reproduction range of the input image are
Matrices determined to be converted to six primary color values indicating the current range
Color image data representing the input image using the
Can be mapped within the color reproduction range of the output device. Therefore, an input image including an area outside the color reproduction range of the output device can be favorably reproduced by the output device.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a color image processing apparatus according to a first embodiment of the present invention.

FIG. 2 is a matrix conversion circuit 10 according to the first embodiment;
3 is a circuit block diagram for realizing No. 3.

FIG. 3 is a CIExy chromaticity diagram.

FIG. 4 is a diagram schematically plotting a color reproduction range of a CRT and a color reproduction range of a hard copy on L ^* , u ^* , v ^* , and a uniform color space.

FIG. 5 is an enlarged view of a main part of FIG.

FIG. 6 is a flowchart illustrating an operation according to the present exemplary embodiment.

FIG. 7 is a diagram illustrating a vector when a maximum saturation color in the magenta direction is detected according to the second embodiment.

[Explanation of symbols]

 101 selector 102, 104 bus 103 matrix conversion circuit 105 sampling circuit 106 buffer memory 107 CPU 201 minimum extraction circuit 202, 203, 204 subtraction circuit 205, 206, 207 latch circuit 208, 209, 210 multiplication circuit 211 matrix conversion circuit

Continuation of front page (56) References JP-A-4-119765 (JP, A) JP-A-63-35078 (JP, A) JP-A-61-214893 (JP, A) JP-A-3-159475 (JP, A) JP-A-2-220566 (JP, A) JP-A-4-1155691 (JP, A) JP-A-3-83455 (JP, A) JP-A-1-168454 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 1/60 H04N 1/46

Claims (3)

(57) [Claims] 1. A color image processing device for mapping color image data representing an input image to a color reproduction range of an output device, wherein red, green, and blue representing the color reproduction range of the output device on a uniform color space. , cyan, magenta, of the six primary colors of yellow
Each direction on the basis, the calculation means for calculating the component values in each direction of the 6 primary colors of the color image data, from the component values of the calculated red indicating the color gamut of the input image, green, 6 for blue, cyan, magenta and yellow
Detecting means for detecting a primary color value; and a matrix coefficient determined so that the six primary color values indicating the color reproduction range of the detected input image are converted into six primary color values indicating the color reproduction range of the output device. And a mapping means for mapping the color image data to a color reproduction range of the output device. 2. The image processing apparatus according to claim 1, wherein the detecting unit is configured to indicate a color reproduction range of the input image with respect to a vector in each direction of six primary colors indicating a color reproduction range of the output device on the uniform color space.
2. The color image processing apparatus according to claim 1, wherein a maximum value of a direction cosine of a vector in each direction of a primary color is detected as a component value in each direction of six primary colors indicating a color reproduction range of the input image. 3. A color image processing method for mapping color image data representing an input image to a color reproduction range of an output device.
A is, in the uniform color space, a color reproduction range of the output device shows
Red, green, blue, cyan, magenta and yellow
The six sources of the color image data with respect to each direction.
Calculating a component value in each direction of the color; and calculating a color reproduction range of the input image from the calculated component values.
Red, green, blue, cyan, magenta, and yellow 6
A detecting step of detecting primary color values, and six primary color values indicating a color reproduction range of the detected input image are
Converted to six primary color values indicating the color reproduction range of the output device
Using the matrix coefficients obtained as described above, the color image
Map image data to the color gamut of the output device
A color image processing method comprising a mapping step .