JP3740705B2 - Color conversion device - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a color conversion device capable of improving the utilization rate of a color space of a lookup table.
[0002]
[Prior art]
In digital copying machines, image input / output devices, digital image processing devices, etc., the types of color signals handled differ between input and output, so color signal conversion (color conversion) is often performed. Many devices using a lookup table have been developed.
[0003]
Conventionally, when performing color conversion of three-dimensional input, input values and representative values of each axis are used as they are as table addresses. For example, when converting an XYZ signal to a CIE L ^* a ^* b ^* signal, first, a representative value is determined for each axis of the XYZ signal, that is, X, Y, Z, and a three-dimensional lookup table is drawn using the representative value as an address. After that, interpolation was performed. Similarly, an example of converting a CIE L ^* a ^* b ^* signal into a CMYK signal corresponding to a printing apparatus is disclosed in Japanese Patent Application No. 6-144268.
[0004]
[Problems to be solved by the invention]
By the way, when a three-dimensional lookup table is constituted by a digital device, it is general to set a rectangular parallelepiped color space in order not to make the structure complicated.
[0005]
On the other hand, the color reproduction range of the printing apparatus has various shapes and is not usually a rectangular parallelepiped. In this case, the 3D lookup table must include the color reproduction range of the printing apparatus. However, if the rectangular parallelepiped includes a complicated shape, the color reproduction range that occupies the color space of the 3D lookup table. The ratio (hereinafter referred to as “utilization rate”) is extremely low, and is generally about 1/6.
[0006]
For example, in a printing method using gravure printing or electrophotography, there is a color reproduction range that protrudes in the b ^* axis direction due to the characteristics of the color material. It does not contribute to color conversion.
In addition, the utilization rate is low even in the XYZ space and the RGB space.
[0007]
Therefore, as an example of improving the utilization factor of the three-dimensional lookup table, as described in Japanese Patent Application No. 6-144268, it is proposed to use grid points outside the color reproduction range for color conversion of a specific color. It had been. However, since this method is effective only for a specific color, it has not become a drastic measure for improving the utilization rate of the entire lookup table.
[0008]
The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a color conversion device that can greatly improve the utilization factor of a three-dimensional lookup table with a simple configuration.
[0009]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, in the invention described in claim 1, input color information having three-dimensional color coordinates is referred to a three-dimensional lookup table having a rectangular coordinate space, and another type of color information is obtained. In the color conversion device that converts the input color information into predetermined coordinate values in the input color information so that the coordinate values of the input color information fall within the coordinate space of the three-dimensional lookup table. Affine transformation means for performing affine transformation in accordance with an equation, and the three-dimensional lookup table includes coordinate values transformed by the affine transformation means and one-dimensional coordinates in the input color information not transformed by the affine transformation means The content is set so as to output color information of another format corresponding to the value .
[0012]
According to the second aspect of the present invention, the input color information having three-dimensional color coordinates is converted into color information of another format with reference to a three-dimensional lookup table having a rectangular coordinate space. In the conversion device, a calculation unit that obtains an interpolation value for the input color information, and a predetermined two-dimensional value in the input color information so that the coordinate value of the input color information fits in the coordinate space of the three-dimensional lookup table. An interpolation unit that has an affine transformation unit that affine-transforms the coordinate value according to a predetermined transformation formula, and performs an interpolation process based on the interpolation value and color information of another format output from the three-dimensional lookup table; The three-dimensional lookup table includes coordinate values converted by the affine transformation means and the input values not transformed by the affine transformation means. Characterized in that the content of which is set to output the color information of another format corresponding to the 1-dimensional coordinate values in the color information.
[0014]
According to a third aspect of the present invention, in the color conversion device according to the first or second aspect , an input device that obtains the input color information and an output device that prints the color information converted into another format The constants of the conversion equation in the affine conversion means are determined in accordance with the color reproduction characteristics of the image.
[0015]
According to a fourth aspect of the present invention, in the color conversion device according to the first or second aspect , the input color information is L ^* a ^* b ^* information, and the three-dimensional lookup table is Color information corresponding to the amount of color material for one-dimensional to four-dimensional subtractive color mixing is output.
[0016]
According to a fifth aspect of the present invention, in the color conversion device according to the first or second aspect , the input color information is L ^* a ^* b ^* information, and further, L ^{* in the} information is L ^* a ^* b ^* information ^. L ′ information, a ′ information and b ′ information corresponding to each of the information, a ^* information and b ^* information are created, and the affine transformation result of the affine transformation means is
(1) L ′ + (1/2) a ′ + C1
(2) L ′ − (1/2) a ′ + C2 and (3) L′−b ′ + C3
(However, C1, C2, and C3 are constants)
It is proportional to at least any one of the calculation results.
[0017]
[Action]
As a result of the affine transformation of the input color information performed by the affine transformation means, the shape of the color reproduction range approaches a rectangular parallelepiped. On the other hand, since the coordinate space of the three-dimensional lookup table has a rectangular parallelepiped shape, the utilization factor of the three-dimensional lookup table is improved .
[0018]
【Example】
A: First Embodiment Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the first embodiment of the present invention. In the figure, reference numeral 1 denotes a representative value / interpolated value calculation means for obtaining a representative value and an interpolated value of an input L ^* a ^* b ^* signal. In this way, the representative value and the interpolation value are obtained because the general three-dimensional lookup table outputs data corresponding to the representative value, and interpolation is performed when the output data is interpolated. This is because a value is required.
[0019]
Next, reference numeral 2 denotes representative value affine transformation means for transforming the coordinates of the representative value output from the representative value / interpolation value conversion means. The representative value after coordinate transformation output from the representative value affine transformation means 2 becomes an address signal of the three-dimensional lookup table 3.
[0020]
Here, the affine transformation in the representative value affine transformation means 2 will be described. 2 and 3 are color reproduction ranges of the printing apparatus used in this embodiment, respectively. FIG. 2 shows a state seen from the a ^* b ^* plane, and FIG. 3 shows a state seen from the L ^* b ^* plane. ing. As can be seen from these drawings, when the rectangular parallelepiped three-dimensional lookup table is configured so as to include the color reproduction range of the printing apparatus (the portion indicated by hatching), the utilization factor has to be lowered.
[0021]
By the way, if the state shown in FIG. 2 is observed well, as shown in FIG. 4A, a quadrilateral having a smaller area than the rectangles Z ₁ , Z ₂ , Z ₃ , and Z ₄ that are the outlines of the three-dimensional lookup table. It can be seen that B ₁ , B ₂ , A ₃ and A ₄ can be configured. When this quadrangle is converted into a rectangle by affine transformation, the result is as shown in FIG. Here, looking at the ratio of the color reproduction range shown in the rectangle shown in FIG. 4B, it is apparent that the ratio is clearly larger than that in FIG. 4A, and the utilization rate is improved.
[0022]
Similarly, in the state shown in FIG. 3, it can be seen that quadrangles C ₁ , C ₂ , C ₃ , and C ₄ smaller than the contour of the three-dimensional lookup table can be configured as shown in FIG. When this quadrangle is converted into a rectangle, it becomes as shown in FIG. 5B, and it can be seen that the utilization rate is improved as in the case described above.
[0023]
Now, it is possible to combine the affine transformations shown in FIGS. 4 and 5 to perform a three-dimensional affine transformation. FIG. 6 shows an example of a three-dimensional affine transformation. That is, in FIG. 6A, the hexahedron A ₁ is smaller than the lookup table space Z ₁ , Z ₂ , Z ₃ , Z ₄ , Z ₅ , Z ₆ , Z ₇ , Z ₈ and includes the color re-range. , A ₂ , A ₃ , A ₄ , A ₅ , A ₆ , A ₇ , A ₈ can be configured, and when this is affine transformed into a rectangular parallelepiped, the result is as shown in FIG. . By performing such conversion, the utilization rate of the lookup table can be greatly increased.
[0024]
The representative value affine transformation means shown in FIG. 1 performs a three-dimensional affine transformation as shown in FIG. Here, the mathematical formula used for the affine transformation is preset from the shape of the color reproduction range of the printing apparatus used. That is, since the color reproduction range of the printing apparatus is known, a mathematical expression for performing affine transformation as shown in FIG. 6 is obtained and set in advance.
[0025]
Next, the three-dimensional lookup table 3 shown in FIG. 1 has grid points corresponding to the representative values subjected to affine transformation, and CMYK signals corresponding to the coordinates of the representative values before affine transformation are obtained at each grid point. Output. That is, the three-dimensional lookup table 3 outputs a C′M′Y′K ′ signal corresponding to the representative value of the L ^* a ^* b ^* signal. Then, the C′M′Y′K ′ signal is supplied to the interpolation means 4, where interpolation processing is performed based on the interpolation value output from the representative value / interpolation value calculation means 1 and is output as a CMYK signal. . In this case, affine transformation is not applied to the interpolation value. This is because the C′M′Y′K ′ signal output from the three-dimensional lookup table 3 is a signal corresponding to the representative value of the L ^* a ^* b ^* signal before the affine transformation, and is interpolated. This is because the interpolation value in this case does not need to be subjected to affine transformation.
[0026]
According to the configuration described above, the utilization factor of the three-dimensional lookup table 3 can be improved to about three times that of the conventional one.
The conversion formula in the representative value affine conversion means 2 is set in accordance with the color reproduction range of the printing apparatus used in this embodiment, but instead of this, a standard color reproduction range is assumed, and A corresponding conversion formula may be set.
[0027]
B: Second Embodiment FIG. 7 is a block diagram showing the configuration of the second embodiment of the present invention. This embodiment shows an example in which an RGB signal that is an input signal is converted into a CMYK signal. The affine transformation means 12 shown in this figure performs affine transformation on the R signal and the B signal and outputs the result to the three-dimensional lookup table 13.
[0028]
The three-dimensional lookup table 13 has grid points corresponding to the G signal, the R signal after affine transformation, and the B signal, and outputs a CMYK signal corresponding to the input RGB signal.
[0029]
In this embodiment, affine transformation is performed on two of the three-dimensional input signals, but a substantially rectangular parallelepiped color reproduction space is formed by the G signal, the affine transformed R signal, and B signal. Therefore, the utilization factor of the three-dimensional lookup table 13 is improved as in the first embodiment. For example, when the conversion formula in the affine conversion means 12 is changed to a formula that matches the printing apparatus to be used, the conventional apparatus has a utilization factor of about 30%, and increases to about 50%.
[0030]
Although this embodiment is an example in which no interpolation is performed, a configuration for performing interpolation as in the first embodiment described above may be provided.
[0031]
C: Third Embodiment FIG. 8 is a block diagram showing the configuration of the third embodiment of the present invention. In this embodiment, an RGB signal as an input signal is converted into an L ^* a ^* b ^* signal. The configuration is substantially the same as that of the first embodiment described above, except that the representative value and the interpolation value are calculated after affine transformation of the input signal.
[0032]
That is, the RGB signal is first affine-transformed by the affine transformation means 22 and then transformed into a substantially rectangular parallelepiped color reproduction region. Next, the representative value and the interpolation value of the RGB signal subjected to the affine transformation are calculated by the representative value / interpolation value calculation means, and the representative value is supplied to the lookup table 23 and the interpolation value is supplied to the interpolation means 24.
In this embodiment, since the representative value and the interpolation value are calculated after the affine transformation, the interpolation value also corresponds to the space after the affine transformation.
Even in the above configuration, the utilization factor of the three-dimensional lookup table 23 is improved as in the above-described embodiments.
[0033]
D: Fourth Embodiment Next, FIG. 9 is a block diagram showing the configuration of the fourth embodiment of the present invention. In this embodiment, an L ^* a ^* b ^* signal is converted into a CMYK signal. The upper bits of the L ^* a ^* b ^* signal are supplied to the representative value affine transformation means 32, and the lower bits are used as an interpolation value. It is supplied to the interpolation means 34.
[0034]
The representative value affine transformation means 32 includes adders 32a, 32b, 32c, a bit shifter 32d, and a sign inverter 32e as shown in the figure. And the calculation which the representative value affine transformation means 32 performs is as follows.
(1) L '+ (1/2) a' + C1
(2) L '-(1/2) a' + C2
(3) L'-b '+ C3
In the above equation, the L ′ signal, the a ′ signal, and the b ′ signal are the upper bits (the number of bits may be set as appropriate) of the L ^* signal, the a ^* signal, and the b ^* signal, and C1, C2, C3 is an appropriate constant.
[0035]
When the affine transformation by the above-described mathematical formula is performed, the color reproduction space has a substantially rectangular parallelepiped shape, and the three-dimensional lookup table 33 has only to store 4096 values, so that the utilization efficiency is significantly increased. For example, in a gravure photo or electrophotographic printing method, the utilization factor of a three-dimensional lookup table increases from 17% to about 50%.
The number of additions in the affine transformation in this embodiment is 3. Therefore, in terms of hardware, this is realized by providing three adders, one bit shifter or appropriate wiring, and one sign inverter. In general, affine transformation requires many product-sum operations and tends to be complicated in configuration, but in this embodiment, affine transformation can be realized with a very simple configuration.
[0036]
In the above-described fourth embodiment, all of the formulas (1) to (3) are used, but even if any one of them is used, the effect of improving the utilization rate can be obtained. Further, the calculation results of the formulas (1) to (3) may not be used as they are, but may be configured to use a proportional value by multiplying by a predetermined coefficient.
[0037]
(Modification)
Of the above-described embodiments, some or all of the functions may be realized by software processing.
[0038]
【The invention's effect】
As described above, according to the present invention, the utilization factor of the three-dimensional lookup table can be greatly improved with a simple configuration.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a first embodiment of the present invention.
FIG. 2 is a diagram illustrating a color reproduction range of a general printing apparatus in a CIE L ^* a ^* b ^* space on an a ^* b ^* plane.
FIG. 3 is a diagram illustrating a color reproduction range of a general printing apparatus in a CIE L ^* a ^* b ^* space on an L ^* b ^* plane.
4 is a diagram showing a case where the color reproduction range shown in FIG. 2 is affine transformed. FIG.
5 is a diagram showing a case where the color reproduction range shown in FIG. 3 is affine transformed. FIG.
FIG. 6 is a diagram for explaining the effect of the present application when a three-dimensional affine transformation is performed in a CIE L ^* a ^* b ^* space.
FIG. 7 is a diagram showing a configuration of a second exemplary embodiment of the present invention.
FIG. 8 is a diagram showing a configuration of a third exemplary embodiment of the present invention.
FIG. 9 is a diagram showing a configuration of a fourth exemplary embodiment of the present invention.
[Explanation of symbols]
1 representative value / interpolation value calculation means 2 representative value affine transformation means 3 three-dimensional lookup table 4 interpolation means 12 affine transformation means 13 three-dimensional lookup table 21 representative value / interpolation value calculation means 22 affine transformation means 23 three-dimensional lookup Table 24 Interpolation means 32 Affine transformation means 33 Three-dimensional lookup table 34 Interpolation means

Claims (5)

In a color conversion apparatus that converts input color information having three-dimensional color coordinates into color information of another format with reference to a three-dimensional lookup table having a rectangular coordinate space,
An affine transformation means for affine transformation of a predetermined two-dimensional coordinate value in the input color information according to a predetermined transformation formula so that the coordinate value of the input color information falls within the coordinate space of the three-dimensional lookup table. Have
The three-dimensional lookup table outputs color information in another format corresponding to the coordinate values converted by the affine transformation means and the one-dimensional coordinate values in the input color information not transformed by the affine transformation means. The color conversion device is characterized in that the contents are set as follows. In a color conversion apparatus that converts input color information having three-dimensional color coordinates into color information of another format with reference to a three-dimensional lookup table having a rectangular coordinate space,
A calculation means for obtaining an interpolation value for the input color information;
An affine transformation means for affine transformation of a predetermined two-dimensional coordinate value in the input color information according to a predetermined transformation formula so that the coordinate value of the input color information falls within the coordinate space of the three-dimensional lookup table. Have
Interpolation means for performing interpolation processing based on the interpolation value and color information of another format output from the three-dimensional lookup table,
The three-dimensional lookup table outputs color information in another format corresponding to the coordinate values converted by the affine transformation means and the one-dimensional coordinate values in the input color information not transformed by the affine transformation means. The color conversion device is characterized in that the contents are set as follows. The constants of the conversion formula in the affine transformation means are determined according to color reproduction characteristics of an input device that obtains the input color information and an output device that prints the color information converted into another format. The color conversion apparatus according to claim 1 or 2 . The input color information is L ^* a ^* b ^* information, and the three-dimensional lookup table outputs color information corresponding to a color material amount of one-dimensional to four-dimensional subtractive color mixture. The color conversion apparatus according to claim 1 or 2 . The input color information is L ^* a ^* b ^* information, and L ′ information, a ′ information, and b ′ information corresponding to each of L ^* information, a ^* information, and b ^* information in the information is created. And the affine transformation result of the affine transformation means is
(1) L ′ + (1/2) a ′ + C1
(2) L ′ − (1/2) a ′ + C2 and (3) L′−b ′ + C3
(However, C1, C2, and C3 are constants)
The color conversion apparatus according to claim 1 , wherein the color conversion apparatus is proportional to at least one of the calculation results.

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