JP4793705B2 - Color conversion device - Google Patents

Description

The present invention relates to a color conversion apparatus and, more particularly, to a color conversion apparatus for converting an image signal for the multi-primary-color display device having four or more primary colors an input image signal.

  In recent years, there are several references on high fidelity color reproduction. For example, Non-Patent Document 1 and Non-Patent Document 2 point out the importance of high fidelity color reproduction in fields such as electronic commerce, digital archives, and telemedicine.

  On the other hand, one of the causes of deterioration in color reproduction accuracy is a narrow color gamut of the display device. In the uniform chromaticity diagram (UCS), the color gamut of HDTV is narrower than the human visible region. Therefore, it is clear that the color range that can be displayed by HDTV is narrower than the visible region and the display region is not sufficient.

  Currently, research is being conducted on display devices with a wider color gamut than HDTV. The wide color gamut display device here is a display device that not only has a wide reproduction range on the chromaticity diagram, but also can reproduce a color of a wide color gamut without reducing the maximum luminance. In order to develop a wide color gamut display device with three primary colors like existing devices, primary colors with high saturation and high brightness are required. In general, the luminous efficiency of the light emitting device decreases as the saturation increases. For this reason, a wide color gamut display device using the three primary colors becomes inefficient. This can be solved by using primary colors that satisfy the following two conditions. That is, a primary color with high saturation but not so high brightness and a primary color with high brightness but not so high saturation are required. However, in this case, four or more primary colors are required.

  As an example of such a multi-primary color display device, there is a six-primary-color rear projection display device manufactured by Olympus, which is one of wide color gamut displays using multi-primary colors. By using four or more primary colors, a wide color gamut display device that is more efficient than the case of the three primary colors can be realized. However, new problems also arise as the primary colors increase. One of them is the degree of freedom of color conversion. In the existing three primary color display devices, since the color tristimulus values XYZ and the three primary colors RGB have a three-to-three relationship, there is no degree of freedom in conversion, and unique conversion is possible. However, as the number of primary colors increases, the tristimulus values and primary colors have a relationship of 3 to 4 or more, and therefore, there is a degree of freedom in converting the signal values of each primary color from the tristimulus values, and the conversion cannot be performed uniquely. This indicates that there are a plurality of combinations of primary colors for displaying a certain XYZ. For this reason, there are also combinations of primary colors with low luminous efficiency.

  On the other hand, the relationship described in the following equation (2) is assumed between the tristimulus values of a certain primary color n and the corresponding input signal values.

Here, X _n , Y _n , and Z _n are tristimulus values of the primary color n when the maximum luminance is output, and _Sn is an input signal value for the primary color n. S _n is 0 ≦ S for taking the value of _n ≦ 1, it can be considered as normalized value of the maximum luminance in primary color n. The S _n will be referred to as relative brightness of the primary colors n later. When Expressions (1) and (2) are combined and expressed using a matrix, conversion of the tristimulus values XYZ from the relative luminance values of the respective primary colors is given by the following Expression (3).

  Further, the conversion from XYZ to the relative luminance of each primary color is expressed by the following expression (4) from the expression (3).

  Here, in Equation (4), when the number of primary colors is 3, the coefficient matrix is a square matrix, so that the inverse matrix is uniquely determined, and conversion can be performed without any problem. However, when the number of primary colors is 4 or more, the matrix becomes a coefficient of 3 × n (n> 3), and thus an inverse matrix cannot be obtained uniquely.

  Some researches have been conducted to deal with this problem, which are disclosed in Non-Patent Documents 4 to 6.

  Non-Patent Document 4 proposes a method of using different color matching functions for each observer without using CIE-XYZ color matching functions for color reproduction, and a method of reproducing the shape of spectral radiance estimated by a multispectral camera. ing. Color conversion can be uniquely performed by matching the number of dimensions of the color matching function with the number of dimensions of the display device or by reproducing the spectral radiance with no degree of freedom. However, it is not realistic to measure the color matching function for each individual or to use a multispectral camera.

  Non-Patent Document 5 proposes a method of uniquely performing color conversion by dividing a color gamut created by primary colors and assigning priorities. This method has an advantage that high-speed conversion can be performed, but the degree of freedom is eliminated only by the priority order for each region, and other effective usage methods of the degree of freedom cannot be considered.

  In Non-Patent Document 6, it is possible to perform conversion with no degree of freedom by using a linear interpolation method using three points with equal luminance. However, as in Non-Patent Document 5, other effective uses other than the degree of freedom cannot be considered.

  The linear programming problem is an optimization problem that maximizes or minimizes the objective function z under the constraint conditions, and the constraint conditions and the objective function are expressed in a linear form. For this problem, Non-Patent Document 7 is helpful. Since the problem can be treated algebraically under the condition of linearity, an optimal solution can be obtained simply as compared with the nonlinear optimization problem. The standard form of the linear programming problem is shown in Equation (5).

  On the other hand, the following two basic theorems are given to linear programming. That is, the first is “if there is a feasible solution, there is always a feasible base solution”, and the second is “if there is an optimal solution, there is an optimum solution among the feasible base solutions. It exists ".

  By using this theorem, an optimal solution can be obtained by a finite number of combinatorial searches. However, as the number of variables increases, the number of combinations increases and processing takes time. Therefore, the simplex method is used. In the simplex method, the optimal solution can be obtained by efficiently using the basic theorem of linear programming by judging the optimality using the relative cost coefficient.

  Next, linear programming in color conversion will be described. In order to use linear programming for color conversion, a conversion function from the relative luminance value of each primary color to XYZ and the range that the relative luminance value can take are set as constraints, and any objective function may be set. The following formula (6) shows a color conversion problem that has been converted to the standard form of linear programming.

Comparing equations (5) and (6), there are two differences compared to the normal linear programming problem. One is the range that the variable can take. In the normal linear programming problem, the range that the variable can take is 0 ≦ X _n , but the range of relative luminance is 0 ≦ S _n ≦ 1 in performing color conversion, and the simplex method is applied as it is. I can't. This can be dealt with by introducing an insufficient variable for S _n ≦ 1 and solving it by incorporating it into the constraint condition as S _n + a = 1, or by using the upper limit method. There is Non-Patent Document 8 as a reference book on this problem.

Another difference is that there is no lack / margin variable (d _j ) in the color conversion problem. In normal linear programming, the initial feasible basis solution is obtained by setting the deficit / margin variable as a non-basis variable (= 0), and the simplex method is advanced using that. In the linear programming used for color conversion, there is no shortage / margin variable and the upper limit value is given to the variable. Therefore, various combinations of two types of non-basis variables, 0 and 1, must be sought. It becomes difficult to find an initial feasible basis solution. For this purpose, it is necessary to use a two-stage simplex method. The two-stage simplex method finds an initial feasible basis solution in the first stage or obtains information that it does not exist. In the second stage, an optimal solution is found from the initial feasible basis solution, or information is obtained that the solution is not bounded (there is a solution that is too small).

Next, color conversion considering power consumption will be described. Since the constraint conditions have been determined as described above, color conversion can be performed only by giving an objective function. Since there is a degree of freedom in multi-primary color conversion, there are also combinations of primary colors that increase power consumption in self-luminous display devices. To determine the value of the primary color to minimize power consumption by using a linear programming method, must represent the relative luminance S _n primaries _n power consumption P _n in the linear formula such as the following equation (7) . In addition, the power consumption of the multi-primary color display device that satisfies Expression (7) is Expression (8).

P _n = c _n × S _n (7)
_{P 1 + P 2 + ··· +} P n = c 1 S 1 + c 2 S 2 + ··· c n S n ··· (8)
The above equation (8) matches the objective function z in equation (6). There is a time gray scale control as one method of the gray scale control in which the power consumption is actually in the linear form as in the equation (8). Although other control methods are also non-linear, they have a feature that the luminance increases as the power consumption increases. The basis solution that can be obtained even if the accuracy of the objective function is not so accurate in linear programming satisfies the constraints. For this reason, even if the relationship between the relative luminance and the power consumption is approximated by the equation (7), there is no problem.

  The above linear programming method can reduce the power consumption of the display device, but the processing time required for color conversion is long in a large display device. For example, in a SHIPP XYZ image, when color conversion is performed by linear programming, the processing time for one SHIPP image is 3 minutes and 10 seconds. On the other hand, in the case of three primary colors, the color conversion time is 14 seconds at the time of matrix calculation. Thus, the color conversion time is significantly increased in the linear programming method as compared with the 3 × 3 matrix calculation.

  In the above color conversion method for such a multi-primary color display device, the purpose of color conversion can be achieved without increasing the power consumption of the display device by using linear programming, but the color conversion time is significantly increased. There are challenges. By solving this by introducing a decision tree, it is required to shorten the color conversion time without increasing the power consumption of the display device.

  In addition, as described above, it is required to faithfully reproduce colors in places such as electronic commerce and telemedicine (see, for example, Non-Patent Document 9). However, in existing image systems, colors that depend on equipment are used. Because of the reproduction, the color displayed on the display looks different depending on the device.

  In order to deal with this problem, there is a color management system (CMS) (see, for example, Non-Patent Document 10). In the CMS, for display color reproduction, a Shaper / Matrix Model using an ICC (International Color Consortium) profile (see, for example, Non-Patent Document 3) (see, for example, Non-Patent Document 11, hereinafter referred to as SM). Is used.

  S. M.M. For the color reproduction of M, the tristimulus values XYZ having the highest luminance of each primary color are used. In order to perform accurate color reproduction, it is necessary that the chromaticity is constant with respect to the change in luminance of each primary color. However, color tracking that shifts chromaticity in a low luminance region occurs in some types of displays, for example, some displays using liquid crystal (see FIG. 8), so that accurate color reproduction cannot be performed (see, for example, Non-Patent Document 12). ). So far, a method corresponding to color tracking (hereinafter referred to as a faithful color reproduction method) has been reported (for example, see Non-Patent Document 13).

  However, the above-described faithful color reproduction method has a problem that it takes a long time for color conversion because a color conversion matrix is created for each pixel.

Masahiro Yamaguchi, Hideaki Haneishi, Nagaaki Oyama “Spectrum-Based High-Color Reproduction Image System-Natural Vision” Technical Report of the Institute of Image Information and Television Engineers, 26, 58, 7-12, 2002 "Development of a high-detail color management system based on human perception-its application to the reproduction of records of museums and museum collections", 1996 and 1997 IPA creative information technology training business results report “International Color Consortium:“ ICC Profile Specification Version 3.2 ”” 1995 (“International Color Consortium,“ ICC Profile Specification, Version 3.2 ”” 1995) Takeshi Terachi, Kenro Osawa, Masahiro Yamaguchi, Nagaaki Oyama "Color Matching Experiments Using Six Primary Color Displays" Color Forum JAPAN 2001, pages 97-100 2001 "Takeyuki AJITO, Kenro OHSAWA, Takashi OBI, Masahiro YAMAGUCHI and Nagaaki OHYAMA," Color Conversion Method for MultiplixMixingPrivateMixingPix 8, no. 3, pp. 191-197 (2001) (Yuki Ajito, Kenro Osawa, Takashi Obita, Masahiro Yamaguchi, Nagaaki Oyama, “Color Conversion Method of Multi-Primary Color Display Device Using Matrix Switch”, Optical Review Magazine, Vol. 8, No. 3, 191-197 (2001) Hideto Motomura, “Color conversion for a multi-primary display using linear interpolation on equine- luminance plane method (LIQUID)”, Journal of 1/2 ID. 371-387 (2003) (Motomura Hideo, “Color Conversion of Multi-Primary Color Display Device Using Linear Interpolation Method on Isoluminance Surface”, SID Journal, Vol. 11, No. 2, 371-387 (2003)) Masatoshi Sakawa, “Optimization of Linear Systems (From One Purpose to Multiple Purposes)” Morikita Publishing 1984 G. B. Dantzig “Upper bounds, secondary constraints and block triangularity in linear programming”, Econometrica, 23, pp. 174-183 (1955) (Gee B. Danzig, "Upper limit, second limit, and blotter triangle in linear programming" Econometrica, 23, 174-183, 1955). Shuichi Kagawa and Hiroaki Sugiura, “Current Status and Future of Color Management Technology”, Mitsubishi Electric Technical Report, vol.76, No.11, pp.739-742, (2002). MD Study Group et al. "Illustration Color Management Practice Rule Book 2005-2006", MD Study Group, Works Corporation, pp.52-53, (2005). Dawn Wallner, "Building ICC profiles-the Mechanics and Engineering", (2000). Yuka Utsumi et al. "Study on luminance dependence of color tone in liquid crystal displays", Journal of the Institute of Image Information and Television Engineers, vol.25, No.72, pp.13-18, (2001). Shimadzu Mamoru, Takaya Masaki, Ohashi Gosuke, Shimohira Mifumi, "High-fidelity color reproduction method for color tracking in displays", IEICE Technical Report, vol.103, No.649, pp.37-40 , (2004).

The present invention has been made in order to solve the above-described problem. When input image data is converted into an image signal for a multi-primary color display device having four or more primary colors to display an image, power consumption is achieved. An object of the present invention is to provide a color conversion apparatus that can shorten the color conversion time without increasing the color and can suppress the color tracking.

In order to achieve the above object, the color conversion device according to the first aspect of the present invention is a predetermined n primary colors (n ≧ 4) that can display tristimulus values XYZ of the XYZ color system on a multi-primary color display device. A color conversion matrix creation method for creating a color conversion matrix for converting to the three primary color signal values in the combination of the three primary colors selected from the above based on the characteristics of each primary color. Obtaining the corresponding three primary color signal values using a predetermined color conversion matrix, obtaining the three primary color gradation values corresponding to the obtained three primary color signal values from the halftone reproduction characteristics of the multi-primary color display device; Determining the tristimulus values XYZ corresponding to the three primary color gradation values from the device profile of the multi-primary color display device, and determining the luminance of the tristimulus values XYZ of the predetermined gradation as the luminance of the tristimulus values XYZ of the reference gradation Meet And calculating the color difference between the tristimulus value XYZ of the predetermined gradation and the tristimulus value XYZ of the reference gradation, and if the calculated color difference exceeds a predetermined threshold value, Creating and storing a color conversion matrix based on the stimulus value XYZ, changing the reference gradation to the predetermined gradation, and changing the predetermined gradation to one gradation or a plurality of gradations. the process for processing repeatedly executed for all gradations of containing, a color conversion matrix creating means to create a more plurality of color conversion matrices to the color conversion matrix creating method executed for all combination of and the three primary colors for all three primary colors, the storage means for storing the plurality of color conversion matrices created by the color conversion Matrigel box forming means, the relative bright of each primary color in the multi-primary-color display device of the n-primary (n ≧ 4) Range and constraints, and the color conversion result of each color conversion a plurality of the tristimulus values X YZ of XYZ color system into image signals of the n-primary by linear programming objective function to maximize or minimize a predetermined, a tristimulus value XYZ which is input, based on, for (n-3) primary colors among the image signals of the n primary colors, minimum luminance value or by converting the image signal of the maximum luminance value, for the remaining three primary colors , it said by converting the halftone Luminance value (n-3) to select a color conversion matrix corresponding to the image signals of the primary colors from the plurality of color conversion matrices, for a multi-primary-color display device of the n-primary And color conversion means for generating the image signal .

According to the present invention, the tristimulus value XYZ of the predetermined gradation and the tristimulus value XYZ of the predetermined gradation and the tristimulus of the reference gradation are matched with the luminance of the tristimulus value XYZ of the reference gradation. The color difference from the value XYZ is obtained. If the obtained color difference exceeds a predetermined threshold value, a process for creating and storing a color conversion matrix based on the tristimulus values XYZ of the predetermined gradation is executed for all three primary colors and all combinations of the three primary colors. To do. For this reason, a color conversion matrix is created for each primary color for gradations where the color difference exceeds the threshold.
Incidentally, as described in claim 5, wherein the color difference can be used to calculate the CIE2000 color difference formula.

At the time of color conversion, it is possible to perform high-precision and high-speed color conversion by selecting an optimal color conversion matrix corresponding to the gradation from among the plurality of color conversion matrices created and performing color conversion.
Specifically, the color conversion means uses a linear programming method that maximizes or minimizes a predetermined objective function, with the relative luminance range of each primary color in the n-primary (n ≧ 4) multi-primary color display device as a constraint. a color conversion results of a plurality of tristimulus values XYZ of the XYZ color system were each color conversion to the image signals of the n-primary Ri good, and tristimulus values XYZ input, based on, of the image signals of the n-primary for out (n-3) primary colors, into image signals of the minimum luminance value or highest luminance values, the remaining three primary information, wherein the (n-3) a color conversion matrix corresponding to the image signal of the primary An image signal for the n-primary multi-primary color display device is generated by selecting from the plurality of color conversion matrices and converting to a halftone luminance value .

In addition, as described in claim 2, the threshold value may be set to a value smaller than the threshold values of the other primary colors for the primary wavelength of the shortest wavelength among the three primary colors. That is, for the primary color having the shortest wavelength among the three primary colors, the threshold value is set strictly so that color conversion can be performed with higher accuracy than the other primary colors. For this reason, the primary wavelength of a short wavelength can be accurately color-converted, and the color appearance can be improved.
Further, as described in claim 3, wherein the color conversion result, the relative luminance range of each primary color in the multi-primary-color display device of the n-primary (n ≧ 4) and constraints consumption in the multi-primary-color display device Ru can be the color conversion result of converting each color a plurality of tristimulus values XYZ of the XYZ color system by linear programming to minimize the objective function to an image signal of the n-primary power as an objective function.
Further, as described in claim 4, wherein the color conversion means, the color conversion result, the decision tree to enter the input tristimulus values to the classification model is inductive learning the classification model had use of Accordingly, the (n-3) primary color image signal out of the n primary color image signals may be converted into the lowest luminance value or the highest luminance value image signal.

  According to the present invention, the color conversion by linear programming is regarded as a discrete classification problem, and the color conversion result by linear programming is learned by induction using the classification model, thereby performing color conversion processing by classification and simple matrix calculation, and color conversion Time can be shortened. Further, for the three primary colors of the base portion, a color conversion matrix is created by the color conversion matrix creating method according to claim 1 and color conversion is performed using the created color conversion matrix, so that color tracking can be suppressed. .

  According to the present invention, when input image data is converted into an image signal for a multi-primary color display device having four or more primary colors to display an image, the color conversion time is shortened without increasing power consumption. In addition, the color tracking can be suppressed.

It is an example of the color conversion system of a multi-primary color display device. It is explanatory drawing which shows the example of calculation of the color conversion based on a decision tree. This is an output example showing the result of color conversion using a decision tree. It is a schematic block diagram of a color conversion apparatus. It is a diagram which shows an example of a halftone reproduction characteristic. It is a flowchart of the color conversion matrix preparation process performed in a color conversion matrix preparation part. It is a flowchart of the color conversion process performed in a color conversion part. It is a chromaticity diagram for demonstrating color tracking.

  When multi-primary color conversion is performed using linear programming, there is a problem that the conversion speed becomes slow. In order to perform color conversion without impairing the advantages of color conversion of linear programming, a method of inductively learning the result of color conversion by linear programming is conceivable. In this case, it is the simplest method to construct a model with the input values as XYZ tristimulus values and the output values as multi-primary color signals. However, it is difficult to build a model for accurately outputting multi-primary color signals that are continuous values. Therefore, in the present invention, the result of color conversion by linear programming is treated as a discrete value, and these are subjected to inductive learning in a classification model to perform color conversion.

In order to achieve high speed while guaranteeing high fidelity color reproduction, it is only necessary to use a combination of a basis / non-basis of linear programming instead of a model that directly outputs color signals. In the color conversion method using linear programming, among the obtained solutions, (the number of primary colors −3) is the highest (tone: 255, that is, the signal value Si is 1.0, i is a number indicating the primary color, and so on. ), Or no light at all (gradation: 0, ie, the signal value Si is 0). These extreme solutions are called non-basic solutions. The optimal solution for linear programming in multi-primary color conversion is that if the optimal non-basis combination ( _n C _n-3 * 2 ^n-3 ) is determined, these combinations are used as the output value of the classification model. By using a 3 * 3 conversion matrix adapted to the obtained output value, it can be handled as a three-primary color system. According to this method, it is possible to perform multi-primary color conversion at high speed while maintaining the fidelity without losing the advantages of the linear programming method by simply performing matrix operation.

  The result of the above linear programming is learned with a classification model. This time, a decision tree is adopted as a model for classifying. Since decision trees are by no means a fast classification model, it is necessary to consider introducing other models in order to achieve higher speed. It is also appropriate to use rule-based C4.5 as a decision tree implementation. Since the rule is given as X ′ <X <X ”, it is considered that the rule-based C4.5 performs area division by a rectangular parallelepiped in the XYZ space.

  In summary, the color conversion device of the multi-primary color display device according to the present invention, as shown in FIG. 1, a first processing means for learning a color conversion result by linear programming into a classification model, a classification model, and a matrix calculation And a second processing means for performing color conversion based on the above, and configured to reduce the processing load of the processing means based on the linear programming.

The processing part for causing the classification model to learn the color conversion result by the linear programming method is ( _n C _{n−) of the} non-basis solution among the optimal solutions obtained by performing color conversion using XYZ as input values and the linear programming method. ₃ * 2 ^n-3 ), the learning of the classification model is performed by the relationship of many XYZ vs. non-basis combinations, and the output value is calculated using a 3 * 3 color conversion matrix that matches the output value. This is for handling as the three primary colors.

  The color conversion processing means based on the classification model and the matrix calculation outputs an optimum combination of non-basis solutions from the input XYZ by the constructed classification model, and generates a 3 * 3 color conversion matrix suitable for the output value. To perform color conversion.

In order to confirm the above-described invention in detail, a specific calculation process is executed for an actual image, and the color conversion processing time is obtained. Processing is divided into model construction and color conversion by model. The total number of possible combinations of non-basic solutions in the n primary color display device (n ≧ 4) is (M = _n C _n−3 * 2 ⁿ⁻³ ), and color conversion is performed by linear programming for a large number of XYZ. The combination of the obtained optimal solutions in the total number M is output, and a classification model is constructed using these results as learning data. On the other hand, color conversion by the model of the present invention is shown in FIG. By inputting the input (X _c , Y _c , Z _c ) into the classification model, non-basic solutions (S ₄ , S ₅ , S, reflecting the learning results are selected from a total of M non-basic solutions. ₆ ) is obtained, and the remaining basis solutions (S ₁ , S ₂ , S ₃ ) are obtained by matrix calculation. Further, a tone value as a digital signal is obtained from the obtained signal value using a tone curve.

In the multi-primary color (n primary color) color conversion, as described in the background art above, the number of equations is three and the number of variables is n. Therefore, the number of variables is n, the number of expressions is 3, and the degree of freedom is n-3. From the above, there are L = _n C _n−3 2 ⁿ⁻³ combinations of non-basis (0 and upper limit value 1). Solving linear programming to find the optimal solution is equivalent to choosing an optimal combination from L ways.

  This is clearly a discrete classification problem. There are various ways to solve the classification problem. Color conversion is calculated in advance by linear programming from an image taken by a camera capable of acquiring SHIPP or XYZ. It is necessary to train the model using the results calculated in advance as training data.

  The procedure for performing color conversion by such a method is as follows. First, if possible, L inverse matrices are preloaded into the memory. Second, by inputting XYZ into the constructed model, one of L combinations is obtained. Third, color conversion is performed using an inverse matrix corresponding to the combination.

  The above method has two problems. The first is to make a mistake in the solution, that is, to output a wrong combination from the above-mentioned L ways, and the second is that the solution cannot be obtained. The first problem may include a problem that an optimal solution cannot be obtained, but there is no particular problem in terms of color conversion. Color conversion is possible if a solution is obtained for any of the L combinations. The second problem can also be solved by passing to linear programming when a solution cannot be obtained.

  Therefore, the present inventors learned the result of color conversion using a decision tree which is one of classification methods. The decision tree is used for the following three reasons.

  First, the output class can handle discrete values. Second, classification can be performed in a linear domain division. Third, as a learning result, it is possible to output a rule for classification as a sentence, so that it is easy to incorporate it into a program.

The decision tree package used is C4.5. The number of learning data was 11102. As a result of learning with the default option, the number of rules was 198. An example of the output result is shown in FIG. 3. In FIG. 3, the values of X, Y, and Z are multiplied by 100 to form an integer type in order to discard fine digits. The output classes are * for a base, 1 and 0 for a non-base, and are Lr, Lg, Lb, Rr, Rg, and Rb from the left. Here, Lr, Lg, and Lb are primary color signals (relative luminance) of r, g, and b on the short wavelength side, and Rr, Rg, and Rb are primary colors of r, g, and b on the long wavelength side, respectively. It is a color signal.

  Since it is a linear programming problem, a linear optimal solution space should be constructed, but as a result, 198 conditions were output. On the other hand, since the noise data is included in the dark portion in the image used for learning, 156 rules were generated as a result of learning the above result by changing the minimum appearance frequency condition option.

  Since the speeding up of the color conversion process has been described, the color conversion calculation time is easily compared between the linear programming method and the classification method. In the linear programming method, when XYZ values are input, multiprimary color signal values are output. On the other hand, when the classification method is used, the output when XYZ is input is a combination pattern of a base and a non-base. In order to obtain the signal values of multi-primary colors, it is further necessary to obtain an inverse matrix from the obtained pattern and perform matrix calculation using it. Since the number of inverse matrices used is limited, they should be prepared in advance. Since only confirmation of speedup was performed, the matrix and the like were generated by random numbers, and the calculation load was made equivalent to color conversion.

  The number of pixels corresponding to the number of pixels of an image captured by an imaging device capable of acquiring XYZ, that is, X, Y, Z calculated randomly at 1392 * 1040 times is used to calculate the color signal value by linear programming and classification. Calculated. The calculation time in the linear programming method was 42.5 seconds. On the other hand, the calculation time by the classification method was 4.5 seconds. It can be said that the speed can be increased by using the classification method. In another example, for a SHIPP image (yacht) consisting of 16 × 3 bits having a resolution of 4,096 × 3,072 pixels, for example, in the case of displaying six primary colors, the color conversion time is 320 seconds to 21 seconds. Shortened to

Further, in the present embodiment, color conversion capable of suppressing color tracking and shortening the color conversion time is performed for the three basic solutions (primary colors) S ₁ , S ₂ , and S ₃ . In the following, it is assumed that the number of primary colors of the applied multi-primary color display device is 6, and is represented by a1 to a6.

FIG. 4 shows a schematic block diagram of the color conversion apparatus 10 that performs color conversion for such three primary colors S ₁ , S ₂ , and S ₃ . As shown in FIG. 4, the color conversion device 10 includes a color conversion matrix creation unit 12, a storage unit 14, and a color conversion unit 16. In this embodiment, the case where the _three basis solutions are S ₁ , S ₂ , and S ₃ is described as an example, but other combinations, for example, S ₂ , S ₄ , and S ₆ are the basis solutions. In this case, color conversion by the color conversion device 10 is performed on the base solution.

  Although details will be described later, the color conversion matrix creation unit 12 obtains coefficients of the color conversion matrix based on device profiles (colorimetric data), halftone reproduction characteristics (TRC) data, and the like stored in the storage unit 14. To create a color conversion matrix.

The color conversion unit 16 uses the color conversion matrix created by the color conversion matrix creation unit 12 to perform color tracking on the input three primary colors S ₁ , S ₂ , S ₃ image data (three primary color data). It is converted into image data (color conversion data) to be suppressed and output.

  The storage unit 14 is a multi-primary color display device that displays an image expressed in six primary colors and can generate color tracking, a device profile of each of the six primary colors, halftone reproduction characteristic data for each of the six primary colors, a color conversion matrix to be described later. A color conversion creation program executed by the creation unit 12 and a color conversion program executed by the color conversion unit 16 are stored in advance.

The device profile is data of tristimulus values XYZ corresponding to all gradations of the three primary colors selected from the six primary colors a1 to a6. Since there are ₆ C ₃ = 20 combinations for selecting the three primary colors from the six primary colors, device profiles are prepared for the number of combinations. The device profile is obtained as follows. For example, the gradation values of the selected three primary colors are changed in several gradation steps (for example, 8 gradation steps) for each primary color and input to the display device, and each time the color output from the display device is measured by the colorimeter. Thus, tristimulus values XYZ for each primary color number gradation are obtained. The tristimulus values XYZ of other gradations are obtained by linear interpolation or the like. Thereby, tristimulus values XYZ corresponding to all three primary color gradations are obtained. Note that tristimulus values XYZ corresponding to all three primary color gradations may be obtained by colorimetry.

  FIG. 5 shows an example of a halftone reproduction characteristic of a primary color of a liquid crystal projector (when the primary colors are RGB three primary colors). As shown in the figure, the halftone reproduction characteristic indicates the correspondence between the RGB gradation value (input value) and the RGB signal value (output value) of the display device, and is obtained in advance for each primary color of RGB. It is done. The halftone reproduction characteristic data is data representing a halftone reproduction characteristic as shown in the figure, that is, table data or a formula indicating a correspondence relationship between RGB gradation values and RGB signal values, and is stored in the storage unit 14 for each primary color. Stored in advance. In this embodiment, in order to perform color conversion by creating a color conversion matrix for a multi-primary color display device of six primary colors, halftone reproduction characteristic data for each of the six primary colors is stored in the storage unit 14 in advance.

  Here, the conventional S.I. M.M. The color conversion from XYZ color system image data to RGB color system image data in M will be described. S. M.M. In M, the input tristimulus values XYZ are converted into RGB signal values (relative values) of 0 to 1 using the following equation.

Here, the values of the coefficients X _R , Y _R , Z _R , X _G , Y _G , Z _G , X _B , Y _B , Z _B of the color conversion matrix are the maximum gradations of the respective primary colors obtained from the device profile. This is a value obtained by subtracting the tristimulus value XYZ having the gradation value 0 as the bias component from the tristimulus value XYZ in the case of the gradation value 255 being. That is, X _R , Y _R , and Z _R are values obtained by subtracting the tristimulus value XYZ corresponding to R of the gradation value 0 from the tristimulus value XYZ corresponding to _R of the gradation value 255, and X _G , Y _G and Z _G are values obtained by subtracting the tristimulus value XYZ corresponding to G of the gradation value 0 from the tristimulus value XYZ corresponding to _G of the gradation value 255, and X _B , Y _B and Z _B are This is a value obtained by subtracting the tristimulus value XYZ corresponding to B of the gradation value 0 from the tristimulus value XYZ corresponding to B of the gradation value 255. In the following, the tristimulus values XYZ of the gradation values n (= 0 to 255) of the primary colors p (= R, G, B) obtained from the device profile, that is, the coefficients of the color conversion matrix are X _pn and Y _pn. , Z _pn . For example the primary colors R, G, tristimulus values XYZ corresponding to the B gradation value 255 as described above, each _{X R255, Y R255, Z R255} , X G255, Y G255, Z G255, X B255, Y B255, Z _B255 .

  Then, using the halftone reproduction characteristic data representing the halftone reproduction characteristics as shown in FIG. 5, RGB gradation values are obtained from the RGB signal values. That is, the R gradation value is obtained from the R signal value using the R halftone reproduction characteristic data, and the G gradation value is obtained from the G signal value using the G halftone reproduction characteristic data. The B tone value is obtained from the B signal value using the B halftone reproduction characteristic data.

  In this way, S.I. M.M. In M, since the tristimulus values XYZ of the highest luminance (maximum gradation) of each primary color are always used for the matrix calculation, the influence of color tracking is not taken into consideration, and the optimum color for the display device in which color tracking occurs. Reproduction is not obtained.

  In this embodiment, as will be described in detail later, a plurality of color conversion matrices are created for each primary color according to the gradation value, and at the time of color conversion, a color conversion matrix suitable for the gradation value is selected and color conversion is performed. This enables high-speed and high-precision color reproduction even in a display device in which color tracking occurs.

  Next, the color conversion matrix creation processing executed in the color conversion matrix creation unit 12 will be described with reference to the flowchart shown in FIG.

  First, in step 100, the color conversion matrix creation unit 12 sets a reference gradation d to be described later. In the present embodiment, as an example, each primary color is represented by 8-bit data, and 256 gradations of 0 to 255 are set, and 255 which is the maximum gradation is set as a reference gradation.

  In step 101, one combination is selected from 20 combinations of three primary colors selected from the six primary colors a1 to a6. In the present embodiment, in order to simplify the description, a case will be described in which the combination of the three primary colors selected first is a1, a2, and a3, and the three primary colors are R, G, and B.

  In step 102, a primary color for which a color conversion matrix is to be created is set. In the present embodiment, since a color conversion matrix is created for each of the three primary colors a1, a2, and a3 (R, G, and B), the color conversion matrix is initially set to a1, for example.

  In step 104, an initial color conversion matrix of a color conversion matrix for converting XYZ color system image data into RGB image data is set.

Here, as an example, the coefficients of the color conversion matrix of the above formula (9) are used as the tristimulus values X _R255 , Y _R255 , Z _R255 , corresponding to the primary colors R, G, B of the gradation value 255 as the reference gradation. _{Set to} X _G255 , Y _G255 , Z _G255 , X _B255 , Y _B255 , Z _B255 . These values are obtained from the device profile stored in the storage unit 14.

In step 106, the sum of the tristimulus values XYZ of R, G, and B tones e obtained from the device profile is added to XYZ in the above equation (9) to obtain RGB signal values. In the present embodiment, the first gradation e is 255, which is the maximum gradation. That is, X in the above (9) to enter the (X _R255 + X _G255 + X _B255), the Y type a (Y _R255 + Y _G255 + Y _B255), the Z a (Z _R255 + Z _G255 + Z _B255) Input to obtain RGB signal values.

  In step 108, RGB gradation values are obtained from RGB signal values using the halftone reproduction characteristic data of each primary color stored in the storage unit 14. That is, the R gradation value is obtained from the R signal value using the R halftone reproduction characteristic data, and the G gradation value is obtained from the G signal value using the G halftone reproduction characteristic data. The B tone value is obtained from the B signal value using the B halftone reproduction characteristic data.

  In step 110, tristimulus values XYZ corresponding to the RGB gradation values obtained in step 108 are obtained from the device profile stored in the storage unit 14.

  In step 112, the color difference between the tristimulus value XYZ of the gradation e obtained in step 110 and the tristimulus value XYZ of the reference gradation d is obtained. As described above, since the first reference gradation d is 255 which is the maximum gradation, the tristimulus value XYZ corresponding to R of the gradation 255 obtained from the device profile is set as the tristimulus value XYZ to be compared. A color difference from the tristimulus value XYZ of the gradation e obtained in step 110 is obtained. When obtaining the color difference, the tristimulus value XYZ of the gradation e is matched with the luminance of the reference gradation d, and the tristimulus value XYZ of both is converted into Lab color system data. Find the color difference. That is, by replacing Y of the tristimulus value XYZ of the gradation e with Y of the tristimulus value XYZ of the reference gradation, and further multiplying X and Z of the tristimulus value XYZ of the gradation e by a predetermined coefficient, The brightness of the tristimulus value XYZ of e is matched with the brightness of the reference gradation d. The predetermined coefficient is a value obtained by dividing Y of the tristimulus value XYZ of the reference gradation by Y of the tristimulus value XYZ of the gradation e. Specifically, when the tristimulus values of the gradation e are Xe, Ye, Ze, and the tristimulus values of the reference gradation d are Xd, Yd, Zd, Xe ′ = Xe × (Yd / Ye), Ye The color difference between the tristimulus value Xe′YeZe ′ and the tristimulus value XdYdZd is determined as = Yd, Ze ′ = Ze × (Yd / Ye). The color difference is obtained by, for example, the CIE 2000 color difference formula (for example, see Non-Patent Document 14 below).

  In step 114, it is determined whether or not the obtained color difference exceeds a predetermined threshold value. Since it is determined by humans whether or not the color reproduction is faithful, a threshold that is a criterion for allowing matrix conversion is set to a color difference (for example, a color difference perceived by human eyes from two colors) (for example, Non-patent document 15 below) can be used. In addition, as a specific threshold value, if the threshold value is set, a value (for example, 0. 0) that the color conversion time and the average color difference are within an allowable range from the relationship between the number of color conversion matrices to be created and the average color difference. 2).

  The threshold is set to a value (for example, 1/2 or less) smaller than the other primary colors for the primary color having the shortest wavelength among the three primary colors. In other words, for the primary color having the shortest wavelength among the three primary colors, the threshold is set strictly so that color conversion can be performed with higher accuracy than the other primary colors. Here, since the shortest wavelength of RGB is B, when the threshold values are set to 0.2 for other primary colors R and G, the threshold value for B is set to 0.1, for example. The threshold is set in this way because, even if the color difference is the same for all three primary colors, the color appearance is better when the primary color of the short wavelength is accurately converted. The threshold setting may be changed as appropriate according to the combination of the three primary colors.

  If the color difference between the tristimulus value XYZ obtained in step 110 and the tristimulus value XYZ of the reference gray level d to be compared exceeds the threshold value, the process proceeds to step 116; 120.

In step 116, the color conversion matrix is updated, and the updated color conversion matrix is stored in the storage unit 14. The color conversion matrix is updated by updating the coefficients X _R , Y _R , and Z _R of the current color conversion matrix to the tristimulus values XeYeZe of the gradation e obtained in step 110.

  In step 118, the reference gradation d is changed to the current gradation e. Thus, when the color difference exceeds the threshold, the reference gradation d is changed to the current gradation e, and when the color difference is equal to or less than the threshold, the reference gradation d is not changed and is left as it is.

  In step 120, it is determined whether or not the processing in steps 106 to 118 has been completed for all the gradations of the primary color R, specifically, whether or not the gradation e has become zero. Then, when the processing of Steps 106 to 118 is completed for all gradations of the primary color R, the process proceeds to Step 124, and otherwise, the process proceeds to Step 122.

In step 122, the gradation e is lowered by one and the process returns to step 106. In the second step 106, the R component of the tristimulus value XYZ entering XYZ in the above equation (1) is set to a value obtained by lowering the gradation by one. That is, X in the above (1) Enter the _{(X R254 + X G254 + X} B254), the Y type a _{(Y R254 + Y G254 + Y} B254), the Z a _{(Z R254 + Z G254 + Z} B254) input. The same processing is performed thereafter.

  If this is executed until the gradation e becomes 0, step 120 is affirmed and the routine proceeds to step 124.

  In step 124, it is determined whether or not the processing in steps 102 to 122 has been completed for all three primary colors of the combination selected in step 101. If completed, the process proceeds to step 125. If not completed, the process proceeds to step 125. Then, the process proceeds to step 102, and the same processing as described above is performed for G and B. As described above, when the same processing as described above is performed for B, which is the shortest wavelength of RGB, the threshold value in step 114 is set to a value smaller than R and G.

  When the processing of steps 102 to 122 is completed for all RGB, it is determined in step 125 whether the processing of steps 101 to 124 is completed for all 20 combinations of three primary colors. If the process proceeds to step 126 and the process has not been completed, the process proceeds to step 101 and the same processing as described above is performed for the combination of the other three primary colors.

  In step 126, the coefficients of all created color conversion matrices are output to the color conversion unit 16, and this routine is terminated. The color conversion unit 16 stores the coefficient of the color conversion matrix of each primary color output from the color conversion matrix creation unit 12.

  As described above, in this embodiment, the luminance of the tristimulus value XYZ of the gradation e is matched with the luminance of the tristimulus value XYZ of the reference gradation d, and the color difference between the two is obtained. The process for creating a new color conversion matrix is performed for all gradations of each primary color in all combinations of the three primary colors. As a result, color conversion matrices are created in a number corresponding to the color difference equal to or greater than the threshold value for each primary color in all combinations of the three primary colors. In the following description, a gradation value having a color difference equal to or greater than a threshold value, that is, a gradation value obtained by changing the creation of the color conversion matrix (a gradation value for changing the color conversion matrix at the time of color conversion) is referred to as a matrix change gradation value. Called.

Next, color conversion processing of the color conversion apparatus 10 that performs color conversion of the _three primary colors (basic solutions) S ₁ , S ₂ , and S ₃ executed in the color conversion unit 16 will be described with reference to the flowchart shown in FIG. explain. In the present embodiment, in order to simplify the description, a case will be described in which the three primary colors S ₁ , S ₂ , and S ₃ are a1, a2, and a3, and these are R, G, and B.

  First, in step 200, RGB gradation values are obtained from RGB signal values using halftone reproduction characteristic data stored in the storage unit. That is, the R gradation value is obtained from the R signal value using the R halftone reproduction characteristic data, and the G gradation value is obtained from the G signal value using the G halftone reproduction characteristic data. The B tone value is obtained from the B signal value using the B halftone reproduction characteristic data.

  In step 202, a color conversion matrix corresponding to the RGB gradation value obtained in step 200 is selected for each primary color. The color conversion matrix is selected from the color conversion matrices created when the three primary colors are R, G, and B.

  For example, when the R matrix change gradation value is 240, 225, 210..., And the R gradation value obtained in step 200 is a value within the range of 255-241, Since this is the gradation before the matrix is changed, a color conversion matrix in which the tristimulus value XYZ value of the gradation value 255 which is the maximum gradation of each primary color is a coefficient is selected. If the R gradation value obtained in step 200 is within the range of 240 to 226, the color conversion matrix created when the matrix change gradation value is 240 is selected. If the R gradation value obtained in step 200 is within the range of 225 to 211, the color conversion matrix created when the matrix change gradation value is 225 is selected. The same applies to the following, and the same applies to the other primary colors G and B. As a result, the color conversion matrix is changed for each primary color with a gradation value at which the color difference is equal to or greater than the threshold value.

In step 204, the tristimulus values XYZ are converted into RGB signal values using the selected color conversion matrix. Specifically, as an example, tristimulus values (X _C -X ₅ , Y _C -Y ₅ , Z _C -Z ₅ ) shown in FIG. 2 are used as input values and converted into RGB signal values. That is, the color conversion matrix coefficients X _R , Y _R , Z _R selected for _R , the color conversion matrix coefficients X _G , Y _G , Z _G selected for _G, and the color conversion matrix coefficient X selected for B Color conversion is performed using a color conversion matrix having _B , Y _B , and Z _B as coefficients.

  In step 206, as in step 200, using the halftone reproduction characteristic data stored in the storage unit 14 again, RGB gradation values are obtained from the RGB signal values and output.

Finally, the primary colors S _1, S _2, S ₃ corresponding to the tristimulus values XYZ of the input gradation value of S _4, S _5, S _6, the primary S _1, S obtained in the process shown in FIG. 7 _2, the exact RGB gradation values corresponding to S _3, the tone value of the primary color S _4, S _5, S ₆ are non-basic solution, obtained by the synthesis. In the case of the example shown in FIG. 2, since the primary S ₄ is a non-basic solution, S _5, S ₆ is (0,1,0), i.e. flashing only primaries S ₅ the best, others are quite Since it is not illuminated, the gradation value is (0, 255, 0).

  As described above, in the present embodiment, for the three primary colors that are the basic solutions, the tristimulus values XYZ for each tone value are matched with the reference tone for each tone value, and then the tristimulus for each tone value is set. A color difference between the value XYZ and the tristimulus value XYZ of the reference gradation is obtained, and if this color difference exceeds a threshold value, a color conversion matrix is newly created and stored. At the time of color conversion, color conversion is performed by selecting a color conversion matrix corresponding to the gradation value of each primary color. In other words, since the optimum color conversion matrix is selected for each gradation and color conversion is performed, high-speed and high-precision color reproduction is possible even in a display device in which color tracking occurs.

  The color conversion threshold is set to a value smaller than the other primary colors for the primary color of the shortest wavelength that affects the appearance of the color among the three primary colors, as a criterion for creating a color conversion matrix. Many matrices are created. For this reason, it is possible to perform color conversion with high accuracy and to improve color appearance.

In the present embodiment, the case where the reference gradation set initially is the maximum gradation 255 has been described. However, the present invention is not limited to this, and other gradation values, for example, the intermediate gradation 128 is used as the reference gradation. It may be set as a key. In this case, the coefficients of the initial color conversion matrix, and _{X R128, Y R128, Z R128} , X G128, Y G128, Z G128, X B128, Y B128, Z B128. Then, for example, the process shown in FIG. 3 while increasing the gradation value from the gradation value 128 one by one, that is, the process of updating the color conversion matrix if the color difference is compared with the threshold and the threshold is exceeded, is executed. After performing the gradation value 255, the same processing may be performed while decreasing the gradation value one by one from the gradation value 127.

  In the present embodiment, the case where the color difference is determined for each gradation while changing the gradation one gradation at a time has been described. However, the present invention is not limited to this, and there are a plurality of gradations such as two gradations, three gradations, and the like. The color difference may be determined while changing each gradation.

(Non-Patent Document 14) G. Sharma, W. Wu, ENDalal: “The CIEDE2000 Color-Difference Formula: Implementation Notes, Supplementary Test Data, and Mathematical Observations”, Color Research and Application, vol. 30, No. 1 ( 2005).
(Non-patent Document 15) Japanese Society of Color Science: "New Color Science Handbook [2nd edition]", University of Tokyo Press (1998).

10 color conversion device 12 color conversion matrix creation unit 14 storage unit 16 color conversion unit

Claims (5)

A color for converting the tristimulus values XYZ of the XYZ color system into three primary color signal values in a combination of three primary colors selected from predetermined n primary colors (n ≧ 4) that can be displayed by a multi-primary color display device A color conversion matrix creation method for creating a conversion matrix based on the characteristics of each primary color, the step of obtaining three primary color signal values corresponding to tristimulus values XYZ of a predetermined gradation using a predetermined color conversion matrix; Obtaining three primary color gradation values corresponding to the obtained three primary color signal values from halftone reproduction characteristics of the multi-primary color display device, and tristimulus values XYZ corresponding to the obtained three primary color gradation values to the multi-primary color display device. And determining the tristimulus value XYZ of the predetermined gradation and the tristimulus value XYZ of the predetermined gradation and the reference scale Key The color difference between stimulation values XYZ, if it exceeds a step of determining both the tristimulus values XYZ after converting into data of Lab color system, a threshold color difference obtained is predetermined tristimulus values of the predetermined tone A process including a step of creating and storing a color conversion matrix based on XYZ, changing the reference gradation to the predetermined gradation, and changing the predetermined gradation to one gradation or a plurality of gradations a color conversion matrix creating means to create a more plurality of color conversion matrices to the process that is repeatedly executed for all the gradations, color conversion matrix creating method executed for all combination of and the three primary colors for all three primary colors,
Storage means you store said plurality of color conversion matrices created by the color conversion matrix creating means,
Wherein the constraints relative luminance range of each primary color in the multi-primary-color display device of the n-primary (n ≧ 4), XYZ color system plurality of three stimuli of the linear programming to maximize or minimize the objective function previously determined Based on the color conversion results obtained by converting the values XYZ into the n primary color image signals and the input tristimulus values XYZ, the (n-3) primary colors of the n primary color image signals are the lowest. It was converted into an image signal of luminance value or highest luminance values, for the remaining three primary colors, the intermediate floor the color conversion matrix corresponding to the image signal of the (n-3) primary colors selected from the plurality of color conversion matrices Color conversion means for generating an image signal for the n-primary multi-primary color display device by converting to a tonal brightness value ;
A color conversion device comprising:   The threshold value is smaller than the threshold values of the other primary colors for the shortest wavelength primary color among the three primary colors. Set to value
  The color conversion apparatus according to claim 1.   The color conversion result is the relative luminance of each primary color in the n-primary (n ≧ 4) multi-primary color display device. With the range as a constraint condition, the objective function is the power consumption in the multi-primary color display device as an objective function. A plurality of tristimulus values XYZ of the XYZ color system are converted into the image of the n primary colors by linear programming that minimizes the number This is the result of color conversion for each color conversion to an image signal.
  The color conversion apparatus according to claim 1 or 2.   The color conversion means is an inductive learning of the color conversion result into a classification model using a decision tree. By inputting the input tristimulus values to a similar model, among the n primary color image signals (N-3) The primary color image signal is converted to the lowest luminance value or the highest luminance value image signal.
  The color conversion apparatus according to any one of claims 1 to 3.   The color difference is obtained by the CIE2000 color difference formula.
  The color conversion apparatus of any one of Claims 1-4.