JP5880231B2 - Color conversion table creation device, output system including color conversion table creation device, and control program for color conversion table creation device - Google Patents

Description

  The present invention relates to a color conversion table creation device that creates a color conversion table indicating color values of the second color system corresponding to the grid points of the first color system, an output system including the color conversion table creation device, and The present invention relates to a control program for a color conversion table creation device.

  For color conversion between different devices, a color conversion table indicating a correspondence relationship between device-independent color values (eg, L * a * b * values) and device values is used. Such a color conversion table is called a LUT (Look Up Table). Usually, when creating an LUT, hundreds of color patches composed of combinations of CMYK are output, and interpolation calculation based on L * a * b * values (colorimetric values) obtained from the measurement results is executed. Therefore, thousands of color correspondences are sought. Here, since the interpolation calculation always involves an interpolation error, according to the above method, the accuracy of color conversion becomes lower than when color patches made up of all combinations of CMYK are measured. In relation to this, the following Patent Document 1 discloses a correction amount based on a difference value between an interpolation L * a * b * value obtained by linear interpolation and an interpolation L * a * b * value obtained by nonlinear interpolation. There has been proposed a profile creation apparatus that creates an LUT that calculates and takes into account the correction amount.

  However, since the profile creation apparatus of Patent Document 1 merely creates an LUT using an interpolation method determined at the time of design, the gradation at the time of designing the printer and the gradation of the actual machine used by the user. Are different from each other, a large error occurs between the interpolation value obtained by the interpolation calculation and the actual output value. For example, FIG. 9 is a graph showing the gradation at the time of designing the printer and the gradation of the actual machine. As shown in FIG. 9, the gradation at the time of designing the printer and the gradation of the actual machine are greatly different in the vicinity of B (Blue) = 85%. Therefore, when interpolation calculation for B = 85% is executed by the interpolation method determined at the time of design, the interpolation value obtained by interpolation calculation (● mark in the figure) and the actual output value (x mark in the figure) ) Will cause a large error.

Japanese Patent Laid-Open No. 11-112822

  The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to provide a case where the gradation at the time of designing the output device and the gradation of the actual device are different from each other. Another object of the present invention is to provide an LUT creation device, an output system including the LUT creation device, and a control program for the LUT creation device that can reduce an error between an interpolation value obtained by interpolation calculation and an actual output value.

  The above object of the present invention is achieved by the following means.

(1) A color conversion table creation device for creating a color conversion table indicating a color value of a second color system corresponding to a grid point of the first color system, the primary color of the primary color of the first color system an acquiring unit corresponding to step chart to acquire the colorimetric values of the second color system, the interpolated value of the second color system, corresponding to one or more primary color lattice points included in the step charts, Interpolated by a plurality of interpolation methods using the colorimetric values corresponding to other primary color grid points included in the step chart, acquired by the acquiring unit, and calculated by the plurality of interpolation methods. The difference between the interpolation value corresponding to each of the one or more primary color grid points and the colorimetric value corresponding to each of the one or more primary color grid points acquired by the acquisition unit is calculated. Based on the difference calculation unit and the difference calculated by the difference calculation unit. A selection unit that selects one interpolation method from the plurality of interpolation methods for each of the one or more primary color grid points, and each of the one or more primary color grid points has the same primary color component. A multi-order color calculation unit that calculates an interpolation value of the second color system corresponding to a multi-order color grid point by the one interpolation method selected for each of the one or more primary color grid points; Yes, and the selection unit selects the interpolation method other than computed the difference is largest interpolation method by the difference calculating unit as the one interpolation method, for a portion of the one or more primary color lattice points A color conversion table creation device that selects, as the one interpolation method, an interpolation method that is the same as the one interpolation method selected for another part of the one or more primary color grid points .
(2) The multi-order color calculation unit calculates an average value of the interpolation values of the second color system calculated by the one interpolation method selected for each of the one or more primary color grid points. The color conversion table creating apparatus according to (1) above, wherein
(3) The color conversion table creation device according to (1) or (2), wherein the number of the one or more primary color grid points is different for each element color of the first color system.
(4) The color conversion according to any one of (1) to (3), wherein the number of the other primary color lattice points is different for each element color of the first color system. Table creation device.
(5) The color conversion table creation device according to any one of (1) to (4), wherein the plurality of interpolation methods include linear interpolation and nonlinear interpolation.
(6) The color conversion table creation apparatus according to (5) above, wherein the nonlinear interpolation includes at least one of Neby interpolation, spline interpolation, Lagrange interpolation, and Newton interpolation.
(7) The color conversion table creation apparatus according to any one of (1) to (6), wherein the difference calculation unit calculates a color difference (ΔE * ab) as the difference.
(8) an output device, and a color conversion table creation device that creates a color conversion table indicating a color value of the second color system corresponding to the grid point of the first color system depending on the device of the output device. In the output system, the output device includes an output unit that outputs a step chart of a primary color of the first color system, and the color conversion table creation device includes the second corresponding to the step chart. An acquisition unit that acquires a colorimetric value of a color system and an interpolation value of the second color system corresponding to one or more primary color grid points included in the step chart are acquired by the acquisition unit, An interpolation unit that calculates by a plurality of interpolation methods using the colorimetric values corresponding to other primary color grid points included in the step chart, and the one or more primary colors calculated by the plurality of interpolation methods Interpolated values corresponding to each of the grid points Based on the difference calculated by the difference calculation unit, a difference calculation unit that calculates a difference from the colorimetric value corresponding to each of the one or more primary color grid points acquired by the acquisition unit, A selection unit that selects one of the plurality of interpolation methods for each of the one or more primary color grid points; and a multi-order color in which a primary color component is equal to each of the one or more primary color grid points. A multi-order color calculation unit that calculates an interpolation value of the second color system corresponding to a grid point by the one interpolation method selected for each of the one or more primary color grid points; The selection unit selects an interpolation method other than the interpolation method having the largest difference calculated by the difference calculation unit as the one interpolation method, and selects the one or more primary color grid points for the one or more primary color grid points. The one selected for the other part of the primary color lattice points described above. Output system characterized by an interpolation method identical interpolation and methods, is selected as the one interpolation method.
(9) The output system according to (8), wherein the output device is a printing device, and the first color system is CMYK.
(10) The output system according to (8) or (9), wherein the second color system is CIE L * a * b *, CIE XYZ, or CIE CAM02.
(11) A control program for a color conversion table creating apparatus for creating a color conversion table indicating a color value of a second color system corresponding to a primary color grid point of the first color system, wherein the first color A procedure (A) for obtaining a colorimetric value of the second color system corresponding to the step chart of the primary color of the system, and the second corresponding to one or more primary color grid points included in the step chart. A procedure for calculating a color system interpolation value by a plurality of interpolation methods using the colorimetric values corresponding to other primary color grid points included in the step chart acquired in the procedure (A) ( B), an interpolation value corresponding to each of the one or more primary color grid points calculated by the plurality of interpolation methods, and the one or more primary color grid points acquired in the procedure (A). A procedure (C) for calculating a difference from the colorimetric value corresponding to each of the procedures (C) And (D) selecting one interpolation method from the plurality of interpolation methods for each of the one or more primary color grid points based on the difference calculated in (1), and the one or more primary color grid points. The interpolation values of the second color system corresponding to the multi-color grid points having the same primary color component as each of the first color grid points are obtained by the one interpolation method selected for each of the one or more primary color grid points. The color conversion table creating apparatus executes the calculation procedure (E), and in the procedure (D), an interpolation method other than the interpolation method having the largest difference calculated in the procedure (C) is used. An interpolation method selected as the interpolation method, and for the part of the one or more primary color grid points, the same interpolation method as the one of the one or more primary color grid points selected for the other part, A control program to select as one interpolation method.
(12) A computer-readable recording medium on which the control program according to (11) is recorded.

  The color conversion table creation device according to the present invention is suitable for the primary color interpolation point of the color system (e.g., CMYK) of the output device based on the difference between the interpolated value obtained by a plurality of interpolation methods and the colorimetric value. An interpolation method is selected, and an interpolation calculation for a multi-order color interpolation point is executed using a combination of the interpolation methods. Therefore, according to the present invention, an appropriate interpolation calculation according to the gradation of the actual device of the output device is executed, so that the gradation property at the time of designing the output device and the gradation of the actual device are different from each other. However, the error between the interpolation value obtained by the interpolation calculation and the actual output value can be reduced.

1 is a block diagram illustrating a configuration of a printing system according to an embodiment of the present invention. It is a block diagram which shows the structure of the LUT creation apparatus which concerns on one Embodiment of this invention. It is the schematic which shows an example of the step chart of C (Cyan) contained in the color chart which concerns on one Embodiment of this invention. It is the schematic which shows an example of the interpolation method selected by the interpolation method selection part which concerns on one Embodiment of this invention. It is the schematic which shows an example of the combination of the interpolation method specified by the interpolation method combination part which concerns on one Embodiment of this invention. It is a flowchart which shows the procedure of the LUT creation process which concerns on one Embodiment of this invention. It is a flowchart which shows the procedure of the interpolation method selection process which concerns on one Embodiment of this invention. It is a flowchart which shows the procedure of the multi-order color calculation process which concerns on one Embodiment of this invention. 6 is a graph showing the gradation at the time of designing a printer and the gradation of an actual machine in comparison. It is a graph which shows by contrast the interpolation value calculated by the LUT creation apparatus which concerns on one Embodiment of this invention, and an actual output value.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a block diagram showing the overall configuration of a printing system S including an LUT creation apparatus according to an embodiment of the present invention. As shown in FIG. 1, the printing system S includes a client device 1, a controller 2, a printer 3, a colorimeter 4, and an LUT creation device 5. These devices include the Internet, a LAN (Local Area Network), It is connected via a communication network N such as a WAN (Wide Area Network). However, the type and number of devices connected to the communication network N are not limited to the example of FIG. Each device may be locally connected without going through the communication network N. Details of each device will be described in order below.

  The client device 1 issues print instructions for various document files to the printer 3 using a printer driver, dedicated software, or the like. A print instruction issued by the client apparatus 1 may be referred to as a “print job”. A print job issued by the client device 1 is transmitted to the controller 2.

  The controller 2 generates image data for print processing by executing image processing such as color conversion, screening, and rasterization for the print job received from the client device 1. The LUT created by the LUT creation device 5 is used for color conversion for a print job. The image data generated by the controller 2 is transmitted to the printer 3. The controller 2 may be built in the printer 3.

  The printer 3 executes a printing process based on the image data received from the controller 2. More specifically, the printer 3 has a charging process for charging the photosensitive drum, an exposure process for forming an electrostatic latent image on the photosensitive drum by laser light, and a toner is attached to the electrostatic latent image on the photosensitive drum. A development process for forming a toner image, a transfer process for transferring the toner image on the photosensitive drum to a sheet by a transfer belt, and a fixing process for heating and fixing the toner image transferred to the sheet by a fixing roller. Execute print processing. However, the printer 3 may employ other printing methods such as an impact method, a thermal transfer method, and an ink jet method. The printer 3 prints a color chart for creating an LUT to be measured by the colorimeter 4 in addition to an output image based on a normal print job. The color chart according to the present embodiment will be further described later.

  The colorimeter 4 measures the color chart printed by the printer 3 to obtain device-independent color values corresponding to individual color patches in the color chart. In the following description, CIE L * a * b * is adopted as a device-independent color system. Therefore, the device-independent color value acquired by the colorimeter 4 may be particularly referred to as “colorimetric L * a * b * value”. However, the device-independent color system may be CIE XYZ, CIE CAM02, or the like. The colorimetric L * a * b * values acquired by the colorimeter 4 are transmitted to the LUT creation device 5.

  The LUT creation device 5 performs an interpolation calculation based on the colorimetric L * a * b * values received from the colorimeter 4, thereby making the device independent of the color system (CMYK) grid points of the printer 3. The color value (L * a * b * value) is obtained. The L * a * b * value obtained by such interpolation calculation may be particularly referred to as “interpolation L * a * b * value”. Further, among the grid points of the color system (CMYK) of the printer 3, the grid point for which the interpolation L * a * b * value should be obtained by interpolation calculation may be particularly referred to as an “interpolation point”. Furthermore, lattice points used for interpolation calculation for individual interpolation points may be referred to as “sampling points” below.

  Then, the LUT creation device 5 creates an LUT for the printer 3 using the colorimetric L * a * b * values and the interpolated L * a * b * values. The LUT here includes an input LUT for converting the device value (CMYK value) of the printer 3 into a device-independent color value (L * a * b * value), and a device-independent color value (L *). Both of the output LUTs for converting the a * b * value) into the device value (CMYK value) of the printer 3 are included. The LUT creation device 5 may be built in the controller 2 or the printer 3.

  FIG. 2 is a block diagram showing a detailed configuration of the LUT creation apparatus 5 according to the present embodiment. As shown in FIG. 2, the LUT creation device 5 includes a colorimetric data acquisition unit 51, an interpolation unit 52, a difference calculation unit 53, an interpolation method selection unit 54, an interpolation method combination unit 55, a multi-order color calculation unit 56, and an LUT output unit. 57 and a storage unit 58, which are connected to each other via a bus 50 for exchanging signals. Details of each part will be described in order below.

  The color measurement data acquisition unit 51 acquires the color measurement L * a * b * value of the color chart printed by the printer 3 from the color measurement device 4. However, the colorimetric data acquisition unit 51 can also acquire the colorimetric L * a * b * values measured by a device other than the colorimeter 4 via the client device 1 or the controller 2.

  The color chart according to the present embodiment is a standard color chart including a step chart of each element color (C, M, Y, K) of the color system of the printer 3, and is, for example, based on ANSI IT8.7 / 3 standard. It is a color chart for compliant multicolor printing. In the following description, the density of each element color (C, M, Y, K) is divided into nine levels of 0%, 10%, 20%, 30%, 40%, 55%, 70%, 85%, and 100%. The color patch (step chart) of the primary color that changes, and the density of each element color (C, M, Y, K) changes in five levels of 0%, 20%, 40%, 70%, and 100%. A color chart including the next color patch is used.

  Here, “primary color” refers to a color consisting only of a single element color component in a device-dependent color system, and “multi-order color” refers to a plurality of elements in a device-dependent color system. A color composed of color components. “Multi-order color” includes “second-order color” composed of only two element color components, “tertiary color” composed of only three element color components, and “quaternary color” composed of four element color components. Color ". The color patches in the color chart correspond to a part of the grid points of the color system (CMYK) of the printer 3. Therefore, in the following, each color patch may be referred to as a grid point.

  The interpolation unit 52 executes an interpolation calculation based on the colorimetric L * a * b * values acquired by the colorimetric data acquisition unit 51, so that each element color (C, M, Y, K) in the step chart is displayed. Interpolated L * a * b * values corresponding to some predetermined grid points are obtained. The interpolation point of the primary color for which the interpolation L * a * b * value is to be obtained by the interpolation calculation of the interpolation unit 52 may be particularly referred to as a “primary color interpolation point”.

  FIG. 3 is a schematic diagram illustrating an example of a C (Cyan) step chart. In the example of FIG. 3, grid points of C = 10%, 30%, 55%, and 85% are set as the primary color interpolation points of C (Cyan). Further, in the example of FIG. 3, C = 0%, 20% grid points are set as sampling points of C = 10% primary color interpolation points, and C = 20%, 40% grid points are set at C = 30%. The sampling point of the primary color interpolation point is C = 40%, the color patch of 70% is the sampling point of the primary color interpolation point of C = 55%, and the color patch of C = 70%, 100% is C = 85. % Sampling point of the primary color interpolation point.

  The primary color interpolation points of other element colors M (Magenta), Y (Yellow), and K (Black) are also determined in the same manner as in the example of FIG. However, the primary color interpolation point of each element color can be changed as appropriate, and only the grid point of density (%) within the specified range may be determined as the primary color interpolation point, or a different density ( %) Grid points may be determined as primary color interpolation points. For comparison with the “primary color interpolation point”, the lattice point of the multi-order color from which the LUT creation device 5 should obtain the interpolation L * a * b * value is particularly referred to as “multi-order color lattice point”. Sometimes.

  Further, the interpolation unit 52 obtains an interpolation L * a * b * value by a plurality of predetermined interpolation methods. Therefore, the interpolation unit 52 calculates the same number of interpolation L * a * b * values as the number of interpolation methods for each primary color interpolation point. The plurality of interpolation methods according to the present embodiment include linear interpolation and nonlinear interpolation. The nonlinear interpolation includes at least one of nebble interpolation, spline interpolation, Lagrangian interpolation, and Newton interpolation. In the following description, linear interpolation, nebble interpolation, and spline interpolation are employed as a plurality of interpolation methods. However, the types and number of the plurality of interpolation methods according to the present embodiment are not limited to the above-described example.

The difference calculation unit 53 uses the interpolation L * a * b * value of the primary color interpolation point obtained by the interpolation unit and the same primary color interpolation acquired by the colorimetric data acquisition unit 51 for each of the plurality of interpolation methods. The difference from the colorimetric L * a * b * value of the point is calculated. For example, the difference calculation unit 53 calculates a color difference (ΔE * ab) represented by the following “Equation 1” as a difference between two L * a * b * values. FIG. 3 shows interpolated L * a * b * values obtained by linear interpolation (L * a * b * _10i , L * a * b * _30i , L * a * b * _55i , L * a * b * _85i ). And the colorimetric L * a * b * values (L * a * b * ₁₀ , L * a * b * ₃₀ , L * a * b * ₅₅ , L * a * b * ₈₅ acquired by the colorimeter 4 ) Shows an example of the calculation result of the color difference (ΔE * ab).

  The interpolation method selection unit 54 selects an appropriate interpolation method for each primary color interpolation point based on the difference obtained by the difference calculation unit 53. More specifically, the interpolation method selection unit 54 selects any one interpolation method other than the interpolation method in which the difference obtained by the difference calculation unit 53 is the maximum. FIG. 4 is a schematic diagram illustrating an example of the interpolation method selected for each of the C (Cyan) primary color interpolation points by the interpolation method selection unit 54.

  In the example of FIG. 4, “linear interpolation” is selected for the primary color interpolation point of C = 10%. Similarly, “linear interpolation” is selected for the interpolation point of C = 30%, “nebble interpolation” is selected for the primary color interpolation point of C = 55%, and 1 of C = 85%. “Spline interpolation” is selected for the next color interpolation point. A detailed selection method of the interpolation method by the interpolation method selection unit 54 will be described later.

  The interpolation method combination unit 55 identifies an appropriate combination of interpolation methods for each of the multi-order color interpolation points. More specifically, the interpolation method combination unit 55 extracts and extracts primary color interpolation points having the same component color (C, M, Y, K) for each of the multi-color interpolation points. The combination of interpolation methods selected by the interpolation method selection unit 54 for each of the primary color interpolation points is specified.

  For example, for a secondary color interpolation point of (C, M) = (10%, 10%), the primary color interpolation point of C = 10% where the component of C (Cyan) is equal to the secondary color interpolation point, And M (Magenta) components that are equal to the secondary color interpolation point, M = 10% primary color interpolation points are extracted, and the interpolation method selection unit 54 selects the primary color interpolation points. (For example, “linear interpolation” and “nebble interpolation”).

  Here, among the combinations of interpolation methods specified by the interpolation method combination unit 55, the interpolation method selected for the primary color interpolation point of C (Cyan) may be referred to as “C direction interpolation method”. . Similarly, the interpolation methods selected for the primary color interpolation points of M (Magenta), Y (Yellow), and K (Black) are referred to as “M-direction interpolation method” and “Y-direction interpolation method”, respectively. , Sometimes referred to as “K-direction interpolation method”.

  The multi-order color calculation unit 56 uses the combination of interpolation methods specified by the interpolation method combination unit 55 to obtain an interpolation L * a * b * value corresponding to each multi-order color interpolation point. More specifically, the multi-order color calculation unit 56 performs the interpolation calculation by the two interpolation methods specified for each of the secondary color interpolation points, and the two interpolation L * a * b * values obtained thereby are calculated. Calculate the average value. According to the example of FIG. 5, the multi-order color calculation unit 56 performs linear interpolation (C direction) and nebble interpolation (M direction) on the interpolation point of (C, M) = (10%, 10%). Two interpolation L * a * b * values are obtained, and an average value of these interpolation L * a * b * values is calculated. Similarly, the multi-order color calculation unit 56 calculates an average value of the three interpolation L * a * b * values obtained for each of the tertiary color interpolation points, and further obtains each of the quaternary color interpolation points. Calculate the average of the four interpolated L * a * b * values. However, the multi-order color calculation unit 56 calculates a single interpolation L * a * b * value by the bicubic method instead of calculating the average value of the interpolation L * a * b * values obtained by the interpolation method in each direction. Can also be requested.

  The color conversion LUT output unit 57 uses the average value of the colorimetric L * a * b * value acquired by the colorimetric data acquisition unit 51 and the interpolated L * a * b * value obtained by the multi-order color calculation unit 56. Thus, an LUT indicating the correspondence between the grid points of the color system (CMYK) of the printer 3 and the device-independent color values (L * a * b * values) is created. The LUT created by the color conversion unit LUT output unit 57 is transmitted to the controller 2.

  The storage unit 58 is a RAM (Random Access) that temporarily stores data as work areas such as a ROM (Read Only Memory) that stores programs, parameters, and the like, an interpolation unit 52, a difference calculation unit 53, and a multi-order color calculation unit 56. Memory) and a storage area including a hard disk drive (HDD) that stores programs, data files, and the like.

  Next, an outline of the operation of the printing system S according to the present embodiment will be described. FIG. 6 is a flowchart illustrating a procedure of processing (LUT creation processing) for creating a color conversion LUT for the printer 3 by the LUT creation device 5 according to the present embodiment.

  As shown in FIG. 6, first, the LUT creation device 5 acquires the colorimetric L * a * b * values of the color chart by the colorimetric data acquisition unit 51 (S101). Subsequently, the LUT creation device 5 executes a process of selecting an appropriate interpolation method for each primary color interpolation point (S102: interpolation method selection process). The interpolation method selection process (S102) is executed by the interpolation unit 52, the difference calculation unit 53, the interpolation method selection unit 54, and the interpolation method combination unit 55. Details of the interpolation method selection process (S102) will be described later.

  Subsequently, the LUT creation device 51 executes a process (S103: multi-order color calculation process) for calculating an interpolation L * a * b * value corresponding to each multi-order color interpolation point. The multi-order color calculation process (S103) is executed by the multi-order color calculation unit 56. Details of the multi-color calculation process (S103) will be described later. Thereafter, the LUT creation device 5 transmits the LUT created in the multi-color calculation process (S103) to the controller 2 (S104), and ends the LUT creation process (end).

  FIG. 7 is a flowchart showing the procedure of the interpolation method selection process (S102) according to this embodiment. As shown in FIG. 7, first, the LUT creation device 5 determines the colorimetric L * a * of the sampling point used for the interpolation calculation of the primary color interpolation point among the colorimetric L * a * b * values acquired in S101. The b * value is read (S201). More specifically, the LUT creation device 5 performs color measurement of primary color grid points in which the density of each element color (C, M, Y, K) is 0%, 20%, 40%, 70%, 100%. L * a * b * values are read (see FIG. 3).

  Subsequently, the LUT creation device 5 obtains an interpolation L * a * b * value by linear interpolation (L = 0) for each primary color interpolation point (S202), and the interpolation L * a * b * value and S201. The color difference (ΔE * ab) with the calorimetric L * a * b * value of the same primary color interpolation point read in step S is calculated (S203). Then, the LUT creation device 5 obtains an interpolation L * a * b * value by spline interpolation (L = 1) for each primary color interpolation point (S202), and the interpolation L * a * b * value and S201. A color difference (ΔE * ab) with the calorimetric L * a * b * value of the read primary color interpolation point is calculated (S203). Further, the LUT creation device 5 obtains an interpolated L * a * b * value for each of the primary color interpolation points by nebule interpolation (L = 2) (S202), and the interpolated L * a * b * value and S201. A color difference (ΔE * ab) with the calorimetric L * a * b * value of the read primary color interpolation point is calculated (S203). Table 1 below shows an example of the calculation result of the color difference (ΔE * ab) obtained in S203.

  Subsequently, the LUT creation device 5 selects an appropriate interpolation method for each of the primary color interpolation points based on the color difference (ΔE * ab) obtained in S203 (S204). More specifically, the LUT creation device 5 selects any one interpolation method other than the interpolation method in which the color difference (ΔE * ab) obtained in S203 is the maximum for each primary color interpolation point. That is, in the example of Table 1, any one interpolation method other than spline interpolation (ΔE * ab = 10) is selected as an appropriate interpolation method for C = 10%.

  Similarly, as an appropriate interpolation method for C = 30%, any one of interpolation methods other than nephew interpolation (ΔE * ab = 6) is selected, and as an appropriate interpolation method for C = 55%, linear interpolation is selected. Any one interpolation method other than (ΔE * ab = 5) is selected, and any one other than nephew interpolation (ΔE * ab = 8) is selected as an appropriate interpolation method for the primary color interpolation point of C = 85%. Select one interpolation method.

  Further, when the number of interpolation methods other than the interpolation method having the maximum color difference (ΔE * ab) is two or more as in the example of Table 1 (that is, the predetermined number of interpolation methods is three or more). In some cases, the LUT creation device 5 selects any one of the interpolation methods by the following first selection method or second selection method.

  First, in the first selection method, an interpolation method that minimizes the color difference (ΔE * ab) obtained in S203 is selected as an appropriate interpolation method for each primary color grid point. The underline attached to the numerical values in Table 1 indicates the interpolation method selected by the first selection method. That is, in the first selection method, linear interpolation (ΔE * ab = 3) is selected as an appropriate interpolation method for C = 10%, and linear interpolation (ΔE * ab is selected as an appropriate interpolation method for C = 30%. = 4), Nebby interpolation (ΔE * ab = 3) is selected as an appropriate interpolation method for C = 55%, and Spline interpolation (ΔE * ab = 3) is selected as an appropriate interpolation method for C = 85%. ) Is selected. By selecting an interpolation method that minimizes the color difference (ΔE * ab) in this way, errors caused by interpolation calculations can be minimized.

  In the second selection method, in principle, an interpolation method in which the color difference (ΔE * ab) obtained in S203 is the smallest is selected, and some primary color interpolation points are compared with other primary color interpolation points. Select the same interpolation method. Table 2 below shows another example of the calculation result of the color difference (ΔE * ab) obtained in S203.

  The underline attached to the numerical values in Table 2 indicates the interpolation method selected by the second interpolation method. That is, in the second selection method, linear interpolation having the smallest color difference (ΔE * ab) is selected for the primary color interpolation points of C = 10%, 30%, and 55%. On the other hand, for the interpolation point of C = 85%, linear interpolation which is the same interpolation method as the other interpolation points is selected instead of the spline interpolation with the smallest color difference (ΔE * ab). By selecting the same interpolation method for some interpolation points as for other interpolation points in this way, the interpolation method for many interpolation points can be unified, so that the calculation speed of interpolation calculation can be improved. it can.

  Thereafter, the LUT creation device 5 determines whether or not the selection of the interpolation method for all the primary color interpolation points has been completed (S205). If selection for all primary color interpolation points has not been completed (NO in S205), the procedure of S201 to S204 is repeated for the remaining primary color interpolation points.

  When the selection for all the primary color interpolation points is completed (YES in S205), the LUT creation device 5 performs an appropriate interpolation method for each of the multi-color interpolation points of the color system (CMYK) of the printer 3. Are identified (S206). More specifically, the LUT creation device 5 extracts and extracts primary color interpolation points having the same component color (C, M, Y, K) as to each of the multi-color interpolation points. The combination of the interpolation methods selected in S204 is specified for each primary color interpolation point.

  FIG. 5 shows an example of combinations of interpolation methods in the C direction and M direction specified for each of the secondary color interpolation points of (C, M). According to the example of FIG. 5, for the secondary color interpolation point of (C, M) = (10%, 10%), a combination of linear interpolation (C direction) and nebble interpolation (M direction) is specified. Similarly, for the secondary color interpolation point of (C, M) = (85%, 10%), a combination of spline interpolation (C direction) and nebble interpolation (M direction) is specified, and (C, M ) = (10%, 85%), a combination of linear interpolation (C direction) and spline interpolation (M direction) is specified for the secondary color interpolation point, and (C, M) = (85%, 85). %) Secondary color interpolation points, a combination of spline interpolation (C direction) and spline interpolation (M direction) is specified.

  Thereafter, the LUT creation device 5 stores the combination of interpolation methods specified for each of the multi-color interpolation points in S206 in the storage unit 58 (S207), and ends the interpolation method selection processing (return).

  FIG. 8 is a flowchart showing a procedure of multi-order color calculation processing (S103) according to this embodiment. As shown in FIG. 8, first, the LUT creation device 5 uses the colorimetric L * a * of the sampling point used for the interpolation calculation of the multi-order color interpolation point among the colorimetric L * a * b * values acquired in S101. The b * value is read (S301). More specifically, the LUT creation device 5 performs colorimetry of multi-order color lattice points in which the density of each element color (C, M, Y, K) is 0%, 20%, 40%, 70%, 100%. Read L * a * b * value.

  Subsequently, the LUT creation device 5 obtains an interpolation L * a * b * value corresponding to each of the multi-color interpolation points using the combination of interpolation methods specified in S206 (S302). More specifically, the LUT creation device 5 executes the interpolation calculation by the two interpolation methods specified for each of the secondary color interpolation points, and the average value of the two interpolation L * a * b * values obtained thereby. Calculate Similarly, the LUT creation device 5 calculates an average value of the three interpolation L * a * b * values obtained for each of the tertiary color interpolation points, and further calculates the four values obtained for each of the quaternary color interpolation points. The average value of the interpolated L * a * b * values is calculated.

  Thereafter, the LUT creation device 5 determines whether or not the calculation of the interpolation L * a * b * values for all the multi-order color interpolation points has been completed (S303), and the calculations for all the multi-order color interpolation points are performed. If not completed (NO in S303), the procedure in S302 is repeated for the remaining multi-order color interpolation points. When the calculation for all multi-order color interpolation points is completed (YES in S303), the LUT creation device 5 determines the colorimetric L * a * b * value obtained in S101 and the interpolation L * a obtained in S302. Using the * b * value, an input LUT (a conversion table from CMYK values to L * a * b * values) of the printer 3 is created (S304).

  Subsequently, the LUT creation device 5 creates an output LUT (a conversion table from L * a * b * values to CMYK values) of the printer 3 based on the input LUT created in S304 (S305). In S305, an output LUT is created by using a known method described in Japanese Patent Laid-Open Nos. 3-158071 and 2003-077873. After that, the LUT creation device 5 stores the LUT of the printer 3 created in S304 and S305 in the storage unit 58 (S306), and ends the multi-order color calculation process (return).

  As described above, the LUT creation device 5 according to the present embodiment calculates the color difference (ΔE * ab) between the interpolated L * a * b * values and the colorimetric L * a * b * values obtained by a plurality of interpolation methods. Based on this, an appropriate interpolation method for the primary color interpolation point of the color system (CMYK) of the printer 3 is selected, and the multi-order color interpolation point of the color system (CMYK) of the printer 3 using a combination of the interpolation methods. The interpolation calculation is executed. FIG. 10 is a graph showing the interpolation value obtained by the LUT creation device 5 (multi-order color calculation unit 56) according to the present embodiment in comparison with the actual output value. As shown in FIG. 10, according to the LUT creation device 5 according to the present embodiment, appropriate interpolation calculation according to the gradation of each printer 3 is executed. Even when the tonality is different from each other, the error between the interpolation value obtained by the interpolation calculation and the actual output value can be reduced.

  The present invention is not limited to the above-described embodiments, and can be variously modified within the scope of the claims. For example, in the above embodiment, the printer 3 is adopted as an output device that should create an LUT. However, the output device according to the present invention may be any device that has a function of outputting an image, for example, It may be a display device such as an LCD (Luxud Crystal Display) or an organic EL (Electroluminescence) display. In the above embodiment, CMYK is adopted as the device-dependent color system of the output device. However, the color system of the output device according to the present invention may be any standard color system. For example, RGB, sRGB, CMY, HSV, HLS, etc. may be used.

  Each means of the color conversion table creating apparatus according to the present invention can be realized by a dedicated hardware circuit for executing the above procedure, or by a CPU (Central Processing Unit) executing a program describing the above procedure. It is feasible. When the present invention is realized by the latter, a program for operating each means may be provided by a computer-readable recording medium such as a floppy (registered trademark) disk or a CD-ROM, or via a communication network such as the Internet. May be provided online. In this case, the program recorded on the computer-readable recording medium is usually transferred to and stored in a ROM, a hard disk or the like. Further, the above program may be provided as a single application software, or may be incorporated in the device as one function of the device constituting the color conversion table creation device.

1 client device,
2 controller,
3 Printer,
4 Colorimeter,
5 LUT creation device,
51 colorimetric data acquisition unit,
52 Interpolator,
53 Difference calculation unit,
54 interpolation method selection section,
55 Interpolation method combination part,
56 Multi-order color calculator,
57 LUT output section,
58 storage unit,
S Printing system.

Claims (12)

A color conversion table creation device for creating a color conversion table indicating color values of a second color system corresponding to grid points of the first color system,
An acquisition unit for acquiring a colorimetric value of the second color system corresponding to the step chart of the primary color of the first color system;
The interpolated values of the second color system, corresponding to one or more primary color lattice points included in the step chart obtained by the obtaining unit, in addition to the primary color lattice points included in the step chart An interpolation unit for calculating by a plurality of interpolation methods using the corresponding colorimetric values;
The interpolation value corresponding to each of the one or more primary color grid points calculated by the plurality of interpolation methods, and the measurement corresponding to each of the one or more primary color grid points acquired by the acquisition unit. A difference calculation unit for calculating a difference from the color value;
A selection unit that selects one interpolation method from the plurality of interpolation methods for each of the one or more primary color grid points based on the difference calculated by the difference calculation unit;
The interpolated value of the second color system corresponding to a multi-color grid point having the same primary color component as each of the one or more primary color grid points is obtained for each of the one or more primary color grid points. possess a multi-color calculator for calculating the selected the one interpolation method, a
The selection unit includes:
An interpolation method other than the interpolation method with the largest difference calculated by the difference calculation unit is selected as the one interpolation method,
For one part of the one or more primary color grid points, the same interpolation method as the one interpolation method selected for the other part of the one or more primary color grid points is selected as the one interpolation method. A color conversion table creation device. The multi-order color calculation unit calculates an average value of interpolation values of the second color system calculated by the one interpolation method selected for each of the one or more primary color grid points. The color conversion table creation device according to claim 1 . 3. The color conversion table creation apparatus according to claim 1, wherein the number of the one or more primary color grid points is different for each element color of the first color system. The number of other primary color lattice point, the color conversion table creating device according to any one of claims 1-3, wherein different for each element color of said first color system. Wherein the plurality of the interpolation method, linear interpolation and color conversion table generation apparatus according to any one of claims 1-4, characterized in that it includes a non-linear interpolation. 6. The color conversion table creation apparatus according to claim 5 , wherein the non-linear interpolation includes at least one of nebble interpolation, spline interpolation, Lagrangian interpolation, and Newton interpolation. Said difference calculation unit, a color conversion table creating device according to any one of claims 1-6, characterized in that to calculate the color difference (ΔE * ab) as the difference. An output system comprising: an output device; and a color conversion table creation device that creates a color conversion table indicating a color value of a second color system corresponding to a grid point of the device-dependent first color system of the output device. There,
The output device includes an output unit that outputs a step chart of a primary color of the first color system.
The color conversion table creation device includes an acquisition unit that acquires a colorimetric value of the second color system corresponding to the step chart, and the first color grid point corresponding to one or more primary color grid points included in the step chart. An interpolation unit that calculates an interpolated value of a two-color system by a plurality of interpolation methods using the colorimetric values corresponding to other primary color grid points included in the step chart acquired by the acquiring unit; , The interpolation value corresponding to each of the one or more primary color grid points calculated by the plurality of interpolation methods, and the one or more primary color grid points acquired by the acquisition unit Based on the difference calculated by the difference calculation unit and a difference calculation unit that calculates a difference from the colorimetric value, one interpolation method is selected from the plurality of interpolation methods for each of the one or more primary color grid points. A selection unit to select, and the one An interpolation value of the second color system corresponding to a multi-color grid point having the same primary color component as each of the above primary color grid points is selected for each of the one or more primary color grid points. wherein possess a multi-color calculation unit for calculating the one interpolation method, was,
The selection unit includes:
An interpolation method other than the interpolation method with the largest difference calculated by the difference calculation unit is selected as the one interpolation method,
For one part of the one or more primary color grid points, the same interpolation method as the one interpolation method selected for the other part of the one or more primary color grid points is selected as the one interpolation method. An output system characterized by: 9. The output system according to claim 8 , wherein the output device is a printing device, and the first color system is CMYK. It said second color system, CIE L * a * b * , the output system according to claim 8 or 9, characterized in that CIE XYZ, or CIE CAM02. A control program for a color conversion table creation device for creating a color conversion table indicating a color value of a second color system corresponding to a primary color grid point of the first color system,
A procedure (A) for obtaining a colorimetric value of the second color system corresponding to the step chart of the primary color of the first color system;
Other primary color grids included in the step chart, obtained by the procedure (A), with the interpolation values of the second color system corresponding to one or more primary color grid points included in the step chart. A procedure (B) for calculating by a plurality of interpolation methods using the colorimetric values corresponding to points;
Interpolated values corresponding to each of the one or more primary color grid points calculated by the plurality of interpolation methods, and corresponding to each of the one or more primary color grid points acquired in the procedure (A). A procedure (C) for calculating a difference from the colorimetric value;
A step ( D ) of selecting one interpolation method from the plurality of interpolation methods for each of the one or more primary color grid points based on the difference calculated in the step (C);
The interpolated value of the second color system corresponding to a multi-color grid point having the same primary color component as each of the one or more primary color grid points is obtained for each of the one or more primary color grid points. A step ( E ) of calculating by the selected one interpolation method is executed by the color conversion table creating device ;
In the procedure (D),
An interpolation method other than the interpolation method having the largest difference calculated in the step (C) is selected as the one interpolation method;
For one part of the one or more primary color grid points, the same interpolation method as the one interpolation method selected for the other part of the one or more primary color grid points is selected as the one interpolation method. to, control program. A computer-readable recording medium on which the control program according to claim 11 is recorded.