JP2012124809A - Color conversion table correction device, color conversion table correction method, color conversion table correction program, and print control unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To create a color conversion table which performs accurate color conversion by easily and securely correcting the color conversion table without influence of artificial factors such as skills of a correcting person.SOLUTION: An ink amount data correction part 33 repeats recorrection by comparing with a predetermined threshold a difference vector indicating the difference between the colorimetric value of a patch printed with corrected specific ink amount data and the value of a device-independent color space indicating a corrected color. The predetermined threshold is divided into plural values in accordance with index values relating to color brightness in the color conversion table.

Description

  The present invention relates to a color conversion table correction apparatus, a color conversion table correction method, a color conversion table correction program, and a print control apparatus.

  In recent years, inkjet printers and laser printers have become widespread as color printers. A color printer uses a color conversion table that converts input color image data into a combination of a plurality of ink amounts. As the color conversion table, for example, a lookup table in which input color image data is associated with a combination of a plurality of ink amounts is often used. In a conventional color conversion table creation method, (i) coordinate values in a device-independent color space corresponding to a large number of input color image data are obtained, and (ii) a plurality of color patches are printed with a plurality of ink amount data. , (Iii) measuring each color patch to obtain coordinate values in the device-independent color space, and (iv) input color image data and ink amount data from the correspondence between the coordinate values in the device-independent color space A look-up table representing the correspondence between and was created. (For example, refer to Patent Document 1).

JP 2002-118762 A

However, the actual number of operations for printing and measuring the color patches is limited, and a color conversion error occurs due to the limited number of color patches. In particular, this color conversion error cannot be ignored for a color whose color change is easy to understand. For example, the actual number of patches for which printing and colorimetry can be performed is about 10 ³ , but in the color conversion table, the correspondence relationship is defined for a much larger number of colors. Accordingly, the coordinate values of these colors are calculated by an interpolation calculation using the colorimetric values of the color patches. In such an interpolation, an error always occurs. Of course, the color conversion table is created in a state including various errors such as a colorimetric error.

  Therefore, even if color conversion is performed with reference to the color conversion table and printing is performed, a color exactly equal to the color indicated by the input color image data is not always printed. For colors such as gray that are easy to recognize color changes, high quality printing cannot be performed if there is such a difference. In order to eliminate such differences, it is conceivable to artificially correct the ink amount data specified in the color conversion table. However, artificial correction requires the skill of the corrector, and the corrector must Variation will occur.

  The present invention has been made in view of the above-mentioned problems, and is a color that performs high-precision color conversion by correcting the color conversion table easily and accurately without being affected by human factors such as the skill of the corrector. It is an object of the present invention to provide a color conversion table correction apparatus, a color conversion table correction method, a color conversion table correction program, and a print control apparatus that can create a conversion table.

  Application Example 1 With reference to a color conversion table that defines the correspondence between color data in a predetermined color space and ink amount data indicating the ink amount of each color used in the printing apparatus, the color conversion table is specified. A color conversion table correction device that corrects specific ink amount data, a color conversion table data acquisition unit that acquires color conversion table data indicating the color conversion table, and a color printed with the specific ink amount data Target data acquisition means for acquiring target data indicating an ideal value of the image, patch printing means for printing a patch by the printing apparatus using the specific ink amount data, and a colorimetric value obtained by measuring the printed patch. A colorimetric value acquisition means and a vector having a reverse component of a difference vector indicating a difference between the ideal value and the colorimetric value within a predetermined device-independent color space; An ink amount data correcting means for calculating a correction vector based on the correction vector, calculating ink amount data for reducing the difference based on the correction vector, and correcting the specific ink amount data based on the calculated ink amount data. The ink amount data correction means is a difference vector indicating a difference between the colorimetric value of the patch printed by the specific ink amount data after correction and the value of the device-independent color space indicating the color after correction. And re-correction is repeatedly performed by comparing with a predetermined threshold value, and the predetermined threshold value is divided into a plurality of values according to an index value relating to color brightness in the color conversion table. Characteristic color conversion table correction device.

  According to the color conversion table correction device described above, specific ink amount data is corrected for a color conversion table that is defined in advance. That is, the ink amount data is corrected so that the color data in a predetermined color space is converted into ink amount data different from the original color conversion table for a specific color. This correction is realized by a correction vector calculated based on a vector having a reverse component of a difference vector indicating a difference between the ideal value and the colorimetric value. That is, the correction is performed by calculating ink amount data for reducing the difference using the correction vector, and updating the specific ink amount data with the calculated ink amount data. Therefore, when color conversion is performed using the corrected color conversion table, as specific ink amount data, data that can be printed in a state where the difference between the ideal value and the colorimetric value is very small is obtained. For this reason, it is possible to perform printing by performing color conversion with very high accuracy.

  Here, the table acquired by the color conversion table data acquisition unit may be prepared in advance, but the acquired color conversion table data is a table in which the accuracy of color conversion is improved by the correction according to the present invention. Therefore, it is not a color conversion table data in which the ideal value and the colorimetric value are strictly equal for all ink amount data, but a table in which a certain degree of error can occur. Further, in the present invention, in order to improve the color conversion accuracy for specific ink amount data, it is preferable to apply the present invention when color conversion with high accuracy is particularly required for the color indicated by the specific ink amount data. .

  The color conversion table data only needs to be able to convert color data in a predetermined color space into ink amount data. For this purpose, the color data in the predetermined color space and the ink of each color used in the printing apparatus are used. It is only necessary to define a correspondence relationship with the ink amount data indicating the amount. Therefore, it may be data in which the color data and the ink amount data are associated with each other for a plurality of colors, or profile data that defines the relationship between the two based on a plurality of parameters.

Further, the predetermined color space may be a color space of a color system different from the ink amount space including the amount of ink used in the printing device as a component, and the color space or device used in an image device different from the printing device. Various color spaces such as an independent color space can be employed. For example, an sRGB space used for a display or an RGB space used for a scanner can be adopted as the former, and an L ^* a ^* b ^* space or an L ^* u ^* v ^* space can be adopted as the latter. . Of course, the color data in the predetermined color space only needs to be data for specifying the color in the color space, and data representing the size of each color component in each color system can be adopted. It is.

  The target data only needs to be able to indicate an ideal value of a color to be printed with specific ink amount data. In other words, it is difficult to create a color conversion table so that the ink amount data obtained by color conversion referring to the color conversion table data is exactly the same as the color before color conversion. It is possible to determine in advance exactly whether to convert to. Therefore, in the present invention, the color to be obtained as a result of the color conversion is set as an ideal value and defined in advance as target data.

  The color printed by the specific ink amount data can be specified by printing by the patch printing unit and the colorimetric value obtained by the colorimetric value acquisition unit. Accordingly, the patch printing unit only needs to be able to control the printing apparatus so as to print a patch having a predetermined area based on the ink amount data, and the colorimetric value acquisition unit can measure the colorimetry acquired by a colorimeter or the like. It is sufficient if the color value can be acquired.

  The ink amount data correction means uses the difference between the ideal value and the colorimetric value in a predetermined device-independent color space when performing correction so that the output color based on the corrected ink amount data approaches the target. . Therefore, it is preferable that the ideal value and the colorimetric value are acquired as values in the device-independent color space. Of course, it may be configured to obtain a value in the device-independent color space through some conversion. Further, after the specific ink amount data is corrected, the exact color is not known unless the result of printing with the corrected ink amount data is measured. However, if the corrected color can be specified in a predetermined device-independent color space, the ink amount data corresponding to the corrected color and at least the difference is highly likely to be eliminated. Can be predicted.

  For example, if the correspondence between the device-independent color data in the predetermined device-independent color space and the ink amount data is defined in advance, the ink amount data corresponding to the corrected color is predicted by interpolation calculation or the like. can do. Therefore, if the corrected color in a predetermined device-independent color space is determined so that the difference between the color indicated by the corrected ink amount data and the ideal value is eliminated, the correction in the ink amount data correcting unit is performed. Can be implemented. Of course, in this correction, it is only necessary to perform correction so as to eliminate the difference between the ideal value and the colorimetric value within a predetermined device-independent color space, and various configurations can be employed.

For example, since there is a difference between the ideal value and the colorimetric value, if the correction vector has at least a component in the reverse direction with respect to the difference vector representing the difference, the ideal value and the color measurement value It can be considered that the direction is to eliminate the difference from the colorimetric value. Therefore, under such an idea, the corrected color is determined by the sum of a correction vector having at least a component in the reverse direction with respect to the difference vector from the ideal value to the colorimetric value and a vector indicating the ideal value. Correction may be performed. As the difference vector, in addition to a vector indicating the difference between the ideal value and the colorimetric value, a vector that is a scalar constant multiple can be adopted. As the predetermined device-independent color space, an L ^* a ^* b ^* space, an L ^* u ^* v ^* space, or the like can be used.

  It is not essential that this correction be completed only once. For example, after correction is performed by the ink amount data correction unit, re-correction is performed if printing by the patch printing unit and colorimetric value acquisition by the colorimetric value acquisition unit are performed based on the corrected ink amount data. can do. That is, by repeating the correction, the difference between the colorimetric value and the ideal value can be gradually eliminated, and the ink amount data can be corrected so that the two are finally very close. At this time, re-correction is repeatedly performed by comparing the above-described difference vector with a predetermined threshold. This predetermined threshold is divided into a plurality of values according to the index value relating to the brightness of the color in the color conversion table, so that correction can be made with accuracy suitable for each division. And the correction is not repeated more than necessary.

  Application Example 2 In the color conversion table correction apparatus, the index value is lightness.

  According to the color conversion table correction device described above, threshold values can be set according to human visual characteristics for each lightness region, and ink amount data is corrected so that it is very close to the ideal value not only numerically but also visually. can do.

  Application Example 3 A difference vector having the smallest difference between the ideal value and the colorimetric value among the difference vectors when the recorrection is performed when the number of times of the recorrection reaches a predetermined number. The color conversion table correction apparatus according to claim 1, wherein a correction vector calculated based on a vector having a reverse direction component is adopted.

  According to the color conversion table correction device described above, the optimum result is obtained from the correction result before a predetermined number of times when the optimum correction result does not fall within the threshold due to the influence of error at the time of colorimetric value acquisition or the influence of the threshold setting. Can be selected.

  Application Example 4 In performing the re-correction, the ink amount data correction unit calculates the correction vector with reference to a difference vector in a correction before the re-correction, and performs the correction. The color conversion table correction device.

  According to the color conversion table correction device described above, various methods can be adopted as specific correction methods when correction is repeated. For example, the difference between the colorimetric value of a patch printed with the ink amount data obtained by the Nth correction (N is a natural number) and the value of the device-independent color space indicating the color (the value after the Nth correction) The correction may be performed with reference to the difference vector in the N-th correction and the difference vector in the M-th correction (M is a natural number equal to or less than N−1). That is, when only the difference between the color after the Nth correction and the colorimetric value is considered in the Nth correction, the Nth correction is very close to the ink amount data after the Mth correction. When the value is obtained, the correction after the (N + 1) th time is repeated substantially the same as the previous correction. That is, the difference may not be resolved no matter how many times it is corrected.

  However, if the N-th correction is performed with reference to the difference in the correction stage before the N-1th correction, even if the ink amount data after the M-th correction is obtained in the N-th correction, a value very close to that is obtained. In the (N + 1) th and subsequent corrections, the difference of NM times is taken into consideration, so that substantially the same correction as the Mth correction is not repeated. Therefore, it is possible to effectively prevent the occurrence of vibration and perform correction for quickly eliminating the above-described difference. Further, when only the difference between the color after the N-th correction and the colorimetric value for the ink amount data is taken into consideration in the N-th correction, the N-th correction is caused by an interpolation error, a colorimetric error, or the like. The above-mentioned difference may be increased by so-called correction (so-called divergence), but the occurrence of such divergence can be effectively prevented by referring to the difference before the (N-1) th time.

  Note that the color conversion table correction device described above may be implemented alone, or may be implemented with other methods in a state of being incorporated in a certain device. It is included and can be changed as appropriate. In addition, the above-described method for performing correction based on the difference between the ideal value and the colorimetric value, when proceeding according to a predetermined procedure, naturally has an invention in that procedure. is there. Therefore, the present invention can also be applied as a method. Further, when trying to implement the present invention, a predetermined program may be executed by the color conversion table correction apparatus. Therefore, the present invention can also be applied as the program. Furthermore, it can be said that the present invention is also used in a print control apparatus that executes printing with reference to the color conversion table corrected by the present invention. Of course, it can be said that the present invention is used even for a method or program for realizing a print control apparatus.

It is explanatory drawing which illustrates the correction process of a color conversion table roughly. It is a block diagram of a color conversion table correction apparatus. It is a flowchart of a color conversion table correction process. It is explanatory drawing explaining the example of a process at the time of specifying patch data. It is explanatory drawing explaining a mode that the amount of ink is correct | amended. It is a block diagram of a printing control apparatus.

Here, embodiments of the present invention will be described in the following order.
(1) Outline of the Present Invention (2) Configuration of Color Conversion Table Correction Device (3) Color Conversion Table Correction Processing (4) Other Embodiments

(1) Outline of the Present Invention FIG. 1 is an explanatory diagram schematically illustrating a color conversion table correction process according to the present invention.
In the present embodiment, correction is performed on color conversion table data (LUT 15a) created in advance. The LUT 15a is sRGB data indicating RGB (red, green, blue) gradation values in the sRGB color space and CMYKlclm data indicating gradation values of each ink color of CMYKlclm (cyan, magenta, yellow, black, light cyan, light magenta). This defines the correspondence with (ink amount data). In other words, the LUT 15a is look-up table data that is referred to when image data specifying the color of an image with sRGB data is input and printing is performed by a printing apparatus equipped with CMYKlclm color inks.

  In the example shown in FIG. 1, each component of the sRGB data is represented by a gradation value of 0 to 255, and the gradation value generated by combining the obtained values by dividing the gradation value range of each component into approximately 16 equal parts. Are registered in the LUT 15a. That is, CMYKlclm data for outputting the same color as sRGB data in which each component of RGB has any value of “0, 16, 32,..., 255” is associated. Yes. In the example shown in FIG. 1, it is possible to adopt a configuration in which each component is expressed by gradation values from 0 to 255 and the output color is expressed by combining the gradation values of each component in CMYKlclm data.

The sRGB data is data conforming to the sRGB standard, and the color corresponding to each combination of RGB values is known. That is, arbitrary sRGB data can be associated with data in a device-independent color space such as an L ^* a ^* b ^* space by a known arithmetic expression or the like. However, the CMYKlclm data is a device-dependent color, and the color corresponding to each combination of CMYKlclm values depends on the printer model, printing conditions, printing medium, and the like.

  Therefore, in general, a plurality of patches are printed using CMYKlclm data while specifying a printing device, printing conditions, printing medium, and the like to which the LUT 15a is applied, and an output color in the CMYKlclm data is converted to a device-independent color space by a colorimeter or the like. Specified as data in Then, by associating the color indicated by the sRGB data with the color indicated by the CMYKlclm data in the device-independent color space, the LUT 15a is created by associating the CMYKlclm data for outputting the same color as the specific sRGB data. .

  Here, the number of colors defined in the LUT 15a is not particularly limited, but is generally determined as described above and is often 173 or more. It is very difficult and practically impossible to perform printing and color measurement on a patch having approximately the same number of colors as such an enormous number of colors. Therefore, it is usual to print out a smaller number than 173, for example, 103 patches and perform colorimetry, and calculate CMYKlclm data corresponding to sRGB data from these colorimetry results by interpolation.

  However, the color conversion accuracy in the LUT 15a created in this way includes errors due to the above-described interpolation calculation, errors during colorimetry, and the like. For specific colors that require very high-accuracy color conversion, Such an error becomes a factor of image quality degradation. As described above, the color indicated by the sRGB data is known, but even if the CMYKlclm data is associated with the color indicated by the sRGB data so as to output exactly the same color, it cannot be said that the image quality is the best. In some cases. For example, since the output color in the printing apparatus is always influenced by the background color of the print medium, in a specific case such as a bluish background color, printing is performed with a color different from the color indicated by the sRGB data. In some cases, printing results that are preferable are obtained. Also in this case, if the accuracy of color conversion is low, the target output color that has been changed as described above cannot be obtained, and this may cause a deterioration in image quality, such as a sense of incongruity when the image is viewed.

The present invention corrects the LUT 15a so that the specific color as described above can be converted to a desired color with high accuracy. In the example shown in FIG. 1, gray is used as the specific color. Yes. That is, the CMYKlclm data associated with the gray defined in the LUT 15a (R = G = B data in the sRGB data, gray data 15d) is corrected. In the present invention, target data 15b in which the ideal value of the data indicating gray is expressed in the L ^* a ^* b ^* space is prepared in advance, and the output color of the CMYKlclm data is the color defined in the target data 15b. Make corrections so that The target data is a predetermined L ^* a ^* b ^* value. However, when the ideal gray value is completely achromatic, an operation to create an L ^* a ^* b ^* value is performed in advance. Even if not, it can be calculated and obtained from the sRGB data.

In the correction according to the present invention, a patch is printed with the CMYKlclm data of the gray data 15d described above, and the print result is measured to correct the CMYKlclm data so that the color difference ΔE between them is smaller than the threshold value αi. That is, in FIG. 1, the colorimetric value is shown as a vector M (L ^* a ^* b ^* space vector), and the L ^* a ^* b ^* value of the target data is shown as a vector T0. If is not less than αi, the CMYKlclm data is corrected.

  Here, the threshold value α of the color difference ΔE is divided into sections, and the value differs depending on the section. For example, if sections are divided according to lightness, the threshold value α1 of the high lightness section and the threshold value α2 of the low lightness section can be set so as to satisfy the relationship of α1 <α2 in consideration of human visual characteristics.

In this correction, the L ^* a ^* b ^* value of the patch printed by the CMYKlclm data is moved in a direction to cancel the difference between the L ^* a ^* b ^* value of the target data 15b and the colorimetric value before and after the correction. Is implemented as follows. In other words, when there is a difference between the vector M and the vector T0, it is predicted how the L ^* a ^* b ^* value corresponding to the CMYKlclm data will be changed, and the difference is eliminated. Calculate the predicted L ^* a ^* b ^* value.

After predicting the L ^* a ^* b ^* values, with reference to the correspondence data 15c, calculates the CMYKlclm data corresponding to the L ^* a ^* b ^* value after prediction by interpolation. The correspondence relationship data 15c is data in which CMYKlclm data and the colorimetric values thereof are associated with each other for gray and surrounding colors, and both are associated with a larger number of colors than the gray data 15d.

Of course, the interpolation calculation referring to the correspondence data 15c also includes an error, but in the correspondence data 15c, since more colors are defined around the gray than the LUT 15a, the L ^* a ^* b ^* value is used. Corresponding CMYKlclm data can be calculated with higher accuracy. In the present invention, the correction is repeated until the color difference ΔE becomes smaller than the threshold value αi as described above. Therefore, in the end, the difference between the print color by the CMYKlclm data and the target becomes very small. As a result, it is possible to obtain an LUT 15a that can perform color conversion with high accuracy for gray and surrounding colors.

(2) Configuration of Color Conversion Table Correction Device A configuration for carrying out the present invention will be described in detail below. FIG. 2 is a block diagram showing the configuration of the computer 10 that functions as the color conversion table correction apparatus according to the present invention. In FIG. 2, the computer 10 can be realized by a general-purpose personal computer, and is connected to a display 11, a printing device 12, a keyboard 13a, and a mouse 13b. In the present embodiment, the color of the image displayed on the display 11 is specified by the sRGB data, and the color of the image printed by the printing device 12 is specified by the CMYKlclm data.

  The computer 10 includes a program execution environment such as a CPU, RAM, and ROM (not shown), and can execute a PRTDRV 21, the LUT correction program 30, an application program (not shown), and the like under the control of an OS (not shown). A user of the computer 10 can perform operation input on the LUT correction program 30 or application program by operating the keyboard 13a, mouse 13b, or the like. In the PRTDRV 21, an image is printed by controlling the printing device 12 in accordance with a print instruction for an image or the like created by an application program (not shown). In the present invention, it is also possible to receive patch data in which the color of a pixel is specified by CMYKlclm data from the patch data generation unit 31 of the LUT correction program 30 and cause the printing apparatus 12 to execute printing.

  In other words, the PRTDRV 21 can implement functions provided by a normal driver for controlling the printing apparatus 12, such as a color conversion function, a halftone processing function, and a function for generating print data to be transferred to the printing apparatus 12. It is. The print data generated by the PRTDRV 21 is transferred to the printing apparatus 12 via the USB I / O 14. That is, the printing apparatus 12 is connected to the computer 10 via USB, and when receiving the print data, records the ink on the print medium according to the print data and forms an image.

  The LUT correction program 30 includes a patch data generation unit 31, a colorimetric value acquisition unit 32, an ink amount data correction unit 33, and a smoothing processing unit 34. Refer to the LUT 15a, target data 15b, and correspondence data 15c. Thus, the correction of the LUT 15a can be performed. In the example shown in FIG. 2, the LUT 15a, the target data 15b, and the correspondence data 15c are recorded in advance in the HDD 15 provided in the computer 10.

  The patch data generation unit 31 is a module that acquires ink amount data, generates the patch data based on the ink amount data, and outputs the patch data to the PRTDRV 21. That is, the patch data generation unit 31 acquires specific ink amount data, and generates patch data indicating a patch having a predetermined area. When this patch data is output to the PRTDRV 21, a patch is printed, and this patch becomes a color measurement target. Here, the ink amount data acquired by the patch data generation unit 31 is recorded in the ink amount data defined in the LUT 15a, the ink amount data generated by the correction by the ink amount data correction unit 33, and the correspondence data 15c. This is ink amount data for specifying a color.

  Of the ink amount data acquired by the patch data generation unit 31, the ink amount data defined in the LUT 15a is the ink amount data defined in the LUT 15a and the correction target, and is shown in FIG. In the example, CMYKlclm data is associated with gray sRGB data. The ink amount data generated by the correction by the ink amount data correcting unit 33 is ink amount data generated at each repetition stage when the correction by the ink amount data correcting unit 33 is repeated as will be described later. The ink amount data for specifying the color recorded in the correspondence relationship data 15c is the ink amount for specifying the color of the patch to be measured when the correspondence relationship data 15c is created as will be described later.

The colorimetric value acquisition unit 32 is a module that acquires the colorimetric values output from the colorimeter 40 via the USB I / O 14. That is, the color meter 40 measures the color of the patch printed by the printing device 12 using the patch data generated by the patch data generation unit 31. In the present embodiment, the colorimeter 40 outputs the color measurement result as an L ^* a ^* b ^* value, and data indicating the L ^* a ^* b ^* value is input to the computer 10 via the USB I / O 14.

  The colorimetric value of the patch printed by the ink amount data specifying the color recorded in the correspondence relationship data 15c is associated with the ink amount data and becomes correspondence relationship data 15c. The color measurement value of the patch printed by the ink amount data defined in the LUT 15a and the ink amount data generated by the correction by the ink amount data correction unit 33 is transferred to the difference vector calculation unit 33a of the ink amount data correction unit 33. It is. This colorimetric value corresponds to the vector M shown in FIG.

The ink amount data correction unit 33 includes a difference vector calculation unit 33a, a difference vector calculation unit 33b, and an interpolation calculation unit 33c. The difference vector calculation unit 33a acquires a colorimetric value from the colorimetric value acquisition unit 32 as described above, and indicates a difference vector (vector M-vector) indicating a difference from the predicted L ^* a ^* b ^* value (vector T). T) is calculated. The ink amount data correction unit 33 repeats the correction as described above, but in the first correction for a certain correction target, the ideal value of the target data 15b recorded in the HDD 15 (the vector T0 shown in FIG. 1 above). Corresponds to the vector T, and the L ^* a ^* b ^* value after prediction is the vector T in the second and subsequent corrections. Further, the difference vector calculated at each correction stage is recorded in a memory (not shown).

  Of course, since the colorimetric value is obtained for a patch printed with the ink amount data defined in the LUT 15a or data obtained by correcting the ink amount data, a target corresponding to the original ink amount data to be corrected is extracted. Thus, a difference vector is calculated for each color to be corrected. That is, the ideal value corresponding to the ink amount data defined in the original LUT 15a is extracted from the target data 15b indicating the ideal values of a plurality of colors, and the difference vector is calculated.

The correction vector calculation unit 33b is a module that extracts the difference vector and target data (vector T0) to be corrected and calculates a correction vector C. In the present embodiment, the vector indicating the L ^* a ^* b ^* value after the prediction is expressed as a vector T0 + vector C. The L ^* a ^* b ^* value after the prediction is input to the interpolation calculation unit 33c.

The interpolation calculation unit 33c is a module that performs interpolation calculation with reference to the correspondence data 15c, and calculates ink amount data corresponding to the input L ^* a ^* b ^* value after prediction. The calculated ink amount data is ink amount data generated by the correction by the ink amount data correcting unit 33 described above. Therefore, one correction is performed by the above processing. The ink amount data correction unit 33 can repeatedly perform such correction. In the repetition process, when the magnitude of the difference vector becomes equal to or less than the threshold value αi, the correction is terminated, The LUT 15a is overwritten with ink amount data corresponding to the predicted L ^* a ^* b ^* value.

  In the embodiment shown in FIG. 2, further correction can be performed on the LUT 15a overwritten as described above. As a further correction, the smoothing processing unit 34 is a module that performs correction so that a change in output color due to a plurality of correction target ink amount data becomes smooth. The smoothing processing unit 34 obtains ink amount data to be corrected and ink amount data that outputs a color close to the output color at the ink amount from the LUT 15a, and evaluates whether the change in the output color is smooth. An evaluation function is calculated.

  Then, the process of calculating the value of the evaluation function by changing the ink amount data to be corrected is repeated to converge the evaluation function to a small value. This evaluation function is calculated by obtaining a predetermined smoothing parameter 15e. The smoothing processing unit 34 only needs to acquire ink amount data having a small evaluation function value by varying the ink amount data, and various algorithms can be employed.

(3) Color Conversion Table Correction Processing Next, the correction processing executed in the above color conversion table correction device will be specifically described along the flowchart shown in FIG. The flow shown in the figure is a processing flow of the LUT correction program 30, and the processing shown in FIG. 3 is executed in a state where the LUT 15a and the target data 15b are created in advance and recorded in the HDD 15. In this process, correspondence data 15c is created before correction (step S100).

  In step S100, the patch data generation unit 31 generates patch data. This patch data is a specific color such as gray and has more colors than the gray color defined in the LUT 15a. FIG. 4 shows a processing example when specifying the patch data. FIG. 4A is a diagram showing the sRGB data defined in the LUT 15a in an orthogonal RGB space. That is, since the value range of the sRGB data is the same for each color component value, the values that the sRGB data can take form a cube indicated by a broken line in the orthogonal RGB space.

  Arbitrary sRGB data is represented by lattice points that exist in the cube and each color component value is an integer. In this cube, gray data corresponds to grid points on a straight line KW connecting the origin K and the vertex W of the cube. In the LUT 15a, sRGB data and corresponding CMYKlclm data obtained by combining values obtained by dividing each color component value range into approximately 16 parts and corresponding CMYKlclm data are registered, so that there are 17 grid points including both ends on the straight line KW.

  FIG. 4 shows a state in which a cube in the RGB space is cut to form a plane P including the straight line KW in order to explain how the number of grid points in the RGB space increases around the 17 grid points. FIG. 4B shows a state in which the plane P is projected onto the RG plane and the lattice points on the plane P are connected by straight lines parallel to the R axis and the G axis. In the operation in step S100, lattice points are extracted more densely than the lattice points shown in FIG. 4B on and around the straight line KW.

  FIG. 4C is an enlarged view showing the vicinity of both ends of the straight line KW in FIG. 4B. In FIG. 4C, grid points corresponding to the sRGB data registered in the LUT 15a are indicated by black circles, and straight lines connecting the grid points when each color component value range is divided into approximately 52 parts are indicated by solid lines. That is, FIG. 4C shows an example in which sRGB data corresponding to lattice points when each color component value range is divided into approximately 52 equal parts in step S100 is extracted on and around the straight line KW.

  If the grid points are extracted from the periphery of the straight line KW in this way, the grid points on the straight line KW and within a certain distance from the straight line KW are extracted and used as sRGB data for creating the correspondence data 15c. Good. FIG. 4C shows an example in which grid points within a broken line at a certain distance from the straight line KW are extracted. For example, white circles are extracted on the R axis.

  When a grid point finer than the grid point corresponding to the sRGB data registered in the LUT 15a is extracted, the ink amount data corresponding to the grid point is calculated with reference to the LUT 15a. The patch data generation unit 31 creates patch data specifying a color based on the calculated ink amount data, and outputs the patch data to the PRTDRV 21. As a result, the printing apparatus 12 prints a plurality of patches. When the patch is printed, colorimetry is performed by the colorimeter 40, and the colorimetric value is acquired by the colorimetric value acquisition unit 32. Since the ink amount data used for printing the patch is known in advance, the ink amount data is associated with the colorimetric value. The data created as a result is the above-described correspondence data 15c.

  In the printing and colorimetry operations described above, the same operation is performed for the black circle lattice points on the straight line KW, that is, the lattice points corresponding to the sRGB data registered in the LUT 15a. The colorimetric value obtained by this work is used as the initial value of the vector M described above. In creating the correspondence data 15c, sRGB data corresponding to lattice points when each color component value range is divided into approximately 52 parts is considered. However, when printing and colorimetry are actually performed, straight lines are considered. Since only KW and surrounding grid points are extracted, the number of patches does not increase so that printing and colorimetry operations cannot be performed.

As described above, when the creation of the correspondence data 15c and the initial value acquisition process of the vector M are performed in the process of step S100, the correction process of the LUT 15a is performed in and after step S105. For this purpose, first, a number N indicating the number of corrections is initialized (step S105). Then, an L ^* a ^* b ^* value that will eliminate the difference between the ideal value and the colorimetric value is predicted according to the following equation (1) (step S110).

Here, the vector T N + 1 indicates the number of corrections N + 1 and the vector Ti indicates the L ^* a ^* b ^* value after prediction at the correction number i. The vector T0 is registered in the target data 15b as described above. L ^* a ^* b ^* value. The vector Mi is the colorimetric value of the patch at the correction number i, while the vector M0 is the colorimetric value of the patch printed with the ink amount data registered in the LUT 15a.

The expression (1) shown in FIG. 3 is an expression for performing the (N + 1) th prediction in consideration of the difference before the Nth correction, and the correction vector of the second term with respect to the target vector T0 shown in the first term. It is a formula that performs prediction by adding C. Here, the correction vector C is calculated from the difference vector (vector M-vector T) between the colorimetric value of the patch and the ideal value or the corrected value (predicted value) before the (N-1) th time. That is, the difference vector calculation unit 33a acquires the colorimetric value and the target data 15b or the corrected L ^* a ^* b ^* value before the N-1th time to create a difference vector, and the correction vector calculation unit 33b C is calculated, thereby calculating the value of the above equation (1).

  For example, in the first prediction, the inverse vector of the difference vector (vector M-vector T) between the colorimetric value of the patch and the target data 15b becomes the correction vector C. In the second and subsequent times, the inverse of the difference vector in the second term. A correction vector C is obtained by multiplying the vector by a weight corresponding to the reciprocal of the magnitude of the previous difference vector and adding it. Such a correction vector is a difference vector indicating a difference between the ideal value and the colorimetric value, or a difference between the colorimetric value and a value in a device-independent color space indicating a color corrected by the ink amount data correcting unit. It can be said that the difference vector is calculated from a vector having a reverse component of the difference vector.

  After performing the prediction as described above, the interpolation calculation unit 33c refers to the correspondence data and calculates the ink amount data corresponding to the obtained predicted vector T N + 1 (step S115). The ink amount data calculated here is the corrected ink amount data. When the ink amount data is calculated, the patch data generation unit 31 acquires the calculated ink amount data, generates patch data, and outputs the patch data to the PRTDRV 21 (step S120). As a result, the printing apparatus 12 prints the patch whose color is specified by the corrected ink amount data.

  When the patch is printed, the colorimetric value acquisition unit 32 acquires a colorimetric value that is a colorimetry result obtained by the colorimeter 40 (step S125). Here, the acquired colorimetric value is an accurate value of the color output based on the corrected ink amount data, and is expressed as a vector MN + 1. Therefore, in order to compare this colorimetric value with the ideal value of the color to be output by the target data 15b, that is, the converted ink amount data referring to the LUT 15a, the difference vector (vector T0−vector MN + 1) is large. Is calculated. Then, it is calculated whether or not the magnitude of the difference vector is equal to or smaller than a predetermined threshold value αi (for example, color difference 1) (step S130).

  If it is not determined in step S130 that the magnitude of the difference vector is equal to or smaller than the predetermined threshold value αi, the number N indicating the number of corrections is incremented (step S140), and steps S110 to S130 are repeated. If it is determined in step S130 that the magnitude of the difference vector is equal to or smaller than the predetermined threshold value αi, the color printed with the corrected ink amount data and the color registered in the target data 15b are very close. The LUT 15a is updated with the corrected ink amount data (step S145). Even if it is not determined in step S130 that the size of the difference vector is equal to or smaller than the predetermined threshold value αi, if the difference vector repeated up to the Nth time has converged (step S135), the size of the difference vector is the largest. The LUT 15a is updated with the ink amount data after the Mth (M <N) th interpolation, which is smaller (step S145). Then, it is determined whether or not the update has been completed for all the ink amount data to be corrected (step S150), and the processes in and after step S105 are repeated until it is determined that the update has been completed for all.

FIG. 5 is an explanatory diagram for explaining how correction is performed in steps S110 to S130 described above. This figure shows a state in which coordinates in the L ^* a ^* b ^* space are projected onto the a ^* b ^* plane. Also, the coordinates specified by the vector T are shown as black circles, and the coordinates specified by the vector M 1 are shown as white circles. A vector T0 indicating the color value registered in the target data 15b is an ideal value, and is a fixed value for each ink amount data to be corrected.

  Assuming that the colorimetric value of the patch printed with the ink amount data (data before correction) registered in the LUT 15a is the vector M0, the predicted value in the first correction is the reverse of the target vector T0 and the difference vector. A vector (vector T0−vector M0) is added. That is, there is a difference corresponding to the difference vector between the target and the colorimetric value. Therefore, if correction corresponding to the inverse vector of the difference vector is performed on the ink amount data, the corrected ink amount data This is based on the idea that the print result at the target approaches the target.

  Actually, there may be a case where the target and the colorimetric value are not very close by one correction due to an interpolation error, a colorimetric error, or the like. In FIG. 5, the vector M1 indicating the colorimetric value of the patch printed with the ink amount data corresponding to the color indicated by the predicted vector T1 in the first correction does not match the target vector T0, and the difference Shows an example where In this embodiment, the correction is repeated a plurality of times in order to eliminate such a difference, but at this time, the prediction is performed in consideration of the difference vector in the past correction, and this correction is formulated. Is the above formula (1).

  That is, the second term of Equation (1) is a linear combination of the inverse vector of the difference vector at each correction stage multiplied by a weighting coefficient that increases as the distance from the target to the colorimetric value decreases. . Specifically, in order to calculate the vector T2 in FIG. 5, the distance d0 of the difference vector (vector M0−vector T0) and the distance d1 of the difference vector (vector M1−vector T0) are calculated, and the reciprocal of both is calculated. The closer the distance, the larger the value. Each reciprocal is normalized by a value obtained by adding the reciprocal of each distance ((1 / d0 + 1 / d1) in FIG. 5) so that each coefficient does not exceed 1.

  According to this equation, the closer the point indicated by the color measurement value vector MN and the point indicated by the target vector T0 are, the more the inverse vector of the difference between them can contribute to the calculation of the correction vector C. In other words, the closer the colorimetric value is to the target, the more likely it is to be able to accurately predict the output color based on the corrected ink amount data, so the closer the colorimetric value is to the target, the greater the contribution of the difference vector. Thus, the output color based on the ink amount data can be brought close to the target quickly.

  On the other hand, the reason for considering other difference vectors without considering only the difference vector with the smallest difference is to prevent vibration and divergence during the repetition of correction. That is, when only the difference between the color after the N-th correction and the colorimetric value for the ink amount data is taken into consideration in the N-th correction, vibration and divergence can occur as described above. For example, when the colorimetric value corresponding to the ink amount data corresponding to the vector TN in FIG. 5 is substantially the same as the vector MN (M is a natural number smaller than N), the same correction is repeated without considering the past history. End up.

  In addition, when the difference is increased by the N-th correction due to an interpolation error, a colorimetric error, or the like, the influence of the N-th correction is large unless the past history is taken into consideration. However, this causes a situation where the colorimetric value does not approach the target. However, in the present embodiment, since the above linear combination is performed including not only the difference vector in the Nth correction but also the difference vector in the previous correction, the above vibration and divergence can be effectively prevented. The output color based on the ink amount data can be brought close to the target quickly.

  After updating the ink amount data to be corrected as described above, the LUT 15a that can perform color conversion with high accuracy on the ink amount data to be corrected and the surrounding colors is obtained. In terms of color conversion accuracy, it is possible to perform very high-accuracy color conversion with the LUT 15a at this stage, but further, the relationship between the ink amount data to be corrected is adjusted to perform higher-quality printing. It can also be possible.

  That is, although the above-described correction enables highly accurate color conversion, the relationship between the correction target ink amount data is not taken into consideration. Therefore, further correction may be performed so that the gradation of the output color to be corrected changes smoothly. In the flowchart shown in FIG. 3, the above-described smoothing processing unit 34 performs such correction (smoothing process: step S155), and the ink amount data to be corrected in the LUT 15a is updated with the data after the smoothing process, and the HDD 15 is updated. (Step S160), and the correction processing of the color conversion table is completed.

(4) Other Embodiments The LUT 15a created as described above can be used in a printing process that is performed on a general-purpose computer. Accordingly, it can be said that a print control apparatus that performs print processing using the LUT 15a also uses the present invention. FIG. 6 is a block diagram illustrating a configuration example of a computer that uses the LUT 15a during printing. The computer 110 is a general-purpose personal computer, and a PRTDRV 210, an input device driver (DRV) 220, and a display driver (DRV) 230 are incorporated in the OS 200. The display DRV 230 is a driver that controls display of image data and the like on the display 180, and the input device DRV 220 receives code signals from the keyboard 310 and the mouse 320 input via the serial communication I / O 190a and receives a predetermined signal. A driver that accepts input operations.

  The APL 250 is an application program that can execute retouching of a color image, and the user can operate the input device for operation under the execution of the APL 250 and cause the printing apparatus 400 to print the color image. The printing apparatus 400 is the same type of printing apparatus as the printing apparatus 12 described above. When printing such a color image, the LUT 15a created by the present invention is referred to. The image data 150a of the color image created by the APL 250 is dot matrix data in which each color component of RGB is expressed in gradation, is data compliant with the sRGB standard, and is stored in the HDD 150.

  The PRTDRV 210 is the same software as the PRTDRV 21, and includes an image data acquisition module 210a, a color conversion module 210b, a halftone processing module 210c, and a print data generation module 210d. Further, the LUT 15 a created according to the present invention is stored in the HDD 150. When the user issues a print execution instruction when executing the APL 250, the image data 150a to be printed is acquired by the image data acquisition module 210a, and the image data acquisition module 210a activates the color conversion module 210b. The color conversion module 210b is a module that converts sRGB gradation values into CMYKlclm gradation values, performs an interpolation operation with reference to reference points defined in the LUT 15a, and converts arbitrary sRGB data into CMYKlclm data.

  When the color conversion module 210b performs color conversion to generate CMYKlclm data, the CMYKlclm data is transferred to the halftone processing module 210c. The halftone processing module 210c is a module that performs halftone processing for converting the CMYKlclm tone value of each dot and expressing it with the recording density of the ink droplets, and is a head for attaching ink at the recording density after conversion. Generate drive data. The print data generation module 210d receives the head drive data and performs a rearrangement process in which the print data generation module 210d is rearranged in the order used by the printing apparatus 400. After the rearrangement process, print data is generated by adding predetermined information such as image resolution, and is output to the printing apparatus 400 via the USB I / O 190b. The printing apparatus 400 prints the image displayed on the display 180 based on the print data.

  In this printing process, color conversion is performed with reference to the LUT 15a created according to the present invention, so that it is possible to perform color conversion with a very high accuracy for a specific color, in the case of the above embodiment, gray and surrounding colors. In addition, it is possible to output a printed matter in which gray gradation changes very smoothly. The above description is a very general-purpose printing process by the PRTDRV 210. Therefore, if the LUT 15a according to the present invention is created, the LUT 15a according to the present invention can be replaced with the LUT 15a according to the present invention without changing the hardware configuration at all. Image quality printing can be performed.

  Moreover, embodiment described above is an example for implement | achieving this invention, The structure is not restricted to the said structure. For example, in the above-described correspondence data 15c, CMYKlclm data and its colorimetric values need only be associated with each other with respect to a larger number of colors than the number of colors measured when creating the LUT 15a. Various methods can be adopted as the method.

That is, as shown in FIG. 4C, the present invention is not limited to a configuration in which lattice points at a certain distance from the straight line KW are extracted around the straight line KW. If the ideal value shown in the target data 15b is closer to the other axis than the straight line KW, a lattice point at a certain distance from the other axis is extracted around the other axis substantially parallel to the straight line KW. It is also possible to extract lattice points not in the RGB space but in the L ^* a ^* b ^* space. It is not essential that the grid points to be extracted are evenly arranged in a predetermined color space. For example, a greater number of grid points may be extracted than other colors around the color for which color conversion is to be performed with high accuracy.

Further, the LUT 15a defines the correspondence between the sRGB data and the CMYKlclm data. However, as the LUT to be corrected according to the present invention, the correspondence between the color data in the predetermined color space and the ink amount data is defined. Any configuration can be adopted. For example, it may be an LUT that defines the correspondence between the coordinate data in the L ^* a ^* b ^* space and the ink amount data. Of course, it is not essential that the ink amount data is CMYKlclm data. If the printing apparatus is capable of mounting other ink, the present invention is applied to an LUT that defines ink amount data for specifying the ink amount. It is possible.

  1 0 ... Computer, 1 1 ... Display, 1 2 ... Printing device, 15 a ... L U T, 1 5 b ... Target data, 1 5 c ... Corresponding relationship data, 1 5 d ... Gray data, 15 e ... Smoothing parameters, 2 1... PRTDRV, 3 0... LUT correction program, 3 1... Patch data generation unit, 3 2... Colorimetric value acquisition unit, 3 3. a ... difference vector calculation unit, 3 3 b ... correction vector calculation unit, 3 3 c ... interpolation calculation unit, 3 4 ... smoothing processing unit, 4 0 ... colorimeter.

Claims (7)

A specific ink amount defined in the color conversion table with reference to a color conversion table that defines the correspondence between the color data in the predetermined color space and the ink amount data indicating the ink amount of each color used in the printing apparatus A color conversion table correction device for correcting data,
Color conversion table data acquisition means for acquiring color conversion table data indicating the color conversion table;
Target data acquisition means for acquiring target data indicating ideal values of colors printed with the specific ink amount data;
Patch printing means for printing a patch by the printing apparatus according to the specific ink amount data;
Colorimetric value acquisition means for acquiring a colorimetric value obtained by measuring the printed patch;
A correction vector is calculated based on a vector having a reverse component of a difference vector indicating a difference between the ideal value and the colorimetric value within a predetermined device-independent color space, and the difference is reduced by the correction vector. Ink amount data correcting means for calculating ink amount data and correcting the specific ink amount data by the calculated ink amount data;
The ink amount data correcting unit generates a difference vector indicating a difference between the colorimetric value of the patch printed by the specific ink amount data after correction and the value of the device-independent color space indicating the corrected color. Based on repeated re-correction by comparing with a predetermined threshold,
The color conversion table correction apparatus according to claim 1, wherein the predetermined threshold value is divided into a plurality of values according to an index value relating to color brightness in the color conversion table.   The color conversion table correction apparatus according to claim 1, wherein the index value is brightness.   When the number of times of performing the re-correction reaches a predetermined number of times, the reverse component of the difference vector having the smallest difference between the ideal value and the colorimetric value among the difference vectors when the re-correction is performed. The color conversion table correction apparatus according to claim 1, wherein a correction vector calculated based on the vector is used.   2. The ink amount data correction unit, when performing the re-correction, calculates the correction vector with reference to a difference vector in a correction before the re-correction, and performs the correction. 4. The color conversion table correction device according to any one of items 1 to 3. A specific ink amount defined in the color conversion table with reference to a color conversion table that defines the correspondence between the color data in the predetermined color space and the ink amount data indicating the ink amount of each color used in the printing apparatus A color conversion table correction method for correcting data,
A color conversion table data acquisition step of acquiring color conversion table data indicating the color conversion table;
A target data acquisition step of acquiring target data indicating an ideal value of a color to be printed with the specific ink amount data;
A patch printing step of printing a patch by the printing device according to the specific ink amount data;
A colorimetric value acquisition step of acquiring a colorimetric value obtained by measuring the color of the printed patch;
A correction vector is calculated based on a vector having a reverse component of a difference vector indicating a difference between the ideal value and the colorimetric value within a predetermined device-independent color space, and the difference is reduced by the correction vector. An ink amount data correction step of calculating ink amount data and correcting the specific ink amount data by the calculated ink amount data;
In the ink amount data correction step, a difference vector indicating a difference between the colorimetric value of the patch printed by the specific ink amount data after correction and the value of the device-independent color space indicating the color after correction. Based on repeated re-correction by comparing with a predetermined threshold,
The color conversion table correction method, wherein the predetermined threshold value is divided into a plurality of values according to an index value relating to brightness of the color in the color conversion table. A specific ink amount defined in the color conversion table with reference to a color conversion table that defines the correspondence between the color data in the predetermined color space and the ink amount data indicating the ink amount of each color used in the printing apparatus A color conversion table correction program for correcting data,
A color conversion table data acquisition function for acquiring color conversion table data indicating the color conversion table;
A target data acquisition function for acquiring target data indicating an ideal value of a color printed with the specific ink amount data;
A patch printing function for causing the printing apparatus to print a patch based on the specific ink amount data;
A colorimetric value acquisition function for acquiring a colorimetric value obtained by measuring the printed patch;
A correction vector is calculated based on a vector having a reverse component of a difference vector indicating a difference between the ideal value and the colorimetric value within a predetermined device-independent color space, and the difference is reduced by the correction vector. An ink amount data correction function that calculates ink amount data and corrects the specific ink amount data by the calculated ink amount data;
In the ink amount data correction function, a difference vector indicating a difference between a colorimetric value of the patch printed by the specific ink amount data after correction and a value of a device-independent color space indicating the color after correction. Based on repeated re-correction by comparing with a predetermined threshold,
The color conversion table correction program characterized in that the predetermined threshold value is divided into a plurality of values according to an index value relating to the brightness of the color in the color conversion table. A print control device that converts color data in a predetermined color space into ink amount data indicating the amount of ink of each color used in the printing device, and causes the printing device to execute printing based on the ink amount data,
Color conversion table data acquisition means for acquiring color conversion table data indicating a color conversion table;
Target data acquisition means for acquiring target data indicating ideal values of colors printed with specific ink amount data;
Patch printing means for printing a patch by the printing apparatus according to the specific ink amount data;
Colorimetric value acquisition means for acquiring a colorimetric value obtained by measuring the printed patch;
A correction vector is calculated based on a vector having a reverse component of a difference vector indicating a difference between the ideal value and the colorimetric value within a predetermined device-independent color space, and the difference is reduced by the correction vector. Ink amount data correcting means for calculating ink amount data and correcting the specific ink amount data by the calculated ink amount data;
The ink amount data correcting unit generates a difference vector indicating a difference between the colorimetric value of the patch printed by the specific ink amount data after correction and the value of the device-independent color space indicating the corrected color. Based on repeated re-correction by comparing with a predetermined threshold,
The print control apparatus according to claim 1, wherein the predetermined threshold is divided into a plurality of values according to an index value relating to color brightness in the color conversion table.

Images