JP2018082374A - Color conversion table creation method and color conversion table creation system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a color conversion table creation method capable of preventing gradation degradation of a color conversion table.SOLUTION: A method of creating a color conversion table used for converting the device color value into an ink amount includes a step of receiving input of colorimetric data, i.e., the colorimetric results of a colorimetric chart printed using a temporary color conversion table, a step of calculating the device color value capable of obtaining a desired color value from the colorimetric data, a step of calculating the reliability of the device color value based on the density of the colorimetric data in the color space, a step of determining an interpolation method performing interpolation calculation of the number values of a correction table created from the device color values, based on the value of the reliability, a step of calculating the number values of the correction table by the interpolation method thus determined, and a step of creating a true color conversion table, by correcting the temporary color conversion table by using the correction table.SELECTED DRAWING: Figure 7

Description

  The present invention relates to a color conversion table creation method and a color conversion table creation system.

  Conventionally, as described in, for example, Patent Document 1, in the color conversion table creation, the maximum value of the primary color values is determined, and a one-dimensional table for each color is created using interpolation calculation processing with the limitation. However, a technique for performing one-dimensional color value correction using the one-dimensional table has been known.

Special table 2013-509059

  However, since the color conversion table creation method described in Patent Document 1 does not consider the error of the interpolation calculation, the one-dimensional table is created in a state where the interpolation value output by the interpolation calculation is deviated from the true value. Sometimes. Therefore, when a color conversion table created based on the one-dimensional table is used at the time of printing, a difference occurs between the color value of the printed color and the true color value (decrease in color reproducibility) and the level. There was a problem that tone deterioration occurred.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

[Application Example 1]
In order to achieve the above object, a color conversion table creation method of the present invention is a method of creating a color conversion table used for converting a device color value into an ink amount, and uses the provisional color conversion table. A colorimetric data input step for receiving input of colorimetric data, which is a colorimetric result of the colorimetric chart printed in this manner, and a device color for calculating the device color value from which the desired color value is obtained from the colorimetric data A value calculation step, a reliability calculation step for calculating the reliability of the device color value based on the sparse density of the colorimetric data in a color space, and a correction created from the device color value based on the value of the reliability An interpolation method determination step for determining an interpolation method for performing an interpolation operation on the numerical value of the table, and an arithmetic operation for calculating the numerical value of the correction table by the determined interpolation method. And a correction table calculation step of generating a true color conversion table by correcting the temporary color conversion table using the correction table. .

  According to this application example, the reliability of the calculated device color value can be calculated, and the color conversion table can be corrected according to the reliability value. For this reason, for example, for device color values with low reliability, by correcting the color conversion table so as to reduce the degree of influence on the correction of the color conversion table and creating the color conversion table, it is possible to prevent deterioration in color reproducibility, or , Gradation deterioration can be prevented.

[Application Example 2]
In the color conversion table creation method described in the application example, it is preferable that the interpolation method is determined by comparing the reliability value with a predetermined threshold value.

  According to this application example, the reliability value for determining the interpolation method can be accurately classified by the predetermined threshold value. Therefore, the interpolation method of the correction table can be reliably determined according to the reliability value.

[Application Example 3]
In the color conversion table creation method described in the application example, the interpolation method is performed by comparing any one of the average value, the maximum value, the minimum value, and the median value of the reliability with the predetermined threshold value. It is preferable to determine.

  According to this application example, the reliability value of a plurality of device color values is accurately determined by comparing the representative value (average value, maximum value, minimum value, or median value) with a predetermined threshold value. Can be divided into Therefore, the correction table interpolation method can be reliably determined according to the overall tendency of the reliability of the plurality of device color values.

[Application Example 4]
In the color conversion table creation method described in the application example, it is preferable to use approximate curve interpolation or spline curve interpolation as the interpolation method.

  According to this application example, approximate curve interpolation or spline curve interpolation can be selected as an interpolation method determined according to the reliability value. As a result, by selecting approximate curve interpolation, the degree of influence of the calculated device color value on the interpolation result can be reduced. Further, by selecting spline curve interpolation, the degree of influence of the calculated device color value on the interpolation result can be increased. Therefore, the degree of influence of the calculated device color value on the interpolation result can be controlled based on the reliability value.

[Application Example 5]
The color conversion table creation system of the present invention includes a colorimetric data input unit that receives input of colorimetric data that is a colorimetric result of a colorimetric chart printed using a temporary color conversion table, and the colorimetric data. A device color value calculation unit for calculating a device color value for obtaining a desired color value; a reliability calculation unit for calculating a reliability of the device color value based on a sparse density of the colorimetric data in a color space; and An interpolation method determining unit that determines an interpolation method for performing an interpolation operation on a numerical value of a correction table created from the device color value based on a reliability value, and a correction table for calculating the numerical value of the correction table by the determined interpolation method A color conversion table that creates a true color conversion table by correcting the provisional color conversion table using a calculation unit and the correction table. Characterized in that it comprises a Le correction processing unit.

  According to this application example, the reliability of the calculated device color value can be calculated, and the color conversion table can be corrected according to the reliability value. For this reason, for example, for device color values with low reliability, the color conversion table is created by reducing the degree of influence on the correction of the color conversion table, thereby preventing deterioration of color reproducibility or gradation deterioration. Can be prevented.

1 is a configuration diagram illustrating an example of a printing system including a printing apparatus to which a color conversion table (LUT) according to the present invention is applied. It is the figure which showed a part of color conversion table. It is the figure which expressed the color conversion table in three dimensions. It is the figure which showed the correlation with the RGB value and CMY value in a three-dimensional color conversion table. It is a block diagram which shows an example of the color conversion table preparation system to which this invention is applied. It is a block diagram which shows an example of a color conversion table correction | amendment part. It is the figure which showed the process which converts RGB image data into CMY image data. It is a figure which shows the correction table which matched the RGB value before correction | amendment, and the RGB value after correction | amendment. It is a figure which shows an example of the color conversion table after correction | amendment. It is a flowchart which shows an example of the color conversion table preparation method. It is a figure which shows an example of a temporary color conversion table. It is a figure which shows the calculation method on the color value space in the interpolation process B. It is a figure which shows the calculation method on RGB space in the interpolation process B. It is a figure which shows the relationship between the magnitude | size of an area in the interpolation process B, and an interpolation error. It is a figure which shows the relationship between the magnitude | size of an area in the interpolation process B, and an interpolation error. It is a figure which shows the example which matched the correction table, the said reliability, and its average value. It is the figure which expressed the approximated curve represented by the two-dimensional coordinate graph about the response | compatibility of R and R 'of a correction table. FIG. 6 is a diagram illustrating a correspondence between R and R ′ in the correction table, which is represented by a two-dimensional coordinate graph and includes a spline curve. It is the figure which showed the method of calculating | requiring R50 and R200 about the correspondence of R and R 'of a correction table with a spline curve on a two-dimensional coordinate graph. It is a figure which shows the result of having calculated | required R50, G50, B50, R200, G200, B200 based on the interpolation process C. It is a figure showing the result of having calculated | required the gradation value which the color conversion table does not hold | maintain in the interpolation process A. FIG. It is a figure which shows the true color conversion table correct | amended based on the correction table.

(Embodiment)
Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Overview of printing system)
FIG. 1 is a configuration diagram illustrating an example of a printing system 100 including a printing apparatus 109 to which a color conversion table (LUT) 108 according to the present invention is applied. A printing apparatus 109 illustrated in FIG. 1 performs printing based on image data that has been subjected to color conversion processing using the color conversion table 108.

  As shown in FIG. 1, the printing system 100 includes a host computer 101 and a printing apparatus 109 that is a color inkjet printer.

  The host computer 101 is a host device of the printing apparatus 109 that issues a printing instruction to the printing apparatus 109, and is configured by a personal computer, for example. Accordingly, the host computer 101 is not shown, but includes a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), a HDD (Hard Disc Drive), a display (display device), and an operation device (display device). Etc.).

  As shown in FIG. 1, the host computer 101 includes an application unit 102 and a printer driver unit 103 as functional configurations.

  The application unit 102 is a print request source for the printing apparatus 109. When a print request is made, the application unit 102 instructs the printer driver unit 103 to perform image data to be printed (hereinafter referred to as original image data) by an instruction operation on the host computer 101 or the like. ). The original image data is data in a standard format (for example, a format conforming to GDI (Graphical Data Interface)) in which print targets such as characters and graphics are expressed in object units. Further, in the original image data, the color of the image is expressed by RGB (Red, Green, Blue) multi-value (for example, 256 gradations of 0 to 255) as an example. Note that the application unit 102 includes a program that describes processing contents, a CPU that executes processing according to the program, a RAM, and the like.

  The printer driver unit 103 is a part responsible for a driver function for the printing apparatus 109, performs various processes on the original image data output from the application unit 102, generates print data for the printing apparatus 109, and performs the printing Data is transmitted to the printing apparatus 109. The printer driver unit 103 includes a driver program describing processing contents, a CPU that executes processing according to the program, various data used for processing, and a memory that stores the driver program and various data.

  As shown in FIG. 1, the printer driver unit (control unit) 103 includes a rendering unit 104, a color conversion unit 105, a halftone processing unit 106, a command generation unit 107, and a color conversion table 108 as functional configurations.

  The rendering unit 104 performs a rendering process on the original image data output from the application unit 102, and converts the original image data into image data in units of pixels. That is, the rendering unit 104 converts the original image data into image data in which each pixel has a multi-value (for example, 256 gradations from 0 to 255) of device color values (in this example, RGB). The pixel unit image data is hereinafter referred to as RGB image data.

  The color conversion unit 105 converts the RGB image data generated by the rendering unit 104 into color data (hereinafter, CMY (Cyan, Magenta, Yellow) as an example) used by the printing apparatus 109 (hereinafter, CMY (Myan, Magenta, Yellow)). , Which is referred to as CMY pixel data). The CMY pixel data is ink amount data in which each pixel has CMY multivalue (for example, 256 gradations of 0 to 255). Therefore, the CMY pixel data is data representing the ink amount of each ink in the printing apparatus 109.

  The halftone processing unit 106 is a part that executes halftone processing for converting image data in units of pixels into image data expressed by the presence or absence of printing dots. In the present embodiment, the halftone processing unit 106 converts CMY pixel data into data expressed by the presence or absence of each CMY print dot (hereinafter referred to as dot data). The print dots are dots formed on the print medium (paper) by ejecting ink from the printing apparatus 109. As an example, a plurality of dots having large, medium, and small sizes are used. In this case, the dot data includes information representing the presence / absence of a large dot, the presence / absence of a medium dot, and the presence / absence of a small dot for each color for each position of the print medium on which ink is ejected.

  The command generation unit 107 is a part that expresses a print request including image data to be printed with a command for the printing apparatus 109. The print request generated by the command generation unit 107 is transmitted from the host computer 101 to the printing apparatus 109 as print data. When the command generation unit 107 generates a print request (print data), the dot data is expressed by a command for the printing apparatus 109 and included in the print data.

  The color conversion table 108 is a table that stores data for converting the color representation of the image data output from the application unit 102 into the color representation of the ink color used in the printing apparatus 109, that is, the color conversion unit. 10 is a table for color conversion processing by 105; In the present embodiment, the color representation in the RGB color space is converted to the color representation in the CMY color space.

  The color conversion table 108 expresses (R, G, B) which is a color value of three-dimensional data expressed by 8 bits (256 gradations) of each RGB color by 8 bits (256 gradations) of each color of CMY. Information assigned to (C, M, Y) which are color values of the three-dimensional data. Therefore, when the color conversion table 108 is used, an arbitrary color expressed in RGB can be expressed in CMY. However, if all the colors (about 16.77 million colors) expressed by 8 bits (256 gradations) of each color have corresponding data, the amount of data becomes enormous. 108 includes data only for colors in which each color is expressed by a predetermined number n of gradations smaller than 256 gradations.

  FIG. 2 is an example showing a part of the color conversion table 108 expressed in a two-dimensional table format. In FIG. 2, the left three columns are RGB values (device colors) of the RGB image data, and the right three columns are CMY values (ink amounts) corresponding to the RGB values in the CMY image data. In the color conversion table 108 expressed in a tabular format, the number of rows indicating gradation values is the third power of the gradation number n. The color conversion unit 105 determines a CMY value as converted data using the RGB value as a primary key in the color conversion table 108. Specifically, for example, when RGB = (50, 50, 50) is input to the color conversion unit 105, the row (2 from the top) in which the RGB value is (50, 50, 50) in the color conversion table 108. CMY values (200, 200, 200) of the (line) are determined as output values (converted data).

  FIG. 3A is a diagram representing the color conversion table in three dimensions. The example shown in FIG. 3A shows a case where each color of RGB is expressed by 18 gradations (n = 18). FIG. 3B shows CMY values associated with the eight vertices (A, B, C, D, E, F, G, H) of the unit cube of FIG. 3A. In this color conversion table, CMY values (C, M, Y) are assigned to grids (R, G, B) obtained by dividing 17 axes of RGB in the RGB color space.

  When the color to be converted is not on the grid, the color conversion unit 105 calculates a corresponding (C, M, Y) value using an interpolation method such as “tetrahedral interpolation”. Interpolation processing for calculating (C, M, Y) values not on the grid of the color conversion table using an interpolation method such as “tetrahedral interpolation” is referred to as interpolation processing A. In this case, for example, when color conversion is performed on the color indicated by the point O in FIG. 3A, the unit cube is associated with eight grid points (A to H) including the point O (C, M, and H). The (C, M, Y) value for the O point can be obtained from the (Y) value (value shown in FIG. 3B).

  The color conversion table 108 is stored in a ROM, HDD, or the like.

  The printing apparatus 109 is a color inkjet printer that executes a printing process in accordance with a printing instruction from the host computer 101. The printing apparatus 109 performs printing by synchronizing the operation in the main scanning direction of the print head 116 provided with the nozzles that eject ink onto the printing medium and the operation in the sub-scanning direction of the paper that is the printing medium. Reciprocal printing for ejecting ink in both directions of the operation 116 is performed.

  As shown in FIG. 1, the printing apparatus 109 includes a controller unit 110 and a printing mechanism unit 112.

  The controller unit 110 includes a program describing processing contents, a CPU that executes processing according to the program, a RAM, a ROM that stores the program, an ASIC, and the like. As a functional configuration, a print control unit 111 is provided as shown in FIG.

  The print control unit 111 is a part that receives print data according to a print instruction and causes the print mechanism unit 112 to execute print processing according to the print data. The print control unit 111 controls each of the main scanning unit 113 and the sub scanning unit 117.

  The printing mechanism unit 112 is a part that executes a printing process on a print medium (paper) in accordance with an instruction from the controller unit 110. As illustrated in FIG. 1, the printing mechanism unit 112 includes a main scanning unit 113 and a sub scanning unit 117. Note that the printing control unit 111 and the printing mechanism unit 112 surrounded by a broken line in FIG. 1 are referred to as a printing unit 119.

  The main scanning unit 113 includes a print head 116 that includes nozzles that eject CMY inks, a carriage 115 that includes the print head 116 and moves in the main scanning direction, a carriage motor 114 that drives the carriage 115, and the like. Is provided.

(Configuration of color conversion table creation system)
Next, the color conversion table creation system 300 will be described with reference to FIGS. 4A to 6B.

FIG. 4A is a configuration diagram showing an example of a color conversion table creation system 300 to which the present invention is applied. The color conversion table creation system 300 includes a color conversion table creation unit 301, a color conversion table correction unit 302, and a profile creation unit 303. The color conversion table creation unit 301 creates a temporary color conversion table (FIG. 2). The color conversion table correction unit 302 creates a correction table (FIG. 6A), corrects the temporary color conversion table based on the created correction table, and creates a true color conversion table (FIG. 6B). The profile creation unit 303 creates a profile based on the true color conversion table. A profile is a table that associates device color values with color values. The processing of the color conversion table creation system 300 is completed after the processing of the color conversion table creation unit 301, the processing of the color conversion table correction unit 302, and the processing of the profile creation unit 303.
The color conversion table creation unit 301 is a part responsible for creating a color conversion table, and for example, associates RGB values with CMY values. The color conversion table correction unit 302 is a part responsible for correcting the color conversion table. For example, the color conversion table correction unit 302 changes the association between RGB values and CMY values so that the relationship between RGB values and color values is linear. The profile creation unit 303 is a part responsible for creating a profile of the color conversion table. For example, the profile creation unit 303 creates a profile so that a certain RGB value becomes a desired color value.

  FIG. 4B is a configuration diagram illustrating an example of a system of the color conversion table correction unit 302 in FIG. 4A. A colorimetric data input unit 304 that accepts input of colorimetric data that is a colorimetric result of a colorimetric chart printed using a temporary color conversion table, and a device color that can obtain a desired color value from the colorimetric data A device color value calculation unit 305 that calculates a value, a reliability calculation unit 306 that calculates a reliability of the device color value based on the sparse density of the colorimetric data in the color space, and a device color value based on the reliability value An interpolation method determining unit 307 that determines an interpolation method for performing an interpolation calculation on the numerical value of the correction table to be created, a correction table calculating unit 308 that calculates a numerical value of the correction table using the determined interpolation method, A color conversion table correction processing unit 309 that generates a true color conversion table by correcting the color conversion table;

  FIG. 5 is a diagram schematically illustrating a process in which the color conversion unit 105 converts the RGB image data 400 into the CMY image data 401 based on the color conversion table 108. When the RGB image data 400 is input to the color conversion unit 105, the color conversion unit 105 outputs CMY image data 401 with reference to the color conversion table 108.

  In the color conversion table 108, when the RGB value and the CMY value do not have a desired correspondence relationship, when a CMY image converted based on the color conversion table 108 is printed, a desired color value cannot be obtained. Therefore, color reproducibility is lowered and gradation is deteriorated. Therefore, by creating a correction table and correcting the correspondence relationship between the RGB values and the CMY values of the color conversion table to a desired correspondence relationship, it is possible to prevent a decrease in color reproducibility and to prevent occurrence of gradation deterioration.

Next, processing for correcting the color conversion table 108 using the correction table will be described.
FIG. 6A is an example of a correction table in which RGB values (device colors) before correction are associated with RGB values (device colors) after correction. 6A shows the RGB values (device color) before correction, and the right three columns show the RGB values (device color) after correction, which are in one-to-one correspondence for each row. . In the correction table, color values (R, G, B) of each one-dimensional data expressed by 8 bits (256 gradations) of each color of R, G, or B are set to R ′, G that match a desired characteristic. Information to be assigned to 'or B' (R ', G', B ') is provided.
Since the correction table is a table for correcting the color conversion table 108, the correction table includes data having the same number of gradations n as the color conversion table 108.

  6B is an example in which the color conversion table 108 of FIG. 2 is corrected based on the correction table of FIG. 6A (true color conversion table). FIG. 6B is a table similar to FIG. 2, and the left three columns are RGB values (device colors), and the right three columns are CMY values (ink amounts). The color conversion table 108 in FIG. 6B is created by referring to the correction table in FIG. 6A and changing the color conversion table 108 in FIG. As described above, the correction table is a table that stores data for correcting the color conversion table 108 of FIG. 2 in accordance with desired color value characteristics.

  Specifically, for example, when RGB = (50, 50, 50), which is an RGB value before correction, is input to the correction table (see FIG. 6A), RGB ′ = (100, 100, 100) is obtained. When the row of RGB = (100, 100, 100) is searched from the color conversion table 108 in FIG. 2, CMY = (150, 150, 150) is obtained. Based on the above processing, if RGB = (50, 50, 50) of the RGB value (device color) before correction is input, the correspondence relationship is changed so that CMY = (150, 150, 150) is obtained. A new color conversion table 108 can be created. That is, the color conversion table 108 (see FIG. 2) that should have been obtained by inputting RGB = (50, 50, 50) and CMY = (200, 200, 200) is converted into RGB = (50, 50, 50). And CMY = (150, 150, 150) (this value is a CMY value obtained when RGB = (100, 100, 100) is input) can be corrected to the color conversion table 108. .

  In addition to the method of storing only the color conversion table 108 of FIG. 6B obtained by correcting the color conversion table 108 of FIG. 2 based on the correction table of FIG. 6A as described above, the correction table of FIG. 6A and the color conversion table of FIG. 108 may be stored, and the CMY value may be obtained from the RGB values before correction in the color conversion unit 105 of FIG.

The correction table is completed by printing a colorimetric chart, performing an interpolation calculation process based on the colorimetric result (colorimetric data), and calculating an RGB value for obtaining a desired color value. The interpolation process for calculating the RGB value from which a desired color value is obtained is referred to as interpolation process B.
Note that calculating RGB values that provide desired color values for all m tone values increases the number of patches and the amount of calculation of the colorimetric chart. Only the gradation value obtained by extracting a predetermined number m ′ (m ′ <m) from the RGB value for obtaining a desired color value is calculated. Hereinafter, m ′ is the number of grids created in the correction table, and the gradation value is the grid position created in the correction table.

  The color conversion table creation method according to the present embodiment in the color conversion table correction unit 302 shown in FIG. 4A is executed in the following procedure. FIG. 7 is a flowchart illustrating an example of a color conversion table creation method. This flowchart is executed using a provisional color conversion table created by a color conversion table creation method as described in Patent Document 1. FIG. 7 shows processing executed by the color conversion table correction unit 302.

  Here, by correcting the temporary gradation conversion table 108 of 6 gradations based on the correction table of 6 gradations, a value representing gray on the RGB values (all RGB values are the same value, hereinafter referred to as a gray axis) is printed. An example will be described in which a true color conversion table 108 is created in which the color values are achromatic. In addition, the number of grids created in the correction table is 4, the grid positions are 0, 100, 200, and 255, and the grid positions for calculating RGB values for obtaining desired color values are 100 and 200. In this example, the obtained color value is not changed at grid positions of 0 and 255.

  FIG. 8 shows an example of a temporary gradation conversion table 108 with 6 gradations. Using the temporary color conversion table 108, the colorimetric chart created with RGB values is converted into CMY values and printed (step S1). Further, the printing result is measured by a colorimeter to obtain a color value (step S2). The color measurement data is input to the color conversion table creation system 300 (Step S3: Color measurement data input step).

Based on the colorimetric data input in step S3, a correspondence relationship between the RGB value and the color value of the colorimetric chart is created, and an RGB value for obtaining a desired color value (achromatic color) is obtained by interpolation processing B. (Step S4: Device color value calculation step).
9A and 9B are explanatory diagrams showing an outline of the interpolation process B. FIG. The color value C0 in FIG. 9A is a desired color value, and the color values C1, C2, and C3 are three points in the colorimetric data surrounding C0. 9B is an RGB value from which C0 is obtained, and RGB1, RGB2, and RGB3 are RGB values corresponding to C1, C2, and C3. C0, C1, C2, C3, RGB1, RGB2, and RGB3 are known, and RGB0 is unknown.

  RGB0 is calculated by the following procedure. First, C0 is expressed by a vector composed of three points C1, C2, and C3 and a predetermined coefficient (a, b). If a calculation formula is shown assuming that a vector connecting from point A to point B is represented as vector (AB), vector (C1C0) = a × vector (C1C2) + b × vector (C1C3). Since C0, C1, C2, and C3 are known, the predetermined coefficients (a, b) are calculated. Next, RGB0 is expressed by a vector composed of three points of RGB1, RGB2, and RGB3 and a predetermined coefficient (c, d). The calculation formula is vector (RGB1RGB0) = c × vector (RGB1RGB2) + d × vector (RGB1RGB2). The values of RGB0 are calculated by substituting the values of a and b into the coefficients of c and d. The unknown RGB0 is calculated by the above linear calculation. Therefore, the interpolation process B is a kind of linear interpolation.

  Next, the area (sparse density) of the triangle having the vertices of three points C1, C2, and C3 used for the calculation of the interpolation process B is calculated, and the reciprocal thereof is set as the reliability of RGB0 obtained by the calculation (step S5). : Reliability calculation step). The reason why the reciprocal of the area is used as the reliability is that the smaller the area, the smaller the interpolation error (difference between the true value and the value obtained by the interpolation process B). On the contrary, if the area is large, the possibility that the interpolation error becomes large increases, and the reliability of the value obtained in the interpolation process B decreases.

Next, the relationship between the size of the area and the interpolation error is shown.
10A and 10B are explanatory diagrams used to show the relationship between the area size and the interpolation error. FIG. 10A and FIG. 10B are examples in which the point M on the curve C is obtained by linear interpolation using two points (point A and point B) on the curve C and different from the point M. Linear interpolation is a process of obtaining a point existing between two points on the assumption that the two points are connected by a straight line instead of a curve. A value obtained by linear interpolation is called an interpolation value.

  In FIG. 10A, when using point A1 and point B1 to obtain point M between them by linear interpolation, the point actually obtained is point M1 on the line segment connecting point A1 and point B1. is there. Since the point M to be originally obtained exists on the curve C and the actually obtained point M1 exists on a line segment connecting the points A1 and B1, a difference E1 is generated between the points M and M1.

  On the other hand, the points A2 and B2 on the curve C shown in FIG. 10B are closer to the point M than the points A1 and B1 shown in FIG. 10A. If an attempt is made to obtain a point M between the points A2 and B2 by linear interpolation, the actually obtained point is the point M2 on the line segment connecting the points A2 and B2. Since the point M to be originally obtained exists on the curve C and the actually obtained point M2 exists on a line segment connecting the points A2 and B2, a difference E2 is generated between the points M and M2. Since the points A2 and B2 are closer to the point M than the points A1 and B1, the difference E2 is smaller than the difference E1. A smaller difference means that it is closer to the value to be obtained.

  Therefore, when obtaining a point in the middle by linear interpolation, the closer the point group used for the calculation is to the value to be obtained, the more accurately it can be calculated. When applied to the interpolation process B, it means that the closer the point groups C1, C2, and C3 used for interpolation are, the more accurately the interpolation value can be calculated. That is, it can be said that the smaller the area of the triangle having the vertices at three points C1, C2, and C3, the more accurate and more reliable the interpolation value obtained in the interpolation process B is. Therefore, the reciprocal of the area of the triangle is used as a numerical value indicating that the interpolation value obtained in the interpolation process B is highly reliable.

FIG. 11 is an example in which an RGB value for obtaining a desired color value (achromatic color) is calculated for each of the four created grid positions, and the reliability and the average value are associated with the RGB values on the gray axis. . The left three columns in the table of FIG. 11 show the RGB values (device colors) before correction, and the fourth to sixth columns from the left show the corrected RGB values (device colors). The seventh column from the left shows the reliability of the RGB value, and is blank because the reliability cannot be calculated in a grid that does not calculate the RGB value from which the desired color value (achromatic color) is obtained. The average value of reliability (hereinafter referred to as reliability average value A) is shown in the bottom row of the column indicating reliability.
In the specific example, when the device color before correction is RGB = (0, 0, 0) and RGB = (255, 255, 255), respectively, since the change is not performed by the correction table, the device color after correction is RGB = (0 , 0, 0) and RGB = (255, 255, 255). In the device color before correction RGB = (100, 100, 100) and RGB = (150, 150, 150), since the RGB value for obtaining a desired color value (achromatic color) was calculated, the device color after correction is RGB = (90, 95, 110) and RGB = (160, 145, 140).

  In the device colors RGB before correction (RGB = (50, 50, 50)) and RGB = (200, 200, 200), since RGB values for obtaining desired color values (achromatic colors) are not calculated, R50 in FIG. G50, B50, R200, G200, and B200 are unknown values at the end of step S5. Since the correction table is finally referred to for correcting the color conversion table 108, R50, G50, RGB = (50, 50, 50) and RGB = (200, 200, 200), which are unknown values. It is necessary to determine the values of B50, R200, G200, and B200. Based on the known values RGB = (0, 0, 0), RGB = (100, 100, 100), RGB = (150, 150, 150) and RGB = (255, 255, 255), unknown values The process for determining R50, G50, B50, R200, G200, and B200, which are

  Specifically, the interpolation process C is based on RGB = (0, 0, 0) and RGB = (100, 100, 100) which are known values, and RGB = (50, 50, 50) existing therebetween. This is a process of calculating the corrected device color. In the interpolation process C, there are several methods for interpolating values existing between them. For example, a method of performing linear interpolation using a known value as an end point, a method of estimating an intervening value using an approximate curve passing through the known value, and the like can be considered.

Next, in order to determine an interpolation method in the interpolation process C, the reliability average value A (which may be the maximum value, the minimum value, or the median value of reliability) is compared with a predetermined threshold (step) S6: Part of the interpolation method determination step). The predetermined threshold value used here is a boundary value divided into a group with high reliability and a group with low reliability, and is a known value obtained by a prior evaluation test or the like.
In accordance with the comparison result, an interpolation method of interpolation processing C for interpolating a correction table composed of 4 grids into a 6-gradation correction table is determined (step S7: part of the interpolation method determination step).

  Specifically, when the predetermined threshold value is 0.20, the reliability average value A is less than the threshold value in this example when the numerical value is compared with 0.15 of the reliability average value A. It is. In this case, it is determined that the interpolation method of the correction table is an interpolation method using an approximate curve. That the reliability average value A is less than the threshold value means that the reliability of the RGB value from which the desired color value (achromatic color) obtained by the interpolation process B is obtained is low overall, This is because it is preferable to interpolate the correction table based on an approximate curve that can reduce the degree of influence on the interpolation result by the RGB value from which the color value (achromatic color) is obtained.

  FIG. 12A is a diagram showing the correspondence between R and R ′ in the correction table of FIG. 11 in a two-dimensional coordinate graph with R as the horizontal axis and R ′ as the vertical axis. R0, R100, R150, and R255 are points representing the value of the vertical axis (R ′) when the horizontal axis value (R) is 0, 100, 150, and 255, respectively. Curve C is an approximate curve created based on R0, R100, R150, and R255. Curve C draws a smooth curve that passes through two points R0 and R255 and does not pass two points R100 and R150.

  On the other hand, when the predetermined threshold value is 0.10, the reliability average value A is equal to or larger than the threshold value when the reliability average value A0.15 is compared with a numerical value. In this case, it is determined that the interpolation method of the correction table is, for example, an interpolation method by spline curve interpolation. That the reliability average value A is equal to or greater than the threshold value means that the reliability of the RGB value from which the desired color value (achromatic color) obtained by the interpolation process B is obtained is high overall, and the desired color This is because it is preferable to interpolate the correction table based on the spline curve interpolation that can increase the degree of influence on the interpolation result by the RGB value from which the value (achromatic color) is obtained.

  FIG. 12B is a diagram showing the correspondence between R and R ′ in the correction table of FIG. 11 in a two-dimensional coordinate graph with R as the horizontal axis and R ′ as the vertical axis. R0, R100, R150, and R255 are points representing the value of the vertical axis (R ′) when the horizontal axis value (R) is 0, 100, 150, and 255, respectively. A curve D represents a spline curve created based on R0, R100, R150, and R255. Curve D draws a smooth curve that passes through four points R0, R100, R150, and R255.

  Next, a correction table is created based on the determined interpolation method (step S8: correction table calculation step). A correction table composed of four grids is interpolated into a six-gradation correction table by the determined interpolation method. Specifically, when the determined interpolation method is approximate curve interpolation, an interpolation value corresponding to each gradation value when expanded to 6 gradations is calculated from the approximate curve, and the value is used as a correction table value.

  FIG. 13A is a diagram showing a method of obtaining R50 and R200 by representing the correspondence between R and R ′ in the correction table of FIG. 11 by a spline curve on a two-dimensional coordinate graph. R0, R100, R150, and R255 are points of the vertical axis value (R ′) when the horizontal axis value (R) is 0, 100, 150, and 255, respectively. A curve D represents a spline curve created based on R0, R100, R150, and R255. R50 and R200 are points on the curve D when the horizontal axis values (R) are 50 and 200, respectively. In order to obtain the vertical axis value (R ′) when the horizontal axis value is 50 or 200 in the correction table composed of four grids of R0, R100, R150 and R255, the vertical axis value of R50 and R200 ( R ′) may be obtained (interpolation process C). Specifically, R50 = 45 is obtained by calculating the vertical axis value at the horizontal axis value 50 on the curve D. Similarly, R200 = 197 is obtained by calculating the vertical axis value at the horizontal axis value 200 on the curve D. The calculation is similarly performed for G and B. As a result, a correction table composed of four grids can be created by interpolating into a six-gradation correction table.

  FIG. 13B is a correction table showing the results of obtaining R50, G50, B50, R200, G200, and B200 of FIG. 11 based on the interpolation process C. The left three columns of the table in FIG. 13B indicate the RGB values (device colors) before correction, and the right three columns indicate the RGB values (device colors) after correction. When RGB before correction = (50, 50, 50), RGB ′ after correction is (45, 47, 55), and when RGB before correction = (200, 200, 200), RGB after correction '= (197, 200, 207).

  The true color conversion table 108 is created by correcting the 6-gradation color conversion table 108 based on the created 6-gradation correction table (step S9: color conversion table creation step). Specifically, the corrected RGB ′ value in the RGB value of the gray axis is acquired with reference to the correction table, and the color conversion table 108 is referred to based on the corrected RGB ′ value. When referring to the color conversion table 108, the interpolation process A is performed to obtain gradation values that are not held in the color conversion table 108, and CMY ′ values for the corrected RGB ′ values are acquired based on the gradation values. . The true color conversion table 108 is obtained by associating the RGB values of the gray axis with the CMY ′ values.

FIG. 14A is a table showing a result obtained by interpolation processing A for gradation values not held in the color conversion table 108 shown in FIG. 6B. 14A shows the RGB values (device colors), and the right three columns show the CMY values (ink amounts). In FIG. 6B, RGB = (50, 50, 50), RGB = (100, 100, 100), RGB = (150, 150, 150), RGB = (200, 200, 200), RGB = (255, 255). 255).
On the other hand, RGB = (45, 47, 55), RGB = (90, 95, 110), RGB = (160, 145, 140), which are tone values for which CMY values (ink amounts) should be calculated based on the correction table. ), RGB = (197, 200, 207) does not exist in the color conversion table 108 of FIG. 6B. Therefore, the CMY value (ink amount) in the gradation value is calculated using the interpolation process A from the color conversion table 108 in FIG. 6B. For example, CMY = (210, 208, 200) for RGB = (45, 47, 55), CMY = (165, 160, 145), RGB = (160) for RGB = (90, 95, 110) 145, 140), CMY = (115, 110, 95), and RGB = (197, 200, 207), CMY = (58, 55, 48).

  FIG. 14B shows a true color conversion table 108 that is a result of correcting the color conversion table 108 of FIG. 14A based on the correction table of FIG. 13B. The left three columns in the table of FIG. 14B indicate RGB values (device colors), and the right three columns indicate CMY values (ink amounts). For example, RGB = (50, 50, 50) becomes RGB ′ = (45, 47, 55) with reference to the correction table of FIG. 13B. RGB ′ = (45, 47, 55) becomes CMY = (210, 208, 200) with reference to the color conversion table 108 of FIG. 14A. Therefore, the true color conversion table 108 associates CMY = (210, 208, 200) with RGB = (50, 50, 50). The true color conversion table 108 is created by performing this operation for all the gradations of the correction table.

  As described above, in the color conversion table creation method according to the present embodiment, the reliability of the device color value from which the desired color value obtained by the interpolation process B is obtained is calculated, and the color conversion table according to the reliability value. Since 108 can be corrected, it is possible to prevent a decrease in color reproducibility or to prevent gradation deterioration.

  In the present embodiment, the device color value reliability average value A from which a desired color value is obtained is determined based on a predetermined threshold value, and is described as a process for switching the interpolation method for the entire correction table. The present invention can also be applied to processing for individually switching the interpolation method of the section between the device color values according to the reliability corresponding to the device color value from which the value is obtained. Specifically, when the device color value before correction and the device color value after correction are represented by two-dimensional coordinates, coordinate values with high reliability (device color values before correction and desired color values can be obtained). For example, linear interpolation is used between the value pairs), and coordinate values with low reliability are used by interpolation using an approximate curve. For example, when using an interpolation method using an approximate curve, a weighted approximate curve calculation method may be used with reliability as a weight. When this calculation method is used, it is possible to create a smooth curve that always passes coordinate values with high reliability and does not pass coordinate values with low reliability.

  In this embodiment, the RGB value is described as an example of the device color value. However, a CMYK value (CMYK value as an image signal value, not an ink amount) may be applied as the device color value.

  The protection scope of the present invention is not limited to the above-described embodiment, but covers the invention described in the claims and equivalents thereof.

  DESCRIPTION OF SYMBOLS 100 ... Printing system 101 ... Host computer 102 ... Application part 103 ... Printer driver part 104 ... Rendering part 105 ... Color conversion part 106 ... Halftone processing part 107 ... Command generation part 108 ... Color conversion table , 109 ... printing apparatus, 110 ... controller unit, 111 ... print control unit, 112 ... printing mechanism unit, 113 ... main scanning unit, 114 ... carriage motor, 115 ... carriage, 116 ... print head, 117 ... sub-scanning unit, 118 ... Paper feed motor, 119 ... Printing unit, 300 ... Color conversion table creation system, 301 ... Color conversion table creation unit, 302 ... Color conversion table correction unit, 303 ... Profile creation unit, 304 ... Colorimetric data input unit, 305 ... Device color value calculation unit, 306... Reliability calculation unit, 30 ... interpolation method determining unit, 308 ... interpolation table calculation section, 309 ... color conversion table correction processing unit, 400 ... RGB image data, 401 ... CMY image data.

Claims (5)

A method of creating a color conversion table used to convert device color values into ink amounts,
A colorimetric data input step for accepting input of colorimetric data that is a colorimetric result of a colorimetric chart printed using the provisional color conversion table;
A device color value calculating step for calculating the device color value from which the desired color value is obtained from the colorimetric data;
A reliability calculation step of calculating the reliability of the device color value based on the sparse density of the colorimetric data in a color space;
An interpolation method determining step for determining an interpolation method for performing an interpolation operation on a numerical value of a correction table created from the device color value based on the reliability value;
A correction table calculation step of calculating a numerical value of the correction table by the determined interpolation method;
A color conversion table creating step for creating the true color conversion table by correcting the temporary color conversion table using the correction table;
A method for creating a color conversion table, comprising: In claim 1,
A method of creating a color conversion table, wherein the interpolation method is determined by comparing the reliability value with a predetermined threshold value. In claim 2,
A color conversion table creation method, wherein the interpolation method is determined by comparing any one of the average value, the maximum value, the minimum value, and the median value of the reliability with the predetermined threshold value. In any one of Claim 1 to 3,
A color conversion table creation method, wherein the interpolation method is approximate curve interpolation or spline curve interpolation. A colorimetric data input unit that accepts input of colorimetric data that is a colorimetric result of a colorimetric chart printed using a provisional color conversion table;
A device color value calculation unit for calculating a device color value from which a desired color value is obtained from the colorimetric data;
A reliability calculation unit that calculates the reliability of the device color value based on the sparse density of the colorimetric data in a color space;
An interpolation method determining unit that determines an interpolation method for performing an interpolation operation on a numerical value of a correction table created from the device color value based on the reliability value;
A correction table calculation unit for calculating a numerical value of the correction table by the determined interpolation method;
A color conversion table correction processing unit that creates the true color conversion table by correcting the temporary color conversion table using the correction table;
A color conversion table creation system comprising:

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