JP2005268994A - Color conversion table creation method, color conversion table creation device, color conversion table creation program, and color conversion table - Google Patents

Abstract

PROBLEM TO BE SOLVED: To create a color conversion table capable of realizing high-precision color reproducibility by accurately reproducing a patch even when color developability varies depending on ink.
In the color conversion table creation method according to the present invention, when reproducing a LUT creation patch, correction is performed in accordance with the color developability of each basic ink (CMY). Therefore, it is possible to prevent the hue of the LUT creation patch reproduced by the color development of each basic ink from being biased. That is, the color reproducibility of the color conversion table created by the interpolation calculation based on the color measurement result of the LUT creation patch can be improved.
[Selection] Figure 7

Description

  The present invention relates to a color conversion table creation method, a color conversion table creation device, a color conversion table creation program, and a color conversion table.

  In recent years, color ink jet printers have been widely used as image output devices. A normal color ink jet printer forms a print image on a recording medium using a plurality of types of ink having hues of cyan (C), magenta (M), and yellow (Y) in addition to black (K) ink. To do. That is, any color in the color image input to the color ink jet printer can be reproduced using these plural types of ink.

In such a color ink jet printer, arbitrary color data in a color image is input, and color conversion processing is performed that specifies the amount of a plurality of types of ink that can reproduce the same color. The correspondence between this color data and the amount of multiple types of ink is described in advance in a color conversion table (LUT: Look Up Table). By referring to the color conversion table, the ink corresponding to the input color data is stored. It is possible to acquire the quantity of (for example, refer patent document 1).
JP-A-10-91089

  It is desirable that the color conversion table faithfully reproduces the hue of the color represented by the color data of the color image. In particular, since human color perception can sensitively sense a hue shift from an achromatic color, it is desirable to faithfully reproduce the hue of a color around an achromatic color. When the correspondence relationship between the color data and the combination of each ink amount is defined, a plurality of patches (color samples) are actually reproduced, and the correspondence relationship is defined based on the color measurement result. At this time, in order to obtain good color reproducibility with respect to an arbitrary color, the plurality of patches are prepared to reproduce a plurality of colors having different hues, saturations, and lightnesses. At this time, by creating a patch close to the color for which the ink amount is to be calculated and measuring the color, it is possible to improve the accuracy of calculating the ink amount.

  By the way, there are cases where the color developability varies depending on the ink used for printing. Color development means the strength of the ink's influence on the reproduced color. For example, when an achromatic color can be reproduced by mixing a plurality of inks, it can be said that an ink with a lower mixing ratio has higher color developability than other inks. In such a patch reproduced using an ink having high color developability, the color balance may be biased to the color of the ink having high color developability. Therefore, there is a case where a patch that is not close to the color for which the ink amount is desired to be reproduced, and there is a problem that the accuracy of specifying the ink amount is lowered. In particular, since the calculation of the amount of ink with high accuracy is required in the vicinity of the achromatic color as described above, it has been a problem to reliably reproduce the patch having the color around the achromatic color.

  The present invention has been made in view of the above problems. A color conversion table capable of realizing high-precision color reproducibility even when color developability varies depending on ink, and creation of a color conversion table capable of generating such a color conversion table. It is an object to provide a method, a color conversion table creation device, and a color conversion table creation program.

Means and actions / effects for solving the problems

  In order to achieve the above object, the invention according to claim 1 is the above-described element color capable of expressing the same color based on a complementary color relationship between each element color used in the image equipment and each basic ink color used in the printing apparatus. Correspondence between the value data and the virtual basic ink value data expressed by the gradation of each basic ink can be calculated. Then, the corrected basic ink value data is calculated by multiplying the calculated virtual basic ink value data by a color correction coefficient corresponding to the color developability of each basic ink. In this way, by creating a patch based on the corrected basic ink value data that takes into account the color developability of each basic ink, a patch close to the intended color is created without being affected by the color developability of each basic ink. can do. In other words, since a patch close to the intended color can be created reliably, the calculation accuracy of the correspondence between the element color value data and the ink value data in the color conversion table creation step can be improved. Therefore, it is possible to create the color conversion table with good color reproducibility.

  In addition, the color correction coefficient can be changed according to the gradation of the virtual basic ink value data. In other words, the virtual basic ink value data is not multiplied by the color correction coefficient uniformly in all gradations. Therefore, the correction amount in the second conversion step can be changed to an appropriate amount according to the gradation of the virtual basic ink value data.

  In color measurement, it is sufficient if the color value of the print result can be acquired, and each Lab color space (L, a, b is usually marked with *, but is omitted in this specification for simplicity. The same applies hereinafter. It is only necessary to obtain colorimetric data indicating coordinates in the device-independent color space. When creating a color conversion table, color measurement data indicating coordinates in the device-independent color space and coordinates in the device-independent color space of colors indicated by element color value data used in other image devices Can be used.

  That is, if the coordinates in the device-independent color space are known for a plurality of colors, the basic ink value data of an arbitrary color and the color component values of each color used in other image devices are calculated by interpolation or the like. Therefore, it is possible to define the color conversion table by calculating the correspondence between the two for any color. Since it is necessary to obtain the coordinates in the device-independent color space for the color component values of each color used in other image devices, the sRGB standard that can calculate the coordinates in the device-independent color space using a predetermined formula It is preferable that this data. Of course, the display color on another image device may be measured.

  As an example of a specific method of changing the color correction coefficient according to the gradation of the virtual basic ink value data, the invention according to claim 2 is a color space in which the color correction coefficient covers the virtual basic ink value data. The shape of the outermost shell and the shape of the outermost shell in the color space covered by the corrected basic ink value data are varied so as to be congruent or similar. That is, the color space covered by the corrected basic ink value data can be formed without distorting the outer shape of the color space covered by the virtual basic ink value data. In this way, the corrected basic ink value data can be generated without changing the relative ratio of the gradation range of each basic ink in the corrected basic ink value data. Accordingly, when performing color separation processing, duty limiting processing, or the like in the color space of the corrected basic ink value data, color mixing or color mixing based on the virtual ink amount ratio of the basic ink in the color space of the virtual basic ink value data is performed. Even if color separation is performed, the hue balance is not greatly lost. That is, the reproduced color of the patch can be accurately maintained even with simple color separation processing and duty limit processing.

  In the invention according to claim 3, the corrected basic ink value data is represented by a tone curve function having the gradation of the virtual basic ink value data as an input gradation. That is, according to the tone curve function, the correction range corresponding to the color development of the basic ink can be increased in the intermediate gradation region, and the correction corresponding to the color development in the low gradation region and the high gradation region. The width can be reduced. Further, since correction is not possible at the lowest gradation and the highest gradation, the outermost shell of the color space of the virtual basic ink value data and the outermost shell of the color space of the corrected basic ink value data are Can be matched.

  Further, in the invention according to claim 4, the correction coefficient is represented by an Nth order function (N is an integer of 1 or more) having the gradation of the virtual basic ink value data as a variable. That is, the corrected basic ink value data obtained by multiplying the correction coefficient by the gradation of the virtual basic ink value data can be expressed by an (N + 1) degree function. That is, the corrected basic ink value data can be varied in a curved manner according to the gradation of the virtual basic ink value data. Therefore, the color space of the virtual basic ink value data is optimized by optimizing each coefficient. Can be matched with the outermost shell of the color space of the corrected basic ink value data.

  According to a fifth aspect of the present invention, there is provided a correction table that defines a correspondence relationship between the virtual basic ink value data and the corrected basic ink value data in the second conversion step. Refer to the correction table. Thus, the corrected basic ink value data is obtained from the virtual basic ink value data. By referring to the correction table created in advance, the processing load in the second conversion step can be reduced.

  In the invention according to claim 6, in the color correction coefficient calculating step, the color developability is calculated from the mixing ratio of the basic ink when the achromatic color is reproduced by mixing the basic ink. Then, the color correction coefficient can be calculated based on the calculated color developability. By doing so, the color correction coefficient can be calculated even when the color developability of each basic ink is unknown.

  Furthermore, it can be said that the above-described apparatus for creating a color conversion table also uses the technical idea of the present invention, and a color conversion table creating apparatus corresponding to claim 1 can be configured as in claim 7. Of course, a configuration corresponding to claims 2 to 6 is also possible. In addition, such a color conversion table creation apparatus may be implemented independently, or may be implemented with other apparatuses and methods in a state of being incorporated in a certain device. The present invention is not limited and includes various aspects, and can be changed as appropriate, such as software or hardware.

  In the case of software that implements a color conversion table creation method and apparatus as an embodiment of the idea of the invention, the software naturally functions and is used in such software. Therefore, the present invention can also be realized as a color conversion table creation program as in the eighth aspect. Of course, a configuration corresponding to claims 2 to 6 is also possible. The recording medium may be a magnetic recording medium, a magneto-optical recording medium, or any recording medium that will be developed in the future. Furthermore, the duplication stages such as the primary replica and the secondary replica are equivalent without any question. The ninth aspect specifies the invention as a color conversion table created according to the present invention. Also in this case, it is possible to adopt a configuration corresponding to claims 2 to 6.

  In addition, even when the communication method is used as a supply method, the present invention is not used. Further, even when a part is software and a part is realized by hardware, the idea of the invention is not completely different, and a part is stored on a recording medium and is appropriately changed as necessary. It may be in the form of being read. In addition, all functions may not be realized by the program itself, but may be realized by an external program or the like. Even in such a case, it is only necessary that each function can be realized by a computer.

Here, embodiments of the present invention will be described in the following order.
(1) Outline of color conversion table creation:
(2) Apparatus and process for creating a color conversion table:
(3) Other embodiments:
(4) Summary:

(1) Outline of color conversion table creation:
FIG. 1 is an explanatory diagram schematically illustrating the steps of a color conversion table creation method according to the present invention. Since this step requires a lot of arithmetic processing, it is preferable to use a computer. In addition, since the LUT creation patch is actually printed, it is preferable to perform printing using a printer that performs printing using the created color conversion table.

The color conversion table in this embodiment is a table that defines the correspondence between RGB data and CMYKlclm data for 17 ³ reference points, and any color can be obtained by performing an interpolation process with reference to these reference points. RGB data and CMYKlclm data can be associated with each other. In this embodiment, RGB data represented by gradations of element colors is sRGB standard compliant data used in a computer display, and each color has 256 gradations (an integer value of 0 to 255). The color is expressed by a combination of RGB colors expressed in gradation with the data). The CMYKlclm data is data for specifying the amount of ink ejected by the printer according to the present embodiment and is 256 gradations for each color (data in which the gradation of each color is designated by an integer value of 0 to 255). Color is expressed by combination.

  In order to perform printing by a printer, it is necessary to associate the RGB data and the CMYKlclm data with each other in the color conversion table. Measure the actual printing result on the printer. Then, a color conversion table is created by associating colors based on the RGB data and CMYKlclm data in the device-independent color space.

In the present embodiment, a Lab color space as the device-independent color space (usually this space is expressed as L ^* a ^* b ^* , but for simplicity in this specification, * is omitted and the same applies hereinafter). In the color conversion table creation step, first, the coordinate value of the Lab color space is specified for each of RGB data and CMYKlclm data. The RGB data conforms to the sRGB standard as described above, and the sRGB data can be converted into coordinate values in the Lab color space by a predetermined conversion formula. In FIG. 1, the converted coordinates are denoted as L ₀ a ₀ b ₀ , and at this stage, a plurality of RGB data are converted into coordinate values in the Lab color space.

In the color conversion table, the reference points are preferably distributed over substantially the entire color gamut of the display and printer in order to enable color conversion for any color expressed by RGB data and CMYKlclm data. . However, since the color gamuts of the display and the printer are generally different, color gamut mapping is performed to convert colors on the display into colors that can be expressed by the printer. In addition, when outputting an image, it is often the case that the actual color, such as skin color or sky blue, is output as it is rather than outputting it as it is. For, the actual color is converted to the memory color. In FIG. 1, the coordinates in the Lab color space obtained by converting the coordinates L ₀ a ₀ b ₀ in this way are represented as L ₁ a ₁ b _1, and the plurality of RGB data are represented by the L ₁ It is associated with a ₁ b ₁ .

  On the other hand, the CMYKlclm data is ink value data that specifies the ink amount and is a device-dependent color. Therefore, the color values in the Lab color space are obtained by measuring the LUT creation patch that has actually been printed by the colorimeter. However, the CMYKlclm data is data that expresses an arbitrary color by appropriately combining the respective ink amounts of the six colors of ink, and a very similar color can be expressed by a number of combinations.

In this embodiment, an LUT creation patch showing 10 ³ representative colors is created and measured, but there are many similar colors even at different coordinates in the ink amount space, so there is no rule. Even if the combination of ink amounts to be measured is determined, it cannot be distributed over substantially the entire color gamut of the printer. By distributing the representative colors reproduced by the LUT creation patch over substantially the entire color gamut of the printer, the RGB data and the CMYKlclm data can be correlated with high accuracy over the entire color gamut of the printer.

Therefore, 10 ³ representative colors are set in advance so as to be distributed in substantially the entire RGB color space, and by printing LUT creation patches based on the 10 ³ sets of RGB values, the color gamut of the printer is set. It is possible to distribute representative colors reproduced by the LUT creation patch over substantially the entire area. However, since the RGB value indicating the representative color is not data indicating the ink amount of CMYKlclm, the LUT creation patch cannot be printed by the printer as it is. Therefore, a process of converting the RGB value indicating the representative color into data indicating the ink amount of CMYKlclm is performed.

When converting RGB values into CMYKlclm data, the RGB values are first converted into virtual CMY values based on the complementary color relationship between RGB and CMY. The conversion formula at this time can be expressed by the following formula (1). The C ink, M ink, and Y ink correspond to the basic ink colors referred to in the present invention, and a three-dimensional color space that is covered by virtual CMY values and that has each color as an orthogonal axis is referred to as a virtual CMY color space.
C = 255-R
M = 255-G Formula (1)
Y = 255-B

Furthermore, since the printer of the present embodiment can use CMYKlclm ink, it is necessary to re-represent the representative color expressed by the virtual CMY value by the CMYKlclm value. Therefore, generally, color separation processing for converting virtual CMY values into six-color ink amounts is performed, and the concept of color separation processing is also used in this embodiment. In the color separation process, the virtual CMY value is converted into a six-dimensional ink value by a predetermined conversion formula. That is, since it is easy to create a conversion formula that converts coordinate values in the three-dimensional space into coordinate values in the six-dimensional space, 10 ³ colorimetric objects are first identified in the three-dimensional virtual CMY color space. In this conversion formula, the process is simplified by performing conversion from 3D to 6D to determine CMYKlclm data.

  In this separation process, conversion is performed so that the ink recording rate can be specified from the CMYKlclm data after the separation process. In the simplest case, 0-100% of the ink recording rate is uniformly assigned to each gradation of 0-255 so that the ink recording rate can be specified from each ink value. It is possible to perform color separation by a conversion equation that performs conversion in consideration of various limitations such as a recording amount limitation and a black ink usage limitation. In any case, the ink recording rate is specified from the CMYKlclm data after the color separation process.

When the CMYKlclm data is specified as described above, 10 ³ sets of coordinate values to be measured are obtained in the ink amount space of 256 tones for each color, and the LUT creation patch for the representative color indicated by the coordinate values is obtained. To print. In an inkjet printer, the number of gradations is 2 to 4 for each dot, that is, the second floor in which ink droplets are recorded and not recorded, the fourth level in which ink droplets are not recorded, and large, medium, and small dots are recorded. Since gradation expression is performed by a tone or the like, halftone processing is performed for each 256-color ink amount and converted into data representing the gradation of each dot in the printer. When printing is performed based on this data, 10 ³ LUT creation patches are obtained. By measuring these colors using a colorimeter, the coordinate values in the Lab color space are obtained for 10 ³ LUT creation patches. Can be identified. In FIG. 1, this coordinate value is shown as L ₂ a ₂ b ₂ .

The coordinate values L ₂ a ₂ b ₂ corresponding to the 256-level CMYKlclm data and the coordinate values L ₁ a ₁ b ₁ corresponding to the 256-level RGB data can be specified by the above steps. To determine the correspondence between RGB data and CMYKlclm data. The coordinate value L ₂ a ₂ b ₂ and the coordinate value L ₁ a ₁ b ₁ do not indicate the same color, but there are 10 ³ coordinate values in the color space, so the coordinate value L ₂ a Arbitrary CMYKlclm data can be calculated from ₂ b ₂ by interpolation calculation, and arbitrary RGB data can be calculated from coordinate values L ₁ a ₁ b ₁ by interpolation calculation. Accordingly, the correspondence between RGB data and CMYKlclm data can be defined by interpolation calculation, and as a result, the above-described color conversion table can be determined.

Although the color conversion table can be determined by the above steps, in order to create a color conversion table with good color reproducibility, the LUT creation patch needs to be reproduced with a desired representative color. In other words, in this embodiment, the representative colors of 10 ³ LUT creation patches are output and measured so that they are distributed in substantially the entire color gamut of the printer, but the color of this LUT creation patch is a specific hue. If it is biased, the correspondence between the RGB data and the CMYKlclm data cannot be accurately defined in the entire color gamut of the printer. This is because when the color of the LUT creation patch is biased to a specific hue, a hue area in which the distribution density of the representative colors indicated by the patch is sparse is generated, and accurate interpolation calculation cannot be performed in the hue area.

  In particular, in order to improve the color reproducibility in the vicinity of an achromatic color in which a person can easily detect a hue shift, it is necessary to reliably create an LUT creation patch for reproducing a color in the vicinity of an achromatic color. However, as described above, there is a problem that the correspondence between the RGB data near the achromatic color and the CMYKlclm data cannot be defined with high accuracy due to the deviation of the hue of the LUT creation patch near the achromatic color. As a factor that causes the color of the LUT creation patch to be biased to a specific hue, there is a difference in color development of CMY inks.

  FIG. 2 shows a list of the contents of CMYK ink. In the figure, each color ink is formed by mixing ion-exchanged water with a coloring material composed of various dyes and pigments exhibiting a desired color, ethylene glycol for viscosity adjustment, and the like. The type of the color material is indicated in the color index (C.I). As described above, the color developability of each ink differs due to the difference in the type and amount of the color material contained in each ink. In addition, the image quality of a printed image can be improved by making the color development of each ink different. For example, the graininess can be reduced by suppressing the color developability of the ink where the ink dots are conspicuous. Further, the ink consumption can be suppressed by increasing the color developability of the ink in which the ink dots are not noticeable. In this embodiment, the Y ink dots are less noticeable than the M ink and C ink dots, and the Y ink color is set higher than the M ink and C ink.

  Thus, if the color developability of Y ink is higher than that of other inks, LUT creation is performed with each ink amount specified based on the coordinates in the virtual CMY color space where the color developability of each ink is assumed to be uniform. When the patch for output is output as it is, a patch for creating an LUT that is biased to the hue of Y ink is formed as a whole. That is, the patch distribution is dense in the hue area close to the Y ink axis, but the patch distribution is sparse in the hue area close to the M ink axis and the C ink axis. Therefore, it becomes impossible to create a color conversion table with good color reproducibility in a hue region close to the M ink axis or the C ink axis. In order to solve this problem, in the present invention, the LUT creation patch is printed with an ink amount that takes into account the difference in color developability of each ink. Hereinafter, the method will be described along the flow of color conversion table creation processing.

(2) Apparatus and process for creating a color conversion table:
In the present invention, in the present invention, the LUT creation patch is printed with an ink amount (CMYKlclm data) that takes into account the difference in color developability of each ink. In the following, the apparatus and processing for that will be described more specifically. . FIG. 3 is a flowchart showing processing for creating a color conversion table (LUT) according to the present invention, and FIG. 4 is a block diagram showing the configuration of a computer for executing the processing.

  The computer 10 includes an arithmetic processing unit 11 that executes arithmetic processing and an HDD 12 that stores data. Further, it is connected to the printer 20 via an interface (not shown), and print data can be output from the computer 10 to execute printing. Further, the computer 10 can capture colorimetric data obtained by colorimetry by the colorimeter 30. This color measurement data can be input using a predetermined input device, input via a recording medium, or input by connecting and transferring data via a predetermined interface. It is.

  In the arithmetic processing unit 11, a predetermined program for creating a color conversion table can be executed to execute arithmetic processing, and the virtual CMY value converting unit 11a obtains the gray axis detection virtual CMY value in step S100. calculate. More precisely, the gray axis detection RGB value corresponding to the gray axis detection patch is set in advance, and the virtual axis CMY value for gray axis detection is calculated by converting the RGB value by the above equation (1). To do. As can be seen from the above equation (1), the gray level detection virtual CMY has a gradation value range of 0 to 255, and the gradation value is defined by an integer. Further, assuming three colors of CMY ink, the ink amount and the gradation value are considered to correspond linearly.

  FIG. 5 shows the coordinates of the gray axis detection virtual CMY values in the virtual CMY color space. The RGB values for gray axis detection are four RGB equivalent coordinates (R = G = B = 127, R = G = B = 159, R = G = B = equally distributed evenly on the gray axis of the RGB color space. 191, R = G = B = 223) and eight sets of RGB values existing on cubic grid points of ± 30 gradations of each color component centered on the calculation target point. It consists of interpolation points. Then, by converting the RGB values for gray axis detection into virtual CMY values by the above equation (1), the coordinates P0 to P8 of the gray axis detection virtual CMY values as shown in FIG. 5 are obtained. The coordinate P0 corresponds to the calculation target point, and the coordinates P1 to P8 respectively correspond to the interpolation points. The calculation target point P0 corresponding to the RGB equivalence coordinates also has an equivalent virtual CMY value according to the above equation (1). Assuming that the virtual CMY value of the calculation target point P0 is (C, M, Y) = (H, H, H), the virtual CMY values of the interpolation points P1 to P8 can be expressed by the following equation (2), respectively. it can.

P1: (H-30, H + 30, H + 30)
P2: (H-30, H + 30, H-30)
P3: (H + 30, H + 30, H + 30)
P4: (H + 30, H + 30, H-30)
P5: (H-30, H-30, H + 30) (2)
P6: (H-30, H-30, H-30)
P7: (H + 30, H-30, H + 30)
P8: (H + 30, H-30, H-30)

  However, from the RGB equivalent coordinates (R = G = B = 127, R = G = B = 159, R = G = B = 191, R = G = B = 223), H = 32, 64, 96, 128 It becomes. As described above, when the gray axis detection virtual CMY value is acquired in step S100, the halftone processing unit 11e identifies the ink droplets to be ejected in the printer 20 based on the gray axis detection virtual CMY value in step S110. Perform halftone processing. The halftone processing unit 11e generates intermediate data specifying the ejected ink droplets for each pixel in the printer 20 based on the CMY data. In the halftone process, a known dither method, error diffusion method, or the like can be used.

  The print data generation / output unit 11f performs processing such as arranging the data after the halftone processing in the order of ink droplet ejection at each nozzle of the printer 20 to perform gray axis detection patches corresponding to the gray axis detection virtual CMY values. Is generated and output to the printer 20. As a result, the printer 20 prints 36 gray axis detection patches corresponding to the gray axis detection virtual CMY values (step S115). After printing the gray axis detection patch, the colorimeter 30 measures the color of the gray axis detection patch (step S120). The colorimeter 30 is a device that acquires Lab coordinate values to be measured as colorimetric data, and the acquired colorimetric data is taken into the conversion function calculation unit 11 b of the computer 10.

Through the above processing, coordinate values (corresponding to L ₂ a ₂ b ₂ in FIG. 1) corresponding to the gray axis detection virtual CMY values are obtained in the Lab color space which is a device-independent color space. . In other words, the conversion function calculation unit 11b corresponds to the gray axis detection virtual CMY values and the coordinate values in the Lab color space obtained by the color measurement for the calculation target point P0 and the interpolation points P1 to P8 in FIG. You can understand the relationship. Then, to get the lightness L ₂ patches for gray axis detection converting function calculating section 11b in step S125 corresponds to the calculated target point P0, the same lightness as this and,, Lab color space as the a ₂ = b ₂ = 0 Specify the coordinates in. Further, the CMY values corresponding to these coordinates are calculated by performing a three-dimensional interpolation operation using the virtual CMY values and L ₂ a ₂ b ₂ values corresponding to the interpolation points P1 to P8.

That is, four sets of CMY values that can actually reproduce an achromatic color are identified corresponding to each calculation target point (H = 32, 64, 96, 128). When the color developability of each ink is uniform, an achromatic color can be reproduced by the virtual CMY value of the calculation target point P0. Therefore, in this case, the virtual CMY value corresponding to the calculation target point P0 and the CMY value obtained by actual measurement match, but generally, the color developability of each ink is not uniform as described above, so that they do not match. Further, when an achromatic color having the same brightness as the calculation target point P0 exists outside the cubic lattice formed by the interpolation points P1 to P8, the calculation accuracy of the three-dimensional interpolation operation is lowered, but the gradation width of ± 30 The possibility of achromatic colors outside the cubic lattice is reduced. Therefore, the three-dimensional interpolation calculation can be performed with high accuracy. It is assumed to refer to the CMY values obtained by the 3-dimensional interpolation operation and the measured gray C _O M _O Y _O value.

Figure 6 shows an example of a measured gray C _O M _O Y _O value calculated as described above. In other words, possible Found Gray C _O M _O Y _O value reproduce achromatic four brightness corresponding to H = 32,64,96,128 are calculated. In step S130, it calculates the color correction coefficient from the relationship between the virtual CMY values for the gray axis detected and measured gray C _O M _O Y _O value. Color correction factor by multiplying the virtual CMY values for the gray axis detection, a coefficient can be obtained the corresponding measured gray C _O M _O Y _O value given by the function of the virtual CMY values and variables.

Figure 7 represents the vertical axis the measured gray C _O M _O Y _O value on the horizontal axis the virtual CMY values for the gray axis detected by Found Gray C _O M _O Y graph calculation results of _O values shown in FIG. 6 ing. Incidentally, the measured gray C _O M _O Y _O values are plotted by ● in the graph. In the figure, the measured gray C _O M _O Y _O value is proportional to the gray axis detection virtual CMY value for any of the C ink, M ink, and Y ink. This is because the color developability of the ink can be considered to be proportional to the amount of ink. Since the achromatic color can be reproduced when the color of the CMY ink becomes uniform, the CMY inks having different color developability exhibit different inclinations.

In addition, the ratio of the slopes of the CMY inks in FIG. By thus specifying the possible actual gray C _O M _O Y _O value reproduce gray, it is possible to obtain the color development of the ratio of the CMY inks. In the present embodiment, since the color developability of Y ink is higher than that of other inks, Y ink can reproduce an achromatic color with a smaller ink amount (tilt) than other inks.

Here, calculating the color correction coefficient that approximates only (indicated by ●) Found Gray C _O M _O Y _O value corresponding to H = 32,64,96,128, the slope of the broken line shown in FIG coloring It is calculated as a correction coefficient. In this case, the color correction coefficient is constant regardless of the gray level of the gray axis detection virtual CMY value. However, in the present invention, in addition to the actually measured gray C _O M _O Y _O values corresponding to H = 32, 64, 96, and 128, a virtual CMY value (0, 0, 0) → C _O M _O as the outermost shell condition. Y _O value (0,0,0), and forcibly added each color two sets of correspondence between the virtual CMY values _{(255,255,255) → C O M O} Y O value (255, 255, 255) From these, the color correction coefficient is calculated. Each outer shell condition is plotted with x in the figure. In the present embodiment, the color correction coefficient that satisfies the above requirements is represented by a quadratic function of the following equation (3).

In the above equation (3), α _C (C), α _M (M), α _Y (Y) are color correction coefficients for each ink color component, and each has a virtual CMY value as a variable. Means. Substituting the virtual CMY values (0, 0, 0) and (255, 255, 255) of the outermost shell condition into the above equation (3), α _c (C) = α _M (M) = α _Y ( Y) = 1, and when this is multiplied by the virtual CMY values (0, 0, 0), (255, 255, 255), the C _O M _O Y _O values (255, 255, 255) are satisfied. Further, a calculation formula obtained by multiplying the color correction coefficients α _C (C), α _M (M), α _Y (Y) by a virtual CMY value is referred to as a conversion function, and the conversion function is expressed by the following formula (4). Yes.

In the above equation (4), f _C (C), f _M (M), and f _Y (Y) indicate conversion functions for the respective ink color components. The conversion functions f _C (C), f _M (M), and f _Y (Y) are stored in the HDD 12. By substituting arbitrary virtual CMY values that are equal to these conversion functions f _C (C), f _M (M), and f _Y (Y), C _O that can actually reproduce an achromatic color. A combination of M _O Y _O values can be calculated. Furthermore, since it can be considered that the color developability of ink does not depend on the amount of other ink, the conversion functions f _C (C), f _M (M), and f _Y (Y) can reproduce achromatic colors. C _O M _O Y _O value combinations not only predict the can predict the C _O M _O Y _O value considering the chromogenic Upon reproduce any color.

For example, when it is desired to reproduce the cyan color of the intermediate gradation (128), it is understood that the ink amount of C ink corresponding to the calculated value of f _C (128) larger than the 128 gradation is necessary. Similarly, when it is desired to reproduce yellow of the intermediate gradation (128), it is understood that the ink amount of Y ink corresponding to the calculated value of f _Y (128), which is smaller than 128 gradations, is sufficient. However, compared towards the high-lightness region (low gradation) is a low-brightness area, Found C _O obtained by M _O Y _O faithfully transform function on values f _C (C), f _M ( Since M) and f _Y (Y) are calculated, it can be said that the color in the high brightness region has better color reproducibility.

When the conversion functions f _C (C), f _M (M), and f _Y (Y) are stored in the HDD 12, the virtual CMY value conversion unit 11a creates 10 ³ sets for creating the LUT creation patch in step S130. A virtual CMY value for LUT creation (hereinafter simply referred to as a virtual CMY value) is acquired. Strictly speaking, LUT creation RGB values corresponding to the LUT creation patch are set in advance, and the virtual CMY values are obtained by converting the RGB values by the above equation (1) (first conversion). Process). The conversion processing unit 11c that has acquired the virtual CMY value substitutes the LUT creation patch by substituting the virtual CMY value into the conversion functions f _C (C), f _M (M), and f _Y (Y) in step S135. color correction C _O M _O Y _O values for creating (hereinafter, simply color correction C _O M _O Y _O value shall be expressed.) is calculated (second conversion step).

FIG. 8 is a graph comparing the virtual MY value and the corrected M _O Y _O value. Also shows a two-dimensional color space with C = C _O = 0 in FIG. A broken line indicates a lattice line of the virtual MY value (M = Y = 64, 128, 196) before the conversion, and a solid line indicates a lattice line of the corrected M _O Y _O value after the conversion corresponding to the MY value. Yes. The virtual Y-value grid lines are respectively corrected downward by the above conversion, and have the largest correction width in the intermediate gradation (Y = 128). On the other hand, the virtual M-value grid lines are each corrected upward by the above conversion, and have the largest correction width in the intermediate gradation (M = 128). However, since the outermost shell condition is satisfied, the shape of the outermost shell is not distorted before and after the conversion in the MY color space.

Further, by converting the virtual MY value into the corrected M _O Y _O value, the red axis given by the virtual condition of M = Y in the virtual MY space is corrected as indicated by the solid line. The solid red axis is set based on the color developability of the MY ink, and the red color can be actually reproduced by mixing the MY inks with the ink amount corresponding to the coordinate value on the red axis.

Figure 9 compared before and after converting the gray axis in the color correction and the virtual CMY color space C _O M _O Y _O color space is a three-dimensional color space. Is not distorted outermost shell shape and the virtual CMY color space color correction C _O M _O Y _O color space even before and after the even converted in FIG. This is because the outermost shell condition is satisfied in any of the CMY components. Also in the figure, the gray axis is corrected so as to be curved in the same manner as the red axis described above. Then, by mixing the CMY inks in the ink amount corresponding to the C _O M _O Y _O value on the gray axis, it is possible to reproduce the actual achromatic. By doing so, for example, the LUT creation patch set under the condition of R = G = B in order to evaluate the achromatic color can be reproduced so as to actually become the achromatic color. Of course, other LUT creation patches can reproduce colors as intended in the RGB color space.

Color correction C _O M _O Y _O value calculated in step S135 is input to the separation processing unit 11d, separation processing is performed at step S140. Separation process is carried out by substituting the color correction C _O M _O Y _O value to a predetermined separation function. This separation function is defined by separation function data 12 a stored in the HDD 12. Of course, the color separation processing may be performed by interpolation processing using LUT instead of assignment to a function.

In color separation, although it is not strict, it is assumed that colors before and after color separation coincide with each other, and the value of CMYKlclm data after color separation is an integer value of 0 to 255. Further, it is considered that the magnitude of the value of CMYKlclm data after color separation and the ink amount correspond linearly. The printer 20 according to the present embodiment can use K ink as a high-order color ink that can be reproduced by mixing CMY inks, and the C _O M _O Y _O value is replaced with the gradation of K ink. Plate processing is performed.

FIG. 10 schematically shows the state of this separation process. In the figure, a relatively high lightness (C _O M _O Y _O value is smaller) substantially color correction C _O M _O Y _O value corresponding to the patch for LUT generation of the achromatic region, and the brightness is low (C _O The state of each ink value before and after the color separation color correction C _O M _O Y _O value corresponding to the LUT creation patch (with a large M _O Y _O value) is shown. Incidentally, separation function of the present embodiment detects the minimum components of the color measurement color correction C _O M _O Y _O value, replacing the CMY inks of the gradation value and the same amount of the same minimum components to the gradation of the K ink It demonstrates as what performs a process. That is, the color separation process is performed assuming that the color mixture ratio of CMY inks for reproducing achromatic colors is 1: 1: 1. That is, the color separation process is performed without taking into account the difference in color developability of CMY inks.

As shown in the figure, it is possible to replace the majority of color measurement color correction C _O M _O Y _O values with the gradation of the K ink in substantially achromatic region. Accordingly, the contribution ratio of K ink in the LUT creation patch reproduced with the ink amount after color separation is high. Further, since the amount of K ink generated is large in the substantially achromatic region where the lightness is low, the contribution ratio of K ink is further increased. According to the conversion functions f _C (C), f _M (M), and f _Y (Y), the color reproducibility by the CMY ink in the low lightness region is slightly inferior to that in the high lightness region. Since the contribution ratio of the achromatic color region LUT creation patch to the color of the CMY ink is extremely small, even if the correction amount based on the color developability of the CMY ink is small, hue deviation does not become a problem. That is, the LUT creation patch in the substantially achromatic region can be accurately reproduced at all brightness levels.

Here, consider the case where the color correction coefficients α _C , α _M , and α _Y are constant in all gradations. That is, the color correction coefficients α _C , α _M , and α _Y are calculated based only on the measured gray C _O M _O Y _O values indicated by ● in FIG. 7, and the color correction coefficients α _C , α _M , α Consider the case where _Y is calculated as the slope of a straight line indicated by a broken line. In this case, a substantially color correction C _O M _O Y _O value indicating the achromatic color, a low brightness area (high gradation) I see the respective components greatly different values. In maximum gradation, color adjustment Y _O value is a gradation of 2 times the color correction C _O M _O.

FIG. 11 shows the state of the color separation process for the color of the substantially achromatic region with low lightness in this case. In the figure, K ink is replaced by the Y _O value of the smallest gradation among the color correction C _{OM O} Y _O values. As a result, a large amount of the chromatic CM component originally having a large gradation remains in spite of the color separation of the substantially achromatic region, and a color having a color is reproduced. That is, when the color correction coefficients α _C , α _M , and α _Y are constant in all gradations, if the color separation process is performed with a virtual color mixture ratio that does not take color development into account, the color reproduction is performed before and after color separation. There arises a problem that the identity is lowered.

In contrast, in the present invention while correcting color correction C _O M _O Y _O value in accordance with the color development property but also satisfy outermost conditions, while keeping the outermost shape of the color space color Correction can be performed. Therefore, even if the color separation processing is performed at a virtual color mixture ratio that does not take color development into account, the hue does not greatly deviate. In particular, in the region near the outermost shell with low lightness, the amount of ink is large and the ink duty is limited, so that the color consistency before and after color separation is important. However, the outermost shell shape of the color space does not necessarily need to be maintained in the same shape, and if it is a similar shape, the ratio of each ink amount is maintained, so that the color separation process can be performed with a virtual color mixture ratio. it can.

  When the color separation process for the K ink is completed, the gradation of the C ink is separated into the gradations of the C ink and the lc ink while considering the ink duty limit, and the gradation of the M ink is changed between the M ink and the lm ink. Separation into gradations. By using light ink in this way, the gradation in the high brightness area can be enriched. It is possible to perform the color separation process between steps S100 to S110. However, by not performing the color separation process, it is possible to perform a pure color comparison between CMY inks. . However, the color separation process may be performed virtually on the condition that 0 is given to the gradation of the Klclm ink between steps S100 and S110. In addition, since the color separation process is not performed in steps S100 to S110, it is not possible to cope with the ink duty limitation. However, since the gray axis detection virtual CMY value is set to only the low gradation region, the ink duty limitation is not possible. Absent.

  In step S145, the halftone processing unit 11e acquires CMYKlclm data after color separation and performs halftone processing. In step S150, the print data generation / output unit 11f generates print data and outputs the print data to the printer 20. Since the processing contents in the halftone processing unit 11e and the print data generation / output unit 11f are the same as those described above, description thereof will be omitted. After printing the LUT creation patch, the colorimeter 30 measures the color of the LUT creation patch (steps S150 and S155). The colorimeter 30 is a device that acquires Lab coordinate values to be measured as colorimetric data, and the acquired colorimetric data is taken into the LUT creation unit 11 h of the computer 10.

With the above processing, coordinate values in the Lab color space (corresponding to L ₂ a ₂ b ₂ in FIG. 1) of the CMYKlclm data corresponding to the colorimetric virtual CMY values are obtained. Become. On the other hand, in step S160 and subsequent steps, processing for acquiring coordinate values in the Lab color space corresponding to RGB data is performed. Note that steps S160 and S165 may be executed before step S100.

In step S160, the sRGB data conversion unit 11g acquires sRGB values prepared in advance, and converts them into coordinate values in the Lab color space using a predetermined conversion formula (corresponding to L ₀ a ₀ b ₀ in FIG. 1). Note that the number of objects to be converted by the sRGB data converter 11g is about 10 ³ , and the object to be converted may be specified in advance by arbitrarily combining coordinates obtained by dividing the RGB color ranges into nine equal parts. In step S165, the sRGB data conversion unit 11g further performs correction in consideration of the color gamut mapping and the memory color. As a result, coordinate values corresponding to L ₁ a ₁ b ₁ in FIG. 1 are obtained.

This coordinate value is taken into the LUT creation unit 11h. In step S165 and step S155, the LUT creation unit 11h acquires L ₁ a ₁ b ₁ and L ₂ a ₂ b ₂ shown in FIG. 1, and in step S170, a plurality of reference points are obtained by interpolation processing. Defines the correspondence between RGB data and CMYKlclm data. Here, the correspondence between RGB data and CMYKlclm data may be defined. Since RGB data and CMYKlclm data corresponding to arbitrary coordinates in the Lab color space can be calculated by interpolation, the correspondence between the two data can be defined for any color via the Lab coordinate values. In step S175, the association and the RGB data and the CMYKlclm data for 17 ^three reference points, and generates table data indicating the correspondence relationship is stored in the HDD12 (LUT12c).

  In this way, by correcting the gradation of the CMY inks for creating the LUT creation patch based on the color developability of each ink, an LUT creation patch having a color close to the intended color in the RGB color space can be obtained. Since it can be reproduced, the accuracy of the interpolation calculation can be improved. In particular, since the reproducibility of the LUT creation patch is good in the substantially achromatic region, the LUT 12c having high color reproducibility in the substantially achromatic region can be created. Therefore, by performing color conversion with reference to the LUT 12c of the present invention, it is possible to output a high-quality monochrome print image with no hue bias.

(3) Other Embodiments In the previous embodiment, when the conversion functions f _C (C), f _M (M), and f _Y (Y) are calculated in step S125, the conversion functions f _C (C), f _{Although M} (M) and f _Y (Y) are stored in the HDD 12, a correction table may be stored instead of the conversion functions f _C (C), f _M (M) and f _Y (Y).

FIG. 12 shows this correction table. Correction table 12d~12f is provided for each component of CMY, color correction C _O M _O Y _O value corresponding to the virtual CMY values of 0 to 255, respectively are defined. The correction tables 12d to 12f can be obtained by sequentially substituting virtual CMY values from 0 to 255 gradations into the conversion functions f _C (C), f _M (M), and f _Y (Y). And a correction table 12d~12f advance, by storing in the HDD 12, when converting the virtual CMY values in the color correction C _O M _O Y _O value in step S135, by referring to the correction table 12d~12f Thus, the same conversion can be performed. By doing so, the above-mentioned virtual CMY values for 10 ³ sets of LUT creation patches are substituted into the conversion functions f _C (C), f _M (M), and f _Y (Y) at a higher speed than the above. Conversion can be performed.

すなわち、一般的なトーンカーブ関数によれば最外殻では補正幅を０とするすることができるため、上記最外殻条件を満足することができる。 Further, the conversion functions f _C (C), f _M (M), and f _Y (Y) may be approximated to the measured gray C _O M _O Y _O value and satisfy the outermost shell condition. The function of can be applied. As an example, a tone curve function can be applied. The following formula (5) shows an example.

That is, according to a general tone curve function, the correction width can be set to 0 in the outermost shell, so that the outermost shell condition can be satisfied.

Further, in the previous embodiment, the gray axis detection RGB value is set on the assumption that the portion where the gray axis exists to some extent is within a distance of ± 30 gradations from the coordinates where the virtual CMY values are equal. However, other methods may be applied. For example, when the color developability of each ink is significantly different or when the color developability cannot be predicted, a gray axis detection patch covering the entire virtual CMY color space may be reproduced. Specifically, about 4 to 10 ³ gray axis detection patches distributed in a uniform grid in the virtual CMY color space may be reproduced. Alternatively, the coordinates of the gray axis may be roughly detected with a coarse grid at first, and then a fine grid may be generated around the detected coordinates to be accurately interpolated. Conversely, when the color developability of each ink is known in advance from the ink components, the conversion functions f _C (C), f _M (M), and f _Y (Y) are calculated based on the color developability. Anyway.

(4) Summary:
As described above, in the color conversion table creation method according to the present invention, when reproducing the LUT creation patch, correction according to the color developability of each basic ink (CMY) is performed. Therefore, it is possible to prevent the hue of the LUT creation patch reproduced by the color development of each basic ink from being biased. That is, the color reproducibility of the color conversion table created by the interpolation calculation based on the color measurement result of the LUT creation patch can be improved.

It is explanatory drawing which illustrates schematically the process of the color conversion table preparation method. It is a table | surface which shows the structure of an ink. It is a flowchart which shows a color conversion table preparation work. It is a block diagram of a computer. It is explanatory drawing of the virtual CMY value for gray axis detection. It is a table | surface which shows the color measurement result of the patch for gray axis | shaft detection. It is a graph which shows the colorimetry result of the patch for gray axis detection. FIG. 5 is a diagram comparing a virtual MY color space and a color correction MY color space. FIG. 6 is a diagram comparing a virtual CMY color space and a color correction CMY color space. It is explanatory drawing explaining the example of a color separation process. It is explanatory drawing explaining the example of a color separation process. It is explanatory drawing explaining the example of a correction table.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Computer, 11 ... Arithmetic processing part, 11a ... Virtual CMY value conversion part, 11b ... Conversion function calculation part, 11c ... Conversion processing part, 11d ... Separation processing part, 11e ... Halftone processing part, 11f ... Print data generation / Output unit, 11g ... RGB data conversion unit, 11h ... LUT creation unit, 12 ... HDD, 12a ... separation function data, 12b ... conversion function data, 12c ... LUT, 12d-12f ... correction table, 20 ... printer, 30 ... colorimeter

Claims (9)

Color measurement is performed on patches for multiple representative colors output by the printing device, and the ink value data expressed by the gradation of the ink amount of each color used in the printing device from each color measurement result and each of the other image devices used In a color conversion table creation method for creating a color conversion table that defines a correspondence relationship with element color value data expressed by element colors,
A first conversion step of calculating virtual basic ink value data capable of expressing the same color from a complementary color relationship between the element colors and the colors of the basic inks used in the printing apparatus;
The corrected basic ink value data is obtained by multiplying the gradation of the virtual basic ink value data by a color correction coefficient that is based on the color developability of each basic ink and varies depending on the gradation of the virtual basic ink value data. A second conversion step for calculating
A patch printing step of printing a patch for the representative color by the ink value data based on the corrected basic ink value data calculated by the first and second conversion steps;
A color conversion table creating step of calculating a correspondence relationship between the ink value data and the element color value data based on the color measurement result of the patch, and storing the correspondence relationship in the color conversion table; To create a color conversion table.   The color correction coefficient is such that the shape of the outermost shell of the color space covered by the virtual basic ink value data and the shape of the outermost shell of the color space covered by the corrected basic ink value data are congruent or similar. 2. The color conversion table creation method according to claim 1, wherein the color conversion table fluctuates according to the gradation of the virtual basic ink value data.   The corrected basic ink value data is represented by a tone curve function having the gradation of the virtual basic ink value data as an input gradation by multiplying the color correction coefficient by the corrected basic ink value data. The color conversion table creation method according to claim 2.   3. The correction coefficient according to claim 1, wherein the correction coefficient is expressed by an Nth order function (N is an integer of 1 or more) having a gradation of the virtual basic ink value data as a variable. Color conversion table creation method.   The second conversion step refers to the correction basic ink value data from the virtual basic ink value data by referring to a correction table that defines the correspondence between the virtual basic ink value data and the corrected basic ink value data. The color conversion table creation method according to claim 1, wherein the color conversion table is obtained.   2. A color correction coefficient calculating step of calculating the color correction coefficient based on the color developability from a mixing ratio of the basic ink when an achromatic color is reproduced by mixing the basic ink. The color conversion table creation method according to claim 5. Color measurement is performed on patches for multiple representative colors output by the printing device, and the ink value data expressed by the gradation of the ink amount of each color used in the printing device from each color measurement result and each of the other image devices used In a color conversion table creation device for creating a color conversion table that defines a correspondence relationship with element color value data expressed by element colors,
First conversion means for calculating virtual basic ink value data capable of expressing the same color from the complementary color relationship between each element color and the color of each basic ink used in the printing apparatus;
The corrected basic ink value data is obtained by multiplying the gradation of the virtual basic ink value data by a color correction coefficient that is based on the color developability of each basic ink and varies depending on the gradation of the virtual basic ink value data. A second conversion means for calculating
Patch printing means for printing a patch for the representative color based on the ink value data based on the corrected basic ink value data calculated by the first and second conversion means;
A color conversion table creating means for calculating a correspondence relationship between the ink value data and the element color value data based on a color measurement result of the patch and storing the correspondence relationship in the color conversion table; Color conversion table creation device. Color measurement is performed on patches for multiple representative colors output by the printing device, and the ink value data expressed by the gradation of the ink amount of each color used in the printing device from each color measurement result and each of the other image devices used In a color conversion table creation program for creating a color conversion table that defines the correspondence with element color value data expressed in element colors,
A first conversion function for calculating virtual basic ink value data capable of expressing the same color from a complementary color relationship between the element colors and the colors of the basic inks used in the printing apparatus;
The corrected basic ink value data is obtained by multiplying the gradation of the virtual basic ink value data by a color correction coefficient that is based on the color developability of each basic ink and varies depending on the gradation of the virtual basic ink value data. A second conversion function for calculating
A patch printing function for printing a patch for the representative color by the ink value data based on the corrected basic ink value data calculated by the first and second conversion functions;
Causing a computer to realize a color conversion table creating function for calculating a correspondence relationship between the ink value data and the element color value data based on the color measurement result of the patch and storing the correspondence relationship in the color conversion table. A color conversion table creation program characterized by Measures multiple patches output by a printing device, and expresses the ink value data represented by the gradation of the ink amount of each color used in the printing device and the element colors used in other imaging devices from the colorimetric results. In the color conversion table that defines the correspondence with the element color value data
A first conversion step of calculating virtual basic ink value data capable of expressing the same color from a complementary color relationship between the element colors and the colors of the basic inks used in the printing apparatus;
The corrected basic ink value data is obtained by multiplying the gradation of the virtual basic ink value data by a color correction coefficient that is based on the color developability of each basic ink and varies depending on the gradation of the virtual basic ink value data. A second conversion step for calculating
A patch printing step of printing a patch for the representative color by the ink value data based on the corrected basic ink value data calculated by the first and second conversion steps;
It is created by calculating a correspondence relationship between the ink value data and the element color value data based on the color measurement result of the patch, and performing a color conversion table creation step of storing the correspondence relationship in the color conversion table. A color conversion table characterized by that.

Images