JPH10248017A - Color calibration method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the color calibration method by which a color of special ink is reproduced without being limited by a color reproduction range of process ink. SOLUTION: The color calibration method applies conversion to process image data used for a printing machine so as to match color reproduction of the printing machine with color reproduction of a calibration output device to obtain process color conversion image data used for the calibration output device (process color conversion process) (S1), and further references the color reproduction data of the calibration output device to obtain special color reference data (special color reference process) (S2) and obtains special color conversion image data used for the calibration output device based on the special color image data used for the printing machine and the special color reference data (special color conversion process) (S3). Then the method synthesizes the process color conversion image data with the special conversion image data to obtain synthesis image data (synthesis process) (S4) and the calibration output device outputs the synthesis image data (output process) (S5).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color proofing method for simulating color reproduction by a printing press in a proofreading output machine. In particular, the present invention provides a color proofing method having excellent color reproducibility of special colors when printing is performed using not only process inks but also special color inks.

[0002]

2. Description of the Related Art Process inks are Y (Yellow), M
(Magenta), C (Cyan), and K (Black). Many color prints are printed using this process ink,
Printing may be performed using a special color ink. For example, if special colors occupy a large area in printed matter,
There are cases where a high printing effect is required, cases where it cannot be expressed only with process ink, and the like. In particular, printing on many flexible packaging materials and carton materials includes both a portion printed with process ink and a portion printed with special color ink.

In general, before printing a printed product as a product on a printing press (printing press) (printing press), a proof print is usually printed on a proof printing press (proof printing). This is because, in the proof print, whether the contents and arrangement of the character patterns are appropriate, whether the color reproducibility is appropriate (color proof), and the like are checked in advance. For this proof printing, for example, digital data is input and a color print is output.
DCP (Direct Digital Color Printer) is used. In conventional color proofing using DDCP, output is usually performed using data that has been converted using a look-up table (LUT). The look-up table is a look-up table for data conversion for obtaining data in which proper color reproduction can be obtained in DDCP with respect to data when printing is performed using process ink in the printing press.

[0004]

In this case, a problem in color reproduction occurs when not only a portion printed with a process ink but also a portion printed with a special color ink, such as printing on a soft packaging material. That is, when the color of the special color ink is not included in the color reproduction range that can be expressed as a printed material with the process ink, appropriate color reproduction cannot be obtained in the printed material. Therefore, an object of the present invention is to provide a color proofing method capable of reproducing the color of a special color ink without being limited to the color reproduction range of the process ink.

[0005]

The above objects are achieved by the present invention described below. That is, the present invention relates to a color proofing method that simulates color reproduction by the printing press in a proof output device, wherein the color reproduction of the printing press and the proof output are performed on process color image data used in the printing press. A process color conversion process for obtaining a process color conversion image data to be used in the proofreading output device by performing a conversion for matching the color reproduction of the proofreading device, and referencing a special color by referring to the color reproduction data of the proofreading device corresponding to the special color sample data. A spot color reference step of obtaining data; a spot color conversion step of obtaining spot color converted image data used in the proofreading device based on the spot color image data used in the printing press and the spot color reference data; the process color converted image data and the spot color It has a combining step of combining the converted image data to obtain combined image data, and an output step of outputting the combined image data with the calibration output device. It is a color proofing wherein "a.

According to the present invention, with respect to process colors,
In the process color conversion process, conversion is performed on the process color image data used in the printing press to match the color reproduction of the printing press with the color reproduction of the proof output device, and process color converted image data used in the proof output device is obtained. . On the other hand, for special colors, special color reference data is obtained by referring to the color reproduction data of the proof output device corresponding to the special color sample data in the special color reference process, and the special color image data and the special color reference data used in the printing press in the special color conversion process. The special color conversion image data used in the proofreading machine is obtained based on the above. As described above, the processing of the process color and the processing of the spot color are independently separated. Then, in the combining process, the process color converted image data and the special color converted image data are combined to obtain combined image data, and in the output process, the combined image data is output by the proofreading output device. Therefore, there is provided a color proofing method capable of reproducing the color of the special color ink without being limited to the color reproduction range of the process ink.

[0007] The present invention also provides a method for obtaining a special color conversion image data used in a proofreading machine based on a plurality of special color sample data and a plurality of special color image data used in the printing press. Is a color proofing method. " According to the present invention, the special color conversion image data used in the proofreading output machine is obtained based on the plurality of special color sample data and the plurality of special color image data used in the printing press by the special color conversion process. Therefore, color reproduction of a plurality of special color inks can be performed.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The color proofing method of the present invention will be described below with reference to embodiments. FIG. 1 is a flowchart showing the processing steps when the color proofing method of the present invention is performed. The process will be described with reference to FIG. In the process color conversion process in step S1, conversion is performed on the process color image data used in the printing press to match the color reproduction of the printing press with the color reproduction of the proof output device, and the process color conversion image used in the proof output device is performed. Data is obtained. The process color image data is image data on which a printing plate of each color process ink is formed when printing is performed by the printing press.
Therefore, the value of each pixel of the process color image data is C,
It consists of a set of three color values of M and Y (or four color values of C, M, Y and K). That is, the value of each pixel can be represented by a color vector.

The process color conversion process is a process of referring to a process LUT (lookup table) for an input color vector and outputting a corresponding color vector, and converting all pixels constituting the process color image data. This is the process of obtaining the already-completed color vector. The process LUT is a data table for matching the color reproduction of the printing press with the color reproduction of the proof output device (described later). When process color conversion image data having a converted color vector as a pixel value is output by the proof output device, The color reproduction matches the color reproduction of the printing press.

In the spot color reference process of step S2, spot color reference data is obtained by referring to the color reproduction data of the proof output device corresponding to the spot color sample data. Since the color of a spot color is usually specified by a color sample (from a customer), the color sample is first quantified using a colorimeter to obtain colorimetric data of the color sample (that is, special color sample data). On the other hand, the color reproduction data of the proofreading output device is a color vector as input data, that is, values of three colors C, M, Y (or C, M, Y, K).
Are associated with the colorimetric data of the printed matter output by the proofreading device based on the input data. Therefore, the corresponding color vector (special color reference data) is obtained by referring to the color reproduction data of the calibration output device based on the color measurement data of the color sample. Conversely, when the special color reference data is used as input data and output by the calibration output device, the reproduced color matches the color of the color sample.

In the spot color conversion process in step S3, spot color converted image data to be used in the proof output device is obtained based on spot color image data and spot color reference data used in the printing press. The special color image data is image data that forms the basis of forming a printing plate of the special color ink when printing is performed by the printing press. The special color ink is an ink specially prepared so that a color that matches the color of the color sample is reproduced in a printed matter when printing is performed in the printing press. That is, one spot color is printed by one printing plate using one spot color ink. Therefore, each pixel value of the special color image data is a scalar. The special color conversion process is a process of converting (usually replacing) each scalar pixel value with the special color reference data that is a color vector to obtain special color converted image data having the converted color vector as a pixel value.

Normally, the color of a special color sample is expressed in 100% (solid), and each pixel value of the special color image data is set to 0% and 10%.
Often expressed as two of 0%. In this case, the pixel value 0% is converted (replaced) into a zero vector and the pixel value 100% is converted into a color vector (special color reference data). The special color image data includes a halftone portion (each pixel value is 0% to 100%
In such a case, corresponding data is obtained for a pixel value for which there is no corresponding data by applying an interpolation method based on the special color reference data, and converted to the data.

In the combining process in step S4, the process color converted image data and the spot color converted image data are combined to obtain combined image data. As described above, in both the process color conversion image data and the special color conversion image data, each pixel value is a value of three colors C, M, and Y (or a value of four colors C, M, Y, and K). Is a color vector consisting of The present invention is not limited by the synthesis method, but is carried out, for example, as follows. First, the pixel values of the pixels corresponding to the process color and the special color image data are compared, and if both are zero vectors, the zero vector is used as the pixel value after the combination of the pixels. When one of the pixels is a zero vector, a color vector that is not a zero vector is set as a pixel value after the combination of the pixel. If both are non-zero color vectors, the color vector (usually the process color) that forms the surface layer of the viewing surface of the printed matter is taken into account for the pixel in consideration of the process color and the printing order of the special color printing plate. Is the pixel value after combining.

In the output step of step S5, the composite image data is output by the calibration output device. Since the composite image data obtained by the above process is used, the color reproduction of the special color ink can be performed without being limited to the color reproduction range of the process ink.

Next, the color reproduction range according to the calibration method of the present invention will be described. FIG. 2 is a diagram schematically showing a color reproduction range according to the calibration method of the present invention. In FIG. 2, the horizontal axis represents the value of a ^{* in} the L ^* a ^* b ^* color system, and the vertical axis represents the value of b ^* . The value of L ^* is an axis in a direction perpendicular to the paper surface of FIG. 2, and FIG. 2 shows L ^* in the L ^* a ^* b ^* color system (color solid) ^.
Indicates a constant cut surface. The inner hexagon indicated by process color 1 represents the boundary of the color reproduction range of the printing press using the process ink, and is a color that can be reproduced inside. The intermediate hexagon shown in the conventional color proofing 2 represents the boundary of the color gamut by the conventional color proofing method.
The color inside is reproducible. The outer hexagon indicated by color proof 3 of the present invention represents the boundary of the color gamut by the color proofing method of the present invention, and the color inside can be reproduced.

The hexagon of the process color 1 and the hexagon of the conventional color proofing 2 theoretically have the same shape, but there are errors such as errors, colorimetric errors, and arithmetic errors of the printed matter. The reality is slightly different. The hexagon of the color proof 3 of the present invention has the widest color reproduction range so as to include the hexagon of the process color 1 and the hexagon of the conventional color proof 2. In general, the color reproduction range of DDCP used for color proofing is at least equivalent to the color reproduction range of the printing press using process inks, although it is natural for the purpose. This is realized by selecting a color material for the ink used in the DDCP that meets the purpose, improving the apparatus itself so as to obtain a required print density, and the like.

Next, the reproduction of a special color will be described with reference to an example. In FIG. 2, the spot color A is inside the hexagon of process color 1. That is, the special color A is the same as the conventional color proof 2
And within the hexagon of color proofing 3 of the present invention. Accordingly, the color reproduction of the printing press of the spot color A can be properly performed by the conventional color proofing or the color proofing of the present invention. The spot color B is outside the hexagon of conventional color proofing 2. However, this special color B is inside the hexagon of color proof 3 of the present invention. Therefore, the color reproduction of the present printing press cannot be properly performed with the special color B by the conventional color proofing, but the color proofing of the present printing press can be properly performed by the color proofing of the present invention. In the conventional color proofing, color reproduction is usually performed using the color of the hexagonal boundary of the color proofing 2 indicated by the arrow b as an approximate color.

The spot color C is outside the hexagon of the conventional color proof 2 and outside the hexagon of the color proof 3 of the present invention. Therefore, the color reproduction of the printing machine cannot be properly performed using the special color C by the conventional color proofing or the color proofing of the present invention.
However, in the conventional color proofing, the color of the hexagonal boundary of the color proof 2 indicated by the arrow c1 is the best approximation color, whereas according to the color proofing of the present invention, the hexagon of the color proof 3 indicated by the arrow c2 is used. Can be made closer to the appropriate color reproduction up to the color of the boundary of.

Next, the DDCP colorimetric data and the process LUT used in the present invention will be described. The DDCP colorimetric data is used as the color reproduction data of the proof output device in the above-described special color reference process, and the process LUT is used in the above-described process color conversion process of matching the color reproduction of the printing press with the color reproduction of the proof output device. . FIG. 3 is a diagram showing a process of creating a process LUT. DDCP colorimetric data is obtained during the process. In FIG. 3, first, chart image data 11 for preparing a print color chart is prepared.
Each pixel value of the image data is, for example, a color vector (C,
M, Y), and each element C, M, Y is 0 to 255
(8 bits). Therefore, when creating a print color chart, for example, 10 charts at approximately 32 intervals are used.
0, 16, 32, 64, 96, 128, 1
60, 192, 224 and 255 are selected and given to each element. That is, the pixel value is 1000 (10 × 10 ×
10) Chart image data 1 consisting of color patches corresponding to the pixel values, which are the types of color vectors (C, M, Y)
Create 1.

The chart image data 11 is input and D
Output is performed by the DCP device 12, and a DDCP output chart 14 is obtained. Further, based on the chart image data 11, printing is performed by the printing press 13 using a printing plate obtained by performing plate making under standard conditions and process ink, thereby obtaining a printing chart 15 of the printing press. Next, using a colorimeter 16, D
The color patches of the DCP output chart 14 are measured and the DDCP
The colorimetric data 17 is obtained. In addition, the colorimeter 16 is used to measure the color of each color patch of the printing chart 15 of the printer to obtain the colorimetric data 18 of the printer. The colorimetric value is, for example, a color system L ^* a ^* b
^* Value. Of course, other color systems RGB, XYZ,
And the like. The colorimetric data of the DDCP colorimetric data 17 and the main unit colorimetric data 18 are (L ^* , a ^* , b
The data of (C, M, Y) corresponding to the value of ( ^* ) is data registered in a table format (that is, as a related value).

Next, a process LUT 19 is created.
The color obtained by printing by the printing press 13 based on the value of (C, M, Y) and the value of (C ′, M ′, Y ′) are input and output by the DDCP device 12. It is assumed that the color and the colorimetric values (L ^* , a ^* , b ^* ) are the same (the color difference is minimum). The process LUT 19 has such (C,
The value of (M, Y) and the value of (C ′, M ′, Y ′) are data registered in a table format (that is, as related values).

As described above, the chart image data 11 is discrete data in which each element of C, M, and Y has a value of, for example, about 32 intervals.
7 and the machine colorimetric data 18 are also discrete data. Therefore, by applying an interpolation method based on these data, each element of C, M, and Y is converted into continuous data at one interval.
(C ′, M ′, Y ′) corresponding to the (C, M, Y) value in which the colorimetric values (L ^* , a ^* , b ^* ) are the same (the color difference is minimum)
Find the value of A method for obtaining such a process LUT 19 is disclosed in JP-A-1-234251 and JP-A-2-214.
No. 266, etc., detailed description is omitted here.

The image data (C, M, Y) for the printing press is converted to image data (C ', M', Y ') for DDCP with reference to the process LUT 19. And that D
DCP image data (C ′, M ′, Y ′) is input and output by the DDCP device 12. The color obtained by the output is the same as the color obtained by printing by the printing press 13 based on the image data (C, M, Y) for the printing press (the color difference is minimum).

Next, the color proofing method of the present invention will be described in more detail. FIG. 4 is an explanatory diagram showing an example in which the color proofing method of the present invention is applied in the plate making process. 4, the same parts as those in FIG. 3 are denoted by the same reference numerals. Hereinafter, the same applies to the drawings for explaining the present invention. CEPS (color electr
An onic prepress system 31 obtains decomposed image data to be printed with each ink color from a document such as a color document (photograph), a line drawing, or image data (operation 1). C, M, C of photo 32 and line drawing (1) 33 printed with process ink
The Y and K decomposed image data and the decomposed image data of the special color (1), the special color (2), the special color (3), and the special color (4) of the line drawing (2) to be printed with the special color ink are obtained.

Data processing by the process color color matching application 36 is performed, and the process LUT 19 is referred to the separated image data of the photo 32 to obtain a matched photo 38 (operation 2; process color conversion process). The matched photograph 37 is represented by color data (C ′, M ′, Y ′) input to the DDCP 12, and is image data composed of color data in the range of the conventional color proofing 2 shown in FIG. Further, data processing by the process color color matching application 36 is performed, and the line drawing (1) 33
The process LUT 19 is referred to for the decomposed image data, and a matched line drawing (1) 38 is obtained (operation 3: process color conversion process). Matched line drawing (2)
38 also represents color data (C ′, M ′,
Y ′) is image data composed of color data in the range of the conventional color proofing 2 shown in FIG.

On the other hand, four color samples 41a to 41a corresponding to the special colors (1), (2), (3), and (4).
1d (customer; specified by the orderer of the printed material). The color of the four color samples 41a to 41d is
2 to obtain four color sample colorimetric data 43 (operation 4). Each of the color sample colorimetric data 43 is, for example, a value of a color system L ^* a ^* b ^* . Next, L ^* a ^* b
^* → Data processing is performed by the CMY conversion application 44, and the above-described DDCP colorimetric data 1 (see FIG. 3)
7, four color sample colorimetric data 43 (L ^* a ^* b
^* ) Is converted into four CMY data 46 (operation 5: spot color reference process). The CMY data 46 is represented by color data (C ′, M ′, Y ′) input to the DDCP,
Data of a range of colors reproduced by P, that is, color data of the range of color proofing 3 of the present invention in FIG.

By the above conversion, the special color (1), special color (2), special color (3),
Since the separated image data (separated image data) of the special color (4) is represented by the color data (C ′, M ′, Y ′), it is combined with the image data (C ′, M ′, Y ′). be able to.
That is, image data represented by elements of the number of spot colors can be represented as image data represented by three (C ′, M ′, Y ′) elements. Separation of the special color plate and data processing by the synthesizing application 47 are performed, and a matched line drawing (2) which is the image data (C ′, M ′, Y ′) thereof
48 is obtained (operation 6; spot color conversion process).

The matched photograph 37, the matched line drawing (1) 38, and the matched line drawing (2) 47 obtained as described above are subjected to data processing by the synthesis application 49 in FIG. 5
0 is obtained (operation 7; synthesis process). Output data 50
Is input to the DDCP device 51, output and output (printed matter)
52 is obtained (operation 8; output process). This output (printed matter)
The color 52 is the same as the color obtained by printing with the printing press using the process ink and the special color ink (the color difference is minimum).

Next, an example of the data processing (operation 5) by the above L ^* a ^* b ^* → CMY conversion application 44 will be described in detail. FIG. 5 shows L ^* a ^* b ^* → CMY
FIG. 9 is an explanatory diagram illustrating a data processing process of a conversion application. As described above with reference to FIG. 4, the color sample 41 is measured by the colorimeter 42, and the color sample colorimetric data (L ^* a ^{*) is obtained.}
b ^* ) 43 is obtained. Also, as described above with reference to FIG.
DDCP colorimetric data 17 is obtained. An example of a change in the discrete CMY values of the DDCP output chart 14 is shown in a rectangular frame. As shown in FIG. 5, L ^* a ^* b ^*
→ As a result of data processing by the CMY conversion application 44, CMY data 46 is obtained. The data processing process includes the following steps.

First, in the step, out of the 1000 sets (1000 color vectors) of DDCP colorimetric data 17, the color sample colorimetric data (L ^* a ^* b ^* ) 43 that is closest (the color difference is the smallest) 27. Select a set of data.
FIG. 6 is a diagram showing a relationship between the color sample colorimetric data (L ^* a ^* b ^* ) 43 and the selected 27 sets of DDCP colorimetric data 17 as a three-dimensional graph. Next, in a step, for example, a point having continuous CMY values of one step, that is, 32 × 32 × 32, between each set of DDCP colorimetric data 17 having discrete CMY values of 32 steps. ×
It is subdivided into eight points, and the L ^* a ^* b ^* values of each point are obtained by an interpolation method (for example, eight-point interpolation). Next, in the step, the L ^* a ^* b ^* value closest to the color sample colorimetric data (L ^* a ^* b ^* ) 43 (with the smallest color difference) is calculated as 32 × 32 × 32 × The user selects one of the eight points and outputs the value of CMY as CMY data 46.

[0031]

As described above, according to the present invention,
There is provided a color proofing method capable of performing color reproduction of a special color ink without being limited to the color reproduction range of a process ink.
In addition, there are a plurality of spot colors, and the spot color conversion process is a process of obtaining spot color conversion image data used in a proof output device based on a plurality of spot color sample data and a plurality of spot color image data used in the printing press according to the present invention. For example, color reproduction of a plurality of special color inks can be performed.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a processing procedure when a color proofing method of the present invention is performed.

FIG. 2 is a diagram schematically showing a color reproduction range according to the calibration method of the present invention.

FIG. 3 is a diagram showing a process of creating a process LUT.

FIG. 4 is an explanatory diagram showing an example in which the color proofing method of the present invention is applied in a plate making process.

FIG. 5 is an explanatory diagram showing a data processing process of an L ^* a ^* b ^* → CMY conversion application.

FIG. 6 shows color sample colorimetric data and selected 27 sets of DDCP.
FIG. 3 is a diagram showing a relationship with colorimetric data as a three-dimensional graph.

[Explanation of symbols]

Reference Signs List 1 Process color 2 Conventional color calibration 3 Color calibration of the present invention 11 Chart image data 12 DDCP device 13 Main printing machine 14 DDCP output chart 15 Main printing chart 16 Colorimeter 17 DDCP colorimetric data 18 Main colorimetric data 19 Process LUT 31 CEPS 32 Photograph 33 Line drawing (1) 34 Line drawing (2) 36 Process color matching application 37 Matched photo 38 Matched line drawing (1) 41a to 41d Color sample 1 to Color sample 4 42 Colorimeter 43 Color sample Colorimetric data 44 L ^* a ^* b ^* → CMY conversion application 46 CMY data 47 Separation of special color plate, composite application 48 Matched line drawing (2) 49 Composite application 50 Output data 52 Output

──────────────────────────────────────────────────続 き Continued on front page (51) Int.Cl. ⁶ Identification code FI H04N 1/46 H04N 1/46 Z

Claims (2)

[Claims] 1. A color proofing method for simulating color reproduction by a printing press in a proofreading output machine, wherein the color reproduction of the printing press and the proofreading output color are performed on process color image data used in the printing press. A process color conversion step of performing conversion to match the color reproduction of the proof output device to obtain process color conversion image data used by the proof output device, and referring to the color reproduction data of the proof output device corresponding to the special color sample data, to obtain special color reference data. A special color conversion step of obtaining special color conversion image data used in the proofreading device based on the special color image data used in the printing press and the special color reference data; and the process color conversion image data and the special color conversion. A combining step of combining image data to obtain combined image data, and an output step of outputting the combined image data with the calibration output device. Color calibration method according to claim. 2. The method according to claim 1, wherein the special color conversion step includes obtaining the special color conversion image data used in the proofreading output device based on the plurality of special color sample data and the plurality of special color image data used in the printing press. 2. The color proofing method according to claim 1, wherein the process is a process.