CN107395919A - A kind of method for realizing digital printing color correction by changing characteristic file - Google Patents

Abstract

The invention discloses a method for realizing digital printing color correction by modifying characteristic files. The method of the present invention is to print the corresponding characteristic color scale image according to the source characteristic file of the digital file to be printed, measure the L ^* a ^* b ^* value of each color block, and aim at the target L ^* a ^* b ^* value of each color block , find out the L ^{* a * b * value of the part of the color block that has a small color difference from the target color block from the printed color block, and then find out the L * a * b * value of the} corresponding color block in the source characteristic file, using the polynomial The least squares regression method is used to establish the mapping relationship, and then the target L ^* a ^* b ^* value is used as an independent variable to calculate the correction value of the corresponding color block in the source characteristic file, and then use the revised characteristic file to replace the original source characteristic file. Assigned to the digital file to be printed for printing. The invention realizes a high-precision and convenient color correction method for digital printing.

Description

A Method of Realizing Color Correction in Digital Printing by Modifying Characteristic Files

technical field

The invention relates to the technical field of quality control in the printing process, in particular to a color correction method for digital printing.

Background technique

Digital printing refers to the use of computers to read digital files and directly complete the reproduction of images without the need to make printing plates. At present, common methods include electrostatic photography and inkjet. When a digital printing machine reads a source file in RGB mode or CMYK mode, the operator needs to specify a characteristic file for the color information in the source file. The operator can select the property file embedded in the source file as the source property file, or designate other property files as the source property file of the source file. The digital printing machine converts the device-related color information in the source file into device-independent color information according to the specified source characteristic file, and then converts it into the color information of the digital printing machine according to the device characteristic file of the digital printing machine, and then prints Output hard copy.

Depending on the needs, the target color of the output hard copy may simulate the color of the original, or it may simulate the color printed by a traditional printing machine. However, due to the performance change of the digital printing machine, the difference in the target color space of the source digital file or the simulated traditional printing machine and the color space of the output device, the actual printed color may deviate from the target color beyond the allowable range. The control software of some digital printing machines will provide a correction interface. According to the current printing effect and target requirements, the operator can adjust the tone curve and other methods to make the printing effect as close as possible to the target requirements; some digital printing machine control software does not have a correction function. Operators need to edit and adjust the source profile or digital printing machine profile in professional color management software such as ProfileEditer. However, these two corrections are based on the subjective perception of the human eye, which have poor accuracy, require high operator experience and skills, and result in unstable results.

Contents of the invention

The technical problem to be solved by the present invention is to propose a color correction method that can make the digital printing color approach the target requirement in the color gamut space of the entire digital printing machine without depending on the operator's subjective feeling.

In order to solve the above-mentioned technical problems, the method of the present invention is aimed at the designated digital printing machine and digital file and its source characteristic file, first prints the characteristic color mark image corresponding to the source characteristic file, according to the actual printed color of the characteristic color mark image The chromaticity value and the chromaticity value of the source characteristic file, establish a mapping relationship in the local color gamut space, and then use the target chromaticity value as an independent variable, obtain the corrected value of the chromaticity value of the source characteristic file according to the mapping relationship, and then use the corrected characteristic The file is printed as the source characteristic file of the digital file to be printed, and the printing color correction is realized. Specifically include the following steps:

Step (1) For the specified digital printing machine, according to the characteristic file A specified by the digital file to be printed, obtain the corresponding characteristic color patch image and characteristic data file, and the characteristic data file records the L ^* of each color block a ^* b ^* values, in order respectively denoted as Under the condition that the characteristic file A is specified as the source characteristic file of the characteristic color standard image, print the characteristic color standard image, measure the L ^* a ^* b ^* value of each color block, and record them in sequence as And the target L ^* a ^* b ^* value of each color block is recorded as Among them, the L ^* a ^* b ^* value refers to the color coordinate value of the CIE 1976 L ^* a ^* b ^* space recommended by the International Commission on Illumination, and n is the number of color blocks in the characteristic color scale image;

Step (2) For the first color block of the characteristic color scale image, calculate respectively and and and The color difference value between, according to the principle of small color difference priority, take the measurement value of 30 color blocks from all color blocks, and record it as then from Find the L ^* a ^* b ^* value of the 30 color blocks corresponding to the position, recorded as

by as an independent variable, with As the dependent variable, the mapping relationship M is established using the method of polynomial least squares regression;

The target of the first color patch of the image with the characteristic color scale is an independent variable, and the correction value of the first color block of the characteristic file is calculated by using the mapping relationship M, which is denoted as

For other color blocks of the characteristic color scale image, according to value, use the same method to calculate the correction value from the second color block to the nth color block of the characteristic file, which is recorded as

Step (3) ends with value, replacing the one recorded in the characterization data file described in step (1) value, and then use the modified characterization data file to make the revised characterization file B. Making use of characteristic data files to make characteristic files adopts mature technology;

(4) Under the condition that the specified characteristic file B is the source characteristic file of the characteristic color patch image, print the characteristic color patch image again, measure the L ^* a ^* b ^* value of each color block, and record them in sequence as Calculate separately and and and Then calculate the average color difference and the maximum color difference, if the average color difference and the maximum color difference are less than the set expected value, perform step (6), or although the average color difference or the maximum color difference does not meet the set expected value, but the operator If the correction is abandoned, step (6) is also performed; otherwise, step (5) is performed. Calculation of chromatic aberration adopts mature technology;

Step (5) ends with is the chromaticity value of the source profile, with In order to print the resulting chromaticity value, the target chromaticity value remains unchanged, and use the same method as steps (2) and (3) to revise the characteristic file again until the requirements are met, or the operator stops the correction and directly executes step (6);

Step (6) Use the digital printing machine described in step (1) to designate the corrected characterization file as the source file of the digital file to be printed, and complete the printing.

Preferably, the color mode of the digital file to be printed includes RGB mode and CMYK mode, and the corresponding specified characteristic files are RGB-L ^* a ^* b ^* characteristic file and CMYK-L ^* a ^* b ^* characteristic file.

Preferably, with as an independent variable, with As the dependent variable, when the mapping relationship M is established by using the polynomial least squares regression method, an 11-term polynomial is used.

Preferably, the target L ^* a ^* b ^* value of the characteristic color standard image is specified by the operator according to the needs, and it can be the L ^* a ^* b ^* value obtained by printing the characteristic color standard image on a traditional printing machine, or it can be the value to be printed The L ^* a ^* b ^* value or other target value of the source property file specified by the number file.

Beneficial effects of the present invention: the color correction of the present invention is realized based on objective detection data. Compared with the correction based on subjective experience, the color correction has high accuracy and gets rid of the dependence on experience; the present invention is based on adjacent local color The chromaticity value of the source characteristic file and the chromaticity value of the actual printing effect in the domain space are corrected for each color contained in the source characteristic file one by one. Compared with the global correction, the color correction accuracy is high; The source profile realizes color correction, and is suitable for various digital printing systems equipped with basic color management functions, with wide adaptability.

Description of drawings

Figure 1 shows the color management process of a common digital printing machine

Fig. 2 is the realization flowchart of color correction method of the present invention

detailed description

The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

Printing technology can be divided into traditional printing technology with printing plate and digital printing technology without printing plate. A digital printing machine includes a control computer and a printing device, etc. The color management process of a common digital printing machine is shown in Figure 1. The control computer reads digital files and converts them into device-independent colors according to the specified source characteristic files. value, and then convert it into a device-related color value according to the characteristic file of the current printing device, and the printing mechanism controls the ink volume according to the color value. In theory, through color management, the printed color can simulate the color of the original, but in actual production, due to various equipment and materials, the printed color may deviate from the color of the original beyond the allowable range and needs to be corrected; another possibility is , the user requires the color printed by the device to simulate the effect of traditional printing as much as possible, and also needs to correct the color. The present invention proposes a method for realizing color correction by modifying source characteristic files, and the specific implementation method is shown in FIG. 2 .

Step (1) For the specified digital printing machine and the digital file to be printed, it is necessary to specify the source characteristic file for the digital file to be printed. The color mode of the digital file to be printed includes RGB mode and CMYK mode, and the corresponding specified characteristic files are RGB-L ^* a ^* b ^* characteristic file and CMYK-L ^* a ^* b ^* characteristic file. The source characteristic file can be the characteristic file embedded in the current digital file, or a certain characteristic file can be specified through the operation interface of the control computer, depending on the settings of the operator.

According to the source characteristic file A specified by the digital file to be printed, obtain the corresponding characteristic color scale image and characteristic data file from the producer of the characteristic file. Generally, the general characteristic files can be obtained from the official website of the producer or international organization , such as www.fogra.org, www.icc.org, etc., download the characteristic file and the corresponding characteristic color scale image and characteristic data file, and the L ^* a ^* b ^* value of each color block is recorded in the characteristic data file , recorded in order as The L ^* a ^* b ^* value refers to the color coordinate value of the CIE1976L ^* a ^* b ^* space recommended by the International Commission on Illumination, and n is the number of color blocks in the characteristic color scale image, such as the color of the ANSI IT8.7/3 standard color card The number of blocks is 928.

On the control computer operation interface of the digital printing machine, specify the characteristic file A as the source characteristic file of the characteristic color standard image, print the characteristic color standard image, measure the L ^* a ^* b ^* value of each color block after drying, and respectively in order recorded as The target L ^* a ^* b ^* value of each color block depends on the operator’s expectations. If the operator wants the printed color to restore the color of the original as much as possible, the target value is the value of each color block recorded in the characteristic file embedded in the original. L ^* a ^* b ^* value, if the operator wants the printed color to simulate the color of the traditional printing machine as much as possible, the target value is the L ^* of each color block obtained after the traditional printing machine prints the same characteristic color standard image a ^* b ^* value, record the target L ^* a ^* b ^* value of each color block in order as

Step (2) For the first color block of the characteristic color scale image, calculate the target value respectively with printed measurements and and The color difference value between, according to the principle of small color difference priority, select the measurement value of 30 color blocks adjacent to the first color block from all color blocks, and record it as Then respectively according to The location, documented from the characterization data file of the Find the L ^* a ^* b ^* value of the 30 color blocks corresponding to the position, recorded as

Considering that when using the method of polynomial regression to establish a mapping relationship, the number of items is too small, and the accuracy of the mapping relationship is low; the number of items is too large, and the established mapping relationship is likely to be in a state of oscillation. The present invention provides a preferred solution to use 11 polynomials. use L ^* , a ^* , b ^* values, construct an input matrix R with 30 rows and 11 columns according to formula (1):

use L ^* , a ^* , b ^* values, construct an output matrix H with 30 rows and 3 columns according to formula (2):

Using the least squares method, according to formula (3) to solve the mapping relationship matrix M:

M＝(R ^T R) ^-1 (R ^T H) (Formula 3)

R ^T in formula (3) represents the transposition of matrix R.

Use the target of the first color patch of the characterized color patch image According to formula (4), the correction value of the first color block of the characteristic file is calculated, which is denoted as

In formula (4) for L ^* , a ^* , b ^* values. For other color blocks of the characteristic color scale image, according to value, use the same method to calculate the correction value from the second color block to the nth color block of the characteristic file, which is recorded as

Step (3) ends with value, replacing the one recorded in the characterization data file described in step (1) Then use the modified characterization data file to create a revised characterization file B. Making characteristic files using characteristic data files adopts mature technology and tool software, such as ProfileMaker of X-Rite Company.

(4) On the operation interface of the control computer, specify the characteristic file B as the source characteristic file of the characteristic color patch image, print the characteristic color patch image again, measure the L ^* a ^* b ^* value of each color block, and record it in order as Calculate separately and and and Then calculate the average color difference and the maximum color difference, if the average color difference and the maximum color difference are less than the set expected value, perform step (6), or although the average color difference or the maximum color difference does not meet the set expected value, but the operator If the correction is abandoned, step (6) is also performed; otherwise, step (5) is performed. Calculating the color difference between two colors, and then calculating the average color difference and the maximum color difference belong to common technical knowledge in this field.

Step (5) ends with is the chromaticity value of the source profile, with For printing result chromaticity value, target chromaticity value No change, use the same method as steps (2) and (3), revise the characteristic file again until the requirements are met, or the operator stops the modification and directly executes step (6).

Step (6) Designate the corrected characterization file as the source characterization file of the digital file to be printed on the control computer operation interface of the digital printing machine, and complete the printing.

Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. If these modifications and variations fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and variations.

Claims (4)

1. A method for realizing digital printing color correction by modifying a characteristic file, characterized in that it comprises the following steps: Step (1) For the specified digital printing machine, according to the characteristic file A specified by the digital file to be printed, obtain the corresponding characteristic color patch image and characteristic data file, and the characteristic data file records the L ^* of each color block a ^* b ^* values, in order respectively denoted as Under the condition that the characteristic file A is specified as the source characteristic file of the characteristic color standard image, print the characteristic color standard image, measure the L ^* a ^* b ^* value of each color block, and record them in sequence as And the target L ^* a ^* b ^* value of each color block is recorded as The L ^* a ^* b ^* value refers to the color coordinate value of the CIE 1976 L ^* a ^* b ^* space recommended by the International Commission on Illumination, and n is the number of color blocks in the characteristic color scale image; Step (2) For the first color block of the characteristic color scale image, calculate respectively and and and The color difference value between, according to the principle of small color difference priority, take the measurement value of 30 color blocks from all color blocks, and record it as then from Find the L ^* a ^* b ^* value of the 30 color blocks corresponding to the position, recorded as by as an independent variable, with As the dependent variable, the mapping relationship M is established by using the polynomial least squares regression method; mature technology is used to establish the mapping relationship by using the polynomial least squares regression method; The target of the first color patch of the image with the characteristic color scale is an independent variable, and the correction value of the first color block of the characteristic file is calculated by using the mapping relationship M, which is denoted as For other color blocks of the characteristic color scale image, according to value, use the same method to calculate the correction value from the second color block to the nth color block of the characteristic file, which is recorded as Step (3) ends with value, replacing the one recorded in the characterization data file described in step (1) Value, and then use the modified characterization data file to make the revised characterization file B; using the characterization data file to make the characteristic file adopts mature technology; (4) Under the condition that the specified characteristic file B is the source characteristic file of the characteristic color patch image, print the characteristic color patch image again, measure the L ^* a ^* b ^* value of each color block, and record them in sequence as Calculate separately and and and Then calculate the average color difference and the maximum color difference, if the average color difference and the maximum color difference are less than the set expected value, perform step (6), or although the average color difference or the maximum color difference does not meet the set expected value, but the operator Give up the correction, and also perform step (6), otherwise perform step (5); calculate the color difference using mature technology; Step (5) ends with is the chromaticity value of the source profile, with In order to print the resulting chromaticity value and keep the target chromaticity value unchanged, use the same method in steps (2) and (3) to revise the characteristic file B again until the requirements are met, or the operator stops the correction and directly executes step (6); Step (6) Use the digital printing machine described in step (1) to designate the corrected characterization file as the source file of the digital file to be printed, and complete the printing. 2. A method for realizing digital printing color correction by modifying characteristic files according to claim 1, characterized in that: the color modes of the digital files to be printed include RGB mode and CMYK mode, and the corresponding specified characteristics The files are RGB-L ^* a ^* b ^* profile and CMYK-L ^* a ^* b ^* profile. 3. A method for realizing digital printing color correction by modifying a characteristic file according to claim 1, characterized in that: as an independent variable, with As the dependent variable, when the mapping relationship M is established by using the polynomial least squares regression method, an 11-term polynomial is used. 4. A method for realizing digital printing color correction by modifying the characteristic file according to claim 1, characterized in that: the target L ^* a ^* b ^* value of the characteristic color mark image is specified by the operator according to the needs, which is traditional The L ^* a ^* b ^* value obtained by printing the characterization color scale image by the printing machine or the L ^* a ^* b ^* value of the source characteristic file specified by the digital file to be printed.