How to achieve color management
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In the color image copying process, to achieve color uniformity from scanning, display, output to printing, it is necessary to implement standardized, standardized, and data-based color management. The so-called standardization refers to the production of standard raw materials and equipment in the production process and standard data. The standardization means that the production process is constructed according to certain procedures. The data is the standard and the standard center must perform the operation. Quantitative indicators, all can be expressed by data values, the three complement each other, the overall scientific combination, to achieve the purpose of mass production, stability and improve product quality.
Color management includes: 1. Color matching between input devices. 2. A match between the original color and the display color. 3. Color matching between output devices. 4. Match between display color and print color. 5. The color matching between the original and the printed matter. Color management is to solve the problem of color conversion matching between various devices. First, establish a standard light source in a standard color environment. The standard light source core components should have a higher color temperature and a higher color rendering index. The International Standard Lighting Commission (CIE) and the national printing industry standards stipulate that the standard light source color temperature of observation reflection samples is 5000K. At 6500K, the color rendering index is usually >95%. 5000K is a CPM (mini) transmission standard light source; Germany JUST fluorescent tube color rendering index is 97%. Samples are observed using a standard light source while requiring constant room light.
Secondly, select the device-independent color space (CIE, LAB). According to the color theory, any white light color can be matched by the color source three source colors R, G, B, but the ratio of the three primary colors is not unique; any neutral Gray can be used to match the three source colors C, M, Y. However, due to the incomplete absorption of light by the colorant, to achieve the desired neutral ash and to meet the actual printing effect, it must be compensated by black ink. In this way, the same color block is expressed by different devices, and the ratios of C, M, Y, and K are different. For example, in a certain color block, the ratio of CMYK when printing and reproducing with Tianjin ink is 64%, 36%, 8%, and 10%, and the ratio of reproduction by inkjet printer is 60%, 30%, 10%, and 10%. The physical quantity CMYK describing the same color is related to equipment and materials. If the above color patches are read by CIE L*a*b*, the CIE L*a*b* values (70, 40, -12) at the time of printing and the CIE L*a*b* at the time of inkjet proofing are to be ensured. The values (70, 40, -12) are the same, then the visual appearance of the colors is consistent, which means that CIE L*a*b* is device-independent, independent of the physical quantity describing the color. The color gamut of the CIE L*a*b* color space is much larger than any other device-dependent color space, so that the color gamut range is not lost in the reference color space during color conversion mapping. Color management is to use the independent device-independent physical quantity CIE L*a*b* to communicate and derive the correspondence between the original color, the screen color and the printing color in the color space, to achieve visual consistency of colors, to achieve different devices and Color conversion. Color conversion refers to the conversion of colors in different color spaces. Through the L*a*b* color space as the inter-transition color space, the conversion between various device colors can be completed, and the infinite combination conversion relationship between the device and the device can be converted into between the device space and the standard color space. The five-five correspondence greatly simplifies the complexity of matching conversion.
Then, a profile describing the color of the device is created to reflect the range and characteristics of the color of the device, and the feature space can be used to complete the color space between the device and the L*a*b* chroma space. Conversion. Color management uses the following process structure to complete the cross-platform and system transfer uniformity of color conversion.
As can be seen from the above process structure diagram, through a core color conversion engine (L*a*b*), various device feature files, ie, RGB information input by the scanner, RGB information of the display, and output of the printing device are output. CMYK information is converted to each other. The collection of various information data is the basic work of color conversion. If it is not done well, other conversion work will be wrong. Therefore, equipment standards must be established for equipment calibration. Use the calibration process to generate a new color signature file that matches the current job. In a color management system, the fundamental purpose of calibration is to match the actual operating state of the device to the conditions described in the device profile.
Implementing a standardized color feature file format, the current ICC format has become a color standard such as Apple's Color Syne; Windows 98 ICM (Image Color Mathing) system defines the color image TIFF, PIC and EPS file format; the combined feature file information can be embedded in the image file In the middle, completely solve the inconsistency of color transfer and maintain the accuracy of color.
The following describes a method of generating a feature file of a color display, a scanner, a color inkjet, a laser printer, and a color printer.
The color display is in the process of input and output in the whole color processing flow. Therefore, whether it can accurately display the input original or accurately predict the color effect of the output will directly affect the WYSIWYG performance of the entire management system. Under the standard light source, measure with the screen chromaticity, correct the screen, and use the supporting software to generate a device feature file for the current working state.
The scanner is a device that reads colors, and the methods for generating scanner signature files are similar. First scan a standard color standard. Currently commonly used IT8 color standard, the color standard consists of 264 color blocks, representing the sampling of the entire CIEL*a*b* color, with a 23-pole neutral gray scale at the bottom. Companies that produce color-coded (Kodak, Fnji, and Agfa) now have microwave variations between the various color standards produced, but these differences can be analyzed without affecting the color management system accuracy of the color code. The color patch on the color scale is measured by the calibrated spectrophotometer to measure its chromaticity value L*a*b*, thereby generating a L*a*b* parameter table of the color patch. This parameter list is usually included when the manufacturer provides the color code. To create a scanner's signature file, use the scanner to scan the color code and obtain the RGB values for each color patch on the color scale so that a conversion between RGB and L*a*b* can be established. A quick lookup table that maps a point in the RGB file generated on the scanner to the L*a*b* space. This is the basic composition and usage principle of the scanner's signature file. The scanner will drift over the length of the application, and this conditional variation of the scanner's profile will bring color management discrepancies. Therefore, the scanner should be calibrated. The professional scanners produced today are based on the Tango and Putian system scanners from Heidelberg. The New Color 7000 scanning software used has a strong color management feature and has its own automatic calibration function. The customer is worried that the accuracy of the scan is guaranteed. The standard display RGB ICC profile can also be added to the customer's RGB feature file for the current working state. It is scanned with RGB direct files to generate L*a*b* images. The mode allows you to modify the image signature file, saturation, brightness, and hue without rescanning. It has great flexibility to embed various device profile files into color images.
With the popularization of spectrophotometers, the color inkjet, laser printer and printer feature files are all color-coded. The digital color code of IT87/3 has a total of 928 colors composed of different CMYK combination values. Blocks, respectively, in the color inkjet, laser printer output and according to their current use of paper, ink printed on the press CMYK digital color code. Then, the L*a*b* chromaticity value of each color block of the printed sample and the printed product is measured by a spectrophotometer, and the result is filled in the record table and saved, and the CMYK color space and the L*a*b* color space are established. The mapping transformation relationship generates a signature file for a particular color printing process.
To effectively manage color across systems and applications, it is important to embed ICC profiles in files. ICC profiles will indicate the correct color space for the file, when another cognitive ICC application opens an image. The application knows if you want to perform color conversion and conversion accuracy. For example, color management is applied in digital proofing. The purpose and function of proofing is to check the pre-press color separation effect and the accuracy of editing and typesetting, and provide customers with signature prints for printing. Digital proofing does not require carriers (such as film and PS version) to transfer graphic information; The carrier's graphic data information is transmitted, so digital proofing is also called no software and no printing proofing method. There are two main types of digital proofing equipment: color inkjet printers or color laser printers. Digital proofing can simulate proofs of traditional proofing, providing standards and evidence for printing. Finally, there are environmental standards, color conversion standards, equipment standards; with the description of the device color feature files, color uniformity through color management software, and WYSIWYG in color management.
In the entire color image production process, from no data, to the establishment of data, recommended standards to data stability, color management under the open structure is possible.