Three core issues of color management system

2022-08-14
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On the three core problems of color management system

the three cores of color management system (3C)

color management must follow a series of specified processes in order to achieve the desired effect. The color management process has three elements: calibration, characterization and color conversion. They are the three cores of color management

1. Equipment calibration

in order to ensure the stability, reliability and sustainability in the process of color information transmission, it is required to The output devices are compared with F to ensure that they reach the best working state.

input correction: it is to correct the brightness, contrast and black-and-white field (RGB balance) of the input devices Correct. For example, the corrected scanner should obtain the same image data for the same original no matter when it is scanned

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display correction: enable the display card to accurately display color on the display according to the color information of image data

output correction: the correction of the control setter is based on the correct image of the display; The calibration of printing machine and proofing machine must make the paper, ink and other printing materials used by the equipment meet the standards

2. Equipment characterization

as mentioned above, one of the cores of color management is to establish equipment files. As a standard, it is a bridge between two color spaces (PCS)

In short, the color management system transfers the data file to the color space of the characteristic file according to the characteristic file of the input device, and then transfers the color information of the data file to the color space of the output device according to the characteristic file of the output device (display, proofing machine), so as to ensure the consistency of color restoration in the workflow

3. Color conversion

the third core of color management is the color conversion module (CMM), which is used to interpret the device feature file and convert the color data of different devices according to the device color described in the feature file

a basic principle of color conversion is that the same color is still the same color on different devices. In order to achieve this goal, it is necessary to have a device independent color system to measure the color on each device, and any device related color space can be represented in this color space. If different device related colors can correspond to the same point in the device independent color space, the conversion between them must be accurate. During color conversion, the color data is first converted to the color value of.Pcs through the device characteristic file, and then converted to the color value of other devices as required

the following describes the application of these three cores in color management through the specific production of ICC characteristic files of input and output devices:

● production of ICC characteristic files of input devices

(1) it8 standard color code

if you want to make ICC characteristic files of input devices, you generally need to use Kodak IT8.7/1it8.7/2 standard color code (hereinafter referred to as color code)

The

it8.7/1 and IT8.7/2 color codes are applicable to the color calibration of input devices (scanners and digital cameras). The materials used in the IT8.7/1

and IT8.7/2 color codes are color transparent positive and color photo paper respectively. IT8.7/1 is applicable to input color correction of transmission scanner; IT8.7/2 is applicable to input color correction of reflection scanner

it color code consists of four parts:

① sampling color area. This part consists of 12 color blocks. It is divided into dark color block, intermediate color block and bright color block by column, and each part has four saturation changes

② color ladder. There are seven columns in this part, three of which represent cyan, magenta and yellow respectively; The other three columns represent red, green and blue respectively; The middle column is neutral

③ neutral gray scale. At the bottom of the color scale is a 22 grade neutral gray ladder varying in degrees. It is mainly used for basic correction of input equipment

④ optional area. Different manufacturers can arrange three columns according to their own needs, such as characteristic colors. The whole color code contains 286 color blocks. These color blocks are the standard color blocks that establish the scanner ICC characteristics folder, which completely express the whole color gamut, and each color block contains a standard chromaticity data

(2) production of scanner ICC characteristic file

the production process of scanner ICC characteristic file is:

① calibration

the purpose of calibration is to adjust the equipment (such as scanner, display, printer and printer) to the standard state to ensure that it meets the production specifications. Most of the equipment will be calibrated when leaving the factory, but the equipment will also change due to environmental changes during use; Due to the aging of equipment, the automatic storage of experimental conditions and results affects the quality of color reproduction, so calibration is very important for color reproduction. All equipment must be calibrated before use

warm up the scanner after it is turned on, and then run the scanning drive software. First scan the it8 original, and use the gray scale at the bottom of the it original to correct the brightness, contrast, focal length, gray balance and middle tone of the scanner

for example: ensure that the gray ladder is the first level in the scanned image. The RGB value is kept between 150 and 255, the RGB value of the twenty second level, that is, the dark part, is kept between 0 and 5, and the RGB value of the eleventh level in the middle part is kept to the right of 125

if the R, G and b values of the scanned image are inconsistent, it indicates that the scanner is biased. Assuming that the RGB values of the eleventh level of the middle tone are measured as r=118, g=125 and b=125, in this case, the red intensity can be increased to maintain a balance with green and blue. This can ensure that the color deviation of the scanned image is small and the color is pure under J

② scan it8 color code

according to the reflection of the scanner; The transmission type selects IT8.7/1 color code or IT8.7/2 color code scanning. In the scan menu, cancel de sharpening, level adjustment and other options related to color management, and then save the scanned RGB image

③ there are many software that generate ICC characteristic files

in the ICC characteristic file making software, such as the software of Heidelberg company and the software of AGFA company

run the ICC characteristic file making software, open the standard file containing the chromaticity data of colored blocks and the scanned image file in the software, and the software will compare, correct and optimize the RGB data in the image file with the lab chromaticity data of the color block standard by professionals, so as to obtain the characteristic file of the scanner. Finally, save this file and end the production of the ICC characteristic file of the input device

in this ICC characteristic file, I records the basic characteristics of the color reproduction of this scanner. Different scanners have different ICC characteristic files. It can be seen that the role of it8 color code in the process of making ICC feature files is like a ruler to measure the accuracy of color

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