The three cores of the color management system
The three cores of the color management system (3C)
Color management must follow a series of prescribed processes 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 of the color information transmission process, it is required to compare the input device, display device and output device to ensure that they reach the best working state.
Input correction: It is to correct the brightness, contrast, black and white field (RGB balance) of the input device. For example, the corrected scanner should obtain the same image data for the same original document whenever it is scanned.
Display calibration: enables the display card to accurately display colors on the display based on the color data of the image data.
Output calibration: The calibration of the collating machine is based on the correct image of the display; the calibration of the printing machine and the proofing machine must make the printing materials such as paper and ink used by the equipment comply with the standard.
2. Characterization of equipment
As mentioned earlier, one of the cores of color management is to establish a device file, which serves as a standard and is a bridge between two color spaces (PCS).
Simply put, 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 output device according to the characteristic file of the output device (display, proofer) Color space to ensure the consistency of color reproduction 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 characteristic file and convert the color data of different devices according to the device color described in the characteristic file.
A basic principle of color conversion is that the same color is guaranteed to be the same color on different devices. In order to achieve this goal, there must be a device-independent color system to measure the colors on each device, and any color space related to the device 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. When performing color conversion, the color data is first converted to by the device characteristic file. The color value of PCS is then converted to the color value of other devices as needed.
The following describes the application of these three cores in color management through the specific production of input and output device ICC characteristic files:
â— Creation of input device ICC characteristic file
(1) IT8 standard color code
If you want to create an ICC characteristic file for an input device, you generally use the Kodak IT8.7 / 1IT8.7 / 2 standard color scale (hereinafter referred to as the color scale).
IT8.7 / 1 and IT8.7 / 2 color scales are suitable for color calibration of input devices (scanners and digital cameras), IT8.7 / 1
The materials used in the IT8.7 / 2 color scale are color transparent positive film and color photo paper, respectively. IT8.7 / 1 is suitable for input color correction of transmission scanners; IT8.7 / 2 is suitable for input color correction of reflection scanners.
The IT color code consists of four parts:
①Sampling color area. This part is composed of 12 × 12 color patches. It is divided into dark color block, middle color block and light color block by column, each part has four saturation changes.
â‘¡Color ladder. There are seven columns in this part, three of which represent cyan, magenta, and yellow; the other three represent red, green, and blue; and the middle column is neutral.
â‘¢ Neutral gray ladder ruler. At the bottom of the color scale are the twenty-two neutral gray scales that vary according to degree of cosmology. Mainly used for basic correction of input devices.
â‘£Optional area. Different manufacturers can arrange three columns according to their needs, such as characteristic colors. The entire color scale contains a total of 286 color patches. These color patches are the standard color patches that establish the ICC characteristic folder of the scanner. They express the entire color gamut more completely, and each color patch contains a standard colorimetric data.
(2) Preparation of scanner ICC characteristic file
The production process of the scanner ICC characteristic file is:
â‘ Calibration
The purpose of calibration is to adjust equipment (such as scanners, monitors, printers, and printing presses) to a standard state to ensure that it meets production specifications. Most devices are calibrated when they leave the factory, but during the use of the device, the quality of color reproduction is also affected by factors such as environmental changes and aging of the device. Therefore, calibration is very important for reproducing colors. All equipment must be calibrated before use.
After the scanner is turned on, it warms up and then runs the scan driver software. It first scans the IT8 manuscript, and uses the gray scale at the bottom of the IT manuscript to correct the scanner's brightness, contrast, focal length, gray balance, and midtones.
For example: to ensure the first level of the gray ladder in the scanned image. The RGB value is maintained between 150 and 255, the RGB value of the twenty-second level, that is, the dark tone portion, is maintained between 0 and 5, and the RGB value of the eleventh level in the middle portion is 125 right.
If the R, G, and B values ​​of the scanned image are inconsistent, it means that the scanner has a color cast. Assume that the RGB values ​​of the eleventh level in the middle tone are actually measured as R = 118, G = 125, B = 125. Increase the intensity of red to maintain balance with green and blue. This can ensure that the scanned image has a small color cast and pure color.
â‘¡ Scan IT8 color code
According to the reflection of the scanner; the type of transmission selects IT8.7 / 1 color scale or IT8.7 / 7 color scale scanning. In the scan menu, cancel the screen removal, sharpening, gradation adjustment and other options related to color management, and then save the scanned RGB image.
â‘¢ Generate ICC characteristic file in ICC characteristic file making software
There are many softwares for making feature folders, for example: Heidelberg software, Agfa software.
Run the ICC feature file creation software, open the standard file containing the color block chromaticity data and the scanned image file in the software, the software will compare, correct and correct the RGB data in the image file with the color block standard Lab chromaticity data. Optimize to get the characteristic file of the scanner, and finally save this file to 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 the IT8 color scale in the process of making ICC characteristic files is like a ruler to measure the accuracy of color.
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