I. Workflow Analysis of Color Management System
Color management is, in a certain sense, a technical field of correct interpretation and processing of color information, namely managing people's perception of color. Objectively speaking, it is the process of converting the color data of an image from one color space to another on the premise that the color distortion is minimal.
In the entire image reproduction process, the equipment involved has its own ability to represent colors, that is, different color spaces (Figure 1). The main purpose of color management is to achieve the conversion of different color spaces to ensure the same image. The appearance of the colors is matched as much as possible from the appearance of the input display and output, and ultimately the color of the original and the reproduction is in harmony. Establishing the device's color profile is the core of color management. It describes the representative color features of each device in the file system, such as the chromaticity characteristic curve, the output color gamut characteristic curve, and the color management system. These representative color features achieve the matching and conversion of the color space of each device, and ultimately achieve WYSIWYG.
1. Colorimetric characteristics of the scanner, the establishment of the scanner's color profile, the RGB value of the input image, according to the description of the file conversion to the standard color space.
2. The chrominance characteristic of the display is established, a display description file is created, and the CMS is converted to a standard color space.
3. Perform color gamut characterization on the output device, create a description file for the output device, and convert the CMYK dot percentages to the standard color space based on the description file.
4. The input, display, and output devices are all in the same standard color space for a uniform color appearance.
Second, the elements of color management
Color management must follow a series of prescribed operating procedures in order to achieve the desired results. The process of color management has three elements. These three elements are referred to as "3C", namely "Calibration"-calibration, "Characterization"-characterization, and "Conversion"-conversion.
1. calibration
In order to ensure the stability, reliability, and sustainability of the color information transfer process, calibration of the input devices, display devices, and output devices is required to ensure that they are in standard operating conditions.
1 input correction
The purpose of the input correction is to correct the brightness, contrast, black and white fields (balance of the three primary colors of RGB) of the input device, and to correct the scanner, for example, when the scanner is initialized to zero, for the same original, anytime Scanning should be able to get the same image data.
2 monitor correction
The display correction causes the display characteristics of the display to conform to the ideal parameter values ​​set in the device's own device description file, so that the display card is based on the color data of the image data to accurately display the colors on the display screen.
3 output correction
Output correction is the last step in the calibration process. It includes corrections to the printer and imagesetter, and corrections to the printer and proofer. The characteristics of the output device are calibrated according to the device description provided by the device manufacturer to make the device Output according to the factory-standard characteristics. In the printing and proofing correction, the printed materials such as paper and ink used in the equipment must meet the standards.
2. Characterization
When all devices are calibrated, it is necessary to record the characteristics of each device. This is the characterization process. Each device in a color desktop system has its own color characteristics, and in order to achieve accurate color space conversion and matching, devices must be characterized. For input devices and displays, using a known standard chromaticity value table (such as the IT8 standard color scale), the chromaticity characteristic curve of the device is made against the chromaticity values ​​of the table and the chromaticity values ​​generated by the input device. For the output device, use the color space diagram to make the device's output color gamut characteristic curve.
Based on the chroma characteristic curve of the input device, a color description file of the input device is made against the device-independent color space. At the same time, using the output device's color description file, these description files are independent from the device color space to the standard device. Color space for conversion bridges.
3. Convert
Based on the calibration of the equipment in the system, the device description file is used to realize the correct conversion of the color space of each device using the standard device-independent color space as a medium. Since the color gamut of the output device is narrower than the color gamut of the original document, scanner, and display, the color gamut needs to be compressed during color conversion. The color gamut compression proposes four methods in the ICC protocol.
1 Absolute Colorimetric Method This method leaves the color in the output color gamut unchanged, and the color beyond the color gamut is replaced by the color of the gamut boundary. For the case that the output color gamut is similar to the input color gamut, an ideal copy can be obtained by this method.
2 Relative Chromatometry This conversion method changes the white point calibration. All colors will be changed according to the change of the calibration point, but the color space compression is not performed. Therefore, all colors beyond the color space range are also bound by the color gamut. The closest color is replaced. In this way, the white point can be calibrated at high speed according to the color of printing paper, which is suitable for color space conversion with a close range of color gamut.
3 Outstanding saturation method This method pursues high saturation and nonlinear compression of saturation. It is not necessarily faithful to the manuscript. Its purpose is to obtain a full color with limited equipment.
4 Sensing Method This method performs gradient optimization while mapping the color gamut. It maintains the relative relationship of the colors, and adjusts the conversion ratio according to the color rendering range of the output device, so as to make the sense of color consistent.