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.
The classification of color space
Figure II color management flow chart
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.
Third, several mainstream color management system
1. Adobe's Photoshop color management system. As Adobe's most famous image processing software, Photoshop has an irreplaceable position in image processing. At the same time, Adobe also strives to develop Photoshop into a complete color management system. The color management features of Photoshop software are controlled by the "Monitor Setup" - Display Settings, "Printing Inks Setup" - Printing Ink Settings, and "Separation Setup" - Separation Settings.
In order to make the color the user sees on the screen be as close as possible to the color of the output proofs, the user's system must first be calibrated. The accurate color workflow must begin with an accurate display correction and correct display settings. Monitor Setup includes monitor correction and monitor setup. In the display, the display type, phosphor powder, color temperature and other parameters are correctly selected. In this way, when the image is converted to the color mode, the correct color conversion can be performed according to the setting of the above parameters.
The basic purpose of the printing ink setup is to provide Photoshop with information on the inks, papers, and printing machinery used in the final printing. Different papers have different ink absorption and dot gain, and different inks have different color rendering properties. The dot gain rates of different printing methods are also different. So you need to set these parameters in Photoshop Inks Setup. After the basic information is obtained from the RGB image data and the display data obtained in the Monitor Setup, the color mode conversion is performed according to the characteristics of the final printing conditions.
The most important item in the separation is the determination of the black version. In theory, multiple CMYK values ​​can be generated from one RGB value separation, and the final printing effect should be the same. However, due to the ink receiving capacity of different printing papers and the characteristics of different printing machines, the theoretical possibility may not be realized. Therefore, it is necessary to set the percentage of printing separation, the amount of black printing, and the percentage of printing total dots.
2. Apple's ColorSync color management system. ColorSync is a color management system released by Apple. It adopts the ICC standard and uses the Lab color space as the standard reference space. It is a system-level color management software applied under the Mac operating system. It is an open color management system, users can use ColorSync's Plug-in module to perform color conversion between different color management systems. The ColorSync system consists of three components, an ICC standard color profile, a color matching method (CMM), and an application software interface (API).
The device description file defines the color characteristic information of the device, and through this information, the color range that can be displayed, captured and reproduced by the used device can be obtained. When using ColorSync to create a file, a Profiles file is stored in the file so that when processing images with the ColorSync description file on other devices, ColorSync compares the device characteristics of the created image with the characteristics of the display. Color space matching operations to get the best color effect.
The color matching method is the core part of ColorSync. It realizes the conversion of different color spaces. More importantly, ColorSync's color conversion mode adopts Linotype HELL's high-quality color matching technology. Users can use a better color gamut compression method to process color. Outside the domain.
Because various application softwares have different color performances, the same image color will have different appearance from image scanning to final copying. In order to be able to grasp the quasi-color at any time during the work, it is necessary to apply these application software to perform effective color matching. The color matching method can be used for color matching through the ColorSync application software to achieve a consistent purpose.
3. Kodak's color management system. Kodak is a color management system that supports both PC and Mac platforms. Unlike Apple's ColorSync and Adobe's Photoshop, Kodak's color management system tends to use color management in a modular way, and it provides several localized modules that can be combined to form a complete color management. system. And it also has color management capabilities for the Photo CD image input process.
Kodak color management system generally includes four modules: KPCMS, DCP, PICC and PCS100. They can be used separately or combined into a complete color management system.
The KPCMS module (Kodak Precision Color Management System) provides a sufficient number of device description files that conform to the ICC standard format. Its input devices include RGB image sources and CMYK image sources, and its output devices include hitting machines and proofers. However, since it cannot edit the custom device description file itself, it has great limitations.
DCP module (device color description file startup software package) is a software package that provides display and output color correction specifically for Photo CD images. When opening a Photo CD image, the user is required to select two device description files. The input device description file is a The description file of the Photo CD device. The output device description file may be a display description file or a color proofer description file. The PICC module (Precision Input Color Characterization) provides a scanner utility calibration routine that enhances the DCP functionality.
For advanced image processing and scanning applications, Kodak provides the PCS100 module to speed up the color conversion process of Photo CD images. With the PCS100 module, direct links between various devices can be generated to skip the intermediate conversion process of the CIE color space, saving time for each conversion and color loss during conversion.
Fourth, how to choose the right color management system
At present, many companies at home and abroad have developed color management systems. Although the International Color Organization (ICC) formulates the standard ICC Profile describing the color expression of devices, the color management systems developed by companies under this standard have the ability to manage the color of devices. Differently, the color space conversion between devices should be based on
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.
The classification of color space
Figure II color management flow chart
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.
Third, several mainstream color management system
1. Adobe's Photoshop color management system. As Adobe's most famous image processing software, Photoshop has an irreplaceable position in image processing. At the same time, Adobe also strives to develop Photoshop into a complete color management system. The color management features of Photoshop software are controlled by the "Monitor Setup" - Display Settings, "Printing Inks Setup" - Printing Ink Settings, and "Separation Setup" - Separation Settings.
In order to make the color the user sees on the screen be as close as possible to the color of the output proofs, the user's system must first be calibrated. The accurate color workflow must begin with an accurate display correction and correct display settings. Monitor Setup includes monitor correction and monitor setup. In the display, the display type, phosphor powder, color temperature and other parameters are correctly selected. In this way, when the image is converted to the color mode, the correct color conversion can be performed according to the setting of the above parameters.
The basic purpose of the printing ink setup is to provide Photoshop with information on the inks, papers, and printing machinery used in the final printing. Different papers have different ink absorption and dot gain, and different inks have different color rendering properties. The dot gain rates of different printing methods are also different. So you need to set these parameters in Photoshop Inks Setup. After the basic information is obtained from the RGB image data and the display data obtained in the Monitor Setup, the color mode conversion is performed according to the characteristics of the final printing conditions.
The most important item in the separation is the determination of the black version. In theory, multiple CMYK values ​​can be generated from one RGB value separation, and the final printing effect should be the same. However, due to the ink receiving capacity of different printing papers and the characteristics of different printing machines, the theoretical possibility may not be realized. Therefore, it is necessary to set the percentage of printing separation, the amount of black printing, and the percentage of printing total dots.
2. Apple's ColorSync color management system. ColorSync is a color management system released by Apple. It adopts the ICC standard and uses the Lab color space as the standard reference space. It is a system-level color management software applied under the Mac operating system. It is an open color management system, users can use ColorSync's Plug-in module to perform color conversion between different color management systems. The ColorSync system consists of three components, an ICC standard color profile, a color matching method (CMM), and an application software interface (API).
The device description file defines the color characteristic information of the device, and through this information, the color range that can be displayed, captured and reproduced by the used device can be obtained. When using ColorSync to create a file, a Profiles file is stored in the file so that when processing images with the ColorSync description file on other devices, ColorSync compares the device characteristics of the created image with the characteristics of the display. Color space matching operations to get the best color effect.
The color matching method is the core part of ColorSync. It realizes the conversion of different color spaces. More importantly, ColorSync's color conversion mode adopts Linotype HELL's high-quality color matching technology. Users can use a better color gamut compression method to process color. Outside the domain.
Because various application softwares have different color performances, the same image color will have different appearance from image scanning to final copying. In order to be able to grasp the quasi-color at any time during the work, it is necessary to apply these application software to perform effective color matching. The color matching method can be used for color matching through the ColorSync application software to achieve a consistent purpose.
3. Kodak's color management system. Kodak is a color management system that supports both PC and Mac platforms. Unlike Apple's ColorSync and Adobe's Photoshop, Kodak's color management system tends to use color management in a modular way, and it provides several localized modules that can be combined to form a complete color management. system. And it also has color management capabilities for the Photo CD image input process.
Kodak color management system generally includes four modules: KPCMS, DCP, PICC and PCS100. They can be used separately or combined into a complete color management system.
The KPCMS module (Kodak Precision Color Management System) provides a sufficient number of device description files that conform to the ICC standard format. Its input devices include RGB image sources and CMYK image sources, and its output devices include hitting machines and proofers. However, since it cannot edit the custom device description file itself, it has great limitations.
DCP module (device color description file startup software package) is a software package that provides display and output color correction specifically for Photo CD images. When opening a Photo CD image, the user is required to select two device description files. The input device description file is a The description file of the Photo CD device. The output device description file may be a display description file or a color proofer description file. The PICC module (Precision Input Color Characterization) provides a scanner utility calibration routine that enhances the DCP functionality.
For advanced image processing and scanning applications, Kodak provides the PCS100 module to speed up the color conversion process of Photo CD images. With the PCS100 module, direct links between various devices can be generated to skip the intermediate conversion process of the CIE color space, saving time for each conversion and color loss during conversion.
Fourth, how to choose the right color management system
At present, many companies at home and abroad have developed color management systems. Although the International Color Organization (ICC) formulates the standard ICC Profile describing the color expression of devices, the color management systems developed by companies under this standard have the ability to manage the color of devices. Differently, the color space conversion between devices should be based on
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