Foreword
As an important direction for the development of the current prepress system, the Color Management System (CMS) has been recognized by many people in the printing industry and related fields. Its birth and development are closely related to the desktop version and digital printing. Due to the open environment of the desktop publishing system, the colors involved in the copying process involve different devices and media, and there is an indefinite relationship between these devices and the media, making the image colors from the original to the display to the final print. It's hard to be consistent. Therefore, color management has become more and more important for software developers and industry insiders.
I. Workflow Analysis of Color Management System
In a sense, color management is a technical field on the correct interpretation and processing of color information, that is, managing people's perception of color; objectively speaking, the color data of an image is taken from a The process of converting a color space to another color space.
In the entire image reproduction process, the devices involved have their own expressive color capabilities, ie, different color spaces (FIG. 1). The main purpose of color management is to realize the conversion of different color spaces so as to ensure that the appearance of the same image from the input to the display and the output is matched as much as possible, so as to achieve the harmony of the color of the original and the copy. Establishing the device's color profile is the core of color management. The profile describes the representative color features of each device in the system, such as the chromaticity characteristic curve, the output color gamut characteristic curve, and the color management system. Using these representative color features to achieve the matching and conversion of the color space of each device, and ultimately achieve WYSIWYG (color management process as shown in Figure 2).
a. Colorimetric characteristics of the scanner, the establishment of the scanner's color description file; RGB value of the input image, according to the description file is converted to the standard color space;
b. Perform colorimetric characterization on the monitor, create a monitor description file, and convert to a standard color space through the CMS;
c. Color characterization of the output device, the establishment of the output device description file; according to the description of the file, the CMYK dot percentage is converted to the standard color space;
d. 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 color management process has three elements, abbreviated as "3C" (ie, "Calibration" - Standard, Characterization - Characterization and "Conversion" - Conversion).
1. Standards In order to ensure the stability, reliability, and sustainability of color information delivery, input devices, display devices, and output devices are required to be calibrated to ensure that they are in calibration.
(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. Taking the correction of the scanner as an example, after the scanner is initialized to zero, the same image data should be obtained for the same document at any time.
(2) Display calibration The display calibration makes the display characteristics of the display meet the ideal parameter values ​​set in its own device description file, so that the display card accurately displays the colors on the display screen according to the color data of the image data.
(3) Output correction Output correction is the final step of the calibration process, including the correction of printers and imagesetters, and the correction of printers and proofers. According to the device description provided by the equipment manufacturer, the characteristics of the output device are corrected so that the device is 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 characteristics of the device are compared against the chromaticity values ​​of the table and the chromaticity values ​​generated by the input device. Curve; For output devices, 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, the color description file of the output device is made using the color gamut characteristic curve of the output device. These description files are bridges from device color space to standard device-independent color space.
3. Conversion Based on the calibration of the equipment in the system, the equipment description file is used to realize the correct conversion between the equipment's color spaces using the standard equipment-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:
a. Absolute Colorimetric Method This method leaves the color change in the output color gamut unchanged, and replaces the color that exceeds the output color gamut with 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.
b. Relative Colorimetric This conversion method changes the white point calibration. All colors will be changed according to the change of the calibration point, but no color gamut compression is performed. Therefore, all colors that exceed the color space range are also gamut. The closest color to the border 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.
c. Outstanding saturation method This method pursues high saturation and nonlinear compression of saturation. This is not necessarily faithful to the manuscript. Its purpose is to wait until the device is constrained to wait until the color is saturated.
d. Perceptual This method performs gradient mapping while performing gamut mapping. It maintains the relative relationship of colors, that is, adjusts the conversion ratio according to the color rendering range of the output device, in order to make the color feel consistent.
Third, several mainstream color management system
Adobe's Photoshop color management system is 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, Printing Inks Setup, and 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 be calibrated first, and the accurate color workflow must begin with the display calibration and display settings. "Monitor Setup" includes two parts of monitor calibration and monitor setup. In the display, the correct selection is made with the display type, phosphor powder, color temperature, etc., so that 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 printing ink setup is to provide Photoshop with ink, paper, and printing machinery information for final printing. Different papers have different ink absorption and dot gain, different inks have different color rendering properties, and dot gains of different printing methods are also different. So you need to set these parameters in "Printing Inks Setup". After obtaining the basic information from the RGB image data and the display data obtained in "Monitor Setup", the color mode conversion can be 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 type of separation, the amount of black printing, and the percentage of printing total dots.
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 Profile file is stored in the file so that when processing images with ColorSync profiles 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, which realizes the conversion of different color spaces. What's more, ColorSync's color conversion mode uses Linotype Hell's high-quality color matching technology. Users can use better color gamut compression methods to handle colors outside of the color space.
Due to the different performance of various application softwares on colors, from the image scanning to the final copying, the same image color will have a different appearance. In order to be able to accurately grasp the colors at work, these application softwares need to perform effective color matching. The color matching method can perform color matching by ColorSync's application software to achieve a consistent purpose.
Kodak's Kodak color management system is a color management system that supports both PC and Mac platforms. Photoshop with Apple ColorSync and Adobe
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