With the development of calculators and external devices, color desktop systems have emerged, from image input devices, monitors to final output methods are increasingly diversified. The application of high-resolution scanners, digital cameras, color printers and digital color printers for the short-term market has become very common. The types of manuscripts have also evolved from traditional reversal films, negative films, and reflections to photo CDs. The calculators and displays used for image processing are also different. In addition, color output devices that meet various needs are increasing, such as color inkjet printers, thermal wax printers, and sublimation printers. The emergence of the Internet has opened up a new field for image output. Since the color image processing system is an open system, the composition of the system is flexible and flexible, that is to say, the scanner produced by Party A, the display produced by Party B, and the imagesetter and printer produced by Party C can be used. The equipment produced by different manufacturers is combined into a system. How to maintain the consistency of color from the original input of the scanner to the proofing and output of the finished product, the quality control of the image color is crucial.
1. The essence and task of color management
The essence of color management is to accurately control and manage the color transmission in the entire desktop publishing system to achieve a perfect color reproduction effect. Color management can make color reproduction and reproduce color independent of the equipment used, that is, the same color data, With any system output, you will get the same color effect.
All hardware devices (scanners, monitors, printers, laser imagesetters, printers, and other output devices) in the production process interpret and define colors in different ways, and errors in the conversion of color information between different types of devices are prone to errors , There is no guarantee that the colors of the files exchanged between the systems remain the same. The main task of color management is to solve the problem of image data conversion in various color spaces, so that the color of the image is minimized during the entire production process. The basic idea is: choose a color reference space that is not related to the device, then characterize each device of the entire system, and finally establish a certain correspondence in the color space of each device.
Second, the content of color management
The content of color management includes device calibration, characterization, and color conversion. Device calibration is a prerequisite to ensure the normal operation of the system, but it does not guarantee that the image and the original are consistent, but to see whether the device reaches the original design pointer; the description of the device characteristics is the color expression ability of different devices and the range of colors presented Description; color conversion is to convert the color space of one device to the color space of another device under consistent conditions, and to establish a color image of the mapping between different color spaces.
Feature description refers to the use of digital methods to describe the color rendering performance of color media such as scanners, displays, color printers, and inks in detail, that is, to describe the color space of various media with an appropriate color system. Feature description is a method to measure and determine the color gamut or reproducible color set of various input and output devices. It can define the color gamut (ColorGamut). The data that describes the color rendering range of the device after the feature description is called the device color. Profile (Profile), Profile is usually provided by the manufacturer when the device is shipped from the factory, and sometimes it can also be used with certain tools (such as the Colorton color measurer of Light Source in the United States) and corresponding color management software (such as Haier ’s) Printopen and Scanopen, etc.) to create Pro-file by themselves.
Color conversion refers to converting the colors in the color space on a device to another color space under known conditions according to the mapping relationship between different colors in different color spaces. Since the color gamut of the output device is generally smaller than the color gamut of the original, scanner, and display, it is necessary to compress the color gamut of the original in the color gamut of the output device. First convert the RGB color information obtained from the scanner to the CIE-Lab color space that is not related to the device, and then convert the CIE-Lab color space to the device's own color space according to the ICCProfile of the next device. Since the CIE-Lab color space has a wide color gamut containing other color spaces, there is no loss of color quality.
3. Calibration (calibration) of the display
Device color correction is to make the input and output devices use color description files to formulate and describe colors according to their respective characteristic curves. It is the basis of the device's color rendering method as a feature description. Before converting color data between different media, be sure to calibrate the device. The calibration determines the deviation of the color rendering performance of the same device under different conditions, so that the color information has temporal consistency during the acquisition and transmission process. The purpose of correcting the display is to make the displayed image and the final output image color as close as possible. The calibration steps are as follows: After the monitor is turned on, warm it up for half an hour to keep the monitor in a stable state; Adjust the indoor light source to a level that can be maintained frequently, and turn off the extra light source to avoid these dynamic changes affecting the display Brightness and contrast: Turn off all desktop patterns and change the background color of the monitor to neutral gray, so that it will not affect the vision during the calibration process and help to adjust the gray balance; Set the Gamma value and adjust first From the Gamma control panel, select the appropriate Target Gamma at the top of the dialog box, that is, the target Gamma value. The general image is recommended to use 1.8. If you want to use a video recorder or film recording to output the image, Gamma is only set to 2.2;  To correct the white field, first create a blank new file in Photoshop, then select a paper with the same whiteness as the printing, click the WhitePoint button, drag the triangle slider until the white in the display matches the white in the pattern as much as possible ; Correct Gamma value, adjust with GammaAdjustment until the visual effect of the two color blocks in the two-color gray bar above the triangle slider Near, there is no obvious boundary;  correct the color balance and gray balance, click Balance, adjust the RGB three-color slider, until there is no color in the gray scale below the slider, it is a gray color scale;  correct the black point, Click "BlackPoint" and drag the RGB tri-color slider until the dark part of the gray scale below the slider feels the same as the dark part of the gray scale in printing. After the above steps, the color calibration process of the display is completed. The color correction result will immediately affect the display of the monitor. At this time, close the Gamma window, and the Gamma window setting will take effect every time the computer is activated. Of course, we can also save several Gamma files in the Control Panel by clicking the Save Setting button for different papers, different monitors and other requirements. After setting the storage, reactivate the machine, click the Load Setting button in the Camma dialog box, and select the appropriate Gamma setting value.
4. Scanner calibration
For scanners, products from different manufacturers have their own unique color correction systems, such as: MICROTEK's DCR (dynamic color rendition) dynamic color correction software, Agfa's FotoTune color management software, Tsinghua Unisplendour's ImageCalibration color management software, etc. Through these color management software, automatic color compensation, effectively solve the problem of color distortion of the scanned image, so that the color image has the best color effect. The use of scanner color management software is very simple. Taking DCR as an example, first put the standard color table AGFA-IT8 (used for scanning reflection drafts) and KODAKQ-60 (used for scanning transmission drafts) provided by MICROTEK in the scanner. Execute the MICROTEK CALIBRATION program, then select CALIBRATE, and press the button to activate the DCR color correction system. When scanning general images in the future, simply check whether DCR is selected. Once selected, DCR will be automatically applied to all scanned color images.
V. System color correction
In general, most systems are equipped with a scanner to input images, perform image processing in a calculator, mix images and text, etc., and then transmit the film to the output center through the MO disc, Zip disc or through the network, and then go to the plate making , Proofing, printing, etc., so when scanning the image, the scanner should be aware of, such as where the scanned image is used, what type of printer, ink, paper, etc. used. Although it is not possible to directly adjust the graphic output machine and printer, you can use image processing software such as Photoshop to set the ink parameters and color separation settings.
1. Printer ink setup (Printinglnk Setup)
Printinglnk Setup (printer ink setup) in Photoshop is an advantageous tool to cooperate with back-end printing, in which we can set variable factors such as the type of ink, dot expansion and color deviation during printing, and these settings are only made between color modes It only takes effect during conversion. In the Printinglnk Setup dialog box, click Ink Color, select a suitable ink standard and printing paper, because the paper's absorption of ink will greatly affect the final result, where the default setting is US standard ink on coated paper The SwopCoated standard on (coated paper), this setting is more suitable for most printing requirements in China: the preset of dot expansion represents the dot expansion rate of the middle tone of the image on the specified paper, and Photoshop then establishes a dot expansion curve based on this data to adjust The dot expansion rate of each tone of the image. Set the dot expansion to a smaller value, the image is brighter, set a larger value, the image is darker. Use a reflection densitometer to measure the color correction bar during proofing, and adjust the dot expansion value according to the test results; the gray balance setting can independently control the CMYK color to compensate for the printing color deviation; if the image is a grayscale image or a color image For the channel, if you want to see the effect of using dot expansion on the screen during the image processing, you can select the Use Dot Gain for Grayscalelmage option.
2. Separation Setup (SeparationSetup)
In the color separation settings, it provides how to control the generation of the CMYK four-color version, including: the black generation method, the under color removal UCR (Under ColorRemoval), the total printing ink volume control (Total lnk Limit) and Under Color Addition (UCA). In the color separation setting dialog box, you can choose the color separation type-UCR / GCR according to the needs and the production quality of the printing plant. Due to technical factors, GCR often causes dirty spots in the highlight part, while UCR is only for the dark part Substitute, there will not be too much deviation, more use: The degree of black version includes Light (light), Medium (medium), Heavy (height), Maxium (maximum), Custom (custom), etc. There is a variety of options, you can customize the black version curve in the image, and other color versions will automatically calculate their own replacement amount;  Black version ink limit refers to the amount of ink used in the black version of UCR / GCR, which is missing in Photoshop The province setting is 100%, usually set it to 80% -90% to basically suppress other colors; Total ink limit means the maximum ink density supported by the printing press, the default setting in Photoshop is 300%; Undercolor Addition (UCA) means that after using the GCR black version generation method, a certain amount of cyan, magenta, and yellow are added to the dark tone area of ​​the image. If only the black version is used in the dark tone area, it may sometimes cause Too bland Tune, appropriately adding yellow green color ink products, will make dark portions of images richer. In addition, this option can be adjusted only when the GCR black version generation method is selected. The adjustable range of UCA is 0% -100%. As the value increases, the amount of CMY gradually increases.
Color management is the most difficult and most important link in the process of color image reproduction. With the continuous improvement of users' color requirements, color management will become more and more important in the prepress publishing system. The International Color Organization ICC has developed a standard ICC Profile that describes the color expression of the device. According to the standard, the color description files created by different color management systems can be converted at will, and cross-platform color communication can be achieved.
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