Color management through the correction system

With the development of computers and external devices, color desktop systems have emerged, and the input devices, displays, and final output methods of images have become increasingly diverse. High-resolution scanners, digital cameras, color printers, and digital color printers for the short-run market have become very popular. The types of manuscripts have also evolved from traditional reversals, negatives, and reflections to photo CDs. Computers and displays used for image processing also vary. In addition, color output devices adapted to various needs are also increasing, such as color inkjet printers, thermal wax printers, and thermal sublimation printers. The emergence of the Internet has opened up a new field for image output. Because the color image processing system is an open system, the composition of the system is flexible, that is, it can use the scanner produced by Party A, the display produced by Party B, the imagesetter and printer produced by Party C. The equipment produced by different manufacturers is combined into a system. How to maintain the consistency of the color from the original input of the scanner to the proofing and the output of the finished product, and the quality control of the color of the image is crucial.

I. The essence and task of color management

The essence of color management is to precisely control and manage color transfer in the entire desktop publishing system, achieving perfect color reproduction effects. Color management enables the color reproduction and reproduction of colors to be independent of the devices used, that is, the same color data, With any system output, you get the same color effect.

All hardware devices (scanners, monitors, printers, laser imagesetters, printers, and other output devices) in the manufacturing process interpret and define colors differently, and color information is easily mistyped between devices of different types. There is no guarantee that the colors of the exchanged files between the systems will remain the same. The main task of color management is to solve the problem of data conversion of images in various color spaces, and minimize the distortion of the colors of the images throughout the entire production process. The basic idea is to choose a device-independent color reference space, then characterize and describe each device in the entire system, and finally establish a definite correspondence relationship in the color space of each device.

Second, the content of color management

The content of color management includes equipment calibration, feature description, and color conversion. Calibration of equipment is the premise to ensure the normal operation of the system, but it does not guarantee that the image and the manuscript remain the same, but to see whether the equipment meets the original design specifications; the description of the equipment characteristics is the color expression ability and the presented color range of different equipments. The description of the color conversion is to convert the color space of one device to the color space of another device under a uniform condition, and establish an anti-color mapping between different color spaces.

Feature description refers to the use of digital methods to describe in detail the color rendering performance of color media such as scanners, monitors, color printers, and inks. That is, the appropriate color system is used to describe the color space of each media. The feature description is a method for measuring and determining the color gamut or the reproducible color set of various input/output devices. The color gamut can be defined. After the characterization, the data describing the color rendering range of the device is called device color. Profiles, Profiles are usually provided by the manufacturer at the time the device is shipped from the factory, and sometimes can be used with certain tools (such as the US Light Source Colorton Color Metrics) and the corresponding color management software (such as Haier's Printopen, Scanopen, etc.) Build your own Pro-file.

Color conversion refers to a one-to-one mapping relationship between different color spaces according to different colors, and the color in a color space on a device is converted into a color space in another known condition. Since the color gamut of the output device is generally smaller than the color gamut of the original document, the scanner, and the display, it is necessary to compress the color gamut of the original in the color gamut of the output device. First, the RGB color information obtained from the scanner is converted into a device-independent CIE-Lab color space. Then, according to the ICC Profile of the next device, the CIE-Lab color space is converted into the device's own color space. Since the CIE-Lab color space has a wide color gamut containing other color spaces, color quality is not lost.

Third, the display calibration (calibration)

Device color correction is to make the input and output devices use color description files to formulate and describe colors in accordance with their respective characteristic curves. It is the basis for characterizing device color rendering methods. Before the color data is converted between different media, the calibration of the device must be performed. The deviation of the color rendering performance of the same device under different conditions is determined by scaling, so that the color information has a temporal consistency in the process of acquisition and transmission. The purpose of correcting the display is to make the colors of the displayed image and the final output image as close as possible. The calibration procedure is as follows: 1 After the monitor is turned on, it is warmed up for half an hour to keep the monitor in a stable state; 2 The indoor light source is adjusted to a level that can be constantly maintained, and the extra light source is turned off to prevent these dynamic changes from affecting the display. Brightness and Contrast: 3 Turn off all the desktop patterns and change the background color of the monitor to neutral gray. This will not affect the vision during the calibration and help to adjust the gray balance. 4 Set the gamma value, first adjust From the Gamma control panel, select the appropriate Target Gamma at the top of the dialog box, which is the target gamma value. The general image recommendation is 1.8. If you want to use a video recorder or film to output the image, Gamma will only be set to 2.2; 5 Field, first create a blank new file in Photoshop, then select a paper with the same degree of whiteness printing, click the White Point button, drag the triangle slider until the white in the display matches the white in the pattern as much as possible; 6 Correct gamma values, adjust with GammaAdjustment until two color patches in the bicolor gray bar above the triangle slider Similar, there is no obvious limit; 7 correct color balance and gray balance, click Balance, adjust the RGB tricolor slider, until there is no color in the gray ladder feet below the slider, a gray level; 8 correct black field, Click "BlackPoint" and drag the RGB tri-color slider until the dark part of the gray scale below the slider matches the dark part of the gray scale in the print. After the above steps, the monitor color calibration process is completed. The school color result will immediately work on the display of the monitor. At this time, the Gamma window is closed. The Gamma window setting will take effect each time the computer is started. Of course, we can also press the Save Setting button for various requirements for different papers, different monitors, etc., and store several gamma files in the Control Panel. After setting the memory, reboot the machine, click the Load Setting button in the Camma dialog box, and select the appropriate Gamma setting.

Fourth, the scanner calibration

For scanners, products from different manufacturers have their own unique color correction systems, such as: Dynamic color rendition (DCR) dynamic color correction software from Microtek, FotoTune color management software from Agfa, and ImageCalibration color management software from Tsinghua Unisplendour. Through these color management software, color compensation is automatically performed, effectively solving 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, the standard color tables AGFA-IT8 (for scanning draft) and KODAKQ-60 (for transmissive writing) provided by MICROTEK are first placed on the scanner. Perform the MICROTEK CALIBRATION program, select CALIBRATE, and press the key to start the DCR color correction system. When the general image is scanned later, simply check to see if DCR is selected. Once selected, the DCR will automatically be applied to all scanned color images.

Fifth, the system color

In the case of a ship, most systems are equipped with scanner input images, image processing in the computer, graphics and text, etc., and then through the MO disc, Zip disc or through the network output to the output center output film, and then go plate-making , proofing, printing, etc., so when the scanner scans the image, it should be aware of where the scanned image is used, what type of printer, ink, paper, etc. are used. Although it is not possible to directly adjust graphic output machines or printers, image processing software such as Photoshop can be used to set ink parameters and color separations.

1. Printinglnk Setup

PrintingLnk Setup in Photoshop is a useful tool for printing with the back end, where we can set the variables such as ink type, dot gain and color shift during printing, and these settings can only be performed between color modes. It only takes effect when converting. In the Printinglnk Setup dialog box, click Ink Color to select a suitable ink standard and printing paper, because the absorption of the ink by the paper will have a great influence on the final result. The default setting is the US standard ink in the coated paper. The SwopCoated standard on coated paper is more suitable for most of the printing requirements in China. The dot gain preset represents the halftone dot enlargement rate of the image on the specified paper. Photoshop then uses this data to create a dot gain curve to adjust The dot enlargement rate of each tone of the image. Set the dot expansion to a smaller value, the image is brighter, set a larger value, and the image is dark. Use the reflection densitometer to measure the color stripe in the proofing and adjust the dot gain value according to the test result. The gray balance setting can independently control the CMYK color to compensate the printing color shift; if the image is a grayscale image or a color image The channel, you want to see the effect of dot gain on the screen during image processing, you can select Use Dot Gain for Grayscalelmage option.

2. Separation Setup

Provides control of the generation of CMYK four-color plates in separation settings, including: Black Generation, Under Color Removal (UCR), Total lnk Limit, and Under color gain UCA (Under Color Addition). 1 In the Separation Settings dialog box, the color separation type can be selected according to the needs and the production quality of the printing factory - UCR/GCR. Due to technical factors, GCR often causes the highlights to be dirty, while UCR is only for the shadowing. Alternatives, there will not be much deviation, use more: 2 Black version of the degree of Light (Light), Medium (Middle), Heavy (Altitude), Maxium (Max), Custom (Custom) and so on Choices, customization is the ability to adjust the black version of the curve in the image, other color version will automatically calculate its own replacement; 3 black version of the ink limit is the UCR/GCR black version of the amount of ink used, Photoshop lack Provincial setting is 100%, usually set it as 80% -90% will basically be able to suppress other colors; 4 total ink limit that the printing press can support the maximum ink density, the default setting in Photoshop is 300%; 5 Under color gain UCA (Undercolor Addition) refers to the use of GCR black version of the production mode, in the dark tone of the image area to add a certain amount of blue, yellow, yellow, if in the dark tone area only black version instead, sometimes caused Too flat Tune, appropriately adding yellow green color ink products, will make dark portions of images richer. In addition, this option can only be adjusted when the GCR black generation method is selected. UCA can adjust the range of 0% -100%, as the value increases, the amount of CMY gradually increases.

Color management is the most difficult and important part of the process of color image reproduction. With the continuous improvement of color requirements of users, the role of color management in prepress publishing systems will become increasingly important. The International Color Organization (ICC) has developed a standard ICC profile that describes the color expression of devices.