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Brief introduction to color management technology


color management system (CMS), as an important direction of the current prepress system development, has been recognized by many people in the printing industry and related fields. Its birth and development are closely related to desktop publishing and digital printing. Due to the open environment of desktop publishing system, different devices and media are involved in the process of color reproduction, and there is an uncertain relationship between these devices and media, which makes it difficult to keep the image color consistent from the original to the display, and then to the final print. Therefore, color management has been paid more and more attention by software developers and insiders

1. Workflow analysis of color management system

in a sense, color management is a technical field about the correct interpretation and processing of color information, that is, to manage people's feelings about color; Objectively speaking, it is the process of converting image color data from one color space to another on the premise of minimum color distortion

in the whole image reproduction process, the devices involved have their own color capabilities, that is, different color spaces (Fig. 1)

┌ CIE xyz

┌ device independent space ┼ CIE lab

│ └ CIE luv

color space ┌ RGB based on additive method

└ device related color space └ CMYK based on subtractive method

Figure 1 Classification of color space

color the main purpose of management is to realize the conversion of different color spaces, so as to ensure the color of the same image from input to display The appearance shown in the output should match as much as possible, and finally achieve the harmony and consistency of the colors of the original and the replica. The establishment of equipment color profile is the core of color management. The description file describes the representative color features of each equipment in the system, such as chromaticity characterization curve, output color gamut characteristic curve, etc., which are used to detect the impact resistance of metal materials under dynamic load. The color management system uses these representative color features to realize the matching and conversion of the color space of each equipment, Finally, what you see is what you get (the color management process is shown in Figure 2)

display printer


platform scanner CMS proofing machine


roller scanner printer

Figure 2 color management flow chart

A. characterize the chromaticity of the scanner and establish the color description file of the scanner; Compare the RGB value of the input image and convert it to the standard color space according to the description file

b. characterize the chromaticity of the display, establish the description file of the display, and convert it to the standard color space through CMS

c. characterize the color gamut of the output device and establish the description file of the output device; Convert the percentage of CMYK points to the standard color space according to the tracing document

d. input, display and output devices are all in the same standard color space, so as to obtain a unified color appearance

second, the elements of color management

color management must follow a series of specified operating processes in order to achieve the desired effect. There are three elements in the color management process, referred to as "3C" (that is, "calibration" -- standard, characterization "-- characterization and" conversion "-- conversion)

1. Standard

in order to ensure the stability, reliability and sustainability in the process of color information transmission, it is required to standardize the input equipment, display equipment and output equipment of global peek that will usher in the peak demand in 2018, so as to ensure that they are in the calibration state

(1) input correction

the purpose of input correction is to correct the brightness, contrast and black-and-white field (the balance of RGB three primary colors) of the input device. Take the calibration of the scanner as an example. When the scanner is initialized and zeroed, the same image data should be obtained for the same original no matter when it is scanned

(2) display correction

display correction makes the display characteristics of the display conform to the ideal parameter values set in its own device description file, so that the display card can accurately display colors on the display screen according to the color information of image data

(3) output correction

output correction is the last step of the correction process, including the correction of printers and imagesetters, as well as the correction of printers and proofing machines. According to the equipment description document provided by the equipment manufacturer, the characteristics of the output equipment shall be corrected to make the equipment output according to the standard characteristics at the time of delivery. When printing and proofing correction, the paper, ink and other printing materials used by the equipment must meet the standards

2. Characterization

when all devices are calibrated, the characteristics of each device need to be recorded, which is the characterization process. Each device in the color desktop system has its own color characteristics. In order to achieve accurate color space conversion and matching, the device must be characterized. For input devices and displays, use a known standard chromaticity value table (such as it8 standard color code), compare the chromaticity value of the table with the chromaticity value generated by the input device, and make the chromaticity characterization curve of the device; For the output device, the output gamut characteristic curve of the device is made by using the color space diagram

on the basis of making the chromaticity characteristic curve of the input device, compare the color space irrelevant to the device, and make the color description file of the input device; At the same time, the color description files of the output device are made by using the color gamut characteristic curve of the output device. These description files are the bridge to transform from the device color space to the standard device independent color space

3. Conversion

based on the calibration of the equipment in the system, the correct conversion between the color spaces of each equipment is realized by using the equipment description file and taking the standard equipment independent color space as the medium. Because the color gamut of the output device is narrower than that of the original, scanner and display, it is necessary to compress the color gamut during color conversion. Four methods of color gamut compression are proposed in the ICC protocol:

A. absolute colorimetry method this method keeps the color in the output color gamut unchanged after conversion, and replaces the color beyond the output color gamut with the color at the boundary of the color gamut. For the case that the output gamut is similar to the input gamut, this method can obtain ideal replication

b. relative colorimetric method this conversion method changes the white point calibration, and all colors will be changed accordingly according to the change of the calibration point, but no gamut compression is done, so all the colors in the super excellent spatial range are also replaced by the colors with the closest gamut boundaries. With this method, white dots can be calibrated at high speed according to the color of printing paper, which is suitable for color space conversion with close range of color gamut

c. highlight saturation method this method pursues high saturation and carries out nonlinear compression on saturation. This is not necessarily true to the original, and its purpose is to wait until the color is saturated under the condition of device limitations

d. sensory this method not only maps the color gamut, but also optimizes the gradient. It maintains the relative relationship of colors, that is, adjust the conversion ratio according to the color rendering range of the output device, so as to achieve the consistency of colors in the sense

III. several mainstream color management systems

Adobe's Photoshop color management system, as the most famous image processing software of adobe company, Photoshop has its irreplaceable position in image processing. At the same time, Adobe also strives to develop Photoshop into a complete color management system. The color management feature of Photoshop software is controlled by monitor setup, printing inks setup and color separation settings

in order to make the color seen by the user on the screen as close as possible to the color of the output sample, the user's system must be calibrated first. The accurate color workflow must start from the display calibration and display settings. "Monitor Setup" includes two parts: display calibration and display setup. The products are supplied to customers from warehouses all over the European continent and the UK, and the display type, phosphor powder, color temperature and other numbers are selected correctly in the display, so that when the image is converted into color mode, the correct color conversion can be carried out according to the settings of the above parameters

the basic purpose of printing ink setting is that Photoshop provides information about the ink, paper and printing machinery used for final printing. Different paper has different absorption and dot enlargement of ink, different ink has different color rendering, and different printing methods have different dot enlargement rates. Therefore, these parameters need to be set in "printing inks setup". After obtaining the basic information from the RGB image data and the display data obtained in the "Monitor Setup", the color mode conversion can be completed according to the characteristics of the final printing conditions

the most important item in color separation is the determination of the amount of black plate. Theoretically, color separation from one RGB value can produce multiple CMYK values, 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, it may not be possible in theory. Therefore, it is necessary to set the type of color separation, the amount of black plates, and the percentage of total printing points accordingly

apple's colorsync color management system colorsync is a color management system released by apple. It adopts ICC standards and takes 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 convert colors between different color management systems through the plug in module of colorsync. Colorsync system consists of three parts, an ICC Standard color description file, color matching method (CMM) and application software interface (API)

the device description file defines the color characteristic information of the device, through which the color range that the device can display, capture and reproduce can be obtained. When using colorsync to create a file, a profile file will be stored in the file. In this way, when processing images with colorsync description files on other devices, colorsync will perform color space matching operations by comparing the characteristics of the device that created the image with the characteristics of the display, so as to obtain the best color effect

color matching method is the core part of colorsync, which realizes the conversion of different color spaces. More importantly, colorsync's color conversion mode adopts the high-quality color matching technology of Linotype hell, so users can use a better color gamut compression method to deal with colors outside the color gamut

due to the different expressive power of various application software on color, the same image color will have different appearance from image scanning to final replication. In order to master accurate colors at all times in work, it is necessary to carry out effective color matching of these application software. Color matching can be carried out through the application software of colorsync, so as to achieve the same purpose

Kodak's color management system Kodak is a color management system that supports both PC and MAC platforms. Unlike apple colorsync and Adobe's Photoshop, Kodak's color management system tends to use a modular approach to color management, and provides several local modules, which can be combined to form a complete color management

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