The third step: color conversion.

Color conversion refers to the color conversion between a device and a device or device and a pigment or a pigment and a pigment. The color range of each device or paint is different: for example, the color display is RGB color, and the color printing is CMYK color; and the color range of different brands (even the same brand) color display may not be the same; similarly different ink manufacturers The ink color range may also be different.

The color conversion in color management does not provide exactly the same color, but makes the device or pigment achieve the best color while allowing the user to know the color obtained. Soft Proofing, for example, uses a color display (RGB color) to simulate four-color printing (CMYK color) colors. The area where the triangle overlaps with the quadrangle in the figure below represents the same color as the display and the four-color printing. The colors outside the range are calculated by the color management system (color matching/gamutmapping) to give the closest color.

Replace it.


This alternative process is not random and disorderly. The International Color Association has proposed four solutions:

(1) Absolute Colorimetric Method This method is to keep the color in the same color range unchanged, and replace the color beyond the color range with the border color. In the case where the input color range and the output color range are similar, this method can obtain an ideal color reproduction.

(2) Relative colorimetric method This method changes the white point calibration, and all colors will be changed according to the change of the calibration point, because all the colors beyond the color space range are also replaced by the color with the closest boundary. This method can be used to calibrate the white point at a high speed according to the color of the printing paper, which is suitable for the color space conversion with a close color range.

(3) Outstanding saturation method This method pursues high saturation and nonlinear compression of saturation, which may not be faithful to the original. Its purpose is to get a saturated color with limited equipment.

(4) Sensibility 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.

5, common color management system

Now there are many color management products, but it can be roughly divided into several levels: First, the color management in the computer operating system (such as ICM in Windows, ColorSync in MAC, etc.), and the second is the color installed in the operating system. Management software (such as Kodak's KPCMS, Agfa's ColorTune, Adobe's Photoshop, etc.), as well as color measurement instruments (such as X-Rite's ColorElite, Grinda's Spectrolino, etc.) and color management hardware (such as Linuo Haier's MacCTU Color Transfer Interface Card).

(1) ColorSync

Mac's ColorSync color management system 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 you use ColorSync to create a file, a Profile file is stored in the file so that when you process an image with a ColorSync profile on another device, ColorSync
By comparing the characteristics of the device that created the image with the characteristics of the display, color space matching operations are performed to obtain 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.

(2) Photoshop

Adobe's Photoshop color management system, Adobe's most famous image processing software, has an irreplaceable position in image processing. 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.

(3) KPCMS

Kodak's color management system KPCMS supports both PC and Mac platforms. Unlike ColorSync and Photoshop, KPCMS tends to use a modular approach to color management and provides several localized modules that can be combined to form a complete color management system. And it has for Photo

The color management function of the CD image input process. The Kodak color management system generally includes four modules: KPCMS, DCP, PICC and PCS100, which can be used separately or combined into a complete color management system.

The KPCMS module (Kodak Precision Color Management System) provides enough 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 printers and proofers. However, since it cannot edit the device description file itself, it has great limitations.

The DCP module (device color profile startup package) is a software package that provides display and output color correction for PhotoCD images. When opening a PhotoCD image, the user is required to select two device description files. The input device description file is a Photo CD device. The description file of 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 standard program that enhances the DCP functionality.

The PCS100 module is used for advanced image processing and scanning applications, accelerating the color conversion process of PhotoCD and ProphotoCD images. With the PCS100 module, direct links between various devices can be generated to skip the intermediate conversion process to the CIE color space, saving time for each conversion and color loss during conversion.

(4)ColorTune

Agfa's ColorTune is a software solution for professional color management. Unlike color profiles of scanners, display screens, or printers that use the universal color management built into the operating system, ColorTune uses a personalized ICC profile to enhance your color workflow. ColorTune helps you set up the color characteristics of scanners and display screens, and the upgraded ColorTune Pro allows you to create the color characteristics of your own output device or printer.

ColorTune guarantees consistent color output on different devices such as scanners, display screens or printers. Although we know that each person's perception of color is different, the ability of each device to process and reproduce colors is also different. With ColorTune, Agfa's professional color image processing technology allows you to fully enjoy the color processing capabilities of all devices.

Agfa's ColorTune not only ensures accurate color, but also has high reliability and high predictability. Once you use ColorTune to adjust your output device, you can reliably copy the colors you want, and even predict the image output on the printer or any other method based on the image on the screen, such as four-color printing, high-speed fax color Printing and so on. ColorTune makes everything in your hands.

(5)ColorElite

X-Rite's ColorElite system is a high-precision, innovative and portable solution that allows creative professionals to color-control all devices, including monitors, scanners, digital cameras and printers. The Pulse ColorElite system includes a new hand-held precision spectrophotometer, an award-winning display calibrator, advanced color management software and a versatile PathfinderTM handheld scanning guide.

(6) CPS

Heidelberg's CPS color management software was developed by world-renowned prepress equipment development and manufacturer Linotype-Hell, and was acquired by Heidelberg in 1997. The CPS color management software is LinoColoe, ViewOpen ICC, Scanopen ICC and PrintOpen ICC. The first is the color management center and scanner driver software, and the latter three are the feature software for the manufacture of screens, input and output devices. (to be continued)

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