Feel the color management around you (1)

Not long ago, an advertiser asked: “The same ad is published in four different magazines using the same video. Why is the effect printed out differently?” In fact, this involves the problem of color management in the process of image copying. . Regarding the color management system, although the related books and magazines have already introduced many, most of them focus on the principle introduction and theoretical discussion, seldom on the actual application and work process. In the process of communicating with some of my peers, I feel that we have some problems with the understanding of color management and the specific applications.

In fact, color management is on our side. Today's color image printing and copying process, whether from image scanning, processing, printing, filming, printing, or CTP direct plate making, to printing, inevitably involves color management. Whether you like it or not, consciously or unconsciously, the color management system will inevitably influence the change of color. The original intention of the color management system is to hope that what is seen on the computer screen is to get the color of the image from a printer or a printer, and to truly achieve "what you see is what you get." In principle, color management is to convert the image color in the color space of different devices, so that the image color can still maintain the visual consistency after being processed by different devices. Correct use of the color management system allows the image to be correctly restored during print copying. However, if used improperly, the color management system itself will also become a factor that affects the correct restoration of image colors.

There is a view that the application of the color management system completely abandons the traditional dot replication theory. In fact, the theory of color management is based on the reproduction of outlets. Therefore, some people think that color management is a traditional way to solve the current problem. Although the application of the color management system can realize the reproduction of colors, merely relying on color management may not be able to realize idealization of image reproduction. Correctly copying and restoring images also requires the use of image processing methods and skills based on dot structure and replication principles.

1, see the role of color management system from Photoshop

In theory, a color management system simply uses one color of one color space to correspond to one color of another color space. However, in a specific application, the color management system that we can feel is just a so-called device color profile. How the color profile of the device constrains and influences the color change in the image copying process requires special attention. Although the role of the color management system runs through the entire color image reproduction process, the key operations are mainly in the prepress stage. Here will mainly from the color management system in the Photoshop processing of the various processes of the specific editing and application, to discuss the role of color management system in the image processing process. 1. Image mode conversion and color separation Open Photoshop, click on the toolbar "Edit" "Color Settings", where "color space" has "RGB", "CMYK", "gray", "spot color" and many other options. They correspond to the color space of computer display, printing or proofing, monochrome (black and white) printing and spot color printing. Among these options, the color profile corresponding to these devices is selected, which is the so-called device color profile. For the actual measured color profile of a printing or printing device, it should be loaded into the corresponding color space.

There are multiple modes of color to be applied during image copying. Different patterns of image colors have different color spaces. Color space usually refers to the spatial model used to describe the composition of colors. Because there are many ideas about how colors are composed, and each of them has its own unique features and applications, a variety of image modes have also been formed, such as RGB, CMYK, and Lab.

Most digital images obtained from image scans or directly from digital cameras use the RGB mode, and computer displays often use RGB mode. This is a color mode formed using the principle of color light synthesis, but color printers and printers use CMYK color materials to form colors by overlapping or mixing of color materials, and cannot directly apply the RGB image mode.
The processing of an image is usually scanned, displayed, printed, color-separated, or directly plated before printing can be completed. Different devices often use different image modes, causing difficulties in the transfer of images between different devices. Therefore, in order to enable the image to be able to pass between devices, it is necessary to first solve the problem of image mode conversion.
There are many common and commonly used image modes. Even the most common RGB mode, there are actually several. The RGB mode obtained directly from the digital camera or scanner has the characteristics of complete information and wide color gamut. But at the same time, it also brings a large amount of information data, which causes the problem of computer processing, display, and transmission speeds. The CMYK color gamut range used for color printing and printing is much smaller than that of the conventional RGB mode. During image processing, when an RGB mode image is converted to a print or print-compatible CMYK color mode, the colors that exceed the CMYK mode color gamut will be replaced or discarded. At the same time, since the color gamut range greatly exceeds the color range of printing or printing, the image displayed by the computer tends to change significantly during the conversion from the RGB mode to the CMYK mode.

And Photoshop uses AdobeRGB1998 for image processing and display. Although AdobeRGB1998 belongs to the same RGB mode, its color gamut is much smaller than that of conventional RGB, and the image is closer to the printing and printing effects on the computer display. Therefore, it is usually selected as the image processing process for printing and printing. Computer display image mode. The simplest method of adjusting the computer display screen to AdobeRGB1998 mode can be accomplished with the GAMMA device provided by Photoshop. Under normal circumstances, as long as you strictly follow the instructions, carefully adjust several times to meet the basic requirements.

As for the aRGB model, there have been reports that it will become a common image mode for web and print. Although this image mode has the advantages of small data volume, ease of network transmission, and computer processing, it also has the problem of easily losing details. At least for now, it is difficult to accept for the publishing and printing industries.

The image mode suitable for printing or printing is CMYK. Its color is composed of the percentage value of the dot area of ​​each color ink composed of C, M, Y, and K. The color of R, G, and B is formed by the percentage of the density of the three primary colors that make up the color, which is very different from the color space of CMYK. The process of converting an RGB mode color to a CMYK mode is often referred to as color separation. The basis for decomposing the R6B color and synthesizing the CMYK color is the color profile of the printing or printing apparatus. Image mode conversion and color separation are important tasks of image processing. In the process of mode conversion and color separation, the device determines the color profile of the device.

Open any picture in Photoshop at the same time with the mouse to move on the picture, the information box instantly displays the R6B data and CMYK data of this color. If you change the options for the CMYK color space, for example, from the Japanese standard to the American or European standard, the CMYK data at this point in the information frame will change, but the RGB data will not change. This shows that Photoshopl~ changes the CMYK data of the image based on the device's color profile. However, while changing the CMYK data, there is no change in the RGB data, which indicates that the color of the image remains unchanged when the CHYK mode is changed. That is to say, when the printing or printing equipment changes, as long as the corresponding device property file is selected, the color of the image remains basically unchanged. This is exactly what people hope the color management system can play in the image replication process.

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