Interchange of Color Spaces in Color Management (I)

In the process of color image copying, in order to achieve color uniformity from scanning, display, output, and printing, it is necessary to implement standardized, standardized, and data-based color management. Color management includes: 1. Color matching between input devices; 2. The match between the original color and the display color; 3. Color matching between output devices; 4. Match between monitor color and print color; 5. The color match between the original and the print. Color management is to solve the problem of color conversion and matching among various devices. We all know that there are many ways to measure a color, and each method is measured in a coordinate system called color space. Since the beginning of the 20th century, the research and definition of the color metric space have been more perfect. So far, with the development and popularization of computer technology, the color space and its mutual transformation and management technology have become increasingly mature, providing a complete technical foundation for the reproduction of our rich and colorful world.

The color space classification is as above: In the prepress processing, generally, the manuscript is converted from the original (generally continuous natural manuscript) to the RGB space to complete the color separation, and then converted to the standard space XYZ independent of the device by the RGB space or LAB, and then complete the conversion of standard space to RGB (such as CRT) or CMYK space (print), and finally output. The color spaces that are often used in color management are separately described below.

First, RGB color space

The basic law of color mixing shows that any color in the natural world can be produced by the mixture of the three primary colors of red, green, and blue. This can be geometrically derived from a space coordinate system consisting of R, G, and B three mutually perpendicular axes. It is called RGB tristimulus space, or RGB color space. For any color, it can be matched with a certain proportion of red, green and blue light.

The RGB system is an indispensable color system in color processing systems. Appear on many occasions, often used: 1. The display, which uses the R, G, and B values ​​sent to drive the R, G, and B electron guns to emit electrons, and excites the R, G, and B colors of phosphors on the phosphor screen to emit light of different brightness, and adds them together. Mix to produce various colors; 2. The scanner, which absorbs the R, G, and B components of light transmitted through the reflection or transmission of the original, and uses it to represent the color of the original.

As we all know, different scanners scan the same image, will get different colors of image data; different models display the same image, there will be different color display results. Therefore, we refer to the RGB color space as the device-dependent color space. The RGB space mentioned here is different from the real three-primary color system space of CIE 1931 RGB, which is device-independent.

Second, Ycc color space

In the image scanning, in addition to using the scanner, we also have an important scan input device, PhotoCd. Since PhotoCd undergoes a mode compression when storing images, PhotoCd uses the Ycc color space. From its main component, with two separate color channels, Ycc color space is used to save the image, which can save storage space. The conversion formula from RGB color space to Ycc color space is as follows: