Color Space 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.

Color space classification as above:

In pre-press processing, generally, the manuscript is converted from the manuscript (generally continuous natural manuscript) to the RGB space to complete the color separation, and then converted to the standard space XYZ or LAB independent of the device through the RGB space, and then the standard is completed. The conversion of space to RGB (such as CRT) or CMYK space (print) is 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 phosphors of the three colors R, G, and B on the phosphor screen to emit light of different brightness and passes through the phase. Add mixing 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:

Third, 1931 CIEXYZ color space

In 1931, the International Commission on Illumination (CIE) established a “standard chroma viewer” after performing a large number of normal people's visual measurements and statistics, thus laying the quantitative basis for modern CIE standard colorimetry.

Because the “standard chroma observer” used to calibrate the spectral colors had negative stimulus values, the calculation was inconvenient and difficult to understand. Therefore, in 1931 CIE used the three imaginary primary colors X, Y, and Z on the basis of the RGB system. A new color system. The tristimulus value matching it to the isospectral spectrum is named "CIE1931 standard chroma observer spectral tristimulus value" and is abbreviated as "CIE1931 standard chroma observer". This system is called "CIE1931 Standard Color System" or "2° Field of View XYZ Color System."

The Yxy color space can be obtained by slightly transforming the CIEXYZ color space, where Y takes the value of Y in the tristimulus values ​​to represent the brightness, and x and y reflect the chromaticity characteristics of the color. The definition is as follows:

In color management, it is very important to choose a device-independent color space. The device-independent color space is defined by the International Commission on Illumination (CIE) and includes both the CIEXYZ and CIELAB standards. They contain all the colors that the human eye can discern. Moreover, the establishment of the CIEYxy colorimetric system creates conditions for the quantitative determination of colors. However, in this space, the distance between two different colors does not correctly reflect the difference in people's color perception, that is to say, the wide capacity of the color in different directions at different positions in the CIEYxy color chart. It is different, this is the inhomogeneity of Yxy color space. The existence of this defect makes it impossible to intuitively evaluate colors in Yxy and XYZ spaces.