Color Space Interchange in Color Management (II)

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.

Fourth, CIELAB uniform color space

In view of the non-uniformity of the CIEXYZ color space, the International Commission on Illumination has recommended several internationally common uniform color space and color difference formulae to countries around the world. The CIELAB color space is one of them. The CIELAB space is a nonlinear transformation space in the CIEXYZ space. The formula is as follows:

The advantage of CIELAB's uniform color space is that it can better reflect the psychology of the object color when the color difference of the color is greater than the visual recognition threshold (just noticeable) but smaller than the color difference value of the adjacent two poles in the Munsell system. Feel the effect. Moreover, CIELAB is intuitive and intuitive to evaluate colors. And because CIELAB is a device-independent color space, in color management, using this feature, it is possible to communicate and calculate the correspondence between the original color, the screen color, and the printing color in the color space so that the colors are visually consistent and different. Color conversion between devices.

Fifth, CMYK color space

The printing industry expresses colorful colors and tones through the overprinting of different dot area ratios of the three primary colors of blue (C), (M), and yellow (Y) inks. This is the CMY color space of the three primary colors. In actual printing, four colors (C), (M), Y (yellow) and BK (BK) are generally used. In the middle of the printing, the tone is increased to a darker tone.

The CMYK color space is related to the equipment or the printing process, and the process method, the characteristics of the ink, the characteristics of the paper, etc., have different printing results under different conditions. So the CMYK color space is called the device-dependent table color space. Moreover, CMYK has a multi-valued nature, that is, for the same color with the same absolute chromaticity, under the same printing process premise, the CMYK digital combination can be used to express and print. This feature has brought a lot of trouble to the color management, also brings a lot of flexibility to the control.

In the printing process, it is necessary to go through a color separation process. The so-called color separation is to convert the RGB color used in the computer to the CMYK color used in printing. There are two complicated problems in the conversion process. One is that the two color spaces are not exactly the same in the range of the expression color. The RGB color gamut is larger and the CMYK is smaller, so the gamut compression is needed; The second is that these two colors are related to the specific equipment, and the color itself is not absolute. Therefore, it is necessary to perform the conversion through a device-independent color space, that is, the XYZ or LAB color space described above can be used for conversion.

The above several color spaces are the most common color conversion spaces in color management, so understanding them and the relationship between them is very important.