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Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two dimension, often specified as hue ( h) and colorfulness ( s), where the latter is alternatively called saturation, chroma, intensity,In modern terminology the word intensity may refer to lightness, not colorfulness. or excitation purity. This number of parameters follows from trichromacy of vision of most humans, which is assumed by most models in color science.
Purity is roughly equivalent to the term saturation in the HSV color model. The property hue is as used in general color theory and in specific such as HSL and HSV color spaces, though it is more perceptually uniform in color models such as Munsell, CIELAB or CIECAM02.
Some separate the three dimensions of color into one luminance dimension and a pair of chromaticity dimensions. For example, the white point of an sRGB display is an , chromaticity of (0.3127, 0.3290), where and coordinates are used in the xyY space.
These pairs determine a chromaticity as affine coordinates on a triangle in a 2D-space, which contains all possible chromaticities. These and are used because of simplicity of expression in CIE 1931 (see below) and have no inherent advantage. Other coordinate systems on the same X-Y-Z triangle, or other , can be used.
On the other hand, some color spaces such as RGB and XYZ do not separate out chromaticity, but chromaticity is defined by a projectivization, and its coordinates, such as and or and , can be calculated through the division operation, such as , and so on.
The xyY space is a cross between the CIE XYZ and its normalized chromaticity coordinates xyz, such that the luminance Y is preserved and augmented with just the required two chromaticity dimensions.
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