2015-04-02

Color terms

Additive color
Color created by mixing a number of different light colors. Adding red, green and blue together forms like sun light so the color becomes white.


Subtractive color
Color created by mixing dyes or inks. The color of dyes or inks is the only visible spectrum reflecting to eyes and other visible spectrum are absorbed. Hence the color mixed by red, green and blue together absorbs all visible spectrum and the color is black.


Hue
Map colors to a cyclic color wheel.

Saturation
Saturation is one type of colorfulness. The saturation of a color is determined by a combination of light intensity and how much it is distributed across the spectrum of different wavelengths.

HSL and HSV
HSL (hue-saturation-lightness) and HSV (hue-saturation-value) are cylindrical-coordinate representations of points in an RGB color model.

Chromaticity
An objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s).

CIE color space
Human eye has three kinds of cone cells, which sense light, with spectral sensitivity peaks in short (S, 420-440nm), middle (M, 530-540nm) and long (L, 560-580nm) wavelengths. Three parameters (S, M, L), corresponding to levels of stimulus of the three types of cone cells, can in principle describe any color sensation, called LMS color space.

A color space maps a range of physical produced colors to an objective description of color sensations registered in the eye, typically in terms of tristimulus values. The tristimulus values associate with a color space can be conceptualized as amounts of three primary colors in a tri-chromatic additive color model.

Most wavelengths will not stimulate only one type of cone cell due to overlap of spectral sensitivity curves. LMS tristimulus values for pure spectral colors would imply negative values for at least one of these three primary colors. To avoid negative RBG values and to have one component describing brightness, "imaginary" primary colors and corresponding color-matching functions have formulated. The resulting tristimulus values are X, Y, Z (XYZ color space).

Humans tend to perceive light within green parts of the spectrum as brighter than red or blue. The luminosity function that describes the perceived brightness of different wavelengths is thus roughly analogous to M.

CIE model defines Y as luminance, Z is quasi-equal to blue stimulation (S cone response), and X is a mix of cone response curves chosen to be non-negative.

The chromaticity of a color is specified by two derived parameters x and y, two of the three normalized values which are functions of all three tristimulus values, X, Y and Z.

The derived color space specified by x, y and Y is known as xyY color space and is widely used to specify colors in practice.

CIE Chromaticity Diagram
The outer curved boundary is the spectral locus, with wavelengths in nanometers. This diagram represents all of the chromaticities visible to average person. All visible chromaticities correspond to non-negative values of x, y and z.

Reference
[1] https://en.wikipedia.org/wiki/Additive_color
[2] https://en.wikipedia.org/wiki/Subtractive_color
[3] https://en.wikipedia.org/wiki/Hue
[4] https://en.wikipedia.org/wiki/Colorfulness
[5] https://en.wikipedia.org/wiki/HSL_and_HSV
[6] https://en.wikipedia.org/wiki/Chromaticity
[7] https://graphics.stanford.edu/courses/cs178-10/applets/threedgamut.html
[8] https://en.wikipedia.org/wiki/CIE_1931_color_space
[9] https://en.wikipedia.org/wiki/Color_space
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