Color characteristics
A color can be characterized by several attributes, which are more or less subjective or based on standardized conventions. The RGB systems defines three primary colors: red, green, and blue. Half and half mixing of two of them defines the secondary colors: yellow, cyan and magenta. An arbitrary mixing of the primaries is associated to a color that can be defined by its relative proportions of red, green and blue. The color space is a cube along the three R, G, and B axes. Although widely used, this system cannot represent all the colors perceived by human eyes.
The mixing of the three primaries can be measured by an angle, often referred to as hue. This angle varies between 0° from pure red to 120° for pure green, passing through yellow, then reaching blue at 240° and back to red at 360° passing through magenta along the so-called purple line.
The saturation of a color is a measure of its purity. A fully saturated color is the purest possible one. A monochromatic light, i.e. a radiation emitted at a single frequency, is fully saturated. When the saturation decreases, the color appears more and more pale until it becomes gray.
The lightness of a color is its relative brightness compared to white under identical illumination conditions. By definition, the lightness of black is 0 and the one of white is 100%.
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Left: 3D representation of the RGB cube seen from its blue corner. The farthest vertex of the upper face is white. By subdividing each axis into N-1 intervals (e.g. N = 256), a regular mesh composed of N³ nodes is realized inside the cubic space, with one color being associated to each node. Right: cut of the RGB cube by a plane normal to the direction of the principal diagonal (the line composed exclusively of grays joining the black vertex to the white one). By construction, the sum R + G + B of the components of the three color channels in this plane is a constant. The angles marked around the cross section differentiate the hues. The center of the figure is a gray of 40% lightness. The saturation of each hue increases from the center to the border. |
Provided mathematical definitions exist between the hue, saturation and lightness, and the three indices R, G, B, the latter can be calculated from the former. For instance, one may set that the colors displayed in the planar cut of the RGB cube shown in the right-hand side of the above figure all have the same lightness, here 40%. With this convention in hands, the lightness is readily obtained as l = (R+G+B)/3. Similarly, defining the hue h as the polar angle in any planar cut of the RGB cube normal to the [111] direction requires but simple geometry to arrive at the relation tan(h) = √3 (G-B)/(2R- G-B). In the RGB system, a color is fully saturated when one of its three primary components is zero. Accordingly, the saturation can be defined, e.g., by s = (M-m)/M where M = max(R,G,B) and m = min(R,G,B). The edges of the [111] planar section of the RGB cube that are perpendicular to the angles h = 60°, 180°, and 300° are fully saturated.