RGB is the most common representation of color in digital image files, partly because the human eye perceives color using red, green, and blue sensitive photoreceptive cones, and partly because it is easy to display on computer monitors (which have red, green, and blue phosphors, roughly corresponding to the cones in the eye).
However, there are many other ways to represent color in images.
The HLS and HSV models, for instance, instead of representing colors as superpositions of three primary colors, have values for the hue of the color, the degree of saturation, and the intensity. This type of representation was developed to be more intuitive than RGB, though it suffers from some difficulties for image computation, such as a discontinuity in hue value at angle 360. However, it may have advantages for some types of image calculations where it is useful to have the hue, saturation, and intensity separately.
HSV Hex Cone
Smith's hue, saturation, value (HSV) model uses the hex-cone shown below, where:
• Saturation is the length of the radial line from white to the pixel color.
• Value is the height from the apex of the hex-cone.
• Hue is the angle in degrees measured from the line between white and red.
1. Green
2. Blue
3. Red
4. White
Under HSV, maximum saturation can occur only if value is at its maximum of 1.0 (255 in a 0-255 scale).
HLS Double-Hex Cone
The hue, lightness, saturation (HLS) model is almost identical to the HSV model except that a double hex-cone is used instead of an inverted hex-cone. In this model:
• Hue and saturation are defined in the same way for both systems.
• Value and lightness are slightly different. Under HLS, maximum saturation can only occur at a lightness of 0.5 (127 in a 0-255 scale). At higher lightness values, the colors become diluted by white light.
1. White
2. Black
Additional Information
Neither HLS nor HSV has an intensity value that compensates for the relative difference in perceived brightness of red, green, and blue of the human eye, so if you create colors of constant HLS lightness or HSV value, they do not appear equally bright.
