Color-related Concepts
Luminance
Luminance is defined as the level of brightness in the
image. Brightness is the attribute of a visual sensation
according to which an area appears to emit more or less light.
The spectral composition of luminance is related
to the brightness sensitivity of human vision.
Lightness
Human vision has a nonlinear perceptual response to
brightness. This perceptual response to luminance is called Lightness.
Intensity
Intensity is the amount of energy transmitted by electromagnetic
radiation - in our case, from or incident on the display
surface.
Hue
Hue is the attribute of a visual sensation
according to which an area appears to be similar to one of the perceived
individual or combination colors of red, green or blue.
Saturation
Saturation is the colorfulness of an area of the image judged in
proportion to its brightness. The more concentrated an
area of the image with one color, the more saturated the
area is with that color. Saturation of an image area
with a particular color can be reduced by using the
color white on that area.
True Colors
True color is the result of the additive effect of the 3
separate components of red,
green and blue. True color is typically implemented by
allocating 8 bits for each of the red, green and blue
components. This results in more than 16 million colors
or 16 million combinations of different
"shades" of the red, green and blue
components. True color is sometimes referred to
as 24-bit color or 16 million colors.
Indexed Colors
Indexed coloring is implemented by the use of color maps
or palettes. These color maps contain a relative small
number of discrete colors. Image data contains the index
number of a color for each pixel and this index is used
to search a lookup table for the corresponding color
from the color map.
For example, in an eight-bit indexed color system with
216 colors, the color map can be fashioned into a 6x6x6
cube with 3 axes - a red, a green and a blue axis. Each
of these axes with have a range of values from 0 to 5.
An RGB image can then be converted to a color map by
choosing the color map index corresponding to the "closest"
RGB triple for
each pixel in the image.
More Information
More information is available at the website of the
Commission Internationale de l'Eclairage. (International
Commission of Illumination) which is a standards
organization providing specifications for the
description of device independent color.
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