Graphics Programs Reference
In-Depth Information
some method of defining not only how each device behaves but also a
method of creating these values.
Next time you hear the terms RGB and CMYK, you should immedi-
ately realize “I don't have enough information.” Having just RGB or
CMYK values does not allow you to reproduce that color or image as it
was intended. The next time someone supplies you with RGB or CMYK
data, be sure to ask what color space it's in. If they don't know you might
want to explain that you don't have an appetite for mystery meat.
Device-Independent Color Spaces
In addition to device-dependent color spaces, there are also device-
independent color spaces. These color spaces encompass all of human
vision. The most common is called CIELAB (or L*a*b; often written as
LAB, although technically the * should be used). Back in 1931, the CIE
(Commission Internationale de L'Éclairage, also known as International
Commission on Illumination), a group or color scientists, conducted a
series of experiments and tests on humans to determine how they per-
ceive color. The tests involved showing groups of volunteers a sample
color under very controlled conditions whereby each subject adjusted the
intensity of red, green, and blue lights until the mix of the three matched
the sample color. This allowed the CIE to specify precisely the stimulus
response of the human eye.
The CIE came up with the term standard observer to describe a hypo-
thetical average human viewer and his or her response to color. Fur-
thermore, the results of these tests produced a mathematical model of a
color space formulated not on any real-world device, but rather on how
we humans (the standard observer) actually perceive color. This core
color model is called CIE XYZ (1931). This is the color model from which
all other device-independent color models are created. Like the RGB color
model with three additive primaries, CIE XYZ uses three spectrally
defined imaginary primaries: X, Y, and Z. These X, Y, and Z primaries may
be combined to describe all colors visible to the standard observer. Also
in 1931, a synthetic space called CIE xyY was created, which itself is
derived from CIE XYZ. In 1976, CIELAB and CIELUV were added to the
mix of these device-independent color spaces. The CIELAB color space is
a synthetic, theoretical color space derived from XYZ. Unlike the origi-
nal, CIELAB has the advantage of being perceptually uniform (sort
of . . .). That simply means that a move of equal value in any direction
at any point within the color space produces a similar perceived change
to the standard observer.
The XYZ color space is based on three quantities or stimuli. The geek
term for describing this is tristimulus values (three stimuli). Technically the
term tristimulus values refers to the XYZ values of the original CIE XYZ
color model although you will often hear people describe tristimulus
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