Page 181 - DCAP504_Computer Graphics

Computer Graphics

receiving the entire distribution C (λ) for a given color, however human brains receive only three values
(the tri-stimulus values).

Tri-stimulus values are the three values that are used to define a color in a specific tri-
chromaticity color model. For example the red color can be specified by the values r(1,
0, 0), g(0, 1, 0), and b(0, 0, 1).

Those three values are the responses of the three types of cones to the color. This reduction of a color to
three values leads to the basic principle of tri-chromaticity theory.

If two colors have the same tri-stimulus values, then they are visually identical.

The Tri-Chromaticity Theory
The tri-chromaticity theory is a contribution of two renowned researchers by name Thomas Young and
Hermann Von Helmholtz.
In this theory, let us consider S as a given light or color stimulus. The effect of S can be expressed by
combining light from three primary colors (red, green, blue) in right proportions:

S= r (red) +g (green) +b (blue)
The above formula uses only red, green, and blue because they are considered as the standard choice in
color matching experiments. Moreover, these three primary colors coincide with the wavelength values
that cause peak response from three types of cones of human visual system. The wavelength of the
three primary colors varies from 350 to 780. The three types of cones that are there in human visual
system are named as β, δ, and ρ cones and these cones are most sensitive to the light in the wavelength
range of 350 to 780.
An important aspect of tri-chromaticity theory is to match all visible colors and the weight values of
three primary colors have to be negative.

When red, green, and blue are used as primary colors, the value of b which is
the weight of the blue component may be negative. A negative value of b
indicates that the effect of the given stimulus S can not be matched normally
through the additive process. However, if S is mixed with some blue light then
the effect of the mix can be matched by a linear combination of red and green.
Some important points about tri-chromaticity theory are that:

1. It follows simple model for interaction of stimulus and three different types of receptors in the
human eye.
2. It can be described in terms of linear vector spaces.
3. It forms basis for colorimetry and design of many aspects of color imaging systems. This theory
does not predict all visual phenomena.
Commission Internationale de l’Eclairage (CIE) XYZ Color Model
Based on the fact that no naturally occurring primaries can be used to match all visible colors, the
International Commission on Illumination defined three imaginary (non-realizable) primaries such as X,
Y, and Z in 1931. The three primaries X, Y, and Z were the result of an affine transformation applied to
three natural primaries to ensure the expression of every single wavelength light or spectral color using
a linear combination of the CIE primaries with no negative weight.

174 LOVELY PROFESSIONAL UNIVERSITY