The RenderMan Interface - Paul Bourke

Section 11
TYPES
The Shading Language is strongly typed and supports the following basic types:
11.1 Floats
Floats are used for all scalar calculations. They are also used for integer calculations.
Floating-point variables are defined as follows:
float a, b=1;
The initialization value may be any scalar expression.
11.2 Colors
The Shading Language implements color as an abstract data type independent of the number
of samples and the color space. The major operations involving color are color addition
(‘+’ operator) corresponding to the mixing of two light sources, and color filtering (‘*’ operator)
corresponding to the absorption of light by a material. In each case these color
operations proceed on a component by component basis.
The number of color samples used in the rendering program is set through the RenderMan
Interface. Once the number of color samples has been specified, colors with the appropriate
number of samples must be passed to a shader. When setting the number of samples, the
user can also specify a transformation from RGB space to this n sample color space. This
allows a shader to convert color constants to the specified color space.
Color component values of 0 correspond to minimum intensity, while values of 1 correspond
to maximum intensity. A color constant of 0 is equivalent to black, and of 1 is
equivalent to white. However, values outside that range are allowed (values less than
zero decrement intensity from other sources). Note that if you are using a color variable to
represent reflectivity, only component values between 0 and 1 are physically meaningful,
whereas color variables that represent radiance may be unbounded.
Color variables may be declared with:
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