The RenderMan Interface - Paul Bourke

Section 14
SHADERS AND FUNCTIONS
The Shading Language distinguishes between shaders and functions. Shaders are procedures
that are referred to by RenderMan Interface procedures. Functions are procedures
that can be called from within the Shading Language. The distinction between shaders and
functions is primarily a consequence of different argument passing conventions.
14.1 Shaders
Shaders are introduced by the keywords light, surface, volume, displacement, or imager
and are followed by the name of the shader and the statements that comprise its definition.
These keywords indicate the type of RenderMan Interface shader that is being defined.
The RenderMan Interface uses the shader type to enforce a type match in subsequent calls
to RenderMan Interface procedures. For example, it is illegal to declare a shader to be of
type light and then instance it using RiSurface.
The arguments to a shader are referred to as its instance variables. All of these variables are
required have default values, and are assumed to be uniform unless declared otherwise.
Shader instance variable values can be changed when a particular shader is instanced from
the RenderMan Interface. For example, consider the shader weird:
surface
weird( float a=0.5; varying float b=0.25 )
{
Ci = color (mod(s,a), abs(sin(a+b)), mod(b,t));
}
This surface shader may be referenced through the RenderMan Interface with the RiSurface
command. For example,
RiSurface ( ”weird”, RI NULL );
instances the above shader with all its defaults.
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