The RenderMan Interface - Paul Bourke

Shader instance variable values can be changed from their defaults by passing their new
values through the RenderMan Interface. This first requires that the type of the variable be
declared. The declaration for weird would be:
RiDeclare( ”a”, ”uniform float” );
RiDeclare( ”b”, ”varying float” );
Of course, the RiDeclare can be eliminated if “in-line declarations” are used (see Section 3).
In either case, uniform instance variables can be set by passing them in the parameterlist of
a shader instance. In the following example, a is redefined while b remains equal to its
default:
RtFloat a = 0.3;
RiSurface( ”weird”, ”a”, (RtPointer)&a, RI NULL );
Shader variables can also be set in geometric primitives. For example, the weird shader
variables could be set when defining a primitive:
RtFloat a;
RtFloat bs[4];
RtPoint Ps[4];
RiPolygon(4, ”P”, Ps, ”a”, (RtPointer)&a, ”b”, (RtPointer)bs, RI NULL)
a is a single float and b is an array containing four values, since it is a varying variable. The
standard variable ”P” is predeclared to be of type varying point.
If a geometric primitive sets a shader variable that is defined in none of the shaders associated
with that primitive, it is ignored. Variables that are set on geometric primitives
override the values set by the shader instance.
Shader instance variables are read-only in the body of the shader, unless they are declared
using the output keyword:
displacement
lumpy ( float a=0.5; output varying float height=0 )
{
float h = a * noise(P);
P += h * normalize(N);
N = calculatenormal(N);
height = h;
}
This displacement shader produces a lumpy surface and also saves the displacement amount
in an output variable height. Output variables of one shader may be read by other shaders
on the same primitive (see Section 15.8) in order to pass information between them.
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