The RenderMan Interface - Paul Bourke

14.2 Functions
Functions are similar to shaders except they can be called from shaders or other functions,
but cannot be instanced from the RenderMan Interface. Functions in the Shading Language
are also much like functions in C. For example, the following defines a function normalize
that returns a unit vector in the direction V:
vector
normalize ( vector V )
{
return V/length(V);
}
Function calls use the traditional C syntax, except for the issues noted below.
Functions may return any of the basic types (float, color, point, vector, normal, matrix,
string). Functions may not return arrays, though they make take arrays as parameters.
Functions must have exactly one return statement, except for those functions declared with
type void, which may not have any return statement.
Function parameters are passed by reference; in other words, parameters are not copied
into private data areas of the function. Nevertheless, function parameters are not writable
unless specified with the output keyword. For example:
void
normalize inplace ( output vector V )
{
V = V/length(V);
}
Functions in the Shading Language cannot be called recursively.
Functions may be polymorphic — that is, you may define multiple functions with the same
name, as long as they take different types or numbers of arguments. Functions may even
be polymorphic based on return type. For example:
float snoise (point p)
{
return 2 * (float noise(p)) - 1;
}
float snoise (float s, float t)
{
return 2 * (float noise(s, t)) - 1;
}
vector snoise (point p)
{
131