The RenderMan Interface - Paul Bourke

Quadrics, patches and patch meshes, and NURBS primitives have well-defined global twodimensional
surface parameters. All the points on the surface of each primitive are functions
of these parameters (u,v). Except for NURBS and polygons, the domain of the surface
parameters is the unit square from 0 to 1. Texture coordinates may be attached to primitives
by assigning four sets of texture coordinates, one set to each corner of this unit square.
This is done by setting the current set of texture coordinates or by defining texture coordinates
with the geometric primitives as described below.
Subdivision surfaces and implicit surfaces have locally defined parameterizations, but no
globally consistent parameterization across an arbitrary surface of that type.
All geometric primitives normally inherit their color and opacity from the graphics state.
However, explicit colors and opacities can be provided when defining the primitive (”Cs”
and ”Os”).
Associated with each geometric primitive definition are additional primitive variables that
are passed to their shaders. These variables may define quantities that are constant over
the surface (class constant), piecewise-constant but with separate values per subprimitive
(class uniform), bilinearly interpolated (class varying), or fully interpolated (class vertex).
If the primitive variable is uniform, there is one value per surface facet. If the primitive
variable is varying, there are four values per surface facet, one for each corner of the unit
square in parameter space (except polygons, which are a special case). On parametric
primitives (quadrics and patches), varying primitive variables are bilinearly interpolated
across the surface of the primitive. Colors, opacities, and shading normals are all examples
of varying primitive variables.
The standard predefined primitive variables are defined in Table 5.1 Standard Geometric
Primitive Variables. Other primitive variables may be predefined by specific implementations
or defined by the user with the RiDeclare function, or may be declared “in-line” as
part of the parameter name itself (see Section 3). Primitive variables which are declared
to be of type point (including the three predefined position variables), vector, normal, or
matrix are specified in object space, and will be transformed by the current transformation
matrix. Any vector or normal variables will be transformed by the equivalent transformation
matrix for vectors or normals. Primitive variables which are declared to be of type
color must contain the correct number of floating point values as defined in RiColorSamples.
More information about how to use primitive variables is contained in Part II: The
RenderMan Shading Language.
5.1 Polygons
The RenderMan Interface supports two basic types of polygons: a convex polygon and a
general concave polygon with holes. In both cases the polygon must be planar. Collections
of polygons can be passed by giving a list of points and an array that indexes these points.
The geometric normal of the polygon is computed by computing the normal of the plane
containing the polygon (unless it is explicitly specified). If the current orientation is lefthanded,
then a polygon whose vertices were specified in clockwise order (from the point
of view of the camera) will be a front-facing polygon (that is, will have a normal vector
which points toward the camera). If the current orientation is right-handed, then polygons
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