The RenderMan Interface - Paul Bourke

Information Name Type Class Floats
Position ”P” point vertex 3
”Pz” float vertex 1
”Pw” h vertex 4
Normal ”N” normal varying 3
Color ”Cs” color varying (3)
Opacity ”Os” color varying (3)
Texture Coordinates ”s” float varying 1
”t” float varying 1
”st” 2 float varying 2
Table 5.1: Standard Geometric Primitive Variables
whose vertices were specified in counterclockwise order will be front-facing. The shading
normal is set to the geometric normal unless it is explicitly specified at the vertices.
The surface parameters of a polygon are its (x,y) coordinates. This is because the height z
of a plane is naturally parameterized by its (x,y) coordinates, unless it is vertical. Texture
coordinates are set equal to the surface parameters unless texture coordinates are given
explicitly, one set per vertex. Polygons do not inherit texture coordinates from the graphics
state.
The rules for primitive variable interpolation and texture coordinates are different for polygons
than for all other geometric primitives. Constant primitive variables are supplied as
a single value for the entire aggregate primitive. Uniform primitive variables are supplied
for each polygon. Both varying and vertex primitive variables are supplied for each polygon
vertex, and are interpolated across the interior without regard to the artificial surface
parameters defined above. Note that interpolating values across polygons is inherently
ill-defined. However, linearly interpolating values across a triangle is always well defined.
Thus, for the purposes of interpolation, polygons are always decomposed into triangles.
However, the details of how this decomposition is done is implementation-dependent and
may depend on the view.
RiPolygon ( RtInt nvertices, ...parameterlist...)
nvertices is the number of vertices in a single closed planar convex polygon. parameterlist
is a list of token-array pairs where each token is one of the standard geometric primitive
variables or a variable which has been defined with RiDeclare. The parameter
list must include at least position (”P”) information. If a primitive variable is of class
vertex or varying, the array contains nvertices elements of the type corresponding
to the token. If the variable is uniform or constant, the array contains a single element.
The number of floats associated with each type is given in Table 5.1, Standard
Geometric Primitive Variables.
No checking is done by the RenderMan Interface to ensure that polygons are planar,
convex and nondegenerate. The rendering program will attempt to render invalid
polygons but the results are unpredictable.
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