The RenderMan Interface - Paul Bourke

Section 5
GEOMETRIC PRIMITIVES
The RenderMan Interface supports only surface- and solid-defining geometric primitives.
Solid primitives are created from surfaces and combined using set operations. The geometric
primitives include:
• planar convex polygons, as well as general planar concave polygons with holes,
• collections of planar convex or general planar concave polygons with holes which
share vertices (polyhedra),
• bilinear patches and patch meshes,
• bicubic patches and patch meshes with an arbitrary basis,
• non-uniform rational B-spline surfaces of arbitrary degree (NURBS),
• quadric surfaces, tori, and disks,
• subdivion surface meshes,
• implicit surfaces,
• 1D points and 2D curves or ribbons.
Control vertex points are used to construct polygons, patches, NURBS, subdivision meshes,
point clouds, and curves. Point positions can be either an (x,y,z) triplet (”P”) or an (x,y,z,w)
4-vector (”Pw”). If the vertex is part of a patch mesh, the position may be used to define
a height field. In this case the vertex point contains only a (z) coordinate (”Pz”), and the
(x,y)s of points of the height field are set equal to the parametric surface parameters of the
mesh.
All primitives have well-defined geometric surface normals, so normals need not be provided
with any primitive. The surface normal for a polygon is the perpendicular to the
plane containing the polygon. The surface normal for a parametric curved surface is
computed by taking the cross product of the surface’s parametric derivatives: (∂P/∂u) ×
(∂P/∂v). As mentioned in the Section 4.2.13, Orientation and Sides, if the current orientation
does not match the orientation of the current coordinate system, normals will be flipped. It
is also possible to provide additional shading normals (”N”) at polygon and bilinear patch
vertices to help make the surface appear smooth.
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