The RenderMan Interface - Paul Bourke

A PatchMesh primitive variable of class vertex has the same number of entries as the
position ”P” (i.e., nu×nv) and is interpolated using the same order and basis matrices.
Any varying primitive variables are interpolated piecewise-bilinearly across the patch
mesh and contain n values, one for each patch corner, where n is defined by:
uwrap vwrap
n = (nupatches + 1) · nvpatches ”nonperiodic” ”periodic”
n = (nupatches + 1) · (nvpatches + 1) ”nonperiodic” ”nonperiodic”
n = nupatches · (nvpatches + 1) ”periodic” ”nonperiodic”
n = nupatches · nvpatches ”periodic” ”periodic”
(with nupatches and nvpatches defined as given above). If a variable is uniform, it
contains nupatches × nvpatches elements of its type, one for each patch (see Figure 5.2).
Primitive variables of class constant have exactly one data element of the appropriate
type.
A patch mesh is parameterized by a (u,v) which goes from 0 to 1 for the entire mesh.
Texture maps that are assigned to meshes that wrap should also wrap so that filtering
at the seams can be done correctly (see the section on Texture Map Utilities). If texture
coordinates are inherited from the graphics state, they correspond to the corners of
the mesh.
Height fields can be specified by giving just a z coordinate at each vertex (using ”Pz”);
the x and y coordinates are set equal to the parametric surface parameters. Height
fields cannot be periodic.
RIB BINDING
PatchMesh type nu uwrap nv vwrap ...parameterlist...
EXAMPLE
RtPoint pts[28];
RtFloat foos[2];
RtFloat bars[6];
RiBasis (RiBezierBasis, 3, RiBezierBasis, 3);
RiDeclare (”foo”, ”uniform float”);
RiDeclare (”bar”, ”varying float”);
RiPatchMesh (”bicubic”, 7, ”nonperiodic”, 4, ”nonperiodic”,
”P”, (RtPointer)pts, ”foo”, (RtPointer)foos,
”bar”, (RtPointer)bars, RI NULL);
SEE ALSO
RiBasis, RiNuPatch, RiPatch
Non-uniform B-spline patches are also supported by the RenderMan Interface. Rational
quadratic B-splines provide exact representations of many different surfaces including general
quadrics, tori, surfaces of revolution, tabulated cylinders, and ruled surfaces.
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