The RenderMan Interface - Paul Bourke

U
V
0 1
2
3
12 13
14
15
4 5
6 7
8 9 10 11
8 9 10 11
4 5
6 7
12 13
14 15 0 1
V
2
3
Typical Left-Handed
Bezier Bicubic Patch
Equivalent Right-Handed
Bezier Bicubic Patch
Figure 5.1: Bicubic patch vertex ordering
This primitive is a compact way of specifying a quadrilateral mesh of patches. Each
individual patch behaves as if it had been specified with RiPatch. type can be either
”bilinear” or ”bicubic”. parameterlist is a list of token-array pairs where each token is
one of the geometric primitive variables or a variable which has been defined with
RiDeclare. The parameter list must include at least position (”P”, ”Pw” or ”Pz”) information.
Patch mesh vertex data is supplied in first u and then v order just as for
patches. The number of control points in a patch mesh is (nu)*(nv).
Meshes can wrap around in the u or v direction, or in both directions. If meshes
wrap, they close upon themselves at the ends and the first control points will be
automatically repeated. As many as three control points may be repeated, depending
on the basis matrix of the mesh. The way in which meshes wrap is indicated by giving
a wrap mode value of either ”periodic” or ”nonperiodic”.
The actual number of patches produced by this request depends on the type of the
patch and the wrap modes specified. For bilinear patches, the number of patches in
the u direction, nupatches, is given by
nupatches =
while for bicubic patches,
⎧ (
⎪⎨
nupatches = (
⎪⎩
{
nu if uwrap = ”periodic”
nu − 1 if uwrap = ”nonperiodic”
nu
nustep
nu−4
nustep
)
if
uwrap = ”periodic”
)
+ 1 if uwrap = ”nonperiodic”
The same rules hold in the v direction. The total number of patches produced is equal
to the product of the number of patches in each direction.
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