The RenderMan Interface - Paul Bourke

5.2 Patches
Patches can be either uniform or non-uniform (contain different knot values). Patches can
also be non-rational or rational depending on whether the control points are (x, y, z) or
(x, y, z, w). Patches may also be bilinear or bicubic. The graphics state maintains two 4x4
matrices that define the bicubic patch basis matrices. One of these is the current u-basis and
the other is the current v-basis. Basis matrices are used to transform from the power basis
to the preferred basis.
RiBasis ( RtBasis ubasis, RtInt ustep, RtBasis vbasis, RtInt vstep )
Set the current u-basis to ubasis and the current v-basis to vbasis. Predefined basis
matrices exist for the common types:
RtBasis RiBezierBasis;
RtBasis RiBSplineBasis;
RtBasis RiCatmullRomBasis;
RtBasis RiHermiteBasis;
RtBasis RiPowerBasis;
The variables ustep and vstep specify the number of control points that should be
skipped in the u and v directions, respectively, to get to the next patch in a bicubic
patch mesh. The appropriate step values for the predefined cubic basis matrices are:
Basis
Step
RiBezierBasis 3
RiBSplineBasis 1
RiCatmullRomBasis 1
RiHermiteBasis 2
RiPowerBasis 4
The default basis matrix is RiBezierBasis in both directions.
RIB BINDING
Basis uname ustep vname vstep
Basis uname ustep vbasis vstep
Basis ubasis ustep vname vstep
Basis ubasis ustep vbasis vstep
For each basis, either the name of a predefined basis (as a string) or a matrix may be
supplied. If a basis name specified, it must be one of: ”bezier”, ”b-spline”, ”catmullrom”,
”hermite”, or ”power”.
EXAMPLE
Basis ”b-spline” 1 [-1 3 -3 1 3 -6 3 0 -3 3 0 0 1 0 0 0] 1
SEE ALSO
RiPatch, RiPatchMesh
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