


OpenGL evaluators provide applications with the capability to
specify polynomial or rational curves and surfaces using control
points relative to the Bezier basis. The curves and surfaces are
then drawn by evaluating the polynomials provided at various values
for the u parameter of a curve or the (u,v) parameters of a surface.
A tensor product formulation is used for the surfaces.
OpenGL evaluators only support rectangular patches, not triangular
patches. Although triangular patches can be converted into
rectangular patches, direct support for triangular patches is likely
to be more efficient.
The tessellation algorithm used is too inflexible for most purposes;
only the number of rows and columns can be specified. Adjacent
patches must then have identical numbers of rows and columns, or
severe cracking will occur. Ideally, a number of subdivisions could
be specified for all four sides of a rectangular patch and for all
three of a triangular patch. This extension goes one step further
and allows those numbers to be specified in floating-point, providing
a mechanism for smoothly changing the level of detail of the surface.
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