nurbs and bezier rendering - plus question.....

[Tim Hall]

In article <5966 at eos.UUCP> prevost at eos.UUCP (Michael Prevost) writes:
>
>1) Is it possible to render bezier patches as shaded surfaces ?

There seems to be alot of new documentation on curves/surfaces in 3.2
so what I say might be obsolete, I'm sure the SGI people will give 
more details.  Prior to 3.2 the answer is no.  What you would have to
do is "cut up" your surface, find the surface normal at each vertex,
and then render the patch as a bunch of triangles.  (This fits
perfectly into the mesh routines)

Some good references for this and curves and surfaces in general:

Curvers ans Surfaces for Computer Aided Geometric Design.
Gerald Farin, Academic Press. ISBN 0-12-249050-9

Geometric Modeling. Michael E. Mortenson.
John Wiley & Sons. ISBN 0-471-88279-8

The Siggraph notes.

>2) Is it possible to restrict the end points of nurbs.

Yes.  The paper "Hierarchical B-Spline Refinement" by David Forsey and
Richard Bartels in the 1988 Siggraph Proceedings is how I dealt with
this.  Basically the paper describes how to insert knots to "protect"
the end points.  So, for a patch, you would cut it up into four smaller
patches centered about your editing point.

The question....

A co-worker and I were wondering why anyone would want two different
patch basis', as the SGI allows you to do.  Rendering a surface like
this would look rather bizarre unless you somehow changed your control
points to match the basis.  ( This isn't a complaint, just wondering
why )

-Tim Hall
tjh at bu-pub.bu.edu