Bilinear Patch Mesh -> Polygons

Bilinear Patch Mesh -> Polygons

Post by Rick LaMo » Sat, 09 Aug 1997 04:00:00

>I have an object defined as a bilinear patchmesh and I was to extract the
>polygons that make up this object.

In general, there is no way to accurately represent a bilinear patch mesh
with planar polygons.  A bilinear patch mesh is made up of bilinear patches.
A bilinear patch is like a four sided polygon with straight edges except
without the restriction of being planar.

To visualize the concept of non-planarity, grasp a sheet of paper with both
hands.  Twist your left hand toward you and your right hand away from you.
The sheet is now non-planar.

If your bilinear patches happen to be planar, you may safely replace them
with four sided polygons.  One tricky difference about polygons and bilinear
patches in the RenderMan interface is vertex ordering.  Polygon vertices are
specified in clockwise order around the perimeter.  Bilinear patch vertices
are specified in "uv" order: upper-left, upper-right, lower-left, lower-right.

I won't go into vertex ordering of meshes here.  If you don't have hardcopy
documentation of RiPatchMesh, the spec is online:

Rick LaMont
Dot C Software, Inc.


1. Converting polygon mesh to patch

I have a polygon mesh which I would like to convert to a patch.
The surface fitting methods I've seen require that the points
be already connected by a curve, and then address the question
of connecting the curves.  I only have a list of vertices and
a normal for each.  Any suggestions/algorithms/references for
converting that data to a patch?  (Obviously, I'd like to maintain
the detail of the original model.)

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