[vtkusers] 3D cells behave differently

[jnorris at uiuc.edu]

Hi all,

I've noticed an inconsistancy in behavior of the 3D cells in vtk3.2beta.
This is probably in older versions as well.

My data is in an unstructured grid.  I need to be able to generate the
surface (via vtkGeometryFilter) and then add a cutting plane.  I do this by
using vtkClipPolyData to clip the surface.  I cap the clipped surface by
using a vtkCutter on the unstructured grid and appending the outputs of the
two filters together via vtkAppendPolyData.  The vtkCutter and
vtkClipPolyData use the same vtkPlane object.  This works great for pyramids,
wedges, voxels, and hexahedrons, but not for tetrahedrons.  With the other
3D cells, the normals of the polygons generated by vtkCutter are opposite
the normal of the cutting plane, so on my clipped surface they are facing
out (which is what I want).  But with tets, they apparently are facing in.
I can't simply pass the output of my vtkCutter through a vtkReverseSense,
because I have to be able to handle grids with mixed cell types, and only
the tets are facing in.

Any ideas?

Thanks,
John

P.S. I had an example that demonstrates this, but it mysteriously quit
working while I was at lunch.  I'll email/post it once it's working again,
if anyone is interested.
-- 
John Norris
Research Programmer
Center for Simulation of Advanced Rockets
http://www.uiuc.edu/ph/www/jnorris