[vtk-developers] FYI vtkPolyData::GetCellEdgeNeighbors()

[Will Schroeder]

I wanted to preserve a discussion that Joachim recently initiated.
Basically he encountered unexpected behavior when invoking
GetCellEdgeNeighbors() (see attached figure from Joachim). Basically,
triangle C is returned as a neighbor to triangle A edge(0,1).

Note that the documentation of vtkDataSet::GetCellNeighbors() and the
specialized method vtkPolyData::GetCellEdgeNeighbors() reads something like:

"Topological inquiry to get all cells using list of points exclusive of the
cell specified (e.g., cellId). Note that the list consists of only cells
that use ALL the points provided."

So it produces the results as documented, but in cases where you have
concave, or self-intersecting (non-compatible meshes) cells you may see
unexpected results.

Other notes:
+ Similar cases can be found for face neighbors using GetCellNeighbors()
+ The other concept that needs to be brought out I think we can call
"symmetry". Given two cells A and B, with E an edge of A. If the neighbor
N(A,E) = B, and N(B,E) = A then the neighbor operation is symmetric.
However if the cell B has no edge E, then the symmetry property fails. In
other words, the intersection of A∩B must contain E (I'm not sure symmetry
is the right term, we have to think about this.) I think we can show that
for convex, non-intersecting (i.e., compatible) meshes, the neighbor
operation is symmetric.

There are a couple of approaches to addressing this: adding new methods,
changing the behavior of the old one, modifying filters like feature edges
to be symmetry aware (it would be a fun addition) etc. Longer term if VTK
is to be used more for explicit geometric modelling (as compared to
volumetric/implicit modeling) then better data structures and API are
probably needed.

AFAIK this is in the mulling stage. Of course any feedback and comments are
encouraged.

Best,
Will

-- 
William J. Schroeder, PhD
Kitware, Inc. - Building the World's Technical Computing Software
28 Corporate Drive
Clifton Park, NY 12065
will.schroeder at kitware.com
http://www.kitware.com
(518) 881-4902
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