[vtk-developers] vtkWedge face node ordering and vtkUnstructuredGridGeometryFilter

[Andreas Buykx]

Hi,

To detect if coinciding faces of neighbouring 3D cells are connected, vtkUnstructuredGridGeometryFilter walks the points of these faces in opposite directions and expects the point ids to match. This fails for vtkWedge attached to e.g. vtkTetra and vtkHexahedron as demonstrated by the python script I attach. The root cause is that the vtkWedge::GetFaceArray and vtkWedge::GetFace return the face points in an ordering that makes the face normal point into the 3D cell instead of out of the  3D cell, like all other 3D cells do. The two other algorithms that generate surface cells (vtkDataSetSurfaceFilter and vtkGeometryFilter) don't suffer from this ordering because the former checks point ordering but explicitly allows both loop directions, and the latter only takes cell connectivity into account without checking point-ordering at all.

I don't know what the reason is to have a different face-point ordering for the vtkWedge, but one of the other consequences is that the outer surface will have inconsistent cell normals as can easily be seen when loading the xml files produced by the tests in paraview, and showing the cell normals. So I would prefer to change the face-point ordering in vtkWedge. If that is not possible for some reason then I propose to make/update the case-blocks in the surface filters cell-type switches to re-order the face-points for the wedge faces. As long as this is not dealt with our FEA models will show edges where they should not be so I will have to solve this issue one way or another, and I'd like to contribute it to VTK as well.

Shall I make a mantis issue for this? Who can help me with the git/gerrit stuff?

Thanks a lot,
Andreas Buykx
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