[vtk-developers] vtkWedge face node ordering and vtkUnstructuredGridGeometryFilter

[Berk Geveci]

Hi Andreas,

Changing the ordering of vtkWedge would be a very large undertaking
given that there is years of documentation, file formats (in VTK and
3rd party), code etc. that depend on a certain ordering. One less
intrusive way would be to change any vtkWedge methods that returns
faces to return them in a different order. I suspect this option is
doable but I'd like to hear from the community. The least intrusive
options is to fix the few filters that have this issue but I feel like
it is a band-aid.

Best,
-berk

On Wed, Apr 1, 2015 at 9:51 AM, Andreas Buykx <A.Buykx at tnodiana.com> wrote:
> 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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