[vtkusers] vtkDataSetSurfaceFIlter doesn not extract boundary faces

[Luca Pallozzi Lavorante]

Hi vtkusers,

regarding my previous post, in which I wrote about visualizing a smooth
polygonal boundary obtained from an unstructured grid, I have verified that
vtkDataSetSurfaceFilter returns all the faces of the cells in the original
grid instead of the boundary cells. There is a similar 2009 post about this
issue by Teresa Azevedo:

http://public.kitware.com/pipermail/vtkusers/2009-October/054301.html

She used vtkGeometryFilter to obtain the boundary (vtkDataSetSurfaceFilter,
which I use,  is a faster version, as stated in the documentation), but
instead the filter returned all the faces of the grid's cells.
Is there anything I am missing? Most of the external edges generated by
vtkDataSetSurfaceFilter are non-manifold because two adjacent triangles on
the boundary share an edge with an internal triangle from the non-boundary
face of a 3D cell (Triquadratic hexahedron). I think this is the cause for
the non-smooth appearance that the rendered surface gets after applying
vtkPolyDataNormals. So I  though I should remove all non-boundary faces in
order to get  a correct normal generation.
I know the 3D cells do share 2D faces in the non-boundary portion of the
grid because I am providing the cells' coordinates myself. How can I get
only the boundary faces? Is there any condition in the
vtkDataSetSurfaceFilter and vtkGeometryFilter classes which maybe my
unstructured grid is not respecting?

Thank you very much,

        Luca
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