[vtkusers] vtkDataSetSurfaceFIlter doesn not extract boundary faces

[Luca Pallozzi Lavorante]

Hi everybody,

I would like to confirm that I have successfully obtained the polygonal
boundary of my vtkUnstructuredGrid using vtkDataSetSurfaceFilter. The
problem was in the way I was building the mesh. Even if the coordinates of
geometric coincident points between adjacent cells were the same, I was
inserting them with different point id's so the cells were considered as
disconnected by the filter.
I apologize for bothering the list with the previous post.

  Thank you

     Luca


On Wed, Nov 30, 2016 at 11:39 AM, Luca Pallozzi Lavorante <
lplavorante at gmail.com> wrote:

> 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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