[vtk-developers] Polyhedral challenge

David Gobbi david.gobbi at gmail.com
Thu Feb 18 15:04:31 EST 2010


Hi Will,

Have you and Hua done the "Clip" part of the cell implementation yet?
If not, I might have some code that will help.  I've been working on a
class for clipping of watertight polydata in VTK.  It does the capping
of the open end after a clip:

- contouring the polydata at the clip plane to get the clip-edges of
the clipped polygons
- joining the new edge line segments into a one or more cap polygons
- identifying complex polygons, i.e. polygons with holes
- triangulating the polygons (*partly done*)

The "triangulating" part is the only part that I haven't finished
(that's why I asked about Delaunay earlier).  I'm using ear-cut
triangulation, which only works for simple polygons, but other than
that everything is done.

   David


On Thu, Feb 18, 2010 at 9:38 AM, Will Schroeder
<will.schroeder at kitware.com> wrote:
> Hua and I have made progress on adding a vtkPolyhedron cell to VTK. We have
> a wiki page describing the tasks (which will take some time
> to implement depending on our funding and
> schedule http://www.vtk.org/Wiki/VTK/Polyhedron_Support). We are using the
> interpolation method of mean value coordinates which Hua has already
> implemented, thus far it is a super algorithm. Comments are welcome.
> We are now starting the integration into vtkUnstructuredGrid. If you are
> interested let me know and at the appropriate point we can make our git
> repository available.
>
>
> On Thu, Jan 21, 2010 at 6:53 AM, Will Schroeder <will.schroeder at kitware.com>
> wrote:
>>
>> FYI-
>>
>> Hua Yang and I are adding a new cell type to VTK: the polyhedral cell.
>> This is mainly to support our CFD friends who like to compute flow solutions
>> using flux approaches (balance mass and energy in and out of a region).
>>
>> One of the key questions is how to interpolate across the interior of such
>> a cell. In the past I have seen two typical approaches 1) tessellate the
>> polyhedron into tetrahedra, and then use the tetrahedra to interpolate; and
>> 2) use a 1/r**2 (or similar weighting function) to interpolate from the cell
>> nodes to an interior point in the polyhedron. Both approaches have problems
>> with continuity, etc.
>>
>> We are also researching other approaches. However in the interest of
>> completeness, if anybody has suggestions for alternatives we'd love to hear
>> about them.
>>
>> Will
>
>
>
> --
> William J. Schroeder, PhD
> Kitware, Inc.
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