[vtkusers] Fwd: Negative Volume after Tetrahedralize/Tessellate

[Joachim Pouderoux]

Hi Felipe,

See documentation of vtkTetra class:

 * vtkTetra [...] The tetrahedron is defined by the four points
 * (0-3); where (0,1,2) is the base of the tetrahedron which, using the
 * right hand rule, forms a triangle whose normal points in the direction
 * of the fourth point.

Which seems to mean that face formed with points 0, 1 and 2 describes the
base face considering points are listed
in clockwise order when seen from the exterior of the cell.
In this case vtkTetra::ComputeVolume() returns a positive volume.

Rule is the same with vtkWedge but it looks like the wedge you provide have
point listed in incorrect order (CCW when seen from exterior).
If you reverse them, tets are correct.

If I am wrong with point ordering, then I guess there is a problem when
constructing wedges in the Clip filter.

Dou you agree?

Best,

Joachim

*Joachim Pouderoux*, PhD

*Technical Expert - Scientific Computing Team*
*Kitware SAS <http://www.kitware.fr>*


2017-03-28 5:11 GMT-04:00 Felipe Bordeu <felipebordeu at gmail.com>:

> Hi,
>
>
> I'm having trouble working with clip feature in ParaView
> ("vtkPVMetaClipDataset"), but I think is a VTK bug. My mesh if 100%
> tetrahedron mesh, then I clip the mesh, the resulting mesh has tets and
> wedge. Then I apply a tetrahedralize to convert the resulting mesh back to
> only tets but the connectivity (numbering) of the element is not correct.
> If I apply mesh quality I get negative volume for some elements.
> I have the same "wrong" result with the Tessellate Filter (with zero level
> of subdivisions).
>
> Can anyone confirm this bug.
>
> Attached a single wedge to demonstrate the bug.
>
> Using Paraview 5.2 in linux (home build) and Windows (binary from web)
>
> Thanks to all
> Felipe
>
>
>
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