[vtkusers] wedge: different node ordering

[Andy Bauer]

Hi Nico,

Unfortunately, #3 is wrong (even though it's in some of the VTK
publications) and the only way to really determine that is by looking at
the wedge cell. I believe that it renders properly because the cell faces
are still the same, just the normals are wrong. Doing analytical operations
may or may not be incorrect (e.g. Jacobian is negative for #3 as you
noticed but maybe slicing may be correct).

In my opinion this is an inconsistency with the cell specification since
every other 3D cell type has ordering such that the normal will point
outward from the cell when using the right-hand rule to determine normals.

As a side note on this, you can see nearly all cell types (not polygons or
polyhedra) using the vtkCellTypeSource (VTK) or the Unstructured Cell Types
source (PV). See
https://blog.kitware.com/new-vtk-source-for-generating-different-cell-types/
for more details.

Cheers,
Andy

On Fri, Jan 19, 2018 at 3:53 AM, Nico Schlömer <nico.schloemer at gmail.com>
wrote:

> Hi everyone,
>
> Recently, a meshio user came up with an interesting problem in wedge node
> ordering when reading VTK files. The VTK specification [2] lists this [3]
> ordering, but a file in the wild had that [4]. They look almost identical
> at first sight, and in fact they both list wo triangles, where the `k`th
> node of the first is connected to the `k`th node of the second. In the
> ordering in the specification, however, the second triangle normal points
> towards the first triangle, while in the the other, the first triangle
> normal points towards the second triangle.
>
> ParaView will happily display both variants, but I'm wondering if the
> second case if formally covered by the specification.
>
> (The issue was discovered when doing FEM computations where the second
> variant led to negative Jacobians.)
>
> Cheers,
> Nico
>
> [1] https://github.com/nschloe/meshio/issues/185
> [2] https://www.vtk.org/wp-content/uploads/2015/04/file-formats.pdf
> [3] https://user-images.githubusercontent.com/9031253/
> 35021239-dbb37d7a-fae4-11e7-8534-167726495bd8.png
> [4] https://user-images.githubusercontent.com/181628/
> 35061681-40a8ee76-fbc2-11e7-9fe3-caeff268738a.png
>
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