[Gmsh] from hexahedrons to tetrahedrons

[Geordie McBain]

> I have a structured mesh with hexahedrons and I would like to
> split each hexahedrons into tetrahedrons.
> Is it possible ?

Hello.  Someone asked me this the other day.  At first I thought it
should be easy, but then I found I didn't know how to do it, in Gmsh
or otherwise.  I had thought that one should be able to simply split
each hexahedron, but then reading the documentation for how it's done
in the VTK library:

  http://www.vtk.org/doc/nightly/html/classvtkDataSetTriangleFilter.html

I saw that one must meet `the issue of face compatibility across
quadrilateral faces (which way to orient the diagonal?)'.

I figure then that it should be possible to export a hexahedral mesh
as VTK, use this VTK function to split to tetrahedra and save as VTK,
then reread the VTK.  I haven't actually gone on to try this.

I would be interested to hear of a pure Gmsh solution, if anyone has one.

> if yes into how many tetrahedrons ?

I think the answer for this should be six, since the volume of a
tetrahedron with three mutually perpendicular edges of unit length is
1/6.