Force embedding into 2D space

[michalhabera]

See FEniCS/dolfinx#159.

An option to force convert 3D mesh to 2D mesh (by stripping last component away) would be very useful.
Some users are preparing planar (2D) meshes in gmsh, which embeds into 3D. Reading 3D mesh into finite element library results in larger system matrices and suboptimal computations for tensors of rank >= 1.

[nschloe]

meshio is really just a translator. If the input mesh has a z-component that's 0 everywhere, then the output mesh will have, too. It'd be weird to mess with that. I suppose this could be fixed either in gmsh itself -- or dolfin.

[mrehor]

Well, I'm afraid that you're already messing with that a bit when reading xdmf3 mesh, see

meshio/meshio/xdmf_io.py

Lines 353 to 354 in ecd790a

	if geometry_type == "XY":
	points = numpy.column_stack([points, numpy.zeros(len(points))])

But this is good. Each mesh that is potentially saved as planar can be read again by meshio. I've created the pull request #298 with a small fix adding the option save_as_planar to meshio.xdmf_io.XdmfWriter.

[nschloe]

Alright now. I've looked at this issue again and found that meshio didn't properly handle 2D meshes at all. @mrehor gave a good example. The original reason for this was that some mesh formats (like VTK) don't support 2D meshes at all.

Now, I/O is handled properly (#301) for those formats that support it (like XDMF). There also is a command-line option for meshio-convert that allows pruning the z-components if it's all 0. This should hopefully satisfy your use case.

[mrehor]

Thanks for fixing this @nschloe, works as expected.