Solid vs Shell meshes

[bobb-uk]

More a question about CSXCAD than OpenEMS, but......

When a model is meshed and output to an STL file [to be imported by the PolyhedronReader], should the internals of the model contain tetrahedrons or is it suffice for the 'outer walls' of the model to be triangle faces ?

Thanks,
Bob.

[bobb-uk]

What I wrote isn't clear: Sorry.
Does the STL model need tetrahedrons or can it be constructed only from surface faces ?

[LubomirJagos42]

I was solving this issue in previous threads since it happens that I was correcting simulations which were importing STL files and they were interpreted as shell in openEMS
Whole trick is that STL file always describe just shell which is composed from triangular faces with their normals. I STL is correct it creates waterproof solid, applications like FreeCAD or similar exporting right solids, but some apps when doing boolean operations don't set normal corrects and there are holes in faces, specially for this there are tools to correct defects of STL files, like Blender don't always export right STL files since it internally works with nurbs curves.
Therefore STL should be solid but doesn't have to be generated right, in FreeCAD there is special checker for this.

[bobb-uk]

Basically, meshes do not need to contain tetrahedrons.
Faces are sufficient for STL models.
Each face normal must point in the correct direction so software can determine which is the outside face and which is the inside face.
All surface faces in the mesh must align to be "water tight". There can be no gaps, no holes and no part should be "non-manifold"

[thliebig]

STL are always only triangulated surfaces. There are no tetrahedrons!
openEMS just requires them to be closed and healthy (e.g. no non-manifold faces).
Because the discretization pretty much works in a way is coordinate xyz inside the object. If yes then e.g. an electric field edge is set to zero (perfect metal) or if it is a dielectric it will contribute to create the filling for the capacitor for the electric field update equation etc.
If the STL is not closed this may go (horribly) wrong and is then only contained/limited by the bounding box...

[thliebig]

This also explains why having edges fall on the surface might get tricky... because it is then undefined if the algo thinks a coordinate is inside or outside... Thus having a STL surface slightly oversize is not a bad idea...
Or use just the basic primitives were the checks are the same, but much more robust... e.g. to check if xyz is inside a sphere is very easy and can even be done including a very small tolerance.