[Gmsh] Skipping surface on conversion from STEP to geo_unrolled

[Christophe Geuzaine]

> On 3 Apr 2019, at 18:23, Gavin Ridley <gavin.keith.ridley at gmail.com> wrote:
> 
> Hi,
> 
> I would just like to note something to the developers regarding a perhaps unnecessary skipping of surfaces in the code.
> 
> I recently have had to convert a STEP file with fairly complicated geometry to a GEO.

Don't do that. By design, Gmsh never translates from one CAD format to another. The "unrolled GEO" feature is there for unrolling complex GEO scripts. It can indeed export a limited subset of geometrical entities created by other CAD kernels, but it's there only for debugging.

If you want to modify a STEP model, set

SetFactory("OpenCASCADE");

at the beginning of your script. Since we import STEP models with OpenCASCADE, this will allow you to modify the file directly with OpenCASCADE. See https://gitlab.onelab.info/gmsh/gmsh/blob/master/demos/boolean/import.geo for an example.

Christophe




> This is done so that I can split one of the surfaces in the STEP file to set different BCs on different parts of the surface. At the moment, it appears that gmsh will skip several surfaces in the conversion from STEP to geo_unrolled unnecessarily: surfaces were left out from the gmsh file. A warning message comes with this.
> 
> It seems, however, that the skipped surfaces work fine if you just go back in and add surfaces manually. Gmsh now reads the geo file correctly.
> 
> So, why are these surfaces being skipped? Is there some ambiguity about their interpretation from STEP? It would be nice to not have to manually go in and add the surfaces.
> 
> -- 
> Thanks,
> Gavin Ridley
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— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine