[Gmsh] How to swith surface normal of a step file and to export this orientation in .mesh format

[Christophe Geuzaine]

On 19 Mar 2013, at 11:14, NENNIG Benoit <benoit.nennig at supmeca.fr> wrote:

> Dear gmsh team,
> 
> I would like to switch orientation of surfaces coming from a part design by FreeCAD and export in step format.
> - I try to reverse normal orientation in the FreeCAD (MyShape.reverse() ??) but in gmsh nothing change (however I am not sure of how is made the export in step and if step can store inward normal vector....)
> - I try the "classical" gmsh version :
>    Physical Surface (1) = {-SufList[]}; 
> It works for .msh format (native gmsh) but in .mesh (inria format) only the label is change into -label but the orientation remain the same...
> - flip all mesh normal is not possible because I have other shape in my mesh, not concerned with this problem.
> 
> Do you have an idea ?
> 

I think we should provide an explicit interface to orient (or just "revert" w.r.t. the natural orientation) the mesh.

The new MSH3 format will not change the orientation of the elements at "write" time, as this makes the on-disk representation different from the "in-memory" representation. The ".mesh" format is already "correct" in that sense.

I've filed this as https://geuz.org/trac/gmsh/ticket/191 


> Thanks!
> 
> Benoit
> 
> ps : In .msh format, I observe that if the surface has strong variation close a small inner corner, the element orientation may be random close to the inner corner if there is only one element on the corner thickness. 
> 

Hmm... When there is no inner vertex we resort to a simple geometrical test at the barycenter of an element. Could you send a simple example?

Thanks,

Christophe




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-- 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine