[Dune] Extract neighbouring element opposite to a certain vertex

[Carsten Gräser]

Hi,

Am 25.02.2016 um 15:29 schrieb Agnese, Marco:
> Hi,
> 
> I have a simplex entity and I want to extract the neighbouring element
> opposite to the vertex with local index vtxIdx. 
> 
> Therefore I need to find the intersection opposite to this vertex.  
> 
> This intersection has local index faceIdx=worlddim-vtxIdx. Is it right?
the exact ordering is encoded in the reference elements and depicted here:

  http://www.dune-project.org/doc/doxygen/dune-geometry-html/group___geometry_reference_elements.html

As you can see face index and opposite vertex index do not coincide in general.

> So if I extract entity.template subEntity<1>(faceIdx) I should be able
> to extract this face. Am I correct? But the face extracted in this way
> is not an intersection therefore I can't use it to find the neighbouring
> entity. Is there a solution here?
> 
> 
> The other solution is to loop over the intersections and extract the
> vtxIdx-th one. But does it work always? Or which is the ordering of this
> intersections?
> 
> I would expect to have them sorted accordingly the face local index (so
> faceIdx) but that seems not to be the case. What am I missing?
First of all your opposite vertex assumption is not correct.
Furthermore cannot be a one to one mapping of faces and intersections
in general because there are more intersection than faces in case of
a nonconforming mesh.

Best,
Carsten

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