CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

On 09/06/2017 03:32 PM, Iasonm wrote:
> Hello,
>
> I have some questions regarding the use of collapse_edge on a surface_mesh.
>
> *Why I need it*
> I am implementing  this
> <http://visgraph.cse.ust.hk/projects/skeleton/skeleton_sig08.pdf>   for
> educational purposes. The part 5 of this paper is called connectivity
> surgery and it simplifies a previously contracted 2D mesh (after the
> contraction process the volume of the mesh is so small that the mesh is
> considered to lay on a plane). This connectivity surgery is in fact a mesh
> simplification process using edge collapsing until no faces are left. On
> each iteration of this process an edge is chosen based on some cost function
> and it gets collapsed. I am thinking of using CGAL's collapse_edge function
> to make the collapse.
>
> *My questions*
> Since collapse_edge alters the connectivity of the mesh after each call to
> collapse_edge I will need to call collect_garbage.
> 1. In the reference manual of collect_garbage it is stated: "By garbage
> collecting elements get new indices. In case you store indices in an
> auxiliary data structure or in a property these indices are potentially no
> longer referring to the right elements.". *Will collect_garbage() alter
> parts of the mesh  that are not in the 1-ring area of the collapsed edge?
> *This is important to me since if it doesn't I can just update the cost
> matrix  (the one that is used to choose which edge will be collapsed on the
> next iteration) to match the changes in the mesh. Otherwise I will need to
> update the whole cost matrix which will be computationally costly.
>
> 2. Do you think there are other problems in using CGAL::Euler::collapse_edge
> on a CGAL::Surface_mesh?
Why do you want to call collect_garbage() at each step? Actually it is
not need. You can work with your mesh without calling it and simply call
garbage_collect() if you want to free some space.

Sebastien.

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