CGAL users discussion list

[Manuel Caroli <>]

Hi Juliano,

Juliano Costa wrote:
> Hello Manuel,
>  
[...]
The *interior* of the (smallest) sphere passing through the two vertices 
of the diagonal of a cube (or cuboid) does not contain any other 
vertices of the cube (or cuboid). This is a degenerate case though. The 
CGAL::Triangulation_3 partitions the convex hull of the point set into 
tetrahedra even in degenerate cases. So the result you get is fine.
>  
> Well, I ignored this issue and proceed with my intend so I used the 
> DelaunayTriangleHierarchy::Finite_facets_iterator like this:
I don't actually get what you mean by "surface" in the code below. Are 
you talking about the convex hull?

>  
> //On inserting points I'm setting the Vertex_hanlde->info with an 
> "unsigned long" value as index. 
>  
> //I need to get the triangles points indexes:
> CPUInt64_t  _indexA,_indexB,_indexC;
> CPUInt64_t* _iPointsIndexes[3];
> _iPointsIndexes[0] = &_indexA;
> _iPointsIndexes[1] = &_indexB;
> _iPointsIndexes[2] = &_indexC;
>  
> //having DelaunayTriangleHierarchy _delaunayTrianguleHierarchy;
> DelaunayTriangleHierarchy::Finite_facets_iterator _fit = 
> _delaunayTriangule.finite_facets_begin(); 
>  
> while(_fit !=  _delaunayTriangule.finite_facets_end())
> {
>      //typedef CGAL::Delaunay_triangulation_3<K,Tds> DelaunayTriangle;
>      DelaunayTriangle::Facet  _facet = *_fIt;
>      unsigned long _facetIndex = _facet.second;
>  
>      //typedef DelaunayTriangle::Cell_handle CGALCellHandle;
>      CGALCellHandle facetCell = _facet.first;
>      char _isInfiniteCell = _delaunayTriangule->is_infinite(_facetCell);
>      //neighbor cell
>      CGALCellHandle neigborCell = _cell->neighbor(_facetIndex );
>      char _isInfiniteNeigbor = _delaunayTriangule.is_infinite(neigborCell);
>  
>      if( (!_isInfiniteCell) && (!isInfiniteNeigbor) )
>      {
>           //I only want the surface right?!?!!
>           continue;
>      } 
>      char _iTriVrr=0;
>     
>      for(char _iPointsLoop=0;_iPointsLoop<4;_iPointsLoop++)
>      {
>            if( _iPointsLoop != _facetIndex )
>            {
>                *_iPointsIndexes[_iTriVrr++] = (unsigned 
> long)(_fIt->first->vertex(_iPointsLoop)->info());
>            }
>      }
>      //Here I got _indexA,_indexB,_indexC which I'm saving on the OBJ file.
>  
>     _fit++;
> } 
>   
> With this code, when I provide an ordinary box or even a sphere the 
> result is just wonderful.
> But when I try this with a small points cloud (10000 3d points of a 
> tunnel for example) the result is a mess.
> I got triangles coming from a side to the another and from the top to 
> the ground.
> I want just the surface.
>  
> I used this tunnel points cloud as example, but my intention is to use 
> the same algorith providing points cloud of any
> 3d object or place  (e.g.: an office room).
>  
> Now I was about to try to integrate the my current source code with the 
> CGAL "alpha shape" example...
>  
> But you've come with a different "light": "use a surface mesher". But a 
> surface mesher wouldn't interpolate "new created points"?
Yes the surface mesher creates its own points so it does not seem 
adapted to your problem.
> So, to make this "surface mesher" what I've be doing so far is useless?
>
> On the CGAL examples for surface mesher  (like 
> examples/Surface_mesher/mesh_an_implicit_function.cpp)
> <http://www.cgal.org/Manual/3.3/examples/Surface_mesher/mesh_an_implicit_function.cpp>the 
> input points comes from a generator surface function 
>  
> // defining the surface
> //  Surface_3 
> <http://www.cgal.org/Manual/3.3/doc_html/cgal_manual/Surface_mesher_ref/Concept_Surface_3.html#Cross_link_anchor_1083> 
> surface
>  
> Can I insert points of an unknown shape?
You might want to have a look at the new Surface reconstruction package: 
http://www.cgal.org/Manual/beta/doc_html/cgal_manual/Surface_reconstruction_points_3/Chapter_main.html
Note that this is only a beta release. The final release will be 
available by mid September.

best

Manuel

>  
> I really appreciate your help!
> Thanks once again!
>  
> Juliano Costa
> Brazil.
>  
>  
> 2009/8/5 Manuel Caroli < 
> <mailto:>>
>
>     Hi Juliano,
>
>     the Triangulation_3 package computes a triangulation of R^3 with the
>     input points as vertices. If you want to compute a triangular mesh
>     approximating a surface you should use a surface mesher. Maybe you
>     could be more specific about your problem.
>
>     best
>
>     Manuel