CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

It does not destroy your triangulation, it simply takes the ownership.
When you're done with the Alpha_shape_2, you can swap [1] it back to 
your original triangulation or directly use it as a triangulation.

Something like the following should work:
Dt dt;
Alpha_shape as(dt);
static_cast<Dt&>(as).swap(dt); // the alpha-shape is no longer valid


Sebastien

[1] 
http://doc.cgal.org/latest/Triangulation_2/classCGAL_1_1Triangulation__2.html#a1788d6ffa3f5986b2b07e7e0b48a2ffc

On 07/27/2015 04:30 PM, Daniel Duque wrote:
> Thanks Sebastien.
>
> I fact, I do have info on the vertices, e.g the bool variable that marks
> a vertex as being inner or outer and lots of other stuff. That info gets
> copied correctly on the new T2 triangulation, but the latter is
> destroyed when building the alpha shape. My problem is then either to
> transfer back the info to the original T triangulation (which I do, but
> in a manner which is not too efficient), or perhaps stopping the alpha
> shape constructor from destroying my triangulation.
>
> I have just noticed there is another way to create an alpha shape, from
> a list of points:
>
> http://doc.cgal.org/latest/Alpha_shapes_2/classCGAL_1_1Alpha__shape__2.html#a5524e49e27ef267165673d0a36bddb14
>
> If I could make an alpha shape from a list of pair< points, vertex
> handle >, I guess I could retrieve the vertex from which each of the
> points came from...
>
> Best,
>
> Daniel
>
>
> On Mon, Jul 27, 2015 at 1:28 AM, Sebastien Loriot (GeometryFactory)
> <
>  
> <mailto:>>
>  wrote:
>
>     What about adding an info in the vertex type?
>
>     See this example:
>     http://doc.cgal.org/latest/Triangulation_2/index.html#title40
>     using this class:
>     
> http://doc.cgal.org/latest/Triangulation_2/classCGAL_1_1Triangulation__vertex__base__with__info__2.html
>
>     Sebastien.
>
>
>     On 07/24/2015 05:40 PM, Daniel Duque wrote:
>
>         Hello everyone.
>
>         I have been using alpha shapes to identify the outer vertices of
>         triangulations
>         for some time, and I am very happy with the method. However, the
>         underlying triangulation
>         is destroyed in the process. This often OK, but can be
>         inconvenient if
>         one just wants to "mark"
>         the nodes as outer, but keeping the triangulation. Right now I
>         am doing
>         this crude treatment
>         which, as you see, copies the triangulation, destroys it, then
>         marks the
>         vertices on the original
>         triangulation as outer if the coincide with the nodes of the
>         alpha shape.
>
>         The crudest part, as you can see, is that this coincidence is
>         implemented by "locate", since
>         I have lost any other connection with the original triangulation. I
>         guess I could hack the
>         copy procedure in order to include a pointer or handle to the
>         original
>         one...
>
>         Any ideas will be welcome.
>
>         Best,
>
>         Daniel Duque
>         UPM
>
>         // T is a global Delaunay triangulation
>
>         void surf_detect(void) {
>
>             Triangulation T2(T);
>
>             FT hh= 1.2 * simu.h();
>
>             FT alpha=hh*hh;
>             Alpha_shape_2 al( T2, alpha );
>
>             for (Alpha_shape_vertices_iterator
>         asv=al.alpha_shape_vertices_begin();
>                  asv!=al.alpha_shape_vertices_end();
>                  ++asv) {
>
>               Point p=(*asv)->point();
>
>         //  mark vertices in the alpha shape as boundary vertices
>         //  implemented by locate (not very efficient, I guess)
>
>               int i;
>               Triangulation::Locate_type lt;
>               Face_handle fc=T.locate(p, lt , i );
>               fc->vertex(i)->boundary.set( true );
>             }
>
>             return;
>         }
>
>
>
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