CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

If I remember correctly, the range
[alpha_shape_edges_begin,_end] is over regular edges in Regularized mode,
and singular and regular edges in general mode (it gives you the 
alpha-shape which is the boundary of the alpha-complex)

So interior edges are not displayed.


You this function so get the classifications:

void
alpha_edges( const Alpha_shape_2&  A)
{
  int r=0,s=0,i=0,e=0;
  for(Alpha_shape_2::Finite_edges_iterator it =  A.finite_edges_begin();
      it != A.finite_edges_end();
      ++it)
  {
    switch ( A.classify(*it) )
    {
      case Alpha_shape_2::REGULAR:  ++r; break;
      case Alpha_shape_2::SINGULAR: ++s; break;
      case Alpha_shape_2::EXTERIOR: ++e; break;
      case Alpha_shape_2::INTERIOR: ++i; break;
    }
  }
  std::cout << "EIRS " << e << " " << i << " "<< r << " " << s << 
std::endl;
}

S.

Benoît Presles wrote:
> Hello Everybody,
>
> In my last email, I submitted you 2 problems I had with the 
> "Alpha_shape_2" class. I still have the first problem but after some 
> tests I realised that the results I obtained about the second problem 
> were OK.
> However, I still get some wrong results for some list of points (cf. 
> attached file):
> First test:
> - I compute the alpha shape (GENERAL) with the optimal alpha value found 
> with the function "find_optimal_alpha(1)" and I display on the screen 
> the alpha edges. With such a value of alpha I should get a closed 
> boundary, but instead I get this result:
> Reading 13 points from file
> Alpha Shape computed
> Optimal alpha: 0.01
> Nb of solid components: 1
> 0.3 0.2 0.2 0.2
> 0 0 0.1 0
> 0 0.2 0 0.1
> 0 0.1 0 0
> 0.2 0.2 0.1 0.2
> 0.1 0.2 0 0.2
> 0.1 0 0.1 0.1
> 0.4 0.1 0.4 0.2
> 0.4 0 0.4 0.1
> 0.3 0.1 0.3 0
> 0.4 0.2 0.3 0.2
> 0.3 0 0.4 0
>
> Second test:
> - I compute the alpha shape (GENERAL) with alpha=0.0025 and I display on 
> the screen the alpha edges.  With such a value of alpha I should get 
> much more edges but instead I get this result:
> Reading 13 points from file
> Alpha Shape computed
> Optimal alpha: 0.01
> Nb of solid components: 0
> 0.3 0.2 0.2 0.2
>
> Third test:
> - I compute the alpha shape (GENERAL) with alpha=0.0026 and I display on 
> the screen the alpha edges.  With such a value of alpha I get too much 
> edges:
> Reading 13 points from file
> Alpha Shape computed
> Optimal alpha: 0.01
> Nb of solid components: 0
> 0.3 0.2 0.2 0.2
> 0.3 0.1 0.3 0.2
> 0 0 0.1 0
> 0 0.2 0 0.1
> 0 0.1 0 0
> 0.2 0.2 0.1 0.2
> 0.1 0.2 0 0.2
> 0.1 0.1 0 0.1
> 0.1 0.1 0.1 0.2
> 0.1 0 0.1 0.1
> 0.4 0.1 0.4 0.2
> 0.4 0 0.4 0.1
> 0.3 0.1 0.3 0
> 0.4 0.2 0.3 0.2
> 0.4 0.1 0.3 0.1
> 0.3 0 0.4 0
>
>
> Thank you for your help,
> Best Regards,
> Ben_P
>
>
>
> Le 11 août 10 à 19:07, Benoît Presles a écrit :
>
> > Hello Everybody,
> >
> > I use the latest patched version of CGAL and I think some bugs could 
> > be present in the "Alpha_shape_2" class (all problems can be 
> > reproduced by using the list of points "fin" attached in this email).
> >
> > First problem:
> > - First, I use the function "find_optimal_alpha" to find the best 
> > alpha to get 1 connected component.
> > - Second, I set this alpha.
> > - Third, I use the function "number_of_solid_components" to get the 
> > number of solid components of A.
> > - Fourth, I compute the alpha edges which I display on the screen.
> >
> > When I do this last step, I get a "CGAL::Assertion_exception":
> > Reading 6 points from file
> > Alpha Shape computed
> > Optimal alpha: 380.5
> > Nb of solid components: 1
> > terminate called after throwing an instance of 
> > 'CGAL::Assertion_exception'
> >  what():  CGAL ERROR: assertion violation!
> > Expr: (classify((*edge_alpha_it).second.first, 
> > (*edge_alpha_it).second.second) == REGULAR)
> > File: /appli/include/CGAL/Alpha_shape_2.h
> > Line: 1170
> > Aborted
> >
> > Second problem:
> > - First, I compute the alpha shape with alpha=50.
> > - Second, I compute the alpha edges which I display on the screen.
> >
> > With such a value of alpha I should get the convex hull of the points, 
> > but instead I get this result:
> > Reading 6 points from file
> > 0 0 1 0
> > 1 0 2.5 0
> > 2.5 0 4.5 0
> > 4.5 0 7 0
> >
> >
> > Attached, you can fine the c++ code (basically the same as in the 
> > example) and a list of points for which the program doesn't work as 
> > expected (both problems).
> >
> >
> >
> > Thank you for your help,
> > Best Regards,
> > Ben_P
> >
> > --
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> >
> > <fin.txt><alphaShape.cpp>