CGAL users discussion list

[Jari Vauhkonen <>]

Hello,

I've been trying to get a boundary of a point set by building an alpha shape
with an optimal alpha value and outputting the points belonging to the 
boundary
of the alpha shape.

My problem is that the alpha shape I created seems to contain only a half of 
the
points set.

See http://cc.joensuu.fi/~jvauhkon/half_alpha.GIF for an example. The red 
points
are the boundary points outputted. I've got around 100 nearly similar point
sets and, as far as I've checked, the result seems to be the same every time.

Here's the central pieces of my code. The standard list (lp) that contains the
input points should be OK. The code is always ran with an optimal alpha value,
although the result is same with a defined alpha. What am I doing wrong?


        // compute alpha shape  
        Alpha_shape_2 as(lp.begin(),lp.end());
        std::cout << "Alpha shape computed in REGULARIZED mode by default" 
                << std::endl;
        if (isOptimalAlpha)
        {
                // find optimal alpha value
                Alpha_iterator opt = as.find_optimal_alpha(5);
                std::cout << "Optimal alpha value to get a connected 
component:" 
                <<  *opt    << std::endl;
                alpha = *opt;
                as.set_alpha(alpha);
        }
        else
                as.set_alpha(alpha);

        Alpha_shape_2::Alpha_shape_vertices_iterator vert_it;
        for ( vert_it = as.alpha_shape_vertices_begin(); vert_it != 
                as.alpha_shape_vertices_end(); vert_it++ ) {
                Vertex_handle h = *vert_it;
                // not a part of the interior & optimal alpha,
                // so has to be a part of the boundary
                if (as.classify(h) != Alpha_shape_2::INTERIOR) {
                        Point p = h->point();
                        fprintf(output, "%f %f\n", CGAL::to_double(p[0]), 
                                CGAL::to_double(p[1]) );
                }
        }

        

Thanks in advance,
  Jari