CGAL 5.4.1 - 2D Alpha Shapes
#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Cartesian_converter.h>
#include <CGAL/Delaunay_triangulation_2.h>
#include <CGAL/Projection_traits_xy_3.h>
#include <CGAL/algorithm.h>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <fstream>
#include <vector>
#include <list>
typedef double coord_type;
typedef K::Point_2 Point;
typedef K::Segment_2 Segment;
// -------------------------------------------------------------------
// Since K::Point_2 is here in fact CGAL::Point_3<FK>, the basic Cartesian_converter
// cannot be used (and thus ExactAlphaComparisonTag cannot be set to 'true') because
// it does not know how to convert from CGAL::Point_3<FK> to CGAL::Point_2<EK>.
// Thus, we must provide a specialization of Cartesian_converter to be able
// to set ExactAlphaComparisonTag to 'true'
namespace CGAL {
template < class K2, class C >
class Cartesian_converter<Epic, K2, C>
typedef CGAL::Projection_traits_xy_3<Epic> Source_kernel;
typedef K2 Target_kernel;
typedef C Number_type_converter;
typedef typename Source_kernel::Point_2 SP2;
typedef typename Target_kernel::Point_2 TP2;
TP2 operator()(const SP2& p) const
return TP2(c(p.x()), c(p.y()));
C c;
} // namespace CGAL
// The partial specialization must be defined before Alpha Shapes-related headers
#include <CGAL/Alpha_shape_2.h>
#include <CGAL/Alpha_shape_vertex_base_2.h>
#include <CGAL/Alpha_shape_face_base_2.h>
// ExactAlphaComparisonTag is false
typedef K Gt;
typedef CGAL::Triangulation_data_structure_2<Vb,Fb> Tds;
// ExactAlphaComparisonTag is true
typedef K Gt;
typedef CGAL::Triangulation_data_structure_2<Vb_TT,Fb_TT> Tds_TT;
typedef CGAL::Delaunay_triangulation_2<Gt,Tds_TT> Triangulation_2_TT;
template <class Alpha_shape,class InputIterator, class OutputIterator>
void alpha_edges(InputIterator begin, InputIterator end,
const typename Alpha_shape::FT& Alpha,
bool mode,
typedef typename Alpha_shape::Alpha_shape_edges_iterator Alpha_shape_edges_iterator;
Alpha_shape A(begin,end);
if (mode) { A.set_mode(Alpha_shape::GENERAL); }
else { A.set_mode(Alpha_shape::REGULARIZED); }
for(Alpha_shape_edges_iterator it = A.alpha_shape_edges_begin();
it != A.alpha_shape_edges_end(); ++it) {
*out++ = A.segment(*it);
template <class OutputIterator>
bool file_input(OutputIterator out)
std::ifstream is("data/fin3", std::ios::in);
if( {
std::cerr << "unable to open file for input" << std::endl;
return false;
int n;
is >> n;
std::cout << "Reading " << n << " points from file" << std::endl;
std::copy_n(std::istream_iterator<Point>(is), n, out);
return true;
int main()
std::list<Point> points;
if(! file_input(std::back_inserter(points)))
return -1;
// ExactAlphaComparisonTag is False
std::vector<Segment> segments;
alpha_edges<Alpha_shape_2>(points.begin(), points.end(),
std::cout << "Alpha Shape computed with ExactAlphaComparisonTag = false" << std::endl;
std::cout << segments.size() << " alpha shape edges" << std::endl;
// ExactAlphaComparisonTag is True
std::vector<Segment> segments;
alpha_edges<Alpha_shape_2_TT>(points.begin(), points.end(),
std::cout << "Alpha Shape computed with ExactAlphaComparisonTag = true" << std::endl;
std::cout << segments.size() << " alpha shape edges" << std::endl;
return 0;