CODEWITHSUNDEEP
polygonintersectioncgal
Philia Jankov
Philia Jankov

Reputation: 13

CGAL Intersection returns false result

I m a bit confused about how the the CGAL::do_intersect works. I thought the function returns true if there is a point in both polygons. As far as I m not mistaken in lies within out and I should see true printed out or what am I missing?

#include <CGAL/Point_2.h>
#include <CGAL/Polygon_2.h>
#include <CGAL/Exact_predicates_exact_constructions_kernel.h>

typedef CGAL::Exact_predicates_exact_constructions_kernel Kernel;
typedef Kernel::Point_2 Point_2;
typedef CGAL::Polygon_2<Kernel> Polygon_2;
int main(int argc, char **argv)
{
  Polygon_2 in, out;
  in.push_back(Point_2(1,1));
  in.push_back(Point_2(1,2));
  in.push_back(Point_2(2,2));
  in.push_back(Point_2(2,1));

  out.push_back(Point_2(0,0));
  out.push_back(Point_2(3,0));
  out.push_back(Point_2(3,3));
  out.push_back(Point_2(0,3));

  std::cout << "IN intersect with OUT is " << (CGAL::do_intersect(in, out) ? "true":"false") << std::endl;
  std::cout << "OUT intersect with IN is " << (CGAL::do_intersect(out, in) ? "true":"false") << std::endl;
  std::cout.flush();
}

Upvotes: 1

Views: 151

Answers (1)

m.s.
m.s.

Reputation: 16324

The vertices in the polygon need to be counter-clockwise. The following code produces the desired output:

IN intersect with OUT is true
OUT intersect with IN is true

#include <CGAL/Point_2.h>
#include <CGAL/Polygon_2.h>
#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
#include <CGAL/Boolean_set_operations_2.h>

typedef CGAL::Exact_predicates_exact_constructions_kernel Kernel;
typedef Kernel::Point_2 Point_2;
typedef CGAL::Polygon_2<Kernel> Polygon_2;
int main(int argc, char **argv)
{
  Polygon_2 in, out;
  in.push_back(Point_2(1,1));
  in.push_back(Point_2(2,1));
  in.push_back(Point_2(2,2));
  in.push_back(Point_2(1,2));

  out.push_back(Point_2(0,0));
  out.push_back(Point_2(3,0));
  out.push_back(Point_2(3,3));
  out.push_back(Point_2(0,3));

  std::cout << "IN intersect with OUT is " << (CGAL::do_intersect(in, out) ? "true":"false") << std::endl;
  std::cout << "OUT intersect with IN is " << (CGAL::do_intersect(out, in) ? "true":"false") << std::endl;
  std::cout.flush();
}

Upvotes: 1

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