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Subject: [Boost-users] [Polygon] using polygon for linesegment intersection
From: gast128 (gast128_at_[hidden])
Date: 2011-04-26 13:30:10

Hello all,

I was trying to use the Boost.Polygon library to test for line segment
intersection. I define 2 polygons (lines) and use the Boolean operators of the
polygon_set. However it gives an empty result:

void TestBoostPolygonIntersectLineSegment()
   using namespace boost::polygon::operators;

   typedef boost::polygon::polygon_data<int> Polygon;
   typedef boost::polygon::point_data<int> Point;
   typedef boost::polygon::polygon_set_data<int> PolygonSet;
   typedef std::vector<Polygon> PolygonVector;
   Point aPts1[] = { Point(0, 0),
                     Point(10, 0) };

   Point aPts2[] = { Point(5, 5),
                     Point(5, -5) };

   Polygon line1(aPts1, aPts1 + count_array(aPts1));
   Polygon line2(aPts2, aPts2 + count_array(aPts2));
   //both share point (5, 0)
   assert(boost::polygon::contains(line1, Point(5, 0)));
   assert(boost::polygon::contains(line2, Point(5, 0)));

   //eh..., expected it to be 1, but anyway
   assert(boost::polygon::size(line1) == 2);
   assert(boost::polygon::size(line2) == 2);

   PolygonSet ps1;
   PolygonSet ps2;
   PolygonSet ps3 = ps1 & ps2;
   //gives no result
   if (!ps3.empty()) //<---
      PolygonVector vec1;

Also the documentation states that the size function gives the number of edges,
but a line has only 1 edge:

assert(boost::polygon::size(line1) == 2); //not 1?

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