Rotate a polygon using boost geometry

Question

I am trying to rotate a polygon using boost geometry. Probably I am doing something wrong. I have a polygon, not centered in the origin, declared like this:

Polygon _poly;
Polygon _poly2;

  Point2D A(4,3);
  Point2D B(4,5);
  Point2D C(6,5);
  Point2D D(6,3);
  Point2D CLOSE(4,3);


  _poly.outer().push_back(A);
  _poly.outer().push_back(B);
  _poly.outer().push_back(C);
  _poly.outer().push_back(D);

Then, I perform a rotation with:

  boost::geometry::strategy::transform::rotate_transformer<boost::geometry::degree, double, 2, 2> rotate(45.0);

But the resulting coordinates of the polygon are not the correct ones:

poly's coordinates: 4 3 4 5 6 5 6 3

rotated coordinates: 4 0 6 0 7 0 6 -2

What I have to do?

Answer 1

You polygon is invalid (see the documentation). This is easy to check with is_valid .

If you don't know the input source, you can always try to correct with boost::geometry::correct :

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#include <iostream>

#include <boost/geometry/geometry.hpp>
#include <boost/geometry/geometries/geometries.hpp>
#include <boost/geometry/geometries/point.hpp>
#include <boost/geometry/algorithms/is_valid.hpp>
#include <boost/geometry/algorithms/transform.hpp>

namespace bg  = boost::geometry;

typedef bg::model::point<double, 2, bg::cs::cartesian> Point2D;
typedef bg::model::polygon<Point2D> Polygon;
//typedef bg::model::box<Point2D> box;

int main() {

    Polygon _poly;
    Polygon _poly2;

    Point2D A(4,3);
    Point2D B(4,5);
    Point2D C(6,5);
    Point2D D(6,3);
    Point2D CLOSE(4,3);

    _poly.outer().push_back(A);
    _poly.outer().push_back(B);
    _poly.outer().push_back(C);
    _poly.outer().push_back(D);

    std::cout << std::boolalpha << bg::is_valid(_poly) << "\n";
    bg::correct(_poly);
    std::cout << std::boolalpha << bg::is_valid(_poly) << "\n";
}

Output:

false
true

In this case you clearly forgot to add the CLOSE point