Difference fails for double precision

[MauriceHubain]

Difference does not work properly with double precision for some corner cases:

output1 = input0 - input1 output2 = output1 - input2

expected behavior and result with float precision: output1 ≈ output2 result with double precision: output2 is empty

#include <boost/geometry.hpp>


template <typename T>
void test_Bug83673_3()
{
  using Point2d         = boost::geometry::model::d2::point_xy<T>;
  using Polygon2d       = boost::geometry::model::polygon<Point2d, false, false>;
  using MultiPolygon2d  = boost::geometry::model::multi_polygon<Polygon2d>;

  MultiPolygon2d input0;
  boost::geometry::read_wkt("MULTIPOLYGON(((1.25499999790794003 0.850000004118476982,-1.25500000209204998 0.849999998970381032,-1.25499999999999989 -0.850000001029619034,1.25499999999999989 -0.849999999999999978,1.25499999790794003 0.850000004118476982)))", input0);
  boost::geometry::correct(input0);

  Polygon2d input1;
  boost::geometry::read_wkt("POLYGON((-0.875 -0.849999999999999978,-0.875 -0.070000000000000201,-1.25499999999999989 -0.070000000000000201,-1.25499999999999989 -0.849999999999999978,-0.875 -0.849999999999999978))", input1);
  boost::geometry::correct(input1);

  MultiPolygon2d output1;
  boost::geometry::difference(input0, input1, output1);


  Polygon2d input2;
  boost::geometry::read_wkt("POLYGON((-1.25499999999999989 -0.850000001029619034,-1.25500000209204998 0.849999998970381032,-1.43000000230740998 1.02499999861144997,-1.42999999978463999 -1.02500000110140999,-1.25499999999999989 -0.850000001029619034))", input2);
  boost::geometry::correct(input2);

  MultiPolygon2d output2;
  boost::geometry::difference(output1, input2, output2);


  std::cout << "test_Bug83673_3" << "<" << typeid(T).name() << ">" << std::endl;
  std::cout << "input0:" << boost::geometry::to_wkt(input0) << "\tarea:" << boost::geometry::area(input0) << std::endl;
  std::cout << "input1:" << boost::geometry::to_wkt(input1) << "\tarea:" << boost::geometry::area(input1) << std::endl;
  std::cout << "output1:" << boost::geometry::to_wkt(output1) << "\tarea:" << boost::geometry::area(output1) << std::endl;

  std::cout << "input2:" << boost::geometry::to_wkt(input2) << "\tarea:" << boost::geometry::area(input2) << std::endl;
  std::cout << "output2:" << boost::geometry::to_wkt(output2) << "\tarea:" << boost::geometry::area(output2) << std::endl;
  std::cout << std::endl;
}


int main()
{
  test_Bug83673_3<float>();
  test_Bug83673_3<double>();

  return 0;
}

Output:

test_Bug83673_3<float>
input0:MULTIPOLYGON(((1.255 0.85,-1.255 0.85,-1.255 -0.85,1.255 -0.85,1.255 0.85)))     area:4.267
input1:POLYGON((-0.875 -0.85,-0.875 -0.07,-1.255 -0.07,-1.255 -0.85,-0.875 -0.85))      area:0.2964
output1:MULTIPOLYGON(((-0.875 -0.07,-0.875 -0.85,1.255 -0.85,1.255 0.85,-1.255 0.85,-1.255 -0.07,-0.875 -0.07)))       area:3.9706
input2:POLYGON((-1.255 -0.85,-1.255 0.85,-1.43 1.025,-1.43 -1.025,-1.255 -0.85))        area:0.328125
output2:MULTIPOLYGON(((-0.875 -0.07,-0.875 -0.85,1.255 -0.85,1.255 0.85,-1.255 0.85,-1.255 -0.07,-0.875 -0.07)))       area:3.9706

test_Bug83673_3<double>
input0:MULTIPOLYGON(((1.255 0.85,-1.255 0.85,-1.255 -0.85,1.255 -0.85,1.255 0.85)))     area:4.267
input1:POLYGON((-0.875 -0.85,-0.875 -0.07,-1.255 -0.07,-1.255 -0.85,-0.875 -0.85))      area:0.2964
output1:MULTIPOLYGON(((1.255 0.85,-1.255 0.85,-1.255 -0.85,1.255 -0.85,1.255 0.85),(-1.255 -0.07,-0.875 -0.07,-0.875 -0.85,-1.255 -0.85,-1.255 -0.07))) area:3.9706
input2:POLYGON((-1.255 -0.85,-1.255 0.85,-1.43 1.025,-1.43 -1.025,-1.255 -0.85))        area:0.328125
output2:MULTIPOLYGON()  area:0

This was tested with boost 1.88. With boost 1.71 we get the same (proper) results for float and double precision.

[barendgehrels]

Thanks for the report. Could you maybe also test with 1.89 (in preparation)? Things have changed a lot...

[MauriceHubain]

I tested it on branch "boost-1.89.0.beta1" and got the same result.

[barendgehrels]

Thanks for testing!