(extracted from #1241)

Hi All,

I believe I hit this same issue and thought I may help by creating a few test cases showing to help:

BOOST_AUTO_TEST_CASE(Boost_Diffence_ClearAll)

{

	using PointXY = boost::geometry::model::d2::point_xy<double>;

	using Polygon = boost::geometry::model::polygon<PointXY>;

	using MultiPolygon = boost::geometry::model::multi_polygon<Polygon>;



	MultiPolygon polyA;

	read_wkt("MULTIPOLYGON((("

	    "-2.0785311235613415 -0.6304193410175202,"

      "-2.0534946127981359 -0.6304193410175202,"

      "-2.0534946127981359 -0.8867932112327471,"

      "-2.3098684830133629 -0.8867932112327471,"

      "-2.3098684830133629 -0.6554558517807265,"

      "-2.2848319722501573 -0.6554558517807265,"

      "-2.0785311235613415 -0.6554558517807265,"

      "-2.0785311235613415 -0.6304193410175202)))", polyA);



	MultiPolygon polyB;

	read_wkt("MULTIPOLYGON((("

			"-2.0785311235613420 -0.6304193410175202,"

			"-2.0534946127981359 -0.6304193410175202,"

			"-2.0534946127981359 -0.6554558517807265,"

			"-2.0785311235613420 -0.6554558517807265,"

			"-2.0785311235613420 -0.6304193410175202)))", polyB);



	MultiPolygon polyAB;

	//Clip off the small polyB appendix top/left in part polyA

	difference(polyA, polyB, polyAB);



	//Without an error we should get:

	//BOOST_CHECK_CLOSE(area(polyAB), area(polyA) - area(polyB), 0.0001);

	//BOOST_CHECK_EQUAL(polyAB.size(), 1u);

	//BOOST_CHECK(polyAB[0].inners().empty());



	//But we get this:

	BOOST_CHECK(polyAB.empty());

}



BOOST_AUTO_TEST_CASE(Boost_Diffence_ClearsTooMuch)

{

	using PointXY = boost::geometry::model::d2::point_xy<double>;

	using Polygon = boost::geometry::model::polygon<PointXY>;

	using MultiPolygon = boost::geometry::model::multi_polygon<Polygon>;



	MultiPolygon polyA;

	read_wkt("MULTIPOLYGON((("

		"-6.1723606999999996 3.2834021000000000,"

		"-6.1723606999999996 2.8006724999999992,"

		"-5.7133718999999994 2.8006724999999992,"

		"-5.7133718999999994 3.2834021000000000,"

		"-5.6896310999999997 3.2834021000000000,"

		"-5.6896310999999997 2.7769316999999996,"

		"-6.1961014999999993 2.7769316999999996,"

		"-6.1961014999999993 3.2834021000000000,"

		"-6.1723606999999996 3.2834021000000000)))", polyA);



	MultiPolygon polyB;

	read_wkt("MULTIPOLYGON((("

		"-6.1723606999999996 2.8006724999999997,"

		"-5.7133718999999994 2.8006724999999997,"

		"-5.7133718999999994 2.7769316999999996,"

		"-6.1723606999999996 2.7769316999999996,"

		"-6.1723606999999996 2.8006724999999997)))", polyB);



	MultiPolygon polyAB;

	//Clip off the small polyB at the bottom center part of polyA

	difference(polyA, polyB, polyAB);



	//Without an error we should get:

	//BOOST_CHECK_CLOSE(area(polyAB), area(polyA) - area(polyB), 0.0001);

	//BOOST_CHECK_EQUAL(polyAB.size(), 2u);

	//BOOST_CHECK(polyAB[0].inners().empty());

	//BOOST_CHECK(polyAB[1].inners().empty());



	//But we get this:

	BOOST_CHECK_CLOSE(area(polyAB), 0.5*(area(polyA) - area(polyB)), 0.0001);

	BOOST_CHECK(polyAB.size() == 1u);

	BOOST_CHECK(polyAB[0].inners().empty());

}

Originally posted by @JohanDore in #1241