Incorrect work boost::geometry::union_ on C++ - c++

Question

I want to use boost::geometry::union_ with polygons on picture, but it cannot see point E (says, that polygon ABCDE has area as ABCD). How can I fix it?

Answer 1

Here's how I'd write your exercise. I designed a similar scene (the points are probably named differently, but that's not the [sic] point):
Note: I've rounded the coordinates to the nearest integral number (and scaled x10 afterwards)
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#include <boost/geometry.hpp>
#include <boost/geometry/geometry.hpp>
#include <boost/geometry/geometries/point_xy.hpp>
#include <iostream>
#include <fstream>
namespace bg = boost::geometry;
namespace bgm = bg::model;
using Point = bgm::d2::point_xy<int>;
using Polygon = bgm::polygon<Point>;
using Multi = bgm::multi_polygon<Polygon>;
int main() {
auto prepare = [](auto name, auto& geo) {
std::string r;
if (!bg::is_valid(geo, r)) {
std::cout << "Correcting " << name << ": " << bg::wkt(geo) << " (" << r << ")\n";
bg::correct(geo);
}
std::cout << name << ": " << bg::wkt(geo) << "\n";
};
Point A{-120, 50}, B{-60, 70}, C{-70, 50}, D{-40, 00}, E{-130, 10};
Point F{-20, -20}, G{40, -10}, H{80, 60}, I{50, 80}, J{-20, 50}, K{30, 30};
Polygon ABCDE({{A, B, C, D, E, A}});
Polygon DFGHIJK({{D, F, G, H, I, J, K, D}});
prepare("ABCDE", ABCDE);
prepare("DFGHIJK", DFGHIJK);
Multi out;
bg::union_(ABCDE, DFGHIJK, out);
prepare("union", out);
{
namespace bs = bg::strategy::buffer;
const double buffer_distance = 3;
bs::distance_symmetric<double> distance(buffer_distance);
bs::join_miter join;
bs::end_flat end;
bs::point_circle circle(6);
bs::side_straight side;
Multi outline;
bg::buffer(out, outline, distance, side, join, end, circle);
std::ofstream svg("output.svg");
boost::geometry::svg_mapper<Point> mapper(svg, 400, 400);
mapper.add(ABCDE);
mapper.add(DFGHIJK);
mapper.add(outline);
mapper.map(ABCDE,
"fill-opacity:0.3;fill:rgb(153,0,0);stroke:rgb(153,0,0);"
"stroke-width:1;stroke-dasharray:1 2");
mapper.map(DFGHIJK,
"fill-opacity:0.3;fill:rgb(0,0,153);stroke:rgb(0,0,153);"
"stroke-width:1;stroke-dasharray:1 2 2 1");
mapper.map(outline,
"fill-opacity:0.1;fill:rgb(153,204,0);stroke:rgb(153,204,0);"
"stroke-width:1");
}
std::cout << "Areas: " << bg::area(ABCDE) << " + " << bg::area(DFGHIJK)
<< " = " << bg::area(out) << "\n";
}
Which prints the expected:
ABCDE: POLYGON((-120 50,-60 70,-70 50,-40 0,-130 10,-120 50))
Correcting DFGHIJK: POLYGON((-40 0,-20 -20,40 -10,80 60,50 80,-20 50,30 30,-40 0)) (Geometry has wrong orie
ntation)
DFGHIJK: POLYGON((-40 0,30 30,-20 50,50 80,80 60,40 -10,-20 -20,-40 0))
union: MULTIPOLYGON(((-40 0,-130 10,-120 50,-60 70,-70 50,-40 0)),((-40 0,30 30,-20 50,50 80,80 60,40 -10,-
20 -20,-40 0)))
Areas: 3600 + 5800 = 9400
It also generates the following SVG:
I hope the way in which I approach it, and specifically the diagnostics code is helpful to you.