Geometry Boost List: [ggl] Unioning many polygons

Subject: [ggl] Unioning many polygons
From: John Swensen (jpswensen)
Date: 2011-10-08 16:21:57

I am finally getting back to my iphone game that will use Boost.Geometry.  I have all the splitting and mirroring of polygons working, but am having trouble unioning polygons back together.  I don't quite get why the results of the boost::geometry::union_ operation are placed in a vector.

I would like to do something like the following

    // Split the polygon along the line
    std::vector<polygon_2d> splits = splitPolygon (line, a);
    std::vector<polygon_2d> mirrors;
    
    // Mirror every polygon
    BOOST_FOREACH(polygon_2d poly, splits)
    {
        polygon_2d tmp = mirrorPolygon (line, poly);
        mirrors.push_back(tmp);
    }
    
    // Union all the split polygons and their mirrors together
    std::vector<polygon_2d> unionPoly;
    BOOST_FOREACH(polygon_2d poly, splits)
    {
        boost::geometry::union_ (unionPoly, poly, unionPoly);
    }
    
    BOOST_FOREACH(polygon_2d poly, mirrors)
    {
        boost::geometry::union_ (unionPoly, poly, unionPoly);
    }

Of course this doesn't work because union_ takes polygon_2d in my case, not a std::vector<polygon_2d> as its first argument.

What is the correct way of unioning a bunch of polygons together?

John