Geometry Boost List: [ggl] Re: Help needed with Intersection operation

Subject: [ggl] Re: Help needed with Intersection operation
From: Christoph Keller (christoph)
Date: 2011-08-19 18:57:02

Hello Angus,

thanks for providing the code. However if I do that using double 
precision, i get a triangle as output, that has little to do with the 
intersection.

I made a mistake posting my Data. One element had a different value. I 
also used correct, as you suggested, but as I expected this changes nothing.

I posted my complete code below. Note that the y-value 
29.884635544079821 cannot belong to the first polygon.

So it seems we found two errors. The question for me is if I should 
develop my own intersection code? I just have to calculate the 
difference/intersection of a lot of triangles in 2D. The library needs 
to handle these cases, they ALWAYS occur in practice.

Greetings,
Christoph

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/polygon.hpp>
#include <boost/geometry/geometries/adapted/boost_tuple.hpp>

BOOST_GEOMETRY_REGISTER_BOOST_TUPLE_CS(cs::cartesian)
typedef boost::tuple<double, double> point;
typedef boost::geometry::model::polygon<point> polygon;

void boostGiveMeError() {
     std::vector<polygon> outDiff, outIntersect, outInnerDiff;
     polygon vecPoly, lpPoly;

     lpPoly.outer().push_back(boost::make_tuple(0.24806946917841693, 
26.015444246572663));
     lpPoly.outer().push_back(boost::make_tuple(31.751930530821582 , 
33.984555753427337));
     lpPoly.outer().push_back(boost::make_tuple(32.248069469178418 
,30.015444246572663));
     lpPoly.outer().push_back(boost::make_tuple(0.24806946917841693, 
26.015444246572663));

     vecPoly.outer().push_back(boost::make_tuple(17.763942722600319, 
32.236057277399681));
     vecPoly.outer().push_back(boost::make_tuple(19.192448808558737, 
30.807551191441263));
     vecPoly.outer().push_back(boost::make_tuple(16.000000000000000, 
30.000000000000000));
     vecPoly.outer().push_back(boost::make_tuple(17.763942722600319, 
32.236057277399681));
     boost::geometry::correct(vecPoly);
     boost::geometry::correct(lpPoly);

     boost::geometry::difference(vecPoly, lpPoly, outDiff);
     for(unsigned i=0; i < outDiff.size(); i++)
         boost::geometry::unique(outDiff[i]);

     double debugMe[5][2];
     for(int i=0; i < 5; i++) {
         debugMe[i][0] = outDiff[0].outer()[i].get<0>();
         debugMe[i][1] = outDiff[0].outer()[i].get<1>();
     }
}

THE DEBUGGER OUTPUT:

-        debugMe    0x0052eddc    double [5][2]
-        [0]    0x0052eddc    double [2]
         [0]    15.543935884491610    double
         [1]    29.884635544079821    double
-        [1]    0x0052edec    double [2]
         [0]    16.000000000000000    double
         [1]    30.000000000000000    double
-        [2]    0x0052edfc    double [2]
         [0]    17.763942722600319    double
         [1]    32.236057277399681    double
-        [3]    0x0052ee0c    double [2]
         [0]    19.192448808558737    double
         [1]    30.807551191441263    double
-        [4]    0x0052ee1c    double [2]
         [0]    15.543935884491610    double
         [1]    29.884635544079821    double

On 08/19/2011 11:37 PM, angusj wrote:
> Christoph, on rereading your post I can see that my previous reply 
> wont be helpful as evidently you did 'close' the polygons (and the 
> link I gave was to the wrong thread too). Anyhow, I have compiled and 
> tested your polygons and can confirm that there appears to be a small 
> problem with the solution to an intersection operation (which returns 
> a single point well beyond any potential intersection).
>    #include<boost/geometry.hpp>
>    #include<boost/geometry/geometries/point_xy.hpp>
>    #include<boost/geometry/geometries/polygon.hpp>
>    #include<boost/geometry/multi/geometries/multi_polygon.hpp>
>    #include<boost/geometry/domains/gis/io/wkt/wkt.hpp>
>
>    typedef boost::geometry::model::polygon ggl_polygon;
>    typedef boost::geometry::model::multi_polygon ggl_polygons;
>
> 	ggl_polygon blue, green;
> 	boost::geometry::read_wkt(
>          "POLYGON((0.24806946917841693 26.015444246572663, 31.751930530821582 33.984555753427337, "
> 		"32.248069469178418 30.015444246572663, 0.24806946917841693 26.015444246572663))", green);
> 	boost::geometry::read_wkt(
>          "POLYGON((17.763942722600319 32.236057277399681, 19.192448808558737 30.807551191441263, "
> 		"16.000000000000000 30.000000000000000, 17.763942722600319 32.236057277399681))", blue);
> 	ggl_polygons output;
> 	bg::intersection(green, blue, output);
> 	cout<<  output;
>
> ------------------------------------------------------------------------
> View this message in context: Re: Help needed with Intersection 
> operation 
> <http://boost-geometry.203548.n3.nabble.com/Help-needed-with-Intersection-operation-tp3250402p3269317.html>
> Sent from the Boost Geometry mailing list archive 
> <http://boost-geometry.203548.n3.nabble.com/> at Nabble.com.
>
>
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