CODEWITHSUNDEEP
c++boostpolygonboost-geometry
Learning is a mess
Learning is a mess

Reputation: 8277

boost::geometry Most efficient way of measuring max/min distance of a point to a polygon ring

I have been using boost::geometry library in a program, mostly for handling polygon objects.

I am now trying to optimize my code to scale better with larger polygons. One my functions checks for a given polygon and a given point (usually inside the polygon) the minimum and maximum distance between the point and polygon outer ring.

I do it by looping on the polygon edges:

polygon pol;
point myPoint;
double min = 9999999, max = 0;
for(auto it1 = boost::begin(bg::exterior_ring(pol)); it1 != boost::end(bg::exterior_ring(pol)); ++it1){
    double distance = bg::distance(*it1, myPoint);
        if(max < distance)
            max = distance;
        if(min > distance)
            min = distance;
    }

I am hoping that there are algorithms faster than this one, linear in the polygon number of edges. Is there such a thing already inside the boost::geometry library?

Upvotes: 4

Views: 3924

Answers (2)

matthieu
matthieu

Reputation: 353

For best performances you should use an RTree with boost::geometry::index. Creating the RTree has a cost, but then computing the ditance of a point to any of the (multi)polygon ring will be much faster. Example code:

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/geometries.hpp>
#include <boost/geometry/index/rtree.hpp>
#include <iostream>
#include <vector>

int main()
{
    namespace bg = boost::geometry;
    namespace bgi = boost::geometry::index;
    typedef bg::model::point<double, 2, bg::cs::cartesian> point;
    typedef bg::model::polygon<point> polygon;

    point p{ 0, 0 };
    // create some polygon and fill it with data
    polygon poly;
    double a = 0;
    double as = bg::math::two_pi<double>() / 100;
    for (int i = 0; i < 100; ++i, a += as)
    {
        double c = cos(a);
        double s = sin(a);
        poly.outer().push_back(point{10 * c, 10 * s});
        poly.inners().resize(1);
        poly.inners()[0].push_back(point{5 * c, 5 * s});
    }
    // make sure it is valid
    bg::correct(poly);

    // create rtree containing objects of type bg::model::pointing_segment
    typedef bg::segment_iterator<polygon const> segment_iterator;
    typedef std::iterator_traits<segment_iterator>::value_type segment_type;

    bgi::rtree<segment_type, bgi::rstar<4> > rtree(bg::segments_begin(poly),
                                                   bg::segments_end(poly));

    // get 1 nearest segment
    std::vector<segment_type> result;
    rtree.query(bgi::nearest(p, 1), std::back_inserter(result));

    BOOST_ASSERT(!result.empty());

    std::cout << bg::wkt(result[0]) << ", " << bg::distance(p, result[0]) << std::endl;

    return 0;
}

Upvotes: 0

sehe
sehe

Reputation: 393694

I'd suggest you can use the builtin strategies for finding the minimum distance between the polygon and the point:

Live On Coliru

#include <boost/geometry.hpp>
#include <boost/geometry/core/cs.hpp>
#include <boost/geometry/io/io.hpp>
#include <boost/geometry/geometries/point_xy.hpp>
#include <boost/geometry/geometries/polygon.hpp>
#include <boost/geometry/algorithms/distance.hpp>

namespace bg = boost::geometry;

using point = bg::model::d2::point_xy<double>;
using polygon = bg::model::polygon<point>;

int main() {
    polygon pol;
    boost::geometry::read_wkt(
            "POLYGON((2 1.3,2.4 1.7,2.8 1.8,3.4 1.2,3.7 1.6,3.4 2,4.1 3,5.3 2.6,5.4 1.2,4.9 0.8,2.9 0.7,2 1.3)"
            "(4.0 2.0, 4.2 1.4, 4.8 1.9, 4.4 2.2, 4.0 2.0))", pol);

    point myPoint(7, 7);
    double min = 9999999, max = 0;

    std::cout << "Minimal distance: " << bg::distance(pol, myPoint);

}

Prints

Minimal distance: 4.71699

Further hints:

You should consider ranking the distances first using comparable_distance. As you can see the sample there suggests looping over all the sampled distances... so I don't think the library has a better offering at this time.

More sophisticated algorithms are planned, of which a number may be related to this problem:

Note also that Boost Geometry Index has a related predicate comparable_distance_far but it's not exposed as of yet.

Summary

You can improve at least a bit by using comparable_distance here for now.

Features have been planned and it looks like there is a good chance that requesting them on the mailing list/Boost Trac will help getting them there.

Upvotes: 2

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