Initializing a polygon in boost::geometry

5
votes

I am new to the generic geometry library that is proposed for inclusion with boost:

http://geometrylibrary.geodan.nl/

I have two vectors vector<int> Xb, Yb that I am trying to create a polygon from. I am trying to get something along the lines of the following code snippet:

 polygon_2d P;

 vector<double>::const_iterator xi;
 vector<double>::const_iterator yi;

 for (xi=Xb.begin(), yi=Yb.begin(); xi!=Xb.end(); ++xi, ++yi)
  P.push_back (make<point_2d>(*xi, *yi));

The above code does not work, complaining that P does not have a push_back member function. How do I initialize the polygon from points that have coordinates vector<int> Xb,vector<int> Yb?

3
c++boostpolygonboost-geometry
A quick note, the likely hood that specific library you mention will survive a review process is highly unlikely. Your best option today is to write a light-weight C++ wrapper for the Generic Polygon Clipper library, as none of the proposed submissions for 2D polygon operations come anywhere near the performance GPC provides. - Matthieu N.
Thanks for the suggestion,Beh. GPC seems like a nice library, however it seems to be missing a feature that is important for me - namely the ability to compute the area of a polygon. - D R
Dzhelil, fortunately the area algorithm is trivial to implement yourself. alienryderflex.com/polygon_area - Stefan Monov
@Beh Tou Cheh - For your and others information, it survived and has become a part of Boost C++ Libraries. - mloskot
Now it has been included in Boost as Boost.geometry - Ivan Xiao

3 Answers

12
votes

Here is example to the extension of your original question you asked as a comment below Kirill's answer: Are intersections between polygons possible?

Yes, polygon-polygon intersections are supported by Boost.Geometry (aka GGL)

#include <iostream>
#include <vector>
#include <boost/geometry/geometry.hpp>
#include <boost/geometry/geometries/cartesian2d.hpp>
#include <boost/geometry/geometries/adapted/c_array_cartesian.hpp>

using namespace boost::geometry;

int main(void)
{
    // Define a polygons and fill the outer rings.
    polygon_2d a;
    {
        const double c[][2] = {
            {160, 330}, {60, 260}, {20, 150}, {60, 40}, {190, 20}, {270, 130}, {260, 250}, {160, 330}
        };
        assign(a, c);
    }
    correct(a);
    std::cout << "A: " << dsv(a) << std::endl;

    polygon_2d b;
    {
        const double c[][3] = {
            {300, 330}, {190, 270}, {150, 170}, {150, 110}, {250, 30}, {380, 50}, {380, 250}, {300, 330}
        };
        assign(b, c);
    }
    correct(b);
    std::cout << "B: " << dsv(b) << std::endl;

    // Calculate interesection
    typedef std::vector<polygon_2d > polygon_list;
    polygon_list v;

    intersection_inserter<polygon_2d>(a, b, std::back_inserter(v));
    std::cout << "Intersection of polygons A and B" << std::endl;
    for (polygon_list::const_iterator it = v.begin(); it != v.end(); ++it)
    {
        std::cout << dsv(*it) << std::endl;
    }

    return 0;
}

Here is the result (the polygon being intersection is moved to south for better visibility):

alt text

I hope it will work for you.

6
votes
append(P, make<point_2d>(*xi, *yi));
0
votes

You can also use a tuple to initialize the polygon

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

and

boost::geometry::assign_points(
    polygon, boost::assign::tuple_list_of
        (300, 330) (190, 270) (150, 170) (150, 110) (250, 30) (380, 50)
        (380, 250) (300, 330)
);