Dilating a polygon boost::geometry

Question

Is there a morphological dilation operation for polygon geometry objects?

For example, say I have a square with side length 1 centered at the origin (a boost::geometry::model::polygon with points (.5, .5), (-.5, .5), (-.5, -.5), (.5, -.5) ). If I "dilate" it with a distance/radius of .5, I would get a square with side length 2, still centered at the origin. That is, all of the edges of the polygon should be "pushed out" along their normal direction.

David Doria David Doria · Accepted Answer · 2016-08-03T20:47:51

The term for this is "buffer", and buffering polygons is only implemented in the overload of buffer() with strategies: http://www.boost.org/doc/libs/1_61_0/libs/geometry/doc/html/geometry/reference/algorithms/buffer/buffer_7_with_strategies.html

Here is an example that dilates a triangle by .1 units.

#include <iostream>
#include <list>

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point_xy.hpp>
#include <boost/geometry/geometries/polygon.hpp>

using coordinate_type = double;
using point_type = boost::geometry::model::d2::point_xy<coordinate_type>;
using polygon_type = boost::geometry::model::polygon<point_type>;

int main()
{
    // Construct
    polygon_type polygon;
    // Counter clock-wise points don't seem to work (no error, but empty output)
//    boost::geometry::append(polygon, point_type {0,0});
//    boost::geometry::append(polygon, point_type {1,0});
//    boost::geometry::append(polygon, point_type {0,1});
//    boost::geometry::append(polygon, point_type {0,0});

    // Points specified in clockwise order
    boost::geometry::append(polygon, point_type {0,0});
    boost::geometry::append(polygon, point_type {0,1});
    boost::geometry::append(polygon, point_type {1,0});
    boost::geometry::append(polygon, point_type {0,0});

    //boost::geometry::buffer(poly, output, .5); // THIS_OPERATION_IS_NOT_OR_NOT_YET_IMPLEMENTED

    const double buffer_distance = .1;
    const int points_per_circle = 36;
    boost::geometry::strategy::buffer::distance_symmetric<coordinate_type> distance_strategy(buffer_distance);
    boost::geometry::strategy::buffer::join_round join_strategy(points_per_circle);
    boost::geometry::strategy::buffer::end_round end_strategy(points_per_circle);
    boost::geometry::strategy::buffer::point_circle circle_strategy(points_per_circle);
    boost::geometry::strategy::buffer::side_straight side_strategy;

    boost::geometry::model::multi_polygon<polygon_type> input;
    input.push_back(polygon);

    boost::geometry::model::multi_polygon<polygon_type> outputMultiPolygon;

    boost::geometry::buffer(polygon, outputMultiPolygon,
                distance_strategy, side_strategy,
                join_strategy, end_strategy, circle_strategy);

    std::cout << outputMultiPolygon.size() << std::endl;

    polygon_type outputPolygon = outputMultiPolygon[0];

    // Print the points of the result (there are many more than in the input because the corners have been rounded)
    for(unsigned int pointID = 0; pointID < outputPolygon.outer().size(); ++pointID) {
        std::cout << boost::geometry::get<0>(outputPolygon.outer()[pointID]) << " " << boost::geometry::get<1>(outputPolygon.outer()[pointID]) << std::endl;
    }

    return 0;
}

Dilating a polygon boost::geometry

1 Answers