扩展多边形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.

Answer 1

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;
}