reverse - develop

Generic function to reverse a geometry. It resembles the std::reverse functionality, but it takes the geometry type into account. Only for a ring or for a linestring it is the same as the std::reverse.

Synopsis

template<typename Geometry>
void reverse(Geometry & geometry)

Parameters

Type	Concept	Name	Description
Geometry &	Any type fulfilling a Geometry Concept	geometry	A model of the specified concept which will be reversed

Header

Either

#include <boost/geometry.hpp>

#include <boost/geometry/algorithms/reverse.hpp>

Conformance

The function reverse is not defined by OGC.

The function reverse conforms to the std::reverse function of the C++ std-library.

Behavior

Case	Behavior
Point	Nothing happens, geometry is unchanged
Segment	Not yet supported in this version
Box	Nothing happens, geometry is unchanged
Linestring	Reverses the Linestring
Ring	Reverses the Ring
Polygon	Reverses the exterior ring and all interior rings in the polygon
Multi Point	Nothing happens, geometry is unchanged
Multi Linestring	Reverses all contained linestrings individually
Multi Polygon	Reverses all contained polygons individually

	Note
	The reverse of a (multi)polygon or ring might make a valid geometry invalid because the (counter)clockwise orientation reverses.

Complexity

Linear

Example

Shows how to reverse a ring or polygon

#include <iostream>

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

BOOST_GEOMETRY_REGISTER_BOOST_TUPLE_CS(cs::cartesian)

int main()
{
    using point = boost::tuple<int, int>;
    using polygon = boost::geometry::model::polygon<point>;
    using ring = boost::geometry::model::ring<point>;

    polygon poly;
    boost::geometry::exterior_ring(poly) = {{0, 0}, {0, 9}, {10, 10}, {0, 0}};
    boost::geometry::interior_rings(poly).push_back({{1, 2}, {4, 6}, {2, 8}, {1, 2}});

    double area_before = boost::geometry::area(poly);
    boost::geometry::reverse(poly);
    double area_after = boost::geometry::area(poly);
    std::cout << boost::geometry::dsv(poly) << std::endl;
    std::cout << area_before << " -> " << area_after << std::endl;

    ring r = {{0, 0}, {0, 9}, {8, 8}, {0, 0}};

    area_before = boost::geometry::area(r);
    boost::geometry::reverse(r);
    area_after = boost::geometry::area(r);
    std::cout << boost::geometry::dsv(r) << std::endl;
    std::cout << area_before << " -> " << area_after << std::endl;

    return 0;
}

Output:

(((0, 0), (10, 10), (0, 9), (0, 0)), ((1, 2), (2, 8), (4, 6), (1, 2)))
38 -> -38
((0, 0), (8, 8), (0, 9), (0, 0))
36 -> -36