boost geometry area with a vector of pairs?

Question

Is there any way of making boost::geometry::area work with a vector of pairs? It says here https://www.boost.org/doc/libs/1_75_0/libs/geometry/doc/html/geometry/reference/algorithms/area/area_1.html that it should work with MultiPoint. So I have called BOOST_GEOMETRY_REGISTER_MULTI_POINT - which works for convex_hull, but it does not work for area. Or do I have to create a Polygon and append the points to it as shown here: https://www.boost.org/doc/libs/1_75_0/libs/geometry/doc/html/geometry/reference/models/model_polygon.html

This is my code:

#include <iostream>
#include <cstdlib>

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/register/point.hpp>
#include <boost/geometry/multi/geometries/register/multi_point.hpp>

BOOST_GEOMETRY_REGISTER_POINT_2D(decltype(std::pair<double, double>{}), double, cs::cartesian, first, second)
BOOST_GEOMETRY_REGISTER_MULTI_POINT(decltype(std::vector<std::pair<double, double>>{}))

using MultiPoint = std::vector<std::pair<double, double>>;
MultiPoint getHull(const MultiPoint& points)
{
  MultiPoint hull{};
  boost::geometry::convex_hull(points, hull);
  return hull; // This returns a vector of pairs of points respresenting a hull
}
double getHullArea(const MultiPoint& points)
{
  return boost::geometry::area(points); // This always return 0
}

int main()
{
    // Unit square
    auto points = MultiPoint{{0, 0}, {0, 1}, {1, 1}, {1, 0}, {0,0}};
    auto hull = getHull(points); // This works as expected
    auto area = getHullArea(hull); // This always return 0
}

Answer 1

Yes it support MultiPoint. And you get the documented behaviour:

So, you get area is 0, like you should be expecting.

Clearly you want to adapt as an areal geometry. Let's assume your pairs are Points that form a Ring.

using Point = std::pair<double, double>;
using Ring  = std::vector<Point>;

BOOST_GEOMETRY_REGISTER_POINT_2D(Point, double, cs::cartesian, first, second)
BOOST_GEOMETRY_REGISTER_RING(Ring)

Now it works as you should expect:

Live On Coliru

#include <iostream>
#include <cstdlib>

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/register/point.hpp>
#include <boost/geometry/geometries/register/ring.hpp>
namespace bg = boost::geometry;

using Point = std::pair<double, double>;
using Ring  = std::vector<Point>;

BOOST_GEOMETRY_REGISTER_POINT_2D(Point, double, cs::cartesian, first, second)
BOOST_GEOMETRY_REGISTER_RING(Ring)

Ring getHull(const Ring& points) {
    Ring hull;
    bg::convex_hull(points, hull);
    return hull;
}

double getArea(const Ring& points) {
    return bg::area(points);
}

int main() {
    for (Ring points : {
             Ring{{0, 0}, {0, 1}, {1, 1}, {1, 0}, {0, 0}},
             Ring{{0, 0}, {0, 2}, {3, 2}, {3, 1}, {1, 1}, {1, 0}, {0, 0}},
         })
    {
        std::cout << "-----\nInput:\t" << bg::wkt(points) << "\n";
        if (std::string reason; !bg::is_valid(points)) {
            std::cout << "Input:\t" << reason << "\n";
            bg::correct(points);
            std::cout << bg::wkt(points) << "\n";
        }

        std::cout << "Hull:\t"      << bg::wkt(getHull(points)) << "\n";
        std::cout << "Area:\t"      << getArea(points)          << "\n";
        std::cout << "Hull Area:\t" << getArea(getHull(points)) << "\n";
    }
}

Prints

-----
Input:  POLYGON((0 0,0 1,1 1,1 0,0 0))
Hull:   POLYGON((0 0,0 1,1 1,1 0,0 0))
Area:   1
Hull Area:      1
-----
Input:  POLYGON((0 0,0 2,3 2,3 1,1 1,1 0,0 0))
Hull:   POLYGON((0 0,0 2,3 2,3 1,1 0,0 0))
Area:   4
Hull Area:      5

As you can see I added a more interesting example: