用成对的向量提升几何面积？

问题描述

有没有办法让 boost::geometry::area 与成对的向量一起工作？它在这里 https://www.boost.org/doc/libs/1_75_0/libs/geometry/doc/html/geometry/reference/algorithms/area/area_1.html 说它应该与 MultiPoint 一起工作。所以我调用了 BOOST_GEOMETRY_REGISTER_MULTI_POINT - 它适用于 convex_hull，但它不适用于 area。或者我是否必须创建一个多边形并将点附加到它，如下所示： https://www.boost.org/doc/libs/1_75_0/libs/geometry/doc/html/geometry/reference/models/model_polygon.html

这是我的代码：

#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},{1,0}};
    auto hull = getHull(points); // This works as expected
    auto area = getHullArea(hull); // This always return 0
}

解决方法

是的，它支持多点。你会得到记录在案的 behaviour:

所以，你得到的面积为 0，就像你应该期待的那样。

显然您想适应为一个面几何体。假设您的对是 Points，形成 Ring。

$@

现在它如您所愿：

Live On Coliru

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)

印刷品

#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,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},{1,0}},Ring{{0,2},{3,})
    {
        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";
    }
}

如你所见，我添加了一个更有趣的例子：