如何使用C ++ boost几何来从两个polgons创建ring（donut）？

Question

我试图计算2个多边形之间的差异（事实上，为简单起见，直线形状）。例如，shape1具有点列表{0 0,100,0,100 100,0 100}，并且shape2具有点列表{25 25,75 25,75 75,25 75}。 所以按照概念，我期待＆＃34; shape1 - shape2＆＃34;将为我提供4个矩形盒子，可以制作戒指或圆环形状。我不知道如何实现这一目标，但我发现＆＃34;提升＆＃34;图书馆在线并尝试过这样：

#include <iostream>
#include <list>

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

#include <boost/foreach.hpp>


int main()
{
    typedef boost::geometry::model::polygon<boost::geometry::model::d2::point_xy<double> > polygon;

    polygon green, blue;

    boost::geometry::read_wkt( "POLYGON((0 0 100 0 100 100 0 100))", green);

    boost::geometry::read_wkt( "POLYGON((25 25 75 22 75 75 25 75))", blue);

    std::list<polygon> output;
    boost::geometry::difference(green, blue, output);

    int i = 0;
    std::cout << "green - blue:" << std::endl;
    BOOST_FOREACH(polygon const& p, output)
    {
        std::cout << i++ << ": " << boost::geometry::area(p) << std::endl;
    }


    output.clear();
    boost::geometry::difference(blue, green, output);

    i = 0;
    std::cout << "blue - green:" << std::endl;
    BOOST_FOREACH(polygon const& p, output)
    {
        std::cout << i++ << ": " << boost::geometry::area(p) << std::endl;
    }


    return 0;
}

然而，没有打印出来...我的计划是找出一种方法将输出结果转换为矩形框{0 0 100 0 100 25 0 25} {0 75 100 75 100 100 0 100} {0 25 25 25 25 75 0 75}和{75 25 100 25 100 75 75 75}，但我感到沮丧的是，上述代码根本没有打印出来。 谁能给我一些指导？ 我附上了一张图片，展示了我想做的事情。事实上，我已经拥有了代表这个甜甜圈的所有点坐标，所以也许我可以跳过＆＃34;差异（）函数＆＃34; ？如果是这种情况，困难的部分将是将甜甜圈转换成矩形段。 （我们假设这里的所有多边形都是直线形状。） 感谢。

Answer 1

您的起始多边形没有所需的方向。您可以使用is_valid自行查找。方便的是，您可以使用correct来解决输入数据中最常见的问题：

std::string reason;
if (!bg::is_valid(geometry, reason))
{
    std::cout << "Correcting " << bg::wkt(geometry) << " to ";
    bg::correct(geometry);
    std::cout << bg::wkt(geometry) << "\n";

    if (!bg::is_valid(geometry, reason))
        std::cout << "UNCORRECTIBLE: " << reason << "\n";
}

当你这样做时，你会发现：

Correcting POLYGON((0 0,100 0,100 100,0 100,0 0)) to POLYGON((0 0,0 100,100 100,100 0,0 0))
Correcting POLYGON((25 25,75 22,75 75,25 75,25 25)) to POLYGON((25 25,25 75,75 75,75 22,25 25))

现在，当你得到差异时，你将得到预期的结果：

auto do_diff = [](std::string caption, auto const& a, auto const& b) {
    multi output;
    bg::difference(a, b, output);

    std::cout << caption << bg::wkt(output) << std::endl;
};

do_diff("green - blue:", green, blue);
do_diff("blue - green:", blue, green);

完整演示

<强> Live On Coliru

#include <iostream>
#include <fstream>

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

#include <boost/foreach.hpp>

namespace bg = boost::geometry; 

int main() {
    typedef bg::model::d2::point_xy<double> point;
    typedef bg::model::polygon<point> polygon;
    typedef bg::model::multi_polygon<polygon> multi;

    polygon green, blue;

    bg::read_wkt("POLYGON((0 0 100 0 100 100 0 100))", green);
    bg::read_wkt("POLYGON((25 25 75 22 75 75 25 75))", blue);

    auto test = [](polygon& geometry) {
        std::string reason;
        if (!bg::is_valid(geometry, reason))
        {
            std::cout << "Correcting " << bg::wkt(geometry) << " to ";
            bg::correct(geometry);
            std::cout << bg::wkt(geometry) << "\n";

            if (!bg::is_valid(geometry, reason))
                std::cout << "UNCORRECTIBLE: " << reason << "\n";
        }
    };

    test(green);
    test(blue);

    auto do_diff = [](std::string caption, polygon const& a, polygon const& b) {
        multi output;
        bg::difference(a, b, output);

        std::cout << caption << bg::wkt(output) << std::endl;
    };

    do_diff("green - blue:", green, blue);
    do_diff("blue - green:", blue, green);

    {
        std::ofstream svg("output.svg");
        boost::geometry::svg_mapper<point> mapper(svg, 400, 400);
        mapper.add(blue);
        mapper.add(green);

        mapper.map(blue, "fill-opacity:0.5;fill:rgb(0,0,153);stroke:rgb(0,0,200);stroke-width:2");
        mapper.map(green, "fill-opacity:0.5;fill:rgb(0,153,0);stroke:rgb(0,200,0);stroke-width:2");
    }
}

打印

Correcting POLYGON((0 0,100 0,100 100,0 100,0 0)) to POLYGON((0 0,0 100,100 100,100 0,0 0))
Correcting POLYGON((25 25,75 22,75 75,25 75,25 25)) to POLYGON((25 25,25 75,75 75,75 22,25 25))
green - blue:MULTIPOLYGON(((0 0,0 100,100 100,100 0,0 0),(25 25,75 22,75 75,25 75,25 25)))
blue - green:MULTIPOLYGON()

并写svg：

Answer 2

  

other answer将结束你的沮丧/困惑，这个答案会让你更有成效：）

而且，是的，你可以跳过差异函数：

bg::read_wkt("POLYGON((0 0 0 100 100 100 100 0 0 0) (25 25 75 22 75 75 25 75 25 25))", donut);

指定内环（反方向！）。

结果：

由该程序生成：

<强> Live On Coliru

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

#include <iostream>
#include <fstream>

namespace bg = boost::geometry; 

int main() {
    typedef bg::model::d2::point_xy<double> point;
    typedef bg::model::polygon<point> polygon;

    polygon donut;
    bg::read_wkt("POLYGON((0 0 0 100 100 100 100 0 0 0) (25 25 75 22 75 75 25 75 25 25))", donut);

    std::ofstream svg("output.svg");
    boost::geometry::svg_mapper<point> mapper(svg, 400, 400);
    mapper.add(donut);

    mapper.map(donut, "fill-opacity:0.5;fill:rgb(0,0,153);stroke:rgb(0,0,200);stroke-width:2");
}