如何在Boost几何中使用多个线串创建盒子？

Posted 2023-02-21

【中文标题】如何在Boost几何中使用多个线串创建盒子？

【英文标题】：How to create box using multiple linestrings in Boost geometry？

【发布时间】：2021-08-11 20:32:02

【问题描述】：

我正在尝试使用多个线串创建一个框。当然这些线串是相互连接的，可以组成一个盒子。 boost中是否有任何功能可以做到这一点？谢谢！

【参考方案1】：

我不确定您到底在寻找什么。所以，让我双向走吧。

单向

如果你有一个盒子，你可以使用分段迭代器来获取分段：

Live On Coliru

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point_xy.hpp>
#include <boost/geometry/geometries/linestring.hpp>
#include <boost/geometry/geometries/box.hpp>
#include <iostream>

namespace bg = boost::geometry;
namespace bgm = bg::model;

using Point      = bgm::d2::point_xy<int>;
using Box        = bgm::box<Point>;
using Poly       = bgm::polygon<Point>;

int main()

    Box const box  Point 0,1, 1,2 ;

    std::cout << "box: " << bg::wkt(box) << "\n";

    // now get the segments:
    Poly p;
    bg::assign(p, box);

    for (auto f = bg::segments_begin(p), l = bg::segments_end(p); f != l; ++f) 
        std::cout << bg::wkt(*f) << "\n";
    

打印

box: POLYGON((0 1,0 2,1 2,1 1,0 1))
LINESTRING(0 1,0 2)
LINESTRING(0 2,1 2)
LINESTRING(1 2,1 1)
LINESTRING(1 1,0 1)

另一种方式

最灵活的是使用envelope算法得到一个盒子：

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#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point_xy.hpp>
#include <boost/geometry/geometries/linestring.hpp>
#include <boost/geometry/geometries/box.hpp>
#include <iostream>

namespace bg = boost::geometry;
namespace bgm = bg::model;

using Point      = bgm::d2::point_xy<int>;
using Box        = bgm::box<Point>;
using LineString = bgm::linestring<Point>;

int main()

    LineString const ls1, 1, 1, 2, 0, 2, 0, 1;

    std::cout << "ls: " << bg::wkt(ls) << "\n";
    
    Box box;
    bg::envelope(ls, box);

    std::cout << "box: " << bg::wkt(box) << "\n";

打印

ls: LINESTRING(1 1,1 2,0 2,0 1)
box: POLYGON((0 1,0 2,1 2,1 1,0 1))

（请注意，点顺序已更正以生成有效的环）

有验证吗？

为了减少灵活性，换句话说，对输入进行更多验证，我会从点手动创建一个多边形。

验证后，您可以通过 WKT 创建一个框...这并不理想，但可能比编写自己的逻辑要好。然而，从 WKT 读取 Box 似乎只在没有任何实际验证的点上应用最小/最大算法 - 因此实际上与获取 bg::envelope 相同。

因此，IMO 解决此问题的最佳方法是将生成的框与手动多边形进行比较，以检查输入是否已经是一个框。

bool is_box(Poly const& p) 
    Box env;
    bg::envelope(p, env);
    return bg::equals(p, env);

例如使用一些输入进行测试：

 LineString1, 1, 0, 1, 0, 2, 1, 2, 1, 1,
 LineString1, 1, 1, 2, 0, 2, 0, 1,
 LineString1, 1, 1, 2, 0, 2, 0, 1, 1, 1,
 LineString1, 1, 0, 2, 0, 1, 1, 1, // triangle

is_box 将为最后一个返回 false（即使在更正多边形之后）

Live On Coliru

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point_xy.hpp>
#include <boost/geometry/geometries/linestring.hpp>
#include <boost/geometry/geometries/box.hpp>
#include <iostream>

namespace bg = boost::geometry;
namespace bgm = bg::model;

using Point      = bgm::d2::point_xy<int>;
using Box        = bgm::box<Point>;
using Poly       = bgm::polygon<Point>;
using LineString = bgm::linestring<Point>;

bool is_box(Poly const& p) 
    Box env;
    bg::envelope(p, env);
    return bg::equals(p, env);


int main()

    for (auto& ls : 
             LineString1, 1, 0, 1, 0, 2, 1, 2, 1, 1,
             LineString1, 1, 1, 2, 0, 2, 0, 1,
             LineString1, 1, 1, 2, 0, 2, 0, 1, 1, 1,
             LineString1, 1, 0, 2, 0, 1, 1, 1, // triangle
         )
    
        std::cout << " --\nls: " << bg::wkt(ls) << "\n";

        Poly p;
        bg::assign_points(p, ls);

        if (std::string reason; !bg::is_valid(p, reason)) 
            std::cout << "Trying to correct...: " << reason << "\n";
            bg::correct(p);

            if (!bg::is_valid(p))
                continue;
        
        std::cout << "poly: " << bg::wkt(p) << "\n";

        std::cout << "is a box? " << (is_box(p) ? "yes" : "no") << "\n";
    

打印

 --
ls: LINESTRING(1 1,0 1,0 2,1 2,1 1)
poly: POLYGON((1 1,0 1,0 2,1 2,1 1))
is a box? yes
 --
ls: LINESTRING(1 1,1 2,0 2,0 1)
Trying to correct...: Geometry is defined as closed but is open
poly: POLYGON((1 1,0 1,0 2,1 2,1 1))
is a box? yes
 --
ls: LINESTRING(1 1,1 2,0 2,0 1,1 1)
Trying to correct...: Geometry has wrong orientation
poly: POLYGON((1 1,0 1,0 2,1 2,1 1))
is a box? yes
 --
ls: LINESTRING(1 1,0 2,0 1,1 1)
Trying to correct...: Geometry has wrong orientation
poly: POLYGON((1 1,0 1,0 2,1 1))
is a box? no