几何，路口 [英] Geometry, intersection

问题描述

我有一个问题。我想相交四有四。

I have a problem. I would like to intersect a quad with a quad.

int main(){
  typedef boost::geometry::model::point_xy<double> TBoostPoint;
  typedef boost::geometry::model::polygon<TBoostPoint> TBoostPoly;
  TBoostPoint point;
  TBoostPoly firstPoly, secondPoly;
  boost::geometry::read_wkt("POLYGON(
                (1.504477611940313, 3.761194029850755), 
                (1.504477611940305, 3.573134328358203),
                (1.316417910447765, 3.573134328358206),
                (1.316417910447769, 3.761194029850752))", firstPoly);
  boost::geometry::read_wkt("POLYGON(
                (1.504477611940313, 3.761194029850755), 
                (1.504477611940305, 3.573134328358203),
                (1.316417910447765, 3.573134328358206),
                (1.316417910447751, 3.761194029850769))", secondPoly);
 std::vector<TBoostPoly> outPoly;
 boost::geometry::intersection(firstPoly,secondPoly,outPoly);
}

outPoly - 是空的，但它并非如此。

outPoly - is empty, but it not so.

推荐答案

有两个主要的问题。

输出不确定，因为输入是无效的。

The output is undefined because the input is invalid.

1. 输入WKT指定的，而不是你所期望的大量无效内圈（包括单点）的，5分（不含收盘点位）的单外圈。修复：

1. The input WKT specifies a lot of invalid inner rings (consisting of single points), instead of what you expected, a single outer ring of 5 points (excl. closing point). Fix it:

bg::read_wkt("POLYGON(( 1.504477611940313 3.761194029850755, 1.504477611940305 3.573134328358203, 1.316417910447765 3.573134328358206, 1.316417910447769 3.761194029850752))", first);
bg::read_wkt("POLYGON(( 1.504477611940313 3.761194029850755, 1.504477611940305 3.573134328358203, 1.316417910447765 3.573134328358206, 1.316417910447751 3.761194029850769))", second); 

几何加速整个假设你从不对文件preconditions错误。如果您在多边形概念页和preconditions读路口你会看到thew全list¹。

Boost Geometry assumes throughout that you never make errors against the documented preconditions. If you read in the polygon concept page and preconditions for intersection you'll see thew full list¹.

如果你没有，你没有友好的错误，只是无声的故障，损坏或者只是错误的答案。是啊。这是不好的。

If you don't, you get no friendly errors, just silent failure, corruption or just wrong answers. Yeah. That's bad.

更糟的是，BGeo甚至没有一个 is_valid 工具来测试大量的需求，直到升压1_57（IIRC）。好消息是，如果升级到该版本或更高版本你的生活就会简单得多。

What's worse, BGeo didn't even have a is_valid facility to test the bulk of requirements until Boost 1_57 (IIRC). The good news is, if you upgrade to this version or later your life will be much simpler.

在这种情况下，你会了解到，多边形没有正确关闭：

In this case you would have learned that the polygons weren't properly closed:

<大骨节病> 住在Coliru

#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/geometry/io/io.hpp>
#include <boost/geometry/algorithms/intersection.hpp>
#include <boost/geometry/algorithms/correct.hpp>
#include <boost/geometry/algorithms/is_valid.hpp>

namespace bg = boost::geometry;

int main(){
    typedef bg::model::d2::point_xy<double> TPoint;
    typedef bg::model::polygon<TPoint>      TPoly;
    TPoly first, second;

    bg::read_wkt("POLYGON(( 1.504477611940313 3.761194029850755, 1.504477611940305 3.573134328358203, 1.316417910447765 3.573134328358206, 1.316417910447769 3.761194029850752))", first);
    bg::read_wkt("POLYGON(( 1.504477611940313 3.761194029850755, 1.504477611940305 3.573134328358203, 1.316417910447765 3.573134328358206, 1.316417910447751 3.761194029850769))", second); 

    std::string reason;
    // polys not closed!
    if (!bg::is_valid(first, reason))  std::cout << "First polygon not valid: "  << reason << "\n";
    if (!bg::is_valid(second, reason)) std::cout << "Second polygon not valid: " << reason << "\n";

    bg::correct(first);
    bg::correct(second);

    // no more output!
    if (!bg::is_valid(first, reason))  std::cout << "First polygon not valid: "  << reason << "\n";
    if (!bg::is_valid(second, reason)) std::cout << "Second polygon not valid: " << reason << "\n";

    std::vector<TPoly> out;
    bg::intersection(first, second, out);

    for (auto& g : out)
        std::cout << "\nresult: " << bg::wkt(g) << "\n";
}

打印：

First polygon not valid: Geometry is defined as closed but is open
Second polygon not valid: Geometry is defined as closed but is open

哎呀。在GEOS不营业！ 正确的（多）自动驾驶修复了这个对我们来说：

Oops. The geos weren't closed! correct(poly) fixes this for us on auto-pilot:

result: POLYGON((1.50448 3.57313,1.31642 3.57313,1.31642 3.76119,1.50448 3.76119,1.50448 3.57313))

---

¹外环必须是逆时针，顺时针内，多边形必须被关闭......类似的东西。

---

¹ outer ring must be counter clockwise, inner cw, polys must be closed... stuff like that.

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