C++如何判断点是否在多边形之内

先上参考链接：

点在多边形内 经典算法

如何判断点在多边形的内部？

如何判定一点是否在给定顶点的不规则封闭区域内?

射线算法基本原理：以point为起点，无穷远为终点，作平行于x轴直线，判断多边形的每一条边与射线是否有交点，若交点总数为奇数，则点在多边形之内，若为偶数，则在多边形之外。

容易理解又很好实现，如下：

#include "opencv2/opencv.hpp"
#include <iostream> 
#include <fstream>
using namespace std;  

std::ifstream polygon_data("./area1.txt", std::ifstream::in);
std::vector<cv::Point2d> area;

void func()
{
    double x = 0, y = 0;
    while(polygon_data >> x >> y)
    {		
        area.push_back(cv::Point2d(x, y));
    }

//	for (int i=0; i < area1.size(); i++)
//	{
//		std::cout << "x=" << area[i].x << "  y=" << area[i].y << std::endl;
//        std::cout << area1[i] << std::endl;
//	}

}
 
int main()
{  
    func(); 
    int nCount = area.size(); // 4
    int nCross = 0;
    cv::Point2d p = cv::Point2d(-10, 50); 
    std::cout << p << std::endl;

    for (int i = 0; i < nCount; i++)
    {
        cv::Point2d p1 = cv::Point2d(area[i]);
//        std::cout << p1<< std::endl;
        cv::Point2d p2 = area[(i + 1) % nCount];
        
        if ( p1.y == p2.y )  //p1p2与射线平行
        continue;

        if ( p.y < min(p1.y, p2.y) ) //射线在p1p2延长线上
        continue;
        
        if ( p.y >= max(p1.y, p2.y) ) //射线在p1p2延长线上
        continue;

        double x = (double)(p.y - p1.y) * (double)(p2.x - p1.x) / (double)(p2.y - p1.y) + p1.x; //p1p2与射线交点
        if ( x > p.x )
        nCross++; 
    }

    std::cout << "nCross =  " << nCross << std::endl;
    if(nCross % 2 == 1)
    std::cout << "Point is in polygon. " << std::endl;
    else
    std::cout << "Point is not in polygon. " << std::endl;
    return 0;
}

完毕！