C# 点在多边形内

//这个算法是参考计算机图形学书上的算法得来的，可以说十分简单高效。   （C++）

//calc point in ploygon or not    

bool PIP(CPoint p,const CPoint data[],int n){   

    bool flag=0;   

    for(int i=0;i<n;i++){   

        if(p.y<data[i].y&&p.y<data[(i+1)%n].y)   

            continue;   

        if(data[i].x<=p.x&&data[(i+1)%n].x<=p.x)   

            continue;   

        int dx=data[(i+1)%n].x-data[i].x;   

        int dy=data[(i+1)%n].y-data[i].y;   

        float t=float(p.x-data[i].x)/dx;//求得交点的t值   

         float y=t*dy+data[i].y;   

        if(y<=p.y&&t>=0&&t<=1)   

        flag=!flag;   

    }   

    return flag;   

}  

 

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c# 测试代码

 

 

namespace _062001
{
    class Program
    {
        static void Main(string[] args)
        {
            Point[] data = new Point[4];
            data[0] = new Point(0, 0);
            data[1] = new Point(0, 5);
            data[2] = new Point(5, 5);
            data[3] = new Point(5, 0);

            Point p = new Point(3, 3);
            if (PIP(p,data,4))
            {
                Console.WriteLine("在区域内");
            }else{
                Console.WriteLine("不在区域内");
            }
        }

        static bool PIP(Point p,Point[] data,int n)
        {
            bool flag =false;
            for (int i=0;i<n;i++)
            {
                if (p.Y <data[i].Y && p.Y<data[(i+1)%n].Y)
                      continue;
                if (data[i].X <p.X && data[(i+1)%n].X <p.X)
                      continue;
                float dx = data[(i+1)%n].X - data[i].X;
                float dy = data[(i+1)%n].Y - data[i].Y;
                float t= (float)((p.X - data[i].X)/dx);
                float y = t*dx + data[i].Y;
                if (y<p.Y && t>=0 && t<=1)
                      flag =!flag;
            }
            return flag;
        }
    }
    class Point
    {
        private float x;

        public float X
        {
            get { return x; }
            set { x = value; }
        }
        private float y;
        public float Y
        {
            get { return y; }
            set { y = value; }
        }

        public Point(float p_x,float p_y)
        {
            x = p_x;
            y = p_y;
        }

    }
   
}