点在多边形内算法的实现

 

点在多边形内算法的实现

cheungmine

2007-9-22

 

本文是采用射线法判断点是否在多边形内的C语言程序。多年前，我自己实现了这样一个算法。但是随着时间的推移，我决定重写这个代码。参考周培德的《计算几何》一书，结合我的实践和经验，我相信，在这个算法的实现上，这是你迄今为止遇到的最优的代码。

这是个C语言的小算法的实现程序，本来不想放到这里。可是，当我自己要实现这样一个算法的时候，想在网上找个现成的，考察下来竟然一个符合需要的也没有。我对自己大学读书时写的代码没有信心，所以，决定重新写一个，并把它放到这里，以飨读者。也增加一下BLOG的点击量。

 

首先定义点结构如下：

/* Vertex structure */

typedef struct                      

{

     double x, y;

} vertex_t;

 

本算法里所指的多边形，是指由一系列点序列组成的封闭简单多边形。它的首尾点可以是或不是同一个点（不强制要求首尾点是同一个点）。这样的多边形可以是任意形状的，包括多条边在一条绝对直线上。因此，定义多边形结构如下：

/* Vertex list structure – polygon */

typedef struct                      

{

     int                num_vertices;      /* Number of vertices in list */

     vertex_t           *vertex;           /* Vertex array pointer */

} vertexlist_t;

 

为加快判别速度，首先计算多边形的外包矩形（rect_t），判断点是否落在外包矩形内，只有满足落在外包矩形内的条件的点，才进入下一步的计算。为此，引入外包矩形结构rect_t和求点集合的外包矩形内的方法vertices_get_extent，代码如下：

/* bounding rectangle type */

typedef struct

{

     double min_x, min_y, max_x, max_y;

} rect_t;

 

/* gets extent of vertices */

void vertices_get_extent (const vertex_t* vl, int np, /* in vertices */

rect_t*  rc /* out extent*/ )

{

     int i;       

     if (np > 0){

         rc->min_x = rc->max_x = vl[0].x; rc->min_y = rc->max_y = vl[0].y;

     }else{

         rc->min_x = rc->min_y = rc->max_x = rc->max_y = 0; /* =0 ? no vertices at all */

     }

 

     for(i=1; i<np; i++)

     {

         if(vl[i].x < rc->min_x) rc->min_x = vl[i].x;

         if(vl[i].y < rc->min_y) rc->min_y = vl[i].y;      

         if(vl[i].x > rc->max_x) rc->max_x = vl[i].x;

         if(vl[i].y > rc->max_y) rc->max_y = vl[i].y;

     }

}

 

     当点满足落在多边形外包矩形内的条件，要进一步判断点（v）是否在多边形（vl：np）内。本程序采用射线法，由待测试点（v）水平引出一条射线B（v，w），计算B与vl边线的交点数目，记为c，根据奇内偶外原则（c为奇数说明v在vl内，否则v不在vl内）判断点是否在多边形内。

具体原理就不多说。为计算线段间是否存在交点，引入下面的函数：

（1）is_same判断2（p、q）个点是（1）否（0）在直线l（l_start，l_end）的同侧；

（2）is_intersect用来判断2条线段（不是直线）s1、s2是（1）否（0）相交；

 

/* p, q is on the same of line l */

static int is_same(const vertex_t* l_start, const vertex_t* l_end, /* line l */

const vertex_t* p,

const vertex_t* q)

{

     double dx = l_end->x - l_start->x;

     double dy = l_end->y - l_start->y;

 

     double dx1= p->x - l_start->x;

     double dy1= p->y - l_start->y;

    

     double dx2= q->x - l_end->x;

     double dy2= q->y - l_end->y;

 

     return ((dx*dy1-dy*dx1)*(dx*dy2-dy*dx2) > 0? 1 : 0);

}

 

/* 2 line segments (s1, s2) are intersect? */

static int is_intersect(const vertex_t* s1_start, const vertex_t* s1_end,

const vertex_t* s2_start, const vertex_t* s2_end)

{

     return (is_same(s1_start, s1_end, s2_start, s2_end)==0 &&

is_same(s2_start, s2_end, s1_start, s1_end)==0)? 1: 0;

}

 

 

下面的函数pt_in_poly就是判断点（v）是（1）否（0）在多边形（vl：np）内的程序：

 

int pt_in_poly (   const vertex_t* vl, int np,  /* polygon vl with np vertices  */

const vertex_t* v)

{

     int i, j, k1, k2, c;

     rect_t        rc; 

     vertex_t     w;

     if (np < 3)

         return 0;

    

     vertices_get_extent(vl, np, &rc);

     if (v->x < rc.min_x || v->x > rc.max_x || v->y < rc.min_y || v->y > rc.max_y)

         return 0;

 

     /* Set a horizontal beam l(*v, w) from v to the ultra right */  

     w.x = rc.max_x + DBL_EPSILON;

     w.y = v->y;

 

     c = 0;        /* Intersection points counter */

     for(i=0; i<np; i++)

     {

         j = (i+1) % np;

 

         if(is_intersect(vl+i, vl+j, v, &w))

         {

              c++;

         }

         else if(vl[i].y==w.y)

         {

              k1 = (np+i-1)%np;

              while(k1!=i && vl[k1].y==w.y)

                   k1 = (np+k1-1)%np;

 

              k2 = (i+1)%np;

              while(k2!=i && vl[k2].y==w.y)

                   k2 = (k2+1)%np;

             

              if(k1 != k2 && is_same(v, &w, vl+k1, vl+k2)==0)

                   c++;

 

              if(k2 <= i)

                   break;

 

              i = k2;

         }

     }

 

     return   c%2;

}

 

本想配些插图说明问题，但是，CSDN的文章里放图片我还没用过。以后再试吧！实践证明，本程序算法的适应性极强。但是，对于点正好落在多边形边上的极端情形，有可能得出2种不同的结果。

下面是python的版本：

#!/usr/bin/python#-*- coding: UTF-8 -*-## pip.py#   point in polygon## 2016-01-07## point(pt) is inside polygon(poly)######################################################################### gets extent of vertices vl#def extent_vertices(vl):    min_x = 0.0    min_y = 0.0    max_x = 0.0    max_y = 0.0    i = 0    for (x, y) in vl:        if not i:            (min_x, min_y) = (x, y)            (max_x, max_y) = (x, y)            i = 1        else:            if x < min_x:                min_x = x            if y < min_y:                min_y = y            if x > max_x:                max_x = x            if y > max_y:                max_y = y    return (min_x, min_y, max_x, max_y)# points p, q are on the same of line l#def same_side(l_start, l_end, p, q):    dx, dy = float(l_end[0] - l_start[0]), float(l_end[1] - l_start[1])    dx1, dy1 = float(p[0] - l_start[0]), float(p[1] - l_start[1])    dx2, dy2 = float(q[0] - l_end[0]), float(q[1] - l_end[1])    d = (dx * dy1 - dy * dx1) * (dx * dy2 - dy * dx2)    if d >= 0:        return 1    else:        return 0# are line segments s1, s2 intersect ?#def intersect_segs(s1_start, s1_end, s2_start, s2_end):    if not same_side(s1_start, s1_end, s2_start, s2_end) and not same_side(s2_start, s2_end, s1_start, s1_end):        return 1    else:        return 0# is point pt(x, y) in polygon poly#   returns:#     0 : not in#     1 : indef pt_in_poly(pt, poly):    np = len(poly)    if np < 3:        return 0    (x, y) = pt    (min_x, min_y, max_x, max_y) = extent_vertices(poly)    if x < min_x or x > max_x or y < min_y or y > max_y:        return 0    # set a horizontal beam line (pt, w) from pt to the ultra right    w = (max_x + max_x*0.1 + 1, y)    c = 0    for i in range(0, np):        j = (i+1) % np        if pt == poly[i]:            return 1        elif intersect_segs(poly[i], poly[j], pt, w):            c = c + 1        elif poly[i][1] == w[1]:            k1 = (np + i - 1) % np            while (k1 != i and poly[k1][1] == w[1]):                k1 = (np + k1 - 1) % np            k2 = (i+1) % np            while (k2 != i and poly[k2][1] == w[1]):                k2 = (k2 + 1) % np            if k1 != k2 and not same_side(pt, w, poly[k1], poly[k2]):                c = c + 1            if k2 <= i:                break            i = k2    return c % 2epsilon = 0.0000001assert pt_in_poly((0, 0), [(-1, -1), (-1, 1), (1, 1), (1, -1)]) == 1, "(1) appliaction error"assert pt_in_poly((-1, -1), [(-1, -1), (-1, 1), (1, 1), (1, -1)]) == 1, "(2) appliaction error"assert pt_in_poly((-1, 1), [(-1, -1), (-1, 1), (1, 1), (1, -1)]) == 1, "(3) appliaction error"assert pt_in_poly((-1 - epsilon, -1), [(-1, -1), (-1, 1), (1, 1), (1, -1)]) == 0, "(4) appliaction error"assert pt_in_poly((-1 + epsilon, -1 + epsilon), [(-1, -1), (-1, 1), (1, 1), (1, -1)]) == 1, "(5) appliaction error"assert pt_in_poly((-1 + epsilon, -1 + epsilon), [(-1, -1), (-1, 1), (1, 1), (1, -1)]) == 1, "(6) appliaction error"assert pt_in_poly((0, 0), [(-1, -1), (-1, 1), (1, 1), (1, 0), (2, -1)]) == 1, "(7) appliaction error"assert pt_in_poly((0, 0), [(-1, -1), (-1, 1), (1, 1), (1, 0), (2, 1), (2, -1)]) == 1, "(8) appliaction error"assert pt_in_poly((0, 0), [(-1, -1), (-1, 1), (1, 1), (1, 0), (2, 1), (2, 0), (2, -1)]) == 1, "(9) appliaction error"assert pt_in_poly((0, 0), [(-1, -1), (-1, 1), (1, 1), (1, 0), (2, 0), (3, 0), (2, -1)]) == 1, "(10) appliaction error"assert pt_in_poly((-2, 0), [(-1, -1), (-1, 1), (1, 1), (1, 0), (2, 0), (3, 0), (2, -1)]) == 0, "(11) appliaction error"assert pt_in_poly((3, 0), [(-1, -1), (-1, 1), (1, 1), (1, 0), (2, 0), (3, 0), (2, -1)]) == 1, "(12) appliaction error"