Path: utzoo!attcan!uunet!cs.utexas.edu!news-server.csri.toronto.edu!csri.toronto.edu!corkum
From: corkum@csri.toronto.edu (Brent Thomas Corkum)
Newsgroups: comp.graphics
Subject: clockwise or counter-clockwise
Message-ID: <1990Oct26.170002.9205@jarvis.csri.toronto.edu>
Date: 26 Oct 90 21:00:02 GMT
Organization: Civil Engineering, University of Toronto
Lines: 92


As very many people have pointed out, the code I posted for determining
whether a 2D polygon is clockwise or counter-clockwise was about 155
lines of code to many. I humbly acknowledge this and just wanted to
post a modified version. I also want to thank the many peoply who 
pointed this out. 

Brent




This routine and sample program computes whether a 2D polygon is clockwise
or counter-clockwise. The function takes vertices passed in an array
(x1,y1,x2,y2,...xn,yn where n=#vertices) and the number of vertices.
Of course for a 3D polygon, clockwise or counter-clockwise depends
on which way you look at it. 

Brent Corkum
corkum@boulder.civ.toronto.edu


This function is also available via anonymous ftp at

boulder.civ.toronto.edu (128.100.14.11)

int the

pub/CCW_OR_CW

directory.


P.S. Feel free to use and distribute this function.

/*---------------------------cut here--------------------------------*/
#include <stdio.h>


main()
{
  float pl[100];
  int numv,i,flag;

  printf("Enter #numvertices: ");
  scanf("%d",&numv);
  for(i=1;i<=numv;++i){
    printf("Enter Vertex %d: ",i);
    scanf("%f%f",&pl[2*(i-1)],&pl[2*(i-1)+1]);
  }
  flag=Isclockwise(pl,numv);
  if(flag) printf("polygon is clockwise\n");
  else printf("polygon is counter-clockwise\n");
}
  


/* 
Function by :

Brent Corkum
Data Visualization Lab
Civil Engineering
University of Toronto
Toronto Ontario Canada
corkum@csri.toronto.edu or
corkum@boulder.civ.toronto.edu (128.100.14.11)

That determines whether a 2d polygons vertices are orderred clockwise
or counter-clockwise. The polygon is passed as an array of vertices
(x1,y1,x2,y2,...,xn,yn where n=numv=#vertices or edges in the closed polygon).
The function returns 1 if the polygon is clockwise and 0 if the polygon
is counterclockwise.
*/

int Isclockwise(pl,numv)
float *pl;
int numv;
{
  int i,j;
  float nz;

  nz = 0.0;

  for(i=0;i<numv;++i){
    j = (i+1)%numv;
    nz += (pl[2*i]-pl[2*j])*(pl[2*i+1]+pl[2*j+1]);
  }

  if(nz>0.0)return(0);
  else return(1);
}