Path: utzoo!utgpu!watserv1!watmath!att!att!dptg!ulysses!andante!alice!td
From: td@alice.att.com (Tom Duff)
Newsgroups: comp.graphics
Subject: Re: clockwise or counter-clockwise 2d polygons
Summary: oops, small typo
Message-ID: <11525@alice.att.com>
Date: 23 Oct 90 13:46:15 GMT
References: <1990Oct22.114039.8799@jarvis.csri.toronto.edu>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 11

My last posting contained a small error.  Perhaps this one
does too.  The correct text of the orientation-testing routine
is (I think):

Isclockwise(float *pv, int nv){
	float *p1, *p2, *endp=pv+2*nv;
	float area=0;	/* acually twice the area */
	for(p1=pv, p2=endp-1; p1!=endp; p2=p1, p1+=2)
		area+=(p2[0]-p1[0])*(p2[1]+p1[1]);
	return area<0 ? 1 : area>0 ? 0 : -1;
}