Path: utzoo!attcan!uunet!cs.utexas.edu!swrinde!ucsd!ucbvax!ucsfcgl!cca.ucsf.edu!wet!smiller
From: smiller@wet.UUCP (Gregory Shane Miller)
Newsgroups: comp.graphics
Subject: Re: Polygon orientation test ?
Summary: easist method folows
Message-ID: <1728@wet.UUCP>
Date: 27 Oct 90 04:42:08 GMT
References: <4158.272311a1@cc.curtin.edu.au>
Organization: Wetware Diversions, San Francisco
Lines: 15


The best way to see if a 2D poly is CCW or CW is too look in the
PREPARATA/SHAMOS Comp. Geometry book in chapter 2.  As I recall
in sect 2.3 or 2.2 (I don't have the book with me) there is a foot
note saying, given any three points from a polygon, eval this 3x3
matrix (eg. find det of the 3x3 matrix).  If returns the twice the
signed area of the three points (eg a triangle).  The sign of the
result tells whether it's oriented CW or CCW.

Check it out.
regards-

-- 
--
G. Shane Miller [ smiller@wet.UUCP ]  Von Neumann eat your heart out!