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From: lambert@spectrum.cs.unsw.oz.au (Tim Lambert)
Newsgroups: comp.graphics
Subject: Re: Need to find polygon direction (cw/ccw)
Message-ID: <LAMBERT.90Dec11000343@nankeen.spectrum.cs.unsw.oz.au>
Date: 10 Dec 90 13:03:43 GMT
References: <548@ub.d.umn.edu>
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In-reply-to: jtevik@gisplot1.nrri.umn.edu's message of 9 Dec 90 19:40:14 GMT

>>>>> On 9 Dec 90 19:40:14 GMT, jtevik@gisplot1.nrri.umn.edu (John Tevik) said:

> Here's one for the FAQ.  I need to find the direction (whether clockwise or
> counter-clockwise) of a 2D polygon.

Calculate the *signed* area:

  A = 0.5*\sum_{i=1,2..n} x_i * (y_{i+1} - y_{i-1})

This is positive if and only if the polygon is anti-clockwise.
(This is faster than calculations involving angles.)

Tim