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From: phil@hpsmdca.HP.COM (Philip Walden)
Newsgroups: comp.graphics
Subject: Re: Need an algorithm to calculate area of polygons
Message-ID: <22240001@hpsmdca.HP.COM>
Date: 11 Oct 89 17:02:51 GMT
References: <761@cwjcc.CWRU.Edu>
Organization: HP Surface Mount Development Center
Lines: 20

>>
>>A = abs(sum (i=1 to n) (x(i+1)-x(i))*(y(i+1)+y(i))/2)
>>
>
>This is Newell's formula (with the abs and the /2 added).  The general form
>works in 3-D and computes three terms which are the components of a vector
>whose magnitude (abs again) is the area.
>
	If the ABS was not done, would the sign of A also be
	an indication of "handedness" of the polygon? Is there
	a proof for this?

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Philip Walden
Hewlett Packard
Surface Mount Development Center
3500 Deer Creek Road
Palo Alto, CA 94304
(415) 857-3899
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