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From: airey@matisse.cs.unc.edu (John Airey)
Newsgroups: comp.graphics
Subject: Re: Need an algorithm to calculate area of polygons
Keywords: Polygon, algorithm
Message-ID: <9885@thorin.cs.unc.edu>
Date: 9 Oct 89 15:03:09 GMT
References: <484@ctycal.UUCP> <619@cditi.UUCP> <2273@uw-entropy.ms.washington.edu>
Sender: news@thorin.cs.unc.edu
Reply-To: airey@matisse.cs.unc.edu (John Airey)
Organization: University Of North Carolina, Chapel Hill
Lines: 19

In article <2273@uw-entropy.ms.washington.edu> bill@stuart.UUCP () writes:
>>In article <484@ctycal.UUCP>, ingoldsb@ctycal.COM (Terry Ingoldsby) writes:
>># I need an algorithm that will calculate (quickly)
>># the area of an arbitrary polygon.
                   ^^^^^^^^^

If arbitrary means possibly concave, the following suggestion is inadequate.

>1/2 * sum( x(i)*y(i-1) - y(i)*x(i-1) ), wrapping the indexes around the ends
>will give the (signed) area of a polygon.  The sign is positive if the
>vertices are given clockwise.  Each summand is the signed area of a triangle
>with one edge an edge of the polygon and the remaining vertex (0,0).
>	Bill Dunlap





john m. airey                                               airey@cs.unc.edu