Path: chuka.playstation.co.uk!news
From: Craig Graham <c_graham@hinge.mistral.co.uk>
Newsgroups: scee.yaroze.programming.3d_graphics
Subject: Re: Poly Intersections
Date: Mon, 04 Jan 1999 09:37:14 +0000
Organization: PlayStation Net Yaroze (SCEE)
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References: <3689671E.AA01BBAB@hinge.mistral.co.uk> <slrn78vqlb.f0.tjs@longford.cs.monash.edu.au>
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Toby Sargeant wrote:

<SNIP - more good code from Toby doesn't get repeated>

> if it's a convex polygon:
>
> compute intersection of plane.
> compute dot product of vector from poly.p1->intersection and poly.p1->poly.p2
> compute dot product of vector from poly.p2->intersection and poly.p2->poly.p3
> if the signs are the same, then the point is inside the polygon.
>
> if it's not convex, there are more complex algorithms that do this fast,
> but I don't know them.

Count axis crossing . I got that one to go pretty well. You go round the poly and
count
the number of times the edges cross the axis' projected out from the point you're
testing
- if theres an odd number of crossings on each axis, the point is in the poly, if
there's an
even number, the point is outside. This works on any N sided polygon.


> Toby.

Star performance - this (and Fred's mail) are the two most techy posts we've
had on here in monthes....

Craig.