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From: jbm@eos.UUCP (Jeffrey Mulligan)
Newsgroups: comp.graphics
Subject: Re: Point within cylinder problem
Message-ID: <5134@eos.UUCP>
Date: 15 Sep 89 19:28:26 GMT
References: <7967@medusa.cs.purdue.edu> <10423@phoenix.Princeton.EDU>
Organization: NASA Ames Research Center, California
Lines: 32

markv@phoenix.Princeton.EDU (Mark T Vandewettering) writes:

>>	Even more helpful would be something that would determine if a
>>point is within a cylinder with a hemisphere at each end.  This can be
>>done by checking the point against a spere at each endpoint, but if
>>there is a way to get this "free" I'd prefer it.

[ stuff deleted ]

(point is inside endcap spheres if

>			DotProduct(P-Base, P-Base) < radius * radius

			OR

>			DotProduct(P-Apex, P-Apex) < radius * radius

>The other case is that the point is inside the cylinder.  We can determine
>this by projecting the point down onto the base plane, and determining
>whether it is within radius of the base point.

>We can do some early trimming by precomputing some stuff.

Note that if either of the dot products given above is greater
than (radius*radius + DotProduct(base-apex,base-apex)),
then the point P cannot lie within the cylinder.

-- 

	Jeff Mulligan (jbm@aurora.arc.nasa.gov)
	NASA/Ames Research Ctr., Mail Stop 239-3, Moffet Field CA, 94035
	(415) 694-3745