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From: planting@colby.WISC.EDU ( W. Harry Plantinga)
Newsgroups: comp.graphics
Subject: Re: Point inside of polygon revisited... (It's easy)
Message-ID: <3772@spool.WISC.EDU>
Date: Tue, 30-Jun-87 09:49:58 EDT
Article-I.D.: spool.3772
Posted: Tue Jun 30 09:49:58 1987
Date-Received: Sat, 4-Jul-87 22:48:50 EDT
References: <948@elrond.CalComp.COM> <260@brandx.rutgers.edu> <1365@xanth.UUCP> <3731@spool.WISC.EDU> <402@pembina.UUCP>
Sender: news@spool.WISC.EDU
Reply-To: planting@colby.WISC.EDU ( W. Harry Plantinga)
Organization: U of Wisconsin CS Dept
Lines: 29
Keywords: ray tracing, graphics, polygon intersection
Summary: a hard case

In article <402@pembina.UUCP> obed!stephen@alberta.UUCP (Stephen 
    Samuel) writes:

>In article <3731@spool.WISC.EDU>, planting@colby.WISC.EDU 
    (W. Harry Plantinga (me)) writes:

> : The other problem that Forrest mentions is that of the precision to
> : which the arithmetic is carried out.

>You are ASSUMING problems here: for example: There is little problem with
>roundoff error if the 'arbitrary line' is parallel to the X axis. All that
>needs to be checked in that case is the 'y' values of the endpoints.
>
>If you feel that I am wrong here, please send me an actual counter-example.

                                            ^
                                           / \
                                          /   \
                 ray   <-----------------/*    \
                                        /       \
                                       <_________>

Does the point lie inside or outside of the triangle?  If it is very
close to the edge of the triangle and if the angle of the edge is
appropriate, the precision of computation needed to get the right 
answer can be surprisingly high.

Harry Plantinga
planting@colby.wisc.edu