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From: avr@cs.purdue.EDU (Andrew V. Royappa)
Newsgroups: comp.sys.sgi
Subject: Re: question on gl
Message-ID: <14154@medusa.cs.purdue.edu>
Date: 2 Apr 91 18:31:34 GMT
References: <14144@medusa.cs.purdue.edu> <1991Apr2.150125.22972@odin.corp.sgi.com> <94881@sgi.sgi.com>
Reply-To: avr@cs.purdue.edu (Andrew V. Royappa)
Distribution: usa
Organization: Department of Computer Science, Purdue University
Lines: 37



>In article <1991Apr2.150125.22972@odin.corp.sgi.com>, dave@sgi.com (dave "who can do? ratmandu!" ratcliffe) writes:
>> In article <14144@medusa.cs.purdue.edu> avr@cs.purdue.EDU (Andrew V. Royappa) writes:
>> >The gl graphics user's guide says that the perspective & viewing
>> >transformations transforms the perspective pyramid into the box 
>> >-1.0 <= x,y,z <= 1.0, to facilitate clipping. Is this in fact
>> >the case, 
>> yep--this *is* the case -1.0 to 1.0 for x, y, *and* z clipping.
>> 
>
>actually we clip against -w and +w, where w is normally 1.0 for orthographic
>projections.  For perspective transformations, w is NOT 1.0, and instead of
>transforming the pyramid into a unit box, we really do clip against the 
>pyramid.  
>
>--------------------
>	Gary Tarolli
>
>

Huh. Well, I'm writing some simple 3D routines using X11, and decided
to use the gl transformations printed at the back of the manual. After
applying the MVP matrix to a line segment, I clip against 
-1.0<=x,y,z<=1.0. I see errors sometime even though the clip appears
to do exactly what I want it do. 

So then, should I clip against the cube only when using the ortho
projections, and against the pyramid when using perspective ?

If yes, could you please give me the exact bounds of the pyramid used by
gl ?

Thank you both for your help.

					Andrew