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From: avr@cs.purdue.EDU (Andrew V. Royappa)
Newsgroups: comp.sys.sgi
Subject: Re: question on gl
Message-ID: <14168@medusa.cs.purdue.edu>
Date: 3 Apr 91 03:26:58 GMT
References: <1991Apr2.150125.22972@odin.corp.sgi.com> <94881@sgi.sgi.com> <14154@medusa.cs.purdue.edu> <1991Apr2.224901.8498@odin.corp.sgi.com>
Reply-To: avr@cs.purdue.edu (Andrew V. Royappa)
Distribution: usa
Organization: Department of Computer Science, Purdue University
Lines: 29


OK, I've read the articles by Bruce Holloway & Gary Tolling. The
articles were somewhat dense; unfortunately, so am I. I'm
still not sure what I'm doing wrong. 

Here's what I`m doing, for *perspective* projection:

(1) extend from 3D to 4D coordinates; multiply by MVP matrices given at
back of gl user's guide. 

(2) divide 4D result by w to return to 3D

(3) clip resulting geometry against -1.0<=x,y,z<=1.0

Now, I'm interested in correctness more than speed (as I'm doing this
in software, over X, I'm obviously not interested in speed).

Since bruceh says that the gl matrices guarantee w > 0, I don't see
any problem regarding correctness with the division in step (2). The
only modelling  matrices (the "M" of the "MVP" above) that I use are
translate, scale and rotate. What could I still be doing wrong ?

Well, this discussion is helping me understand this stuff a lot
better.

				Thanks again,

					Andrew