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From: hartlemp@mentor.cc.purdue.edu (Michael P Hartley)
Newsgroups: rec.games.programmer,alt.msdos.programmer,comp.os.msdos.programmer
Subject: Re: 3D Rotation
Summary: easier
Message-ID: <10162@mentor.cc.purdue.edu>
Date: 11 Apr 91 20:29:37 GMT
Article-I.D.: mentor.10162
References: <1991Apr05.224711.12750@lynx.CS.ORST.EDU> <1991Apr8.181450.15309@cs.cornell.edu>
Followup-To: rec.games.programmer
Organization: Purdue University
Lines: 34

In article <1991Apr8.181450.15309@cs.cornell.edu>, ressler@CS.Cornell.EDU (Gene Ressler) writes:
> In article <41021@cup.portal.com> Bobster@cup.portal.com (Robert Jules Shaughnessy) writes:
> >
> >     I also have had this problem. I think you should fallow what is said
ect...
> >yyou view an object more clearly as you pass through them.
> 
> To beat a dead horse ;-), what is really wanted (at least in concept) is to
> _clip_ to the perspective view volume in _3D_, then draw.  (There is
> no `behind the observer' for orthogonal projections.) This volume is the
> (infinite) double pyramid formed by projecting lines through
> the observer point to the corners of the viewport (think `screen').
 ect...
> Check out the 3D version of the Liang Barsky clipping algorithm.
> It nicely truncates 3d lines to just the viewport lobe
> of the pyramid.  Hence when you fly through objects, they
> 
> Happy landings, Gene

   Another way to do it is to transform the perspective (ie pyramid) into 
a parallel orthogonal canonical space.. clipping is extremely easy then.  Check
in Foley "Computer Graphics: Principles and Practice"  chapters 5 and 6.  Every
thing you always wanted to know about 3D projections onto a 2D view-reference-
plane.

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 *    / | \            Michael Hartley         (Yes, I did melt that)       *
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