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From: thant@horus.sgi.com (Thant Tessman)
Newsgroups: comp.sys.sgi
Subject: Re: subroutines perpec and lookat
Summary: lookat thinks y is up
Message-ID: <38766@sgi.SGI.COM>
Date: 25 Jul 89 15:21:36 GMT
References: <8907250331.AA20644@prism.gatech.edu>
Sender: daemon@sgi.SGI.COM
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 23

In article <8907250331.AA20644@prism.gatech.edu>, ccsupos%prism@GATECH.EDU ("SCHREIBER, O. A.") writes:
> 
> 	
> In reference to the above help message, I have straightened out
> my handling of the transformation matrices. However, the twist
> given to the projection does not always correspond to the one that
> is input. Has anyone ever had this problem?
> Thanks in advance.
> Olivier Schreiber


This is probably due you assuming that z is up (a perfectly reasonable
assumption, one would think) but that the 'lookat' for historical reasons 
thinks that y is up, not z.

To use lookat with geometry where z is up, you can follow lookat with
rotate(-900, 'x'); and pretend that z is y and y is -z.  Or you can
look how the lookat matrix is built (Appendix C of the Graphics Library
User's Guide, Volume II) and build your own.

Hope this helps.

thant@sgi.com