subroutines perpec and lookat

[Thant Tessman]

In article <8907250331.AA20644 at prism.gatech.edu>, ccsupos%prism at 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 at sgi.com