question: lookat() command and twist

[Matthew A Machlis]

I am developing a 3-D realtime simulation on a Personal IRIS which has
head-tracking capability to display what the user is actually looking at.
I am using the lookat() command to actually do the transformation involved
in head-tracking.  I have run into an unfortunate aspect of the lookat
command: everything is fine when I am looking straight ahead (along the
positive z-axis) with zero twist.  But as the "look vector" is rotated
downward (towards the negative y axis), still with zero twist, the 
direction which is "up" in terms of what is displayed on the screen
rotates 180 degrees as the head rotates through straight down.
How can I compensate for this twisting of the up direction with lookat?

PLEASE... if you have any information or think you might be able to help
but don't understand exactly what I am talking about, email me.  Thanks
in advance.
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Matt Machlis
MIT Space Systems Laboratory
(617)253-2272