3d rotation question

[Keith W. <kw...@cs...>]

The camera view in my program will reflect a first-person perspective of 
a person's body and head in a world.  The person can move their "body" 
by going forward and backward or rotating left and right (yaw).  The 
person can also individually move their head around their neck by 
"pitching" up and down.

I want this system to do some specific things, some of which I have 
figured out, others of which I haven't.

Moving forward and backward is easy, since I can get the forward vector 
of the object, discard the Y axis and just translate by a distance or 
speed scalar of X and Z to move over horizontal ground (discarding Y 
because looking up shouldn't make a person fly up into the sky when they 
move forward).

Rotating around Y isn't too hard.  I tried using yaw but of course that 
makes it yaw around the local Y axis which gets funky when the head is 
pitched up or down.  However, this is fairly easy by just orienting 
around the Y Euler angle.  I have found that is works properly even when 
the camera is pitched away from a horizontal viewing axis.

However, I have the following problem.  I would like to make the head 
"snap" back to 0 vertical (no pitch) when I move the body forward or 
backward (so instead of wandering around with one's head pointed an 
arbitrary pitch the head should snap back to horizontal as soon as the 
body and camera are moved forward or backward).

I can't figure out how to take the present orientation (a quaternion of 
course) and remove the vertical component of this orientation to flatten 
it to 0 without affecting the horizontal orientation (so the body is 
still looking along the same vertical azimuthal equator of the world but 
at 0 degree altitude).  How do I do this?

Thanks.

________________________________________________________________________
Keith Wiley                                kw...@cs...
http://www.unm.edu/~keithw                 http://www.mp3.com/KeithWiley

"Yet mark his perfect self-contentment, and hence learn his lesson,
that to be self-contented is to be vile and ignorant, and that to
aspire is better than to be blindly and impotently happy."
                                           --  Edwin A. Abbott, Flatland
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