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From: sundar@ai.mit.edu (Sundar Narasimhan)
Newsgroups: comp.windows.x
Subject: Re: 3d rotation in room coords?
Message-ID: <13709@life.ai.mit.edu>
Date: 5 Mar 91 18:01:55 GMT
References: <9103050342.AA20065@maui.coral.com>
Sender: news@ai.mit.edu
Reply-To: sundar@ai.mit.edu
Organization: MIT Artificial Intelligence Laboratory
Lines: 39

In article <9103050342.AA20065@maui.coral.com>, tomt@maui.coral.COM (Tom Tulinsky) writes:
|> 
|> I'd like to allow the user to do 3d rotations of an object in the
|> 'room' coordinate system; i.e. y is toward the ceiling, x is to his
|> right, and z is out of the screen (rt-handed).  I want this to be true
|> even after he has done some rotations....  The naive way of
|> implementing rotations accumulates them so that a 90 deg. y rotation,
|> the x axis is now pointing out of the screen, where the room z axis
|> is.  
|> 
|> I've tried everything I can think of, and can't get it to work.
|> 
|> My latest version, (using the Vogle package from uunet, somewhat like
|> SGI GL) does the y rotation, then tries to calculate where the xroom
|> vector is, and uses an arbitrary rotation routine to rotate about it.
|> The result: it just interchanges the order of rotations:  the X
|> axis is constant but the Y axis is rotated by xrot.  (also it only
|> works for y<0).
I understand what you want to do, but not what your implementation
does entirely. Here's what I have used in the past for something 
like this. (I'm assuming from your code, that you want your 
mouse to control viewer position).

Think of viewing the object from the surface of a sphere. Your eye
point can be specified by 2 co-ordinates (you can think of them
as latitude and longitude). Have your mouse motion map to these
co-ordinates. (For example, you can have up down motion map to
-90 to +90 degrees latitude wise, and your right-left motion map to
-180 to +180 degrees longitude wise). Once you have these co-ordinates
it is a simple matter to compute where the eye point is, and then
other things needed for graphics like the view vector and so on.
Maintain your up vector to be always up toward the ceiling.*

Hope this helps. 

* There is a subtlety associated with keeping the up vector pointing
up all the time (especially as your eye point gets way up and you are
looking almost directly down). If what you want are realistic fly-bys
then you can't keep the up-vector constant.