1) arccos range is only from 0 to pi so I cannot distinguish between 90 and -90 degrees of rotation (which I would like to be represented as 90 and 270 degrees respectively).

2) arccos is apparently undefined at some points. I'm getting error notices for arccos as I'm rotating around. What I'm thinking is that this is because the camera's up vector may not a perfect unit vector (ie its <0,1.001,0> or something like that).

Anyways I would like to know if there's a way to find the angle of rotation between two vectors in 3d space without using an arccos function or any other method that would experience similar problems.

I think this site

http://www.euclidean...tween/index.htm

is presenting some sort of alternative dealing with quaterions and matrices but I do not understand it very well.

**Edited by 1101, 07 May 2010 - 07:10 AM.**