I'm probably not going to rewrite it but I'm intrigued by the problem, what is the optimal way to represent a direction in 3D, i.e. with as little space, redundancy and invalid value cases as possible, and if possible no normalization checks and renormalization operations needed?

(With direction I mean just the direction, i.e. not including position in space nor any roll (rotation around the direction vector itself). For instance a line (ray) can be represented by a point and a direction.)

An idea is to use a rotation in the x-y-plane (represented by a signed int A that is the nominator in the fraction A/PI, giving the interval [+- PI]), plus a half-circle rotation in z-space (represented by a signed int B in B/(PI/2), giving the interval [+- PI/2]). There are no invalid nor un-normalized values so that's good. But it's not fully optimal in terms of bit representation:

- the z-space-rotation has twice the angular precision of the x-y-rotation which is a little irregular,

- even more irregular is that the closer the direction gets to the Z axis the greater the angular precision,

- at the directions +Z and -Z, A has no meaning (all values of A are equally valid).

And using this representation in actual 3D code seems less than efficient. Every time it's used two divisions and floating point conversions seem to be required. And how would you efficiently apply an arbitrary rotation to it?

Your thoughts?

**Edited by ChristerSwahn, 02 July 2012 - 03:43 AM.**