Grassia's Practical Parameterization paper

This paper presents formulas for computing, differentiating and
integrating rotations with an exponential map. In addition, they also 
show how to achieve numerical stability for small rotation angles. They 
use exponential maps to solve the issue of maintaining unit-length in
quaternions under, for instance, interpolation.

Key ideas: Exponential maps are more compact. They contain 
singularities, but ones which can generally be avoided. Possible to 
interpolate using cubic splines.

Contributions: Numerically stable formulation of S3->R3 conversion,
formulas to solve for "freely rotating bodies" and ball-and-socket 
joints (2 and 3-DOF rotations).