Compute 3-dimensional axis-angle from a 4-dimensional one.
In a 3-dimensional axis-angle, the 4th dimension (the rotation) is represented by the norm of the rotation axis vector, which means we transform to .
We usually assume active rotations.
- aarray-like, shape (4,)
Axis of rotation and rotation angle: (x, y, z, angle).
- aarray-like, shape (3,)
Axis of rotation and rotation angle: angle * (x, y, z) (compact representation).