Global alignment
Task:  Compute globally consistent set of
rotations {Ri} such that
   Rjpij Rkpik     or   min |Rjpij - Rkpik|2
1. Initialize “first” frame Ri = I
2. Multiply “next” frame by pairwise rotation Rij
3. Globally update all of the current {Ri}
Q: How to parameterize and update the {Ri} ?