  The goal of rigid registration is to align a source surface $ X $ to a target
surface $ Y $. The alignment process involves iteratively transforming $ X $
closer and closer to $ Y $, such that $ X=Z^0 \rightarrow Z^1 \rightarrow Z^2
\rightarrow ... \rightarrow Z^T=Y $. By applying two simplifications, we can
trace the minimization problem back to solving a system of linear equations.
