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\begin{sf}
SURFACE REFLECTION

Get new incident direction $\mathbf{n_k}$ from surface scheme.
Calculate the matrix $\mathbf{Q_k}=\mathbf{Q_{k-1}q_k}$
, where
\[
\mathbf{q_k}=\frac{\mathbf{O(u_k,s_k)R(n_{k-1},n_k)}}
  {O_{11}(\mathbf{u_{k},s_k})R_{11}} ,
\]
and $\mathbf{Q_0}=\mathbbm{1}$.
\end{sf}
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