Lecture 26 discusses multiple reflections in precomputed radiance transfer and hierarchical radiosity, with subdivision oracles and the push-pull method for shooting and gathering radiosity. (At 12 minutes, the factor of y_i(Psi_k) is not needed in the integral, since the integral for computing t_i^0 already involves that factor. The word "octtree" should be replaced many times by "quadtree", since we are doing quadtree subdivision.)