Lecture 19 explains the hemicube estimates for patch-to-patch form factors for "Galerkin" piecewise constant radiosity and for vertex-to-vertex form factors for "point collocation" piecewise linear radiosity. It also continues the proof that the global lines form factor computation is correct. (At 26:48 minutes, "cos(theta) = length/1" should be "cos(theta) = 1/length" and at 40:40 minutes, the sound track saying "is two at these other vertices" should say "is zero at these other vertices").