Lecture 21 gives an analytic formula for an unobstructed point-to-polygon form factor. (At 9:43 minutes, "Sum beta*cos(alpha)" should be "(1/2) Sum beta*cos(alpha)" since the area of a sector of a unit circle is half the subtended angle. In the formula for cos(alpha), V1 cross V2 must be normalized, because it it not a unit vector, even if V1 and V2 are.) Multipass global illumination methods are explained in terms of L D S G and E notation.