What is theta in this case? The angle between the hemisphere normal and a random vector? And is the hemisphere normal defined by the middle expression?
@cou I think you are right about your definition of theta, but I'm not sure what you mean by the hemisphere normal being defined by the middle expression?
I think I missed the explanation -- what's the purpose of a cosine-weighted sampling? Is it just importance sampling based on the assumption that things close to the horizon are less likely to hit, or something else?
I believe the most of this slide has nothing to do with cosine-weighted sampling- the title should be changed, or an explanation note provided since on its own without context it is misleading.