For spherical lighting, why does the bottom of the hippo appear dark? Wouldn't we play the same game of putting a hemisphere at each point of the mesh and projecting the light source onto it to see how much light each point receives, and the points on the bottom would "see" the same kinds of light that the top points "see," right? Or is the hippo actually rendered with a hemispherical light source?
For ambient occlusion, is this robust to other types of lighting or lighting changes? For example, in a stormy night type of scene where lighting is generally low except for flashes of lightning, would ambient occlusion be a good choice?