I understand that the ambient lighting is a hack to emulate the fact that light/photons are bouncing in every direction so no area is completely black (generally), and that specular highlights depend on the "shininess" of the material + light source, but I'm a bit more confused on what exactly diffuse reflection is :o
My thinking is that diffuse lights up every surface that makes contact with the light source, which should appear brighter than any surface hidden from the light. Since specular concentrates around the highlights, and ambient is the same everywhere, omitting diffuse reflections would make surfaces that face toward and away from the light have the same brightness. I imagine that the significance of diffuse reflection would be more easily noticed on objects that are very dull and have no highlights. We would need diffuse in order to have any plausible sense of depth here.
How accurately can you calculate different materials and shaders computation costs before actually running them on a given object?