Now I'm using your wacky this-is-not-physics wall material, where diffuse reflectivity depends on what is reflected, and a reflection occurs if and only if light shines on a surface.

Yep - this is bizarro world. But, ummm, so what? 

The whole point of GI is to calculate a physically correct global illumination solution. This means that what the shaders do interacts with the lighting. If the shaders are programmed to calculate nonsense, then the lighting is nonsense.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)