By background photo, do you mean something different than all the ones I've posted so far, something less than a full sphere? Every outdoor one I did was lit by a photo.

IDL is not influenced by objects that subtend a small angle of view or a shallow angle. The dominance of IDL comes 90% from objects that subtend an angle within 90 degrees of the surface normal.

A photo facing the camera behind a subject figure contributes practically nothing to the sides of a subject figure, and nothing whatsoever to the front of the figure.

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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)