Quote - Since I have to wait for the render to finish, I thought I'd just ask - is the "floor" supposed to be the ground, the room bottom surface, or the backdrop, or some combination? Did you intend for the backdrop to be above the "floor"?

The room box was moved down one foot to prevent it intersecting the backdrop's floor.  If it's not moved down the renderer becomes unhappy and tries to render both surfaces in the same "space", no good.  Although I suppose some rays will inevitably get wasted in the area between the box and the backdrop I don't particularly care.  Incidentally that's another example of why considering normals in GI is not that wise, imo - the backside/underside of the backdrop prop will behave differently from the front side, very counter intuitive.  Default behavior really ought to be "ignore normals" regardless of any material settings.

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