Attached Link: ShadersPlus @ www.digitalcarversguild.com

You can make anything a shadow catcher, but normally that works only with ray tracing. I you want to 'catch shadows' with GI you need the plugin from DCG that does just that (and lots and lots of other things). If you want just a white background, and a white floor, the horizon line should not be a problem at all. The 'building' of a white room could be just a bent plane from the spline modeller. Or a plane that bends downwards at the end, or something. Good luck! Hoof