Clearly they hoped they'd save money by doing the sequence infront of a glass painting (which actually does cast some realistic light reflections around) but then found that some of the angles didn't match up later and used an optical composite of a matte (only they didn't bother with the continuity as much as they should).
It looked great in 1980 but back then most people saw a film once or twice (not the umpty thrumpty zillion times we have all seen these films since).