Lecture: Reverse engineering M.C. Escher
Reverse engineering M.C. Escher's pictures using the Droste effect, and then putting them together again, but differently. See Bart de Smit's talk at the Symposium held on the occasion of the dutch translation of Roger Penrose's book 'Road to Reality':
The event is a recent one from January this year. Penrose is sitting in the audience, and I suppose his question is somewhat cheeky as he had a personal relationship to Escher himself...
Here's Roger himself in his talk 'Eschermatics' from last year
in which he tells a bit about his (regrettably) short contacts with Escher.