Vurdlak's Mighty Optical Illusions is a potpourri of interesting visual effects, from the mundane to the highly contrived. The warped chessboard's provenance is detailed here. Vurdlak's website was a serendipitous come-uppance resulting from a reference to Sandro Del-Prete in the Random (Blog) of Neil Bickford, where I was reading up on the latest Gathering for Gardner, number nein.