Depth perception can be upset when rules and knowledge are in conflict, as you can see in these short videos of well-known visual illusions.
This perspective window by Adelbert Ames upsets size-shape scaling to produce remarkable effects. Running time: 1min 19sec.
Ames Window Flash video (6MB)
Three objects - a fruit crate, a face mask in relief and a hollow face mask - are illuminated alternately from above and below. This demonstrates how our knowledge of faces can over-ride our perception of depth from shadows. Running time 1min 43sec.
Face Crate Flash video (6MB)
Charlie Chaplin Mask
The rear of a mask of Charlie Chaplin is hollow but can appear convex, like a normal face. Running time 1min 58sec.
Charlie Chaplin Mask Flash video (8MB)
Videos and commentary are copyright © 2006 Richard Gregory.