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.

Ames' Window
This perspective window by Adelbert Ames upsets size-shape scaling to produce remarkable effects.  Running time: 1min 19sec.

Ames Window Flash video (6MB)

Face Crate
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

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.