Anamorphic Illusions
You've probably seen this before. Julian Beever.
http://users.skynet.be/J.Beever/pave.htm
for example, view this from right side up.
http://users.skynet.be/J.Beever/globe.html
then view the same pic from upside down.
http://users.skynet.be/J.Beever/globe-wrongview.html
My favourite one is the batman one
http://users.skynet.be/J.Beever/batman.html
http://users.skynet.be/J.Beever/pave.htm
for example, view this from right side up.
http://users.skynet.be/J.Beever/globe.html
then view the same pic from upside down.
http://users.skynet.be/J.Beever/globe-wrongview.html
My favourite one is the batman one
http://users.skynet.be/J.Beever/batman.html