Trompe-l'œil

Trompe-l'œil (French for "trick the eye" from tromper - to deceive and l'œil - the eye) is an art technique involving extremely realistic imagery in order to create the optical illusion that the depicted objects really exist.

Julian Beever is an extremely talented chalk artist who draws trompe-l'œil drawings with chalk on pavements.

http://users.skynet.be/J.Beever/pave.htm

The illusion only works if viewed from a certain viewpoint.