Anamorphic illusions change as you look at it in different angles.

Julian Beever is known all over the world for his pavement drawings, more especially his 3D illusions, drawn in a special ways to create an impression of 3D when seen from one particular viewpoint.

Felice Varini creates anamorphic illusions of geometric shapes on walls, glass and other materials. You can only see the illusions right from the perfect angle or else it just looks like a bunch of fragments on a wall. He was nominated in 2001 for the Marcel Duchamp Prize.