Anamorphosis

Derived from the Greek words "ana" (again), and "morphe" (shape), it means a distorted image whereby the observer is first deceived by a barely recognizable image, and is then directed to a viewpoint dictated by the formal construction of the painting. The spectator must play a part and re-form the picture. Similar characteristics can be found in illusionistic wall and ceiling painting, and in the use of accelerated and retarded perspective in architecture, theatrical stage design, and urban design. Source: Ralph Mayer, "A Dictionary of Art Terms and Techniques"