Anamorphosis is one of the most interesting special effects things in the history of painting before the computers came to be. =)

The anamorphic pictures created through anamorphosis are usually paintings, but the techniques are also used in sculpture. They were often severely distorted or abstract-looking from "normal" viewpoints.

"Oblique" images needed to be viewed from an unusual vantage point (usually from some side) to see the image in correct form.

The more famous "catoptric" pictures were distorted other way: To view them, one needed to place a cylindrical (or conical or pyramidal or other such shape) mirror of specified diameter on the middle of the picture, and then view the picture with the mirror.

The most famous picture with hidden anamorphic pictures is Hans Holbein's "The Ambassadors", that shows two ambassadors - and a distorted picture of a human skull.

The techniques have also been used to "hide" pictures of "risky" nature (pictures of unpopular political figures, and, you guessed it, pr0n too).

In modern times, anamorphosis plays a large part of street paintings and some signs of various natures.

(liveforever says Harry Potter merchandise also uses this in interesting ways.)

(Sources: My memory and