In the arts, anamorphosis is a visual trick which is used to create an image which appears to be distorted, until the viewer shifts position or uses a special instrument to look at the picture. At first glance, an image created with this technique can appear confusing, puzzling, or mystifying, with some images being so subtle that something just looks slightly off, while others are almost impossible to comprehend. Anamorphosis is used in a variety of ways in art, and you may be able to see some examples of this technique if you visit a museum or a bookstore which carries art books.
The concept of anamorphosis arose around the 14th century, when people first began to understand how perspective worked. If you examine art from before the 14th century, you may note that the art often has a flattened aspect, with no perspective. As artists began to learn about perspective and realism in painting, they also began to explore the possibilities of manipulating perspective to create visual tricks or illusions.
In the case of perspective anamorphosis, in order to understand the image, the viewer must change his or her position relative to the painting. By changing perspective and relaxing the eyes, the viewer can see the hidden image inside the larger image. Perspective anamorphosis is often used to create trompe l'oeil paintings, especially large paintings such as those which line the roofs of cathedrals. Mirror anamorphosis, on the other hand, requires the use of a device like a mirror for the viewer to discern the image.
Artists didn't just explore anamorphosis for fun. By hiding images in paintings with anamorphic techniques, painters could distribute salacious or controversial material, and remain confident that the material would be hidden to most prying eyes. In a sense, anamorphosis permits a sort of visual cryptography, with the author using a key to embed a message in the work, and a viewer using the same key to interpret it.
If you ever approach a work of visual art which seems to be distorted, you may be looking at an example of anamorphosis. You could try changing your position relative to the painting to see if it is a case of perspective anamorphosis, or you may need to position a mirror next to the art to see the hidden message it contains. Museums with anamorphic artworks usually include a discussion of anamorphosis in the description, so that viewers are not frustrated; if you enjoy looking at this type of art, several publishers also produce entire books of anamorphic artwork.
Some famous examples of anamorphosis include "The Ambassadors," which includes a hidden human skull, the vault of the Church of Saint Ignazio, painted in the 1600s by Andrea Pozzo, and "Leonardo's Eye."