An Anamorphic Image is an image that appears normal only when viewed from some particular perspective or when viewed through some transforming optical device such as a mirror. An example of the former that everyone is familiar with are the markings on roadways which are actually distorted in one direction so that they look normal from the driver's point of view.
The image on the left is what is painted on the road; the image on the right is what the driver sees. This is called a "plane anamorph".

        Artists in the renaissance were often fascinated with such perspective distortions and incorporated them into their paintings. Undoubtedly the most famous plane anamorphic painting is The Ambassadors (1533) by Hans Holbein. The gray smudge in the lower centre of the painting is seen as a skull when viewed from the right at a shallow angle above the surface.

If you would like to see another example of a plane anamorph click HERE.