A distorted image that appears normal when seen from a particular viewpoint or with a mirror or lens. Anamorphosis has been used by artists to disguise caricatures, erotic scenes and other hidden images from a casual viewer.

  1280px-Hans_Holbein_the_Younger_-_The_Ambassadors_-_Google_Art_Project   1280px-Holbein_Skull

Left:  Hans Holbein the Younger. The Ambassadors. Oil on oak panel. 1533. National Gallery of Art, London.  Right: anamorphic memento mori skull in the foreground of The Ambassadors, viewed from the correct (oblique) angle.