The Ambassadors is the most famous example of anamorphosis in painting. This Demonstration illustrates the effect by changing the viewer's viewpoint and zooming in on the distorted diagonal shape at the bottom of the picture.
Painted in 1533 by Hans Holbein the Younger, The Ambassadors is famous for the trick of perspective that allows what first appears to be a diagonal streak across the bottom of the canvas to be seen from a particular angle as a skull.
In this Demonstration the image is applied as a texture to a flat square, directly below the observer's original viewpoint. The "turn" control parametrically adjusts both this viewpoint and the orientation of the image, ending at values that reveal the skull. The "zoom" control changes both the width of the field of view and its center, zooming in on the important region of the painting. The user may also directly interact with the image by clicking and dragging to turn it, holding the control key while dragging to zoom in/out, and holding the shift key while dragging to move around.
For more information, see this painting in the Google Art Project.