Anamorphosis
from Michael’s Visual Phenomena & Optical Illusions
The neighboring picture by Hans Holbein is called “The Ambassadors” (1533, from Wikimedia Commons). Every thing is painted very realistic, in trompe-l’œil style. But what is the strange shape in the bottom foreground?
You could try a slanting look on the image from top right. Easier: below I have cut out this shape; with the slider at the left you can change its angle, with the slider at the right its horizontal scale. This allows you to make the shape recognisable. A tip: memento mori.
Holbein painted the shape with a strong perspective distortion, an example of an anamorphotic display. Now in the National Gallery in London, the painting originally hung at the bottom of a staircase, so descending that staircase the angle was just right.
Anamorphotic images can also involve mirroring on curved surfaces, cylinders or cones. The present demonstration only rotates and scales (affine transform), so the perspective transformation is not fully compensated (but good enough :)