Anamorphic art
The Ambassadors (1533) by Hans Holbein the Younger
The skull in this painting (best seen standing to the right of it), hanging in the National Gallery in London is regarded as the most famous piece of anamorphic art, but the anamorphism is not immediately obvious and many people need to be told where to look to find it.
When the painting was first in the possession of the National Gallery, it took some years before anyone solved the puzzle of the "splodge" in the front of it, appearing to spoil an otherwise detailed painting.Many sixteenth century European portraits include skulls as reminders of death An anamorphic image is one that can only be interpreted when viewed from a particular angle or through a transforming optical device like a mirror.

Such images interest artists, graphic designers and mathematicians alike, and all will be meeting on 12-13 December to explore anamorphic images and discuss the technical aspects of how to create and interpret them.
https://www.newscientist.com/gallery/dn16197-anamorphic-art/
to view 12 more images....