Unconventional Staring

Charles the first
Art of Anamorphosis
:

An anamorphosis is a deformed image that appears in its true shape when viewed in some “unconventional” way.
In past centuries, anamorphic designs were used as “secret images”, sometimes to hide erotic drawings, sometimes images of a politically-sensitive nature, such as this portrait of the British King Charles the First which circulated amongst Royalist sympathisers after the King’s execution in 1649.

I don’t see any damn erotic images in the above.

I can imagine museum goers bending sideways and upside-down trying to find the “hidden” images.