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An artwork doesn't necessarily have to reveal its image when viewed normally.
A technique called catoptric anamorphosis deliberately distorts a picture.
Looking directly at the flat artwork can make the subject appear stretched, curved, or nearly impossible to recognize.
But place the correct cylindrical mirror in the middle and something remarkable happens.
The distorted marks are reflected into a recognizable image.
Historical examples of this optical trick survive from centuries ago.
Creating one required manipulating perspective so that the mirror—not the original sheet—presented the intended picture.
It's essentially an old-fashioned visual code that requires a physical object to decode it.
© The Metropolitan Museum of Art