Paul E. Baclace writes:

I'm very curious as to how humans can directly decode encrypted pictures. Do they stare at it for 10 minutes and go "ah, there it is".

SIRD stereograms might qualify as an encryption method, although many have been able to view these patterns using a brute-force search by selectively diverging the eyes. I don't see how this generalizes to a k of d secret sharing analogue, unless the viewer is assumed to have k+1 eyes. nathan