P3D Re: Hollow Face Illusion

[Bruce Springsteen <bsspringsteen@xxxxxxxxx>]

Having successfully lured out Georgeoff (The Bar-B-Q Bard of Silicon
Valley) and found the common ground with Oliver Dean, back to stereo
business.

It's one of those delightful coincidences that George mentioned the hollow
mask illusion at this time.  In the antique mall in downtown Lawrence last
Saturday, I saw one for the first time.  I'd read about this illusion many
times before - and that it was frequently used in domestic religious art. 
The piece I saw was a large ceramic inverted relief of the head of Jesus,
and indeed it appeared to turn to follow you across the room - a dandy
little illustration of the idea that you are always in His sight, or
something like that.  The illusion was quite strong when viewed
monocularly.

But my stereo vision tended to break the spell.  I don't know whether so
much 3D viewing has made me harder to fool with a concave face, but the
thing looked concave at all times when viewing with both eyes,
well-executed though it was and carefully painted to help the deception. 
I think at best, this is an effect that is not hindered too much by
stereopsis; at worst it can divulge the deception.  My wife, who is not in
regular stereo practice, was fooled with one eye *or* two - and quite
astounded.

Some time ago, I read an old article in "Scientific American" about the
tendency to see a concave face as convex.  I can locate that reference for
interested folks.  The subject is also neatly addressed with anaglyphic
illustrations in R. L. Gregory's "The Intelligent Eye".

Bruce



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