P3D "Hollow Face" illusion

[fj834@xxxxxxxxxxxxxxxxxxxxx (Dr. George A. Themelis)]

I bought an interesting item from Edmund Scientific.  It is called "The
Living Image" and it is essentially a hollow mask (the inside of a mask)
mounted on a display with a fluorescent tube lighting it underneath.  This
display has some interesting properties, of interest to stereo vision.

Viewed at a distance the depth appears reversed, i.e., instead of the
hollow (back) end, one normally sees the front of the mask, a face, looking
at them.  The illusion is so strong that most people have to come very
close or even touch the mask to get back the correct hollow image.

I understand why this is happening.  The human face is one of the most
recognizable objects.  Rarely do we get to see a concave (reversed, hollow)
face.  Faced with a concave face, the brain will attempt to reverse it. 
Furthermore, the light from the bottom reinforces this illusion.  Normally
the light source (like the sun) comes from the top.  Directing the light
from the bottom tends to reverse the sense of depth (to restore the
direction of the shadows).

           REALITY                         Light
                                             |
                                             V
          ----------                     ----------
           \       |                    /         |
  Bright -> \      |            Bright /          |
             \     |     <=>          /           |
             /     |                  \           |
    Dark -> /      |             Dark  \          |
           /       |                    \         |
          ----------                     ----------
               ^
               |
             Light                        ILLUSION


This is explained in the diagram above.  Consider the object in the left. 
If the light is coming from below then an observer in the left side of the
object will see the bright and dark areas as indicated by the arrows.  If
the observer's brain does not accept the light coming from the bottom then
it will reverse the depth, as shown in the in the right, and still maintain
the bright and dark areas.

That's the easy part!  Now read the next posting for a fascinating property
of the "Living Image" which is more difficult to explain!

George Themelis