Necker Cube

["Mark E. Shoulson" <cbmvax!uunet!ctr.columbia.edu!shoulson>]

A Necker Cube is that picture of a cube in 2D, y'know, wherein you can't
tell which way it is because you don't know how the lines cross?  Lessee
how good my ASCII is:

      +-----------------+
      |\                |\
      | \               | \
      |  +--------------+--+
      |  |              |  |
      |  |              |  |
      |  |              |  |
      |  |              |  |
      +--+--------------+  |
       \ |               \ |
        \|                \|
         +-----------------+

Well, you get the picture.  Those two spots where the two squares intersect
are ambiguous, in that you don't know which line is in front, and that
means you don't know which square is in front.  If you draw it so you can
tell, but with the disambiguation in an impossible fashion, you get the
famous irrational cube (q.v. various Escher prints.  _Belvedere_ is a good
example, as the guy in the dungeon is holding an irrational cube and
there's a picture of a Necker Cube on the floor with the key intersections
circled).  Or is the Necker Cube the same as the irrational one?  I don't
think so.

~mark