P3D Wallpaper and other illusions

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

Boris, is showing a very interesting effect in his web page.  He has a
picture of a cross-link symmetric fence shot close up and looking straight.
When the eye wonders in the background, parts of the fence are being
fused, creating a confusing illusion regarding the placement of the fence
in space.

Boris, correctly attributes this to the static nature of stereo
photography.  Given the limited depth of focus of the human eye and the
motion parallax, this is not observed very often in real life, but it
observed and that's why it even has a name.  It is known as the "wallpaper
illusion" (and I can see how it got the name)  And, as Ron Labbe pointed
out, this is the principle of the "Magic Eye" stereograms.

Even though it makes good educational material, in stereo photography it is
a good idea to avoid these situations. In general, it is not a good idea to
have discrete levels of depth widely separated.  For example, something
really close and the rest far away.  Then the eye will have to jump from
one level to the other, which is kind of uncomfortable in stereo viewing
(even though it might be OK in real life).  I have experimented with
different compositions and concluded that in general it is more pleasing if
there is a graduation in depth and if the different depth cues add to the
stereoscopic fusion.  Not to say that this is the only way to photograph
things.  But it is the one that sually works the best.

Jim Crowell mentioned the "telephone cord" illusion. I have experienced the
same thing when my daughter carries the phone handset from the kitchen to
the living room for privacy.  It is fascinating to observe these every day
stereoscopic illusions!

-- George Themelis


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