Re: SAY YES TO "PSEUDO"!

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

Larry seems to believe that a pseudo stereo is every bit as natural as a
normal stereo and it is only our lack of familiarity with the visual
experience that gives us troubles.  This view, IMO, fails to acknowledge
the strong effect that monocular cues have in "seeing" depth.  

Most scenes, either real or artificial, have monocular depth cues that
Received: by bobcat.etsu.edu; id AA11257; Mon, 3 Feb 1997 18:53:55 -0600
Date: Mon, 3 Feb 1997 18:53:55 -0600
Message-Id: <97020400590033639@xxxxxx>
Errors-To: 3d-moderators@xxxxxxxxxxxxxxx
Reply-To: photo-3d@xxxxxxxxxxxxxxx
Originator: photo-3d@xxxxxxxxxxxxxxx
Sender: photo-3d@xxxxxxxxxxxxxxx
Precedence: bulk
From: photo-3d@xxxxxxxxxxxxxxx
To: Multiple recipients of list <photo-3d@xxxxxxxxxxxxxxx>
Subject: PHOTO-3D digest 1865
X-Listprocessor-Version: 6.0c -- ListProcessor by Anastasios Kotsikonas
X-Comment:   The Stereoscopic Image (Photo-3D) Mailing List  
Status:   

coexist happily with stereopsis.  In a pseudo stereo these cues conflict
stereopsis.  The image we finally "see" and the degree of our confusion
depends on how these monocular clues interact with the depth reversal in a
pseudo.  The most successful pseudos are the ones where the depth reversal
is compatible with the monocular cues.  The worse are those which are
incompatible, resulting in total confusion and inability to even "see"
depth in the worse case.

It is interesting to see how our brain deals with this conflict.  But it is
a stretch to say that this conflict is a natural event.

Take for example a computer wire model of a cube.  Using orthogonal
projection (parallel rays) we can generate a stereo pair that has zero
linear perspective.  Reversing the pair we still see a cube, only our view
point has changed.  Let's introduce linear perspective by using a central
projection.  The sides of the cube that are closer to the observation point
appear now larger.  If we reverse the pair we will now see an object of
unequal sides.  The sides that are closer to us are smaller and the sides
that are far away are larger.  This is _not_ a cube in my geometry books
and I see nothing indicating "bogus science" (even with smileys) here.

I am at a loss dealing with these comments, perhaps Larry can explain:

"Zero perspective results in the definition of flatness, no matter the
source for the image... Provide any image whether from a photo or generated
in a computer the proper perspectives and you have some form of stereo
image... ANY stereopsis can be viewed in either relationship. If there is
zero perspective change, both variations will appear identically flat.  Any
time a stereo image is reversed there is an equal reversal in all it's
stereo related aspects... The inverse of a cube is still a cube. It doesn't
become something else."

Perhaps we are using a different definition of "linear perspective"?

At the end, the stereo pair is a representation of either a real physical
object or a model of a physical object.  There is a definite choice of
what is right and what is left.  Even in the case of the simple cube wire
model without any linear perspective, the creator has selected the
appropriate observation point.  The reversed image while not in conflict
with any monocular depth information, still is not the one originally
created.  While there is depth information in the reversed image, this is
opposite to the depth information in the real scene so it is justified to
call it "pseudo" meaning "false", as opposed to the real or true image/
model.

George Themelis


------------------------------