Re: Cardboard cutouts and backdrops...

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Fri, 29 Aug 1997 14:38:53 -0500
>From: "P3D Gregory J. Wageman" <gjw@xxxxxxxxxx>
>Subject: Re:  Cardboard cutouts and backdrops...

>John R. elaborated:
>>The usual suspects - motion (as was just discussed),

>Hmm, I'm not sure I'd call this a (physiological) "cue" exactly.  To me
>it seems more of a psychological expectation based on experience.  But
>it's certainly something to get "used to".

I tend to think of it as almost a continuum. There are a large number of
depth cues - some of them apparently actively suppressed from conscious
analysis (for example chromatic aberration and some other color/depth
cues, and possibly keystoning), some which normally work autonomously
but which you can pay attention to if you think about it (for example
disparity between left and right views), and others which make considerable
use of higher level thought processes.

>>accommodation (focus) coupled with convergence in a manner consistent
>>with natural vision,

>Is this really a cue that one must "learn to decouple"?  The fact that
>a stereo slide is planar while disparity is preserved produces the effect,
>but the implication of "learning to decouple" these is that one couldn't
>succesfully view a stereo slide in a viewer until then.

As the editorial in Information Display noted, there are degrees of 
"realism", and complete realism is unattainable by current techniques.
Cues based on focus are relatively weak, so it's possible for many people
to enjoy a stereo photo with no prior training, but there may still be
a slight feeling that "this isn't *quite* real".

>I think rather than "learning to decouple" what you are really saying
>is that, over time, we become familiar with the *forced* decoupling of
>vergence and accomodation, the lack of expected motion, etc.  If so, I
>agree. :-)

By decoupling, I meant that stereo photographs often provide conflicting
depth cues, which indicate different three-dimensional relationships to
different processing functions in the brain. If you can learn to pay more
attention to the cues that the photographer was able to control (i.e.
horizontal disparity), and less attention to the cues which don't quite
work (i.e. focus), you will probably enjoy your stereo viewing more.
That's probably about what you were saying. :-)

John R


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