Re: eyes & keystoning

[T3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Thu, 29 May 1997 14:32:40 -0500
>From: T3D John Bercovitz <bercov@xxxxxxxxxxx>
>Subject: eyes & keystoning

>John R writes:
>> Come to think of it, I wonder whether the human 
>> visual system ever uses keystoning as a depth cue.

>I don't know.  It still amazes me that we can turn 
>our eyes and our heads and not get dizzy.  8-)

>So how do you visualize keystoning as being a depth
>cue?  

Another thought experiment:
Suppose you (as an observer) are holding your head rigidly in place and 
upright, and a featureless, inflexible ribbon (parallel edges) is strung
in front of your face, horizontally, and 1-2 feet away. The ribbon is so
long that it disappears out of your field of view in both directions.
There is no feature on the ribbon to catch your attention, to judge the
degree of convergence. I think by the "classical" models of stereo vision,
you wouldn't really have any way of judging how far away the ribbon is.
(Well, you might have the accommodation of the lenses of your eyes - perhaps
the edges of the ribbon could be made somewhat "fuzzy" to reduce the
usefulness of that cue.)

But if your eyes are converged at some distance closer than infinity,
they will pick up a keystoning effect on the edges of the ribbon. The
amount of keystoning will depend on the distance to the object and the
degree of convergence of the eyes. Some very sketchy experiments a few
minutes ago indicate that it could possibly be useful as a depth cue.
(The challenge, of course, is to eliminate other depth cues, so that only
keystoning remains as a possibility).

The human visual system is obviously used to dealing with keystoning when
viewing nearby objects in stereo - the question is whether there's any
circuitry in there that specifically takes advantage of the phenomenon.

Does any of that sound plausible?

John R


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