Re: Stereo Window and Convergence Examples

[P3D Jim Crowell <crowell@xxxxxxxxxxxxxxx>]

At 9:48 AM -0700 6/24/97, P3D John W Roberts wrote:
>   "There is a clear linkage between two of the depth signalling mechanisms
>   we have described - (1) the convergence of the eyes serving as a *range
>   finder*, and (2) the difference between the two images giving *disparity*.
>   The angle of convergence adjusts the scale of the disparity system. When
>   the eyes fixate a *distant* object, disparities between the images are
>   seen as representing *greater differences in depth* than when the eyes
>   are converged for near vision.
>
>   "If this did not occur, distant objects would look closer together in
>   depth than near objects of the same depth separation, for the disparity
>   is greater the nearer the objects."
>
>I think part of the problem is that there has been no effort here to
>rigorously define "depth perception". The "convergence is weak" crowd seem
>to be implicitly assuming that "depth perception is nothing more or less
>than the ability to sense that one object is nearer or farther than another
>object". The author cited above appears to take the view that depth perception
>is the ability to judge the distance to an object. I would tend to agree more
>with the author of "Eye and Brain" - I think of depth in terms of distance
>rather than order of proximity.

It's definitely a good idea to keep the distinction between absolute &
relative distance clear...however, my understanding is that vergence state
doesn't have a very strong effect on perceived absolute distance either.
Certainly that agrees with my experience.

To really get at the effect of vergence you need somethine like a
haploscope (with a mirror slanted at roughly 45 degrees in front of each
eye, so your right eye is looking at an image off to the right & left eye's
object is to the left).  That way you can move things around & change your
vergence angle without altering the retinal images in any way (or you can
alter them in a controlled way independent of vergence, if you want to look
at interactions with other distance cues).

BTW, while the Gregory book is a good introduction to these issues, relying
on him as a final authority is probably not a very good idea; he's
definitely not current.

-Jim C.

----------------------
Jim Crowell
Caltech Division of Biology
216-76
Pasadena, CA
Tel: (818) 395-8337
Fax: (818) 795-2397
jim@xxxxxxxxxxxxxxx



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