P3D Re: "Too much depth"

[Chris Jones <c.jones@xxxxxxxxx>]

At 10:33 01/11/99 -0700, Gabriel wrote:
>The brain processes "stereo" information the same way, regardless of
distance.
>
<later>
>
>I would agree that there are different mechanisms at work at different
>distances but stereopsis would predominate in all cases.

Those two statements don't match. And what is stereopsis predominating
over? Monocular depth perception? 

>>This is pretty much common sense, since it gradually becomes less seeing
>>the same object displaced laterally against the background and more a case
>>of seeing two very different views of the same object.
>
>In stereopsis, the exact same mechanism is at play, that is, disparity, 
>regardless of distance. In the close-up example you give above, the two
>very different views of the same object is still a disparity factor.

Yes but the distinction I was making was that the close-up example isn't
parallax anymore.

>>For close objects the mechanism appears more complex, and appears to rely
>>more on vergence movements, proprioception (here the feedback of eye
>>position) and some implicit interpretation of the object's structure.
>
>Stereopsis still predominates.

But there is more than one kind of stereopsis. The distant case I described
is known as "quantitative" or "fine" stereopsis, and the close-up case is
"qualitative" or "coarse" stereopsis. It's a different process even if the
results are broadly similar. And can arise from different cues.

>>For very close objects, you can get a pretty good idea of depth just from
>>shadows and perspective through a single eye.
>
>Not true. Try this. Hold up two pencils with the leads pointing toward
>each other. Close one eye and try bringing them together till they touch.
>Now try it with both eyes. Which way was easier?

The latter, of course :)

I should have been more specific - I was referring to the example
previously given of wrinkles on a hand, for which I really don't notice a
significant enhancement from using two eyes instead of one (with eyes still
trying to hold focus).

However for your example, do you touch the pencils while keeping your eyes
still at constant focus? My guess would be no. But more than that - the
example you give isn't relevant to still stereo photography, which is what
I was (not explicitly, admittedly) referring to. More on that below, but
consider still photography - would a "natural" snapshot of moving two
pencils together at a small distance be useful, or make a "good" image?
It's unlikely you'd be able to fuse much of it.

Now from my research it also seems that not only are there fine and coarse
methods of static stereopsis, but a different mechanism for dynamic
stereopsis where objects are in motion. Velocity cues are brought into
play, and it appears the brain has a particular ability to combine velocity
information from both eyes to implicitly reconstruct a velocity vector in
3D space.

I'm afraid I haven't read very extensively on dynamic stereopsis (it's
outside my field of research) but I have read enough to see that it uses
neural mechanisms not used in static stereopsis. If you're interested
however, I can pass on some references to it given in other articles.



-- 

Chris Jones 
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