P3D Re: "Too much depth"

[Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

  I 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.

  Chris Jones replies,
>Those two statements don't match. And what is stereopsis predominating
>over? Monocular depth perception? 

I'm not sure why you think they don't match. In my second statement, the
different mechanisms I'm referring to, are not "stereoscopic" depth cues,
but rather monocular depth cues. 

  I wrote,
>>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.

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

I'm not following this. The parallax you refer to is a retinal disparity.
At close distances it's still a retinal disparity. Therefore distance doesn't
come into play. Granted at farther distances the parallax is probably more
important but the "key" word is that they're BOTH retinal disparities. This
retinal disparity simply is the difference in perspective regardless if the
disparity is within the object or the edges of the object (as in parallax).

  I wrote,
>>>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.

  Chris replied,
>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.

That's interesting but I haven't seen any examples or proof of this yet
(that it is a different process). Wouldn't it still be retinal disparity
at play here?

>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).

I don't know about that. I do see a significant difference. 

>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.

I'm not sure why it wouldn't be relevent. Even a static stereo snapshot
would yield much more depth information than viewing the real pencils
with one eye. As for the focus I'm not sure this is a significant factor
in this particular case even if it's in close range.

> 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?

Yes, I think so.

>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.

Unless there is some new developments in perception research, all I know
is that motion depth cues are indeed very strong depth perception cues, but
this is known as a monocular depth cue (since you can do it regardless
if your using one or two eyes).

>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.

I am all ears, or should I say all eyes!

Gabriel