P3D Re: "Too much depth"

[abram klooswyk <abram.klooswyk@xxxxxx>]

The art of boring is to say everything (a quotation),
and the art of confusing is to omit essentials, I add.
I'm afraid I sometimes combine both arts.

Chris Jones wrote (P3d 3574, 31 Oct 1999):
>(...) it seems that different mechanisms dominate
at different distances.
>(...)
>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.

>>(That stereopsis is independent of eye movements is known
>>since the mid 19th century).

>It may have been what was suspected but does not appear to be the case.
For
>a much clearer (and more detailed) explanation than I have been able to

>provide, you might wish to try reading an article "Neural mechanisms
>underlying stereoscopic vision" by F Gonzalez and R Perez (Progress in
>Neurobiology, 55 (3) 191-224, June 1998).

This friendly chat almost seems to deteriorate in a scientific
discussion (:-)).

First, depth and distance perception is not the same.

Depth perception or stereopsis is seeing distance *differences*,
relative distances, it depends almost entirely on horizontal
disparity in both eyes. It is expressed as an angle.
Two points at different distances in space are separated by
an angle, but the stereopsis mechanism doesn't know how far
away they are absolutely, for a certain metric difference in
distance at for example 3 meter gives the same binocular
disparity angle as twice that difference at 6 meter from
the eyes.

Distance perception (or "egocentric distance" perception)
classically was supposed to depend mainly on convergence
perception, which was believed to set the scale for the
stereopsis mechanism, so that the visual system as a whole
also knows how great the differences in distance are in
length "units".

Convergence perception still seems to be regarded as a factor
in this scaling process, but this last decade it turns out
that other factors, especially vertical disparity, are
more important, see for example:
Rogers BJ, Bradshaw MF "Vertical disparities, differential
perspective and binocular stereopsis",
Nature 1993 Jan 21;361(6409):253-5.

An abstract of this article, and also of the one quoted by
Chris Jones, can be found online:
http://www.ncbi.nlm.nih.gov/PubMed/
If you followed the links [Related Articles] after you have
found the first, you soon get buried in abstracts of
similar research (PubMed is a great service to the world).

Gonzalez F, Perez R in their abstract say:
>>(...) Because horizontal disparity alone is insufficient
to compute reliable stereopsis, additional information about
fixation distance and angle of gaze is required. Thus, while
there is unequivocal evidence of cells capable of detecting
horizontal disparities, it is not known how horizontal
disparity is calibrated. Sensitivity to vertical disparity
and information about the vergence angle or eye position may
be the source of this additional information. <<

Erkelens CJ, van Ee R ("A computational model of depth
perception based on headcentric disparity".
Vision Res 1998 Oct;38(19):2999-3018 ) are less reserved:
>>It is now well established that depth is coded by local
horizontal disparity and global vertical disparity.<<
(This is only a summary in an abstract of course)

Not long ago Jim Crowell, who works at the School of
Optometry, University of California, Berkeley,  used
to answer vision physiology questions on this list.
(see the Archives, http://www.calcite.rocky.edu/photo-3d/)
He was co-author of a recent article on the subject here
discussed:
Backus BT, Banks MS, van Ee R, Crowell JA, Vision Res 1999
Mar;39(6):1143-70 , "Horizontal and vertical disparity, eye
position, and stereoscopic slant perception."

They say:
>>The slant of a stereoscopically defined surface cannot be
determined solely from horizontal disparities (...)
There are four other signals that, in combination with
horizontal disparity, could in principle allow an unambiguous
estimate of slant: the vergence and version of the eyes,
the vertical size ratio (VSR), and the horizontal gradient
of VSR. (...)
We found that VSR and sensed eye position are both used to
interpret the measured horizontal disparities. When the two
are placed in conflict, the visual system usually gives more
weight to VSR. However, when VSR is made difficult to measure
by using short stimuli or stimuli composed of vertical
lines, the visual system relies on sensed eye position.<<

In the research there is no indication in my opinion "that
different mechanisms dominate at different distances", except
for the obvious geometrical facts, such as the fact that
stereopsis degrades quadratically with distance.

Chris Jones:
>The distant case I described is known as "quantitative" or
>"fine" stereopsis, and the close-up case is "qualitative"
>or "coarse" stereopsis.

(Angular) stereoscopic acuity doesn't vary much at different
distances.
Close-up the fine stereopsis works too, fortunately!

Abram Klooswyk