P3D Re: Curious eye defect

[Andrew Woods <andrew@xxxxxxxxxxxxxxxxxxxxx>]

Jim Norman <JNorman805@xxxxxxx> wrote:
>...So he gave me a test involving the use of polarized
>glasses and transparency overlays to check just how good my stereo perception
>really is.  The first involved a macro picture of a fly, enlarged to perhaps
>three inches long.  
>... Then he asked me to
>state which of about six little figures stood out from a line of about six
>such figures.  On line "C"  I perceived only a flat line of figures.  On line
>"B" I perceived one figure very slightly ahead of the others, whereas on line
>A, one figure stood out prominently.

This sounds like the "Titmus Fly Test" - a black and white vectograph
produced by Stereo Optical Co. (Chicago, IL) for use by opthamologists
for binocular vision testing and training.

Except that the version I saw had a different arrangement of images for the
second half of the test than you describe.  The version I saw had 'I think'
nine sets of 4 filled circles.  The circles were arranged together as if they
were on the four points of a diamond.  Each of the sets had one circle which 
stuck out from the rest.  From Set 1 to Set 9, the amount by which the
odd-circle-out gradually reduced.

I tested 6 Underwater ROV (Remotely Operated Vehicle) pilots with this
test and they all scored 7 or above on this test.

>The doc asked me to reach for the wing of the fly, which
>protruded, I sensed, about a half-inch from the "window".  
...
>He said that people with normal stereo vision reach for the fly's
>wing as if it were six ot eight inches off the page, and have no trouble
>seeing one figure in each of the three lines of figures standing way out in
>front.

I find it hard to understand why people with intermediate stereo vision would
perceive the wing at an intermediate distance.
If the person had no stereo-vision, the fly would appear flat.  
If the person has normal stereo-vision, simple geometry could be used
to determine where the wing appears (as a function of the wing's horizontal
parallax, the viewing distance and to a lesser extent the person's eye
separation).
Surely with intermediate stereo-vision, the wing would still appear
in the same place but the person's stereo-vision would have less depth
resolution - as is measured by the rows of figures.

Andrew Woods           http://info.curtin.edu.au/~iwoodsa

27 days to go to the Stereoscopic Displays and Applications Conference
       http://info.curtin.edu.au/~iwoodsa/stereoscopic


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