P3D Re: Depth perception cues

[Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

John Roberts wrote (awhile back):
>Is "interposition" broken down any further?

This is with regards to the major sources of depth
information from the book, "Perception." They didn't
break it down further. They do mention...

 Interposition: When one object occludes, or obscures,
 part of another from view, the occluded object is 
 automatically perceived as the more distant one. This
 cue is called interposition, and from the standpoint
 of pictorial representation, it is probably the most
 primitive. There is evidence that by 7 months of age
 human infants can judge relative distance solely on
 the basis of interposition.

 Interposition is such a strong depth cue that it can 
 override retinal disparity when the two cues conflict.

The last line I find interesting. I wonder how this
effects the perception of pseudo-stereo.

I've observed an example of how interposition can
override disparity cues when viewing Pulfrich 3-D. As
most know, Pulfrich is basically stereopsis vision (it
isn't a motion parallax cue). It uses motion and
attenuation to obtain disparities and thus binocular
vision. An interesting kitchen (or rather computer room)
experiment that one might want to try that I believe
shows interposition overriding retinal disparity (at
least to me) is as follows.

First get a pair of sunglasses (or even anaglyph
glasses) and cover the right eye with one side of the
glasses, leaving the left eye unobstructed. Move the
cursor from left to right on a blank area of your
screen. You'll notice it float off the screen. Doing
this over the text, you'll notice the same effect.

Now move the cursor right to left on a blank area.
This time the cursor will seem to recede. Everything
so far seems fine. Now move it over the text. One
might think that it will still recede but this won't
be the case. The cursor won't seem to recede because
it occludes the text and the brain overrides any
disparity information. 

Gabriel



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End of PHOTO-3D Digest 3246
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