Re: Convergence as a cue to depth perception

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Tue, 17 Jun 1997 21:31:23 -0500
>From: "P3D Gregory J. Wageman" <gjw@xxxxxxxxxx>
>Subject: Re:  Convergence as a cue to depth perception

>John R wonders:

>>If you see an object (say 100-300 feet away) with no closer or more distant
>>cues (for example against a clear blue sky), does it become extremely difficult
>>to estimate the distance? ...

>A while back I travelled to Amarillo, Texas on business.  Behind the
>motel where I stayed was rangeland.  There were some cows in "the distance",
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>and beyond that a string of power poles of the TxT type (crossbars atop
>poles, the poles being interconnected by crossbracing) going out to the
>horizon.  There were no other objects to give a sense of scale.
>...My depth perception completely failed to convey the relative distances
>between myself, the cows and the power poles.  The powerlines were
>actually *miles* away, and the cows relatively close (~300 feet).

A very clear and vivid description!

>So to answer the question, I believe even WITH a more distant cue, it
>is very hard to estimate distances over a few hundred feet away.

I'm inclined to agree, barring lateral motion on the part of the observer.

Some not-very-well-controlled observation of fireflies at night indicates
that I feel pretty confident of the distance out to about 50-100 feet -
beyond that the estimate of distance is not very precise. During daytime,
additional cues are available, and distances can often be judged more
accurately (but not always, as in the case you describe).

In previous studies of the relative roles of convergence and retinal
disparity, I wonder whether the scanning mechanism of human vision (and
human stereo vision) is taken into account. When I view a stereo photo,
I may see depth in parts that I haven't converged on, but if I take a while
to study the photo, I'll probably eventually converge my eyes on just about
every major item in the photo, in the process of building up a 3-dimensional
mental image. Modern eye-tracking devices are probably capable of observing
3D convergence in viewing a scene, but I don't know whether anyone's used
them for that.

John R


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