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Re: (meteorobs) Dark Adaption

At 08:01 2/14/2001 +1100, you wrote:
>Even within a static viewpoint
>there are many *cues* to depth in the visual field, from object overlap,
>surface texture to haze obscuration.

Ummm... let's take a really good example and go look at some NEAR pix of
Eros' surface. If you have no idea whatsoever how large any of those
features really are, then you cannot possibly begin to guess how far NEAR
was from the surface when the pic was snapped. THAT'S about as close to a
"static" 2D example as we're likely to come, I suspect.

  But more importantly, vision is a
>dynamic process; we view from a moving platform.  Thus dynamical parallax
>information gives a powerful direct cue to the third dimension.  Animals
>that do not have binocular vision use head movements to determine distance.
>While it is true that this is neither as certain or as immediate as
>binocular vision, it is incorrect to imply that single eye vision is
>strictly 2D. Even within the retina, computations are performed, so what we
>see, even with one eye from a static location, IS more a model than a
>photograph.

Actually... I rather suspect that what really happens is that the brain
shifts the eye's focal point back and forth in an effort to gain an idea of
distance. If the brain receives conflicting information in the form of a
totally unfamiliar object whose dimensions are completely unknown from
memory, it has no idea, except by changing the focal point, just how far
away, or how large, the object is. A good example of this is when we look
through a telescope at a known close-by object: by manually shifting the
focus back and forth, we eventually gain an understanding of the distance.
Then by comparing the conceived distance to the size of the object, we gain
an understanding of the object's size. Our brain does the same with our
eyes. Anybody remember waking up really, really drunk, or perhaps stunned
by a fall or blow to the head? Remember how long it took you to "focus" on
a familiar object? Until that "known" object, with known size and distance,
actually produced the right "numbers" in your head, you felt totally out of
it, right?

>To make this more on topic, when we look at the "starry dome" we are seeing
>a 2D surface.  There are NO cues to depth to the human eye.  It is often
>assumed that a meteor is also a 2D phenomenon because we cannot utilise
>binocular vision over such huge distances.  However I contend that there
>*are* cues to depth, and that these tend to be used correctly in the most
>part.

Yes, I agree.. but I propose that the PRIMARY cue, for such 2D objects for
which no known distance or size information is previously available, is the
brain's attempt to shift focus until the object makes sense.

>Again,
>if the assumption is that an object doesn't change size, then the apparent
>size on the retina is a cue to relative distance.

Umm.. I propose the major cue is the brain's foreknowledge of where it has
to shift the eye's focal point.

  However the information
>on angular velocity alone can be used to calculate orientation (the radiant) 
>(and Pete Gural and I have written programs to do this).

Yes, but with only one source of information (that is, a single "eye"), how
do you differentiate between one object moving x degrees across the FOV and
another object but at a very different distance, both of which present the
same size to the retina and the same angular movement?

SteveH
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