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Re: (meteorobs) Dark Adaption
> depth perception
> (viewing things in 3D) depends entirely on having 2 eyes, e.g., 2 slightly
> separated points of view. With only a single viewpoint, your vision
> becomes 2D - flat, and unless you have, in your head, a very good model of
> wherever you happen to be (tough to do in the dark!) in order to
> compensate for the loss of depth perception you will crash into things,
> take missteps, and fail to grasp things on the first try.
The above statement isn't quite true. It would be mostly true if it said
"with only a single STATIC viewpoint". Even within a static viewpoint
there are many *cues* to depth in the visual field, from object overlap,
surface texture to haze obscuration. 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. People who develop from birth with only one functioning eye,
have quite adequate depth perception from use of these cues. Losing use of
an eye later in life presents varying degrees of handicap.
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. The cue is changing angular velocity. If one *assumes* a constant
3D velocity, (and with no other information this seems to be be a basic
unconscious assumption in visual perception), then the changing angular
velocity gives precise information on it's orientation in space (its
radiant). In the real world (not that meteors aren't real!) there is
usually a paired phenomenon, that of changing size of the object. 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. 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). Unfortunately
the basic assumption is false in the later stages when deceleration becomes
marked. In the early stages however, where deceleration is much less marked,
the assumption is basically true. In most cases where a fireball eye
witness has indicated to me a 3D direction of motion for the fireball,
this has turned out to be basically true. I should add that I have never
had this experience. Clearly the only cue in this case would be changing
angular velocity and demonstrates this it can actually be utilised in
human visual perception.
There are various experiments that demonstrate the use of angular velocity
as a cue to the third dimension. In addition to orientation, the time
of closest approach comes out of these calculations. Diving gannets,
babies blink response to an approaching object, and other phenomena are
demonstrations of this in part.
Cheers, Rob
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