Re: Stereogram on the "Stereo Window"

[P3D Gregory J. Wageman <gjw@xxxxxxxxxx>]

>Greg Wageman is using the disparity to explain the stereo window.
>This explanation is easy to understand in projection (or the computer 
>screen) where the two images overlap.  In a viewer things are different.

It also applies when free-viewing a Realist slide.  What then changes
(besides the interposition of lenses) when using a viewer?

>I agree that the window is located at screen level and it is not moved.
>However it has moved with respect to the scene and changed in size.  

I contend that the scene has moved with respect to the window, and it
most definitely has NOT changed size.  Given that the chips are not
moving either closer or farther, the angle subtended by any object
on the chip cannot have changed and since magnification is also
constant therefore NOTHING has changed in size.  All you've done is
move where the corresponding image points fall on your retina.  You've
altered disparity, and hence apparent depth, but not the *actual* size.

>Greg et al. say that the window is fixed in real space (and I am
>asking where?  7 ft., 15 ft., somewhere else?) and the scene has
>moved with respect to the window.  It is hard to accept that the
>rock has moved closer to the camera without increasing in size
>and depth and tha the mountains have moved in half infinity.

For the projection, computer and freeviewing cases, the real window
is at the screen distance or mask distance.  For viewing, it's
more complicated because there are additonal lenses involved besides
the eye's, and I freely admit I have no idea how to calculate
it.  It clearly doesn't appear to be at the 'real' distance, since
I can't focus that close, but it doesn't appear to be at 7 ft.,
either; more like 1-1.5 ft.

The rock hasn't moved "closer to the camera".  It still subtends
the same angle on film/screen as before.  It seems like a paradox
but really it isn't.  You're playing with the geometry of those
"converging cones" created by aperture masks and centers of
perspective.

>I accept the fact that by putting the window at 15 feet I reduce
>the separation of infinity points and have my eyes converge when
>looking at infinity.  However this does not make the mountains be
>any closer to me than infinity.  Convergence is overriden by
>other stronger cues.

I agree.

	-Greg W.


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