Re: PHOTO-3D digest 1377

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

Neil Harrington writes:

>>If I understand it right, the offset lenses simply cause the left image to
>>have more information from the right side of the scene, and the right image
>>from the left side of the scene.  

>Beyond the window, yes.  You don't see that that means their angles of view
>must converge?  (Of course I mean angles of view as if seen from the center
>of each frame.)

As John Bercovitz explained it, lines drawn from the center of the image
through the center of the lens converge at the window, and then
diverge.  I guess what I have a problem with is your use of the phrase
'converging angles of view', which seems to imply some kind of angular
rotation, when there is in fact none.  Yes, it is a matter of semantics,
but sometimes the choice of semantics makes things much easier to
exlain to a neophyte.  I prefer to think of the offset as a selective
masking at the film plane.

If you think of it as the *apertures* being shifted *outward* from
the center of the lenses, and remember that the images are inverted, it
is easy to see the shifting the left aperture to the left causes that
image to have more information from the right side of the scene.  If
the lens didn't invert the image, shifting the left aperture left would
cause it to capture more information from the *left* side of the scene,
contrary to what you assert in another message.

Let me try a painful ASCII diagram:

    ---------------
    | |         | |
    | |  \ /    | |
    | |   |     | |
    | |  ---L   | |  "L" represents the center of the lens
    | |   o     | |
    | | r   l   | |
    ---------------
    ^ ^         ^ ^
    | |         | |
    | unshifted | unshifted aperture (lens centered)
    shifted     shifted aperture (lens offset)

Doing that on both sides in opposite directions creates the binocular
disparity that positions the window closer than infinity.

I mistakenly implied that one could set the left and right window borders
at different distances by offsetting the apertures by different amounts.
This is incorrect, it would simply result in conflicting disparity between
the two images, and retinal rivalry.  To offset the left and right sides of
the windows by different amounts, the apertures would have to be wider or
narrower on one side with respect to the center of the lens before shifting.

	-Greg


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