Intrinsic/extrinsic parallax

[T3D Larry Berlin <lberlin@xxxxxxxxx>]

john bercovitz writes:
>Joel just put up a larger version of the drawing showing
>the geometry that results when you pull an object in closer.  
>You can view the drawing or retrieve it from:
>
>http://www.frii.com/~rkymtmem/tech3d/inxp.gif
>
>Bear in mind this is an extreme case done to make it clear 
>to me the nature of the distortion that results.  Notice 
>that in both the before and the after cases the objects are 
>very close relative to the separation of the eyes.  That is,
>(distance)/(eye separation) is not a large number.

This is an interesting drawing but it departs from the discussion in several
ways. For starters we are discussing moving a close object to a farther
depth in the scene which is an opposite movement to that depicted. For
another, the two objects remain the same relative size unless the more
distant one is deliberately shrunk to scale its effect. (extreme cases)
Since this drawing has such a large change in scale of the two objects it's
hard to determine anything about relative shifts of a similar sized object.

Perhaps an easy way to visualize this is to picture a relatively small
stereo pair image that uses an infinity point of significantly less than the
interocular distance. Then spread the two images farther apart but stay
within the interocular distance. What distortions happen to the image? Are
they perceptible to the eye? Other than the fused pair appearing farther
away, the image seems appreciably the same image.

Larry Berlin wrote:
>>> Intuitively the object that is moved deeper into the image will have a
>>> relative object Z dimension greater than would ordinarily be observed.

John Bercovitz writes:
>>I agree.  In fact if just look at the geometry I drew, and go from the
>>right view to the left view instead of from the left view to the right
>>view, that's what you'll see.

********** Of course an exaggerated Z dimension was the stated objective
originally. This compensates for the apparent flatness that the camera
records which the eye in real situations can compensate for.

I still find the size disparity between the objects in the drawing gets in
the way of visualizing this. Especially since the size change is opposite
that which I might use were I rearranging image objects. (larger close up
and smaller farther away)

Larry Berlin


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End of TECH3D Digest 41
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