P3D review of stereo reconstruction errors

[john bercovitz <bercov@xxxxxxxxxxx>]

Ortho is such a catch-all word I thought it might be worthwhile 
to review the most common errors which cause non-orthoness 8-).

1) Perspective change
This is viewing from the wrong distance.  It causes a change in the
third or depth dimension but not the other two.  So if you step back 
from a screen, all dimensions (including the distance from you to any
object in the scene) stretch in proportion to how far back you step.

2) Stereobase change
This is a mismatch between the separation of your eyes and the 
separation of the camera's lenses.  If the camera's lenses are 
farther apart than your eyes, when you view the results, you 
will see what a giant sees because a giant's eyes are farther 
apart than yours.  In other words, the scene and everything in it 
will appear smaller than normal in direct proportion to the amount 
of mismatch.  

Stereobase mismatch is not fatally harmful in itself in my opinion 
because it does not distort or warp space; it merely rescales it.  
In that sense it can be very useful if one is aware of it and uses 
it intelligently.

3) Toe-in
This comes from not having the two image planes in the camera
co-planar.  If you have two cameras making your stereo camera, 
and you angle them in at a nearby object, then the reconstructed 
space will be bulged towards you in the middle.
See: http://info.curtin.edu.au/~iwoodsa/spie93pa.html

John B


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