Re: PHOTO-3D digest 1378

[P3D Neil Harrington <nharrington@xxxxxxxxxxxxx>]

Allan Carrano writes:

>     I thought I understood the placement of stereo windows.  Then I read 
>     recent posts (Wageman, Harrington,et.al.), and now I'm confused.
>     
>     I found myself initially understanding and agreeing with Greg 
>     Wageman's assessment:
>     
>     >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.  This binocular 
>     >disparity of the left and right sides of the window border causes the 
>     >window to have an apparent location in the scene closer than 
>     >infinity.  
>     
>     This relates to the reality of looking through a real window.  Stand 
>     in front of one in your home.  Close one eye and look.  Then close the 
>     other eye and look.  The right eye saw more of the left side of what 
>     was out the window and visa versa for the left eye.

Right.  Keep in mind, though, that the angles of view of your two eyes must
converge and cross _at_ the window in order for the effect you describe to
occur.  This is true regardless of whether your eyes themselves are
converged on the window or not.  That is, even if the axes of your eyes'
lenses are parallel, e.g. looking through the window at some distant object,
the angles of view of your two eyes through the window, the angles of view
_in their entirety_, must converge.  You can easily demonstrate this to
yourself with a diagram: draw two separate points for eyes and a line
representing a wall in front of them, with a gap in the line for the window.
Then draw lines from each eye through the ends of the gap, the "sides" of
the window, representing the full field of view for that eye.  As you can
see, it's only because the two fields of view converge and cross at the
window that the left eye sees more of what's on the right (beyond the
window) and vice versa.
    
>     Then Neil Harrington says:
>     
>     >...As long as the frame apertures are spaced slightly farther apart 
>     >than the lenses, the camera will have slightly converging angles of 
>     >view (taking each view as a whole) while the lens axes remain 
>     >parallel.   
>     
>     I would have thought the frame apertures would be spaced slightly 
>     closer than the lenses, so the left eye (lens) would see more of 
>     what's on the right and the right eye (lens would see more of what's 
>     on the left, as with the real world "window reality check" described 
>     above.

Another diagram will show you that what you describe couldn't possibly
happen.  Draw two points for lenses with lines behind them positioned to
represent the film frames.  Then draw lines in the obvious way to indicate
the overall field of view for each frame.  You can see that if the film
apertures are closer together than the lenses as you suggest, the left-lens
side will "see" more of the left, not the right, and the right side will see
more of the right.  Or to put it another way, the fields of view will
diverge instead of converging.  This is the opposite of what you want, of
course.
    
>     I even looked up John Bercovitz's web reference for the diagram of the 
>     apertures separation relative to the lenses, and sure enough the 
>     apertures are shown to be farther apart.  
>     
>     Am I interpreting lens and frame apertures incorrectly?
>     
>     What am I missing? 

I think drawing that diagram showing fields of view for each lens will make
it clear to you.



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