Re: The Keystone Effect

[P3D Andrew Woods <andrew@xxxxxxxxxxxxxxxxxxxxx>]

Robert B. Linnstaedt (Linnstaedt@xxxxxxx) wrote:
> What!! ?? !! The term has to do with projection and perspective distortion.
> 
> As I understand, the keystone effect or "keystoning" refers to distortion
> created by projection onto a screen not parallel to the image plane.  Another
> way to state this is,  the light beam is not perpendicular to the screen.
>  The classic example is when one projects from too low, aiming the projector
> upward toward the screen.  A square image becomes narrow at the bottom and
> wide at the top; 

Keystoning can be produced both at the camera AND the projector.
If the two lenses are not parallel and the two imaging planes (film, CCD)
are not in the same plane, keystoning will be caused at the camera.  
See my paper "Image Distortions in Stereoscopic Video Systems" on my web 
page "http://info.curtin.edu.au/~iwoodsa/spie93pa.html" for an analysis.  
In this case however the narrowing of the images is either
on the left side (for one of the images) and the right side (for the other
image)

Keystoning caused by the camera could be removed by projecting the two 
images with the appropriate reverse projection keystone distortion.

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