Keystone correction
From Wikipedia, the free encyclopedia
Keystone correction, colloquially also called keystoning, is a function that allows multimedia projectors that are not placed perpendicular to the horizontal centerline of the screen (too high or too low) to skew the output image, thereby making it rectangular. Without such correction the image will appear as a horizontal trapezoid, the shape of an architectural keystone; hence the name of the feature.
It is often necessary for a projector to be placed in a position outside the line perpendicular to the screen and going through the screen's center, for example, when the projector is mounted to a ceiling or placed on a table top that is lower than the projection screen. Keystone correction is a feature included with many projectors that provides the ability to intentionally "distort" the output image to recreate the original rectangular image provided by the video or computer source, thus eliminating the skewed output that would otherwise result due to angled projection.
It is quite rare for a projector to have the ability to correct horizontal keystoning distortion (too far left or right of the vertical centerline of the screen). Typically this is easily corrected by moving the projector left or right as necessary.
[edit] Functionality
In modern projectors keystone correction technology is performed digitally (rather than optically) via the internal (LCD) panels or (DLP) mirrors of the projector, depending on the technology used. Thus, when applying keystone correction to an image, the number of individual pixels used is reduced, lowering the resolution and thus degrading the quality of the image projected. Home theater enthusiasts would argue that keystoning should not be used because of the impact it has on image quality. However, it is a useful technology in scenarios where the projector cannot be mounted directly in front of the screen, or even on projectors utilizing lens shift technology where the projector must be mounted outside the frame of the screen.
