Keystone correction is needed when a projector is mounted in a position that is not in line with the screen. When a projector is not perfectly in line with the screen, the image appears trapezoid. With keystone correction, the projector will skew the image vertically or horizontally to correct the trapezoid image back to a rectangular image on the projection screen. It's a digital process where the projector uses scaling and compression to alter the image.This will result in compromised image clarity.