METHOD AND APPARATUS FOR APPLYING CORRECTION TO A SIGNAL USED TO MODULATE A BACKGROUND VIDEO SIGNAL TO BE COMBINED WITH A FOREGROUND VIDEO SIGNAL

An apparatus and method which control the gain of the key signal dynamically and thereby compensate for the effects of an unevenly lit backing or lens vignetting. This dynamic control occurs over a video frame and has the effect of modulating the key signal. In this connection, there are four possible gain controls as follows: a) a horizontal ramp to increase the level of either side (left or right) of the key signal; b) a vertical ramp to increase the level of the top or bottom of the key signal; c) a horizontal parabola to raise or lower the gain of both sides of the key signal simultaneously to nullify lens effects; and d) a vertical parabola to raise or lower the gain of both the top and bottom of the key signal simultaneously to nullify lens effects.