Keyed, true-transparency image information combine

Abstract

A keyed, true-transparency combine and keyer receive prioritized image information signals and their corresponding input key signals. On the one hand, others have changed the order of the channels carrying the image signals as the priority of the image signals changes. On the other hand, here we interchange the order of a plurality of substantially identical keyer units within a keyer as the priority of the image signals changes. In interchanging the keyer units, true transparency processed key signals are also generated. In so doing that, the i-th keyer unit modifies the value of its input key signal Bk.sub.i using values of input key signals from higher priority channels. Thereby a true transparency processed key signal Pk.sub.i is generated for the i-th priority channel. In one embodiment, a key-taken signal is generated by multiplying a key-requested signal and a key-available signal while a key-now-available signal is generated by subtracting the key-taken signal from the key-available signal. In another embodiment, a key-taken signal is generated by subtracting a key-now-available signal from a key-available signal while a key-now-available signal is generated by multiplying the key-available signal by one minus the key-requested signal. In either embodiment, the generated key-now-available signal from a higher priority i-th channel is provided to a lower priority (i+1)-st channel as the key-available signal for the lower priority channel and the processed key signal Pk.sub.i is generated in response to the key-taken signal.