a digital video key signal generator for digital video special effects uses two cascaded programmable lookup tables . one table is addressed by the two chroma component signals and the second table is addressed by the luma signal and the output of the first table . the generator is particularly suitable for use with ccir rec . 601 digital signals , and can be used for a wide variety of compositing functions , using luma keys , chroma keys , combinations thereof , and multiple chroma keys simultaneously .