I believe that predated digital compositing being used. However electronic analog compositing is still much easier than film compositing (like what was used to

I believe that predated digital compositing being used. However electronic analog compositing is still much easier than film compositing (like what was used to combine passes for Star Wars). For instance, blue and green screen was originally for video. You would do it using an Ultimate brand device that was completely analog. That device launched in 1976, but it's creator had been building a analog compositing devices since at least 1964 when he won an Oscar for it. If you are curious about how it worked, I would imagine patents can be found and read. Ultimatte just generate an alpha channel (then called a key and it was actually a separate b/w video signal). Other devices would act in the key to combine the foreground and background together, etc.