Wonderful, thank you very much. I almost forgot I started this thread 6 years ago, and you decide to revive it on my birthday 😃 Great information, thank you. Could you maybe be go to more details with the optical printer part? I got information from several different websites, and they usually say mutually exclusive things. I assume different companies have used different processes or different processes were used in different eras?

Let’s examine a case I mentioned here 6 years ago, feel free to correct me (PLEASE do 😃 )
It’s 1976 and we want to film a shot in which we see our heroes in the cockpit of the Millennium Falcon watching outside, where they see space, the Death Star and the Tie Fighter.

So we need these elements

shot of the Falcon interior, in front of a blue screen
shot of the star field
shot of the Death Star, assuming again in front of a blue screen
shot of the Tie Fighter in front of a blue screen (I assume)
correct?
Now we need 2 mattes for every one of these elements, correct? How do we get them? I read something about rephotographing the bluescreen footage over a red filter onto a high contrast black and white film, and making negative copies of this again on high contrast black and white film, until we get 2 mattes - one has white parts where the original object was and black where the blue screen was, and the other is the negative of that, meaning it has black over the original object (e.g. ship) and everything else is white. Is that correct? Also should they be “white” or “transparent”? Or does white color become transparent during the compositing?