Recently I saw this documentary on youtube that showed the actual process of creating one of the scenes from Return of the Jedi. They showed a person operating an optical printer. But there is something I didn't quite understand about the process. Please take a look at this clip and I'll explain what I don't understand:
Around 4:55 you can see the optical printer superimposing the mask and the foreground footage. Before the mask aligns you can see the blue screen. I thought that negatives are composited together in the optical printer. If you can clearly see the blue color, this can only be a print. Either that or they adjusted the colors in this video so you can see the actual image (but I doubt it, since this documentary was also shot on film).
Also take a look at 2:50.
The person is creating a matte box for this shot with cards. I remember a couple of instances in SW (in older video transfers) where you can actually see this box around the ships (the black changes it's level in that area). But what is this person putting the cards in front of? Is it a screen of some sort? Is this rephotographed from a rear projected screen or something? It can't be some kind of a gate because it's too big. And again, the image behind is clearly a print. Did they copy prints unto a new piece of negative stock? Wouldn't that cause gamma issues?
If it's true that they used prints in optical printers, why is that? Wouldn't it be better to compose low gamma negatives unto a fresh piece of intermediate stock (gamma 1?) in order to preserve the contrast? I don't get the gamma math here.