EJones216 said:
It’s about generating the cleanest possible matte on bluescreen, which in opticals isn’t as forgiving or adjustable as electronic/digital. Once that’s done, the additional passes can bring the element closer to its intended look without needing to worry about corrupting the matte. (this multi-pass technique is still done today in CG effects though I imagine for different reasons, as well as Laika’s stop-motion films which do separate exposures with and without greenscreen to have both the matte and no green spill)
This was true for Vinton/Laika in the early 2000’s as well, when their animation was still mostly shot on 35mm but posted on video. The post company in Portland that I worked for at the time did the Vinton/Laika telecine transfers. It was common for VFX shots that would require later Flame compositing, to be shot on a motion control rig with a beauty pass, a matte pass, a practical light pass (if applicable), and a background plate pass. That was for shots that had repeatable camera moves and little-to-no hand-animation such as static miniatures or product photography. I don’t think it was done much in those film days, but for hand-animated shots requiring both beauty- and matte-passes, the lighting would have to be alternated between animated frames and the camera would shoot two frames per pose. The beauty frames and the matte frames would have to be sorted and grouped together in post.
