alt Hacker NewsWith regards to the sodium vapor process, an idea has been percolating in the back of my head ever since I saw that video. But I don't really have the budget to try it out.
theory: make the mask out of non-visable light
illuminate the backing screen in near Infra-Red light. (after a bit of thought I chose near-IR as opposed to near-UV for hopefully obvious reasons)
point two cameras at a splitting prism with a near IR pass filter(I have confirmed that such thing exists and is commercially available)
Leave the 90 degree(unaltered path) camera untouched, this is the visible camera.
Remove the IR filter from the 180 degree(filter path) camera, this is the mask camera.
Now you get a perfect non-color shifting mask(in theory), The splitting prism would hurt light intake. It might be worth it to try putting the cameras really close together , pointed same direction, no prism, and see if that is close enough.
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This approach was used in the 1950s/60s with ultraviolet light (rather than IR) to create a traveling matte. I'm not sure why visible-light techniques won out. Easier to make sure that the illumination is set up correctly, maybe?
Don't humans and other warm objects also radiate IR?
I'll do you one better, which requires no special cameras (most have IR filters) nor double cameras or prisms.
Shoot the scene in 48 or 96 fps. Sync the set lighting to odd frames. Every odd frame, the set lights are on. Every even frame, set lights are off.
For the backing screen, do the reverse. Even frames, the backing screen is on. Odd frames, backing screen is off.
There you go. Mask / normal shot / Mask normal shot / Mask ... you get the idea.
Of course, motion will cause normal image and mask go out of sync, but I bet that can be remedied by interpolating a new frame between every mask frame. Plus, when you mix it down to 24fps you can introduce as much motion blur and shutter angle "emulation" as you want.
Debevec tried a version of this: https://arxiv.org/abs/2306.13702