Is there any way to extract a semi-transparent overly from a static shot? Say, of a person waking through the frame...Surely it's possible to remove any part of the footage that stays the same, and just leave the part that changes (In this case the semi-transparent person)I think that there would be limitations if this were even possible. For example, if the theoretical person were walking towards the camera, some parts of the shot may never be seen without being partly obscured by the overlay... I have no idea. Is there any way to do this? I'm aware there is a process for removing a semi transparent watermark, but is there something similar that works when the background is static and it's the overlay that's in motion?
So, what you're saying is, you have a stable shot of someone walking, and you want to use a shot of the background from the same angle as base for what gets removed from the walking shot, or, greenscreen but keying out background instead of a color?
I'm no expert, but I think this falls under the category of "things that obviously really should exist but don't and it's hard to understand why they don't", which sadly, many special effects fall under.
If I understand your problem correctly, I think your best bet is either hand-rotoscoping the person out of the shot, or just using a greenscreen.