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This only works if the camera isn't fixed, though. In the frame from the post it might erase the dashboard, the car roof, and so on.

Just define a small area of the screen to run on then. Subtitles are typically within a very small portion of the screen

Nope, every channel of CCTV seems to have its own subtitle convention.

Which is irrelevant to stripping subs from one movie at a time.