Until your comment I didn't realise I'd also read it wrong (despite getting the gist of it). Attempted rephrase of the original sentence:
Imagine history documentaries where they take an old photo, free objects from the background, and then move them round to give the illusion of parallax.
Imagine history documentaries where they take an old photo, free objects from the background, and then move them round to give the illusion of parallax.