Could they not nust change a single pixel in the entire movie and use that as the identifier? Why would that be more expensive to do during production? It's not more expensive than giving your product a product ID, isn't it? Surely, modern software and production can do this cheaply.

I'm no expert on this, that's why I'm asking.

You don't that's the thing. I should have clarified my post.

You can add invisible watermarks to a bitstream to track the file back to the disc. Basically like an unique identifier. If you were to share that ripped Blu-ray illegally, the law enforcement could proof that you (as the buyer of the disc) ripped it.