You can hash the entire video. It would be very, very difficult (impossible?) to create another video with a hash collision that changed even a couple pixels from the original hash. You just need the hash computed from that unaltered video uploaded at time of encoding to the blockchain. It doesn’t change that someone could have created an entire AI video and uploaded it to the same proof of existence, but it does mean you could verify that a given video was observed on the blockchain at time x, and that it has not been altered in some way. Time of video creation would also be included in the hash (and metadata) so that you could verify that the video was created, by what program, at what time; and you’d have (ideally a less than 5s later) time stamp of it being verified on the blockchain.

There’s likely a better way to do this as well using zero knowledge proofs and/or homomorphic encryption but there probably exists a way to do this which would be tractable.