[quietbritishjim]

It's not really about difficulty (although encoding is definitely far more difficult), it's about there being multiple valid encodings.

If you have two red boxes on a black background in one frame, and a single red box in the middle on the next, UIUI there are at least two ways to encode this: "left box moved right a bit; right box disappears" and "right box mounts left a bit; let box disappears".

In a complex scene, there is a huge space of possible ways to encode a frame that give identical outputs, and even more that are lossy. Choosing one that compresses well but is visually similar (and not too slow to find) is a quality issue for the codec. It wouldn't make sense to specify the algorithm for that in the spec.

[femto]

I agree. We are in danger of arguing about semantics/language, despite the effects of the words we are using being the same. I guess that's why Information Theorists use mathematics over words.