[femto]

The concept is similar, in that with MPEG it is the encoder that is the harder of the two, since it has to deal with the noise and real-world effects in the source image.

What I should have written is that the "hard" part, which is generally left unspecified, is the part that removes redundancy. An MPEG encoder removes redundancy whilst its decoder adds redundancy. An FEC/communications encoder adds redundancy whilst its decoder removes redundancy.

[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.