MPEG compression question

FWIW, P-frames and B-frames may get *worse* quality (I mean, higher quantization) than the I-frames. Depending on the scene complexity && the bitrate constraints, then, some artifacts will be inevitable, regardless of how "smart" the encoder was designed to be.

The encoder should take into account the fact that the decoder will only be working with compressed frames. If it fails to do this, then yes, it may cause artefacts to remain because, as far as it is concerned, no change has taken place or needs to be encoded. sneaker_ger's point is that the MPEG standards don't define how an encoder encodes, they only specify what kind of input the decoder expects to receive, so we can't say what any particular, or all, MPEG encoder(s) may or may not do. There could be other reasons for whatever you're seeing, of course, but since you probably won't provide a sample we may never find out.

Where whatever frames may have been encoded so far, (before or following), then the encoder has to still target the un-encoded frame, relative to the 'so-far-encoded' frames, otherwise, what's to point of life, the universe or anything.