MPG question

[user5]

    MPGs don't have the quality that uncompressed AVIs have though in many cases the difference is unnoticeable
depending on the content. If the frames of an MPG need editing then I was wondering if it would be possible to do
that while the frames are still in the MPG without extracting them.
    I'm not talking about changing the video's contrast or something like that. I'm talking about changing individual
pixels so that the MPG looks better when it plays.
    Is this even possible?

[user5]

    I did some preliminary research on this and it seems as if it might be possible but it's beyond my current skills.
    Alterations would be done to some PNG frames in one program for example and then those PNGs would be
mapped to the frames in the compressed video frame by frame or pixel by pixel.
    It involves timestamps, keyframes and all that jazz. This would profoundly mean that the limitations of compressed
videos would be possibly removed. This may be too good to be true. It may not be possible due to the nature of
compressed videos, I don't know. Be cool if it was possible. Back to the drawing board.

[domasz]

If I understood you correctly you want to:
1) compress AVI to MPEG
2) compare MPEG to AVI
3) save difference as PNG files
4) play MPEG with overlaid PNG images

This would of course visually look exactly like an uncompressed AVI but this will not create smaller files. While the size of MPEG will be < than AVI the sum of MPEG+PNG will be pretty much the same as AVI.

[dpethes]

What is your use case? If the compressed file's quality isn't sufficient, then re-encode with higher quality. New MPEG standards even allow for lossless compression. This would be more effective than applying another layer of lossless png-s on top of compressed data, as you lose the advantage of inter prediction this way.