.avi video file compression

[JPnyc]

amazes me. Just for kicks I DLed some software and decompressed a 350mb .avi video file. The resulting file was well over 20gb, and didn't look ANY better than the compressed .avi did. That's incredible to me.

[smurfy]

No offence Joe, but aren't you doing that backwards?
If you START with an uncompressed file and compress it with no loss of visual or audio quality, then you can claim it's great.
But if you start with a compressed file and end with a similar apparent quality file, that could just mean the "de-compression" did nothing but create a huge file.

[JPnyc]

Well since the avi was flawless in quality to begin with, I guess I meant that I don't see how it could be any better. I simply had no idea that the file, decompressed, could be that huge!

[fink]

But if the original decompressed file was viewed next to the compressed file you'd probably see some difference in quality. You can make the file larger but once a media file is compressed there's no way of getting back the data or quality that was lost during the first conversion/compression. Like Smurfy says all you did was make a larger file but didn't do anything to improve it's quality. that's why it's called "lossy" compression... because some data is lost or thrown out and can't be retrieved.

[Tuttle]

Given that an uncompressed .avi file is basically a stream of .bmp files, it's not hard to see why they're massive. Even if you just add some basic encoding that only stores the differences between frames you slash the file size. (But that makes seeking a nightmare -- to jump to 1 hour in the media player has to render that whole hour to itself to be able to work out what to show you. That's where key frames come in, to provide a starting point nearer to where you want to be. But anyway.) And then you get onto the lossy schemes which exchange a bit of quality for even better compression.