[MEncoder-users] divx v's x264

[Diogo Franco]

Em Sex, 2008-06-06 às 09:46 +1000, Alex Samad escreveu:
> See I find this last statement counter intuitive. It has been mentioned
> on this thread that once you set the bitrate the file size should be the
> same no matter what codec is used.
When we say that a lossy video codec allows a smaller filesize is that
you can use a smaller bitrate without losing quality, because the better
efficiency allows to encode more data in less bits. A sample video
encoded at e.g. 400 kbps with x264 using cavlc will lose a bit more
quality than the same video encoded using cabac.

If you think about video compression like a file compressor, you can
imagine that on a video compressor you say what is the size of the file
that you want and the compressor decides how much data to discard to
make the video fit on that filesize. The better the efficiency (and the
larger the target filesize), less data is discarded.

Note that x264 has a lossless mode, which is when you use a quantizer of
0, but you will get really huge files. Also, that is part of the High
Profile, not of the Main Profile.

You can see more information about Arithmetic Coding (CABAC's algorithm)
and a comparison with Huffman Coding (similar to MPEG-4 ASP and CAVLC)
on http://en.wikipedia.org/wiki/Arithmetic_coding