[MPlayer-users] smaller file size after mencoder recoding - why?

[D Richard Felker III]

On Fri, Jan 17, 2003 at 02:43:07PM +0300, Vladimir Mosgalin wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> Hi everyone.
> 
> Can somebody explain why mencoder pass results in smaller files?
> I take almost any avi (i've found this to work with any avi not coded
> with mencoder), then run
> mencoder file.avi -o file_.avi -ovc copy -oac copy -idx
> 
> file_.avi will be smaller then file.avi
> 
> i've tested this on some clips (20-50m size, mpeg4/divx), difference is
> 100-500k. I really don't understand this. Yes, mencoder repacks audio
> and video without recoding, but how come that there were 500k of junk
> inside properly-coded avi? Or wasn't it junk? Maybe mencoder throws out
> something useful?

Try running it twice:

mencoder file.avi -o file_.avi -ovc copy -oac copy
mencoder file_.avi -o file__.avi -ovc copy -oac copy

You should find file_.avi and file__.avi identical, so mencoder isn't
losing frames or something. Most likely the program used to encode the
original file.avi just sucked. I've been able to recover over 10 megs
like this with mencoder on some broken files... :) It's especially
nice when the file is just barely over 703M.

Rich