[MPlayer-users] vbitrate estimation using 3 pass encoding (divx)

[christopher j bottaro]

On Sunday 05 January 2003 05:48 pm, Arpi wrote:
> isn't it vpass=1 and vpass=2 ?

heh yes, typo sorry...=)

> size doesn't matter, in ideal case size won't change. quality may do.

hmm, i see...

> > ok, now here is my real question.  suppose i do a first (audio) pass and
> > it tells me i should use a vbitrate of 635.  ok thats all fine and good,
> > but if i ended up cropping scaling, or reducing the fps in the other
> > passes, that vbitrate of 635 can now be raised...right?  if so, by how
> > much?  how do i
>
> no! that value comes from the length (in time) of your movie and the free
> space left after saving audio on your CD.
> quality may will be raised by scaling etc, see docs.

i'm still not understand this part.  "that value comes from the length (time) 
of the movie and free space left after saving the audio on the cd", but isn't 
it also dependent on the fps and size (dimensions) of the movie?

i'm drawing this conclusion from the fact when i do a single one time pass 
encoding of an dvd chapter to divx, if i lower the fps or scale it, the 
resulting divx is smaller than it would have been if i didn't lower the fps 
or scale it.

since i have a hard time explaining myself sometimes, consider this:

mencoder -oac mp3lame -lameopts vbr=3 -ovc lavc -lavcopts 
vcodec=mpeg4:vhq:vbitrate=1200 -dvd 1 -chapter 1-1 -o test1.avi

so lets say that test1.avi turns out to be 40mb.  well now if i run the exact 
same thing, but lower the fps like this:

mencoder -oac mp3lame -lameopts vbr=3 -ovc lavc -lavcopts 
vcodec=mpeg4:vhq:vbitrate=1200 -dvd 1 -chapter 1-1 -o test2.avi -ofps 20

now test2.avi is say 30mb.  to get it back up to 40, i could raise the 
vbitrate by something...but what?

so what i'm saying is that the vbitrate output by the first (audio) pass is 
the bitrate required for the video + the audio to fit on one cd.  but aren't 
assumtions being made about the fps and dimensions of the video?  maybe not 
assumtions but the estimated bitrate is determined from the orginal 
dimensions of the dvd?

> quality may will be raised by scaling etc, see docs.

heh, can you tell me where?  i've read "2.4 Encoding with MEncoder" like a 
billion times...=)

thanks for the help,
-- christopher