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

[D Richard Felker III]

On Sun, Jan 05, 2003 at 06:10:55PM -0600, christopher j bottaro wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> 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.

Bitrate is exactly what it sounds like, the number of bits to be used
per second. Given the bitrate, the final file size depends only on the
length of the movie (in time). Changing the framerate or the scale
doesn't change the length of the movie, so it shouldn't affect the
file size.

With that being said...sometimes the encoder will not use all the bits
you tell it to (with the vbitrate option) if it doesn't think they're
necessary. This may be what's happening in your case with low
framerate or scaling. But all that means is that the actual bitrate
isn't coming out as high as you requested. See the bitrate mencoder
prints when it finishes and that will tell you what the real average
bitrate ended up being.

Rich