[MPlayer-users] variable bitrate, fixed quality

[D Richard Felker III]

On Wed, Jul 09, 2003 at 02:52:22PM +0200, Nikolaus Rath wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> Hello,
> 
> First: Thank you for your excellent explanation. Now I understand
> the whole thing much better. All the MPEG4 docs I've found so far
> don't describe the rate control. Maybe you should include this posting
> into the ffmpeg documentation.
> 
> D. Richard Felker, III <dalias at aerifal.cx> wrote:
> >> According to an xvid page (http://roeder.goe.net/%7Ekoepi/xvid.shtml,
> >> the "XViD Options Explained" PDF), the xvid codec supports such an
> >> operation. The page says that I should set the "curve compression" to
> >> 0 or 1 and the "payback delay" to 1. But these are the parameter
> >> names in a different xvid gui, I can't find them in mencoder. So maybe
> >> someone can tell me the equivalent mencoder options?
> > 
> > The corresponding options for lavc are vqcomp, vqblur, vqsquish,
> > vratetol, vrc_maxrate, vrc_minrate. However, the defaults are rather
> > good.
> 
> Nevertheless, I would be happy if you could explain them too. :-)

They're explained in the mplayer man page.

> > Increasing vqcomp (default is 0.5) to something near 1 will make
> > quality more steady for the whole movie. Decreasing it will make
> > bitrate more steady (close to constant bitrate).
> 
> I increased vqcomp up to 1. But the bitrate was still exactly as
> specified. That is why I'm thinking that some of the other options
> have wrong values (maybe mencoder sets them, so the defaults in ffmpeg
> don't apply).

The bitrate will always be (almost) exactly as you specify. That's the
way it works. What vqcomp affects is how those available bits get
distributed among frames. With vqcomp=0, all frames will get
approximately the same number of bits, whereas with vqcomp=1, they'll
all get approximately the same quantizer (or at least that's how I
understand it...). Putting vqcomp all the way up at 1 is probably not
a great idea; in that case, you might as well use vqscale=whatever.

So basically, if you're not happy with the quality, you need to tell
it to use a higher bitrate. Normally people select bitrate with the
goal of fitting the final filesize to one or two (or sometimes even
three) cds, rather than playing around with it until they find a
perfect balance between quality and filesize.

BTW, you still haven't told me whether you're using 1pass or 2pass
encoding. If you're only doing 1pass, you should expect the output to
look rather bad...

Rich