[MPlayer-users] variable bitrate, fixed quality

[Nikolaus Rath]

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.

Yes. But an explanation like

                 vqcomp=<value>
                     quantizer compression,  depends  upon  vrc_eq  (pass 1/2)
                     (default: 0.5)

isn't really helpful if you aren't an MPEG developer.
Your explanation below is indeed very good. I think that this type of
explanation should be in the manpage.

>> > 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.

Based on your explanation of rate control, this means that with
vqscale=1 all frames with low quantizers get an increased quantizer
and all frames with high quantizers get a decreased quantizer, while
with vqscale=0 those with high bitrates get increased quantizers and
those with low bitrates get decreased quantizers during the second
pass. Is this correct?

Then there is a difference between vqscale=n and vqcomp=1. With
vqcomp=1 you get the best possible constant quantizer at the desired
bitrate while you have to specify the quantizer manually (bitrate is
ignored) in the vqscale=n mode.

> 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...

2-pass mode, of course.

   --Nikolaus