[MPlayer-dev-eng] 2-pass vqcomp [was: PSNR plot]

[Rich Felker]

On Fri, Apr 29, 2005 at 02:22:32PM -0500, Zoltan Hidvegi wrote:
> Jeff Clagg wrote:
> > > PSNR may be high, even if 2% of the movie is almost unwatchable.  I
> > > did not like this so much, that recently I only use constant QP lavc
> > > encoding with well chosen lmin=lmax and v[bi]_q(factor|offset).
> > 
> > Not to derail this topic, but the default vqcomp is a bit low. 0.6 or
> > 0.7 is usually better, and more closely matches constant QP. You should
> > definitely play with this before you cosider ditching 2pass altogether.
> 
> That's an interesting topic in itself, so I've changed the subject.
> Actually, I've gone up all the way to vqcomp=1, and I got better
> global PSNR and more even quality.  But there was (and maybe still is)
> a bug in lavc which sometimes causes the QP go up to 31 even when you
> use vqcomp=1, this usually happens when some section is completely
> black, or there are some very dark sections in the movie, and it is
> even more likely to happen in 3-pass mode.  That's the main reason I
> gave up on 2-pass, even though when it works, it seems that 3-pass
> gives you less dispersed PSNR distribution than constant QP.

Giving up on 2pass sounds like nonsense. I've used 1pass mode before
and I can assure you the results are completely unusable... It may
work if your whole movie is roughly constant-complexity, but if you
have any segments significantly too complex to keep the requested
average bitrate it will look like total crap.

If you're worried about qp shooting up to 31, just set qmax to 8 or
something...

Rich