[MPlayer-users] one-pass encoding results better than two-pass

[Jason Garrett-Glaser]

On Mon, Jan 5, 2009 at 12:34 AM, Matyas Sustik <mplayer.list at sustik.com> wrote:
> I have encoded a 1080i broadcast using the following filters:
>
> pp=ci,crop=1904:1072:10:0,hqdn3d=4:3:6,softskip,scale=952:536:0:0:0.00:0.60,harddup
>
> with bitrate set to 3000.  On a 20 seconds long sample the one pass encoding
> produced a file with 8703945 bytes and quality:
> x264 [info]: SSIM Mean Y:0.9805128
> x264 [info]: PSNR Mean Y:44.861 U:48.798 V:49.399 Avg:45.849 Global:45.537
> kb/s:3259.63
>
> The two pass encoding (same filter and x264enc options) produced a file with
> 9437678 bytes.
> x264 [info]: SSIM Mean Y:0.9803121
> x264 [info]: PSNR Mean Y:44.632 U:48.669 V:49.280 Avg:45.642 Global:45.489
> kb/s:3068.16
>
> So it would seem from this experiment that the one pass encoding produced a
> smaller file with the same or better quality.  (Visual inspection cannot tell
> them apart.)  Am I doing something wrong?  This is not what one would expect,
> right?  Should I rerun this with other samples?  The current one had a lot of
> fast moving actions.
>
> Matyas

By default, x264 uses psy optimizations which lower SSIM and PSNR but
improve visual quality.  Your first pass settings probably use turbo,
which will among other things disable them, raising PSNR/SSIM.

But more importantly, your first pass is 200kbps higher than the
second--of course it can be higher quality.  You can't compare quality
between two encodes at different bitrates.

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