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

[Jason Garrett-Glaser]

> I am not following you there.  The one pass encoding had the smaller file
> size and higher ssim/psnr.  That I found unexpected.  Incidentally, I do not
> know how the reported kb/s:3259.63 is calculated from the filesize.  I trust
> the file system reported size though.

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

I would call that roughly 190kbps higher bitrate.  At least that's
what my calculator says, yours may disagree.

> Single pass encoding:
> File size: 21902932
> x264 [info]: SSIM Mean Y:0.9822603
> x264 [info]: PSNR Mean Y:44.466 U:47.888 V:49.304 Avg:45.431 Global:44.716
> kb/s:2734.11
>
> Two passes:
> File size: 23831479
> x264 [info]: SSIM Mean Y:0.9838375
> x264 [info]: PSNR Mean Y:44.659 U:48.036 V:49.531 Avg:45.624 Global:45.243
> kb/s:2990.10
>
> 3 passes:
> File size: 23903141
> x264 [info]: SSIM Mean Y:0.9837665
> x264 [info]: PSNR Mean Y:44.587 U:47.986 V:49.520 Avg:45.557 Global:45.237
> kb/s:2999.61
>
> The first two run results I can accept without much suspicion.  However the
> benefit of the second pass is non-existent for me: the increase of less than
> 0.002 SSIM costs almost 10% in file size. But the third seems again to be
> suspicious featuring a larger file with lower SSIM.

If you don't know how to read metrics, don't read them, or at least
ask before you do.  SSIM is inverse logarithmic.  If SSIM goes from
0.98 to 0.99, the quality of the encode *doubled*.
(1-0.9822603)/(1-0.9838375)=9.7%, so you got a 10% quality improvement
between first and second pass.  However, do note that part of the
point of multipass encoding is to *better distribute* the quality, not
just to get better SSIM/PSNR.  Also do note that the lambda (amount of
quality gained per bitrate) varies depending on source and bitrate.

The third pass is useless because third passes are generally useless.
They mainly exist for obsessive-compulsive people who need their
7-pass encodes.

Dark Shikari