Jason Garrett-Glaser wrote: > 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. > I did not care for those numbers printed by mencoder as I mentioned earlier. But you bring up an interesting question: what exactly does mencoder report in the units kb/s? How does that relate to the bitrate=3000 option I was passing to the codec? >> 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. You may misspoke there, there is no logarithm in SSIM. Maybe you thought of PSNR. I guess that you may wanted to point out that SSIM is not linear (but that does not automatically mean it is logarithmic). That point is well taken; it was only my gut feeling at that point that the gain in SSIM/PSNR is not justified by the file size increase. I note that the above metrics are arithmetic/structural and not perception based (as the definition goes). Correlation with perception makes them useful and I read that SSIM is the current favourite in that regard. But lossy video compression quality measurement will never be as objective as say temperature measurement. So to get a better answer to the question how the increased file size and SSIM/PSNR relate, and whether two pass is useful or not, I attempted to find the one pass encoding bitrate option that produces the same file size and then the same SSIM. (These are two separate problems.) These runs take a while on my machine so I have the following so far: one pass encoding with bitrate=3275 produces: File size: 23814217 x264 [info]: SSIM Mean Y:0.9833172 x264 [info]: PSNR Mean Y:44.766 U:48.092 V:49.478 Avg:45.713 Global:45.019 kb/s:2987.82 > If SSIM goes from > 0.98 to 0.99, the quality of the encode *doubled*. See my remark above regarding perception. If by doubled one means that 1/(1-SSIM) is doubled, then yes, by definition... Thanks for your attention. Matyas - Every hardware eventually breaks. Every software eventually works. _______________________________________________ MPlayer-users mailing list MPlayer-users@xxxxxxxxxxxx https://lists.mplayerhq.hu/mailman/listinfo/mplayer-users
