[MPlayer-dev-eng] 2 pass encoding - need explanation

[Attila Kinali]

On Sat, 22 Feb 2003 16:23:55 -0500
D Richard Felker III <dalias at aerifal.cx> wrote:

> It does!! 0.27 is a fairly large difference in PSNR!!

Hmm.. Interesting. Then PSNR behaves totaly different from
what i am used of SNRs (mostly "normal" signals for digital
transmission).

But, then would be the encoding with the lower quantisizers
the worse one ?
 
> BTW, you should try with vqscale=2 on the first pass and see if that
> works well, rather than vqscale=5. The latter will encode the whole
> first pass at very low quality so it probably won't estimate
> complexity well.


I did it and got folowing result:

---
1.pass: vqscale=2 / 2.pass: vbitrate=615

1.pass: -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vpass=1:vbitrate=615:vqscale=2
2.pass: -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vpass=1:vbitrate=615

Video stream:  615.134 kbit/s  (76891 bps)  size: 381105923 bytes  4956.394 secs  118836 frames
Audio stream:  128.000 kbit/s  (15999 bps)  size: 79295296 bytes  4955.956 secs
PSNR: Y:44.34, Cb:47.23, Cr:47.96, All:45.17
---

Which makes it slightly better then the 
1.pass: vbitrate=615 / 2.pass: vbitrate=615 case. 
The maximal quantisizer values are between the two others
(ie, it's not allways max=4 but not as often at max=5)

Though, i cant see any difference in quality between the three
encodings.

			Attila Kinali

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