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

[Attila Kinali]

Heyo,

I recently had a discussion with Jonas about the usage of
vqscale in the first pass of a two pass encoding.
So i wanted to test it and got following results:

Source:
VIDEO:  [MP43]  320x240  24bpp  23.98 fps  615.4 kbps (75.1 kbyte/s)

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

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

Video stream:  615.120 kbit/s  (76889 bps)  size: 381096793 bytes  4956.394 secs  118836 frames
Audio stream:  128.000 kbit/s  (15999 bps)  size: 79295296 bytes  4955.956 secs
PSNR: Y:44.30, Cb:47.20, Cr:47.93, All:45.14
---
---
1.pass: vqscale=5 / 2.pass: vbitrate=615

1.pass: -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vpass=1:vbitrate=615:vqscale=5
2.pass: -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vpass=1:vbitrate=615

Video stream:  615.094 kbit/s  (76886 bps)  size: 381080897 bytes  4956.394 secs  118836 frames
Audio stream:  128.000 kbit/s  (15999 bps)  size: 79295296 bytes  4955.956 secs
PSNR: Y:44.02, Cb:46.98, Cr:47.74, All:44.87
---

Now, the two results seem to be pretty much the same from size and PSNR,
but a analysis plotpsnr.pl shows, that the second test (with vqscale)
has the quantisizer size most time at 4 and less (just a few times at
the beginning at 5 and 6) while the first test has it at 5 and less.

If i'm not mistaken, then the file size and the quality of an encoding
is mostly dictated by the used quantisizer values (assuming that the
same MVs are choosen)

But why is there so less difference in the file size (beside the
file with the lower quantisizer is smaller!) and why doesnt the 
PSNR differ at all ?

Can anyone explain that ?

I uploaded the two psnr logs, if you want to have a look at them:
http://people.ee.ethz.ch/~adogan/psnr_vbitrate.log.gz
http://people.ee.ethz.ch/~adogan/psnr_vqscale.log.gz

		Attila Kinali
-- 
Emacs ist für mich kein Editor. Für mich ist das genau das gleiche, als wenn
ich nach einem Fahrrad (für die Sonntagbrötchen) frage und einen pangalaktischen
Raumkreuzer mit 10 km Gesamtlänge bekomme. Ich weiß nicht, was ich damit soll.
		-- Frank Klemm, de.comp.os.unix.discussion