Why shoud the vbv-related parameters be used in the MB Rate Control in X264?

xkfz007 · 13th November 2013, 03:53

In the ABR RC mode of x264, we just need give the parameter --bitrate, and this is Frame Level Rate control. But when I want to use the MB Level RC (not MB-tree), I have to give the parameters of vbv-bufsize and vbv-maxrate. I can't understand why? Is it impossible to use the MB Level RC with just the parameter --bitrate and HOW?
Can anyone give me some hints or explanation or whatever?

xkfz007 · 13th November 2013, 09:31

ladies and gentlemen, please help me!!

Warperus · 13th November 2013, 09:58

AFAIK VBV sets local restrictions on bitrate to prevent buffer underflow so it's needed in rate control if you set some VBV restrictions.
AFAIK x264 can encode disregarding VBV restrictions, it happens if you don't set it in command line. I donno how it is done internally, but you can track what happens if these parameters are not set in command line.
For simplicity you can use MB Level RC with some insane values for VBV bufsize/maxrate like 1TB/1TB per second. It's not best idea in the world, but I suppose it's good enough for quick dirty experiments.

13th November 2013, 03:53	#1 \| Link
xkfz007 Registered User Join Date: Sep 2013 Posts: 38	Why shoud the vbv-related parameters be used in the MB Rate Control in X264? In the ABR RC mode of x264, we just need give the parameter --bitrate, and this is Frame Level Rate control. But when I want to use the MB Level RC (not MB-tree), I have to give the parameters of vbv-bufsize and vbv-maxrate. I can't understand why? Is it impossible to use the MB Level RC with just the parameter --bitrate and HOW? Can anyone give me some hints or explanation or whatever?

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13th November 2013, 09:31	#2 \| Link
xkfz007 Registered User Join Date: Sep 2013 Posts: 38	ladies and gentlemen, please help me!!

13th November 2013, 09:58	#3 \| Link
Warperus Registered User Join Date: Apr 2010 Location: Sain-Petersburg, Russia Posts: 139	AFAIK VBV sets local restrictions on bitrate to prevent buffer underflow so it's needed in rate control if you set some VBV restrictions. AFAIK x264 can encode disregarding VBV restrictions, it happens if you don't set it in command line. I donno how it is done internally, but you can track what happens if these parameters are not set in command line. For simplicity you can use MB Level RC with some insane values for VBV bufsize/maxrate like 1TB/1TB per second. It's not best idea in the world, but I suppose it's good enough for quick dirty experiments.