2 B frames encoding time anomaly

OpasanNekiLik · 3rd June 2011, 09:45

Could someone explain to me the anomaly why x264 encoder needs more time for encoding with 2 B frames than with 3 B frames?

I made test where I encoded a video sequence with various number [0..16] of B frames between keyframes. The best quality was achieved with 2 and 3 B frames but for some reason 2 B frames require more encoding time than 1 or 3 B frames.

Test can be found here.

detmek · 3rd June 2011, 11:38

New b-pyramid algorithm increases encoding speed, even for --b-adapt 2. If 3 B frames activates b-pyramid, maybe that is a reason.

akupenguin · 5th June 2011, 01:23

Pyramid is slower than non-pyramid, because pyramid allows the use of more references, namely the 2nd future ref in disposable b-frames prior to the B-ref. Pyramid is enabled by default when B>=2. Pyramid with B==2 has a greater fraction of "disposable b-frames prior to the B-ref" than B==3 does. And indeed, the speed bump goes away if you set ref=1 and/or pyramid=none.

Pyramid=strict is faster than pyramid=normal because strict forbids the use of some refs that normal allows. This is not an optimization, just a loss of an opportunity.

3rd June 2011, 09:45	#1 \| Link
OpasanNekiLik Registered User Join Date: Aug 2010 Posts: 12	2 B frames encoding time anomaly Could someone explain to me the anomaly why x264 encoder needs more time for encoding with 2 B frames than with 3 B frames? I made test where I encoded a video sequence with various number [0..16] of B frames between keyframes. The best quality was achieved with 2 and 3 B frames but for some reason 2 B frames require more encoding time than 1 or 3 B frames. Test can be found here.

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3rd June 2011, 11:38	#2 \| Link
detmek Registered User Join Date: Aug 2009 Posts: 463	New b-pyramid algorithm increases encoding speed, even for --b-adapt 2. If 3 B frames activates b-pyramid, maybe that is a reason.

5th June 2011, 01:23	#3 \| Link
akupenguin x264 developer Join Date: Sep 2004 Posts: 2,392	Pyramid is slower than non-pyramid, because pyramid allows the use of more references, namely the 2nd future ref in disposable b-frames prior to the B-ref. Pyramid is enabled by default when B>=2. Pyramid with B==2 has a greater fraction of "disposable b-frames prior to the B-ref" than B==3 does. And indeed, the speed bump goes away if you set ref=1 and/or pyramid=none. Pyramid=strict is faster than pyramid=normal because strict forbids the use of some refs that normal allows. This is not an optimization, just a loss of an opportunity.