[Bug 796984] vaapih265enc: Support hevc encode low latency B frame.

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=796984

--- Comment #17 from Fei <fei.w.wang at intel.com> ---
(In reply to sreerenj from comment #16)
> @Fei: Could you please explain a bit about the logic behind the low-delay-b
> implementation?
> 
> What I can see is that the patch just changes the slice type from P to B if
> low-delay-b enabled. Is it supposed to maintain 2 ref pic lists (list_0 and
> list_1) in this case?

Yes, In this case the 2 ref pic lists is needed. I guess driver will do low
delay b when it detect frame type is B and there are 2 ref pic lists. And I
encoded a yuv into h265 stream with low-latency-b enabled, and checked the
stream with intel-video-analyzer, all P frame changed into predictive B which
is expected. Here is reference code(option p2b) from libva-utils:
https://github.com/intel/libva-utils/pull/116/

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