[Ffmpeg-devel] [PATCH] increase max numbers of B frames

Erik Slagter erik
Tue Feb 21 18:44:21 CET 2006

On Tue, 2006-02-21 at 13:44 +0100, skal wrote:

> > PBBBPBBBP: again, each frame is bigger than it would be in 2 consecutive, 
> > and now there's a decent chance that 67%->75% B-frames isn't enough to 
> > pay off. Though it's a bit more useful than in mpeg4, due to B-pyramid 
> > (which I won't detail here) and miscellaneous other improvements in the 
> > codec.
>    Maybe this point is worth some details: h264 permits use the middle
>    B-slice as reference, making the reference anchors not so far from
>    the remaining inbetween b-slices (non-refs). So, you're back to 
>    deciding PbBbP over PbPbP for the refs, which is quite similar to
>    the PBP vs. PPP decision, as far as refs placement is concerned.
>    Well, this is quite an uncharted (and slippery) land, but some
>    are advocating use of GOPs as big as 32 (!):

A very crude and non-representative test ;-) reveals that

- without b_pyramid: 3 b frames is optimal (1784 kb/s)
- with b_pyramid: 5 b frames is optimal (1636 kb/s)

Both show no significant improvement with larger amounts of B frames
inserted. Nice to know :-)

For this test I took a short clip, with forced B frames, otherwise the
encoder would not select more than 1 B frame at a time...

I intend to do something similar with gop sizes.
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