[MEncoder-users] B-frames and GOP with MPEG-2 for DVD

[Loren Merritt]

On Thu, 7 Jun 2007, RC wrote:
> On Thu, 7 Jun 2007 Nicolas Hesler wrote:
>
>> So why have a b frames option in the first place, if it only degrades
>> the quality?  For smaller encodes?  So as a rule, never use B-Frames
>> unless you're concerned with space?

You're _always_ concerned about space. Otherwise you wouldn't be 
compressing the video. B-frames improve quality-per-bitrate at all 
bitrates except lossless, but not on all content. Sufficiently high motion 
can make B-frames useless, but a good codec can disable B-frames in 
just those regions.

> B-frames are a tricky issue.  At lower bitrates they help significantly
> with gradual scene transitions (fades), but they can hurt the overall
> quality even more in some cases.
>
> It seems they mainly exists in (hardware) encoders for
> simple, single-pass ratecontrol, and so lots of standards mandate their
> use.

As the author of x264's ratecontrol, I can say that B-frames make 1pass 
CBR ratecontrol harder (both in implementation complexity and in chance 
of violating VBV), and have no effect at all on the difficulty of 
unrestricted VBR.

The only reason I can think of that any standard might mandate the use of 
B-frames is to allow fast-forwarding by skipping the B-frames, if you 
don't have a fast enough decoder to process all frames at greater than 
realtime.

--Loren Merritt