[FFmpeg-devel] Scalable Video Encoding and FGS

Erik van Zijst ffmpeg
Fri Oct 19 19:05:58 CEST 2007

Loren Merritt wrote:
> On Fri, 19 Oct 2007, Erik van Zijst wrote:
>> In MPEG1 b-frames are roughly 70 times smaller than i-frames [1], so
>> cutting the bitrate in half, requires removal of 70 b-frames per i-frame
>> (sure, these figures old and mpeg1-specific, but what matters is the
>> concept).
>> [1]
>> "The typical data rate of an I-frame is 1 bit per pixel while that of a
>> P-frame is 0.1 bit per pixel and for a B-frame, 0.015 bit per pixel." --
>> http://bmrc.berkeley.edu/frame/research/mpeg/mpeg_overview.html
> That't a factor of 7, not 70. And What matters is the ration between I+P 
> and B framse, not just I and B.

It's 1 bit/pixel (I) versus 0.015 b/p (B), which is almost a factor 70 
(assuming Berkeley's figures are correct). I versus P is 10-fold, so 
lowering the bitrate requires dropping a substantial number of predicted 

If these ratios are still somewhat representative for current codecs 
(not sure if they are!), dropping all predicted frames in a 
30-frame-per-GOP stream may not even half the bitrate..


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