[MPlayer-users] What are b_frames for and should I use them for my mpeg2 encodings(for DVD) ?

[Giacomo Comes]

On Fri, May 20, 2005 at 11:45:24AM -0400, Rich Felker wrote:
> On Fri, May 20, 2005 at 08:15:44AM -0400, Giacomo Comes wrote:
> > On Fri, May 20, 2005 at 09:56:55AM +0100, VJ wrote:
> > > Hi,
> > >    As the subjects says, I want to learn what B-frames are, what is
> > > their role and will they benefit my mpeg2 encodings which eventually
> > > end up on burnt DVD's?
> > 
> > This is not a technical answer, but what you can expect as an user.
> > 
> > With B frames your mpeg will usually be:
> > smaller
> > with a lower PSNR
> > take a longer time to encode.
> 
> the first and second are totally false without more information.

try it youself:
(I'm encoding only one chapter for speed reason)

mencoder -of mpeg -oac copy -ovc lavc -lavcopts vcodec=mpeg2video:vrc_buf_size=1835:keyint=18:aspect=16/9:vrc_maxrate=7500:vbitrate=7500:mbd=2:psnr -o DVD.mpg dvd://1 -chapter 3-3 -ofps 30000/1001 -vf harddup

check the PSNR and the file size.

now add 1 B-Frame

mencoder -of mpeg -oac copy -ovc lavc -lavcopts vcodec=mpeg2video:vmax_b_frames=1:vrc_buf_size=1835:keyint=18:aspect=16/9:vrc_maxrate=7500:vbitrate=7500:mbd=2:psnr -o DVD.mpg dvd://1 -chapter 3-3 -ofps 30000/1001 -vf harddup

and check again the PSNR and the file size.

PNSR will be lower and the file will be smaller.

The previous example is really basic: only 1 pass and no special libavcodec options,
but I have the same kind of results if I add more options/pass

> personally i strongly recommend against using b frames unless you have
> a very good reason. they rarely help quality:bitrate ratio, and they
> slow down not only encoding but also decoding, among adding lots of
> other complexities if you want to remux the video later. however,
> strict dvd compliance may require them...

B-Frames are mandatory only for VCD.

Giacomo