Re: [rtcweb] H.261 encoding samples at typical bitrates - sign language example

Justin Uberti <juberti@google.com> Thu, 05 December 2013 21:06 UTC

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From: Justin Uberti <juberti@google.com>
Date: Thu, 5 Dec 2013 13:05:57 -0800
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To: Basil Mohamed Gohar <basilgohar@librevideo.org>
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Subject: Re: [rtcweb] H.261 encoding samples at typical bitrates - sign language example
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Ow, my eyes...

The 256 kbps and lower clips are unusable. The 512 kbps clip is borderline,
but might be usable if the framerate was cut in half.

Remember also that these test clips are far better than what would be
obtained from consumer webcams (i.e. good lighting, no shake, no temporal
noise), so real-world performance is likely to be worse than what you see
here.


On Mon, Dec 2, 2013 at 8:23 PM, Basil Mohamed Gohar <
basilgohar@librevideo.org> wrote:

> On 12/02/2013 10:03 PM, Basil Mohamed Gohar wrote:
> > Let's let any further discussions about the usability of H.261, or any
> > other codec for that matter, use actual examples going forward.
> >
> > The following is a VERY quick test of ffmpeg's h261 encoder in the
> > context of the IETF's rtcweb working group's discussion of an MTI
> > (mandatory-to-implement) video codec.
> >
> > sine_irene_cif.y4m taken from derf's collection:
> >
> > http://media.xiph.org/video/derf/y4m/sign_irene_cif.y4m
> >
> > ffmpeg version N-58565-gc122e69
> >
> > bitrate=64k,128k,256k,512k
> >
> > ffmpeg -i sign_irene_cif.y4m -codec:v h261 -b:v $bitrate -g 30
> > sign_irene_cif.y4m-$bitrate.h261
> >
> > http://media.basilgohar.com/rtcweb/sign_irene_cif.y4m-64k.h261
> > (real rate: 157.8kbits/s)
> >
> > http://media.basilgohar.com/rtcweb/sign_irene_cif.y4m-128k.h261
> > (real rate: 165.6kbits/s)
> >
> > http://media.basilgohar.com/rtcweb/sign_irene_cif.y4m-256k.h261
> > (real rate: 289.5kbits/s)
> >
> > http://media.basilgohar.com/rtcweb/sign_irene_cif.y4m-512k.h261
> > (real rate: 541.8kbits/s)
> >
> > I apologize for the bitrate inflation, but if I had more time I can
> > tweak the settings for a more accurate number.  These are simply the
> > rates that ffmpeg produced with such a short clip at the given requested
> > rates.
> >
>
> I've updated the encoding settings as follows to get more accurate
> resulting bitrates, but ffmpeg's h261 encoder seems to bottom-out at
> around ~140kbps, so the only examples from above that are close (after
> using the new settings) are 256k and 512k.
>
> for bitrate in {1..512}k; do ffmpeg -i ../sign_irene_cif.y4m -codec:v
> h261 -b:v $bitrate -minrate $bitrate -maxrate $bitrate -bufsize $bitrate
> -qmax 1024 -g 30 -y sign_irene_cif.y4m-$bitrate.h261; done;
>
> All the above posted examples can be viewed with mplayer and a bash
> command line using the following, if you're interested:
>
> mplayer
>
> http://media.basilgohar.com/rtcweb/sign_irene_cif.y4m-{64,128,256,512}k.h261
>
> The full integer range of bitrates from 1 to 512 can be found here:
>
> http://media.basilgohar.com/rtcweb/h261/
>
> Target bitrate and actual bitrate start to match around 150kbps with
> these new settings.
>
> I am currently exploring other codecs with the same methodology and will
> share the results accordingly.
>
> --
> Libre Video
> http://librevideo.org
> _______________________________________________
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> rtcweb@ietf.org
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>