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

[Basil Mohamed Gohar <[e-mail]>]

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