Re: [rtcweb] H261/MPEG-1 video quality

Maik Merten <maikmerten@googlemail.com> Fri, 15 November 2013 15:17 UTC

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Date: Fri, 15 Nov 2013 16:17:12 +0100
From: Maik Merten <maikmerten@googlemail.com>
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Subject: Re: [rtcweb] H261/MPEG-1 video quality
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As pointed out by Stephan and after investigating a bit on the format 
specifics I feel compelled to state that H.261 and MPEG-1 are indeed 
different enough from each another so that performance evaluations on 
one codec cannot directly be transferred to the other. I'm still tempted 
to assume that CIF H.261 can deliver acceptable quality for talking head 
scenarios at ~256 kbit/s - however, I've yet to find a reliable H.261 
encoder implementation with up-to-date psychovisual optimizations to 
generate actual test vectors.

Best regards,

Maik

Am 15.11.2013 14:52, schrieb cowwoc:
>
> Excellent work Adam. I can't speak for others, but at 254 kbps
> (corrected figure from your follow-up post) H.261 is definitely "good
> enough" and better than an audio-only connection.
>
> Gili
>
> On 14/11/2013 6:16 PM, Adam Roach wrote:
>> I sent a reply to this earlier, but just now realized that it went
>> only to Justin, not to the list.
>>
>>
>> On 11/14/13 13:59, Justin Uberti wrote:
>>> Thanks, this is interesting. Is the ffmpeg 261 encoder limited to
>>> CIF/QCIF, or can you specify arbitrary sizes?
>>
>> It looks like the ffmpeg mpeg-1 coder works for arbitrary sizes. I'm
>> not sure what the difference between mpeg-1 and H.261 are, though, so
>> we could be talking apples and oranges (or at least apples and pears)
>> here. I'll note that mpeg-1 came out in 1991, which is a good 22 years
>> in the past. I'm not drawing IPR conclusions for you, but invite you
>> to ponder the implications yourself.
>>
>> Following Maik's lead with the mpeg-1 js decoder, I put this together:
>>
>> https://dl.dropboxusercontent.com/u/53717247/mpg/maven.html
>>
>> ...with this commandline:
>>
>>   ffmpeg -i maven.mp4 -f mpeg1video -flags qprd -mbd rd -cmp rd
>> -subcmp rd -mbcmp rd -precmp rd -trellis 2 -g 100 -vb 256k maven.mpg
>>
>> I don't really understand most of those options (I just cribbed them
>> from Maik's example) or whether any of them would introduce more
>> latency than is reasonable for a real-time conversation, but I will
>> observe:
>>
>>  1. The encoder claims that it was performing on the order of 90 - 100
>>     fps on my (admittedly modern) system;
>>  2. The resolution is 640x360 (somewhat larger than DCIF);
>>  3. The video is not, to my eye, unusable (draw your own conclusions,
>>     as it's clearly not as nice as modern codecs);
>>  4. At 74 seconds and 4.7 MBytes (i.e., 37.6 Mbits), this encoding
>>     works out to 508 kbits/second total.
>>
>>
>> Source video here, and NASA is acknowledged as the source of the
>> material contained therein: http://www.youtube.com/watch?v=ijAO0FFExx0
>>
>> /a
>>
>>
>>
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>
>
>
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