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RE: Notify user agent available fragment

From: Davy Van Deursen <davy.vandeursen@ugent.be>
Date: Mon, 3 May 2010 08:48:29 +0200
To: "'Jeroen Wijering'" <jeroen@longtailvideo.com>
Cc: <public-media-fragment@w3.org>
Message-ID: <000001caea8c$a40e1db0$ec2a5910$@vandeursen@ugent.be>
Hi Jeroen,

On mei 01, 2010 at 11:04, Jeroen Wijering wrote:
> For example, one video on a server might contain an AUDIO track (and 
> thus the possibility to retrieve audio fragments), while the other 
> video might not contain this track. 

As Jack said, track discovery is out of scope for the MFWG and will probably
be solved (in a browser context) by using the Media Multitrack API, which is
currently being developed within the HTML5 Working
Group [1].

> As a second example, that first 
> video might have a VIDEO track with keyframe-interval of two seconds 
> and the second video might have a VIDEO track with keyframe-interval 
> of four seconds. The server might not be able to return a 2-second 
> fragment of the second video without transcoding (which should be 
> avoided).

This is solved by returning the context to the UA within the HTTP response.
More specifically, the Content-Range-Mapping response header contains
information regarding how accurate the server was able to extract the media
fragment. For example, if we use the following Range header: "Range:
t:npt=11-19", then the Content-Range-Mapping response header might contain
for example "Content-Range-Mapping: t:npt 10-20/0-50". The latter indicates
that the server was not able to extract the media fragment exactly as
requested by the UA (i.e., from 10 to 20 seconds instead of from 11 to 19
seconds). Further, the content-range-mapping header also indicates the start
and end time of the parent resource. Having this information, the UA can now
interpret this response and for example hide the first and last second of
the received fragment during playback of that fragment. This way, the
end-user will experience the exact media fragment he/she requested (i.e.,
11-19 seconds).

Best regards,


[1] http://www.w3.org/WAI/PF/HTML/wiki/Media_MultitrackAPI 

Davy Van Deursen

Ghent University - IBBT
Department of Electronics and Information Systems - Multimedia Lab
URL: http://multimedialab.elis.ugent.be/dvdeurse
Received on Monday, 3 May 2010 06:49:10 UTC

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