W3C home > Mailing lists > Public > public-media-fragment@w3.org > October 2008

Re: video use-case

From: Dave Singer <singer@apple.com>
Date: Mon, 6 Oct 2008 17:04:34 -0700
Message-Id: <p0624081bc51057634e85@[]>
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Cc: Media Fragment <public-media-fragment@w3.org>

At 10:54  +1100 7/10/08, Silvia Pfeiffer wrote:
>On Tue, Oct 7, 2008 at 10:26 AM, Dave Singer <singer@apple.com> wrote:
>>  At 9:16  +1100 7/10/08, Silvia Pfeiffer wrote:
>>>>   For example, in a media file that has an index and is
>>>>   in time-order, a user-agent wanting a time-based subset may be able to
>>>>  use
>>>>   byte-range requests to get the index, and then the part(s) of the file
>>>>   containing the desired media. (We do this today for MP4 and MOV files).
>>>  Yes, byte-ranges are possible. However, the Web server is the only
>>>  component in the system that knows about converting a time offset to a
>>>  byte range. Therefore, you have to first communicate with a URI
>>>  reference to the server which subsegment you would like to have, the
>>>  server can then convert that to byte ranges, return to the UA which
>>>  byte range he has to request, and then we can do a normal byte-range
>>>  request on the full URI.
>>>  When you say that you do this today for MP4 and MOV files, how do you
>>>  communicate the fragment to the Web server?
>>  MP4 and MOV files have tables in the moov atom which give complete time and
>>  byte-offset indexing for every video and audio frame.  Atoms are also sized.
>>   You can gamble, ask for 1K at the start of the file; if it's laid out for
>>  incremental playback, the moov atom will be the first or second atom;
>>   you've got its size now, and can download the rest.  If it wasn't, you have
>>  the size of those atoms and can skip past them and ask for the next.  Once
>>  you have the moov atom, you know exactly what bytes you need to go anywhere
>>  in time (and yes, even sync points are marked, so you know how far to back
>>  up if someone does a random access).  If video and audio are interleaved in
>>  time order, the data you need will be all contiguous.
>This still does not solve the client-server problem. Say, a UA wants
>to play back a MOV file from sec 45-88. The UA does not know how to
>map that to a byte offset and therefore to a byte-range request.

that's what i telling you;  it does.  it can find the index, 
typically in one request.  given the index, it can figure it out...

>UA has to ask the server for this information.

no, really this woks

>The server can ask the
>local MOV file for the byte mapping from the time mapping by analysing
>the tables in the moov atom as you described. Then it can tell the UA
>this information which in turn can do a byte-range request.
>What we are talking about with a temporal media fragment request
>through a URI is the very first step: the UA needs to request from the
>Web server the media fragment.

not always...

>>  This is, of course, very difficult for VBR un-indexed files.  Pretty easy,
>>  of course, for CBR files.
>As long as it is all handled by the Web server, it's no different in
>process. The mapping to byte ranges may be as complicated as possible
>- the UA and the network don't care, they just handle whatever they
>are being told by the Web server.

sure, if we extend http to cover time-range requests

David Singer
Received on Tuesday, 7 October 2008 00:06:26 UTC

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