W3C home > Mailing lists > Public > public-texttracks@w3.org > September 2012

Re: Metadata in the VTT file header (bug 15851), use cases (and a need to close this)

From: Philip Jägenstedt <philipj@opera.com>
Date: Wed, 12 Sep 2012 18:18:40 +0200
To: "Ian Hickson" <ian@hixie.ch>, "David Singer" <singer@apple.com>
Cc: public-texttracks <public-texttracks@w3.org>
Message-ID: <op.wki59en9sr6mfa@kirk>
On Thu, 30 Aug 2012 02:13:03 +0200, David Singer <singer@apple.com> wrote:

> On Aug 29, 2012, at 16:53 , Ian Hickson <ian@hixie.ch> wrote:
>> On Wed, 29 Aug 2012, David Singer wrote:
>>> 5) Time alignment. When WebVTT is used as the caption source for a
>>> system where timestamps are from an arbitrary origin (e.g. a continuous
>>> MPEG-2 Transport stream) we need a way to say that 'timestamp X in this
>>> VTT file aligns with Timestamp Y in the media stream' so as to get
>>> synchronization.  This is naturally put into the header.
>> If there's a WebVTT file with fixed timestamps and a media stream with
>> arbitrary timestamps, then the only place where it makes sense to put  
>> the
>> synchronisation information is in the media stream. Putting it in the
>> WebVTT stream makes no sense; if you are able to adjust that stream then
>> why not just adjust the timestamps?
> Pardon?  You're suggesting completely re-writing the timestamps in the  
> mpeg-2 transport stream so as to … do exactly what?  What we need is a  
> mapping, not a need to re-write whole streams.

MPEG-2 TS doesn't seem particularly relevant to the Web. In any case,  
adjusting all of the WebVTT timestamps using JavaScript would be trivial,  
as would rewriting the entire WebVTT file on the server.

>> Thus this is not a use case for a name-value pair metadata header in  
>> VTT.
> Again, I disagree.
> Can you explain why you want to resist what many of us see as a natural  
> direction to go?  You even proposed a syntax for it, yet you seem to be  
> reaching for reasons not to do it.

For the record, I'm also not very enthusiastic about adding key-value  
metadata to WebVTT. Duplicating language and kind seem like a nice way to  
confuse, and browsers would just ignore the metadata anyway. The suggested  
syntax for multi-line values also looks pretty exotic to me.

Philip Jägenstedt
Core Developer
Opera Software
Received on Wednesday, 12 September 2012 16:19:30 UTC

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