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Re: ISSUE-346: Need ttp:mediaDuration parameter [TTML2]

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Thu, 25 Sep 2014 10:11:16 +0000
To: John Birch <John.Birch@screensystems.tv>, Timed Text Working Group <public-tt@w3.org>
Message-ID: <D049A4E7.11F87%nigel.megitt@bbc.co.uk>
No processing requirements that use ttp:mediaDuration are stated either in
the ttp:mediaDuration section at [1]  or anywhere else in the document,
right now. Taken in isolation this would suggest that it is metadata and
should not be in the parameter namespace, however I think it is merely a
reflection of work in progress.



My expectation is that, regardless of how they are calculated, the
effective media begin time and media end time will be used to define the
temporal period within which any ISDs should be created for display
alongside the related media, and that mediaDuration would therefore be
referenced in the Intermediate Synchronic Document Construction process at
[2]. There's an editorial note on that section - possibly when that note
has been resolved into spec text this will become clearer.



I've proposed in the "Issue-270 and Issue-335" thread that starts at [3]
that this period might better be defined explicitly in terms of mediaBegin
and mediaEnd parameters expressed in the time base of the document. If
mediaDuration is needed and the time base and marker modes permit time
calculations (e.g. it is not SMPTE - discontinuous) then it can be
calculated as (mediaEnd - mediaBegin).

[3] http://lists.w3.org/Archives/Public/public-tt/2014Sep/0065.html

I'm interested in the requirement/use case though: when you say "allows
time values within a document to be 'offset' relative to the start of
associated media", what calculations are you envisaging that a processor
should perform, exactly, and in what circumstances might they be useful?

Kind regards,


On 25/09/2014 10:38, "John Birch" <John.Birch@screensystems.tv> wrote:

>I am uncertain about the 'requirement' for a duration parameter?
>What is the current situation... does TTML1 work as if the media duration
>is 'indefinite' ?
>In which case what additional benefit does having a defined duration
>Of far more importance (IMHO) is a media start value - that allows time
>values within a document to be 'offset' relative to the start of
>associated media.
>Best regards,
>John Birch | Strategic Partnerships Manager | Screen
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>P Before printing, think about the environment-----Original Message-----
>From: Timed Text Working Group Issue Tracker
>Sent: 21 September 2014 13:35
>To: public-tt@w3.org
>Subject: ISSUE-346: Need ttp:mediaDuration parameter [TTML2]
>ISSUE-346: Need ttp:mediaDuration parameter [TTML2]

>Raised by: Glenn Adams
>On product: TTML2
>In order to perform ISD processing, it is necessary to know the duration
>of root external extent. When associate with a related media object, this
>is the simple duration of the related media object. If this is known at
>authoring time, then it is an important parameter that should be
>specified. I propose defining a ttp:mediaDuration parameter attribute
>that takes either an offset-time form of a time expression or the keyword
>"indefinite", where the latter is used (or implied) when no related media
>object exists or its simple duration is unknown or indefinite. If this
>parameter is not specified, then it would be treated as if indefinite
>were specified.
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Received on Thursday, 25 September 2014 10:11:51 UTC

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