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

From: John Birch <John.Birch@screensystems.tv>
Date: Thu, 25 Sep 2014 10:29:06 +0000
To: Nigel Megitt <nigel.megitt@bbc.co.uk>, Timed Text Working Group <public-tt@w3.org>
Message-ID: <0981DC6F684DE44FBE8E2602C456E8AB016C0DB117@SS-IP-EXMB-01.screensystems.tv>
Thanks, good info there...

RE: 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?

I have in mind the easy re-purposing of subtitle documents that were originally created to match 'tape' style video clips (i.e. clips that include a pre-roll slate). This is still very common for distribution of media, e.g. commercials.
Subtitle documents are currently created against the full media duration (and in effect use SMPTE timecode in discontinuous modality to achieve correct subtitle synchronisation) - note these are not necessarily TTML format subtitle documents.

Use of an offsets for timing from the beginning of media would allow easy removal of the pre-roll (which is typical when such media migrate to a media file representation) by simply changing one value.

Admittedly a 'fringe' use case....

I have a tendency to view SMPTE / discontinuous as legacy, and media time as file-based... and of course using media-time timing requires more information about the related media timing.
For me, for media time documents) moving to a completely offset based approach (i.e. from start of media) is more logical than establishing a specific range of timecode values within a continuous media timebase (by using start and end or start and duration values). YMMV.
[Additionally, as discussed briefly at EBU F2F I believe there is utility in a nested timing model with 'soft / sticky' boundaries. This for me would all be 'offset' based.]

Best,
John

John Birch | Strategic Partnerships Manager | Screen
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P Before printing, think about the environment-----Original Message-----
From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
Sent: 25 September 2014 11:11
To: John Birch; Timed Text Working Group
Subject: Re: ISSUE-346: Need ttp:mediaDuration parameter [TTML2]

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.

[1]
https://dvcs.w3.org/hg/ttml/raw-file/default/ttml2/spec/ttml2.html#paramete

r-attribute-mediaDuration

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.

[2]
https://dvcs.w3.org/hg/ttml/raw-file/default/ttml2/spec/ttml2.html#semantic

s-region-layout-step-1

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,

Nigel


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
>bring?
>
>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
>
>John Birch | Strategic Partnerships Manager | Screen Main Line : +44
>1473 831700 | Ext : 2208 | Direct Dial : +44 1473 834532 Mobile : +44
>7919 558380 | Fax : +44 1473 830078 John.Birch@screensystems.tv |
>www.screensystems.tv | https://twitter.com/screensystems

>
>Visit us at
>SMPTE Annual Technical Conference, Loews Hollywood Hotel, Stand 107,
>October 21-23 Languages & the Media, Hotel Radission Blu, Berlin,
>November 5-7
>
>P Before printing, think about the environment-----Original
>Message-----
>From: Timed Text Working Group Issue Tracker
>[mailto:sysbot+tracker@w3.org]
>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]
>
>http://www.w3.org/AudioVideo/TT/tracker/issues/346

>
>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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>Screen Subtitling Systems Ltd. Registered in England No. 2596832.
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This message may contain confidential and/or privileged information. If you are not the intended recipient you must not use, copy, disclose or take any action based on this message or any information herein. If you have received this message in error, please advise the sender immediately by reply e-mail and delete this message. Thank you for your cooperation. Screen Subtitling Systems Ltd. Registered in England No. 2596832. Registered Office: The Old Rectory, Claydon Church Lane, Claydon, Ipswich, Suffolk, IP6 0EQ
Received on Thursday, 25 September 2014 10:29:38 UTC

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