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Re: ISSUE-335 (Negative times for offsets): In order to handle offsets between start time in TTML docs and start time in video, allow negative times to be used in fragment begin times. [TTML.next]

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Fri, 15 Aug 2014 12:54:46 +0000
To: Timed Text Working Group <public-tt@w3.org>, Courtney Kennedy <ckennedy@apple.com>
Message-ID: <D013BF3C.FED8%nigel.megitt@bbc.co.uk>
Hi Courtney,

Iım puzzled by the implied workflow here: if the subtitle file and the
video have been created, at what point is the subtitle file modified to
include the new offset? And if someone or some system is making such an
edit why not simply make the times in the TTML correct against the video,
rather than adding an offset?

Iıve seen this issue arise before, when packaging TTML documents in ISO
BMFF (or some other wrapper). In that case the packaging is likely to
happen after production of all the media that would be wrapped so it seems
like the best way to capture any offset is using the facilities provided
by the wrapper rather than editing the content itself. Certainly ISO BMFF
appears to offer enough parameters/attributes to support that use case.

I guess the key structural point is that there is a need to signal
equivalence of some time reference in the TTML with some other time
reference in a specific rendition of some related media. At the moment
this is expected to happen externally to the TTML document: why would we
bring it inside the document, given that no explicit link exists from
within a TTML 1 SE document to a related media object?

Kind regards,


On 14/08/2014 16:33, "Timed Text Working Group Issue Tracker"
<sysbot+tracker@w3.org> wrote:

>ISSUE-335 (Negative times for offsets): In order to handle offsets
>between start time in TTML docs and start time in video, allow negative
>times to be used in fragment begin times. [TTML.next]
>Raised by: Courtney Kennedy
>On product: TTML.next
>Use case:  
>Subtitles files may be created separately from video and audio for any
>particular piece of content.  Subtitles may be created in different
>facilities and at different points in time than the original content.  As
>a result of this decoupling, sometimes the subtitles file will use a
>different start time than the video and audio.
>Time expressions in sub-elements are relative to the time expressions in
>their parent elements, as described in section 10.2.4 of the TTML
>When subtitles have non-zero start times relative to the video they are
>to be synchronized with, the parent div element can have an offset in the
>begin attribute which, when applied to the times in the samples within
>the div element, will produce time expressions that synchronize with
>The following example uses this offset to indicate that the titles are
>using start time of 01:00:00:00, and require adjustment before their
>values express the actual time they should appear in the video.
><div begin="-01:00:00:00">
>   <p begin="01:00:05:00" end="01:00:10:00">
>   This text should appear at 00:00:05:00
>   </p>
Received on Friday, 15 August 2014 12:55:21 UTC

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