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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: Timed Text Working Group Issue Tracker <sysbot+tracker@w3.org>
Date: Thu, 14 Aug 2014 15:33:14 +0000
Message-Id: <E1XHx1e-0005Hp-72@shauna.w3.org>
To: public-tt@w3.org
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]

http://www.w3.org/AudioVideo/TT/tracker/issues/335

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.  

Proposal:  

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 specification.

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 video.


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>
</div>
Received on Thursday, 14 August 2014 15:33:15 UTC

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