ISSUE-335: 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.

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.

State:
OPEN
Product:
TTML2
Raised by:
Courtney Kennedy
Opened on:
2014-08-14
Description:
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>
Related Actions Items:
No related actions
Related emails:
  1. {agenda} TTWG Meeting 6/11/2014 (from nigel.megitt@bbc.co.uk on 2014-11-05)
  2. {minutes} TTWG Meeting 16/10/2014 (from nigel.megitt@bbc.co.uk on 2014-10-16)
  3. {agenda} TTWG Meeting 16/10/2014 (from nigel.megitt@bbc.co.uk on 2014-10-15)
  4. {agenda} TTWG Meeting 9/10/2014 (from nigel.megitt@bbc.co.uk on 2014-10-08)
  5. {agenda} TTWG Meeting 2/10/2014 (from nigel.megitt@bbc.co.uk on 2014-10-01)
  6. Re: [TTML2] Document relationship to media timings - re Issue-346, Issue-270, Issue-335 (from nigel.megitt@bbc.co.uk on 2014-09-26)
  7. Re: [TTML2] Document relationship to media timings - re Issue-346, Issue-270, Issue-335 (from glenn@skynav.com on 2014-09-26)
  8. Re: [TTML2] Document relationship to media timings - re Issue-346, Issue-270, Issue-335 (from nigel.megitt@bbc.co.uk on 2014-09-26)
  9. Re: [TTML2] Document relationship to media timings - re Issue-346, Issue-270, Issue-335 (from nigel.megitt@bbc.co.uk on 2014-09-26)
  10. Re: [TTML2] Document relationship to media timings - re Issue-346, Issue-270, Issue-335 (from glenn@skynav.com on 2014-09-26)
  11. [TTML2] Document relationship to media timings - re Issue-346, Issue-270, Issue-335 (from nigel.megitt@bbc.co.uk on 2014-09-26)
  12. RE: ISSUE-346: Need ttp:mediaDuration parameter [TTML2] (from John.Birch@screensystems.tv on 2014-09-25)
  13. Re: ISSUE-346: Need ttp:mediaDuration parameter [TTML2] (from glenn@skynav.com on 2014-09-25)
  14. Re: ISSUE-346: Need ttp:mediaDuration parameter [TTML2] (from nigel.megitt@bbc.co.uk on 2014-09-25)
  15. RE: ISSUE-346: Need ttp:mediaDuration parameter [TTML2] (from John.Birch@screensystems.tv on 2014-09-25)
  16. Re: ISSUE-346: Need ttp:mediaDuration parameter [TTML2] (from nigel.megitt@bbc.co.uk on 2014-09-25)
  17. {agenda} TTWG Meeting 25/9/2014 (from nigel.megitt@bbc.co.uk on 2014-09-24)
  18. Re: Issue-270 and Issue-335 (from nigel.megitt@bbc.co.uk on 2014-09-24)
  19. Re: Issue-270 and Issue-335 (from cyril.concolato@telecom-paristech.fr on 2014-09-24)
  20. Re: Issue-270 and Issue-335 (from nigel.megitt@bbc.co.uk on 2014-09-24)
  21. Re: Issue-270 and Issue-335 (from nigel.megitt@bbc.co.uk on 2014-09-24)
  22. Re: Issue-270 and Issue-335 (from glenn@skynav.com on 2014-09-23)
  23. Re: Issue-270 and Issue-335 (from nigel.megitt@bbc.co.uk on 2014-09-23)
  24. Re: Issue-270 and Issue-335 (from glenn@skynav.com on 2014-09-22)
  25. Issue-270 and Issue-335 (from nigel.megitt@bbc.co.uk on 2014-09-22)
  26. {minutes} TTWG Meeting 21/8/2014 (from nigel.megitt@bbc.co.uk on 2014-08-21)
  27. {agenda} TTWG Meeting 21/8/2014 (from nigel.megitt@bbc.co.uk on 2014-08-20)
  28. 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 John.Birch@screensystems.tv on 2014-08-15)
  29. 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 ckennedy@apple.com on 2014-08-15)
  30. 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 glenn@skynav.com on 2014-08-15)
  31. 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@bbc.co.uk on 2014-08-15)
  32. 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 glenn@skynav.com on 2014-08-15)
  33. 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 ckennedy@apple.com on 2014-08-15)
  34. 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@bbc.co.uk on 2014-08-15)
  35. 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 ckennedy@apple.com on 2014-08-15)
  36. 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@bbc.co.uk on 2014-08-15)
  37. 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 sysbot+tracker@w3.org on 2014-08-14)

Related notes:

If we do permit negative times, then it may be possible to restrict them to use in a begin attribute on the tt:body element. Alternatively, I have in the past proposed a ttp:mediaOffset parameter which could specify a positive or negative time offset between the begin of the document interval and the media time origin. Such a parameter could be used in conjunction with (or in the definition of) referenceBegin in [1].

[1] http://www.w3.org/TR/2013/REC-ttml1-20130924/#time-expression-semantics-media

My preference is to introduce ttp:mediaOffset rather than generalizing begin's time expressions.

Glenn Adams, 14 Aug 2014, 15:50:54

Duplicate of Issue 270.

Glenn Adams, 21 Sep 2014, 10:47:02

Addressed in https://dvcs.w3.org/hg/ttml/rev/3cbc109b90bd

Glenn Adams, 21 Sep 2014, 12:39:32

This is not a duplicate of Issue-270 but is orthogonal to it. In fact the solution proposed is the opposite approach to that suggested in Issue-270.

Specifically in the case of a specified media offset, I am puzzled about how this plays with wrapper formats like ISO BMFF, which may specify their own offset. Are the two offsets intended to be independent (and therefore additive to each other) or duplicates (and therefore in one case metadata)?

Nigel Megitt, 22 Sep 2014, 13:30:38

Reopening this issue because we did not achieve consensus for the implemented edit and during our F2F in Santa Clara [1] an alternative approach in which the mediaBegin time is stated was agreed, with the caveat that playrate change consequences need to be worked through.

[1] http://www.w3.org/2014/10/27-tt-minutes.html#item14

Nigel Megitt, 19 Nov 2014, 17:12:45

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