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]

Hi Nigel,

Ok, we can close this then.

Best Regards,
Courtney 

Sent from my iPhone

> On Aug 15, 2014, at 9:03 AM, Nigel Megitt <nigel.megitt@bbc.co.uk> wrote:
> 
> Hi Courtney,
> 
> I agree it’s a real world situation, but I don’t understand why your proposal is better than just putting the TTML file through a transformation processor that adjusts all the times – hence my questions. 
> 
> At the point when you know what offset value to put into the document you know what all the correct times should be, don’t you?
> 
> If yes, you can already solve this problem with TTML. 
> If no, how do you assign the offset value?
> 
> Am I missing something extra?
> 
> Kind regards,
> 
> Nigel
> 
> 
> From: Courtney Kennedy <ckennedy@apple.com>
> Date: Friday, 15 August 2014 16:31
> To: Nigel Megitt <nigel.megitt@bbc.co.uk>
> Cc: Timed Text Working Group <public-tt@w3.org>
> Subject: 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]
> 
> HI Nigel,
> 
> This is a real world situation that I have encountered with some content.  For whatever reason, the producers of the subtitles cannot use the same start time as the producers of the video and audio.  I think there is a benefit to have all the information within the subtitles file rather than having it in a sideband file which can get lost or separated from the subtitles file.
> 
> Best Regards,
> Courtney
> 
>> On Aug 15, 2014, at 5:54 AM, Nigel Megitt <nigel.megitt@bbc.co.uk> wrote:
>> 
>> Hi Courtney,
>> 
>> I¹m 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?
>> 
>> I¹ve 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,
>> 
>> Nigel
>> 
>> 
>> 
>> 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]
>>> 
>>> 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>
> 
> _____________________________________________
> Courtney Kennedy 408.974.3386, mobile: 408.771.8615
> Engineering Manager, Media Sharing
> Apple, Inc.
> 
> 
> 
> 
>