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]

On Fri, Aug 15, 2014 at 11:58 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
wrote:

>   I think we should keep it open and add support for ttp:mediaTimeOffset
> which could be negative.
>
>
>  Can you explain your rationale please?
>

At first order, it is a much more simple solution that performing
transformation processing to edit times. Think of it as a useful
convenience function. I've had requests for both positive and negative
media offset expressions from a number of sources.


>
>
>    On Fri, Aug 15, 2014 at 11:30 AM, Courtney Kennedy <ckennedy@apple.com>
> wrote:
>
>>  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.
>>
>>
>>
>>
>>
>>
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