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 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<mailto:ckennedy@apple.com>>
Date: Friday, 15 August 2014 16:31
To: Nigel Megitt <nigel.megitt@bbc.co.uk<mailto:nigel.megitt@bbc.co.uk>>
Cc: Timed Text Working Group <public-tt@w3.org<mailto: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<mailto: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<mailto: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.