RE: EBU Segmentation document

Hi Michael,

I’m interested in why you are unhappy with times in the document that would lie outside the sample epoch?

Most container formats state something along the lines of… “content is not valid outside the sample duration”.  Which is fine, and IMHO having e.g. begin and end times that exceed the sample epoch does not violate this concept. Having a begin and end time outside the epoch does not necessarily mean a decoder has to activate the content before the sample epoch (or after it).

Further I’m interested in how other track types are handled… for example audio is often sampled at a different rate to video… consequently audio and video samples are unlikely to align. E.g. How do containers handle the overlap of audio segments across sample boundaries?

I much fear we might be discussing and designing a solution to  a ‘non problem’.

I have reservations about using IDs to link documents… there are any number of potential issues there.

Allowing temporal overlap does at least mean that each document can be independent, whilst retaining some ability for a decoder to optimise for continuity of presentation, regardless of play direction. Further it re-inforces the concept that the subtitle durations are independent of the video (and audio) samples… E.g. subtitle temporal accuracy and continuation of presentation should ideally occur at the specified time points regardless of video playback frame rates or stutters due to adaptive adjustments etc.

Best regards,
John


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From: Michael Dolan [mailto:mdolan@newtbt.com]
Sent: 06 December 2013 00:48
To: 'Timed Text Working Group'
Subject: RE: EBU Segmentation document

ACTION-250: https://www.w3.org/AudioVideo/TT/tracker/actions/250


As I suggested below, I think the use cases in the BBC/EBU contribution can be met without relaxing the temporal constraint as proposed.  It involves two metadata attributes that are used to signal that there exists a temporal continuity between the last synchronic document in one instance document and the first synchronic document in another instance document.

ttm:continues=[xs:string]  This indicates that the identified content may be continued in another document with later, adjacent temporal extent.  The value of the attribute is an identifier that uniquely identifies the content in both documents. There may be more than one of these per document, but the string values must be unique. The scope of uniqueness of the identifier value is the current instance document plus the immediately following (temporally) instance document.

ttm:continuedFrom=[xs:string]  This indicates that the identified content may have been continued from another document with earlier, adjacent temporal extent.  And further, the content that this attribute applies to is identical in every way in both documents. The presentation engine may ignore the content in the current document. This does not mean the two synchronic documents are identical; just the identified content. The value of the attribute is an identifier that uniquely identifies the content from an earlier, temporally adjacent document. There may be more than one of these per document. The scope of uniqueness of the identifier value is the current instance document plus the immediately preceding (temporally) instance document.

For example:

Doc #1:
<p begin="00:07:51:00" end="00:07:52:01" ttm:continues="theLastLine">text that spans documents</p>

Doc #2:
<p begin="00:07:52:01" end="00:07:53:00" ttm:continuedFrom="theLastLine">text that spans documents</p>

Let me know if I have missed some use case or if this won’t work.

Regards,
                Mike

From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
Sent: Thursday, November 21, 2013 8:46 AM
To: John Birch; Michael Dolan; 'Timed Text Working Group'
Subject: Re: EBU Segmentation document

I also agree!

Although… a use case we should consider is bi-directional playing/scanning of content – this was mentioned to me last week (apologies I forget who but I have a pretty good idea it was one of 2 people I'm thinking of!). To facilitate this any forward-facing information should be mirrored as backwards-facing, i.e. the 'continuation of previous' and begin times preceding sample begin time should be permitted.

In the previous discussions was there any consideration of the scope of xml:id within the set of [continued] documents? Normally xml:id is unique per document, but there would be an argument here for extending the uniqueness within the set of connected documents.

Kind regards,

Nigel


On 21/11/2013 16:34, "John Birch" <John.Birch@screensystems.tv<mailto:John.Birch@screensystems.tv>> wrote:

I very much agree…

Although… *even* in a perfect decoder world I would suggest that ‘a priori’ knowledge of the end time of content (i.e. an end time that is effective in a subsequent document) would be useful in improving decoder efficiency. I.e. knowing that display of content spans the next sample boundary can be used to optimise the decoder. The indication that current content was a continuation of previous content is arguably perhaps less relevant.

Best regards,
John

John Birch | Strategic Partnerships Manager | Screen
Main Line : +44 1473 831700 | Ext : 270 | Direct Dial : +44 1473 834532
Mobile : +44 7919 558380 | Fax : +44 1473 830078
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From: Michael Dolan [mailto:mdolan@newtbt.com]
Sent: 21 November 2013 16:26
To: 'Timed Text Working Group'
Subject: RE: EBU Segmentation document

This general topic has come and gone over the years, and a remnant of an early discussion remains in draft TTML2, Appendix L [1].

This proposal is different in that the segments are all valid and complete TTML documents. This is good since more recent analysis suggests that creating document ”pieces” doesn’t actually solve a problem. We should consider removing Appendix L in TTML2.

The focus of this appears to be the management of the temporal extent.  I recommend the title and introduction be clarified as “segment” is misleading.

This issue was also discussed in the past.  A suggestion at the time was to add two attributes that: 1) indicated the content is continued (repeated) in a subsequent document; and 2) indicated that the content is a continuation of an earlier document. This preserves the contained temporal extent of each document.

In a perfect decoder world, this should not be necessary if the end/start times of adjacent documents are identical.  The resulting synchronic documents at the temporal boundary would be identical and the decoder should not glitch and remove the content and flash.  The attributes would “help” the decoder do the right thing.

Regards,

                Mike

[1] https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html#streaming



From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
Sent: Monday, November 11, 2013 6:14 AM
To: Timed Text Working Group
Subject: EBU Segmentation document

All,

As per Action-235 please see this draft document on segmentation of EBU-TT, for reference with respect to Issue-288. Note that it is not finalised within EBU and may be published as a purely informative document rather than a normative one. There is therefore no formal errata document, however some areas have been commented on and may change, including the substantive change described below.

Section 4.1.5 states that content outside a sample's temporal extent shall not be displayed. However in the draft EBU-TT-D specification it is permitted in fault scenarios for processors to make use of the knowledge of any subtitles that extend outside the sample's temporal extent to be displayed, as a 'graceful recovery' option only, in the case that the sample that should be active has not been received.

Kind regards,

Nigel





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