Re: EBU Segmentation document

Hi Mike,

Some comments:

1) Intermediate Synchronic Document (ISD) and Segmentation

The use of the concept of Intermediate Synchronic Document in this 
context can be confusing. IMO the main problem is unrelated.

Let's take a source document with only two subtitles/p elements that 
shall be broken into smaller target documents:

<p begin="00:00:00" end="00:00:10" xml:id="sub1" region="r1">Foo</p>
<p begin="00:00:01" end="00:00:03" xml:id="sub1" region="r2">Bar</p>

You would generate three ISD´s:

IDS 1 = [0s,1s]
ISD 2 = [1s,3s]
ISD 3 = [3s,10s]

Even if you make 3 documents samples of the exact duration of the IDS's 
(1s,2s and 7s) the concept of the ISD does not give a hint about a 
“continued” subtitle. In this case the subtitle with the xml:id "sub1" 
will be shown in all three samples.

If you generate samples with a fixed length of 1s (for example) subtitle 
with the xml:id  "sub1" will continue over 9 documents (not counting the 
first one).

2) xml:id versus new metadata
It can be discussed why we should not take xml:id as unique identifier 
instead of two new metadata elements. I think is possible to overload 
the semantics of xml:id in a specific context. For example it could make 
the constraint that xml:id in a stream of subtitle documents has to 
identify always the same subtitle.

We had the discussion in the EBU-TT context and there were more than one 
opinion that it would be difficult (or not desired) to manage identity 
over more than one document.

You could argue that a sample is in this case not stateless anymore 
because you need all previous send documents to check this constraint.

3) Validation and fallback behavior
Regardless which approach will be taken: the rendering client is still 
responsible to check if two "subtitles" of different documents are the 
"same". The region and the computed style set must be the same. An 
identifier is just a hint.

If you give a hint for a “continued” subtitle: what happens if this is 
not correct e.g. one subtitle in document n is marked as the same as in 
document n-1 but although it as the same content the subtitle has not 
the same computed styleset (e.g. another background color).


Best regards,

Andreas


Am 06.12.2013 01:47, schrieb Michael Dolan:
>
> 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:
>
> <pbegin="00:07:51:00"end="00:07:52:01" 
> ttm:continues="theLastLine">text that spans documents</p>
>
> Doc #2:
>
> <pbegin="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
>
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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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