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Re: ISSUE-346: Need ttp:mediaDuration parameter [TTML2]

From: Glenn Adams <glenn@skynav.com>
Date: Thu, 25 Sep 2014 10:12:54 -0600
Message-ID: <CACQ=j+cnyKa8Wvtf3rnNR6PcgDVsbY492x_V1o537fa9g0gjzA@mail.gmail.com>
To: John Birch <John.Birch@screensystems.tv>
Cc: Timed Text Working Group <public-tt@w3.org>
On Thu, Sep 25, 2014 at 8:36 AM, John Birch <John.Birch@screensystems.tv>
wrote:

>  I cannot see how mediaduration can always be known… it certainly cannot
> be ‘a priori’ knowledge in the streaming live video scenario.
>

Please read the drafted text at [1].

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


>  Consequently I do not see how TTML to ISD conversion can be normatively
> dependent on a parameter that is potentially unavailable.
>

It just means that in such a case, it a duration must be approximated at
some point in time or that everything must be explicitly timed (i.e., no
reliance on indefinite duration containers) or that conversion is not
possible.


>
>
> Am I misunderstanding and it is proposed that media duration is mandatory?
> (only when using media timebase?)
>

No. Read the draft text.


>
>
> Re: SMIL assumes they are relative to beginning of related media.
>
>
>
> Whilst I am certainly no expert on SMIL, I was under the impression that
> SMIL supports the concept of a sync-base that allows the origin point of
> timed events to be offset from a specific time point in the ‘host document’.
>

True, but we didn't support in TTML the full generality of SMIL timing
options. Let's just say that, at least for media time base, it is assumed
by TTML1 that document times are related to BEGIN(media), not
ORIGIN(media). This is no different from assuming that the origin of the
root container extent (in TTML1) is aligned with the origin of the related
media object (video).


>  TTML does not explicitly form a timing relationship with a related media
> within the TTML document, that relationship is established by normative
> prose in the specification – and thus could be elaborated upon.
>

Let's just say that in TTML1 this relationship is only discussed in the
Note in Appendix N.2, but it has been widely assumed and used in other
contexts, e.g., VTT, translation to HTML, etc.:

The above formalisms assumes that the *Root Temporal Extent* corresponds
with the beginning of a related media object. If this assumption doesn't
hold, then an additional offset that accounts for the difference may be
introduced when computing media time M.


>
>
> Re: However, apparently SMPTE time base authors have been using the origin
> of the time line of related media.
>
>
>
> I am not sure what you mean by ‘origin of the time line of related media’.
> The use of SMPTE timebase with ‘continuous’ is (rightly IMHO) deprecated.
>

Granted that for discontinuous mode my comment doesn't apply. Consider my
comment to apply only to smpte continuous mode *and* in the case that a
discontinuous mode document has been converted to smpte continuous or media
time base.


> To effectively use SMPTE timebase you must be in discontinuous mode to
> engage the **reading of timecode values from the frames of the related
> media** - rather than use the play time of the related media – even if
> expressed as SMPTE12M. Timecode is and always has been a labelling scheme
> (that can – if certain constraints are met – also be used to calculate
> durations).
>

Yes, this is true for "SMPTE timecode" usage in the normal discontinuous
based interpretation, but not for SMIL clock-time expressions.


> Since these constraints cannot be guaranteed in the TTML context (i.e. it
> has no knowledge of external media timecode contiguity) then
> ‘discontinuous’ must be used.
>

I don't want to turn this thread into a discussion of the utility of smpte
continuous mode, but I should say that I don't discount its utility as you
appear to do.


> Further, if continuous was used with smpte timebase in a TTML document,
> then the time values in the TTML would have to be relative to the first
> frame of the media – even while being expressed in SMPTE12M… and
> consequently would typically be significantly offset from the timecode
> labels that might exist within the media itself (e.g. by 10 hours or 3
> hours etc.).
>

I am not making that assumption: that time values in a smpte continuous
mode TTML document are relative to the first frame of the media. In fact,
I'm assuming that there may be an arbitrary offset between the two, just as
I'm assuming there may be an arbitrary (positive or negative) offset in the
media time base scenario.


> Yet further, those values would also have to take into account a
> potentially variable length slate that might be added, extended or
> truncated during the processing of media files, necessitating a change of
> the TTML document every time the related media object was processed (which
> is of course undesirable). This is of course all very broadcast media
> centric – but of course that is the domain of smpte timecode!
>

I'm not following this, since I don't know the meaning of "variable length
slate".


>
>
> Since SMPTE must be discontinuous,
>

Nope, we have smpte continuous mode in TTML1 and just today decided to keep
it in TTML2. [1]

[1] http://www.w3.org/AudioVideo/TT/tracker/issues/349


> the ‘origin of a timeline’ is irrelevant? (as the Document Processing
> Context emits labelled synchronisation events).
>

It is definitely not irrelevant in media time base, in smpte continuous
mode, or in conversion of smpte discontinuous mode to media or smpte
continuous time base.


>
>
> Best regards,
>
> John
>
>
>
>
>
>
>
>
> *John Birch | Strategic Partnerships Manager | Screen *Main Line : +44
> 1473 831700 | Ext : 2208 | Direct Dial : +44 1473 834532
> Mobile : +44 7919 558380 | Fax : +44 1473 830078
> John.Birch@screensystems.tv | www.screensystems.tv |
> https://twitter.com/screensystems
>
>
>
> *Visit us at SMPTE Annual Technical Conference, Loews Hollywood Hotel,
> Stand 107, October 21-23 Languages & the Media, Hotel Radission Blu,
> Berlin, November 5-7 *
>
> *P** Before printing, think about the environment*
>
> *From:* Glenn Adams [mailto:glenn@skynav.com]
> *Sent:* 25 September 2014 14:15
> *To:* John Birch
> *Cc:* Timed Text Working Group
> *Subject:* Re: ISSUE-346: Need ttp:mediaDuration parameter [TTML2]
>
>
>
>
>
>
>
> On Thu, Sep 25, 2014 at 3:38 AM, John Birch <John.Birch@screensystems.tv>
> wrote:
>
> I am uncertain about the 'requirement' for a duration parameter?
> What is the current situation... does TTML1 work as if the media duration
> is 'indefinite' ?
> In which case what additional benefit does having a defined duration bring?
>
>
>
> The TTML to ISD conversion process will normatively use this parameter. It
> is also used by the proposed CP5 for conversion to HTML.
>
>
>
>
> Of far more importance (IMHO) is a media start value - that allows time
> values within a document to be 'offset' relative to the start of associated
> media.
>
>
>
> We have an interesting issue here: what are authors using for the origin
> of document times. SMIL assumes they are relative to beginning of related
> media. However, apparently SMPTE time base authors have been using the
> origin of the time line of related meda. But this is unrelated to
> ttp:mediaDuration.
>
>
>
>
> Best regards,
> John
>
> John Birch | Strategic Partnerships Manager | Screen
> Main Line : +44 1473 831700 | Ext : 2208 | Direct Dial : +44 1473 834532
> Mobile : +44 7919 558380 | Fax : +44 1473 830078
> John.Birch@screensystems.tv | www.screensystems.tv |
> https://twitter.com/screensystems
>
> Visit us at
> SMPTE Annual Technical Conference, Loews Hollywood Hotel, Stand 107,
> October 21-23
> Languages & the Media, Hotel Radission Blu, Berlin, November 5-7
>
> P Before printing, think about the environment-----Original Message-----
> From: Timed Text Working Group Issue Tracker [mailto:sysbot+tracker@w3.org
> ]
> Sent: 21 September 2014 13:35
> To: public-tt@w3.org
> Subject: ISSUE-346: Need ttp:mediaDuration parameter [TTML2]
>
>
> ISSUE-346: Need ttp:mediaDuration parameter [TTML2]
>
> http://www.w3.org/AudioVideo/TT/tracker/issues/346
>
> Raised by: Glenn Adams
> On product: TTML2
>
> In order to perform ISD processing, it is necessary to know the duration
> of root external extent. When associate with a related media object, this
> is the simple duration of the related media object. If this is known at
> authoring time, then it is an important parameter that should be specified.
> I propose defining a ttp:mediaDuration parameter attribute that takes
> either an offset-time form of a time expression or the keyword
> "indefinite", where the latter is used (or implied) when no related media
> object exists or its simple duration is unknown or indefinite. If this
> parameter is not specified, then it would be treated as if indefinite were
> specified.
>
>
>
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>
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> any action based on this message or any information herein. If you have
> received this message in error, please advise the sender immediately by
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Received on Thursday, 25 September 2014 16:13:42 UTC

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