{Minutes} TTWG Meeting 2020-11-26

Thanks all for attending today's TTWG meeting. Minutes can be found in HTML format at https://www.w3.org/2020/11/26-tt-minutes.html


In text format:

   [1]W3C

      [1] https://www.w3.org/


                Timed Text Working Group Teleconference

26 November 2020

   [2]Previous meeting. [3]Agenda. [4]IRC log.

      [2] https://www.w3.org/2020/11/19-tt-minutes.html

      [3] https://github.com/w3c/ttwg/issues/160

      [4] https://www.w3.org/2020/11/26-tt-irc


Attendees

   Present
          Atsushi, Cyril, Nigel, Pierre

   Regrets
          Andreas, Gary

   Chair
          Nigel

   Scribe
          nigel

Contents

    1. [5]This meeting
    2. [6]MPEG Liaison regarding ISO/IEC 14496-30 (carriage of
       TTML in MP4) #167
    3. [7]Meeting Close

Meeting minutes

  This meeting

   Nigel: Just one topic on the agenda really, the MPEG liaison
   … Does anyone want time to discuss Patent Policy 2020?

   Cyril: No, waiting on position internally - I will follow up.

   Nigel: Any other business?

   Pierre: Pull Request on IMSC Test Repository

   Nigel: Okay

  MPEG Liaison regarding ISO/IEC 14496-30 (carriage of TTML in MP4) #167

   github: [8]https://github.com/w3c/ttwg/issues/167


      [8] https://github.com/w3c/ttwg/issues/167


   Nigel: The member-tt reflector archive doesn't allow access to
   the attachment.

   Atsushi: I've emailed the systeam about it, it may take some
   time

   Nigel: Nevertheless hopefully we all have the liaison by email?

   Cyril: Also I checked internally: MPEG has a spreadsheet of
   liaison orgs and who is appointed.
   … From W3C side it says Jeff Jaffe is the officer. If that is
   not the case W3C should send a message back asking to update
   it.

   <atsushi> [9]https://www.w3.org/2001/11/StdLiaison


      [9] https://www.w3.org/2001/11/StdLiaison


   Atsushi: From W3C side there are 3 WGs related, and we have on
   the page I linked above
   … 9 so from our side, it is complicated.
   … We have liaison between WG and WG, so I am not sure how we
   organise liaisons in such
   … a complex scenario, but W3C does need to think about it.
   … And also if we have time to look at the liaison table we have
   WG11 from ISO pointed to MPEG but it seems
   … SC29 organise the WG coordination so we need to update that
   also.

   Cyril: Yes WG11 does not exist anymore.
   … I was also checking, it includes Jean Claude Dufourd who is
   no longer involved.
   … Vlad could be the liaison for the font part maybe.
   … But I think in general the right way is to send to the
   secretariat of SC29 and
   … optionally to send to the Chair at the same time if we know
   who that is.

   Atsushi: Usually the team contact is used for each WG. For
   general liaisons we point to Jeff as CEO.
   … I will talk to Philippe after the Thanksgiving holiday about
   how to update the table.

   Nigel: Thanks for the important admin. Looking at the substance
   of this liaison.
   … It looks like the request is for how to express the
   relationship between the different timelines
   … by aligning their zero points.

   Cyril: Last time we discussed this, Pierre suggested using
   "document time zero" as the origin, rather than "timeline".
   … I mentioned the " [document temporal coordinate space] " too
   which is defined in TTML2.

   Nigel: Yes, I noticed that term earlier too.

   Cyril: I had an action from last time to draft an update.
   … (to part 30).
   … [shares screen]
   … [reads through proposed update to 14496-30]
   … I had a question last time. The previous text talked about
   "body" but we should avoid it because it is not
   … clear to me if the outermost element is the region or the
   body, given ISD construction.
   … I noted last time not to use "document timeline"
   … Possibly use ISD though that could be confusing.
   … The other suggestion was to use a formula, saying time X
   output from the TTML decoder maps to the ISOBMFF timeline
   … I'm not necessarily looking for feedback on the rest from
   TTWG.
   … The idea is to say all TTML documents share the same timeline
   with the same origin that
   … matches the presentation timeline in the ISOBMFF file.
   … And then say that the sample times clip the document times,
   and that's it.
   … The rest is not really relevant.

   <Zakim> nigel, you wanted to ask about discontinuous - is it
   allowed?

   Nigel: Does 14496-30 constrain the timebase? If smpte
   discontinuous is allowed then we have to do something.

   Cyril: We touched on this last time - it would be great
   feedback to say it would make life easier if support were
   removed.

   Nigel: Specifically it's for discontinuous markerMode.

   Cyril: Yes then the times are treated as labels and can't be
   compared, right?

   Nigel: Right
   … The whole notion of timing changes because you have to have
   some other thing that issues labels that you can compare.
   … It doesn't really fit with the timing model of ISOBMFF, I
   think.
   … It would be useful to clarify, if it does not mention at the
   moment.
   … Having done that, I think that [document temporal coordinate
   space] is the correct terminology to use.
   … And I wonder what is supposed to happen if `clock` timebase
   is used?!

   Cyril: My understanding is that smpte timebase is not used
   extensively but wallclock times may be used in a dash
   environment.

   Nigel: I'm not sure how they would, but conceivably.
   … The BBC specifies an epoch for the DASH packaging that's
   equivalent to the UNIX epoch, so all times are relative to the
   start of 1970.
   … But in the TTML they'd be media timebase, otherwise it's a
   bit painful going over midnight boundaries.

   Cyril: Okay, how do you want to proceed Nigel?
   … We want to respond to MPEG?
   … The next MPEG meeting is early January.
   … It would be great to have a response of some sort by then,
   even just to get the ball rolling

   Nigel: I hoped we would get somewhere today and then draft the
   response as a later step.

   Cyril: What do we want to respond? To say "use [document
   temporal coordinate space] " if you want to refer to a
   timeline?
   … Or use "computed times" to find a match to document time?

   Nigel: I think it's misleading to think about body or region
   etc
   … Instead any time anything changes as defined by the TTML
   document, the time of the change
   … is a coordinate in the [document temporal coordinate space]
   so it makes sense to say that.

   Cyril: That makes sense, yes.

   Nigel: Unless there are unusual time modes in ISOBMFF I can't
   see how it makes sense to use anything other than media
   timeBase.
   … That's what it was defined for.

   Cyril: yes
   … The clock time base could have use cases, but I'm not sure
   how to use it.

   Nigel: Q: If you had a clock time expression, how would you
   relate that to the presentation timeline in ISOBMFF?

   Cyril: I'd ask back how do you relate the TTML to the related
   media object.

   Pierre: I think this is too complex - just use the ISD times.

   Nigel: My point is that unless you can relate an ISOBMFF
   presentation timeline to a clock time then it makes no sense to
   use clock timeBase.

   Cyril: That makes sense, we should clarify media timebase only.

   Nigel: Yes

   Cyril: OK, assuming that, then Pierre you said it's simple to
   use ISD start and end times and we should use that.

   Pierre: Anything else is misleading. If you have seq and par
   containers then having a time in the middle of a document
   doesn't mean anything.

   Cyril: I'm reluctant to start talking about ISD in Part 30 but
   if the group thinks it is needed then we could.

   Pierre: If you're looking to reduce implementor confusion then
   the only thing that makes sense is the time coordinates
   … on the ISDs. That's the interface between ISOBMFF and TTML.
   How you generate those coordinates in the TTML is a TTML
   authoring issue.

   Nigel: I agree, I'm not sure if we have a clearly defined term
   for the computed time that applies to the beginning of each
   ISD.

   Pierre: 11.3.1.3 Intermediate Synchronic Document Construction
   defines this.

   [10]11.3.1.3 Intermediate Synchronic Document Construction

     [10] https://www.w3.org/TR/ttml2/#semantics-region-layout-step-1


   Cyril: easier to use bullet 2 [resolve timing] than [construct
   intermediate document]

   Pierre: That's fine as well.

   Nigel: That is formally where it is defined.
   … So those resolved times T0, T1 etc are times on the document
   temporal coordinate space.

   Cyril: It sounds obvious, that's the only possibility here.

   Nigel: That's it I think, it makes sense and is super clear.

   Cyril: Yes I think that could work.

   Pierre: I would avoid as much as possible talking about how you
   author a TTML document, in the ISO document.

   Cyril: I would say "each document in the sample produces a set
   of times T0, T1 etc and they're all placed on this timeline,
   and time zero
   … on the timeline is time zero on the presentation timeline".
   … What I'm not sure is, it seems a bit circular. You need to
   know what is the active time duration of the document instance.
   … You have that by looking at the first and last times.

   Pierre: You're getting a string of digits, in seconds.
   … Imagine the rule is to take the output of the ISD
   construction process, and each Ti is an offset into the ISOBMFF
   track timeline.
   … If it says 2s that's literally 2s into the ISOBMFF timeline.
   … Then I can author a TTML document that generates that 2s.

   Nigel: I can see Cyril's concern that "active time duration" is
   not clear and some implementers might think that Ti is relative
   to
   … the beginning of the active time duration, not an absolute
   value.

   Cyril: §8.1.3 has text that seems to be a bit confusing when it
   comes to root temporal extent and document temporal coordinate
   space.

   Nigel: I can see "active time interval" too.
   … I think the text about active time interval and the root
   temporal extent wording on body is orthogonal to this question
   of alignment of timelines.
   … What I read that text to mean is that if any descendant of
   body might extend temporally beyond the end of the body's
   duration, then
   … it will not generate an ISD at that point; this is separate
   to the alignment of those ISD times Ti.

   Cyril: How do you define active time interval then, for the
   document?
   … It needs to be clear if ISOBMFF refers to it. I still think
   there is some ambiguity.

   Nigel: What ambiguity do you see?

   Cyril: It's a bit circular. To compute the active time duration
   you need T0 .. Tn and the active duration is Tn-T0.

   Nigel: But I don't think you care. What you need to know is how
   those values Tn relate to the ISOBMFF track timeline.

   Cyril: We need to say which values lie outside the period
   during which an ISOBMFF sample is active.

   Nigel: Have to think about truncation as well as active vs not
   active ISD times.

   Cyril: Is it correct to say that the active time interval of
   the document is the active time interval of the body element
   possibly clipped by the root temporal extent?

   Nigel: I wouldn't.

   Pierre: What's the point of doing that?

   Cyril: If someone asks me what the active duration is I want to
   be able to answer that.

   Pierre: Why would they want to understand this? Seriously, they
   should read the spec.
   … Something that's not said here under resolved timing is that
   the first thing you do is interpret every begin, end and dur
   according
   … to SMIL semantics, as per 12.4.
   … That will give you unambiguously on every element an absolute
   computed begin and end.
   … That is implied in this [resolve timing] step. Then what you
   call active duration doesn't really matter.
   … All that really matters are those time coordinates.

   Nigel: I'm with that.

   Pierre: We could improve the text for sure!

   Cyril: I don't want ISOBMFF, when it defines the clipping, to
   conflict with any defined active time duratoin.

   Pierre: It's the same in IMF, where it's called the playable
   region which is a subset of the time spanned by all the ISDs,
   often.
   … [sorry I've got to drop off the call now]

   Cyril: I have enough I think.

   Nigel: One more thing that may or may not be helpful, but I
   believe it is true that the root temporal extent is defined
   … to be the same as the ISOBMFF sample period.

   Cyril: I think so, yes.
   … It is defining the external presentation context, the
   ISOBMFF.

   Nigel: Exactly.
   … I think the two key points are temporal alignment, and
   clipping.

   Cyril: Yes, I think people get clipping, but maybe not
   alignment, especially in the live case.
   … There may be no sample with presentation time zero, and the
   ISO document was talking about "start of track" which for
   … some people might be ambiguous.

   Nigel: Makes complete sense.
   … In terms of the liaison response, to help you draft 14496-30,
   what is useful to send back?

   Cyril: Two things:
   … 1. Constrain to media timeBase, because clock or smpte
   doesn't match your expectation.
   … 2. We advise MPEG to use the resolved timing procedure in
   TTML2 which produces a list of time coordinates
   … to align with the track timeline. We suggest using that
   wording.

   Nigel: Okay makes sense, I will draft something and share here
   before sending back.

   SUMMARY: @nigelmegitt to draft response based on conversation

  Meeting Close

   Nigel: It's amazing how fast time goes when you're talking
   about time! We're out of it for today. [adjourns meeting]


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