IRC log of tt on 2020-11-26

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Meeting: Timed Text Working Group Teleconference
16:03:11 [nigel]
scribe: nigel
16:03:19 [nigel]
Agenda: https://github.com/w3c/ttwg/issues/160
16:03:44 [nigel]
Previous meeting: https://www.w3.org/2020/11/19-tt-minutes.html
16:03:52 [nigel]
Present: Pierre, Atsushi, Nigel
16:03:56 [nigel]
Chair: Nigel
16:04:03 [nigel]
Regrets: Andreas, Gary
16:04:43 [nigel]
Topic: This meeting
16:04:55 [nigel]
Nigel: Just one topic on the agenda really, the MPEG liaison
16:05:02 [nigel]
Present+ Cyril
16:05:23 [nigel]
.. Does anyone want time to discuss Patent Policy 2020?
16:05:40 [nigel]
Cyril: No, waiting on position internally - I will follow up.
16:05:52 [nigel]
Nigel: Any other business?
16:06:00 [nigel]
Pierre: Pull Request on IMSC Test Repository
16:06:02 [nigel]
Nigel: Okay
16:06:37 [nigel]
Topic: MPEG Liaison regarding ISO/IEC 14496-30 (carriage of TTML in MP4) #167
16:06:46 [nigel]
github: https://github.com/w3c/ttwg/issues/167
16:07:25 [nigel]
Nigel: The member-tt reflector archive doesn't allow access to the attachment.
16:07:36 [nigel]
Atsushi: I've emailed the systeam about it, it may take some time
16:08:11 [nigel]
Nigel: Nevertheless hopefully we all have the liaison by email?
16:08:25 [nigel]
Cyril: Also I checked internally: MPEG has a spreadsheet of liaison orgs and who is appointed.
16:08:46 [nigel]
.. 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.
16:09:00 [atsushi]
https://www.w3.org/2001/11/StdLiaison
16:09:11 [nigel]
Atsushi: From W3C side there are 3 WGs related, and we have on the page I linked above
16:09:30 [nigel]
.. 9 so from our side, it is complicated.
16:09:57 [nigel]
.. We have liaison between WG and WG, so I am not sure how we organise liaisons in such
16:10:08 [nigel]
.. a complex scenario, but W3C does need to think about it.
16:10:27 [nigel]
.. And also if we have time to look at the liaison table we have WG11 from ISO pointed to MPEG but it seems
16:10:38 [nigel]
.. SC29 organise the WG coordination so we need to update that also.
16:10:45 [nigel]
Cyril: Yes WG11 does not exist anymore.
16:10:55 [cyril]
cyril has joined #tt
16:11:14 [nigel]
.. I was also checking, it includes Jean Claude Dufour who is no longer involved.
16:11:24 [nigel]
.. Vlad could be the liaison for the font part maybe.
16:11:37 [nigel]
.. But I think in general the right way is to send to the secretariat of SC29 and
16:11:47 [nigel]
.. optionally to send to the Chair at the same time if we know who that is.
16:12:05 [cyril]
s/Dufour/Dufourd/
16:12:14 [nigel]
Atsushi: Usually the team contact is used for each WG. For general liaisons we point to Jeff as CEO.
16:12:26 [nigel]
.. I will talk to Philippe after the Thanksgiving holiday about how to update the table.
16:12:44 [nigel]
Nigel: Thanks for the important admin. Looking at the substance of this liaison.
16:13:49 [nigel]
.. It looks like the request is for how to express the relationship between the different timelines
16:13:54 [nigel]
.. by aligning their zero points.
16:14:17 [nigel]
Cyril: Last time we discussed Pierre suggested using "document time zero" as the origin, rather than "timeline".
16:14:49 [nigel]
.. I mentioned the " [document temporal coordinate space] " too which is defined in TTML2.
16:14:57 [nigel]
Nigel: Yes, I noticed that term earlier too.
16:15:17 [nigel]
Cyril: I had an action from last time to draft an update.
16:15:26 [nigel]
.. (to part 30).
16:15:47 [nigel]
.. [shares screen]
16:16:44 [nigel]
.. [reads through proposed update to 14496-30]
16:17:17 [nigel]
q+ to ask about discontinuous - is it allowed?
16:18:00 [nigel]
.. I had a question last time. The previous text talked about "body" but we should avoid it because it is not
16:18:10 [nigel]
.. clear to me if the outermost element is the region or the body, given ISD construction.
16:18:26 [nigel]
.. I noted last time not to use "document timeline"
16:18:36 [nigel]
.. Possibly use ISD though that could be confusing.
16:19:01 [nigel]
.. The other suggestion was to use a formula, saying time X output from the TTML decoder maps to the ISOBMFF timeline
16:19:10 [nigel]
.. I'm not necessarily looking for feedback on the rest from TTWG.
16:19:23 [nigel]
.. The idea is to say all TTML documents share the same timeline with the same origin that
16:19:32 [nigel]
.. matches the presentation timeline in the ISOBMFF file.
16:19:45 [nigel]
.. And then say that the sample times clip the document times, and that's it.
16:19:51 [nigel]
.. The rest is not really relevant.
16:19:53 [nigel]
q?
16:19:56 [nigel]
ack n
16:19:56 [Zakim]
nigel, you wanted to ask about discontinuous - is it allowed?
16:20:36 [nigel]
Nigel: Does 14496-30 constrain the timeline? If smpte discontinuous is allowed then we have to do something.
16:20:51 [nigel]
Cyril: We touched on this last time - it would be great feedback to say it would make life easier if support were removed.
16:21:05 [nigel]
Nigel: Specifically it's for discontinuous markerMode.
16:21:16 [nigel]
Cyril: Yes then the times are treated as labels and can't be compared, right?
16:21:18 [nigel]
Nigel: Right
16:21:49 [nigel]
.. The whole notion of timing changes because you have to have some other thing that issues labels that you can compare.
16:22:01 [nigel]
.. It doesn't really the timing model of ISOBMFF, I think.
16:22:29 [nigel]
.. It would be useful to clarify, if it does not mention at the moment.
16:22:42 [nigel]
.. Having done that, I think that [document temporal coordinate space] is the correct terminology to use.
16:24:29 [nigel]
.. And I wonder what is supposed to happen if `clock` timebase is used?!
16:24:53 [nigel]
Cyril: My understanding is that smpte timebase is not used extensively but wallclock times may be used in a dash environment.
16:25:01 [nigel]
Nigel: I'm not sure how they would, but conceivably.
16:25:30 [nigel]
.. 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.
16:25:50 [nigel]
.. But in the TTML they'd be media timebase, otherwise it's a bit painful going over midnight boundaries.
16:26:00 [nigel]
Cyril: Okay, how do you want to proceed Nigel?
16:26:05 [nigel]
.. We want to respond to MPEG?
16:26:12 [nigel]
.. The next MPEG meeting is early January.
16:26:28 [nigel]
.. It would be great to have a response of some sort by then, even just to get the ball rolling
16:26:52 [nigel]
Nigel: I hoped we would get somewhere today and then draft the response as a later step.
16:27:12 [nigel]
Cyril: What do we want to respond? To say "use [document temporal coordinate space] " if you want to refer to a timeline?
16:27:21 [nigel]
.. Or use "computed times" to find a match to document time?
16:28:26 [nigel]
Nigel: I think it's misleading to think about body or region etc
16:28:44 [nigel]
.. Instead any time anything changes as defined by the TTML document, the time of the change
16:28:58 [nigel]
.. is a coordinate in the [document temporal coordinate space] so it makes sense to say that.
16:29:03 [nigel]
Cyril: That makes sense, yes.
16:29:48 [nigel]
Nigel: Unless there are unusual time modes in ISOBMFF I can't see how it makes sense to use anything other than media timeBase.
16:29:57 [nigel]
.. That's what it was defined for.
16:29:58 [nigel]
Cyril: yes
16:30:20 [nigel]
.. The clock time base could have use cases, but I'm not sure how to use it.
16:31:08 [nigel]
Nigel: Q: If you had a clock time expression, how would you relate that to the presentation timeline in ISOBMFF?
16:31:27 [nigel]
Cyril: I'd ask back how do you relate the TTML to the related media object.
16:32:37 [nigel]
Pierre: I think this is too complex - just use the ISD times.
16:32:54 [nigel]
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.
16:33:02 [nigel]
Cyril: That makes sense, we should clarify media timebase only.
16:33:04 [nigel]
Nigel: Yes
16:33:19 [nigel]
Cyril: OK, assuming that, then Pierre you said it's simple to use ISD start and end times and we should use that.
16:33:40 [nigel]
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.
16:34:02 [nigel]
Cyril: I'm reluctant to start talking about ISD in Part 30 but if the group thinks it is needed then we could.
16:34:27 [nigel]
Pierre: If you're looking to reduce implementor confusion then the only thing that makes sense is the time coordinates
16:34:48 [nigel]
.. on the ISDs. That's the interface between ISOBMFF and TTML. How you generate those coordinates in the TTML is a TTML authoring issue.
16:36:06 [nigel]
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.
16:36:22 [nigel]
Pierre: 11.3.1.3 Intermediate Synchronic Document Construction defines this.
16:36:45 [nigel]
-> https://www.w3.org/TR/ttml2/#semantics-region-layout-step-1 11.3.1.3 Intermediate Synchronic Document Construction
16:37:02 [nigel]
Cyril: easier to use bullet 2 [resolve timing] than [construct intermediate document]
16:37:09 [nigel]
Pierre: That's fine as well.
16:37:18 [nigel]
Nigel: That is formally where it is defined.
16:38:17 [nigel]
.. So those resolved times T0, T1 etc are times on the document temporal coordinate space.
16:38:24 [nigel]
Cyril: It sounds obvious, that's the only possibility here.
16:38:37 [nigel]
Nigel: That's it I think, it makes sense and is super clear.
16:38:41 [nigel]
Cyril: Yes I think that could work.
16:38:57 [nigel]
Pierre: I would avoid as much as possible talking about how you author a TTML document, in the ISO document.
16:39:23 [nigel]
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
16:39:35 [nigel]
.. on the timeline is time zero on the presentation timeline".
16:39:59 [nigel]
.. 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.
16:40:07 [nigel]
.. You have that by looking at the first and last times.
16:40:21 [nigel]
Pierre: You're getting a string of digits, in seconds.
16:40:41 [nigel]
.. Imagine the rule is to take the output of the ISD construction process, and each Ti is an offset into the ISOBMFF track timeline.
16:40:53 [nigel]
.. If it says 2s that's literally 2s into the ISOBMFF timeline.
16:41:02 [nigel]
.. Then I can author a TTML document that generates that 2s.
16:44:29 [nigel]
Nigel: I can see Cyril's concern that "active time duration" is not clear and some implementers might think that Ti is relative to
16:44:44 [nigel]
.. the beginning of the active time duration, not an absolute value.
16:45:46 [nigel]
Cyril: ยง8.1.3 has text that seems to be a bit confusing when it comes to root temporal extent and document temporal coordinate space.
16:45:53 [nigel]
Nigel: I can see "active time interval" too.
16:47:50 [nigel]
.. 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.
16:48:25 [nigel]
.. 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
16:48:51 [nigel]
.. it will not generate an ISD at that point; this is separate to the alignment of those ISD times Ti.
16:49:09 [nigel]
Cyril: How do you define active time interval then, for the document?
16:49:38 [nigel]
.. It needs to be clear if ISOBMFF refers to it. I still think there is some ambiguity.
16:49:45 [nigel]
Nigel: What ambiguity do you see?
16:50:08 [nigel]
Cyril: It's a bit circular. To compute the active time duration you need T0 .. Tn and the active duration is Tn-T0.
16:51:03 [nigel]
Nigel: But I don't think you care. What you need to know is how those values Tn relate to the ISOBMFF track timeline.
16:51:46 [nigel]
Cyril: We need to say which values lie outside the period during which an ISOBMFF sample is active.
16:52:10 [nigel]
Nigel: Have to think about truncation as well as active vs not active ISD times.
16:52:40 [nigel]
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?
16:52:43 [nigel]
Nigel: I wouldn't.
16:52:49 [nigel]
Pierre: What's the point of doing that.
16:53:02 [nigel]
Cyril: If someone asks me what the active duration is I want to be able to answer that.
16:53:20 [nigel]
Pierre: Why would they want to understand this? Seriously, they should read the spec.
16:53:40 [nigel]
.. Something that's not said here under resolved timing is that the first thing you do is interpret every begin, end and dur according
16:53:48 [nigel]
.. to SMIL semantics, as per 12.4.
16:54:07 [nigel]
.. That will give you unambiguously on every element an absolute computed begin and end.
16:54:28 [nigel]
.. That is implied in this [resolve timing] step. Then what you call active duration doesn't really matter.
16:54:36 [nigel]
.. All that really matters are those time coordinates.
16:54:42 [nigel]
Nigel: I'm with that.
16:54:51 [nigel]
Pierre: We could improve the text for sure!
16:55:11 [nigel]
Cyril: I don't want ISOBMFF, when it defines the clipping, to conflict with any defined active time duratoin.
16:55:28 [nigel]
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.
16:56:11 [nigel]
.. [sorry I've got to drop off the call now]
16:56:19 [nigel]
Cyril: I have enough I think.
16:56:55 [nigel]
Nigel: One more thing that may or may not be helpful, but I believe it is true that the root temporal extent is defined
16:57:03 [nigel]
.. to be the same as the ISOBMFF sample period.
16:57:06 [nigel]
Cyril: I think so, yes.
16:57:20 [nigel]
.. It is defining the external presentation context, the ISOBMFF.
16:57:25 [nigel]
Nigel: Exactly.
16:57:50 [nigel]
.. I think the two key points are temporal alignment, and clipping.
16:58:10 [nigel]
Cyril: Yes, I think people get clipping, but maybe not alignment, especially in the live case.
16:58:30 [nigel]
.. There may be no sample with presentation time zero, and the ISO document was talking about "start of track" which for
16:58:36 [nigel]
.. some people might be ambiguous.
16:58:42 [nigel]
Nigel: Makes complete sense.
16:59:18 [nigel]
.. In terms of the liaison response, to help you draft 14496-30, what is useful to send back?
16:59:21 [nigel]
Cyril: Two things:
16:59:33 [nigel]
.. 1. Constrain to media timeBase, because clock or smpte doesn't match your expectation.
16:59:54 [nigel]
.. 2. We advise MPEG to use the resolved timing procedure in TTML2 which produces a list of time coordinates
17:00:21 [nigel]
.. to align with the track timeline. We suggest using that wording.
17:00:38 [nigel]
Nigel: Okay makes sense, I will draft something and share here before sending back.
17:00:54 [nigel]
SUMMARY: @nigelmegitt to draft response based on conversation
17:01:15 [nigel]
Topic: Meeting Close
17:01:47 [nigel]
Nigel: It's amazing how fast time goes when you're talking about time! We're out of it for today. [adjourns meeting]
17:01:50 [nigel]
rrsagent, make minutes v2
17:01:50 [RRSAgent]
I have made the request to generate https://www.w3.org/2020/11/26-tt-minutes.html nigel
17:08:59 [nigel]
s/Last time we discussed Pierre suggested/Last time we discussed this, Pierre suggested
17:09:44 [nigel]
s/Does 14496-30 constrain the timeline/Does 14496-30 constrain the timebase
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s/It doesn't really the timing model of ISOBMFF/It doesn't really fit with the timing model of ISOBMFF
17:14:22 [nigel]
s/What's the point of doing that./What's the point of doing that?
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rrsagent, make minutes v2
17:16:03 [RRSAgent]
I have made the request to generate https://www.w3.org/2020/11/26-tt-minutes.html nigel
17:20:59 [nigel]
scribeOptions: -final -noEmbedDiagnostics
17:21:07 [nigel]
zakim, end meeting
17:21:07 [Zakim]
As of this point the attendees have been Pierre, Atsushi, Nigel, Cyril
17:21:08 [Zakim]
RRSAgent, please draft minutes v2
17:21:08 [RRSAgent]
I have made the request to generate https://www.w3.org/2020/11/26-tt-minutes.html Zakim
17:21:12 [Zakim]
I am happy to have been of service, nigel; please remember to excuse RRSAgent. Goodbye
17:21:16 [Zakim]
Zakim has left #tt
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rrsagent, excuse us
17:26:44 [RRSAgent]
I see no action items