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