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15:00:53 <Zakim> Meeting: Timed Text Working Group Teleconference
15:02:44 <nigel> Agenda: https://github.com/w3c/ttwg/issues/124
15:02:48 <nigel> scribe: nigel
15:03:05 <nigel> Previous meeting: https://www.w3.org/2020/06/25-tt-minutes.html
15:04:03 <nigel> Present: Cyril, Gary, Pierre, Nigel
15:04:08 <nigel> Chair: Gary, Nigel
15:04:23 <cyril> cyril has joined #tt
15:04:33 <nigel> Present+ Andreas
15:04:36 <nigel> Topic: This meeting
15:05:08 <nigel> Present+ Atsushi
15:05:15 <nigel> Nigel: Today we have 3 main topics.
15:05:16 <atai> atai has joined #tt
15:05:25 <nigel> .. 1. IMSC 1.2, not sure if there's a lot we can cover
15:05:38 <nigel> .. 2. TTML2 2nd Ed
15:05:46 <nigel> .. 3. TTML Profile Registry
15:05:57 <nigel> .. Is there any other business, or points to make sure we cover during those?
15:06:09 <nigel> Cyril: Yes, as an AOB we could discuss what happened at MPEG and the issue raised about
15:06:16 <nigel> .. the notion of empty documents, as an FYI maybe.
15:06:44 <nigel> Topic: IMSC 1.2 ARIB liaison response
15:06:59 <nigel> Nigel: I haven't had time to collate our thoughts into a draft liaison response yet.
15:07:09 <nigel> .. Not aware of any other points to discuss on this issue?
15:10:00 <nigel> Topic: TTML2 Implementation Report
15:10:12 <nigel> Cyril: I have an action to check the details of the IR are correct, but the other thing to note
15:10:23 <nigel> .. is that we will need implementations.
15:10:33 <nigel> Nigel: We have one, but only for one presentation feature, at the moment!
15:11:07 <nigel> .. I had the impression that the TTT tool will be updated to pass all the validation tests?
15:11:16 <nigel> Cyril: Yes, but the timeline might be a bit far.
15:11:32 <nigel> Nigel: Right, others are welcome of course.
15:11:46 <nigel> Topic: CSS font-matching algorithm may introduce fingerprinting issues w3c/ttml2#1202 (PING review)
15:12:11 <nigel> Nigel: Just an FYI that Sam Weiler did contact me, and the action is with me to look for a time
15:12:21 <nigel> .. when interested parties can have a discussion to try to resolve the issue.
15:12:36 <nigel> .. I plan to issue a doodle or similar to try to find such a time.
15:12:52 <pal> pal has joined #tt
15:12:58 <nigel> .. Anyone who wants to join that, please do let me know - I imagine at least Pierre and me.
15:13:01 <nigel> Pierre: Yes, of course.
15:13:52 <nigel> Nigel: I don't think there's much more to say on this topic at the moment - Andreas made
15:14:03 <nigel> .. some really helpful suggestions on the github issue, but I don't think they added any
15:14:12 <nigel> .. normative language, and that seems to be the sticking point with Sam at the moment.
15:14:23 <nigel> Topic: TTML Profile Registry
15:14:46 <nigel> Nigel: Editor Glenn has stepped down as Editor, and I've stepped up just to fill the gap,
15:14:55 <nigel> .. but would be very happy if anyone else wants to volunteer.
15:15:11 <nigel> .. I take the hint from Cyril that we can consider all of our documents to be joint ventures.
15:15:49 <nigel> .. Mike Dolan remains an Editor also.
15:16:07 <nigel> .. I want to flag up that we would like to discuss the infamous long-standing issue 71, and
15:16:14 <nigel> .. Mike can join us on the call next week for that.
15:18:54 <nigel> .. [summarises issue as being about the + and | operators in the codecs parameter not being defined]
15:19:00 <nigel> .. Two main decisions:
15:19:11 <nigel> .. 1. How to define - do we need to reference something in TTML2 or not?
15:19:23 <nigel> .. (and if in TTML2 what if that means a change to TTML2)
15:19:38 <nigel> .. 2. Where to define - in the current registry text or elsewhere in the document.
15:19:51 <nigel> .. This has implications about needing to update our registration with IANA or not.
15:20:27 <nigel> -> https://github.com/w3c/tt-profile-registry/issues/71 w3c/tt-profile-registry#71
15:21:01 <nigel> Cyril: It would be easy to discuss if we had a concrete text but I don't know if I will have
15:21:04 <nigel> .. time to produce the text.
15:21:48 <nigel> Nigel: Yes, though we may need to understand where the likely area of consensus will be first.
15:22:15 <nigel> Cyril: Regrets from me for next week and the week after - but please do proceed without me.
15:22:50 <nigel> Topic: AOB - MPEG
15:23:00 <nigel> Cyril: MPEG has been virtually meeting this week.
15:24:00 <nigel> .. I'll paste a link to the discussion that started this AOB.
15:24:01 <cyril> link https://github.com/MPEGGroup/CMAF/issues/9
15:24:21 <nigel> s|link https://github.com/MPEGGroup/CMAF/issues/9||
15:24:30 <nigel> -> https://github.com/MPEGGroup/CMAF/issues/9 CMAF issue
15:24:37 <nigel> Cyril: This may affect TTML and WebVTT.
15:25:00 <nigel> .. In CMAF there is a duration, and it is common for there to be no subtitles at some times,
15:25:06 <nigel> .. especially at the end, e.g. during the credits.
15:25:18 <nigel> .. You have to do some padding, but you can also ask the same question between subtitles
15:25:30 <nigel> .. especially in live cases. When you have no subtitle content for some period, what should be delivered?
15:25:48 <nigel> .. In ISO there are 3 specs involved: CMAF, ISOBMFF and MPEG-3 Part 30.
15:26:01 <nigel> .. There is a tool in ISOBMFF called duration is empty, which allows you to say there is no
15:26:22 <nigel> .. sample in an MPEG-4 segment. It's not explicitly allowed or disallowed in either Part 20 or CMAF.
15:26:36 <nigel> .. There's a practice where most people use empty documents when there is no content to
15:26:48 <nigel> .. be streamed. This has been documented as a possibility in MPEG-4 Part 30.
15:26:55 <pal> q+
15:27:00 <nigel> .. The issue raised is if it should be a recommendation in CMAF.
15:27:12 <nigel> .. The discussion is ongoing and MPEG welcomes feedback on how people should do it.
15:27:30 <nigel> .. My opinion is that if the industry uses empty documents probably we should go for that.
15:27:46 <nigel> .. But in terms of design it would be better to use the duration is empty flag. If you have a
15:28:02 <nigel> .. subtitle generator followed by a packager followed by a DASHer, then the DASHer part
15:28:13 <nigel> .. should not need to know the flavour of subtitle format used, ideally. It may not be how
15:28:14 <atai> q+
15:28:18 <nigel> .. it is implemented in many workflows.
15:28:22 <nigel> ack pal
15:28:34 <nigel> Pierre: As a data point, in IMF, an upstream master format, there is a requirement to use
15:28:41 <nigel> .. filler timed text documents, presumably empty.
15:28:42 <nigel> q+
15:28:54 <nigel> Cyril: I thought there was one single document in IMF
15:29:05 <nigel> Pierre: No, there can be any number but there can't be a part of the timeline with no document
15:29:08 <nigel> .. associated with it.
15:29:21 <nigel> .. I have seen people put in dub cards and associated an empty document, but I've also
15:29:31 <nigel> .. seen people point to an empty part of an existing document. Both are permitted.
15:29:42 <nigel> Cyril: That would be an interesting data point for the MPEG issue.
15:29:58 <nigel> Pierre: It's a slightly different issue for masters, but the lesson is that defining at the package
15:30:11 <nigel> .. level the absence of essence is ambiguous because, what is video black? There is not one
15:30:23 <nigel> .. definition of that. The decision, heavily influenced by it being a master format, is that
15:30:36 <nigel> .. you cannot have empty essence tracks at any point in the timeline, to remove any ambiguity.
15:30:49 <nigel> Cyril: I understand and agree. It makes sense. The context may be different in the streaming
15:30:56 <nigel> .. context, where there may be absence of things.
15:31:15 <nigel> Pierre: Yes, and consumers may not mind so much about black picture or audio artefacts at boundaries.
15:31:25 <nigel> .. What is the objection to having empty documents, technically?
15:31:45 <nigel> Cyril: It highly depends on the workflow, right. In MP4Box, the way it is architectured, the
15:31:56 <nigel> .. DASHing part is completely media unaware. Having to know that you are DASHing
15:32:03 <nigel> .. TTML and having to create an empty document for that.
15:32:08 <nigel> Pierre: How would that work with video?
15:32:21 <nigel> Cyril: Same thing. When you DASH and are missing something you could use duration is empty.
15:32:30 <nigel> Pierre: Is that what MP4Box uses today, for video?
15:32:40 <nigel> Cyril: The question was never raised because you don't have gaps in your timeline. The
15:32:43 <nigel> .. streams are not sparse.
15:32:58 <nigel> Pierre: Again when you get to dub cards, audio can be sparsed. Historically people have
15:33:10 <nigel> .. chosen not to do that.
15:33:13 <nigel> ack atai
15:33:29 <nigel> Andreas: Question: I thought in part 30 the usage of an empty doc as a filler is mandatory.
15:33:34 <nigel> Cyril: No, there's a "may".
15:33:53 <nigel> Andreas: OK, I think we (IRT) always recommended to use it when we worked with streaming
15:34:06 <nigel> .. encoder and packager manufacturers. We would say they should use an empty document.
15:34:20 <nigel> Cyril: Just to respond to that, it makes sense if the authoring system provides empty documents.
15:34:31 <nigel> .. I wrote in the issue, and Nigel commented on it, that it would be weird for the authoring
15:34:40 <pal> q+
15:34:42 <nigel> .. system to provide heartbeat empty documents when there is no content, but it seems
15:34:46 <nigel> .. I was wrong.
15:34:59 <nigel> Andreas: I know that TTML does a lot of things in MP4 that are really different, and how
15:35:07 <pal> q-
15:35:08 <nigel> .. WebVTT is used is more common to how MP4 tracks are built.
15:35:29 <nigel> .. In the EBU Tech3381 spec there is some clarification on part 30 that may be helpful for EBU-TT-D.
15:35:44 <cyril> q+
15:35:44 <nigel> .. An empty document is defined. But it doesn't say it must be used.
15:36:00 <nigel> .. One last question: is it possible to use 2 options, duration zero and an empty document
15:36:10 <nigel> Cyril: I don't think it is possible.
15:36:13 <nigel> q?
15:36:17 <nigel> ack nige
15:36:47 <nigel> Nigel: Tech3381's empty document is defined as "a processor must be able to accept" or
15:37:05 <nigel> .. similar wording - I think in a formal sense that empty document is not conformant to EBU-TT-D.
15:37:09 <nigel> .. Just a side note.
15:38:12 <nigel> .. On the point of defining content that the DASHer can use while being type-unaware,
15:38:21 <nigel> .. how does it handle init segments now?
15:39:38 <nigel> Cyril: Just describing MP4Box, there is an import phase that is media aware, and it creates
15:39:52 <nigel> .. an init segment once. Then the DASHer is copying things over without looking at the media
15:39:54 <nigel> .. any more.
15:40:09 <nigel> .. To the question about an empty segment, yes, while being media unaware, the DASHer
15:40:23 <nigel> .. could architecturally have a way to deliver empty documents. That is an option.
15:40:42 <nigel> .. I'd like to come back to the point about empty EBU-TT-D documents. Did I understand
15:40:52 <atai> q+
15:41:08 <nigel> .. that there are different views of what an empty TTML document is?
15:41:09 <cyril> ack cyril
15:41:21 <nigel> Nigel: Clearly yes in the sense that there are any number of documents you can construct
15:41:36 <nigel> .. that have no presentational effect, for example those just containing metadata.
15:41:55 <nigel> .. Specifically in EBU-TT-D there is a cardinality constraint so that body must contain
15:42:02 <nigel> .. at least one div and div must contain at least one p.
15:42:21 <nigel> .. That means that the conformant way to generate an empty document is to have no body.
15:42:35 <nigel> .. But I know one encoder that doesn't do this, it puts out an empty div.
15:43:13 <nigel> Cyril: It could crash a player?
15:44:20 <nigel> Nigel: I don't know, that's part of the issue, but it's conformant EBU-TT-D so a player should
15:44:29 <nigel> .. be able to deal with a document with no body.
15:44:47 <nigel> Pierre: We should call the "empty" document a "minimal" one!
15:44:51 <nigel> q?
15:44:53 <nigel> ack at
15:45:09 <nigel> Andreas: The reason we defined the empty document in Tech3381 is because part 30 is unclear about it.
15:45:28 <pal> q+
15:45:29 <nigel> .. It is clear that anybody else not using EBU-TT-D may make their own interpretation of an empty document.
15:45:47 <nigel> .. In 3381 we clearly said what an empty doc is, I'm not sure exactly about conformance with EBU-TT-D.
15:45:58 <nigel> .. The other point is that Tech3381 is referenced by HbbTV for DASH, so there may be
15:46:08 <nigel> .. other standards that refer to that definition of an empty document.
15:46:27 <nigel> Cyril: Yes, the question is not to remove it, but maybe to document it clearly in CMAF, if it is the practice in use.
15:46:40 <nigel> Andreas: I know streaming encoders use it, maybe not all of them, but definitely some of them do it.
15:46:43 <cyril> q+
15:46:43 <nigel> ack pal
15:47:00 <nigel> Pierre: I like the idea of being unambiguous. The other thing I wanted to suggest is,
15:47:11 <nigel> .. what about MPEG actually formally asking TTWG for an opinion?
15:47:22 <nigel> .. They could propose a spec to TTWG and ask for an answer.
15:47:37 <nigel> Cyril: I suggested liaising with different entities, and they could include W3C, for sure.
15:47:51 <nigel> Pierre: In the past I've seen MPEG just do something, making a quick decision, but in this case,
15:47:57 <nigel> .. maybe we should try something different.
15:48:08 <nigel> Cyril: That's why I brought it here and brought Nigel into the tickets.
15:48:21 <nigel> Pierre: If you need help in MPEG taking time to do this well then I'm happy to help.
15:48:24 <nigel> .. It is not a simple topic.
15:48:37 <nigel> Cyril: Feel free to raise any point you think is difficult to handle in the ticket; that will give
15:48:43 <nigel> .. me arguments to say we should take our time.
15:48:59 <nigel> .. I have one other small point about TTML in MP4, related to the MPEG meeting.
15:49:10 <nigel> .. Mike Dolan came with a contribution about the use of TTML in DASH, possibly in the
15:49:25 <nigel> .. context of ATSC, indicating that it is ambiguous how to create MP4 segments containing
15:49:30 <nigel> .. TTML, with respect to timing.
15:49:42 <nigel> .. We had discussions about the DASH media presentation timeline, the period timeline,
15:50:00 <nigel> .. and the MP4 presentation timeline. Overall I think we agreed, and Steve Perrot from BBC
15:50:12 <nigel> .. gave an example, of how it should be working. We noticed something suboptimal in
15:50:17 <nigel> .. the MP4 spec, Part 30:
15:50:40 <cyril> The top-level internal timing values in the timed text samples based on TTML express times on the track presentation timeline – that is, the track media time as optionally modified by the edit list. For example, the begin and end attributes of the <body> element, if used are relative to the start of the track, not relative to the start of the sample. This is shown in the figure below, using W3C TTML syntax
15:50:43 <nigel> s/Steve Perrot/Stephen Perrott
15:51:25 <nigel> Cyril: The reason this was written this way is two-fold.
15:51:43 <nigel> .. I think, timing values when they're nested in time container are relative to the parent
15:51:45 <nigel> .. container?
15:51:50 <nigel> Nigel: Depends on the value of timeContainer.
15:51:53 <nigel> Cyril: par and seq?
15:51:56 <nigel> Nigel: Yes
15:52:16 <nigel> Cyril: We thought only the top level is significant. But times can be on the body, div, p, span etc.
15:52:39 <nigel> .. The second sentence with the example about body refers to a figure that does not have
15:53:07 <nigel> .. timing on body, so we have raised a defect report. Do you think we have a better way
15:53:09 <nigel> .. to say the first part?
15:53:30 <nigel> Nigel: I would say that the computed times on TTML elements are on the track presentation timeline.
15:53:52 <nigel> .. In other words, don't delve into the time computation algorithms defined in TTML, but just
15:53:57 <nigel> .. reference the resulting values.
15:56:07 <nigel> .. You could also say that the media timebase coordinates are coincident with the track presentation timeline.
15:56:20 <nigel> .. Also top level timings are not only on body but also region.
15:56:35 <nigel> Cyril: Individuals can propose wording but a proposal from TTWG would be good.
15:57:01 <nigel> Nigel: I'm sure we could try to do that, but MPEG should come and ask us.
15:57:22 <nigel> Cyril: Yes, but we could liaise to say "we've been made aware, here's an answer". I know it's not good practice.
15:57:52 <nigel> Pierre: Bad things have happened from this in the past!
15:58:01 <nigel> Cyril: We have a big overlap here with MPEG membership.
15:58:14 <nigel> Andreas: I would support what Cyril says, to try to help solve the problem regardless of
15:58:22 <nigel> .. the best formal practice.
15:58:34 <nigel> Pierre: The reason I am resistant is not because I don't want to offer a good solution.
15:58:45 <nigel> .. Rather, if we offer a solution as TTWG, then we will need to be there in person at some
15:58:58 <nigel> .. impossible time at some point in the future to defend that input. If we are not willing to
15:59:04 <nigel> .. do that, then it's not going to happen!
15:59:27 <nigel> Cyril: Honestly, this will be people known to this group.
15:59:42 <nigel> .. For example Mike has a view on this, and we could invite him here.
15:59:48 <nigel> Pierre: That would already be a lot better.
16:00:35 <nigel> Nigel: This is really important for us, thank you for raising it.
16:00:55 <nigel> Cyril: If I'm not here next week, it would be good to ask Mike about this too.
16:01:02 <nigel> Pierre: Can you summarise this all?
16:01:13 <nigel> Cyril: We will write it as a defect report. I will ask if it can be public.
16:01:28 <nigel> Pierre: I encourage you to ask it to be forwarded to TTWG so people are aware this is happening.
16:01:31 <nigel> Cyril: I will ask. Thank you.
16:01:36 <nigel> Pierre: Thanks for bringing this up.
16:01:42 <nigel> Nigel: Thanks from me too.
16:01:46 <nigel> Topic: Meeting close
16:02:18 <nigel> Nigel: We're out of time today, thanks everyone. See you next week if you can make it! [adjourns meeting]
16:02:22 <nigel> rrsagent, make minutes v2
16:02:22 <RRSAgent> I have made the request to generate https://www.w3.org/2020/07/02-tt-minutes.html nigel
16:18:49 <nigel> s/Editor Glenn has/Glenn has
16:20:19 <nigel> s|CMAF issue|CMAF issue: Explicit Padding of Subtitles and Timed Text tracks to comply with CMAF track/CMAF presentation model
16:21:09 <nigel> s/duration is empty/duration-is-empty/g
16:22:40 <nigel> s/duration zero and an empty document/duration-is-empty and an empty document
16:23:48 <nigel> s/deal with a document with no body./deal with a document with no body. I agree this is a player question primarily.
16:31:16 <nigel> rrsagent, make minutes v2
16:31:16 <RRSAgent> I have made the request to generate https://www.w3.org/2020/07/02-tt-minutes.html nigel
16:32:28 <nigel> scribeOptions: -final -noEmbedDiagnostics
16:32:31 <nigel> zakim, end meeting
16:32:31 <Zakim> As of this point the attendees have been Cyril, Gary, Pierre, Nigel, Andreas, Atsushi
16:32:33 <Zakim> RRSAgent, please draft minutes v2
16:32:33 <RRSAgent> I have made the request to generate https://www.w3.org/2020/07/02-tt-minutes.html Zakim
16:32:36 <Zakim> I am happy to have been of service, nigel; please remember to excuse RRSAgent.  Goodbye
16:32:40 <Zakim> Zakim has left #tt
16:33:11 <nigel> rrsagent, excuse us
16:33:11 <RRSAgent> I see no action items