14:59:51 RRSAgent has joined #tt 14:59:51 logging to https://www.w3.org/2021/05/27-tt-irc 14:59:54 RRSAgent, make logs Public 14:59:55 Meeting: Timed Text Working Group Teleconference 15:01:40 scribe: nigel 15:01:44 Present+ Nigel, Pierre 15:01:46 Chair: Nigel 15:02:02 Regrets: Gary, Cyril 15:03:07 Previous meeting: https://www.w3.org/2021/05/13-tt-minutes.html 15:09:04 Present+ Atsushi 15:09:30 Agenda: https://github.com/w3c/ttwg/issues/186 15:09:42 Regrets+ Andreas 15:10:03 Topic: This meeting 15:10:15 Nigel: We're low on number today, so not sure we can cover all the agenda items. 15:10:39 .. In particular, I think we need Cyril and/or Glenn for the Shear calculations issue. 15:10:52 .. We may be able to discuss the ISD pull request. 15:11:17 .. The fingerprinting PR looks ready to merge. 15:11:56 Atsushi: I raised the TPAC joint meeting topic thanks to my calendar reminder - it's 4 months out. 15:12:13 .. We may need to raise the actual meeting 1 month before, and start thinking about it 2-3 months before. 15:12:24 .. It's just a heads-up for gathering requirements and ideas. 15:13:12 Nigel: The one thing I'd really flag up is to work with the CSS community to try to make progress on line-based formatting. 15:13:41 .. I suggest I make a comment on the fingerprinting vectors PR, to say "ready to merge" 15:13:51 .. and we postpone discussion about shear 15:14:06 .. but that Pierre and I could use this time usefully to discuss the ISD PR. 15:14:21 .. I think that will be all for today, but anything else? 15:14:33 Pierre: no, let's do it! 15:15:04 Topic: Fix #1232 by clarifying the [resolve timing] procedure w3c/ttml2#1233 15:15:10 github: https://github.com/w3c/ttml2/pull/1233 15:16:08 Nigel: [summarises current state] 15:16:21 Pierre: What is the problem with the spec as it stands? 15:16:46 Nigel: I think it's a problem of ease of understanding, mainly. 15:17:19 .. It seems to me from observation that different readers have different understandings of the ISDs that get generated. 15:17:47 Pierre: Maybe that's the crux of the issue. There's a huge difference between what an implementation will generate and how the spec 15:18:02 .. defines an ISD. Today the spec does not require an implementation to generate anything. 15:18:07 .. It just defines an ISD. 15:18:17 Nigel: It gives a procedure for generating them. 15:18:28 Pierre: I don't think it compels an implementation to generate that sequence. 15:18:43 .. The reason is that in some cases the implementation doesn't want an ISD, it just wants to know the rendering at time X. 15:18:54 .. It's output is not a sequence of ISDs, just one rendering. 15:19:13 .. I agree with you that in the marketplace I've seen confusion about the rendering process of TTML in general. 15:19:27 .. But I think that's different than what the current text says, which I think is fine. 15:19:37 .. It could be improved with an explanation or notes. 15:20:09 Nigel: I agree - I've written an implementation that doesn't touch ISDs at all, which is fine. 15:20:45 Pierre: Conceptually, in my mind, not at an implementation or conformance level, a TTML document can be represented by a sequence of ISDs. 15:20:58 .. Here's a procedure to generate that conceptual sequence of ISDs. 15:21:17 .. You might ask why you care about those ISDs. If you're trying to render something it's a really useful concept. 15:22:01 Nigel: This came out of MPEG work in progress where they want to write a spec that explicitly references the set of ISDs that gets generated. 15:23:02 .. So it seems important that everyone agrees on what the ISDs are. 15:25:35 Pierre: A really confusing consequence of the current language is that if a body has a begin time then there's no ISD before then. 15:25:41 .. Or is there an empty one? 15:26:01 Nigel: It's dancing on the head of a pin to try to identify the difference between an empty ISD or no ISD. 15:26:22 Pierre: There's a huge difference between saying that a document defines behaviour from a to b, and saying that it is always 15:26:32 .. defined from 0 to infinity but may contain empty ISDs. 15:26:52 .. If you generalise it to go from 0 to infinity then that makes the MPEG spec easier to define, because there are no special cases. 15:27:21 .. Whatever temporal interval the ISOBMFF sample selects, there's always something there. The current text as proposed, 15:27:41 .. which might be what Glenn intended, though it's hard to tell, implies that outside the begin and end of the body, somebody 15:27:55 .. could read the spec and say that the renderer returns an error, document not defined. 15:28:33 Nigel: It's the document processing context that defines what happens outside the root temporal extent. 15:30:03 Pierre: It's doing the industry a disservice to say we are not going to define it. 15:30:20 Nigel: I'm nervous that defining ISDs when they're not there could break some applications. 15:30:43 .. For example I think EBU-TT Live defines behaviour based on the root temporal extent. 15:32:38 .. We could go back to my proposal from our F2F at Apple a few years back, and define attributes on the tt element that allow the 15:33:36 .. beginning of the first ISD and the end of the last one to be defined, and default them to zero and infinity respectively. 15:34:18 Pierre: It's not clear to me why the root temporal extent is defined by the body, given that regions have timing. 15:34:39 Nigel: I don't think body does define the root temporal extent, it has a part to play, but the document processing context can modify it 15:34:43 .. however it likes. 15:35:38 Pierre: The tt element defines the root temporal extent. 15:35:47 -> https://www.w3.org/TR/ttml2/#document-structure-vocabulary-tt 8.1.1 tt 15:36:47 .. We could do a significant amount of work to define this more clearly. 15:37:04 .. It's worth exploring the proposal you made about defining it on the tt element - it would be totally explicit. 15:37:56 Nigel: Note that it's a proposal for clip times rather than an offset relative to which the document times are computed. 15:38:08 Pierre: It's a statement that the document behaviour is not defined outside of those. 15:38:33 .. Independently of that I would be tempted to define that there's an ISD for every time between 0 and infinity. 15:38:46 Nigel: I think that would be a substantive change compared to now. 15:39:03 Pierre: I encourage us not to imply that it is different to that. It won't help MPEG. 15:40:31 Nigel: There's also confusion that's been raised before about whether the root temporal extent is an interval or a duration. 15:40:46 Pierre: We could go down the path to really try to reconcile these terms. We've failed before but we could try again. 15:41:03 .. In the context of what MPEG is working on is if there is an ISD defined at every point between 0 and infinity. 15:41:08 .. Then their job is super easy. 15:41:26 Nigel: It's also easy if they know that there might not be. 15:41:31 Pierre: It creates special cases. 15:41:56 Nigel: Realistically, the additional case is "if no ISD is defined, then the presentation is the same as if an empty ISD were active". 15:42:16 Pierre: What is an empty ISD? 15:42:29 Nigel: It's one that generates no presentation. 15:42:36 Pierre: Does it have an empty body or no body? 15:42:41 Nigel: Does it matter? 15:42:54 Pierre: I think that's something that should be defined in TTML, not elsewhere. 15:42:58 Nigel: That's probably true. 15:45:31 Pierre: Can you find it in EBU-TT Live? 15:45:52 Nigel: [looks at the document] it defines the terms "Document resolved begin time" and "Document resolved end time". 15:46:10 Pierre: The concepts of the ISDs that get created and those are not dependent on each other. 15:46:57 .. One concept is the period of time during which some behaviour is defined. 15:47:15 .. The other is what ISDs get created either within that defined behaviour period or outside it. 15:47:30 .. Imagine you're building a renderer: you'd want something to get returned for every point of time. 15:47:50 .. Separately you would want to know if the author defined something for the time coordinate you're interested in. 15:48:41 Nigel: Right, and the application may override whatever the author defined. 15:49:14 Pierre: I'm really worried that the current text introduces additional complexity by implying that there is no ISD defined outside 15:49:30 .. the root temporal extent. 15:49:42 .. If I specify the begin and end on body, does that define the root temporal extent? 15:49:54 s/t./t, which is murkily defined. 15:51:49 Nigel: It can't be both ways. The way root temporal extent is defined permits the processing context to vary it, so if a processing 15:52:08 .. context says "no, it's always zero to infinity", then that's fine, and that's what would get applied in the proposed text. 15:52:23 .. It can't be that the flexibility pins applications down too much, given this flexibility. 15:52:48 Pierre: The bottom line for me is I don't see how introducing into the definition of the ISD construction process a vague term helps any 15:52:55 .. any user, especially MPEG. 15:53:18 Nigel: That's one of the roots of the problem: there's already a vague term - it can't be more vague than completely undefined! 15:53:40 Pierre: I think leaving it undefined is better than introducing a term that has insufficient definition. 15:54:54 Nigel: Okay, if there isn't consensus to merge this change, that's fine. I think it's an improvement, but there's a limit to how much I'm prepared 15:54:56 .. to argue for it. 15:55:27 Pierre: I'm totally game to really work through what the definition of root temporal extent is and define it clearly. 15:55:39 Nigel: That sounds like a F2F or virtual F2F session, like at TPAC. 15:55:42 Pierre: Absolutely. 15:56:13 SUMMARY: No consensus to merge this pull request at present. 15:56:39 Nigel: I plan to give this 2 weeks, and if we don't have consensus to merge, then close it. It's been open only 2 days so far, 15:56:51 .. so others might have interesting things to say about it, who haven't had opportunity yet. 15:58:39 Topic: Meeting close 15:58:46 Nigel: Thanks for the chat today. 15:59:05 .. I raised an issue to create a list of topics to potentially discuss at TPAC. 15:59:23 -> https://github.com/w3c/ttwg/issues/191 Create a list of F2F/TPAC topics w3c/ttwg#191 15:59:38 .. We'll adjourn here today, see you in 2 weeks. [adjourns meeting] 15:59:41 rrsagent. make minutes 16:03:51 s/rrsagent. make minutes// 16:03:56 rrsagent, make minutes 16:03:56 I have made the request to generate https://www.w3.org/2021/05/27-tt-minutes.html nigel 16:04:45 s/ow on number/ow on numbers 16:08:09 rrsagent, make minutes 16:08:09 I have made the request to generate https://www.w3.org/2021/05/27-tt-minutes.html nigel 16:10:39 scribeOptions: -final -noEmbedDiagnostics 16:10:44 zakim, end meeting 16:10:44 As of this point the attendees have been Nigel, Pierre, Atsushi 16:10:45 RRSAgent, please draft minutes v2 16:10:45 I have made the request to generate https://www.w3.org/2021/05/27-tt-minutes.html Zakim 16:10:49 I am happy to have been of service, nigel; please remember to excuse RRSAgent. Goodbye 16:10:53 Zakim has left #tt 16:11:04 rrsagent, excuse us 16:11:04 I see no action items