- From: Nigel Megitt <nigel.megitt@bbc.co.uk>
- Date: Thu, 29 Apr 2021 16:36:52 +0000
- To: TTWG <public-tt@w3.org>
- Message-ID: <32FC2C14-55AE-4B34-965A-0069F8AF4AD7@bbc.co.uk>
Thanks all for attending today's TTWG meeting. Minutes can be found in HTML format at https://www.w3.org/2021/04/29-tt-minutes.html
In text format:
[1]W3C
[1] https://www.w3.org/
Timed Text Working Group Teleconference
29 April 2021
[2]Previous meeting. [3]Agenda. [4]IRC log.
[2] https://www.w3.org/2021/04/01-tt-minutes.html
[3] https://github.com/w3c/ttwg/issues/184
[4] https://www.w3.org/2021/04/29-tt-irc
Attendees
Present
Atsushi, atsushi_, Chris_Needham, Cyril, Gary, Glenn,
Nigel, Pierre, Rob_Smith
Regrets
-
Chair
Gary, Nigel
Scribe
cpn, nigel
Contents
1. [5]This meeting
2. [6]TTML2 - Merging open pull requests
3. [7]TTML2 - Shear calculations and origin of coordinate
system. w3c/ttml2#1199
4. [8]Mention fingerprinting vectors in privacy
considerations. w3c/ttml2#1189
5. [9]WebVTT Add unbounded TextTrackCue.endTime w3c/webvtt#493
6. [10]TPAC 2021
7. [11]AOB: TTML and WebVTT at MPEG
8. [12]WebVTT Unbounded Cue Use Cases w3c/webvtt#496
9. [13]Meeting close
10. [14]Summary of resolutions
Meeting minutes
This meeting
Nigel: Today, we have 2 points on TTML2 and I'd like to take a
minute to cover [15]https://github.com/w3c/ttml2/issues/1189
… We also have a WebVTT pull request to check on, for unbounded
cue end times.
… In AOB we currently have TPAC 2021.
[15] https://github.com/w3c/ttml2/issues/1189
Cyril: I can give an update on TTML and WebVTT discussions in
MPEG if we have time in AOB
Nigel: Thank you. Any other business?
group: [no other business]
Nigel: Since Gary is a few minutes late today I propose we
cover the TTML2 issues first.
Chris: That's ok
Rob: Good for me.
Nigel: Welcome, it's a W3C WG meeting, usual rules apply, we're
nice to each other, etc. almost goes without saying.
TTML2 - Merging open pull requests
Nigel: We have two open pull requests that have been open for
more than 2 weeks and have an approval review.
… By our normal rules, that constitutes consensus.
Glenn: Are these the 2 on TTML2?
Nigel: Yes
Glenn: I've just merged the first and will have merged the
second in a minute or two.
Nigel: Great.
Glenn: I'd also point out that the first of those, to change
the links examples to https is technically not necessary
because they're not links.
Nigel: The point was to demonstrate editorially that we want to
promote use of secure protocols.
Glenn: Yes.
Glenn: (later update) I've merged the second pull request also.
TTML2 - Shear calculations and origin of coordinate system.
w3c/ttml2#1199
github: [16]https://github.com/w3c/ttml2/issues/1199
[16] https://github.com/w3c/ttml2/issues/1199
Nigel: Following my proposal about line wrapping and block
shear, Glenn you wanted to add something?
Glenn: In TTPE we implemented this in a branch. We computed the
line measure for the block prior to performing
… paragraph line breaking operations. We computed that measure
based on the block shear that would apply to the block, so it
has the
… result of resulting the measure by some %age related to the
sines or cosines of the angle.
… It did not require any kind of recursive update to the
measure for formatting purposes.
… Once you've computed the line measure then it's done and you
don't need to recompute.
… I will go back to your issue and see if you're thinking of
something I had not considered.
… We didn't have any problem of the sort that you mentioned.
Nigel: I'm confused by that so will await your further input.
SUMMARY: @skynavga to consider computational issue further
Mention fingerprinting vectors in privacy considerations.
w3c/ttml2#1189
github: [17]https://github.com/w3c/ttml2/issues/1189
[17] https://github.com/w3c/ttml2/issues/1189
Nigel: I've reviewed the fingerprinting vectors just before
today's meeting
… I came up with 3 actions, all to add notes to the Privacy
section
… [describes the actions in [18]https://github.com/w3c/ttml2/
issues/1189#issuecomment-829305696]
[18] https://github.com/w3c/ttml2/issues/1189#issuecomment-829305696]
Glenn: Do any of those involve change to the font
fingerprinting text?
Nigel: No, they're just adding points to P7 and P3. It doesn't
modify P10
Nigel: I'm happy to draft a PR, unless someone else wants to
SUMMARY: @nigelmegitt listed the proposed changes and
highlighted them to the group for consideration
WebVTT Add unbounded TextTrackCue.endTime w3c/webvtt#493
github: [19]https://github.com/w3c/webvtt/pull/493
[19] https://github.com/w3c/webvtt/pull/493
Gary: When we last spoke about the unbounded end time for the
TextTrackCue there wasn't much concern.
… The hold-up was whether there were tests and whether it was
approved on the HTML spec side.
… Last week the [PRs on] tests and the HTML side were merged so
now WebVTT is the last hold-out for the API change,
… which is this pull request right now.
Rob: I concur. Firstly this is my first TTWG meeting, thank you
for the invite. Thanks Gary for the quick summary.
… I'd add that having done the platform tests I notice there's
a dependency between the TextTrackCue tests and the VTTCue
tests
… because the TextTrackCue relies on VTTCue for the
constructor. There's a crossover or interdependency that
results.
… The WebVTT change is required to make the Web Platform Tests
work as well.
Gary: Thanks Rob that makes sense.
… In terms of process, I haven't heard any objections from
anyone. Do we need to wait before merging for the resolution
period?
Nigel: Our normal practice is to consider the opening of a PR
as effectively a Call for Consensus
… so that the Decision Review period expires 2 weeks after
opening, obviously assuming there is consensus at that time.
… What this means is that, since this PR has been open since
November, if the Chair declares consensus, then the Editor can
go ahead and merge.
… I'd just point out that there was a commit just 23 hours ago
so that should be considered.
Gary: I plan to look through it again after this meeting and in
that case go ahead and merge.
Rob: That recent addition was to address Philip Jaegenstedt's
comment that there was a reference to unbounded TextTrackCue
which was
… not used. We tried to keep it as simple as possible and ended
up removing all references to unbounded cues in the process.
… The way it is at the moment it is the one thing we are adding
and do not mention, so I thought it worth adding.
… I couldn't find any similar example and struggled with the
wording so would welcome guidance.
Gary: Earlier I thought it was fine but will take a closer
look.
Rob: Thank you
Gary: Then for the pull request the resolution is for me to
look over it and merge after approval.
Rob: Gary, if there's change to be made on the wording I'm
happy to work with you to get that resolved and complete this.
Gary: Thank you Rob
Resolution: Gary to do final Editorial pass and merge
TPAC 2021
Nigel: TPAC dates have been circulated; TPAC will be virtual,
and be held from 18-29 October.
… The week from 25 to 29 October will be dedicated to the Joint
and Group meetings.
… The question for us is: Do we wish to meet?
… We have until 10 September to declare meeting dates.
… I think we do have some agenda points for joint meetings or a
group meeting that we wanted to cover over the past couple of
years
… but never got around to.
Gary: Last year we wanted to meet with CSS WG but forgot to
confirm that.
Nigel: Yes.
… On a personal level, the timing is terrible for me this year!
… We have some time, but I wanted to give plenty of notice.
… If anyone does have any agenda topics for a meeting in
October, do let me and Gary know.
AOB: TTML and WebVTT at MPEG
Cyril: A quick update.
… As you know, MPEG is working on an amendment to 14496-30. The
initial purpose of the amendment
… was to clarify terminology around TTML. Specifically it is
not always clear how to package TTML in an MP4 track,
… if the document is longer than the sample in which it is
contained, for example.
… So the clarifications are those we discussed some time ago.
… It is progressing well. The changes since last time are
additions of more examples.
… You should have access to the documents but if not then I
will share with you.
… We're adding more examples where we detail first typical use
cases, how the ISDs are created and then how they are
… packaged into MP4 samples and presented. What we've clarified
in the latest round is, with respect to IMSC 1.1 and its HRM,
… the HRM in IMSC indicates how it works on a document, so you
take the sequence of ISDs and apply the HRM on that sequence.
… We're proposing to extend the HRM to all the ISDs produced by
the entire track of documents, as an extension.
… We're recommending behaviour when the last ISD of a sample is
the same as the first ISD of the next sample, avoiding
flickering for example.
… That's it for TTML for example.
Nigel: I'd like more information on how to access the document,
if you're able to help with that, bearing in mind the liaison
we have.
Cyril: I will work offline on that, alongside Mike Dolan.
Nigel: Was there any consideration about how to apply edit
lists?
Cyril: Nigel, your colleague Stephen supplied some comments and
he was right. We're working on changes for that.
… For all the operations on clipping, imagining the TTML
document needs clipping to the time interval of a sample, the
question is what
… timeline do you use. The agreement was that it is the
composition time before the edit list is applied. Then the edit
list puts that on the timeline.
… Concrete example. TTML document 0s - 10s. Sample interval 2s
- 5s.
… That's the sample's composition time interval.
… So you display only part of the document.
… Then the edit list shifts this sample to 0s.
… The first thing that will be presented in this case is the
content of the document from 2s - 5s which is presented
beginning at time 0s.
… We think this is applicable to WebVTT as well, the same
clarification to use composition time instead of presentation
time.
Nigel: That's really good, thank you.
Cyril: Moving on to WebVTT. There was a contribution and the
document proposed changes to the carriage of WebVTT to cover
… the carriage of indefinite cues.
… There are two aspects. Firstly the draft was not completely
clear and more improvements will be needed.
… In particular we have two tools to indicate an indefinite
cue.
… 1. Sample duration = 0 means "indefinite cue" but it is only
allowed as the last sample in the track.
… When you're producing live WebVTT and packaging it you won't
know when the last sample will be so you cannot use it, or you
use it
… all the time and have plenty of samples with duration zero
which is not allowed in the spec.
… 2. The sample has a definite duration, e.g. valid for at
least 2s, so you put that as the duration and after that you
extend the duration
… of the sample by marking it as redundant and having a
duration that is 2 additional seconds, and you keep sending
that until something
… changes, or the end of the presentation.
… There is no strong recommendation yet to use that. The
discussion is ongoing.
… It is all based on the assumption that there is a way in
WebVTT to identify an indefinite cue.
… To my knowledge there is no syntax yet to indicate indefinite
cue duration, and it's a hard problem if you consider backwards
compatibility.
Nigel: Right!
Rob: Can I ask what is the definition of an indefinite cue?
Cyril: Good question. How is it defined in HTML and the WebVTT
JS API?
Gary: It's another word for "unbounded"
Cyril: OK replace indefinite with unbounded.
Glenn: It's a term from SMIL and we use it in TTML to refer to
a duration with no specific end.
Rob: Thank you.
… So then I think w3c/webvtt#496 is relevant to this.
WebVTT Unbounded Cue Use Cases w3c/webvtt#496
Rob: I'm an Invited Expert and Chair the Web Video Map Tracks
(WebVMT) Community Group
… and have been working on WebVMT based on WebVTT but designed
to express location,
… and has expanded to cover metadata. Particularly
location-aware video devices with sensors on them.
… The unbounded cue idea I proposed here was to address the
live capture use case that I was considering for WebVMT.
… There's been some work done proposing a syntax which could be
ported back to WebVTT, because the text track cue structure
… is shared between WebVTT and WebVMT. There have been
discussions about recording sensor values and interpolating
them between
… time samples, and handling discontinuities, which all seem
relevant, so it might be worth having a discussion about that
at some stage.
… I can post a link to last year's TPAC discussion which
probably summarises this fairly well, and also feeds into issue
496 which I raised,
… and welcome feedback on that.
<gkatsev> [20]unbounded webvtt cue syntax use cases and
discussion w3c/webvtt#496
[20] https://github.com/w3c/webvtt/issues/496
Chris: My question follows from what Rob was just saying.
… It seems that we have a number of different constituencies
that are interested in this.
… Rob with WebVMT, WebVTT with unbounded cues in MPEG4, and in
the Media Timed Events IG, emsg boxes,
… and those can have unbounded duration as well. In the last
meeting that we had there,
… Nigel suggested that we ought to do a use case and
requirements gathering so that we understand what
… problem we are trying to solve when we make syntax changes.
… My question is where do we want the home for that discussion
to be?
… Following on from that I would like a clear definition of the
precise questions.
… My proposal is to take this on under the MEIG Timed Events
activity
… so what I've tentatively done is put it on the agenda for the
next MEIG call which will be on
… May 11, Tuesday, the purpose of which is to introduce the
topic to MEIG more generally and
… then use that to identify who needs to be involved in the
discussion and point people towards that timed events activity.
… I wanted to ask if you think that's a good approach.
Nigel: I think that's a good idea, yes.
Nigel: Regarding specific questions, there's backwards
compatibility. MPEG4 has an approach where you can keep
extending. That's similar to the TTML approach. At least one
proposal out there relies on a particular error condition
behaviour, so it's not clear what the backwards compatibility
story is. Need to establish what's acceptable.
Gary: I agree with Nigel, I think discussing in MEIG is a great
place.
… For now, if anyone has specific comments, putting it in
webvtt issue 496 would be good to keep things in one place
until we decide
… there's a better place.
… I agree that backwards compatibility is important and we need
to figure out what behaviour we want and if we also want
forwards compatibility.
… For example the current proposed syntax of not having end
time is technically backwards compatible but old clients just
won't see those
… cues. They won't break but it is not necessarily good
behaviour.
… In addition, any syntax change is going to be hard because we
need to gauge implementer interest.
… In recent years they have been very reluctant. If noone wants
to implement then is there really any point in doing this?
Chris: That's great, due to time I didn't want to go into too
much tech detail. This is the conversation I hope we can have.
Rob: There are existing solutions to this using bounded cues as
opposed to unbounded ones and they still work so if there is an
… issue with backwards compatibility the user already has the
choice to use those existing structures.
Meeting close
Nigel: We're a few minutes over, and we've covered all the
agenda and more today. So thank you everyone, we're adjourned
for today.
… [adjourns meeting]
Summary of resolutions
1. [21]Gary to do final Editorial pass and merge
Minutes manually created (not a transcript), formatted by
[22]scribe.perl version 131 (Sat Apr 24 15:23:43 2021 UTC).
[22] https://w3c.github.io/scribe2/scribedoc.html
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Received on Thursday, 29 April 2021 16:37:16 UTC