- From: Nigel Megitt <nigel.megitt@bbc.co.uk>
- Date: Thu, 13 Oct 2022 16:22:24 +0000
- To: TTWG <public-tt@w3.org>
- Message-ID: <737CF550-1B80-41A7-A4F6-D4C54E4EDCF3@bbc.co.uk>
Thanks all for attending today’s TTWG meeting. Minutes can be found in HTML format at https://www.w3.org/2022/10/13-tt-minutes.html
We tried using Zoom instead of Webex for today’s meeting – any feedback on this switch appreciated. If you were on the call, was there any difference in the experience? Did you want to join the call but have difficulty?
Those minutes in text format:
[1]W3C
[1] https://www.w3.org/
Timed Text Working Group Teleconference
13 October 2022
[2]Previous meeting. [3]Agenda. [4]IRC log.
[2] https://www.w3.org/2022/10/06-tt-minutes.html
[3] https://github.com/w3c/ttwg/issues/229
[4] https://www.w3.org/2022/10/13-tt-irc
Attendees
Present
Andreas, Atsushi, Hew, Nigel, Pierre
Regrets
Gary
Chair
Nigel
Scribe
nigel
Contents
1. [5]This meeting
2. [6]IMSC-HRM - Coalesce empty ISDs into non-empty ISDs
w3c/imsc-hrm#50
3. [7]DAPT-REQs
4. [8]Charter FO Status update.
5. [9]Meeting close
Meeting minutes
<Hew> Reminder that the meeting is on Zoom today...
This meeting
Nigel: Today we have DAPT issues, IMSC-HRM (one pull
request/issue)
… and any update on the charter FO.
… Is there any other business, or anything to make sure we
cover?
Hew: I can give some feedback on DAPT-REQs
Nigel: That'd be great, thank you
group: no other business
IMSC-HRM - Coalesce empty ISDs into non-empty ISDs w3c/imsc-hrm#50
<Github> [10]https://github.com/w3c/imsc-hrm/pull/50 : Coalesce
empty ISDs into non-empty ISDs
[10] https://github.com/w3c/imsc-hrm/pull/50
github: [11]https://github.com/w3c/imsc-hrm/pull/50
[11] https://github.com/w3c/imsc-hrm/pull/50
Nigel: This PR is primarily to deal with the clears and short
gaps between ISDs - is that right?
Pierre: We're trying to address the industry practice of 2
frame gaps between successive
… subtitles or captions.
… Today the HRM imposes a very high cost to those short gaps.
… This PR addresses that.
… I think we're down to just one question:
… Should there be any cost to displaying an empty ISD?
… i.e. painting it and presenting it
… I am arguing that because the model assumes that subtitles
are painted and presented on
… a graphics plane that is then overlaid on to the related
video object, then displaying nothing
… costs nothing because you don't display that pane altogether.
Andreas: Makes sense to me.
Pierre: Another goal of this PR is to make as little change as
possible.
… That's where my mind is, why I'm arguing that there is no
cost with presenting and painting empty ISDs.
Andreas: As I understand there's no algorithm that would
calculate a clearing action, right?
… What it means to have an empty ISD that follows an ISD with
content, that would then be a clearing action.
Pierre: My suggestion is that clearing happens only in the
buffers where things are going to be painted.
… Before drawing a new non-empty ISD you have to clear the back
buffer, and that has a cost.
… You first clear that, then draw the non-empty ISD, then when
the presentation time comes in you
… flip the buffer and display it.
… If you clear it before then there's no cost because you just
don't display the buffer.
… Clearly the "clear" is related to the cost of clearing the
buffer onto which an ISD is painted.
… What I'm suggesting is that in fact there is no need to paint
empty ISDs so there's no cost to not displaying
… the graphics plane, period.
Nigel: That's an argument I hadn't understood before, so good
to know.
Pierre: By the way I'm just trying to come up with something
that requires as little change as possible
… to the model. I hadn't thought exactly this 2 weeks ago for
instance.
Nigel: Good to clarify that you're not saying that the cost of
CLEAR is zero.
Pierre: Yes, the cost of CLEAR is not zero, but there's nothing
to clear for empty ISD.
Nigel: Then the question is if we always need at least one
buffer to be composited, and
… therefore that we need to count the cost of clearing a buffer
at least once for each clear.
… Or can we just say "stop compositing" at no cost.
… I think the conservative approach is to say that we assume
there is always exactly one buffer
… being composited, and therefore we need to count the cost of
preparing an empty buffer if there is
… to be an empty ISD after the current one.
Pierre: Within the model that's not possible with only 2
buffers.
… If you clear the back buffer for the empty ISD and then
present it then you have to wait
… for the empty ISD to be presented which defeats the purpose.
You would need a third buffer.
… What I have in mind is that the screen is being refreshed by
the video.
… There's no cost to not painting on the next refresh. Not
drawing an overlay is a no-op.
… When the next frame comes in you draw neither of the two
buffers.
Andreas: The HRM is a theoretical construct to calculate
presentation cost of ISDs.
… That doesn't mean that is how it is implemented in practice.
… You could view it like you do but of course in practice it
could be like Nigel says when empty ISDs come in.
… Follow a logic that is not yet implemented in the HRM.
Pierre: Sure, exactly, I agree.
… I think what we're trying to do here is to tease out where
the main complexity is, how it scales.
… I think I'm arguing, I've convinced myself, in general, you
could have a cost to not displaying pixels,
… you could have a third buffer, maybe your implementation is
on still images and clearing has a cost,
… but in the case of video the cost of not displaying an ISD is
not substantial and can therefore be safely
… ignored.
Nigel: Sudden moment of clarity for me that the current pre-PR
HRM is what you have to end up with
… if you have a 2 buffer model.
… I think what we're discussing here is that the idea is the
implementation flips from compositing
… buffer A to B to A to B etc
… but in my mind I'd imagined that there was a compositing
plane C
… and the task is to blit A to C, B to C, A to C etc which
would have a very different set of constraints.
… I'm not sure which it is right now.
Pierre: The presentation buffer Pn-1 is directly connected to
the display in figure 2.
… This was designed with a simple TV architecture in mind,
where one or the other is displayed on screen,
… or read directly by the display circuitry.
Nigel: I see what you mean about fig 2.
… But again, fig 2 has no switch about whether the presentation
buffers are connected to the display or not.
Pierre: We could add that to the model, that with empty ISDs
nothing is displayed.
… The PR has text that goes beneath that figure, but we could
modify the figure to make it clearer.
Nigel: Yes. I'm just wondering which is preferable, to insert a
switch to the display
… or to posit a 3rd buffer.
Andreas: A new buffer would add more complexity to the model,
right?
Nigel: I don't really think so.
Andreas: For me the question is if for some implementations
there would be some cost when an
… empty ISD is going through the change, and if this cost is
high enough to be taken into account
… for the calculation of complexity.
Pierre: Exactly.
Andreas: The cost is negligible and not go into the
calculation.
Nigel: I think that introducing the third buffer and counting
the cost of a CLEAR would not
… fail anything that passes today, but there are some documents
that could potentially fail it,
… and we should catch those.
Pierre: Concerned about the editing complexity that would
introduce.
Nigel: I haven't done the exercise, it seems like it _should_
be simple, but maybe not.
Pierre: If you want to try making the edit, go ahead. I'm not
sure what it will look like.
… You need to introduce the cost of drawing into the back
buffer.
Nigel: I would have to make the assumption of a cost-free blit
into the third, composition buffer.
Pierre: If you want to give it a shot, as we pointed out,
adding the cost of a clear for an empty ISD
… will land up with results that are closer to what we have
today. It's not going to
… invalidate documents that are valid today. That's not my
concern.
… It's updating the text in a way that does not cause more
trouble than it solves.
… I'm happy to modify figures - I have the master files, though
they may be checked in.
Andreas: I want to echo a bit Pierre's concern more about the
complexity of the specification and
… unexpected consequences of adding a new concept.
… The issue is that the whole model is already not so easy to
get, from non experts
… or people who really need to implement it.
… Adding a new component just increases the complexity of the
model.
… From an understanding point of view.
… When we consider adding it, we should compare the benefit of
adding it to the
… benefit we get from getting closer to reality for this
particular case.
… We heard that even if we add a component it would not really
change the results very much.
Nigel: Either proposal is a change to the model.
… I'm concerned about clarity of that change.
… The risk is that it's so subtle that it is not actually
noticed properly.
… I'm not sure if the mathematical results would be identical.
Pierre: They would not be because you'd count one extra CLEAR
per sequence of ISDs.
… Neither proposal would invalidate a set of valid documents,
which we don't want to do.
Nigel: I'm interested to know what is the edge case between the
two ideas where
… one would validate a document and the other would say it is
invalid.
Pierre: The cost of that extra clear would be included in the
cost of painting the following non-empty ISD, right?
Nigel: I think so.
Pierre: Let me try something.
… Scenario: non-empty ISD, empty ISD, non-empty ISD
… The model limits the complexity by saying painting the second
non-empty ISD takes some time
… and the system has some time to do that, and what we're
saying now is that the
… time available to paint the second non-empty ISD is the time
between the presentation time of
… difficult to say without graphics on screen - [shares screen]
… [discussion of 2 buffer model plus 3 buffer model by looking
and pointing at the rendering time figure]
Pierre: We could schedule some time for a call next week?
Nigel: That's a good shout - I should be able to do that.
Pierre: I'd really like to get to the point where we ask for
feedback, and
… we need to complete this issue before we do so.
Nigel: Yes
Pierre: I am available at the same time next week
Nigel: Me too
SUMMARY: Discussions to continue offline and possibly in
additional call
DAPT-REQs
Hew: Our AD operations and product owner reviewed DAPT-REQs and
felt
… that it captured their current workflows and that there would
be no problem
… capturing what they do, within it.
… They raised possible future questions.
… For example richer pan options if you want to get creative in
e.g. 5.1 audio or other distribution approaches.
… They wondered about extensible synth voice options to include
tone.
Nigel: Emotional tone maybe? Pitch is already available.
Hew: Yes, that could well be what they meant.
… If you think about dramatic AD particularly they try to carry
the tension or emotion of the programme.
Hew: That was it.
Nigel: Great, thank you, always very helpful to get reviews.
Charter FO Status update.
Nigel: As discussed last week I submitted the Chair's report,
and I got an acknowledgement back.
… I expect the Chair's report to be appended to the document.
… Atsushi, anything to add?
Atsushi: The report will be made available to the FO Council in
the next couple of days.
… Stepping down from Council is ongoing - I am not sure when
they will actually start to discuss it.
… I believe they have 2.5 weeks for stepping down prior to Team
report so they should start shortly.
Nigel: Thanks for the update. Any questions?
… I see nobody on the queue.
… Q from me: did the Objectors respond to the draft Team
report?
Atsushi: I have not seen a response. WG participants can see
the updated report at the same URL.
Nigel: Thank you.
Meeting close
Nigel: Thanks everyone! [adjourns meeting]
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Received on Thursday, 13 October 2022 16:22:46 UTC