Meeting minutes
This meeting
Nigel: Without Cyril, not sure we should look at the DAPT issues.
Nigel: So that leaves IMSC-HRM and any news on the Charter FO.
… Any other business?
Andreas: Nothing from me
Coalesce empty ISDs into non-empty ISDs w3c/imsc-hrm#50
<Github> https://
github: https://
Nigel: Pierre and I had a design chat about this last week
Pierre: Yes, I'd like to share my latest thoughts. I've spent a lot of time on this.
… [shares screen]
… [screen shows updated model with a switch between the presentation buffer and the display, to allow
… clearing to happen by not compositing]
… The path I'm going down as a proposal is that every ISD that comes in, decide if it is empty or not.
… If it is not empty, go through the same process as the current HRM.
… If it is an empty ISD, toggle the output of the current model, to turn off the output of the presentation buffer.
… The idea is that an empty ISD is empty so there is nothing to display.
… What it really means is that the cost of processing empty ISDs is zero because there is nothing to render.
… Separately Nigel and I discussed whether or not there should be additional constraints on the
… duration of empty ISDs, to catch errors seen in IMSC files where very small gaps are introduced
… erroneously.
… Nigel also mentioned that setting minimum time constraints on an ISD might reflect refresh rates etc.
… I'm not so concerned about this because there's already text about the refresh rate possibly not being
… as fast as needed to show every ISD.
… Those are my thoughts.
Nigel: Thanks for that. How would it work here if there were a 25th of a second gap between two
… non-empty ISDs?
Pierre: The switch would open, so nothing would be available for display, at the end of the non-empty ISD.
… While the previous non-empty ISD is being displayed, the next non-empty ISD is being drawn into
… the alternate presentation buffer. When the very short empty ISD occurs, the switch opens,
… independently of the presentation buffer.
… When the empty ISD ends it closes and the next non-empty ISD, drawn into the alternate presentation buffer,
… is moved into the active presentation buffer and is displayed.
Andreas: When a non-empty ISD follows another non-empty ISD the presentation buffer will stay as it was
… before, but it will be toggled so nothing is sent to the display?
…
Pierre: Exactly.
Andreas: When a non-empty ISD comes up is there an additional action to clear the display?
Pierre: The assumption is that it costs nothing to display nothing.
… In a traditional graphics system with a graphics plane with a front and back buffer, the assumption is
… that disabling compositing with the graphics plane is a zero cost operation.
Andreas: OK
Pierre: In modern systems that is true.
Andreas: Maybe there is no cost associated with it, but you may have to delete a DOM node or whatever, in HTML,
… so there is something that needs to be done.
Pierre: If you're using cues, when no cue is displayed there is no work.
… There's work in preparing a non-empty cue but there's no such thing as an empty cue, just no cue.
Andreas: That's right, but there is no duration with the ISD that you send to the display, right?
Pierre: The way the HRM is written, the output of the presentation buffer is "made available" to the display.
… That's really broad, "display", here. It depends on how fast the output rendering device can go.
… The HRM says today that it is conceivable that a non-empty ISD, that has content, is so short, that it will
… never be displayed because it comes between two refresh cycles of the display. That's already acknowledged
… by the HRM.
Nigel: If I understand right, this model will not catch very short empty ISDs?
Pierre: I've agonised over this.
… I really like the idea of catching errors, making an integrated SHALL statement in the model.
… But even with non-empty ISDs we can't guarantee they will be displayed.
… I've been agonising about a SHALL or a SHOULD for the minimum duration of a sequence of empty ISDs.
… The other question is what should the lower bound of that duration be?
… Another important point for the notes:
… Going back to the minimum duration of empty ISDs, I'm uncomfortable:
… Netflix has a min duration of 2 frames, I've heard 3, maybe 1, but nobody ever says what the frame rate is!
… So I'm not sure what the right lower bound is.
… Having a SHOULD for any validator to warn if there is an empty ISD shorter than 1/30s, but making
… it a normative SHALL requirement is another thing.
Nigel: That's a strong argument: if we don't have data points to support SHALL statements we ought not to have them.
Pierre: A SHOULD would not be bad to have, but it would be a heuristic. I'd say right now, that neither
… 608 or STL can have gaps shorter than 1/30s but it is possible with EBU-TT-D say.
Nigel: [asks about the edge case of a very short non-empty ISD and empty ISD followed by a non-empty ISD]
Pierre: The cost of clear is still present for erasing the presentation buffer, that doesn't change.
Nigel: Yes, I see.
Pierre: The minimum duration of a non-empty ISD has not changed.
Nigel: No, it has, because it now also includes the duration of a following empty ISD whereas before it did not.
Pierre: Right, absolutely, you could have a non-empty ISD that's a millionth of a second followed by a
… relatively long empty ISD - before, that was not possible.
Nigel: Worth noting, it may already be present, that this is not about the readability complexity,
… but the presentation complexity.
Pierre: I'm going to note that, maybe make sure there's a statement like that in the document.
… I'll update the pull request.
Nigel: Thank you!
Pierre: Thank you for hearing me out.
SUMMARY: @palemieux to update the pull request
Rechartering Formal Objection Council status update
Atsushi: No update, it's possible that in near future I will hear back.
Pierre: Thank you for that.
Atsushi: The latest document for FO is at the same URL as the previous one so you can see the FO Council report.
Team Report on Timed-Text WG Charter Formal Objection
Nigel: Thanks, that incorporates the Chairs response from me and Gary.
Meeting close
Nigel: Thanks everyone, that's completed those parts of the agenda that we could do today.
… Let's adjourn a little early. [adjourns meeting]