- From: Nigel Megitt <nigel.megitt@bbc.co.uk>
- Date: Tue, 20 Sep 2016 17:18:51 +0000
- To: Timed Text Working Group <public-tt@w3.org>
- Message-ID: <5941EAB8802D6745A7D363D7B37BD1F773F8719C@bgb01xud1012>
Thanks all for a very productive day once again. Minutes are available in HTML format at https://www.w3.org/2016/09/20-tt-minutes.html - I will also provide this link from the group wiki page alongside the minutes for day 1.
In text format:
[1]W3C
[1] http://www.w3.org/
Timed Text Working Group Teleconference
20 Sep 2016
See also: [2]IRC log
[2] http://www.w3.org/2016/09/20-tt-irc
Attendees
Present
Andreas, Dae, Glenn, Nigel, Pierre, Rohit, Thierry
Regrets
Chair
Nigel
Scribe
nigel
Contents
* [3]Topics
1. [4]TTML Profiles and Media Registration
2. [5]Github 1 year on - how 's it going?
3. [6]Reorganisation
4. [7]IMSC Implementation
5. [8]TTML2
6. [9]TTML2 viewports and other terminology
7. [10]TTML2 initial element
8. [11]TTML2 - timing
9. [12]TTML2 - condition
10. [13]TTML2 simplification options.
11. [14]TTML2 issues
12. [15]TTML2 - sequences of documents
13. [16]Audio Description
14. [17]Text Track and Text Track Cue interface
15. [18]TTML2 editorial actions
16. [19]Meeting close
* [20]Summary of Action Items
* [21]Summary of Resolutions
__________________________________________________________
<scribe> scribe: nigel
nigel: Let's continue from yesterday according to the agenda we
planned yesterday.
... Any other agenda points not yet raised?
rohit: Reminder that we have an agenda point to discuss github
experience.
TTML Profiles and Media Registration
nigel: Philippe you made some progress?
plh: Yes, it was submitted - the status in IANA today is that
it has been sent
... for Expert Review. I would expect an answer within a week
or two. On the action
... item I have logged the details.
action-477?
<trackbot> action-477 -- Philippe Le Hégaret to Progress the
update to ttml media type registration with iana -- due
2016-09-15 -- OPEN
<trackbot>
[22]http://www.w3.org/AudioVideo/TT/tracker/actions/477
[22] http://www.w3.org/AudioVideo/TT/tracker/actions/477
plh: I submitted it on Sep 15, got an ack on Sep 17. As soon as
I hear back from
... them I will let you know, and if there are expert comments
I will put the
... commenter in contact with Mike. If everything goes well we
should be all
... set in 2-3 weeks. I don't know how long it takes to update
the registry.
nigel: Thank you!
Github 1 year on - how 's it going?
nigel: I wanted to raise this as a placeholder - I think it has
gone well for IMSC
... but we have had a lot more discussion about TTML. Any
views?
glenn: It's working for me okay.
plh: A question: how far are you from a TTML2 CR?
glenn: Less than 3 months
... We'll ask for Wide Review in 6-8 weeks.
plh: My worry is that you give the other groups enough time to
do the review.
... With the reorg happening Ralph Swick will be responsible
for organising
... horizontal reviews. My recommendation to facilitate your
work on that is
... For Security and Privacy there's a self assessment
questionnaire - you answer
... the questions and send them to the right mailing list as
part of the review;
... if they do not respond then it's fine.
nigel: Even if they don't want to comment I would expect at
least an answer to
... say that, from politeness and also so we know the status.
plh: That's fair enough. Bear in mind this is shifting ground
and we're changing
... the way we do this. Ralph will be harmonising this.
... For the TAG, you can ask for a review using their
spec-reviews github repo
action-480?
<trackbot> action-480 -- Nigel Megitt to Request schedule time
for horizontal review of ttml2 -- due 2016-09-26 -- OPEN
<trackbot>
[23]http://www.w3.org/AudioVideo/TT/tracker/actions/480
[23] http://www.w3.org/AudioVideo/TT/tracker/actions/480
nigel: Please could I delegate this action to you Thierry?
thierry: Yes. This is new to me also.
nigel: I've moved the action to Thierry.
thierry: One idea we have raised is only to request review on
the new features.
plh: That will help them but they may tell you about a problem
with the older
... features.
thierry: And then they will destroy TTML1?
plh: Possibly they could!
... Here are some links to help you with this:
... Having difficulty joining IRC so I'll send these by email.
Reorganisation
plh: With the W3C reorganisation I am becoming responsible for
all the WGs in
... W3C, so I will be responsible for ensuring all the WGs are
able to move forward
... on the Charter deliverables. I am allowed to extend
Charters with Director's approval.
... It is now the responsibility of Wendy Seltzer to look after
what the future web
... is going to look like and propose any changes to the
Charter. Ralph is responsible
... for the entire technical coordination of W3C including
horizontal reviews, TAG
... and other technical issues that groups cannot resolve. If
you come to me
... with a major technical issue I will take that to Ralph for
you.
nigel: Thank you for your help over many years!
andreas: +1
plh: I am still responsible for the TTWG! Nigel can still come
to me and complain
... if he has issues - now all the other Chairs can do this
too!
... So I won't be doing github issues etc myself unless it's
super quick.
... Team Contact and Chair training are now under my
responsibility too.
nigel: Thanks - any questions or any more?
plh: It's been a pleasure, feel free to contact me and I'll try
to be even more responsive.
glenn: So no more ttml.js maintenance?
plh: If anyone wants to fork that and come to me for some
information on it that's fine
... but I can't spend official time coding any more.
[24]https://github.com/plehegar/ttml-js
[24] https://github.com/plehegar/ttml-js
plh: You're welcome to rename it - it's under CC0 so it's
public domain. I'm happy
... if someone wants to do something to put a pointer on my
repo to other places to look.
... Excuse me, I need to go to another WG now.
nigel: Thank you very much!
IMSC Implementation
andreas: [projects Hbb4All presentation]
... I just wanted to give an update on Hbb4all. An EU funded
project that ends this year.
... A strong focus on EBU-TT-D deployment. I always promoted as
TTML because
... it is better known.
... 6 or 7 player implementations, 3 broadcaster apps, one
being for TTAF.
... jwplayer and videojs implementations hopefully that will be
open sourced.
... Also a Samsung native implementation of TTML for HbbTV 2,
which I think
... was a public prototype implementation last year.
... Also, we demonstrated automatic production of subtitles,
DVB TS -> audio processing ->
... speech recognition -> punctuation and captialization ->
TTML generation
... Lots of other partners also.
... subtitling.irt.de - 2 demo live streams of EBU-TT-D in
DASH.
... It uses the dash.js implementation so for example you will
see line gaps between background areas.
... We did some interop testing for each partner with each
other's TTML, to see
... how it is rendered. There are certain issues. Example with
background colour
... not coming out if RGBA is not implemented properly.
... Also font being proportional even though monospace
specified.
... This will be published as a report.
... Another one is overlap of lines - a big problem is
application of lineHeight
... especially with the "normal" setting. There is no clear
message from any group
... using TTML, or there are contradicting messages.
glenn: You mean other than the specification note to use 125%?
andreas: That's a recommendation only. In EBU-TT-D we recommend
not using
... "normal" because it is not implemented the same everywhere.
... The gaps between lines differ between different renderings
again because of
... lineHeight="normal".
... Another one is where implementations try to do a nice
looking thing even
... though it is not specified, like including linePadding when
it is not in the document.
... I also saw a lot of implementations at IBC sometimes called
DFXP because they
... are not always aware that it is the same as TTML
effectively.
nigel: Thank you!
... While we're on this topic it would be a good time for me to
mention about
... EBU-TT Live.
... I mentioned this at the Web & TV IG yesterday also:
[25]http://ebu.github.io/ebu-tt-live-toolkit
... The project is an open source implementation of the EBU-TT
Live spec
... - we're about half way through implementation. The spec is
at v0.9 now
... and we hope to move to v1 in early 2017.
[25] http://ebu.github.io/ebu-tt-live-toolkit
glenn: Do any of the efforts for live define rollup?
nigel: This doesn't - EBU-TT doesn't permit any form of
animation, and in any
... case it is not clear how to use animation for smooth
roll-up. In my opinion
... the best way to do this is still to allow implementation
specific behaviour as
... written in the Note in TTML1.
... By the way, the context here is a sequence of TTML
documents in the absence
... of any wrapper that can provide additional semantics. For
our work here this
... has two impacts for TTML2, one is sequence identification
and numbering
<tmichel> taking a 15 minutes break
TTML2
nigel: Shows a scratch diagram with three real world concepts,
the Root Container Region, the Related Media (e.g. encoded
video) and the Media Player, each being an independent concept.
... default of the Media Object Region which coincides with
TTML1.
... We should make sure that we all share an understanding of
those different
... viewports and that they match what we need in real world
use cases.
glenn: Right. I think we can look at the Safe Crop Area
proposal independent of
... that, because it is orthogonal.
nigel: Right.
andreas: I think we need to gather all these concepts together
and clarify them.
... Especially in relation to IMSC which talks about video
frames.
nigel: [26]https://github.com/w3c/ttml2/pull/184
[26] https://github.com/w3c/ttml2/pull/184
glenn: So you're defining in the document coordinate space an
area that
... presentation processors should keep sacrosanct.
nigel: Yes.
glenn: I don't see that as a problem. Putting all four
parameter values in a single
... attribute can work. I wonder if we should constrain the
values to percentages.
nigel: Yes we could do that.
glenn: I find that a lot of implementations are based on
percentages.
... The one part I have a problem is the presentation
semantics.
... I think we can dispatch the Visible rendering area
definition, or I'd like to.
nigel: If possible that would be okay.
glenn: I'd like to describe the process in the 2nd paragraph
differently and
... use should rather than shall.
... [discussion] If tts:extent is not defined on the tt element
then the root container region is defined as being coincident
with the related media object region.
... If the Related media object region changes during
presentation then I don't
... agree that this triggers a re-layout.
nigel: I don't agree. If percentage extents and font sizes are
used then it is
... possible that a re-layout is needed.
glenn: We always assumed that would not happen.
nigel: Is that written in the spec somewhere? I've never read
it anywhere.
glenn: I think it's implied, I'm pretty sure it's not explicit.
rohit: There's some wording in the text on the tt element about
the behaviour
... if there is no tts:extent on the tt element.
pal: IMSC 1 says explicitly that you shall map to the related
media object.
andreas: So IMSC is saying that you map it to the image frame
of the related media object.
glenn: Correct, but the language there does not talk about
changing on the fly.
... This is a key point, whether the processing context can
change over time
... or not.
pierre: If you don't specify tts:extent on tt then there's no
notion of pixels at all.
... The root container sets the coordinate space for the
document. Every length
... is related to the root container region always.
glenn: There's another way to think about that, which is when
you establish
... the correspondence in the beginning then you can create
pixel values.
pierre: That is one implementation.
glenn: Then after that if scaling occurs then you still use the
same dimensions.
group: [discussion] conclusion that the requirement is to
define that implementation ensures that everything that falls
within the safe crop area is displayed on the screen.
glenn: If a tts:extent is present then it establishes the
document coordinate space.
... If an extent is absent then you need to establish a
document coordinate space (once and for all time) that overlaps
the Related Media Object Region.
pierre: That's one implementation - some implementations do not
use pixel coordinates.
nigel: For me it's okay to have a document coordinate space
that is always only in percentages.
glenn: It's important to know the pixels for laying out fonts
etc.
pierre: Each implementation decides what pixel grid it wants to
render on.
dae: If you have 800x600 video and the user clicks zoom so we
now have 400x300 what's the document coordinate space now?
glenn: That would cut off content on the edges.
dae: So in that case 50% would still mean 400 (or 300)?
glenn: Exactly.
andreas: Is this also how IMSC 1 defines it?
pierre: IMSC 1 makes the mapping of the root container
dependent to the related media object.
nigel: The section about best endeavours is there to allow for
presentation
... processors to do whatever processing they want to do to
achieve the required
... outcome of ensuring the content in the safe crop area is
displayed (i.e. is in the visible rendering area)
... To draw this to a close, I have an action to modify the
first paragraph in
... pull req §11.3.3 to avoid referencing the undefined term
"TTML content"
glenn: In the second paragraph I would rather not use shall but
use a declarative approach.
nigel: This is actually a processor requirement so it's a
shall. There is a defined
... feature for it, which in the case of a presentation
processor, refers to the
... semantics in §11.3.3 so it would be possible to profile out
this behaviour
... using that feature designator.
glenn: Okay I'll look at that.
andreas: I'm a bit worried about the number of terms and their
definitions and that
... we may not all share the same understanding. Everybody has
a certain view,
... but it may be that we are not all on the same level right
now. This is a problem
... if different implementors understand it differently.
pierre: You should bring any differences here. ittp:aspectRatio
in IMSC 1 sets the
... aspect ratio of the root container.
... For example if it were 4x3 then a reasonable implementation
is to create
... a 4x3 pixel array, render the document on that, and
composite that (potentially
... also scaling isomorphically) onto the related media object.
andreas: Yes, centred.
pierre: If I don't specify ittp:aspectRatio then I can choose
whatever shape pixel
... array corresponds to the related media object, choosing
whatever actual
... resolution I want.
... If we think it is important to explain how this works then
we can generate
... examples if you think that's important. Is that your
concern?
andreas: Yes it's part of it, but it's also the other terms.
nigel: And in IMSC 1 it is deliberately left vague whether the
related media object
... is the encoded video or the media player for example. And
different implementations can make different choices.
pierre: Yes, and that's something we should try to define
possibly in specs that
... that reference IMSC 1.
andreas: For this group I would like to make sure we have clear
definitions of:
... related media object, related video object, related media
object region,
... image frame of the related video object, document
coordinate system,
... viewport, viewport target.
nigel: Let's take a 10 minute break and restart at midday.
... Okay, we're back...
TTML2 viewports and other terminology
andreas: I would like to have the reference terms from TTML1,
IMSC, EBU-TT-D
... and then look at the terms in TTML2.
nigel: Ah, that would take a bit more preparation than I think
we have, but it
... sounds like a good review exercise. What we can do now is
look at the new
... terms in TTML2 and do that task later or on our own.
... Glenn what are the important new concepts?
glenn: viewport and viewport target.
... I don't personally like that definition of viewport because
it is too general.
... In our context it should be closely related to the root
container region.
... What I was thinking at the time is that it is possible to
think of different
... viewports, one for presenting the related video object and
another for
... presenting the root container region, but for us the root
container region
... is the only concept that has any significance.
... In TTML1 there was a long outstanding bug to improve the
definition of pixel.
... At the same time we began introducing mechanisms involving
aspect ratio.
... where we introduced display aspect ratio in IMSC. I needed
appropriate
... terminology for discussing aspect ratios. Then I had to
distinguish between
... time of authoring and time of presentation, where aspect
ratios may differ.
... Then there's the vw and vh unit definition which
effectively assume a viewport term.
... We already have this defined in the XSL-FO space.
... The first note in 11.3.1.4 refers to the page-viewport-area
which here
... corresponds to the root container region. They play a
crucial role in defining
... a local coordinate space in which the children are
positioned and rendered.
pierre: Why do we need to define viewport given that the
definition of the vw and vh units do not use it?
glenn: Well we need that to define viewport target.
nigel: Why do we need viewport target?
glenn: This is to relate the root container region to some
other context such as
... encoded video's image area or the area including the black
bars etc.
... Viewport target is a formalisation of an initial attempt to
provide a conceptual
... model for associating a root container region with
something other than the video frame.
... I don't know if it's something that we want to nail down in
TTML2. To support
... it fully we're going to need some new parameters like
viewport target parameter.
... What's in the spec right now is preliminary and needs more
conceptual
... thought as well as syntactic support.
pierre: I don't see why we need anything other than root
container region.
glenn: If we are talking about the outer world we need it...
pierre: That's a separate step.
glenn: Yes it is. The question is does the author want to give
some hints about it.
pierre: Everything that relates to generating pixels should
always be related to
... the root container region, and then we should separate out
the concept of
... how the root container region relates to any real world
thing.
glenn: I have no problem with that, but we have only one
terminology section.
pierre: True, we don't want more than one of those.
nigel: It sounds like there's more work to do here and some
group members may
... have questions in their minds about how far we need to go
in TTML2 to relate
... the root container region to some physical thing.
glenn: I'll proceed with this - it's possible I'll pull it out
depending on effort constraints
rohit: If we have root container region and some aspect ratio
defined, is there
... anything else that needs to be normative?
glenn: IMSC defined DAR, so now we have all three aspect ratios
and it's possible
... to generate a conflict.
rohit: Let's say we get past that issue, is there anything else
that needs to be
... normative?
glenn: Then I think we need normatively use some definitions in
other parts of
... the spec to give semantics to definitions.
... If we do want a viewport target parameter then we need
normative
... syntactic definitions. We could segregate some of this into
an informal annex.
nigel: Where is the use case or requirement coming from to
define viewport
... target in the document?
glenn: Someone said it at some point but I can't point to any
formal submission.
... If nobody thinks it is useful then I would just as soon not
spend time on it.
... We do start to touch on this with safeCropArea.
nigel: All we are doing there is admitting the idea that not
all of the root container
... region might be displayed.
glenn: The only other thing is nigel's suggestion to rename vw
and vh.
dae: it would seem more consistent to use rw and rh since they
are related to root container region.
nigel: My concern is that vw and vh will be misunderstood and
misused.
group: discusses impact of changing vw and vh to rw and rh,
concludes that we will change.
nigel: Thanks for the constructive approach everyone.
... I've updated issue
[27]https://github.com/w3c/ttml2/issues/181
[27] https://github.com/w3c/ttml2/issues/181
TTML2 initial element
pal: So initial was introduced to set defaults for style
attributes; and there's already an inheritance
... scheme for initial. Why not make non-inheritable style
attributes inheritable instead?
glenn: That would break CSS and XSL-FO models very badly, for
example all the background properties.
pierre: You could just reference a style from a region, where
the desired defaults are in that style.
glenn: That's not style inheritance, that's referential
styling.
pierre: Okay, so you could enable referential styling for
backgroundColor with no impact.
... That's my proposal, to use referential styling instead of
initial.
... The style resolution process already does this.
... I then don't need to introduce a new vocabulary.
glenn: That forces users to adopt a particular authoring style
and it may not be efficient in some contexts.
... For example say you have tts:showBackground which defaults
to "always" which you may want to change to
... "whenActive" so that's a good example for using initial to
set it to a different value instead of its normal default.
... In the case of an anonymously generated region by putting
an extent and origin on a p or div, it would not be possible to
have
... them style the region that was created anonymously via the
use of a style attribute. Any style attribute on p for example
... do not apply to the region, only to the p.
... The region created picks up the initial values as defined.
nigel: This touches on the question of whether region style
attributes modify the existing region or create a new one.
... Another syntactical approach could be to introduce say a
tts:regionStyle attribute to reference a style for the created
region.
pierre: [Notes that he objected to this solution to meeting the
default value requirements back in January]
Andreas: I see a use case for this - the only thing is are
there problems in current implementations that make this
necessary,
... because every additional feature adds to the implementation
burden. Does anyone have a problem in TTML1 that makes this
... necessary?
glenn: I have noticed that people often fully specify styles
that are redundant. I view initial as a highly useful authoring
tool to
... reduce the size of documents and the complexity.
... I use initial all the time, and it's an option to profile
it out if necessary.
pierre: Is there anything you can do with initial that you
cannot do with referential styling?
glenn: No. Indeed you could also use full inline styling.
rohit: If you specify all styling attributes you also would not
care about it.
glenn: I found this in change proposal 17 going back to issue
#207 in tracker...
nigel: From the discussion, to summarise, we have consensus to
leave initial in the TTML2 spec.
... Thanks, there's no issue to update on this, though this
initial element does appear to satisfy
[28]https://github.com/w3c/ttml2/issues/36
... Let's take lunch now, return at 1400.
[28] https://github.com/w3c/ttml2/issues/36
TTML2 - timing
nigel: I don't feel we have a lot to do here.
glenn: That's my take too - cleaning up some language may be
adequate.
... Adding some notes about SMIL may be helpful. We need to
firm up the
... idea of a document timeline.
nigel: I have three things to raise here:
... 1. I would like us to have a defined document timeline for
which the document
... defines some activity.
... 2. Consider removing SMIL normative references and
replacing them with our own normative text.
... 3. Consider adding a time expression model controlled by a
parameter on the tt element
... in which all the time expressions are flat and on the
document timeline,
... so no calculations are needed for nested timed elements. We
would need to
... be careful to clarify that elements can only active in this
new model when
... their parent elements are active.
glenn: We could do that, being aware of the potential
additional time taken to
... advance the specification.
nigel: Note that there's no feature designator right now for
nested times in TTML.
... I would add a feature designator for the new flat timing
model to allow
... profiling for implementations that only support flat
timing, which would be
... much simpler to write.
... This also would offer a path to a nice solution for the
first problem, which
... would simply be to allow a document's active period to be
defined by begin
... and end/dur elements on the body element.
andreas: You could simply profile out nested timed elements.
nigel: You could but then you would need a different way to
define the document
... timeline.
rohit: From a use case perspective I prefer flat timing.
dae: I don't see a use case for nested timing.
nigel: If we decided to adopt this then I would volunteer to do
the editing work.
glenn: Post TTML2 it might be more feasible to identify some
specific extensions
... like a new timing model and write a spec just for that, in
a modular approach.
... We would have to put a framework in place in order to deal
with the use
... of modules.
pierre: I've never seen nested timings used, and probably
hardly any
... implementations do it properly.
... I've never seen content for distribution that has nested
timing.
... I think our time is better spent clarifying the timing
model.
glenn: I've read the SMIL timing and synchronisation section
probably 20 times
... and implemented it at least 3 times, and every time I have
different questions
... and implementations, so I think it's difficult to get it
right.
... Absent some reviewed test content I don't think we have
dealt with the questoin
... of correctness in a way that's satisfactory.
nigel: As a minimum I think we need a feature designator for
nested timing.
andreas: Yes.
pierre: Why not just require non-use of nested times?
rohit: I think you can specify that by saying that on the
traversal from root to
... leaf only one element specifies a begin or end time.
nigel: From the discussion, I think we're not generally
supportive of adding
... a flat timing model now but we are interested in
reproducing the SMIL semantics
... and making them normative in TTML2.
glenn: That would take a lot of time, so I'm thinking it should
be a post-TTML2
... task because there are too many things that are higher
priority.
... I wouldn't object to anyone else going ahead and doing that
work but
... I'm not sure I would have time to review it and sign off on
it.
andreas: The first thing is that SMIL is hard to understand for
everyone; the
... second is if we have any existing problems that we need to
solve. If not, then
... is there an urgent need to clarify it in TTML2? Perhaps the
clarifications could
... be done in a separate document for example.
nigel: We could certainly do that as a Working Group Note say.
pierre: The situation in most implementations is probably that
there are bugs.
rohit: People who author do so with a "flat timing" mindset.
pierre: Most players assume that, and are just based on leaf
elements having
... begin and end attributes.
... My suggestion for going beyond that is if there's part of
the industry that
... wants more than a flat simple model then getting out of
SMIL would help
... get beyond that. If we don't care about that for now then
we don't have to
... do anything for TTML2.
glenn: There are not a lot of timing tests in the TTML test
suite.
... but there are a few that would be useful to run through
existing code.
rohit: One possibility is that you produce ISD sequence and
check if two
... implementations produce identical ISD sequences.
nigel: By the way I still have a problem with making the ISD
serialisation normative in the spec.
... it's a useful concept but I don't think it needs to be
normative.
glenn: I'd be prepared to make it informative.
nigel: Going back to my first point, I would like to be able to
indicate without
... changing the SMIL timing semantics or the syncbase the time
from which
... any given document defines some output behaviour.
glenn: You could just use the first ISD time.
nigel: That only defines the time at which some content is
displayed. I want to
... be able to define a time at which the document becomes
active even if at
... the beginning for some period there is no content
displayed.
andreas: [explains concept on whiteboard]
glenn: It sounds like you want to introduce the idea of a
supra-document timeline
... that covers multiple documents' timelines.
... [expresses problem as needing to put documents onto a
separate timeline via the whiteboard]
andreas: What problems exist with the current mechanism?
nigel: It would solve some specific cases for assembling live
subtitles onto some
... kind of "global" timeline, also potentially for feeding
into a packaging mechanism.
... I would make an "entry point" and "exit point" parameter on
the tt element,
... which would be optional. It would not modify any of the
existing time
... resolution or computation semantics, but effectively tell
the presentation
... processor to "seek" into a particular moment on the
document timeline, and
... "stop" on a different moment, if specified.
andreas: [expresses concern that this could duplicate
functionality in other layers]
rohit: This also has some analogies in IMF for example.
nigel: I would make this optional so that it can be used when
there is no external
... wrapper format.
andreas: Possibly it would be interesting to view this as a
proposal.
nigel: Ok I'll do that! [break for coffee, return at 1550]
andreas: Over coffee I had another idea that might solve your
use case Nigel...
... If there's no wrapping information there's no way to get
the information for external processing context.
... So what you could do is define a new element outside the tt
element that could be used as a wrapper,
... then have a tt element inside it. So you just define a
wrapper element for the tt document.
... This could be a short note or whatever defining the new
element and the semantic.
nigel: Ok, why would that be better than having a parameter on
the tt element?
andreas: The semantic of the tt element doesn't change at all,
and it would work for any kind of document.
rohit: This would make TTML2 documents look very different from
TTML1 documents.
andreas: You could wrap TTML1 documents in this also.
... And it doesn't conflict with current semantics.
nigel: One issue with this is that the active begin and end
times should be in the document's timebase and
... you would have to inspect the contents to discover what the
outer wrapper values actually mean.
... As a general concept I can see it could be useful but for
this it feels to me on first glance a bit "heavy".
TTML2 - condition
nigel: [29]https://github.com/w3c/ttml2/issues/128
... The issue here is how you use the condition mechanism to
set different attributes on regions or styles
... for example based on an externally passed in parameter or
media query.
[29] https://github.com/w3c/ttml2/issues/128
glenn: Style attributes can point to multiple styles.
nigel: True, but region attributes cannot.
glenn: I need to review this and think about some alternative
solutions. Region can refer to style elements.
... That's probably what I would do. It would be useful to put
some motivational examples on to
... describe why one would use condition.
... I do have some open questions by what it means 'ignore the
semantics' when the condition is false.
... For example let's say you have a chain of styles that refer
to each other, if one of the middle ones in the
... chain has a false condition do you ignore it and all the
rest of the chain, or just the styles defined on that
... style element. I haven't decided in my mind the best
approach to that.
... For every element we probably have to say something about
what it means to ignore that content.
rohit: One use case would be on the content side, to replace
the "forced" functionality.
nigel: Agreed.
... Glenn you mentioned recently that adopting the 'modify'
semantic for inline anonymous region would
... require a reversal of the direction of reference. Does that
have any impact here?
glenn: The way that content and animation are associated using
xlink:href to point from an animation to
... content, in SVG. When we started with TTML2 animation we
started with that same approach. The SVG
... spec probably defines SMIL animation better than SMIL does.
... We changed it so that content elements point at animate
elements, so to pick up an animation for an
... out of line animation then the content would have an
animate attribute pointing at the id for the animate element.
... If we want to animate the current region then the logical
place for the animation would be as a child
... element of the content, for example a <p tts:extent="..."
etc.> you would synthesise a set element that
... is a child of a p, but targeting the region of the p not
the p itself. Now we need the animate element to
... point at the element to be animated.
... I don't really like using xlink for this purpose so I am
contemplating using a target attribute on the animate element.
... However we do have xlink already because we added support
URL linking, so using the xlink version would
... be feasible here and may be desirable for consistency.
nigel: In that case you could resolve this issue by having set
elements that change region attributes as well
... as condition attributes, so that would effectively
conditionally apply styling etc to regions using that
mechanism.
... The next question is does a set element have to have
timing?
glenn: No, it is a timed element so it has times whether
defined or not, but it could have implied times
... that would be inherited from its parent.
nigel: Presumably its 'rehomed' parent not its lexical parent,
the animation element?
glenn: That's a good point.
nigel: I've added a note to the issue on github there.
... [30]https://github.com/w3c/ttml2/issues/168
[30] https://github.com/w3c/ttml2/issues/168
glenn: I think we already have consensus on doing this.
TTML2 simplification options.
nigel: I don't have any specific proposals for this but have
been wondering if there are any options for
... structuring the TTML2 spec to reference TTML1 and add to
it, for example. I think it's almost certainly
... not something that we would agree to right now, but it's
occurred to me that we could consider it.
group: [discussion of possibilities, predictable lack of
willingness to vary from current path]
andreas: For implementers it might be helpful to have an EBNF
notation for TTML1 and TTML2.
glenn: I would leave that as an exercise for the reader. It
would not add any precision because all the
... information is already present for every syntactic feature.
For example for attribute values we use a more
... general EBNF but for element definitions I used a different
syntax, I think from how SVG and SMIL did it.
... Points at §2.3 for a definition.
nigel: Are we going to hit an accessibility problem with the
spec by using colours only to indicate
... deprecated or obsoleted features?
glenn: Whenever there is one it is also written out in the
specification text.
nigel: Ok there's no problem there then.
rohit: Does this cover the expression syntax too? Or is that
classic EBNF? if it is we should call it out.
glenn: Actually I think we followed CSS, where for example ||
means "in any order" and && means a particular
... order (but we don't happen to use any of those).
... I guess we should check if we say in the conventions if we
make use of those.
rohit: I don't think we do. We should be explicit that we're
using EBNF or whatever.
nigel: Please could you add an issue to the github repository
for that please Rohit?
rohit: Sure, more than happy.
TTML2 issues
rohit: [31]https://github.com/w3c/ttml2/issues/168
[31] https://github.com/w3c/ttml2/issues/168
glenn: Mike Dolan's response to this was that it should be
illegal or an error. I think we can easily update
... the formula in Annex N to accommodate fractional seconds in
clock-time expression in smpte mode.
... If it is allowed it should probably only be permitted in
smpte continuousmode, since they by definition
... can not be labels as are used in smpte discontinuous mode.
nigel: I've heard a question like this before. I think you have
to quantise to a specific frame value, and I don't
... know how you choose whether to use floor or ceiling or
whatever to do it.
glenn: [Go to H.3 in TTML2] clearly you could end up with
fractional frames, and it does not look like we
... have defined any floor or ceiling rules here.
nigel: Whatever we do we could end up generating a frame value
that turns out not to be valid according to
... dropMode, so I'm beginning to come round to Mike's point of
view that we should just not allow this and
... consider it to be an error, since this seems to be
nonsensical.
rohit: That sounds like a resolution.
nigel: It would also be coincident with EBU-TT (not permitting
fractional seconds in smpte timebase)
rohit: Do we also not allow offset time expressions with
anything other than ms and t metrics?
nigel: You are also permitted fractions in offset times.
rohit: Then exclude fraction components too.
glenn: I noticed another problem in H.3 - it does not refer to
subframes, but we do have a subframe component.
... That was intended to match an earlier smpte spec that
allows you to specify field numbers.
nigel: I don't see this problem - subframes are counted in H.3
... I would propose to prohibit offset-time expressions in
TTML2 even if that's a breaking change. It would be better for
the world! At least we would put it out for review.
glenn: I would accept a "shall not" when version="2" for TTML2.
nigel: I've updated the issue with this.
rohit: Re notation, I created
[32]https://github.com/w3c/ttml2/issues/186
[32] https://github.com/w3c/ttml2/issues/186
TTML2 - sequences of documents
nigel: We have already discussed this last year and have
[33]https://github.com/w3c/ttml2/issues/122
... I propose that we move on unless there's a change to what
we agreed - we just need to create a pull
... request for this and do it.
[33] https://github.com/w3c/ttml2/issues/122
glenn: In the context of entry and exit time parameters it
would be preferable to have them all either in
... the metadata namespace or the parameter namespace.
nigel: Arguably there's a philosophical difference since a
processor would not take action based on the
... sequence id and number when processing a single document,
but it would to based on the entry and
... exit time parameters.
Audio Description
nigel: I made a submission early on Monday morning for
requirements for audio description.
... Describes the requirements document; [describes expected
TTML2 structure on flipchart]
... It seems that TTML2 could easily be expanded to fit these
requirements by adding pan and gain
... attributes, an audio mixing model and an additional
interpolation mode for animate. BBC has a
... prototype AD mixer using the Web Audio API that could
easily be modified to run off a TTML2 document
... that supports these extra features.
glenn: TTV can validate the audio element so that could be
another implementation.
nigel: There's currently no AD document spec other than a
proprietary one that I'm aware of so this would
... be helpful to the world also.
... If anyone has interest in taking this further please
respond to my reflector email with any comments
... on the requirements or any other implementation thoughts.
<glenn> +Present glenn
Text Track and Text Track Cue interface
andreas: Having raised this in yesterday's Web & TV IG joint
meeting the question is what role this group
... would want to play.
glenn: I think it would have to play a major role.
andreas: Tomorrow there will be an unconference session on
this. I think it would be good if not only
... driven by TTWG.
glenn: +1
andreas: But also not a lot of people will want to do the work
and have the expertise so they may well point
... to this group. So tomorrow if there is interest to move
this forward one proposal would be that the
... requirements would be documented and taken to the Web & TV
IG.
nigel: To clarify: ask them to validate the requirements?
andreas: I'm not sure how this would work - we may possibly
document requirements and possible
... solutions, and then discuss there for contacting other
working groups from W3C that are dealing with
... the HTML specs.
<glenn>
[34]https://dvcs.w3.org/hg/ttml/raw-file/default/ttml2-api/Over
view.html
[34] https://dvcs.w3.org/hg/ttml/raw-file/default/ttml2-api/Overview.html
nigel: We would need some active participants from Web & TV IG
to take this forward otherwise it could be
... that nothing happens until next year's TPAC! Maybe we need
to go more directly to e.g. Web App WG.
andreas: I was wondering if the attendees of tomorrow's session
might be able to bring some work to this
... working group?
nigel: I wonder if we could reuse the TTCG for this?
andreas: I would rather set draft a document in the TTWG and
encourage others to join. We might isolate
... this work from the rest of the group's work. This was
similarly an issue with the mapping document work.
nigel: It's not in our Charter so we would need to publish as a
Note only, or if we want a Rec then recharter,
... or use this group as an incubator and donate a document to
another group that might have it in scope
... of their charter.
thierry: We could set up a new CG for this - it would be more
neutral than doing it in TTWG and wouldn't
... have the same encumbrances for joining.
andreas: Yes, we could do that.
TTML2 editorial actions
nigel: We're really out of time for this, but Glenn are there
any editorial actions you would like help with?
glenn: No, not right now, I will ask when there are some.
nigel: Okay, thanks.
Meeting close
nigel: Thanks everyone - we've covered a huge amount over two
days including every topic we had on our
... agenda and more besides, with a really constructive
atmosphere.
Glenn: That's thanks to your chairing.
Andreas: Thanks for chairing!
nigel: [blushes] Thanks everyone, have a great rest of TPAC
everyone. [adjourns meeting]
Summary of Action Items
Summary of Resolutions
[End of minutes]
__________________________________________________________
Minutes formatted by David Booth's [35]scribe.perl version
1.144 ([36]CVS log)
$Date: 2016/09/20 17:16:09 $
[35] http://dev.w3.org/cvsweb/~checkout~/2002/scribe/scribedoc.htm
[36] http://dev.w3.org/cvsweb/2002/scribe/
--
Nigel Megitt
Executive Product Manager, BBC Design & Engineering
Telephone : +44 (0)3030807996
BC2 C1 Broadcast Centre, Media Village, 201 Wood Lane, London W12 7TP
Received on Tuesday, 20 September 2016 17:19:21 UTC