- From: Glenn Adams <glenn@skynav.com>
- Date: Sat, 20 Sep 2014 08:42:32 -0600
- To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
- Cc: Nigel Megitt <nigel.megitt@bbc.co.uk>, TTWG <public-tt@w3.org>, "public-texttracks@w3.org" <public-texttracks@w3.org>
- Message-ID: <CACQ=j+e0=DkfWc08R5edaw9RF+Pjf1H4MNtkPPBcSpXdkH6t4Q@mail.gmail.com>
On Sat, Sep 20, 2014 at 7:30 AM, Silvia Pfeiffer <silviapfeiffer1@gmail.com>
wrote:
> Most interesting read. Seems like there is not that much that doesn't
> map in either direction.
>
> A few additions / corrections about WebVTT that relate to this
> discussion and that were missed (though Simon caught most):
>
> 1.) Mapping WebVTT - 608/708
> This wasn't mentioned, but I wanted to make sure people are aware that
> there is already a spec that provides a basic mapping between WebVTT
> and 608 and 708 captions at
> https://dvcs.w3.org/hg/text-tracks/raw-file/default/608toVTT/608toVTT.html
> .
> It does indeed require some CSS.
>
> 2.) Repetition of cue settings:
> There's a plan to allow for some file metadata to set different
> default values for settings, so repetition is not necessary.
>
> 3.) CSS works by using ::cue and ::cue-region - the first for cues and
> the second for regions.
> These selectors work from a Web page (wherever CSS is specified there)
> and override default styling of WebVTT cues.
>
> 4.) cue overalpping:
> It's actually not true that cues cannot overlap in time. They can and
> are expected to. In fact, chapter cues are specifically defined as a
> particular kind of overlapping cues that are hierarchically
> structured, or in your words: are nested.
>
> 5.) cue ids:
> I think it might be possible to map numeric identifiers to XML
> identifiers - at least it's possible to use them like this for CSS:
> ::cue(#\31) { color: green; }
> This references a cue with id=1. That might work for mapping to TTML
> xml:id too.
>
it would be necessary to prefix a numeric identifer with at least one alpha
to use with xml:id
>
> 6.) visibility and opacity:
> are explicitly mentioned as CSS properties usable in ::cue and
> ::cue-region.
>
> 7.) WebVTT does paint-on:
> It's even implemented in browsers. You just need to use the timestamps
> and right now also <c> elements between them, e.g.
> <00:00:01.000><c>From</c><00:00:04.000><c> here</> etc.
>
> I might have missed some more things - if in doubt, just ask.
>
> Cheers,
> Silvia.
>
>
>
> On Wed, Sep 17, 2014 at 12:08 PM, Nigel Megitt <nigel.megitt@bbc.co.uk>
> wrote:
> > Minutes from today's joint TTWG/TTCG meeting (day 1 of 2) available in
> HTML
> > format at http://www.w3.org/2014/09/16-tt-minutes.html
> >
> > In text format:
> >
> > [1]W3C
> >
> > [1] http://www.w3.org/
> >
> > - DRAFT -
> >
> > Timed Text Working Group Teleconference
> >
> > 16 Sep 2014
> >
> > See also: [2]IRC log
> >
> > [2] http://www.w3.org/2014/09/16-tt-irc
> >
> > Attendees
> >
> > Present
> > elindstrom, tmichel, Frans_EBU, pal, Cyril, courtney,
> > andreas, glenn, nigel, Loretta
> >
> > Regrets
> > Chair
> > nigel
> >
> > Scribe
> > nigel
> >
> > Contents
> >
> > * [3]Topics
> > 1. [4]Introductions
> > 2. [5]Agenda
> > 3. [6]Work done so far
> > 4. [7]Logical step through
> > 5. [8]Agenda
> > 6. [9]Document Structure
> > 7. [10]Layout
> > 8. [11]Summary of the day
> > * [12]Summary of Action Items
> > __________________________________________________________
> >
> > <trackbot> Date: 16 September 2014
> >
> > [13]https://www.w3.org/wiki/TimedText/geneva2014#Day_1_0900-170
> > 0
> >
> > [13] https://www.w3.org/wiki/TimedText/geneva2014#Day_1_0900-1700
> >
> > Introductions
> >
> > <scribe> scribeNick: nigel
> >
> > Introductions - Nigel, BBC
> >
> > andreas: IRT
> >
> > Cyril: Telecom ParisTech university; GPAC
> >
> > elindstrom: Opera software
> >
> > zcorpan: Opera software
> >
> > tmichel: W3C, staff contact for the TTWG
> >
> > pal: Movielabs
> >
> > courtney: Apple
> >
> > glenn: Representing various over the years, currently Cox,
> > previously Samsung and Microsoft
> >
> > frans_EBU: Coordinator of EBU group on subtitling
> >
> > Agenda
> >
> > nigel: goes through agenda on wiki page, all happy with that.
> > ... We need to think about how we capture our output, and who
> > will edit the note.
> >
> > courtney: I'm happy to edit the note.
> > ... I don't have a document yet, I've been working on the code
> > first, and have some issues to tackle, and a spreadsheet for
> > attributes.
> >
> > glenn: For browser implementations mapping direct from TTML to
> > HTML would be more efficient
> > ... If the purpose is for direct display then this mapping
> > would be better, but if we want to interchange to WebVTT then
> > that translation would still be useful.
> >
> > courtney: I'm interested in captions both inside and outside
> > browser environments so I'm not focused on HTML solely.
> >
> > andreas: From the mapping we have done we will quite quickly
> > see the overlap - maybe there's a cut and paste into HTML as
> > glenn mentioned.
> >
> > pal: Re WebVTT outside browsers?
> >
> > courtney: Yes, e.g. in an ISO MP4 file that is rendered in a
> > video player.
> >
> > pal: So do we need CSS in practice? To present WebVTT in
> > subtitles and captions?
> >
> > courtney: You certainly can, but it depends on how fancy you
> > want to be. You can do basic 608 without CSS.
> >
> > andreas: you need CSS to do colours, and that's certainly
> > required in Europe.
> >
> > courtney: We define for example a simple mapping from CSS to a
> > property list. I think the better approach is to stick with CSS
> > and
> > ... have a way to embed it in an MP4 file track, and also in a
> > WebVTT file.
> >
> > pal: Will the mapping we do today include that?
> >
> > courtney: yes
> >
> > Cyril: +1
> >
> > courtney: I've been thinking that one TTML file will map to a
> > WebVTT file + a CSS file
> >
> > glenn: That's what I've been thinking, and there's a reusable
> > overlap into HTML/CSS
> >
> > nigel: I've created a wiki page at
> > [14]https://www.w3.org/wiki/TimedText/TTMLtoWebVTT
> >
> > [14] https://www.w3.org/wiki/TimedText/TTMLtoWebVTT
> >
> > <zcorpan> can you paste the number in irc?
> >
> > Work done so far
> >
> > andreas: presents work so far
> > ... This work has been supported by the HBB4ALL project, whose
> > target is to roll out accessibility to IP connected devices,
> > including subtitles, signing and audio (video) description,
> > ... with a focus on hybrid broadcast.
> > ... This is based on EBU-TT-BasicDe as a very restricted TTML
> > feature set.
> > ... In fact it's a subset of EBU-TT-D which is a subset of TTML
> > plus a couple of small extensions.
> > ... It has a video frame with a safe area, 10% in from each
> > edge.
> > ... Alignment is top or bottom only, vertically.
> > ... Horizontally, centred, left or right.
> > ... For Germany, it's left to right, top to bottom writing
> > direction.
> > ... There are 8 different text foreground colours, as from
> > WSTeletext.
> > ... All subtitles have the same background color, font-family,
> > font-size and line height.
> > ... Line breaking is done manually with the <br> element at
> > authoring.
> >
> > glenn: How is the background padding extended on either side of
> > the text?
> >
> > andreas: That's just in the example image, it's not actually
> > present.
> > ... How is this mapping achieved? Positioning, Styling, Timing.
> > ... Positioning:
> > ... [shows video frame with image of Verona]
> > ... In TTML and EBU-TT there's a root container. In EBU-TT it's
> > always the height and width of the video. WebVTT uses the
> > viewport concept,
> > ... which I understand to be the height and width of the video
> > also.
> > ... For the safe area, we define the tt:region, with top-left
> > being 10% 10% in x y as specified by the origin.
> > ... The CSS property is topleft
> > ... The extent is 80% 80%, which in CSS is the width and height
> > of the block level element eg the div
> > ... To place a subtitle the region is defined once in the head
> > and then referenced by the tt:p element. This is similar to a p
> > in html.
> > ... The paragraph gets the width of the region, and the height
> > is calculated by the number of lines inside the p element.
> > ... Vertical alignment is displayAlign: bottom or top.
> >
> > nigel: Will there be CSS mappings for all of these in this
> > presentation?
> >
> > andreas: This is setting out the features to map, we should
> > consider them in scope for our mapping later.
> > ... I didn't use the advanced concepts in WebVTT of cue
> > alignment, so I didn't use them. I wanted something that would
> > certainly work in current browsers.
> > ... In WebVTT I've put the cues in for the text. For a width of
> > 80% the cue box has size: 80%
> > ... The height is defined by the number of lines, just like the
> > p element.
> > ... This is per cue, so the settings seem to need to be
> > repeated every time. I don't know a way to define it once and
> > have it carried through.
> >
> > courtney: If you use a region you can do that.
> >
> > andreas: I didn't use a region.
> >
> > courtney: Then you have to repeat it.
> >
> > andreas: So that's positioning. We can define the position of
> > the box from the left of the video frame, with 10%, using
> > position:10% align:left
> > ... The align setting is important. It works very differently
> > than in TTML e.g. if you set align:middle and position:10% then
> > the reference point for the middle isn't the cue
> > ... start but is the middle of the cue.
> > ... So to centre the text then you have position:50%
> > align:middle
> > ... For vertical alignment it's a bit trickier. To come 10% up
> > from the bottom you can set line:90% or a line number value.
> > ... But this doesn't align the end of the cue box, but aligns
> > the top of the cue box. So that doesn't work.
> > ... What you actually need is position:100% - margin - height
> > of cue-box.
> > ... That works if you have a lot of control over the font
> > height and can calculate the position this way.
> > ... In most cases that's a bit risky. So then I changed to the
> > other possibility, to use line alignment
> > ... The first line in the cue generates the line grid, then you
> > can position the cue box with positive line numbers from the
> > top
> > ... or negative line numbers starting from -1 from the bottom.
> > ... [example shows text one line up from bottom]
> > ... You have to have the snap to lines flag set - this happens
> > automatically if you use line numbers.
> > ... For one line you can have line:-2, or for a two line
> > subtitle, line:-3. Needs a bit of calculation.
> > ... A dirty trick possibly is always to set it to -1 and let
> > the renderer push it up. Possibly this is not recommended but
> > it may work.
> > ... Styling:
> > ... In EBU-TT-BasicDE there's a default style defined once in
> > the head, and a div element that references the defaultStyle.
> > ... In WebVTT you can define a general cue selector ::cue and
> > use almost the same property names and values.
> > ... For font-size some calculation is needed. 60% font size in
> > TTML comes out at 5.33% of the height of the video, which is
> > 100% in CSS.
> > ... A separate CSS file is needed to contain the ::cue
> > selector.
> > ... For inline styles in TTML we set the colour attributes on a
> > style referenced from a span.
> > ... In CSS you can use the pseudo-selector ::cue(c.textWhite) {
> > color: #ffffff; background-color:rgba(0,0,0,0.7); }
> > ... Then in the VTT c.textWhite cue class
> > ... Timing:
> > ... In TTML put a begin and end on, with media timeBase,
> > reference sync is zero. In EBU-TT-BasicDE the fractional
> > seconds are limited to 3 digits.
> > ... This is the same for WebVTT cues.
> >
> > pal: What are the rules for CSS styles when combined with
> > locally set rules? Which takes precedence between author and
> > user choices?
> >
> > courtney: We would consider user choices to override author
> > styles.
> >
> > pal: If you're displaying it on a web page, then web styles
> > taking over seems like not the right thing to do.
> >
> > andreas: It's not clear to me how the CSS that applies to the
> > web page interacts with the VTT cues. From testing there's no
> > relationship.
> > ... The video is a separate viewport with independent styling,
> > from my testing anyway.
> >
> > Cyril: I think that's not expected. I remember that the cues
> > are sourced in the HTML page so the styles should be applied.
> >
> > andreas: I tried it out in Opera.
> >
> > zcorpan: The styling was implemented in presto - I'll put
> > together a quick demo and paste the link
> >
> > andreas: One important point is that we put the background
> > color just behind the text not the box. From what I read
> > there's no possibility
> > ... in WebVTT to put the background only on block level
> > elements, e.g. the whole region/p/div etc.
> > ... It only puts the background behind each glyph. I think
> > there's a WebVTT background box concept but it doesn't seem to
> > apply to the block level.
> >
> > glenn: So TTML allows the background to be specified on the
> > containing block and possibly differently on the span or the p
> > within the larger block.
> > ... So this example (showing two spans each with its own
> > background color) wouldn't be possible?
> >
> > andreas: That's right. In Europe both possibilities are in use.
> > ... We need to be aware of this restriction in the mapping.
> >
> > <zcorpan>
> > [15]http://w3c-test.org/webvtt/rendering/cues-with-video/proces
> > sing-model/basic.html has styling
> >
> > [15]
> >
> http://w3c-test.org/webvtt/rendering/cues-with-video/processing-model/basic.html
> >
> > zcorpan: This shows how a stylesheet applies to WebVTT cues -
> > the stylesheet is in the HTML page and the cues use those
> > styles
> > ... There's a white video behind it.
> >
> > pause for 4 minutes, back at 10:33 (CET)
> >
> > <zcorpan> wrt to the positioning discussion, there are open
> > bugs on the webvtt spec for both changing how positioning works
> > and for adding something that allows for exact positioning.
> > [16]https://www.w3.org/Bugs/Public/buglist.cgi?quicksearch=webv
> > tt%20positioning&list_id=43983
> >
> > [16]
> >
> https://www.w3.org/Bugs/Public/buglist.cgi?quicksearch=webvtt%20positioning&list_id=43983
> >
> > <zcorpan>
> > [17]https://www.w3.org/Bugs/Public/show_bug.cgi?id=25632
> >
> > [17] https://www.w3.org/Bugs/Public/show_bug.cgi?id=25632
> >
> > nigel: we're reassembling...
> >
> > courtney: Here's what I've discovered from writing mapping
> > code.
> > ... There's an issue that we don't have an official WebVTT spec
> > yet - we're working off drafts that aren't versioned.
> > ... When Andreas was talking he was using browser supported
> > features. This is causing a bit of an issue. The mapping I've
> > been doing is off the most
> > ... current WebVTT spec version.
> > [18]http://dev.w3.org/html5/webvtt/
> > ... Here are 3 categories of issue:
> > ... 1. TTMl is more hierarchical than WebVTT
> > ... 2. The two specs define different properties implicitly vs
> > explicitly.
> >
> > [18] http://dev.w3.org/html5/webvtt/
> >
> > 3. The basic problem of converting units (value type
> > conversions)
> >
> > scribe: Hierarchical vs Flat:
> > ... WebVTT has a flat structure with no nested elements. TTML
> > provides a hierarchical structure.
> > ... Metadata: in TTML you can nest metadata hierarchically
> > [shows ttm:agent holmes and Dr Watson]. In WebVTT you get a
> > list with no relationships between them.
> > ... Proposal for WebVTT is hierarchical metadata keys
> >
> > nigel: Is that just metadata or presentation issues too?
> >
> > courtney: It may be less of an issue for presentation issues
> > but there are cases where we run into a similar problem.
> > ... Another example: Calculating relative timings
> > hierarchically in TTML and linearly in WebVTT.
> >
> > Cyril: I think some profiles restrict that.
> >
> > andreas: Yes, EBU-TT-D doesn't allow nested timing.
> >
> > Cyril: That raises the question which profile are we looking
> > at?
> >
> > Courtney: Yes, we can simplify the problem by specifying a
> > profile.
> >
> > glenn: It's useful, though it may take longer, to start from
> > the general case and identify where in the absence of a profile
> > there are issues.
> > ... For example re timing and even styles we could define a
> > mapping based on the sequence of Intermediate Synchronic
> > Documents, to remove the timing issues.
> > ... Just documenting these issues is useful.
> >
> > nigel: We decided last week to use TTML1SE and WebVTT.
> >
> > andreas: for styling there's some hierarchical structure in
> > WebVTT too, by application of class nodes that are nested.
> >
> > courtney: Yes you can have nested styles within a cue but if
> > you want the same style for 10 cues you can't put them in a
> > fragment and declare it at the fragment level.
> > ... Implicit vs Explicit:
> > ... Some functionality is explicitly described by attributes or
> > parameters in one spec but implicitly derived in the other.
> > ... For example, horizontal writing direction. In TTML there's
> > a way to specify horizontal direction but in WebVTT there isn't
> > (unless it's vertical) - it's inferred from the font.
> >
> > glenn: tts:direction is designed to work in relation to the
> > Unicode bidi control characters
> > ... absent of those you can still infer directionality based on
> > the content of the element, though it's harder with mixed
> > content.
> > ... So the direction attribute in TTML doesn't really say
> > 'write right to left' but does specify the default writing
> > direction in the absence of bidi.
> >
> > courtney: WebVTT has bidi too, and rtl and ltr entities.
> >
> > andreas: In Unicode the information is already there.
> >
> > glenn: You have to look at the history of Unicode - people
> > didn't want to use nestable control codes so they wanted CSS
> > attributes to do the same thing.
> >
> > <zcorpan>
> > [19]http://dev.w3.org/html5/webvtt/#h4_processing-model says
> > how to determine direction
> >
> > [19] http://dev.w3.org/html5/webvtt/#h4_processing-model
> >
> > zcorpan: The horizontal direction is taken from the text in the
> > cue, not from the font (in WebVTT)
> > ... You can override it with unicode bidi characters if you
> > want.
> >
> > nigel: Seems like there's no issue to log in our issues list.
> >
> > <zcorpan> "Apply the Unicode Bidirectional Algorithm's
> > Paragraph Level steps to the concatenation of the values of
> > each WebVTT Text Object in nodes, in a pre-order, depth-first
> > traversal, excluding WebVTT Ruby Text Objects and their
> > descendants, to determine the paragraph embedding level of the
> > first Unicode paragraph of the cue. [BIDI]"
> >
> > glenn: TTML has the CSS features as well as the plain text.
> >
> > courtney: Example 2: line breaks - need to be explicit in TTML
> > but can be just new lines in WebVTT.
> >
> > Cyril: That's due to the parser - XML requires this.
> >
> > andreas: Later on we can look at xml:space attributes. From the
> > tests I've seen with xml:space="preserve" then line breaks
> > should be preserved.
> >
> > <zcorpan> XML doesn't require it really
> >
> > glenn: In XSL-FO there are 4 different properties. We define an
> > explicit mapping of xml:space to sets of those values, in TTML.
> > We didn't expose the full XSL-FO model.
> >
> > courtney: Value Type Conversions
> >
> > <glenn> tnx 4 reminder
> >
> > courtney: Example 1 - times
> > ... TTML has different time expressions, WebVTT always has
> > hh:mm:ss.sss with fractional seconds.
> > ... Fortunately the ttp: namespace defines all the required
> > metadata to do the conversions.
> > ... Though I'm not sure that's the case with lengths and
> > position values
> > ... Again TTML allows a broader set of units - pixels, em,
> > cells, %ages
> > ... I'm assuming lineHeight is sort of like em. For some TTML
> > documents I think you need the authored video dimensions to do
> > the mapping.
> >
> > pal: I think if you use %age or c you don't need the video
> > dimensions. If you're going to use pixels then implementations
> > should use tts:extent on the root as well.
> >
> > glenn: By specifying extent on the root you can derive a pixel
> > dimension - this doesn't tell you the pixel relationship to the
> > video though.
> >
> > andreas: An issue is that in general the root container pixel
> > dimensions are not necessarily coincident with the video
> > dimensions.
> > ... The document has no way to specify this in TTML, in
> > general.
> >
> > pal: CFF-TT and EBU-TT-D relate the root container to the
> > video. IMSC introduces an aspect ratio. All the profiles
> > specify how the mapping goes.
> >
> > andreas: For general TTML documents this is an issue.
> >
> > courtney: Attribute mappings
> > ... Some are straightforward.
> > ... Though WebVTT IDs can be purely numeric, and xml:id doesn't
> > allow that. So some modification or convention may be needed,
> > e.g. "cue"+number.
> > ... We could define the best practice.
> > ... Both use BCP47 language values
> > ... Preserve space needs further discussion.
> > ... Styling attributes: colors, fonts etc are fairly
> > straightforward.
> >
> > pal: Is there a subset of CSS that's supported for WebVTT?
> >
> > <zcorpan> [20]http://dev.w3.org/html5/webvtt/#css-extensions is
> > the subset
> >
> > [20] http://dev.w3.org/html5/webvtt/#css-extensions
> >
> > andreas: In WebVTT there's a subset of properties that are
> > permitted. E.g. padding is not allowed.
> >
> > courtney: One requirement set is what's needed for CEA608. It
> > would be useful to have a standard set of CSS classes that can
> > be used for any CEA608 translations into WebVTT.
> > ... There are some properties with no WebVTT equivalent:
> > display, overflow, padding, showBackground.
> > ... For alignment, displayAlign maps to the latest version of
> > the WebVTT spec.
> >
> > andreas: I tried it out, and it would work perfectly.
> >
> > courtney: But they're not widely supported yet. The mapping is
> > nicer at least.
> >
> > <zcorpan> "the properties corresponding to the 'background'
> > shorthand" is allowed, if that is what showBackground does
> >
> > zcorpan: any other properties will be ignored than those listed
> > in the spec.
> > ... I'm not sure how the TTML features map to those but there
> > is a defined subset in the spec.
> >
> > courtney: To expand on that, things like textDecoration in TTML
> > you can have underline set on a cue, but for the rest of it
> > you'd have to go to CSS to do?
> >
> > <zcorpan> <u>
> >
> > zcorpan: For underline you can use CSS or the <u> element
> > inside a cue.
> >
> > courtney: visibility and zIndex - I can't see how to do those
> > in WebVTT.
> > ... extent can be done with a cue box size or a region size.
> > ... A lot of the timing in the ttp: namespace metadata doesn't
> > map to the WebVTT because the timing that's allowed is a lot
> > simpler.
> >
> > zcorpan: visibility and zIndex is not possible in WebVTT.
> >
> > nigel: can't you do visibility with opacity?
> >
> > zcorpan: yes you can do visibility.
> >
> > courtney: there are also the attributes "use", "value" and
> > "type".
> >
> > glenn: Those are in the profile definition mechanism - they're
> > not content or style based.
> >
> > Cyril: does this mean they don't have to be mapped?
> >
> > courtney: since there are no profiles in WebVTT I guess not.
> >
> > glenn: This is all part of the TTML way to specify what a
> > processor needs to support, based on SMIL and SVG originally.
> > ... I think it can probably be ignored but needs more thought.
> >
> > andreas: If we do not find a direct mapping between WebVTT and
> > TTML that doesn't mean that we can rule it out for the mapping
> > ... because there's some intent in the source document and we
> > have to check if theres something that needs to be done.
> >
> > courtney: Ruby: there's no simple mapping from WebVTT to TTML
> > for ruby.
> >
> > glenn: In TTML1 you have to do the work at authoring time and
> > use regions to place the ruby in the right place.
> > ... I've recently specified in TTML2 the ruby markup.
> >
> > Cyril: There may be several ways to define the same thing, so
> > we should try to use a canonical representation as the mapping
> > source.
> > ... For example there are several ways of expressing timing -
> > maybe a requirement before mapping is a single syntax. I'm not
> > sure if this is possible.
> >
> > courtney: it may be an interesting way to break the problem up.
> >
> > Cyril: A problem I've seen before is that when attributes need
> > to be resolved at runtime based on context, e.g. frame rate,
> > video size etc there's not much that can be done.
> > ... We maybe need to classify those attributes that can be
> > mapped offline vs those that need full context to resolve.
> >
> > courtney: that's my presentation.
> >
> > Cyril: There's also the question of which TTML profile to use.
> > But also there are different classes of WebVTT: valid or not?
> > parsable or not?
> > ... Invalid documents may be presented okay by browsers. We
> > should say which class we're looking at.
> > ... Then WebVTT can represent metadata, chapters, subtitles,
> > captions etc. so we should indicate which ones we're mapping,
> > if not all.
> >
> > Logical step through
> >
> > nigel: Processing model
> >
> > Cyril: how does TTML handle overlapping times?
> >
> > glenn: there's arbitrary overlap permitted.
> > ... The first step I'd advocate is to create the intermediate
> > synchronic documents and map to WebVTT.
> >
> > Cyril: In WebVTT there's the concept of cues becoming active
> > and then bumping up existing visible cues.
> >
> > some discussion of how this is handled in TTML
> >
> > andreas: Formally the concept of creating the ISDs makes a lot
> > of sense - we need to make sure everyone understands what that
> > means.
> >
> > glenn: I agree. For example one thing that may not be obvious
> > is that style inheritance is only defined on ISDs so one has to
> > perform the ISD creation prior to style inheritance.
> > ... I've also added a function on the TTV tool to generate the
> > set of ISDs.
> >
> > nigel: We have a choice here to map ISDs or specific bits of
> > cue text.
> > ... This impacts efficiency and metadata.
> >
> > pal: This depends on the use case - if we just have the goal of
> > getting equivalent presentation then efficiency and metadata
> > are secondary concerns.
> >
> > elindstrom: from a browser perspective we're interested in
> > accurate presentation.
> >
> > courtney: I've been thinking about it the opposite way - from a
> > TTML to WebVTT conversion preserving semantics.
> >
> > andreas: Would it be possible to take Courtney's attribute list
> > and make it a structured document, take it as a header, explain
> > the problem scenario,
> > ... and indicate what the options and recommendations are from
> > the WG?
> > ... If you try to map abstractly the logical model then it's
> > very hard. Something more concrete may be a better start.
> >
> > pal: This is a question of how complicated we want to make it -
> > I haven't heard of anyone wanting to use WebVTT as a
> > master/archive/mezzanine format.
> >
> > glenn: There's a use case for distribution though.
> >
> > pal: I can see the use case of converting the TTML experience
> > into a WebVTT experience.
> >
> > glenn: Part of this may be timing oriented in the sense that
> > user agents may potentially add TTML renderers directly, which
> > would reduce the future needs.
> > ... But there may still be WebVTT-only presentation devices.
> >
> > pal: The issue for me is about the non-presentation-based usage
> > of WebVTT.
> >
> > elindstrom: I don't expect that to be a huge use case.
> >
> > nigel: Seems like we've been considering TTML -> WebVTT here.
> > Does the same consideration apply the other way?
> >
> > courtney: WebVTT does roll-up - I'm not sure how we do that
> > with TTML.
> >
> > glenn: we may need to consider using the set element in TTML1.
> >
> > pal: When you say roll-up you mean where there's an animation
> > displayed?
> >
> > glenn: yes, gradually moving up.
> >
> > pal: To do that explicitly in TTML you need animation, but what
> > is possible is to have a region that contains line A at t=0 and
> > at t=1 line B is added, moving line A up.
> > ... This doesn't require any animation.
> >
> > glenn: Yes correct but it doesn't do the whole 608 animation.
> >
> > pal: Then the question is do we need to explicitly define the
> > roll-up animation.
> >
> > glenn: Yes, we put in a note that implementation might do that.
> >
> > courtney: What about paint-on?
> >
> > glenn: That's no problem.
> > ... Does WebVTT support smooth roll-up as opposed to discrete
> > line based roll-up?
> >
> > courtney: I think it does yes, I'll have to confirm.
> >
> > nigel: As a general point here we can leave it open to the
> > converter where it's left unstated in the source spec.
> >
> > courtney: There's a scroll setting on the region in WebVTT that
> > specifies this.
> >
> > nigel: Is there anything else regarding processing model that
> > may affect how we do the conversions?
> > ... So far we have: ISDs, smooth vs discrete scrolling.
> > ... I guess discontinuous markerMode in TTML may be
> > non-mappable too.
> >
> > glenn: I've been thinking about this too - I think it would be
> > modelled by playing back the related media that triggers the
> > discontinuous smpte events and recording the
> > ... elapsed time to make a conversion from discontinuous to
> > continuous.
> > ... There's also the clock based timing which is also
> > interesting! In appendix N we mapped all the timing models to a
> > potentially continuous timeline.
> >
> > nigel: I think we should exclude discontinuous marker mode and
> > maybe clock mode too, as being non-mappable from TTML1 to
> > WebVTT.
> >
> > glenn: I think there may be some TTML2 work that can support
> > this.
> >
> > nigel: I propose to make our mapping explicitly related to
> > TTML1 and if there's anything that helps in TTML2 we can update
> > it later.
> >
> > glenn: Or we can simply reference the ISD creation process.
> >
> > <zcorpan> "If region's text track region scroll setting is 'up'
> > and region already has one child, set region's
> > 'transition-property' to 'top' and 'transition-duration' to
> > '0.433s'." - smooth rollup in webvtt with scroll:up.
> > [21]http://dev.w3.org/html5/webvtt/#h4_processing-model
> >
> > [21] http://dev.w3.org/html5/webvtt/#h4_processing-model
> >
> > nigel: Maybe we can do both, and reference the ISD generation
> > process and make a note that in TTML 1 the process isn't
> > defined in a way that facilitates
> > ... conversion to WebVTT for discontinuous and clock mode
> > times.
> >
> > courtney: If we refer to ISD conversion rather than TTML1
> > what's the reference document?
> >
> > glenn: I'm working on this for TTML2.
> >
> > courtney: Is there a draft document to refer to?
> >
> > andreas: If you make the ISD concept central to the mapping it
> > must be fully elaborated so that everyone can understand it.
> >
> > glenn: I agree but I think there's no way to avoid it other
> > than to create an alternative flavour of the same thing.
> > ... This is the only way to solve the timing hierarchy problem.
> > ... It also gets around the style inheritance process.
> >
> > andreas: Formally I agree but it's hard to communicate the ISD
> > - it wouldn't be a valid TTML document. So the converter
> > wouldn't be from TTML.
> >
> > glenn: We do have examples of ISDs in the TTML1 spec, which is
> > something I'm adding in TTML2.
> >
> > andreas: ISD creation is specified in TTML1 so I think we can
> > use what's there. Is anything else needed?
> >
> > glenn: Yes, the only thing absent is the specification of a
> > serialised form. We only used ISDs as a didactic construct for
> > explaining the formatting model.
> > ... In TTML2 I plan to make interchange of ISDs possible in a
> > standard way.
> > ... It would also be useful for this exercise. Now I have an
> > implementation already those things combine to make this
> > progressable.
> >
> > pal: For mapping can we simply assume that an ISD is a valid
> > TTML document that happens to be static?
> >
> > glenn: almost - it's not quite the same because there's some
> > transformation, e.g. the body element is copied and reparented
> > to the region elements that are temporally active.
> >
> > courtney: My feeling is that this is just trading off one set
> > of problems for another.
> >
> > pal: I was hoping that ISD could just be used to mean 'the
> > state of a TTML document between successive events".
> >
> > Cyril: do we have a presentation on ISDs?
> >
> > glenn: No, though I could do it verbally.
> >
> > andreas: Maybe if it's in the TTV software we could have a look
> > at some simple examples?
> > ... So we don't get stuck here, can we start on attribute
> > mappings that have to be done either way?
> >
> > courtney: I'd prefer to stick with TTML rather than ISDs and
> > defer some of these problems.
> >
> > nigel: +1. Most of the problems are just about timing.
> >
> > glenn: Unfortunately that's not true - there's also the problem
> > that associates content with regions and then performing region
> > style inheritance.
> > ... In the ISD document the content has been associated with
> > individual regions and then region style inheritance, and if
> > you don't go through the ISD process then the latter breaks.
> >
> > nigel: I think you can do the style computation without making
> > the ISD.
> >
> > glenn: There's a risk of duplication of effort.
> >
> > courtney: I think you can map directly.
> >
> > nigel: I want to defer timing issues to ISDs and do everything
> > else directly.
> >
> > glenn: To be clear I didn't mean previously that we need to
> > serialise the ISDs
> >
> > Cyril: We talked earlier about categories - we need to think
> > about metadata etc.
> >
> > pal: I've not heard those use cases.
> >
> > Cyril: Can we assume that metadata-only WebVTT files are out of
> > scope of this?
> >
> > glenn: I guess the issue is searchability - if there are use
> > cases that need searchability e.g. characters, roles, other
> > agents, then we might need to consider that.
> > ... If we're strictly talking about presentation than maybe we
> > don't need to consider that.
> > ... In WebVTT can you use metadata to define larger classes for
> > presentation?
> >
> > courtney: The only thing I've encountered along those lines is
> > voice, which may be one example. The approach I've taken is
> > just to map what is possible to map.
> > ... In the document we can describe what's well defined and
> > note what can't be supported.
> >
> > andreas: I agree - we should publish something sooner and limit
> > certain parts to a canonical representation if there are
> > multiple ways to express the same thing.
> > ... We can decide on a feature by feature basis what to limit,
> > for example.
> >
> > Cyril: we didnt talk about which mapping direction we're
> > talking about.
> >
> > nigel: it's both.
> >
> > andreas: Additionally there are, e.g. in Germany, cases where
> > browsers aren't used to present content, and renderers only
> > understand TTML.
> > ... So we need to go both ways.
> >
> > nigel: Adjourns for lunch - return at 1330 CET.
> >
> > <zcorpan> i will call in 14:00. then 15:00-15:30 i will be
> > absent again
> >
> > <zcorpan> correction. i will call in now but be absent between
> > 14:00-14:30 and 15:00-15:30
> >
> > trackbot, this is ttml
> >
> > <trackbot> Sorry, nigel, I don't understand 'trackbot, this is
> > ttml'. Please refer to
> > <[22]http://www.w3.org/2005/06/tracker/irc> for help.
> >
> > [22] http://www.w3.org/2005/06/tracker/irc%3E
> >
> > trackbot, start meeting
> >
> > <trackbot> Meeting: Timed Text Working Group Teleconference
> >
> > <trackbot> Date: 16 September 2014
> >
> > <scribe> chair: nigel
> >
> > <scribe> scribeNick: nigel
> >
> > Agenda
> >
> > nigel: We may switch things around tomorrow due to changes to
> > flights etc.
> >
> > We will capture output at
> > [23]https://www.w3.org/wiki/TimedText/TTMLtoWebVTT where I
> > enter 'wiki' in the minutes
> >
> > [23] https://www.w3.org/wiki/TimedText/TTMLtoWebVTT
> >
> > Document Structure
> >
> > courtney: Can we go through TTML elements?
> >
> > Cyril: can we map the tt element to the top of a WebVTT
> > document?
> >
> > glenn: explains TTML structure down to style attributes.
> >
> > Cyril: Suggests defining a style class in WebVTT corresponding
> > to each style in TTML
> >
> > glenn: yes, we can do this.
> >
> > courtney: Yes. Right now the CSS document is separate, but in
> > the future it could be embedded.
> >
> > pal: Will there be feedback into WebVTT from this?
> >
> > courtney: There are competing desires here - yes, in principle.
> >
> > Cyril: can we go through these?
> >
> > glenn: Let's keep going with structure.
> > ... Takes group through region properties - including style
> > attributes for origin and extent, and referential approach.
> > ... Each region has an id. If there are no regions defined
> > there's a default, covering 100%.
> >
> > Cyril: How different is this from WebVTT regions?
> >
> > courtney: WebVTT regions can not have styles, but the layout
> > information translates pretty directly.
> >
> > glenn: For example tts:opacity is a region-specific property.
> > backgroundColor can apply to regions independently of the
> > content in the region.
> > ... There are a number of style properties that only apply to
> > regions.
> >
> > andreas: Can a region be compared to a div element in HTML?
> >
> > glenn: yes.
> >
> > andreas: So this is the only element that can be positioned
> > absolutely within the root container.
> >
> > glenn: moves to body
> >
> > Cyril: Will we have an output document structure with headers
> > and bodies, with two subsections - for styling and layout?
> >
> > Courtney: yes.
> >
> > Glenn: That's not a bad way to do it.
> >
> > courtney: Part of this will describe the separate CSS and
> > WebVTT document.
> >
> > glenn: takes us down through body, div, p and span.
> > ... div can contain div; p can not contain p; p can not contain
> > div; div can not contain text.
> >
> > Cyril: so p is equivalent to a cue?
> >
> > courtney: seems that way.
> >
> > glenn: Timing can be specified on body, p, div, p, span and br.
> >
> > Cyril: cues can have nested timing in spans.
> >
> > pal: is there a reason why each p can't map to a cue?
> >
> > glenn: my mental model of a cue is that it is not overlapping
> > in time with other cues. I think this makes things easier.
> >
> > pal: But if we can map a p to a cue then the mapping is
> > simpler.
> >
> > courtney: What else would it map to?
> >
> > glenn: Are you still assuming time has been flattened down and
> > sliced?
> >
> > pal: Yes.
> >
> > glenn: So there are no overlaps. At that point content that is
> > selected into regions is present and everything else has been
> > filtered out.
> > ... every piece of content is associated with a single region
> > in TTML.
> >
> > Cyril: same in WebVTT.
> >
> > glenn: So the concept is to start from body, work down, and
> > associate each piece of content with a region.
> > ... So if there's a region we're not interested in we can
> > filter out that content.
> > ... So there may be multiple <p>s all mapping into a single
> > cue.
> >
> > courtney: With WebVTT you'd define regions, and for each cue
> > reference the region id.
> >
> > glenn: That's exactly how it works in TTML but with the ability
> > to inherit region from an ancestor.
> >
> > Cyril: So you can in principle flatten the TTML structure and
> > remove the <div>s.
> >
> > glenn: You can't remove the <div>s because they specify breaks
> > and style.
> >
> > Cyril: But you could propagate down.
> >
> > nigel: You can paint the background of a div so if you remove
> > it then some information is lost.
> >
> > andreas: is there a layout impact of div?
> >
> > glenn: It implies a breaking boundary in the line progression
> > direction and it may contain styling.
> >
> > group: discusses slicing apart divs into multiple <p>s each of
> > which generates a cue.
> >
> > Cyril: so if I start by resolving all the style references on a
> > p, flattening out all the styles, then...
> >
> > glenn: so you can now enumerate all the <p>s and <div>s and
> > assign each to a cue.
> >
> > courtney: I think we should do that in the document.
> >
> > glenn: Okay but you may end up with a lot of cues all with the
> > same timing. If there's no intrinsic limitation on that then we
> > can go ahead.
> >
> > Cyril: Layout: so div affects layout?
> >
> > glenn: Yes, divs can't (spatially) overlap each other within
> > the same region.
> >
> > andreas: but the only fixed dimension defined is for the
> > region, so the height of each p and div depends on the content
> > flowed into them.
> > ... So there's no difference between the block level boxes that
> > are generated by divs and ps.
> >
> > Cyril: We could create artificial regions for divs that have a
> > background color
> >
> > nigel: we may have some non-mappable functionality here, if a
> > region, a div, and a p all have different background colors.
> >
> > glenn: Also if the div contains a div and both divs contain a
> > p, and all the background colors are different, then you end up
> > with different background paint areas
> >
> > andreas: Can a div create a space that isn't occupied by a p?
> > If a p covers only 50% of the height of the region then its
> > parent div will just have the height of its contained <p>s
> > ... and not expand to the height of the region.
> >
> > glenn: So it will have the same background color as the p
> >
> > courtney: you can't specify an extent on a div or a p?
> >
> > glenn: no that's right.
> >
> > andreas: the width is defined by the region and the height by
> > the flowed in content.
> >
> > Cyril: so you can't have a div with a different background
> > color from its child <p>s?
> >
> > glenn: That's right because we don't have a margin before or
> > after.
> >
> > nigel: I think we've just resolved that <p>s map to cues
> > (repeating Glenn's earlier joke)!
> >
> > glenn: In TTML2 we have padding on content elements not just on
> > region, which might impact this, but thinking about it, it
> > should be okay because it's not margin.
> >
> > courtney: What are content elements?
> >
> > glenn: body, div, p, span, br.
> >
> > Cyril: What if spans have timing that's shorter than their
> > parent p?
> >
> > glenn: If there's an explicit end on the span that makes its
> > active end prior to the active end of its parent then it would
> > depend on the fill mode - it's either freeze or remove.
> > ... I'd have to check what we said about this, from SMIL.
> >
> > andreas: in WebVTT you can have non-ended cues, that last
> > until... when?
> >
> > glenn: In TTML if there's an explicit end on the parent
> > container and the child ends prior to that then there would be
> > two ISDs, one
> > ... covering the first period and the other covering the second
> > period, and the span wouldn't be present in the second period.
> >
> > nigel: +1
> >
> > Cyril: so you can have a span that contains text that activates
> > and deactivates part way through the cue.
> >
> > glenn: Yes, that would be possible in TTML.
> >
> > Cyril: Can we do that in WebVTT?
> >
> > courtney: I don't think so - there's only styling changes part
> > way through a cue.
> > ... So spans with time on them - would we have to separate them
> > into separate cues?
> >
> > Cyril: I don't think that would work because they'd appear on
> > different lines.
> > ... You'd have to go down to the ISD level.
> >
> > nigel: Can you have spans with timing?
> >
> > Cyril: only to switch the text on, not off.
> > ... So not every p is a cue, it's a bit more complicated!
> >
> > glenn: If you split everything into ISDs that do not overlap
> > then these problems can be resolved.
> > ... We need to look more at the details and work out if there's
> > a problem here.
> > ... The only thing we didn't cover is animation. There's a set
> > element in TTML1 that can also delineate ISD boundaries.
> > ... In TTML2 we're adding continuous animation using the
> > animate element
> >
> > In TTML2 ISDs there may be some internal animation within the
> > ISD.
> >
> > andreas: it's also worth noting that every element can have
> > metadata attached.
> >
> > glenn: metadata, except for the ttm:agent attribute which can
> > appear on any content element only, and the region, which
> > reference agent definitions in the header,
> > ... other metadata elements are all local not referential.
> >
> > andreas: TTML also allows child elements that are not in a TTML
> > namespace so it can be extended. A TTML processor is required
> > to prune these out and not reject
> > ... the document. But it doesn't have to display.
> >
> > courtney: Does anyone know if we can have metadata in CSS
> > within a style class?
> >
> > andreas: you can have comments.
> >
> > glenn: they're ignored in the CSS object model.
> >
> > zcorpan: you can have custom properties that can be used for
> > any purpose including metadata.
> >
> > <zcorpan> [24]http://dev.w3.org/csswg/css-variables/
> >
> > [24] http://dev.w3.org/csswg/css-variables/
> >
> > nigel: Can we go through the WebVTT structure and see how that
> > maps?
> >
> > courtney: WebVTT files have a header section that starts with
> > WEBVTT
> >
> > [25]http://dev.w3.org/html5/webvtt/
> >
> > [25] http://dev.w3.org/html5/webvtt/
> >
> > courtney: Then there can be metadata, such as language,
> > copyright etc.
> >
> > Cyril: so when you parse the file, big objects are separated by
> > double line separators.
> > ... Every piece of text separated by two lines is either a cue
> > or is a comment not for display.
> >
> > andreas: but comments are not defined?
> >
> > <zcorpan> [26]http://dev.w3.org/html5/webvtt/#webvtt-comments
> > comments are defined here
> >
> > [26] http://dev.w3.org/html5/webvtt/#webvtt-comments
> >
> > Cyril: no. For example in MP4 carriage you could remove it, or
> > put it in a previous or next segment - it won't be displayed.
> >
> > courtney: In the header section you can also include region
> > definitions.
> >
> > nigel: so you can't have untimed cues?
> >
> > Cyril: yes. Can you in TTML?
> >
> > nigel: yes you can - they have the duration of the whole
> > document (assuming there's no inherited time from a parent time
> > container etc)
> >
> > Cyril: this is in flux in the WebVTT standard, using keywords
> > like 'Next' for 'until the next cue'.
> >
> > glenn: during the conceptual ISD mapping process every piece of
> > content gets timed. Ultimately the active period of the related
> > media object will determine that time,
> > ... in the absence of any other information.
> >
> > andreas: We also have to think about multiple <br> in TTML
> > documents, which are allowed, but shouldn't generate multiple
> > line breaks because they wouldn't
> > ... be displayed in WebVTT.
> >
> > Cyril: so you could define line numbering or put non-breaking
> > spaces on otherwise empty lines. I'm not sure how the
> > backgrounds would be painted for spaces.
> > ... records issue on wiki
> >
> > andreas: You can use empty spans on each line.
> >
> > courtney: Identifiers are used - each cue can have an
> > identifier, which would show up before the begin and end time
> > lines.
> > ... Also regions have ids that can be referenced in cues.
> >
> > Cyril: Those cue ids come from SRT - in SRT each cue has to be
> > a monotonically increasing number with no gaps.
> > ... it's very common to have WebVTT files with numeric
> > identifiers.
> >
> > andreas: and the ids can have spaces in between, which isn't
> > permitted in xml:id
> >
> > courtney: so we should have a convention for mapping to TTML
> > Ids.
> >
> > nigel: Can VTT cue ids be duplicated?
> >
> > courtney: no.
> >
> > nigel: the reason for mentioning it is that if we do TTML ISD
> > -> Cue then the same TTML id may resolve to multiple cues.
> >
> > courtney: there's something to think about here with slicing
> > VTT cues into time slices.
> >
> > Cyrill: As long as all the spans in a p aligns with the end
> > times of the p then you can keep it as a single cue.
> >
> > nigel: that's a special case - think of live word by word
> > subtitles.
> >
> > Cyril: cues have to be laid out in start time order.
> > ... Within a cue you can have internal timing values, that I
> > think also have to be in increasing time order (I'm not sure
> > about that).
> > ... can you have TTML spans that display in reverse time order
> > compared to the document order?
> >
> > glenn: Yes, there are no constraints.
> >
> > Cyril: what about in profiles?
> >
> > pal: I haven't seen any profile that constrains that out.
> >
> > glenn: if the TTML time container is a par (parallel) time
> > container than a child can start after one of its preceding
> > siblings.
> > ... the order in the content will define the order of
> > presentation order (spatially).
> >
> > pal: IMSC 1 allows a document to be labelled progressively
> > decodable which forbids timing on descendants of <p>s.
> >
> > courtney: So that needs to be in the document, i.e. temporal
> > ordering within the document.
> >
> > andreas: EBU-TT-D doesn't constrain this but recommends time
> > ordering. Most legacy formats are sequentially ordered in time
> > as well.
> >
> > Cyril: even if the <p>s were out of order in time that wouldn't
> > be a problem, but out of order <span>s would be a problem.
> >
> > pal: But going to ISD level would avoid that.
> >
> > Cyril: adds this issue to the wiki
> >
> > nigel: Do we have to worry about rtl direction when sorting
> > spans into order in WebVTT?
> >
> > glenn: I would expect that when a span is active all text
> > content of active spans are merged and then directionality is
> > applied on the result.
> >
> > courtney: let's leave the identifier mapping convention until
> > later.
> >
> > nigel: Voice spans are straightforward aren't they?
> >
> > courtney: I think voice maps to agent pretty well.
> >
> > nigel: +1
> > ... What about styling based on voice cue selectors?
> >
> > courtney: You could define a TTML style for each agent.
> > ... Along those lines you can put styling directly on a span -
> > in WebVTT I think you'd have to define CSS classes for those.
> >
> > Cyril: you may not have to scan the whole document but could
> > create a random hash for every time one is encountered.
> > ... I'm also interested in streaming, transcoding live streams.
> >
> > glenn: If it's not been converted into an ISD sequence then you
> > can't avoid parsing the whole document (unless it's
> > progressively decodable).
> > ... You never know if the last markup element will be timed
> > prior to the rest.
> >
> > Cyril: WebVTT documents are always progressively decodable.
> > ... go to example just before section 2 - this has multiple
> > lines in the header. In this case Regions, but it could be
> > copyright, anything else.
> > ... So some parts of the header map to regions and others to
> > metadata.
> > ... continuing on document structure.
> > ... Each cue has a timestamp for start and end, followed by
> > optional settings.
> >
> > Courtney: There are additional settings available.
> >
> > Cyril: they are a combination of styling and layout.
> >
> > nigel: What about at the end of the document?
> >
> > Courtney: there's nothing to mark the ends of documents.
> >
> > Cyril: that's a feature - you can concatenate two WebVTT files,
> > and if the timestamps obey the time rules then it's valid.
> > ... The second header would be ignored.
> >
> > pal: what about styles?
> >
> > andreas: We also need to think about error handling -
> > processing of invalid documents.
> >
> > nigel: Can we simply constrain our mapping to input documents
> > that are valid?
> >
> > Cyril: maybe not - we could consider the WebVTT to TTML mapping
> > to do what a presentation processor would do when given an
> > invalid document
> > ... The behaviour is well defined.
> >
> > nigel: Let's take a break until 1545...
> >
> > <zcorpan> re "nigel: Can VTT cue ids be duplicated?" - yes,
> > there is no requirement about uniqueness for cue identifiers.
> > however region identifiers need to be unique and don't allow
> > spaces
> >
> > <zcorpan> hmm. sorry, looks like cue id requires uniqueness
> > also. i think that changed from a few years ago
> >
> > <zcorpan> looks like the spec allows a cue id to be duplicated
> > as region id
> >
> > Restarting...
> >
> > Layout
> >
> > andreas: We should start with the positioning of a <p> element
> > relative to a region.
> >
> > courtney: The positioning is the piece that will map into
> > WebVTT. There are several region attributes in TTML that can
> > not go in WebVTT.
> >
> > group: discussion of xml:lang on <region> and how it may get
> > inherited by content elements in TTML.
> > ... discussion of style attributes on region - which must be
> > included?
> >
> > courtney: Maybe we should go through each attribute.
> >
> > <tmichel> I just joined Zakim using SIP. It works for me using
> > code ttml#
> >
> > <zcorpan> i still get "this passcode is not valid"
> >
> > glenn: I have a list of style attributes that apply to region.
> > ... there are 12 in TTML1, and of those, 9 apply only to
> > region.
> > ... Styles that apply both to region and other content elements
> > are backgroundColor, display and visibility.
> > ... the ones that apply only to region in TTML1 are
> > displayAlign, extent, opacity, origin, overflow, padding,
> > showBackground, writingMode and zIndex.
> > ... Note that at least one of these will be opened up to
> > content elements in TTML2, which is padding.
> > ... We may also open up opacity to content elements, which
> > would allow the definition of opacity for an element and its
> > content as a collection.
> >
> > andreas: Should we rule out the attributes that will change in
> > TTML2?
> >
> > glenn: In fact opacity and padding are extended to all content
> > elements in TTML2.
> > ... In both cases they aren't being removed from region, so
> > they are still applicable to region in ttml2.
> >
> > courtney: So let's start with those. I believe that only 3 map
> > to a region in WebVTT: displayAlign, extent and origin.
> >
> > andreas: And they can be mapped to properties of the region?
> >
> > nigel: can't you do visibility by setting a style with opacity
> > zero?
> >
> > courtney: you can do that but only on a cue, not on a region.
> >
> > nigel: So another way to say the same thing is that there's no
> > region selector for styling?
> >
> > courtney: Yes.
> >
> > nigel: does the lack of zIndex imply that in WebVTT overlapping
> > regions are prohibited?
> >
> > courtney: I don't think they're prohibited.
> >
> > glenn: In TTML2 on this subject we have a request for
> > expressing z ordering for content to be able to handle 3D.
> >
> > pal: That sounds similar but it's a different concept.
> >
> > Loretta: I'm trying to see if the magic layout algorithm
> > applies to region as well.
> > ... In general there's no notion of zIndex in WebVTT.
> >
> > nigel: Is there an alternative way to achieve backgroundColor
> > on regions in WebVTT?
> >
> > courtney: I don't think so, you can only do it for cues.
> >
> > Cyril: adds non-mappable showBackground on region and zIndex to
> > the wiki.
> >
> > courtney: overflow is always hidden for regions too.
> >
> > glenn: Can wrapping be prevented so that overflow may be
> > relevant?
> > ... Or what happens if you put too much content into a region
> > i.e. too many lines?
> > ... It sounds like extent, origin and displayAlign are
> > currently expressible. The other 9 attributes seem to be
> > absent.
> > ... display seems to be only worthwhile in conjunction with
> > animate.
> >
> > nigel: It seems that the pseudo classes past and future have
> > some relationship to animate.
> >
> > andreas: Wants to note that when we finish on the TTML
> > attributes we should go the other way round.
> >
> > courtney: Let's do the non-style attributes on a region
> > first...
> > ... You can put timing on a region in TTML - there's no
> > equivalent in WebVTT. attributes begin, end, dur, timeContainer
> >
> > glenn: timeContainer is on regions for the processing of
> > animate elements that are children of region.
> >
> > <Loretta> Does the cue-region pseudo-element let us apply CSS
> > styles to regions?
> >
> > nigel: What's the action on that - to add it to the
> > non-mappable list?
> >
> > Cyril: why have timing on regions?
> >
> > glenn: The main reason is to provide timing for background
> > painting when no content is active, and also to specify the
> > timing for animate elements that are children of that region.
> >
> > Cyril: I'm not sure it's not mappable - you can have empty cues
> > applied to a region, with the equivalent times of the TTML
> > region.
> > ... Then that would activate the region in the same way - what
> > happens then is a later question, e.g. background painting.
> >
> > glenn: Actually the timing of a region in TTML can be used to
> > temporally clip the flow of content into that region, so it's a
> > bit more than that.
> > ... The question really is: do implementations use animate?
> >
> > pal: I'm going to check the examples I have.
> > ... another thing is how do you achieve dynamic positioning for
> > text? One way is to create one region per subtitle.
> > ... In that case you may be tempted to put the timing on the
> > region.
> >
> > Loretta: What are you trying to do here?
> >
> > pal: In TTML1 there's no per-cue positioning, e.g. of each <p>.
> > One way to achieve that effect is to define one region per
> > subtitle and position each region
> > ... individually.
> >
> > andreas: From the layout perspective, there's a chance that
> > timings are put on region elements.
> >
> > courtney: Shall we talk about the things that do map?
> > ... On a WebVTT region the available settings are: width,
> > lines, region-anchor, viewport-anchor and scroll.
> > ... I believe that extent in TTML maps to width and lines.
> > ... We have the dimension issues for value units, e.g. if it's
> > in %age then it's okay but in pixels you need the size to do
> > the unit conversion.
> > ... I think that displayAlign and origin in TTML, in
> > combination, map to a combination of regionAnchor and
> > viewportAnchor in WebVTT. The two specs have
> > ... different ways to achieve the same thing. In WebVTT you
> > define a point within the video frame that maps to a point
> > within the region and they don't necessarily
> > ... have to be the same thing. Origin + displayAlign allows you
> > to achieve the same effect.
> >
> > nigel: I thought there was some freedom in WebVTT about the
> > precise positioning, whereas in TTML there's no freedom of
> > movement - is that right?
> >
> > Loretta: I'm still wading through the WebVTT algorithm.
> > Certainly for cues things get moved around to be as close as
> > possible to the stated location.
> >
> > nigel: Yes, I'm not sure if that applies to regions as well as
> > cues.
> >
> > Loretta: Yes, I think it may do - I'm still checking.
> >
> > courtney: I think we should take that offline and research it.
> >
> > andreas: I see a problem with the lines value - this defines
> > the height of the region. A line is defined by the height of
> > the first line of the cue, so a region does not
> > ... always have the same height, as it depends on the first
> > line's size. This is a hard topic to research in general, how
> > this will resolve.
> >
> > nigel: What's a concrete example of that problem?
> >
> > andreas: In general the mapping from TTML to WebVTT may not be
> > possible because for each cue selected into the same region the
> > line height could be different,
> > ... which will result in the region changing height.
> >
> > Loretta: presumably WebVTT would expand the region to
> > accommodate the 5 lines and TTML would clip?
> >
> > glenn: That would depend on lineHeight, fontSize and overflow
> > attributes in TTML.
> > ... Right now we don't have an object-fitting algorithm such as
> > in CSS.
> >
> > Loretta: Is there a way of setting font-relative dimensions?
> >
> > glenn: yes, they can be defined in ems or cells. Ems would be
> > font-relative.
> >
> > andreas: Why is region height important for WebVTT when no
> > background can be drawn?
> >
> > Loretta: the height is important because that determines when
> > scrolling will start.
> >
> > nigel: This seems very similar to the overflow attribute in
> > TTML - if some lines fall out of a region, which ones should an
> > implementation hide?
> >
> > glenn: That's an implementation issue.
> >
> > andreas: Can you explain the difference between the region
> > anchor point and the viewport anchor point?
> >
> > courtney: the region anchor setting defines a point that is
> > fixed in location relative to the region, in case the region
> > has to grow.
> > ... the viewport anchor setting defines where in the video the
> > region must overlap.
> > ... It needs to be understood in relation with the
> > display-align setting.
> >
> > Loretta: right, we need two points. It's like sticking a pin
> > through the region and in the viewport, and any changes to
> > region size keep that point invariant.
> >
> > courtney: the region viewport anchor setting has two points
> > defined, the point within the video and the point within the
> > region.
> > ... Then there's an additional point that is held constant when
> > the region is resized.
> >
> > ack
> >
> > nigel: I think we need to understand the region mapping
> > algorithm from WebVTT - to origin and extent, and if that's a
> > single value or if there are multiple values,
> > ... which in TTML we can do using set elements on the region.
> > ... I think we need a strawman algorithm for this mapping so
> > that we can look at it.
> >
> > andreas: I propose a gist on github for example.
> >
> > courtney: I'll take it as an action item to come up with a
> > strawman proposal.
> >
> > glenn: A moment ago I thought I heard something about origin
> > being in the centre in TTML - was that the question?
> >
> > Courtney: yes, would you do that with displayAlign?
> >
> > glenn: origin is always top left. You can use displayAlign to
> > define where lines are drawn from - in which direction. Right
> > now there's no anchor mechanism in TTML.
> > ... Sean did come up with a change proposal, which I will have
> > to try to dig out.
> >
> > courtney: It's always top left?
> >
> > glenn: yes.
> >
> > nigel: In scope terms, do we need to consider the placement of
> > text within regions, and also the placement of text not in
> > regions?
> >
> > <glenn>
> > [27]https://www.w3.org/wiki/TTML/changeProposal015#region_ancho
> > r_points
> >
> > [27]
> > https://www.w3.org/wiki/TTML/changeProposal015#region_anchor_points
> >
> > glenn: on the prior point, change proposal 15 has a section on
> > this.
> > ... This is proposed for TTML2, but not implemented yet.
> >
> > courtney: In WebVTT cues can have positioning - in TTML1 they
> > don't. So in the mapping to TTML we need to translate to a
> > region.
> >
> > glenn: In TTML2 we are defining inline region definitions, so
> > div and p in TTML2 can take a child region element, including
> > extent and origin.
> >
> > andreas: This is sometimes misused in operation!
> > ... In mapping from WebVTT with no region and snap to lines is
> > active, from the WebVTT spec it looks like margins need to be
> > added top and bottom. Is that correct?
> > ... If the first line is not to be at the bottom and the last
> > line must not be at the bottom, that is.
> > ... We need clarifications of this for accurate mapping.
> > ... will add to the Issues list on the wiki
> >
> > Summary of the day
> >
> > nigel: We've looked at existing work from Andreas and Courtney,
> > thought about the processing models and document structures,
> > ... identified that style attributes should mostly transfer
> > straightforwardly, thought about metadata a bit, and spent a
> > while on layout.
> > ... Tomorrow we have some time set aside for testing, and I
> > suggest we combine the test case generation with the mapping
> > algorithms.
> > ... Thank you everyone, see you tomorrow.
> >
> > adjourns meeting.
> >
> > Summary of Action Items
> >
> > [End of minutes]
> > __________________________________________________________
> >
> >
> > Minutes formatted by David Booth's [28]scribe.perl version
> > 1.138 ([29]CVS log)
> > $Date: 2014-09-16 15:07:16 $
> > __________________________________________________________
> >
> > [28] http://dev.w3.org/cvsweb/~checkout~/2002/scribe/scribedoc.htm
> > [29] http://dev.w3.org/cvsweb/2002/scribe/
> >
> >
>
>
Received on Saturday, 20 September 2014 14:43:26 UTC