W3C home > Mailing lists > Public > public-tt@w3.org > September 2014

Re: {minutes} Geneva F2F day 1 16/09/2014

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Sat, 20 Sep 2014 23:30:50 +1000
Message-ID: <CAHp8n2=J09udw=kVssp_5aAtvn483o=X-NnjkSPEEfGQrZH71g@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: TTWG <public-tt@w3.org>, "public-texttracks@w3.org" <public-texttracks@w3.org>
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
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.

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.


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
>    [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 13:31:39 UTC

This archive was generated by hypermail 2.3.1 : Thursday, 5 October 2017 18:24:17 UTC