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

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Mon, 22 Sep 2014 15:04:13 +0000
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
CC: TTWG <public-tt@w3.org>, "public-texttracks@w3.org" <public-texttracks@w3.org>
Message-ID: <D045F6C9.119EE%nigel.megitt@bbc.co.uk>

>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.

Is there a selector that can be used to distinguish cues within different
WebVTT resources referenced from the same HTML, so that it is possible to
style them differently?

> 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.

Just to be clear, I don't think anyone asserted that WebVTT cues cannot
overlap in time; however we didn't really explore that case very
thoroughly. And there was a strategy proposed to simplify the conversion
process from TTML by explicitly creating non-overlapping entities that
could be converted into WebVTT cues in a way that would guarantee correct
output but possibly at the expense of document size and repetition in some
cases. That strategy helped in some complex cases.



>6.) visibility and opacity:
>are explicitly mentioned as CSS properties usable in ::cue and
>::cue-region.

Thanks we didn't find that.


>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.

Thanks we didn't find that.

Cheers,


Nigel


>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/bas

>>ic.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%20positioni

>>ng&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 Monday, 22 September 2014 15:04:48 UTC

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