- 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>
>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