- From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
- Date: Sat, 20 Sep 2014 23:30:50 +1000
- To: Nigel Megitt <nigel.megitt@bbc.co.uk>
- Cc: TTWG <public-tt@w3.org>, "public-texttracks@w3.org" <public-texttracks@w3.org>
Most interesting read. Seems like there is not that much that doesn't
map in either direction.
A few additions / corrections about WebVTT that relate to this
discussion and that were missed (though Simon caught most):
1.) Mapping WebVTT - 608/708
This wasn't mentioned, but I wanted to make sure people are aware that
there is already a spec that provides a basic mapping between WebVTT
and 608 and 708 captions at
https://dvcs.w3.org/hg/text-tracks/raw-file/default/608toVTT/608toVTT.html
.
It does indeed require some CSS.
2.) Repetition of cue settings:
There's a plan to allow for some file metadata to set different
default values for settings, so repetition is not necessary.
3.) CSS works by using ::cue and ::cue-region - the first for cues and
the second for regions.
These selectors work from a Web page (wherever CSS is specified there)
and override default styling of WebVTT cues.
4.) cue overalpping:
It's actually not true that cues cannot overlap in time. They can and
are expected to. In fact, chapter cues are specifically defined as a
particular kind of overlapping cues that are hierarchically
structured, or in your words: are nested.
5.) cue ids:
I think it might be possible to map numeric identifiers to XML
identifiers - at least it's possible to use them like this for CSS:
::cue(#\31) { color: green; }
This references a cue with id=1. That might work for mapping to TTML xml:id too.
6.) visibility and opacity:
are explicitly mentioned as CSS properties usable in ::cue and ::cue-region.
7.) WebVTT does paint-on:
It's even implemented in browsers. You just need to use the timestamps
and right now also <c> elements between them, e.g.
<00:00:01.000><c>From</c><00:00:04.000><c> here</> etc.
I might have missed some more things - if in doubt, just ask.
Cheers,
Silvia.
On Wed, Sep 17, 2014 at 12:08 PM, Nigel Megitt <nigel.megitt@bbc.co.uk> wrote:
> Minutes from today's joint TTWG/TTCG meeting (day 1 of 2) available in HTML
> format at http://www.w3.org/2014/09/16-tt-minutes.html
>
> In text format:
>
> [1]W3C
>
> [1] http://www.w3.org/
>
> - DRAFT -
>
> Timed Text Working Group Teleconference
>
> 16 Sep 2014
>
> See also: [2]IRC log
>
> [2] http://www.w3.org/2014/09/16-tt-irc
>
> Attendees
>
> Present
> elindstrom, tmichel, Frans_EBU, pal, Cyril, courtney,
> andreas, glenn, nigel, Loretta
>
> Regrets
> Chair
> nigel
>
> Scribe
> nigel
>
> Contents
>
> * [3]Topics
> 1. [4]Introductions
> 2. [5]Agenda
> 3. [6]Work done so far
> 4. [7]Logical step through
> 5. [8]Agenda
> 6. [9]Document Structure
> 7. [10]Layout
> 8. [11]Summary of the day
> * [12]Summary of Action Items
> __________________________________________________________
>
> <trackbot> Date: 16 September 2014
>
> [13]https://www.w3.org/wiki/TimedText/geneva2014#Day_1_0900-170
> 0
>
> [13] https://www.w3.org/wiki/TimedText/geneva2014#Day_1_0900-1700
>
> Introductions
>
> <scribe> scribeNick: nigel
>
> Introductions - Nigel, BBC
>
> andreas: IRT
>
> Cyril: Telecom ParisTech university; GPAC
>
> elindstrom: Opera software
>
> zcorpan: Opera software
>
> tmichel: W3C, staff contact for the TTWG
>
> pal: Movielabs
>
> courtney: Apple
>
> glenn: Representing various over the years, currently Cox,
> previously Samsung and Microsoft
>
> frans_EBU: Coordinator of EBU group on subtitling
>
> Agenda
>
> nigel: goes through agenda on wiki page, all happy with that.
> ... We need to think about how we capture our output, and who
> will edit the note.
>
> courtney: I'm happy to edit the note.
> ... I don't have a document yet, I've been working on the code
> first, and have some issues to tackle, and a spreadsheet for
> attributes.
>
> glenn: For browser implementations mapping direct from TTML to
> HTML would be more efficient
> ... If the purpose is for direct display then this mapping
> would be better, but if we want to interchange to WebVTT then
> that translation would still be useful.
>
> courtney: I'm interested in captions both inside and outside
> browser environments so I'm not focused on HTML solely.
>
> andreas: From the mapping we have done we will quite quickly
> see the overlap - maybe there's a cut and paste into HTML as
> glenn mentioned.
>
> pal: Re WebVTT outside browsers?
>
> courtney: Yes, e.g. in an ISO MP4 file that is rendered in a
> video player.
>
> pal: So do we need CSS in practice? To present WebVTT in
> subtitles and captions?
>
> courtney: You certainly can, but it depends on how fancy you
> want to be. You can do basic 608 without CSS.
>
> andreas: you need CSS to do colours, and that's certainly
> required in Europe.
>
> courtney: We define for example a simple mapping from CSS to a
> property list. I think the better approach is to stick with CSS
> and
> ... have a way to embed it in an MP4 file track, and also in a
> WebVTT file.
>
> pal: Will the mapping we do today include that?
>
> courtney: yes
>
> Cyril: +1
>
> courtney: I've been thinking that one TTML file will map to a
> WebVTT file + a CSS file
>
> glenn: That's what I've been thinking, and there's a reusable
> overlap into HTML/CSS
>
> nigel: I've created a wiki page at
> [14]https://www.w3.org/wiki/TimedText/TTMLtoWebVTT
>
> [14] https://www.w3.org/wiki/TimedText/TTMLtoWebVTT
>
> <zcorpan> can you paste the number in irc?
>
> Work done so far
>
> andreas: presents work so far
> ... This work has been supported by the HBB4ALL project, whose
> target is to roll out accessibility to IP connected devices,
> including subtitles, signing and audio (video) description,
> ... with a focus on hybrid broadcast.
> ... This is based on EBU-TT-BasicDe as a very restricted TTML
> feature set.
> ... In fact it's a subset of EBU-TT-D which is a subset of TTML
> plus a couple of small extensions.
> ... It has a video frame with a safe area, 10% in from each
> edge.
> ... Alignment is top or bottom only, vertically.
> ... Horizontally, centred, left or right.
> ... For Germany, it's left to right, top to bottom writing
> direction.
> ... There are 8 different text foreground colours, as from
> WSTeletext.
> ... All subtitles have the same background color, font-family,
> font-size and line height.
> ... Line breaking is done manually with the <br> element at
> authoring.
>
> glenn: How is the background padding extended on either side of
> the text?
>
> andreas: That's just in the example image, it's not actually
> present.
> ... How is this mapping achieved? Positioning, Styling, Timing.
> ... Positioning:
> ... [shows video frame with image of Verona]
> ... In TTML and EBU-TT there's a root container. In EBU-TT it's
> always the height and width of the video. WebVTT uses the
> viewport concept,
> ... which I understand to be the height and width of the video
> also.
> ... For the safe area, we define the tt:region, with top-left
> being 10% 10% in x y as specified by the origin.
> ... The CSS property is topleft
> ... The extent is 80% 80%, which in CSS is the width and height
> of the block level element eg the div
> ... To place a subtitle the region is defined once in the head
> and then referenced by the tt:p element. This is similar to a p
> in html.
> ... The paragraph gets the width of the region, and the height
> is calculated by the number of lines inside the p element.
> ... Vertical alignment is displayAlign: bottom or top.
>
> nigel: Will there be CSS mappings for all of these in this
> presentation?
>
> andreas: This is setting out the features to map, we should
> consider them in scope for our mapping later.
> ... I didn't use the advanced concepts in WebVTT of cue
> alignment, so I didn't use them. I wanted something that would
> certainly work in current browsers.
> ... In WebVTT I've put the cues in for the text. For a width of
> 80% the cue box has size: 80%
> ... The height is defined by the number of lines, just like the
> p element.
> ... This is per cue, so the settings seem to need to be
> repeated every time. I don't know a way to define it once and
> have it carried through.
>
> courtney: If you use a region you can do that.
>
> andreas: I didn't use a region.
>
> courtney: Then you have to repeat it.
>
> andreas: So that's positioning. We can define the position of
> the box from the left of the video frame, with 10%, using
> position:10% align:left
> ... The align setting is important. It works very differently
> than in TTML e.g. if you set align:middle and position:10% then
> the reference point for the middle isn't the cue
> ... start but is the middle of the cue.
> ... So to centre the text then you have position:50%
> align:middle
> ... For vertical alignment it's a bit trickier. To come 10% up
> from the bottom you can set line:90% or a line number value.
> ... But this doesn't align the end of the cue box, but aligns
> the top of the cue box. So that doesn't work.
> ... What you actually need is position:100% - margin - height
> of cue-box.
> ... That works if you have a lot of control over the font
> height and can calculate the position this way.
> ... In most cases that's a bit risky. So then I changed to the
> other possibility, to use line alignment
> ... The first line in the cue generates the line grid, then you
> can position the cue box with positive line numbers from the
> top
> ... or negative line numbers starting from -1 from the bottom.
> ... [example shows text one line up from bottom]
> ... You have to have the snap to lines flag set - this happens
> automatically if you use line numbers.
> ... For one line you can have line:-2, or for a two line
> subtitle, line:-3. Needs a bit of calculation.
> ... A dirty trick possibly is always to set it to -1 and let
> the renderer push it up. Possibly this is not recommended but
> it may work.
> ... Styling:
> ... In EBU-TT-BasicDE there's a default style defined once in
> the head, and a div element that references the defaultStyle.
> ... In WebVTT you can define a general cue selector ::cue and
> use almost the same property names and values.
> ... For font-size some calculation is needed. 60% font size in
> TTML comes out at 5.33% of the height of the video, which is
> 100% in CSS.
> ... A separate CSS file is needed to contain the ::cue
> selector.
> ... For inline styles in TTML we set the colour attributes on a
> style referenced from a span.
> ... In CSS you can use the pseudo-selector ::cue(c.textWhite) {
> color: #ffffff; background-color:rgba(0,0,0,0.7); }
> ... Then in the VTT c.textWhite cue class
> ... Timing:
> ... In TTML put a begin and end on, with media timeBase,
> reference sync is zero. In EBU-TT-BasicDE the fractional
> seconds are limited to 3 digits.
> ... This is the same for WebVTT cues.
>
> pal: What are the rules for CSS styles when combined with
> locally set rules? Which takes precedence between author and
> user choices?
>
> courtney: We would consider user choices to override author
> styles.
>
> pal: If you're displaying it on a web page, then web styles
> taking over seems like not the right thing to do.
>
> andreas: It's not clear to me how the CSS that applies to the
> web page interacts with the VTT cues. From testing there's no
> relationship.
> ... The video is a separate viewport with independent styling,
> from my testing anyway.
>
> Cyril: I think that's not expected. I remember that the cues
> are sourced in the HTML page so the styles should be applied.
>
> andreas: I tried it out in Opera.
>
> zcorpan: The styling was implemented in presto - I'll put
> together a quick demo and paste the link
>
> andreas: One important point is that we put the background
> color just behind the text not the box. From what I read
> there's no possibility
> ... in WebVTT to put the background only on block level
> elements, e.g. the whole region/p/div etc.
> ... It only puts the background behind each glyph. I think
> there's a WebVTT background box concept but it doesn't seem to
> apply to the block level.
>
> glenn: So TTML allows the background to be specified on the
> containing block and possibly differently on the span or the p
> within the larger block.
> ... So this example (showing two spans each with its own
> background color) wouldn't be possible?
>
> andreas: That's right. In Europe both possibilities are in use.
> ... We need to be aware of this restriction in the mapping.
>
> <zcorpan>
> [15]http://w3c-test.org/webvtt/rendering/cues-with-video/proces
> sing-model/basic.html has styling
>
> [15]
> http://w3c-test.org/webvtt/rendering/cues-with-video/processing-model/basic.html
>
> zcorpan: This shows how a stylesheet applies to WebVTT cues -
> the stylesheet is in the HTML page and the cues use those
> styles
> ... There's a white video behind it.
>
> pause for 4 minutes, back at 10:33 (CET)
>
> <zcorpan> wrt to the positioning discussion, there are open
> bugs on the webvtt spec for both changing how positioning works
> and for adding something that allows for exact positioning.
> [16]https://www.w3.org/Bugs/Public/buglist.cgi?quicksearch=webv
> tt%20positioning&list_id=43983
>
> [16]
> https://www.w3.org/Bugs/Public/buglist.cgi?quicksearch=webvtt%20positioning&list_id=43983
>
> <zcorpan>
> [17]https://www.w3.org/Bugs/Public/show_bug.cgi?id=25632
>
> [17] https://www.w3.org/Bugs/Public/show_bug.cgi?id=25632
>
> nigel: we're reassembling...
>
> courtney: Here's what I've discovered from writing mapping
> code.
> ... There's an issue that we don't have an official WebVTT spec
> yet - we're working off drafts that aren't versioned.
> ... When Andreas was talking he was using browser supported
> features. This is causing a bit of an issue. The mapping I've
> been doing is off the most
> ... current WebVTT spec version.
> [18]http://dev.w3.org/html5/webvtt/
> ... Here are 3 categories of issue:
> ... 1. TTMl is more hierarchical than WebVTT
> ... 2. The two specs define different properties implicitly vs
> explicitly.
>
> [18] http://dev.w3.org/html5/webvtt/
>
> 3. The basic problem of converting units (value type
> conversions)
>
> scribe: Hierarchical vs Flat:
> ... WebVTT has a flat structure with no nested elements. TTML
> provides a hierarchical structure.
> ... Metadata: in TTML you can nest metadata hierarchically
> [shows ttm:agent holmes and Dr Watson]. In WebVTT you get a
> list with no relationships between them.
> ... Proposal for WebVTT is hierarchical metadata keys
>
> nigel: Is that just metadata or presentation issues too?
>
> courtney: It may be less of an issue for presentation issues
> but there are cases where we run into a similar problem.
> ... Another example: Calculating relative timings
> hierarchically in TTML and linearly in WebVTT.
>
> Cyril: I think some profiles restrict that.
>
> andreas: Yes, EBU-TT-D doesn't allow nested timing.
>
> Cyril: That raises the question which profile are we looking
> at?
>
> Courtney: Yes, we can simplify the problem by specifying a
> profile.
>
> glenn: It's useful, though it may take longer, to start from
> the general case and identify where in the absence of a profile
> there are issues.
> ... For example re timing and even styles we could define a
> mapping based on the sequence of Intermediate Synchronic
> Documents, to remove the timing issues.
> ... Just documenting these issues is useful.
>
> nigel: We decided last week to use TTML1SE and WebVTT.
>
> andreas: for styling there's some hierarchical structure in
> WebVTT too, by application of class nodes that are nested.
>
> courtney: Yes you can have nested styles within a cue but if
> you want the same style for 10 cues you can't put them in a
> fragment and declare it at the fragment level.
> ... Implicit vs Explicit:
> ... Some functionality is explicitly described by attributes or
> parameters in one spec but implicitly derived in the other.
> ... For example, horizontal writing direction. In TTML there's
> a way to specify horizontal direction but in WebVTT there isn't
> (unless it's vertical) - it's inferred from the font.
>
> glenn: tts:direction is designed to work in relation to the
> Unicode bidi control characters
> ... absent of those you can still infer directionality based on
> the content of the element, though it's harder with mixed
> content.
> ... So the direction attribute in TTML doesn't really say
> 'write right to left' but does specify the default writing
> direction in the absence of bidi.
>
> courtney: WebVTT has bidi too, and rtl and ltr entities.
>
> andreas: In Unicode the information is already there.
>
> glenn: You have to look at the history of Unicode - people
> didn't want to use nestable control codes so they wanted CSS
> attributes to do the same thing.
>
> <zcorpan>
> [19]http://dev.w3.org/html5/webvtt/#h4_processing-model says
> how to determine direction
>
> [19] http://dev.w3.org/html5/webvtt/#h4_processing-model
>
> zcorpan: The horizontal direction is taken from the text in the
> cue, not from the font (in WebVTT)
> ... You can override it with unicode bidi characters if you
> want.
>
> nigel: Seems like there's no issue to log in our issues list.
>
> <zcorpan> "Apply the Unicode Bidirectional Algorithm's
> Paragraph Level steps to the concatenation of the values of
> each WebVTT Text Object in nodes, in a pre-order, depth-first
> traversal, excluding WebVTT Ruby Text Objects and their
> descendants, to determine the paragraph embedding level of the
> first Unicode paragraph of the cue. [BIDI]"
>
> glenn: TTML has the CSS features as well as the plain text.
>
> courtney: Example 2: line breaks - need to be explicit in TTML
> but can be just new lines in WebVTT.
>
> Cyril: That's due to the parser - XML requires this.
>
> andreas: Later on we can look at xml:space attributes. From the
> tests I've seen with xml:space="preserve" then line breaks
> should be preserved.
>
> <zcorpan> XML doesn't require it really
>
> glenn: In XSL-FO there are 4 different properties. We define an
> explicit mapping of xml:space to sets of those values, in TTML.
> We didn't expose the full XSL-FO model.
>
> courtney: Value Type Conversions
>
> <glenn> tnx 4 reminder
>
> courtney: Example 1 - times
> ... TTML has different time expressions, WebVTT always has
> hh:mm:ss.sss with fractional seconds.
> ... Fortunately the ttp: namespace defines all the required
> metadata to do the conversions.
> ... Though I'm not sure that's the case with lengths and
> position values
> ... Again TTML allows a broader set of units - pixels, em,
> cells, %ages
> ... I'm assuming lineHeight is sort of like em. For some TTML
> documents I think you need the authored video dimensions to do
> the mapping.
>
> pal: I think if you use %age or c you don't need the video
> dimensions. If you're going to use pixels then implementations
> should use tts:extent on the root as well.
>
> glenn: By specifying extent on the root you can derive a pixel
> dimension - this doesn't tell you the pixel relationship to the
> video though.
>
> andreas: An issue is that in general the root container pixel
> dimensions are not necessarily coincident with the video
> dimensions.
> ... The document has no way to specify this in TTML, in
> general.
>
> pal: CFF-TT and EBU-TT-D relate the root container to the
> video. IMSC introduces an aspect ratio. All the profiles
> specify how the mapping goes.
>
> andreas: For general TTML documents this is an issue.
>
> courtney: Attribute mappings
> ... Some are straightforward.
> ... Though WebVTT IDs can be purely numeric, and xml:id doesn't
> allow that. So some modification or convention may be needed,
> e.g. "cue"+number.
> ... We could define the best practice.
> ... Both use BCP47 language values
> ... Preserve space needs further discussion.
> ... Styling attributes: colors, fonts etc are fairly
> straightforward.
>
> pal: Is there a subset of CSS that's supported for WebVTT?
>
> <zcorpan> [20]http://dev.w3.org/html5/webvtt/#css-extensions is
> the subset
>
> [20] http://dev.w3.org/html5/webvtt/#css-extensions
>
> andreas: In WebVTT there's a subset of properties that are
> permitted. E.g. padding is not allowed.
>
> courtney: One requirement set is what's needed for CEA608. It
> would be useful to have a standard set of CSS classes that can
> be used for any CEA608 translations into WebVTT.
> ... There are some properties with no WebVTT equivalent:
> display, overflow, padding, showBackground.
> ... For alignment, displayAlign maps to the latest version of
> the WebVTT spec.
>
> andreas: I tried it out, and it would work perfectly.
>
> courtney: But they're not widely supported yet. The mapping is
> nicer at least.
>
> <zcorpan> "the properties corresponding to the 'background'
> shorthand" is allowed, if that is what showBackground does
>
> zcorpan: any other properties will be ignored than those listed
> in the spec.
> ... I'm not sure how the TTML features map to those but there
> is a defined subset in the spec.
>
> courtney: To expand on that, things like textDecoration in TTML
> you can have underline set on a cue, but for the rest of it
> you'd have to go to CSS to do?
>
> <zcorpan> <u>
>
> zcorpan: For underline you can use CSS or the <u> element
> inside a cue.
>
> courtney: visibility and zIndex - I can't see how to do those
> in WebVTT.
> ... extent can be done with a cue box size or a region size.
> ... A lot of the timing in the ttp: namespace metadata doesn't
> map to the WebVTT because the timing that's allowed is a lot
> simpler.
>
> zcorpan: visibility and zIndex is not possible in WebVTT.
>
> nigel: can't you do visibility with opacity?
>
> zcorpan: yes you can do visibility.
>
> courtney: there are also the attributes "use", "value" and
> "type".
>
> glenn: Those are in the profile definition mechanism - they're
> not content or style based.
>
> Cyril: does this mean they don't have to be mapped?
>
> courtney: since there are no profiles in WebVTT I guess not.
>
> glenn: This is all part of the TTML way to specify what a
> processor needs to support, based on SMIL and SVG originally.
> ... I think it can probably be ignored but needs more thought.
>
> andreas: If we do not find a direct mapping between WebVTT and
> TTML that doesn't mean that we can rule it out for the mapping
> ... because there's some intent in the source document and we
> have to check if theres something that needs to be done.
>
> courtney: Ruby: there's no simple mapping from WebVTT to TTML
> for ruby.
>
> glenn: In TTML1 you have to do the work at authoring time and
> use regions to place the ruby in the right place.
> ... I've recently specified in TTML2 the ruby markup.
>
> Cyril: There may be several ways to define the same thing, so
> we should try to use a canonical representation as the mapping
> source.
> ... For example there are several ways of expressing timing -
> maybe a requirement before mapping is a single syntax. I'm not
> sure if this is possible.
>
> courtney: it may be an interesting way to break the problem up.
>
> Cyril: A problem I've seen before is that when attributes need
> to be resolved at runtime based on context, e.g. frame rate,
> video size etc there's not much that can be done.
> ... We maybe need to classify those attributes that can be
> mapped offline vs those that need full context to resolve.
>
> courtney: that's my presentation.
>
> Cyril: There's also the question of which TTML profile to use.
> But also there are different classes of WebVTT: valid or not?
> parsable or not?
> ... Invalid documents may be presented okay by browsers. We
> should say which class we're looking at.
> ... Then WebVTT can represent metadata, chapters, subtitles,
> captions etc. so we should indicate which ones we're mapping,
> if not all.
>
> Logical step through
>
> nigel: Processing model
>
> Cyril: how does TTML handle overlapping times?
>
> glenn: there's arbitrary overlap permitted.
> ... The first step I'd advocate is to create the intermediate
> synchronic documents and map to WebVTT.
>
> Cyril: In WebVTT there's the concept of cues becoming active
> and then bumping up existing visible cues.
>
> some discussion of how this is handled in TTML
>
> andreas: Formally the concept of creating the ISDs makes a lot
> of sense - we need to make sure everyone understands what that
> means.
>
> glenn: I agree. For example one thing that may not be obvious
> is that style inheritance is only defined on ISDs so one has to
> perform the ISD creation prior to style inheritance.
> ... I've also added a function on the TTV tool to generate the
> set of ISDs.
>
> nigel: We have a choice here to map ISDs or specific bits of
> cue text.
> ... This impacts efficiency and metadata.
>
> pal: This depends on the use case - if we just have the goal of
> getting equivalent presentation then efficiency and metadata
> are secondary concerns.
>
> elindstrom: from a browser perspective we're interested in
> accurate presentation.
>
> courtney: I've been thinking about it the opposite way - from a
> TTML to WebVTT conversion preserving semantics.
>
> andreas: Would it be possible to take Courtney's attribute list
> and make it a structured document, take it as a header, explain
> the problem scenario,
> ... and indicate what the options and recommendations are from
> the WG?
> ... If you try to map abstractly the logical model then it's
> very hard. Something more concrete may be a better start.
>
> pal: This is a question of how complicated we want to make it -
> I haven't heard of anyone wanting to use WebVTT as a
> master/archive/mezzanine format.
>
> glenn: There's a use case for distribution though.
>
> pal: I can see the use case of converting the TTML experience
> into a WebVTT experience.
>
> glenn: Part of this may be timing oriented in the sense that
> user agents may potentially add TTML renderers directly, which
> would reduce the future needs.
> ... But there may still be WebVTT-only presentation devices.
>
> pal: The issue for me is about the non-presentation-based usage
> of WebVTT.
>
> elindstrom: I don't expect that to be a huge use case.
>
> nigel: Seems like we've been considering TTML -> WebVTT here.
> Does the same consideration apply the other way?
>
> courtney: WebVTT does roll-up - I'm not sure how we do that
> with TTML.
>
> glenn: we may need to consider using the set element in TTML1.
>
> pal: When you say roll-up you mean where there's an animation
> displayed?
>
> glenn: yes, gradually moving up.
>
> pal: To do that explicitly in TTML you need animation, but what
> is possible is to have a region that contains line A at t=0 and
> at t=1 line B is added, moving line A up.
> ... This doesn't require any animation.
>
> glenn: Yes correct but it doesn't do the whole 608 animation.
>
> pal: Then the question is do we need to explicitly define the
> roll-up animation.
>
> glenn: Yes, we put in a note that implementation might do that.
>
> courtney: What about paint-on?
>
> glenn: That's no problem.
> ... Does WebVTT support smooth roll-up as opposed to discrete
> line based roll-up?
>
> courtney: I think it does yes, I'll have to confirm.
>
> nigel: As a general point here we can leave it open to the
> converter where it's left unstated in the source spec.
>
> courtney: There's a scroll setting on the region in WebVTT that
> specifies this.
>
> nigel: Is there anything else regarding processing model that
> may affect how we do the conversions?
> ... So far we have: ISDs, smooth vs discrete scrolling.
> ... I guess discontinuous markerMode in TTML may be
> non-mappable too.
>
> glenn: I've been thinking about this too - I think it would be
> modelled by playing back the related media that triggers the
> discontinuous smpte events and recording the
> ... elapsed time to make a conversion from discontinuous to
> continuous.
> ... There's also the clock based timing which is also
> interesting! In appendix N we mapped all the timing models to a
> potentially continuous timeline.
>
> nigel: I think we should exclude discontinuous marker mode and
> maybe clock mode too, as being non-mappable from TTML1 to
> WebVTT.
>
> glenn: I think there may be some TTML2 work that can support
> this.
>
> nigel: I propose to make our mapping explicitly related to
> TTML1 and if there's anything that helps in TTML2 we can update
> it later.
>
> glenn: Or we can simply reference the ISD creation process.
>
> <zcorpan> "If region's text track region scroll setting is 'up'
> and region already has one child, set region's
> 'transition-property' to 'top' and 'transition-duration' to
> '0.433s'." - smooth rollup in webvtt with scroll:up.
> [21]http://dev.w3.org/html5/webvtt/#h4_processing-model
>
> [21] http://dev.w3.org/html5/webvtt/#h4_processing-model
>
> nigel: Maybe we can do both, and reference the ISD generation
> process and make a note that in TTML 1 the process isn't
> defined in a way that facilitates
> ... conversion to WebVTT for discontinuous and clock mode
> times.
>
> courtney: If we refer to ISD conversion rather than TTML1
> what's the reference document?
>
> glenn: I'm working on this for TTML2.
>
> courtney: Is there a draft document to refer to?
>
> andreas: If you make the ISD concept central to the mapping it
> must be fully elaborated so that everyone can understand it.
>
> glenn: I agree but I think there's no way to avoid it other
> than to create an alternative flavour of the same thing.
> ... This is the only way to solve the timing hierarchy problem.
> ... It also gets around the style inheritance process.
>
> andreas: Formally I agree but it's hard to communicate the ISD
> - it wouldn't be a valid TTML document. So the converter
> wouldn't be from TTML.
>
> glenn: We do have examples of ISDs in the TTML1 spec, which is
> something I'm adding in TTML2.
>
> andreas: ISD creation is specified in TTML1 so I think we can
> use what's there. Is anything else needed?
>
> glenn: Yes, the only thing absent is the specification of a
> serialised form. We only used ISDs as a didactic construct for
> explaining the formatting model.
> ... In TTML2 I plan to make interchange of ISDs possible in a
> standard way.
> ... It would also be useful for this exercise. Now I have an
> implementation already those things combine to make this
> progressable.
>
> pal: For mapping can we simply assume that an ISD is a valid
> TTML document that happens to be static?
>
> glenn: almost - it's not quite the same because there's some
> transformation, e.g. the body element is copied and reparented
> to the region elements that are temporally active.
>
> courtney: My feeling is that this is just trading off one set
> of problems for another.
>
> pal: I was hoping that ISD could just be used to mean 'the
> state of a TTML document between successive events".
>
> Cyril: do we have a presentation on ISDs?
>
> glenn: No, though I could do it verbally.
>
> andreas: Maybe if it's in the TTV software we could have a look
> at some simple examples?
> ... So we don't get stuck here, can we start on attribute
> mappings that have to be done either way?
>
> courtney: I'd prefer to stick with TTML rather than ISDs and
> defer some of these problems.
>
> nigel: +1. Most of the problems are just about timing.
>
> glenn: Unfortunately that's not true - there's also the problem
> that associates content with regions and then performing region
> style inheritance.
> ... In the ISD document the content has been associated with
> individual regions and then region style inheritance, and if
> you don't go through the ISD process then the latter breaks.
>
> nigel: I think you can do the style computation without making
> the ISD.
>
> glenn: There's a risk of duplication of effort.
>
> courtney: I think you can map directly.
>
> nigel: I want to defer timing issues to ISDs and do everything
> else directly.
>
> glenn: To be clear I didn't mean previously that we need to
> serialise the ISDs
>
> Cyril: We talked earlier about categories - we need to think
> about metadata etc.
>
> pal: I've not heard those use cases.
>
> Cyril: Can we assume that metadata-only WebVTT files are out of
> scope of this?
>
> glenn: I guess the issue is searchability - if there are use
> cases that need searchability e.g. characters, roles, other
> agents, then we might need to consider that.
> ... If we're strictly talking about presentation than maybe we
> don't need to consider that.
> ... In WebVTT can you use metadata to define larger classes for
> presentation?
>
> courtney: The only thing I've encountered along those lines is
> voice, which may be one example. The approach I've taken is
> just to map what is possible to map.
> ... In the document we can describe what's well defined and
> note what can't be supported.
>
> andreas: I agree - we should publish something sooner and limit
> certain parts to a canonical representation if there are
> multiple ways to express the same thing.
> ... We can decide on a feature by feature basis what to limit,
> for example.
>
> Cyril: we didnt talk about which mapping direction we're
> talking about.
>
> nigel: it's both.
>
> andreas: Additionally there are, e.g. in Germany, cases where
> browsers aren't used to present content, and renderers only
> understand TTML.
> ... So we need to go both ways.
>
> nigel: Adjourns for lunch - return at 1330 CET.
>
> <zcorpan> i will call in 14:00. then 15:00-15:30 i will be
> absent again
>
> <zcorpan> correction. i will call in now but be absent between
> 14:00-14:30 and 15:00-15:30
>
> trackbot, this is ttml
>
> <trackbot> Sorry, nigel, I don't understand 'trackbot, this is
> ttml'. Please refer to
> <[22]http://www.w3.org/2005/06/tracker/irc> for help.
>
> [22] http://www.w3.org/2005/06/tracker/irc%3E
>
> trackbot, start meeting
>
> <trackbot> Meeting: Timed Text Working Group Teleconference
>
> <trackbot> Date: 16 September 2014
>
> <scribe> chair: nigel
>
> <scribe> scribeNick: nigel
>
> Agenda
>
> nigel: We may switch things around tomorrow due to changes to
> flights etc.
>
> We will capture output at
> [23]https://www.w3.org/wiki/TimedText/TTMLtoWebVTT where I
> enter 'wiki' in the minutes
>
> [23] https://www.w3.org/wiki/TimedText/TTMLtoWebVTT
>
> Document Structure
>
> courtney: Can we go through TTML elements?
>
> Cyril: can we map the tt element to the top of a WebVTT
> document?
>
> glenn: explains TTML structure down to style attributes.
>
> Cyril: Suggests defining a style class in WebVTT corresponding
> to each style in TTML
>
> glenn: yes, we can do this.
>
> courtney: Yes. Right now the CSS document is separate, but in
> the future it could be embedded.
>
> pal: Will there be feedback into WebVTT from this?
>
> courtney: There are competing desires here - yes, in principle.
>
> Cyril: can we go through these?
>
> glenn: Let's keep going with structure.
> ... Takes group through region properties - including style
> attributes for origin and extent, and referential approach.
> ... Each region has an id. If there are no regions defined
> there's a default, covering 100%.
>
> Cyril: How different is this from WebVTT regions?
>
> courtney: WebVTT regions can not have styles, but the layout
> information translates pretty directly.
>
> glenn: For example tts:opacity is a region-specific property.
> backgroundColor can apply to regions independently of the
> content in the region.
> ... There are a number of style properties that only apply to
> regions.
>
> andreas: Can a region be compared to a div element in HTML?
>
> glenn: yes.
>
> andreas: So this is the only element that can be positioned
> absolutely within the root container.
>
> glenn: moves to body
>
> Cyril: Will we have an output document structure with headers
> and bodies, with two subsections - for styling and layout?
>
> Courtney: yes.
>
> Glenn: That's not a bad way to do it.
>
> courtney: Part of this will describe the separate CSS and
> WebVTT document.
>
> glenn: takes us down through body, div, p and span.
> ... div can contain div; p can not contain p; p can not contain
> div; div can not contain text.
>
> Cyril: so p is equivalent to a cue?
>
> courtney: seems that way.
>
> glenn: Timing can be specified on body, p, div, p, span and br.
>
> Cyril: cues can have nested timing in spans.
>
> pal: is there a reason why each p can't map to a cue?
>
> glenn: my mental model of a cue is that it is not overlapping
> in time with other cues. I think this makes things easier.
>
> pal: But if we can map a p to a cue then the mapping is
> simpler.
>
> courtney: What else would it map to?
>
> glenn: Are you still assuming time has been flattened down and
> sliced?
>
> pal: Yes.
>
> glenn: So there are no overlaps. At that point content that is
> selected into regions is present and everything else has been
> filtered out.
> ... every piece of content is associated with a single region
> in TTML.
>
> Cyril: same in WebVTT.
>
> glenn: So the concept is to start from body, work down, and
> associate each piece of content with a region.
> ... So if there's a region we're not interested in we can
> filter out that content.
> ... So there may be multiple <p>s all mapping into a single
> cue.
>
> courtney: With WebVTT you'd define regions, and for each cue
> reference the region id.
>
> glenn: That's exactly how it works in TTML but with the ability
> to inherit region from an ancestor.
>
> Cyril: So you can in principle flatten the TTML structure and
> remove the <div>s.
>
> glenn: You can't remove the <div>s because they specify breaks
> and style.
>
> Cyril: But you could propagate down.
>
> nigel: You can paint the background of a div so if you remove
> it then some information is lost.
>
> andreas: is there a layout impact of div?
>
> glenn: It implies a breaking boundary in the line progression
> direction and it may contain styling.
>
> group: discusses slicing apart divs into multiple <p>s each of
> which generates a cue.
>
> Cyril: so if I start by resolving all the style references on a
> p, flattening out all the styles, then...
>
> glenn: so you can now enumerate all the <p>s and <div>s and
> assign each to a cue.
>
> courtney: I think we should do that in the document.
>
> glenn: Okay but you may end up with a lot of cues all with the
> same timing. If there's no intrinsic limitation on that then we
> can go ahead.
>
> Cyril: Layout: so div affects layout?
>
> glenn: Yes, divs can't (spatially) overlap each other within
> the same region.
>
> andreas: but the only fixed dimension defined is for the
> region, so the height of each p and div depends on the content
> flowed into them.
> ... So there's no difference between the block level boxes that
> are generated by divs and ps.
>
> Cyril: We could create artificial regions for divs that have a
> background color
>
> nigel: we may have some non-mappable functionality here, if a
> region, a div, and a p all have different background colors.
>
> glenn: Also if the div contains a div and both divs contain a
> p, and all the background colors are different, then you end up
> with different background paint areas
>
> andreas: Can a div create a space that isn't occupied by a p?
> If a p covers only 50% of the height of the region then its
> parent div will just have the height of its contained <p>s
> ... and not expand to the height of the region.
>
> glenn: So it will have the same background color as the p
>
> courtney: you can't specify an extent on a div or a p?
>
> glenn: no that's right.
>
> andreas: the width is defined by the region and the height by
> the flowed in content.
>
> Cyril: so you can't have a div with a different background
> color from its child <p>s?
>
> glenn: That's right because we don't have a margin before or
> after.
>
> nigel: I think we've just resolved that <p>s map to cues
> (repeating Glenn's earlier joke)!
>
> glenn: In TTML2 we have padding on content elements not just on
> region, which might impact this, but thinking about it, it
> should be okay because it's not margin.
>
> courtney: What are content elements?
>
> glenn: body, div, p, span, br.
>
> Cyril: What if spans have timing that's shorter than their
> parent p?
>
> glenn: If there's an explicit end on the span that makes its
> active end prior to the active end of its parent then it would
> depend on the fill mode - it's either freeze or remove.
> ... I'd have to check what we said about this, from SMIL.
>
> andreas: in WebVTT you can have non-ended cues, that last
> until... when?
>
> glenn: In TTML if there's an explicit end on the parent
> container and the child ends prior to that then there would be
> two ISDs, one
> ... covering the first period and the other covering the second
> period, and the span wouldn't be present in the second period.
>
> nigel: +1
>
> Cyril: so you can have a span that contains text that activates
> and deactivates part way through the cue.
>
> glenn: Yes, that would be possible in TTML.
>
> Cyril: Can we do that in WebVTT?
>
> courtney: I don't think so - there's only styling changes part
> way through a cue.
> ... So spans with time on them - would we have to separate them
> into separate cues?
>
> Cyril: I don't think that would work because they'd appear on
> different lines.
> ... You'd have to go down to the ISD level.
>
> nigel: Can you have spans with timing?
>
> Cyril: only to switch the text on, not off.
> ... So not every p is a cue, it's a bit more complicated!
>
> glenn: If you split everything into ISDs that do not overlap
> then these problems can be resolved.
> ... We need to look more at the details and work out if there's
> a problem here.
> ... The only thing we didn't cover is animation. There's a set
> element in TTML1 that can also delineate ISD boundaries.
> ... In TTML2 we're adding continuous animation using the
> animate element
>
> In TTML2 ISDs there may be some internal animation within the
> ISD.
>
> andreas: it's also worth noting that every element can have
> metadata attached.
>
> glenn: metadata, except for the ttm:agent attribute which can
> appear on any content element only, and the region, which
> reference agent definitions in the header,
> ... other metadata elements are all local not referential.
>
> andreas: TTML also allows child elements that are not in a TTML
> namespace so it can be extended. A TTML processor is required
> to prune these out and not reject
> ... the document. But it doesn't have to display.
>
> courtney: Does anyone know if we can have metadata in CSS
> within a style class?
>
> andreas: you can have comments.
>
> glenn: they're ignored in the CSS object model.
>
> zcorpan: you can have custom properties that can be used for
> any purpose including metadata.
>
> <zcorpan> [24]http://dev.w3.org/csswg/css-variables/
>
> [24] http://dev.w3.org/csswg/css-variables/
>
> nigel: Can we go through the WebVTT structure and see how that
> maps?
>
> courtney: WebVTT files have a header section that starts with
> WEBVTT
>
> [25]http://dev.w3.org/html5/webvtt/
>
> [25] http://dev.w3.org/html5/webvtt/
>
> courtney: Then there can be metadata, such as language,
> copyright etc.
>
> Cyril: so when you parse the file, big objects are separated by
> double line separators.
> ... Every piece of text separated by two lines is either a cue
> or is a comment not for display.
>
> andreas: but comments are not defined?
>
> <zcorpan> [26]http://dev.w3.org/html5/webvtt/#webvtt-comments
> comments are defined here
>
> [26] http://dev.w3.org/html5/webvtt/#webvtt-comments
>
> Cyril: no. For example in MP4 carriage you could remove it, or
> put it in a previous or next segment - it won't be displayed.
>
> courtney: In the header section you can also include region
> definitions.
>
> nigel: so you can't have untimed cues?
>
> Cyril: yes. Can you in TTML?
>
> nigel: yes you can - they have the duration of the whole
> document (assuming there's no inherited time from a parent time
> container etc)
>
> Cyril: this is in flux in the WebVTT standard, using keywords
> like 'Next' for 'until the next cue'.
>
> glenn: during the conceptual ISD mapping process every piece of
> content gets timed. Ultimately the active period of the related
> media object will determine that time,
> ... in the absence of any other information.
>
> andreas: We also have to think about multiple <br> in TTML
> documents, which are allowed, but shouldn't generate multiple
> line breaks because they wouldn't
> ... be displayed in WebVTT.
>
> Cyril: so you could define line numbering or put non-breaking
> spaces on otherwise empty lines. I'm not sure how the
> backgrounds would be painted for spaces.
> ... records issue on wiki
>
> andreas: You can use empty spans on each line.
>
> courtney: Identifiers are used - each cue can have an
> identifier, which would show up before the begin and end time
> lines.
> ... Also regions have ids that can be referenced in cues.
>
> Cyril: Those cue ids come from SRT - in SRT each cue has to be
> a monotonically increasing number with no gaps.
> ... it's very common to have WebVTT files with numeric
> identifiers.
>
> andreas: and the ids can have spaces in between, which isn't
> permitted in xml:id
>
> courtney: so we should have a convention for mapping to TTML
> Ids.
>
> nigel: Can VTT cue ids be duplicated?
>
> courtney: no.
>
> nigel: the reason for mentioning it is that if we do TTML ISD
> -> Cue then the same TTML id may resolve to multiple cues.
>
> courtney: there's something to think about here with slicing
> VTT cues into time slices.
>
> Cyrill: As long as all the spans in a p aligns with the end
> times of the p then you can keep it as a single cue.
>
> nigel: that's a special case - think of live word by word
> subtitles.
>
> Cyril: cues have to be laid out in start time order.
> ... Within a cue you can have internal timing values, that I
> think also have to be in increasing time order (I'm not sure
> about that).
> ... can you have TTML spans that display in reverse time order
> compared to the document order?
>
> glenn: Yes, there are no constraints.
>
> Cyril: what about in profiles?
>
> pal: I haven't seen any profile that constrains that out.
>
> glenn: if the TTML time container is a par (parallel) time
> container than a child can start after one of its preceding
> siblings.
> ... the order in the content will define the order of
> presentation order (spatially).
>
> pal: IMSC 1 allows a document to be labelled progressively
> decodable which forbids timing on descendants of <p>s.
>
> courtney: So that needs to be in the document, i.e. temporal
> ordering within the document.
>
> andreas: EBU-TT-D doesn't constrain this but recommends time
> ordering. Most legacy formats are sequentially ordered in time
> as well.
>
> Cyril: even if the <p>s were out of order in time that wouldn't
> be a problem, but out of order <span>s would be a problem.
>
> pal: But going to ISD level would avoid that.
>
> Cyril: adds this issue to the wiki
>
> nigel: Do we have to worry about rtl direction when sorting
> spans into order in WebVTT?
>
> glenn: I would expect that when a span is active all text
> content of active spans are merged and then directionality is
> applied on the result.
>
> courtney: let's leave the identifier mapping convention until
> later.
>
> nigel: Voice spans are straightforward aren't they?
>
> courtney: I think voice maps to agent pretty well.
>
> nigel: +1
> ... What about styling based on voice cue selectors?
>
> courtney: You could define a TTML style for each agent.
> ... Along those lines you can put styling directly on a span -
> in WebVTT I think you'd have to define CSS classes for those.
>
> Cyril: you may not have to scan the whole document but could
> create a random hash for every time one is encountered.
> ... I'm also interested in streaming, transcoding live streams.
>
> glenn: If it's not been converted into an ISD sequence then you
> can't avoid parsing the whole document (unless it's
> progressively decodable).
> ... You never know if the last markup element will be timed
> prior to the rest.
>
> Cyril: WebVTT documents are always progressively decodable.
> ... go to example just before section 2 - this has multiple
> lines in the header. In this case Regions, but it could be
> copyright, anything else.
> ... So some parts of the header map to regions and others to
> metadata.
> ... continuing on document structure.
> ... Each cue has a timestamp for start and end, followed by
> optional settings.
>
> Courtney: There are additional settings available.
>
> Cyril: they are a combination of styling and layout.
>
> nigel: What about at the end of the document?
>
> Courtney: there's nothing to mark the ends of documents.
>
> Cyril: that's a feature - you can concatenate two WebVTT files,
> and if the timestamps obey the time rules then it's valid.
> ... The second header would be ignored.
>
> pal: what about styles?
>
> andreas: We also need to think about error handling -
> processing of invalid documents.
>
> nigel: Can we simply constrain our mapping to input documents
> that are valid?
>
> Cyril: maybe not - we could consider the WebVTT to TTML mapping
> to do what a presentation processor would do when given an
> invalid document
> ... The behaviour is well defined.
>
> nigel: Let's take a break until 1545...
>
> <zcorpan> re "nigel: Can VTT cue ids be duplicated?" - yes,
> there is no requirement about uniqueness for cue identifiers.
> however region identifiers need to be unique and don't allow
> spaces
>
> <zcorpan> hmm. sorry, looks like cue id requires uniqueness
> also. i think that changed from a few years ago
>
> <zcorpan> looks like the spec allows a cue id to be duplicated
> as region id
>
> Restarting...
>
> Layout
>
> andreas: We should start with the positioning of a <p> element
> relative to a region.
>
> courtney: The positioning is the piece that will map into
> WebVTT. There are several region attributes in TTML that can
> not go in WebVTT.
>
> group: discussion of xml:lang on <region> and how it may get
> inherited by content elements in TTML.
> ... discussion of style attributes on region - which must be
> included?
>
> courtney: Maybe we should go through each attribute.
>
> <tmichel> I just joined Zakim using SIP. It works for me using
> code ttml#
>
> <zcorpan> i still get "this passcode is not valid"
>
> glenn: I have a list of style attributes that apply to region.
> ... there are 12 in TTML1, and of those, 9 apply only to
> region.
> ... Styles that apply both to region and other content elements
> are backgroundColor, display and visibility.
> ... the ones that apply only to region in TTML1 are
> displayAlign, extent, opacity, origin, overflow, padding,
> showBackground, writingMode and zIndex.
> ... Note that at least one of these will be opened up to
> content elements in TTML2, which is padding.
> ... We may also open up opacity to content elements, which
> would allow the definition of opacity for an element and its
> content as a collection.
>
> andreas: Should we rule out the attributes that will change in
> TTML2?
>
> glenn: In fact opacity and padding are extended to all content
> elements in TTML2.
> ... In both cases they aren't being removed from region, so
> they are still applicable to region in ttml2.
>
> courtney: So let's start with those. I believe that only 3 map
> to a region in WebVTT: displayAlign, extent and origin.
>
> andreas: And they can be mapped to properties of the region?
>
> nigel: can't you do visibility by setting a style with opacity
> zero?
>
> courtney: you can do that but only on a cue, not on a region.
>
> nigel: So another way to say the same thing is that there's no
> region selector for styling?
>
> courtney: Yes.
>
> nigel: does the lack of zIndex imply that in WebVTT overlapping
> regions are prohibited?
>
> courtney: I don't think they're prohibited.
>
> glenn: In TTML2 on this subject we have a request for
> expressing z ordering for content to be able to handle 3D.
>
> pal: That sounds similar but it's a different concept.
>
> Loretta: I'm trying to see if the magic layout algorithm
> applies to region as well.
> ... In general there's no notion of zIndex in WebVTT.
>
> nigel: Is there an alternative way to achieve backgroundColor
> on regions in WebVTT?
>
> courtney: I don't think so, you can only do it for cues.
>
> Cyril: adds non-mappable showBackground on region and zIndex to
> the wiki.
>
> courtney: overflow is always hidden for regions too.
>
> glenn: Can wrapping be prevented so that overflow may be
> relevant?
> ... Or what happens if you put too much content into a region
> i.e. too many lines?
> ... It sounds like extent, origin and displayAlign are
> currently expressible. The other 9 attributes seem to be
> absent.
> ... display seems to be only worthwhile in conjunction with
> animate.
>
> nigel: It seems that the pseudo classes past and future have
> some relationship to animate.
>
> andreas: Wants to note that when we finish on the TTML
> attributes we should go the other way round.
>
> courtney: Let's do the non-style attributes on a region
> first...
> ... You can put timing on a region in TTML - there's no
> equivalent in WebVTT. attributes begin, end, dur, timeContainer
>
> glenn: timeContainer is on regions for the processing of
> animate elements that are children of region.
>
> <Loretta> Does the cue-region pseudo-element let us apply CSS
> styles to regions?
>
> nigel: What's the action on that - to add it to the
> non-mappable list?
>
> Cyril: why have timing on regions?
>
> glenn: The main reason is to provide timing for background
> painting when no content is active, and also to specify the
> timing for animate elements that are children of that region.
>
> Cyril: I'm not sure it's not mappable - you can have empty cues
> applied to a region, with the equivalent times of the TTML
> region.
> ... Then that would activate the region in the same way - what
> happens then is a later question, e.g. background painting.
>
> glenn: Actually the timing of a region in TTML can be used to
> temporally clip the flow of content into that region, so it's a
> bit more than that.
> ... The question really is: do implementations use animate?
>
> pal: I'm going to check the examples I have.
> ... another thing is how do you achieve dynamic positioning for
> text? One way is to create one region per subtitle.
> ... In that case you may be tempted to put the timing on the
> region.
>
> Loretta: What are you trying to do here?
>
> pal: In TTML1 there's no per-cue positioning, e.g. of each <p>.
> One way to achieve that effect is to define one region per
> subtitle and position each region
> ... individually.
>
> andreas: From the layout perspective, there's a chance that
> timings are put on region elements.
>
> courtney: Shall we talk about the things that do map?
> ... On a WebVTT region the available settings are: width,
> lines, region-anchor, viewport-anchor and scroll.
> ... I believe that extent in TTML maps to width and lines.
> ... We have the dimension issues for value units, e.g. if it's
> in %age then it's okay but in pixels you need the size to do
> the unit conversion.
> ... I think that displayAlign and origin in TTML, in
> combination, map to a combination of regionAnchor and
> viewportAnchor in WebVTT. The two specs have
> ... different ways to achieve the same thing. In WebVTT you
> define a point within the video frame that maps to a point
> within the region and they don't necessarily
> ... have to be the same thing. Origin + displayAlign allows you
> to achieve the same effect.
>
> nigel: I thought there was some freedom in WebVTT about the
> precise positioning, whereas in TTML there's no freedom of
> movement - is that right?
>
> Loretta: I'm still wading through the WebVTT algorithm.
> Certainly for cues things get moved around to be as close as
> possible to the stated location.
>
> nigel: Yes, I'm not sure if that applies to regions as well as
> cues.
>
> Loretta: Yes, I think it may do - I'm still checking.
>
> courtney: I think we should take that offline and research it.
>
> andreas: I see a problem with the lines value - this defines
> the height of the region. A line is defined by the height of
> the first line of the cue, so a region does not
> ... always have the same height, as it depends on the first
> line's size. This is a hard topic to research in general, how
> this will resolve.
>
> nigel: What's a concrete example of that problem?
>
> andreas: In general the mapping from TTML to WebVTT may not be
> possible because for each cue selected into the same region the
> line height could be different,
> ... which will result in the region changing height.
>
> Loretta: presumably WebVTT would expand the region to
> accommodate the 5 lines and TTML would clip?
>
> glenn: That would depend on lineHeight, fontSize and overflow
> attributes in TTML.
> ... Right now we don't have an object-fitting algorithm such as
> in CSS.
>
> Loretta: Is there a way of setting font-relative dimensions?
>
> glenn: yes, they can be defined in ems or cells. Ems would be
> font-relative.
>
> andreas: Why is region height important for WebVTT when no
> background can be drawn?
>
> Loretta: the height is important because that determines when
> scrolling will start.
>
> nigel: This seems very similar to the overflow attribute in
> TTML - if some lines fall out of a region, which ones should an
> implementation hide?
>
> glenn: That's an implementation issue.
>
> andreas: Can you explain the difference between the region
> anchor point and the viewport anchor point?
>
> courtney: the region anchor setting defines a point that is
> fixed in location relative to the region, in case the region
> has to grow.
> ... the viewport anchor setting defines where in the video the
> region must overlap.
> ... It needs to be understood in relation with the
> display-align setting.
>
> Loretta: right, we need two points. It's like sticking a pin
> through the region and in the viewport, and any changes to
> region size keep that point invariant.
>
> courtney: the region viewport anchor setting has two points
> defined, the point within the video and the point within the
> region.
> ... Then there's an additional point that is held constant when
> the region is resized.
>
> ack
>
> nigel: I think we need to understand the region mapping
> algorithm from WebVTT - to origin and extent, and if that's a
> single value or if there are multiple values,
> ... which in TTML we can do using set elements on the region.
> ... I think we need a strawman algorithm for this mapping so
> that we can look at it.
>
> andreas: I propose a gist on github for example.
>
> courtney: I'll take it as an action item to come up with a
> strawman proposal.
>
> glenn: A moment ago I thought I heard something about origin
> being in the centre in TTML - was that the question?
>
> Courtney: yes, would you do that with displayAlign?
>
> glenn: origin is always top left. You can use displayAlign to
> define where lines are drawn from - in which direction. Right
> now there's no anchor mechanism in TTML.
> ... Sean did come up with a change proposal, which I will have
> to try to dig out.
>
> courtney: It's always top left?
>
> glenn: yes.
>
> nigel: In scope terms, do we need to consider the placement of
> text within regions, and also the placement of text not in
> regions?
>
> <glenn>
> [27]https://www.w3.org/wiki/TTML/changeProposal015#region_ancho
> r_points
>
> [27]
> https://www.w3.org/wiki/TTML/changeProposal015#region_anchor_points
>
> glenn: on the prior point, change proposal 15 has a section on
> this.
> ... This is proposed for TTML2, but not implemented yet.
>
> courtney: In WebVTT cues can have positioning - in TTML1 they
> don't. So in the mapping to TTML we need to translate to a
> region.
>
> glenn: In TTML2 we are defining inline region definitions, so
> div and p in TTML2 can take a child region element, including
> extent and origin.
>
> andreas: This is sometimes misused in operation!
> ... In mapping from WebVTT with no region and snap to lines is
> active, from the WebVTT spec it looks like margins need to be
> added top and bottom. Is that correct?
> ... If the first line is not to be at the bottom and the last
> line must not be at the bottom, that is.
> ... We need clarifications of this for accurate mapping.
> ... will add to the Issues list on the wiki
>
> Summary of the day
>
> nigel: We've looked at existing work from Andreas and Courtney,
> thought about the processing models and document structures,
> ... identified that style attributes should mostly transfer
> straightforwardly, thought about metadata a bit, and spent a
> while on layout.
> ... Tomorrow we have some time set aside for testing, and I
> suggest we combine the test case generation with the mapping
> algorithms.
> ... Thank you everyone, see you tomorrow.
>
> adjourns meeting.
>
> Summary of Action Items
>
> [End of minutes]
> __________________________________________________________
>
>
> Minutes formatted by David Booth's [28]scribe.perl version
> 1.138 ([29]CVS log)
> $Date: 2014-09-16 15:07:16 $
> __________________________________________________________
>
> [28] http://dev.w3.org/cvsweb/~checkout~/2002/scribe/scribedoc.htm
> [29] http://dev.w3.org/cvsweb/2002/scribe/
>
>
Received on Saturday, 20 September 2014 13:31:41 UTC