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RE: TTML Agenda for 15/05/13 - Proposed updates to charter

From: Sean Hayes <Sean.Hayes@microsoft.com>
Date: Fri, 17 May 2013 10:03:48 +0000
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>, Michael Jordan <mijordan@adobe.com>
CC: "public-tt@w3.org" <public-tt@w3.org>
Message-ID: <E9A92BD0A4FC934EB7935470A46D15241F6513F4@DB3EX14MBXC325.europe.corp.microsoft.com>
Hi Silvia, thanks for your observation and I agree with what you say, however I think you are missing the intent here actually. This has nothing to do with the operational model you started, and is in fact an entirely practical mechanism for inter-conversion.

Both formats already have their own internal object model. VTT has the following classes, which is the target of the parse algorithm, although their functionality is not fleshed out much beyond being a bridge to the HTML DocumentFragment
WebVTT Class Object		
WebVTT Italic Object	
WebVTT Bold Object		
WebVTT Underline Object	
WebVTT Ruby Object		
WebVTT Ruby Text Object	
WebVTT Voice Object		
WebVTT Class Object		
WebVTT Text Object		
WebVTT Timestamp Object

TTML has an infoset which contains the following:

Document Information Item
Element Information Item
Attribute Information Item
Character Information Item

Which is a target for the parser and intermediate document form in TTML.

It is my observation that these could easily be unified. Since most of the VTT Objects are essentially an Element Information Item + an Attribute Information Item. The only part that wouldn't actually work today is the reduced infoset doesn't include the processing instruction necessary to convert the timestamp object, but that's a relatively minor fix on the TTML side.

There is code available to do the TTML -> infoset translation for the intermediate document (the algorithm is documented in http://www.w3.org/wiki/TTML/changeProposal005), and I presume the same is true for WebVTT

It is relatively easy to write code to serialise out a WebVTT file from the intermediate TTML infoset (possibly with a side car CSS file); and while the file might not be optimal, it would at least give the same rendering. And, I expect likewise it would be relatively simple to write a TTML file from the WebVTT object set.

But I don't think that pushing the burden of conversion on to the authors is the best way forward here. I hope that once other UA implementers see as we have the amount of commonality here, and that one can reuse much of the same implementation for both TTML and VTT, and further that it is in fact actually simpler to do the TTML rendering as it requires no change to the CSS model; they may indeed feel it less of a burden to add TTML support.

Both specifications are already working on a mapping to HTML5 DocumentFragments and CSS from their version of the object mode.  What I am proposing is really a unification of that effort. Ideally in such a way that does not involve requiring any custom CSS implementation for cues. What I believe would be ideal would be for there to be one documented rendering model for one common object model; at which point implementation of both TTML and WebVTT are simply a matter of plugging in a different parser to the same back end.   This would involve splitting the VTT spec in half, one part for parsing, the other for rendering, and similarly for TTML; and then writing the rendering part up as its own document in a way that suits both purposes. It would also provide a target for other caption formats to use directly too. This may be a bit idealist at this point, but I believe it is certainly doable, and in the best interests of everyone in the community.

As VTT continues to grow and adds the additional features of TTML, such as region support, named metadata and style sets, the model may need to grow slightly, I would prefer that it grow in such a way as to not break this inter conversion.

Hope this clarifies


-----Original Message-----
From: Silvia Pfeiffer [mailto:silviapfeiffer1@gmail.com] 
Sent: 17 May 2013 02:07
To: Michael Jordan
Cc: public-tt@w3.org
Subject: Re: TTML Agenda for 15/05/13 - Proposed updates to charter

I'd like to reply to the idea of having an "common object model" for translating cues between formats.

I think that is an effort that is bound to result in yet another format specification rather than in something that is actually practically useful.

I'd like to put that on the background that I've worked on creating an abstract model for what captions are before. I've analysed typical caption features and put that together on a Web page:
http://www.w3.org/community/texttracks/wiki/Caption_Model .

But you will notice that by doing so, I had to follow existing ideas of how captions are rendered. In particular, I closely followed the
CEA708 model.

The next step would be to specify these ideas in an object model diagram of sorts. But what use would such a object model be?

Think about a sw developer who has a TTML file and wants to translate it to a WebVTT file to be rendered in a Web browser that only supports WebVTT. What does he gain by having a "common object model"? IMHO:
nothing. It will not help him make that translation, because the specifics of TTML and WebVTT are different from the specifics of the object model. Thus, we have just increased the complexity of his task from two to three formats and solved nothing.

In fact, wasn't the idea of TTML exactly that: creating a language in XML that would be able to translate all other caption formats that existed previously and would exist afterwards?

I therefore plead to you: let's not waste any more time on an abstract task such as creating a "common object model" and instead do something that is actually useful to a developer.

Best Regards,

On Fri, May 17, 2013 at 5:53 AM, Michael Jordan <mijordan@adobe.com> wrote:
> Some notes regarding proposed updates to charter :
> http://www.w3.org/2013/05/timed-text-charter.html
> In the mission statement, the phrase "based on implementation 
> experience and interoperability feedback" is too detailed to include 
> in the mission and gets into the "how."
> Under Scope :
> Change wording of "Such formats MUST be useable for online media 
> captioning, described video (aka video/audio description) and
> should address the media accessibility user requirements." to "Such 
> formats MUST support requirements for online media captioning, 
> described video (aka video/audio description) and must address the 
> media accessibility user requirements."
> When does "broadcast production" end? The term is used vaguely here.
> " Publish a Recommendation for a new Timed Text Markup Language (TTML) 
> 1.1
> specification to include the Note described above." What is the note 
> referenced in this sentence?
> "The Group is expected to produce annual updates for the 
> Recommendation with previously specified but now
> newly supported features by browser vendors." This should be a 
> separate deliverable, "Annual recommendation updates..."
> "Establish a semantic mapping between TTML and WebVTT in order to 
> facilitate browser implementation and market adoption." Per our 
> discussion on this morning's call, the idea of a direct semantic 
> mapping between TTML and WebVTT may not be as desirable as a mapping 
> of both semantic structures to a common object model, which can be 
> used to translate cues from either source to HTML5 and CSS.
> Should wrapping up the TTML 1.0 Second Edition come before the new 
> TTML 1.1 specification in the scope list?
> Under Liaisons with W3C Groups. Should we liaise with the group 
> working on Encrypted Media Extensions or is that be covered under our 
> HTML Working Group dependency?
> Should browser vendors be included under External Groups?
> On 5/15/13 1:03 PM, "Sean Hayes" <Sean.Hayes@microsoft.com> wrote:
> our teleconference is scheduled with reference to Boston Time, the 
> correct time of this teleconference in your locale may change. Please 
> check
> http://timeanddate.com/worldclock/fixedtime.html?month=05&day=15&year=
> 2013&hour=10&min=0&sec=0&p1=43
> Thursdays 10:00am-11:00am Boston local
> Zakim Bridge +1.617.761.6200, conference 8865 ("TTML")
> IRC: server: irc.w3.org, port: 6665, channel: #tt Web gateway to 
> :http://www.w3.org/2001/01/cgi-irc
> Chair: Sean Hayes
> Agenda+ Assign Scribe
> Agenda+ Proposed updates to charter :
> http://www.w3.org/2013/05/timed-text-charter.html
> Agenda+ Progress on publication of SE
> Agenda+ HTML5 mapping (now as change proposal [1])
> Agenda+ Approval of 1.1 Change proposals [2] and [3].
> Tracker (Issues and Actions): http://www.w3.org/AudioVideo/TT/tracker
> Profile draft:  http://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10-sdp-us
> Change proposals:
> [1] http://www.w3.org/wiki/TTML/changeProposal005
> [2] http://www.w3.org/wiki/TTML/changeProposal001
> [2] http://www.w3.org/wiki/TTML/changeProposal002
> TTML Wiki
> http://www.w3.org/wiki/TimedText
> Second edition draft:
> https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html?c
> ontent-type=text/html%3bcharset=utf-8
Received on Friday, 17 May 2013 10:04:32 UTC

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