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Re: Liaison response - template on MIME type parameter for TimedText

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Tue, 6 May 2014 09:55:14 +0000
To: Michael Dolan <mdolan@newtbt.com>, "public-tt@w3.org" <public-tt@w3.org>
Message-ID: <CF8E6AEF.1D22D%nigel.megitt@bbc.co.uk>
There's a more substantive issue IMHO. Namespace and schemaLocation do not
adequately identify a particular variant of TTML, nor is it clear how they
should be derived. There are two issues, at least:

1. If a profile uses a mutable namespace (which I would normally expect)
then different versions of the same profile will have the same namespace
and schemaLocation and thus not be distinguishable.
2. It is unclear whether all of the namespaces used in a document must be
listed or just those that are relevant for downstream processing. If a
processor sees a namespace that was not listed what should it's behaviour
be? I do not believe that we are trying to create something here that
should have an impact during processing of the document; rather the
parameter can be used either to select an appropriate document or to
decide whether or not to download and process a document or ignore it.
Similarly it should not be a requirement for the MP4 packager to parse the
document to work out all of the namespaces that are actually used.

Returning to the current alternative proposal, I have a question about the
stpp.TTML.[short name1]+[short name 2]+[etc] idea: what is the order of
precedence if we use the + symbol?

I can imagine stpp.TTML.DFXP+IMSC being interpreted either as "this
document is a DFXP TTML document and an IMSC TTML document" or "this
document is a DFXP TTML document and some other unregistered thing called
IMSC". Are we free to say that the + operator has higher precedence than
the . "operator" to ensure that the former interpretation holds?

This isn't as obvious as it looks at first: in TTML §4 [1] we say abstract
document types can be validated by pruning all content in non-TT
namespaces. Consider an alternate format XX that has a similar rule that
prunes all non-XX namespace content. Someone constructs a document that
contains both TT namespace elements and XX namespace elements at the top
level. The document now serves dual purposes (ignoring for the moment
whether or not this is a good idea), and could potentially be described
either as "stpp.TTML.tt1f+stpp.XX" or as "stpp.TTML.tt1f+XX" - which of
these is correct depends on how we and MPEG collectively define the syntax
in the codecs parameter.

[1] http://www.w3.org/TR/ttml1/#profiles

In the current proposal there is no way to signal conformance to two
mutually independent XML formats that can legally be combined in the same

(in case you're wondering, no, I don't actually want to do this but I'm
just observing the effect of the undefined parts of the current proposal)


On 05/05/2014 14:58, "Michael Dolan" <mdolan@newtbt.com> wrote:

>No one (I don't think) is opposed to addressing the issue.  And, I think
>has accepted the ownership of its parts of it. The questions are about the
>technical details.
>There already is a registered media type - "application/ttml-xml".  So,
>of my questions was essentially why not build on that?  Dave seems to
>building on the MP4 4C code and I'm still studying the two approaches.
>As you know, the namespaces and schemaLocations are already signaling at
>MP4 layer. They can be extracted into a codecs string, but there are
>apparently objections to the resulting length of the string.  I thought
>was the only real issue being addressed, no?
>	Mike
>-----Original Message-----
>From: Cyril Concolato [mailto:cyril.concolato@telecom-paristech.fr]
>Sent: Monday, May 05, 2014 4:43 AM
>To: public-tt@w3.org
>Subject: Re: Liaison response - template on MIME type parameter for
>Hi all,
>Some points worth highlighting/repeating:
>- Aside from the MP4 problem, the problem is general. Consider a DASH MPD
>pointing to a TTML file (not packaged in a MP4). How can a player know
>it'll be able to play it meaningfully without downloading the entire TTML
>file? Does it actually need to know or will it always be able to do
>something with a TTML file? The solution to this problem will provide the
>basis for the solution to the MP4 problem. Has the TTML WG considered
>defining a 'codecs' or 'profile' MIME parameter for TTML?
>- the codecs parameter of MP4 files is useful for identifying the content
>each track in the file before the file is fully downloaded.
>In Adaptive Streaming contexts such as DASH, the initialization segment
>also be downloaded in a first step to get information. So the codecs
>parameter does not have to provide all information. It could just indicate
>that the track contains some flavor of TTML and let the initialization
>segment provide more details. In that case, the solution could be as
>as: codecs="stpp.ttml"
>- If more than TTML is needed, remember that a typical workflow for
>packaging/dashing is: get one (or more) TTML file, package it into an
>(existing) MP4 file, produce an MPD from that MP4 file. Only in the last
>step is the 'codecs' parameter generated. In MSE cases, you don't even
>the MPD but you need to get (most likely in JS) the codecs parameter to
>create the source buffer. Ideally, the TTML 'codecs' string generator
>not have to look at the content of the TTML document, only at the track
>sample entry (similar to AVC codecs generation).
>03/05/2014 20:01, David Singer a écrit :
>> On May 3, 2014, at 9:06 , Glenn Adams <glenn@skynav.com> wrote:
>>> On Fri, May 2, 2014 at 12:23 PM, David Singer <singer@apple.com> wrote:
>>> On Apr 30, 2014, at 16:40 , Michael Dolan <mdolan@newtbt.com> wrote:
>>>> Nigel, Dave and all-
>>>> Is there a TTWG Proposal?
>>>> Would ³stpp² be registered somewhere where it would be unambiguous
>other Codecs strings unrelated to TTML?  I don¹t mean the sample entry 4C
>code, but in the Codecs string namespace.  Wouldn¹t it have to be
>³application/mp4+stppŠ.²?  Or why not use ³application/ttml+xml² (and
>similar for WebVTT)?
>>> The codecs string starts with the 4CC of the sample entry.  After that,
>whoever defined that 4CC gets to say what¹s the next element.  And they
>to say what¹s after that, and so on.
>>> stpp says ³some sort of XML²,
>>> since VTT isn't XML, then would it use something different from stpp?
>> yes, we have different 4CCs for text-based and XML-based formats.
>> stpp is for XML-based
>>> and is owned by MPEG.  So indeed the step that goes from Œstpp¹ to
>sort of TTML¹ is owned by MPEG, and MPEG still needs to resolve this.
>>> The thrust is that IF MPEG solves that, THEN there will be names to
>>> identify TTML dialects, that could go next.  So you would see
>>> codecs=stpp.<some MPEG magic to say it is TTML
>>> I would expect something more simple, e.g.
>>> stpp.vtt
>>> stpp.ttml.tt1f
>>> stpp.ttml.tt1p
>>> stpp.ttml.tt1t
>>> stpp.ttml.sdpu
>>> stpp.ttml.st10
>>> stpp.ttml.st13
>> indeed, MPEG is likely to say that TTML means ³go to the W3C, thou
>sluggard, and be wise²
>>> where we have a registry for mapping the third IDs above to TTML
>designators, e.g.
>>> tt1f -> http://www.w3.org/ns/ttml/dfxp-full
>>> tt1p -> http://www.w3.org/ns/ttml/dfxp-presentation
>>> tt1t -> http://www.w3.org/ns/ttml/dfxp-transformation
>>> tt1u -> http://www.w3.org/ns/ttml/sdp-us s10f ->
>>> http://www.smpte-ra.org/schemas/2052-1/2010/profiles/smpte-tt-full
>>> s13f -> 
>>> http://www.smpte-ra.org/schemas/2052-1/2013/profiles/smpte-tt-full
>>> we definitely do not want to create a new syntax/language for use in
>>> codecs that describes some way to combine profiles; that function is
>>> already defined by the ttml profile definition document syntax
>> we¹re not, but we have to indicate somehow that a document is compatible
>with more than one profile.  we also are not restricted here to a
>4-character-name, so we can use slightly longer names if they are
>> we can¹t split into several entries as you suggest;  there is one entry
>per track, those separated by commas.
>> codecs=stpp.ttml.tt1f,stpp.ttml.tt1p
>> means two tracks, whereas
>> codecs=stpp.ttml.tt1f+tt1p
>> means one track compatible with two TTML profiles.  Big difference. (I
>> am not wedded to plus, but comma and period are both taken already)
>>> for example.
>>> For the TTWG to say ³yes, we¹ll take on dialect naming and forming that
>second-level parameter² is important; it then means that if MPEG finds a
>clean solution to the first level, the actual problem in hand is solved.
>I¹d like the MP4 people to realize before the July meeting that this is
>urgent, and come up with ideas and maybe online discussion ASAP.
>>> This is all provisional ‹ on the TTWG getting agreement not only
>internally, but with the partners; and on us all liking the final result,
>>> Makes sense?
>>>> I understand how one could signal profiles of TTML that a document
>conformed to concurrently, as in the example ­ all of TTML and EBU-TT.
>the signaling requirements go beyond that ­ there is often multiple
>namespaces in use in one document that are not, as an aggregate, a single
>³profile². So, these must be explicitly signaled as well since nearly all
>profiles permit foreign namespaces.  To accommodate this, the ³short
>have to be defined as ³profiles of namespaces² I think.
>>>> For example, if a document uses the CFF-TT text profile of the TTML
>profile, plus SMPTE-TT #608 (US captions), plus CFF-TT metadata, and it
>compatible with IMSC, SDP-US and EBU-TT, then it might look like:
>>>> Regards,
>>>>                  Mike
>>>> p.s. I would give the SC29 Secretary a hint about the target of the
>liaison (MPEG v JPEG).  And, you understand you will not receive a reply
>until mid-to-late July, right?
>>>> From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
>>>> Sent: Wednesday, April 30, 2014 11:48 AM
>>>> To: watanabe@itscj.ipsj.or.jp
>>>> Cc: Timed Text Working Group
>>>> Subject: Liaison response - template on MIME type parameter for
>>>> TimedText
>>>> Dear Mr. Watanabe,
>>>> Thank you for your liaison N14444 of April 2014.
>>>> We think that we can indeed find a solution together.  We are
>>>> looking into creating a table of formal "short names" for the
>>>> profiles of W3C TTML and the profiles of formats derived from it (such
>as SMPTE-TT, EBU-TT, and so on).  If MPEG were to propose how to step from
>the four-character-code of the sample entry (XMLSubtitleSampleEntry and
>XMLMetaDataSampleEntry) to something that identifies "a document
>with one or more profiles of TTML", then we could propose a string
>of a set of one or more these short names as the next parameter.
>>>> For example, say W3C defines two profile short names "W3CTTML" and
>>>> "EBUTTML", and MPEG defines the name "TTML" as referring to the
>>>> overall family, one might see
>>>> codecs=stpp.TTML.W3CTTML+EBUTTML,avc1
>>>> as a codecs string of a file carrying AVC (H.264) and TTML subtitles
>>>> that are additionally EBU-TT conformant.
>>>> We would check with those deriving from TTML (e.g. at SMPTE, EBU and
>DECE) if this approach and design are acceptable, before we formalise
>>>> Kind regards,
>>>> Nigel Megitt, David Singer (chairs, Timed Text Working Group, W3C)
>>>> --
>>>> Nigel Megitt
>>>> Lead Technologist, BBC Technology, Distribution & Archives
>>>> Telephone: +44 (0)3030807996
>>>> Internal (Lync): 0807996
>>>> BC4 A3 Broadcast Centre, Media Village, 201 Wood Lane, London W12
>>>> 7TP
>>>> ----------------------------
>>>> http://www.bbc.co.uk
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>>>> ---------------------
>>> David Singer
>>> Manager, Software Standards, Apple Inc.
>> David Singer
>> Manager, Software Standards, Apple Inc.
>Cyril Concolato
>Multimedia Group / Telecom ParisTech

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Received on Tuesday, 6 May 2014 09:55:47 UTC

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