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

From: Michael Dolan <mdolan@newtbt.com>
Date: Mon, 5 May 2014 06:58:32 -0700
To: <public-tt@w3.org>
Message-ID: <032101cf686a$1b8225d0$52867170$@newtbt.com>

No one (I don't think) is opposed to addressing the issue.  And, I think W3C
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,  one
of my questions was essentially why not build on that?  Dave seems to prefer
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 the
MP4 layer. They can be extracted into a codecs string, but there are
apparently objections to the resulting length of the string.  I thought that
was the only real issue being addressed, no?



-----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 that
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 of
each track in the file before the file is fully downloaded. 
In Adaptive Streaming contexts such as DASH, the initialization segment can
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 simple
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 need
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 should
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 “stpp” be registered somewhere where it would be unambiguous from
other Codecs strings unrelated to TTML?  I don’t mean the sample entry 4C
code, but in the Codecs string namespace.  Wouldn’t it have to be
“application/mp4+stpp….”?  Or why not use “application/ttml+xml” (and
similar for WebVTT)?
>> The codecs string starts with the 4CC of the sample entry.  After that,
whoever defined that 4CC gets to say what’s the next element.  And they get
to say what’s after that, and so on.
>> stpp says “some sort of XML”,
>> 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 ‘stpp’ to ‘some
sort of TTML’ 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 “go to the W3C, thou
sluggard, and be wise”
>> where we have a registry for mapping the third IDs above to TTML profile
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
> we’re 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 mnemonic.
> we can’t 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 “yes, we’ll take on dialect naming and forming that
second-level parameter” is important; it then means that if MPEG finds a
clean solution to the first level, the actual problem in hand is solved.
I’d 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, of
>> 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.  But
the signaling requirements go beyond that – there is often multiple
namespaces in use in one document that are not, as an aggregate, a single
“profile”. So, these must be explicitly signaled as well since nearly all
profiles permit foreign namespaces.  To accommodate this, the “short names”
have to be defined as “profiles of namespaces” I think.
>>> For example, if a document uses the CFF-TT text profile of the TTML Full
profile, plus SMPTE-TT #608 (US captions), plus CFF-TT metadata, and it was
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 compatible
with one or more profiles of TTML", then we could propose a string composed
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 this.
>>> 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
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>> David Singer
>> Manager, Software Standards, Apple Inc.
> David Singer
> Manager, Software Standards, Apple Inc.

Cyril Concolato
Multimedia Group / Telecom ParisTech
Received on Monday, 5 May 2014 13:59:10 UTC

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