- From: Cyril Concolato <cyril.concolato@telecom-paristech.fr>
- Date: Mon, 5 Oct 2015 13:25:08 +0200
- To: Michael Dolan <mdolan@newtbt.com>, 'Nigel Megitt' <nigel.megitt@bbc.co.uk>, 'Glenn Adams' <glenn@skynav.com>
- Cc: 'TTWG' <public-tt@w3.org>
Le 02/10/2015 17:24, Michael Dolan a écrit :
> >> b) is partially addressed as the 'stpp' row is still there.
>
> Summarizing the last email on this – it is there because it is in use
> today, for better or worse. Some options: 1) delete it and thus make
> existing use illegal; or 2) allow it but clearly deprecate its use.
>
I think you are confusing two things here:
- 'stpp' is a 4CC in the ISOBMFF. People may use 'video/mp4;
codecs="stpp"' to identify a TTML track in an MP4 file. That is not
wrong as TTML is an XML-based Subtitle Format, and it is currently the
only standard way to signal it. Players encountering such tracks should
not expect only TTML as they might problem in the future if another XML
subtitle format is stored in MP4. It is MPEG's business to: a) define a
more precise 'codecs' parameter for TTML-based tracks; b) inform its
users to adopt the more precise 'codecs' syntax. I expect that the more
precise value will be: 'video/mp4; codecs="stpp.<TTML-identifier>",
where <TTML-identifier> is replaced by 'tt1f', 'tt1p', ... where the
current approach 'video/mp4; codecs="stpp"' being replaced by
'video/mp4; codecs="stpp.tt1f"' (or with another identifier)
- defining 'stpp' as a TTML profile identifier is wrong, because: a)
there is no "profile designator", "profile definition document", "public
specification" defining the associated profile (such profile is not
defined in ISOBMFF!); and b) even if it such a profile were to be
defined, its use with the approach describe above, i.e. 'video/mp4;
codecs="stpp.stpp"' (note the duplication of 'stpp') will be misleading!
My recommendations are:
- to TTWG: delete the 'stpp' row in the registry
- to MPEG:
- clarify that player implementations encountering 'video/mp4;
codecs="stpp" (without TTML profile identifier) should look at the file
content to make sure it is TTML inside
- define a TTML-track codecs syntax
- recommend packagers to use the new 'codecs' syntax
HTH,
Cyril
--
Cyril Concolato
Multimedia Group / Telecom ParisTech
http://concolato.wp.mines-telecom.fr/
@cconcolato
Received on Monday, 5 October 2015 11:25:31 UTC