Embedding the track kind in WebVTT files

Hi all,

WebVTT files are/will be used in environments where there might not be 
an HTML page pointing to it. Some examples are:
- WebVTT content embedded and multiplexed with video and audio in an MP4 
file
- Plain text WebVTT files referenced by an adaptive streaming manifest 
(DASH MPD, HLS playlist, ...)

All embedding environments typically enable marking a track with a 
language (e.g. an audio track) but not all can carry the kind types 
(subtitles, captions, metadata). In order to avoid losing that 
information in those environments, I think the WebVTT file should 
declare its kind. Also, when producing WebVTT files, it should be 
possible to indicate the language, kind or even label in the file 
itself, to avoid mistakes.

I would like to propose to define standard WebVTT metadata headers, as 
follows:
- WebVTT metadata header name: kind
- WebVTT metadata header value: as defined by kind attribute of the 
track element in the HTML 5 specification

- WebVTT metadata header name: language
- WebVTT metadata header value: as defined by srclang attribute of the 
track element in the HTML 5 specification

- WebVTT metadata header name: label
- WebVTT metadata header value: as defined by label attribute of the 
track element in the HTML 5 specification

Note that some people are already using non standard name/values for 
that (e.g. 
http://wiki.webmproject.org/webm-metadata/temporal-metadata/webvtt-metadata)

Of course, this introduce redundancy when the embedding environment also 
sets those attributes and might be problematic if the values differ. 
Resolving conflicts should be specified by the embedding environment. In 
HTML, the information in the HTML page should probably have precedence, 
to be able to use a caption track as metadata, if the HTML author wants to.

Comments ?

Regards,
Cyril

-- 
Cyril Concolato
Maître de Conférences/Associate Professor
Groupe Multimedia/Multimedia Group
Telecom ParisTech
46 rue Barrault
75 013 Paris, France
http://concolato.wp.mines-telecom.fr/

Received on Thursday, 12 September 2013 09:31:06 UTC