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[Bug 13359] <track> A way is needed to identify the type of data in a track element

From: <bugzilla@jessica.w3.org>
Date: Fri, 27 Jan 2012 20:58:42 +0000
To: public-html-a11y@w3.org
Message-Id: <E1Rqssc-0004uq-0m@jessica.w3.org>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=13359

--- Comment #37 from Bob Lund <b.lund@cablelabs.com> 2012-01-27 20:58:36 UTC ---
(In reply to comment #36)
> So I went to add this, but the description in comment 30 is too vague to get
> interoperability here.
> 
> Could you describe the exact normative steps to generate a string from in-band
> text tracks for each of the formats we should be supporting?

Below is some example normative text. Let me know if this needs to be reworked
in any way.

We've also developed a more general mechanism for exposing all track metadata
in media resource with in-band tracks (see
http://html5.cablelabs.com/tracks/media-container-mapping.html). This is
defined for Ogg, WebM, MPEG-2 TS and MPEG-4 File Format media types. Basically,
the metadata is exposed via a text track to script .

This would be an alternative to the normative text below. This alternative is
more general and also applies to video and audio tracks so in that sense I
think it is a better solution. However, it may be too much to consider at this
point.

Requested normative text
=====================================
The attribute (let's call it text_track_type) MUST be set to a string depending
on the MIME type of the media resource:

video/ogg - The string contains the values of the Ogg Skeleton message headers
content-type, role and name for this text track: '{"content_type":string
containing content-type message header, "role":string containing the role
message header, "name":string containing the name message header}'[1].

video/mp2t - The string contains the PMT elementary stream type and descriptors
for this text track: '{"stream_type":string representation of the PMT
stream_type field, "es_descriptors":[es_descriptor, ...]}'. Each es_descriptor
is {"tag":string representation of the PMT descriptor tag,
"desc_contents":Base64 encoded representation of the PMT elementary stream
descriptor}[2]

video/webm - The string contains the TrackEntry and CodedID for this text
track: '{"tracktype":string representation of the TrackType element contained
in the TrackEntry element, "codecid":string representation of the CodecID
element contained in the TrackEntry element}'[4].

video/mp4 - The string contains the track hdlr box and trak box contents for
this track: '{"trak-mdia-hdlr-hdlr_type":string representation of the hdlr box
hdlr_type field, "trak-mdia-hdlr-name":hdlr box name field, "trak":Base64
representation of the trak box (and all child boxes)}' [4]



[1] http://wiki.xiph.org/SkeletonHeaders
[2] ITU H.222.0 (5/2006) Table 2-33
[3] http://matroska.org/technical/specs/index.html
[4] ISO/IEC 14496-12

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Received on Friday, 27 January 2012 20:58:48 GMT

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