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

https://www.w3.org/Bugs/Public/show_bug.cgi?id=13359

--- Comment #58 from Bob Lund <b.lund@cablelabs.com> 2012-07-03 15:40:46 UTC ---
(In reply to comment #49)
> (In reply to comment #45)
> > 
> > A timed metadata track contains text if moov:trak:mdia:minf:stbl:stsd contains
> > MetaDataSampleEntry('mett'). The type of text data is identified by a MIME
> > type. In this case let TYPE = 'mp4-mett' and VALUE =
> > TextMetaDataSampleEntry.mime_format ([1] section 8.5.2.2).
> > 
> > A timed metadata track contains XML if moov:trak:mdia:minf:stbl:stsd contains
> > MetaDataSampleEntry('metx'). The format of the XML document is identified by
> > its namespace. In this case let TYPE = 'mp4-metx' and VALUE =
> > XMLMetaDataSampleEntry.namespace.
> 
> I can't reconcile this with the MPEG4 spec, mostly because the MPEG4 spec seems
> to be impenetrable.
> 
> First, there's no such thing as a "moov:trak:mdia:minf:stbl:stsd box", it seems
> that the formal way to refer to this would be "the first stsd box in the first
> stbl box in the first minf box in the first mdia box in the first trak box in
> the first moov box, if such a box exists". Is that right? Second, I can't work
> out how to even formally phrase the stuff about "mett" and "mime_format" and so
> forth. I can't find the definition of SDL, and the SDL in the above-cited MPEG
> spec is the only place that mentions "mett". The MPEG files don't seem to
> actually contain anything called "TextMetaDataSampleEntry", that seems to just
> be a concept used in the spec's syntax definitions (?). Really not sure how to
> write this.

Here is proposed text, based on what is already in
http://www.whatwg.org/specs/web-apps/current-work/#text-track-in-band-metadata-track-dispatch-type.
The attachment shows the change with the same formatting as the existing text.



If the media resource is an MPEG-4 file

Let sample entry be the contents of the stsd box, which is contained in the 
stbl box, which is  contained in the minf box, which is contained in the mdia
box, which is contained in the trak box. Let type be the type field of the stsd
box. If type is neither "mett" nor "metx" then the text track in-band metadata
track dispatch type must be set to the empty string. Otherwise, if the type is
"mett" then sample entry is a TextMetaDataSample and the text track in-band
metadata track dispatch type must be set to the concatenation of the string
"mett", a U+0020 SPACE character, and the value of the mime_format field of
TextMetaDataSampleEntry. Otherwise, if the type is "metx" then sample entry is
an XMLMetaDataSampleEntry and the text track in-band metadata track dispatch
type must be set to the concatenation of the string "metx", a U+0020 SPACE
character, and the value of the namespace field of
XMLMetaDataSampleEntry.[MPEG4]

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Received on Tuesday, 3 July 2012 15:41:02 UTC