[whatwg] Proposal for @label attribute associated with kind=metadata TimedTextTracks

[Eric Winkelman]

On Monday, March 21, 2011 11:17 AM, Tab Atkins Jr. [mailto:jackalmage at gmail.com] wrote:

> >> Use Case:
> >>
> >> Many video streams contain in-band metadata for application signaling,
> and other uses.  By using this metadata, a web page can synchronize an
> application with the delivered video, or provide other synchronized services.
> >>
> >> An example of this type of metadata is EISS (
> http://www.cablelabs.com/specifications/OC-SP-ETV-AM1.0-I06-110128.pdf
> ) which is used to control applications that are synchronized with a television
> broadcast.
> >>
> >> In general, a media stream can be expected to carry several types of
> metadata and the types of metadata may vary in time.
> >>
> >> Problem:
> >>
> >> For in-band metadata tracks, there is neither a standard way to represent
> the type of metadata in the HTMLTrackElement interface nor is there a
> standard way to represent multiple different types of metadata tracks.
> >>
> >> Proposal:
> >>
> >> For TimedTextTracks with kind=metadata the @label attribute should
> contain a MIME type for the metadata and that a track only contain Cues
> created from metadata of that MIME type.
> >>
> >> This implies that streams with multiple types of metadata require the
> creation of multiple metadata track objects, one for each MIME type.
> >
> >
> > I don't understand. Are you saying that right now all tracks that are
> > of kind=metadata are made available through a single TextTrack? Cause
> > I don't think that's the case.
> >
> > Or are you worried about text track files that contain more than one
> > type of metadata? If the latter, then how is the browser to know how
> > to separate out the individual cues from a single track into
> > multipled?
> >
> > Can you clarify?
> 
> I'm also somewhat confused.  The OP mentions in-band metadata, but then
> proposes adding something to out-of-band <track kind=metadata>
> elements.

I'm not proposing adding anything to out-of-band <track kind=metadata> elements.  In-band metadata tracks are added to the DOM by the media player, and have the same @label attribute that out-of-band tracks do.  I'm suggesting a use for that @label attribute that solves a problem I've encounter using metadata tracks.

> I'm not familiar enough with in-band metadata tracks to know if it would be
> useful to expose additional information about them, but for out-of-band
> tracks I suspect that any information you may need is application-specific,
> and thus can be served with a data-* attribute.

I agree, there are a number of solutions for out-of-band metadata tracks, but  my concern is specifically in-band metadata tracks.  

If an in-band kind=metadata track appears, what kind of information does it contain?  Can you tell by looking at the DOM?  Can you tell by looking at the cue's text?

Eric