Re: Tech Discussions on the Multitrack Media (issue-152)

In all cases we need both a label and an identifier. A label tells you some information about the track, but does not identify it: two tracks might have the same label. An identifier uniquely identifies the track but doesn't necessarily tell you anything about it.

As Eric says ISO/QT files have IDs and type: I'm not sure if the types in ISO files go further than audio/video/text etc. though.

A DASH manifest has an id for each track (aka Group) as well as some very flexible ways to label the tracks that would need to map to the @kind in the Track API.


On Mar 3, 2011, at 2:55 PM, Eric Carlson wrote:

On Mar 3, 2011, at 2:47 PM, Bob Lund wrote:

Is the MP4 and QT numeric ID unique with respect to the other tracks in a particular file or does it actually identify the type of media in the track?

  Tracks in MPEG-4 and QuickTime containers have separate ID and type fields. IDs are unique and immutable, and so can be used to uniquely identify a track even after a file has been edited (but not re-export, obviously). Type identifies the type of media in a track.


From: Eric Carlson []
Sent: Thursday, March 03, 2011 3:35 PM
To: Silvia Pfeiffer
Cc: Bob Lund; Mark Watson; Philip Jägenstedt;<>
Subject: Re: Tech Discussions on the Multitrack Media (issue-152)

On Mar 3, 2011, at 2:19 PM, Silvia Pfeiffer wrote:

I agree - the label is in all practical means going to have to be


 I think MP-4 also has numeric identifiers, but please correct me if I'm wrong.
  Correct, every track in MPEG-4 and QuickTime containers has a unique numeric ID.


On Fri, Mar 4, 2011 at 8:50 AM, Bob Lund <<>> wrote:

We've contemplated this problem with respect to in-band metadata tracks in live streams over MPEG-2 TS. We'd like to see a descriptive label that identifies the type of metadata (and can be learned by the user agent from the in-band track). Can the DASH manifest and ISO file carry such a label?

Bob Lund

-----Original Message-----
From:<> [] On Behalf Of Mark Watson
Sent: Thursday, March 03, 2011 2:38 PM
To: Silvia Pfeiffer
Cc: Philip Jägenstedt;<>
Subject: Re: Tech Discussions on the Multitrack Media (issue-152)

>From a DASH manifest you do. And from an ISO file there is a track id. Don't know about Matroska.

But in any case, in the absence of a track id from the container the browser could just create numeric track ids and you would be no worse off than your original proposal.

It is the more complex things like adaptive streaming presentations were the tracks can change. If you have a simple file I expect they don't change during playout.


On Mar 3, 2011, at 12:38 PM, Silvia Pfeiffer wrote:

My only concern with addressing by id rather than position is the availability of an id from on-band. Do we get such an identifier for tracks from all containers?


Sent from my iPhone

On 04/03/2011, at 4:51 AM, Mark Watson <<>> wrote:

On Mar 2, 2011, at 4:38 PM, Silvia Pfeiffer wrote:

On Thu, Mar 3, 2011 at 3:40 AM, Mark Watson <<>> wrote:

I have another question related to an earlier discussion on this thread: do I understand correctly that in order to apply CSS styling there needs to be an HTML markup element to apply the styling to ? i.e. that you couldn't apply styling to control the rendering of an in-band track that appears only as a MediaTrack object and not as a <track> element (with associated HTMLTrackElement object) ?

In that case it seems we need to be able to refer to an in-band track in the main resource from a <track> element in order to apply styling. For example the MediaTrack API could expose an "id" attribute (at least for in-band tracks) which could be used to refer to this track from a <track> element.

I didn't want to let this one go unnoticed. In the current spec for
<track> we address the cues inside the <track> - independent of
whether they come from in-band or from an external file - through a
::cue CSS pseudo-selector.

Maybe something like this can be introduced for the tracks, too?
This could then avoid having a HTML markup element for tracks and
allow us to style them similar to how we style cues. Can this work:
::track(number) where number is the position in the IDL tracks list
in the HTMLMediaElement? Some kind of default rendering would need
to be described, too.

I don't have a strong opinion on the solution, just that it should be possible to control the rendering of in-band tracks as well as those which are explicitly declared in the markup.

Do I understand correctly that to style WebVTT with CSS you attach a styles to the HTMLMediaElement containing the :cue pseudo-selector and these apply to both in-band and explicitly defined timed text tracks ?

To apply the same to other tracks, then it still might be better to require some kind track id and then the pseudo-selector would be :track(id). I say this because I expect that in-band tracks might be added or removed during playback and so the numbering will change. For example, accessibility tracks may be available for the main content but not for adverts. Or in a long-lived live stream things will change from show to show.

I expect we will need an event to indicate when the in-band tracks change.




Received on Thursday, 3 March 2011 23:14:03 UTC