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How do we enable/select tracks for playout?

From: Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com>
Date: Tue, 19 Mar 2013 12:39:34 +0100
Message-ID: <51484E76.5000200@ericsson.com>
To: "public-media-capture@w3.org" <public-media-capture@w3.org>
I'd like to discuss enable/disable of MediaStreamTrack's a bit, and 
specifically how it works with the media element.

Currently, the app can enable/disable a MediaStreamTrack by toggling the 
"enabled" attribute [1].

If we look at the video case, you get the impression that you could 
create a MediaStream with two video tracks (e.g. front and rear camera) 
and then toggle which one of them that is displayed in a video element 
by enabling/disabling the different tracks.

But that is not the case. The current definition of videoTrackList and 
videoTrack for the media element in the html draft [2] instead says that 
you'll need to set the "selected" attribute of the videoTrack you want 
to play in the videoTrackList of the _video element_. Toggling the 
enabled bit of the MediaStreamTrack's will not change which track that 
is played (if I understand correctly).

So, in a scenario where the video is sent over the network (using 
PeerConnection), and the sending app want to change which video is 
played at the remote end, it would actually have to signal to the app at 
the remote end to set "selected" on the other videoTrack on the video 
element. Toggling enabled/disabled at the sending side will not help.

I've previously argued that this is strange, and that we should remove 
enable/disable from MediaStreamTrack (and instead have it on consumers - 
Recorder, PeerConnection, media element - only). This could be one way 
to go.

Another could be that we keep enable/disable on the MediaStreamTrack, 
but instead redefine how media elements work when the source is a 
MediaStream. It could be defined so that the enabled MediaStreamTrack is 
always played, and if more than one track is enabled one of the enabled 
ones is (randomly) picked for playout.

A third could be that we keep the current way. I find it a bit 
confusing, but it could work. Note (again in the network case) that if 
we have reliable signaling/detection of disabled/muted, the receiving 
application could listen to "onmute", and if that fires at the currently 
playing video MediaStreamTrack the app could select another for play in 
the video elements videoTrackList.

What do you think?

Stefan

[1] http://dev.w3.org/2011/webrtc/editor/getusermedia.html#attributes-1
[2] 
http://www.w3.org/html/wg/drafts/html/master/embedded-content-0.html#audiotracklist-and-videotracklist-objects
Received on Tuesday, 19 March 2013 11:39:58 UTC

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