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Re: How do we enable/select tracks for playout?

From: Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com>
Date: Thu, 21 Mar 2013 11:23:42 +0100
Message-ID: <514ADFAE.9000804@ericsson.com>
To: public-media-capture@w3.org
It seems to me that we are converging on:

* Enable/disable of a MediaStreamTrack is something different from 
selecting which video track that is being played in a video element
* Those should be separate things (you can select a disabled track for 

This basically means that we don't need to change any of what has 
already been defined:
* MediaStreamTrack keeps its "enabled" attribute
* media elements allow for selecting a specific video track for playout

The one outstanding question is how to deal with the case of several 
enabled video tracks in a MediaStream when being attached to a video 
element. Step 11 of the "resource fetch algorithm" of the media element 

"If either the media resource or the address of the current media 
resource indicate a particular set of audio or video tracks to enable, 
then the selected audio tracks must be enabled in the element's 
audioTracks object, and, of the selected video tracks, the one that is 
listed first in the element's videoTracks object must be selected."

For the audio part it is pretty clear - all tracks that are "enabled" 
should be used (and mixed) for playout. Video is less so. If there is 
more than one video track that is "enabled", which one should be 
selected? There is no "first".


On 3/21/13 7:46 AM, Adam Bergkvist wrote:
> On 2013-03-20 18:12, Martin Thomson wrote:
>> On 20 March 2013 07:33, Adam Bergkvist <adam.bergkvist@ericsson.com>
>> wrote:
>>> On 2013-03-19 14:00, Jim Barnett wrote:
>>>> The final decision of what to display should be up to the sink.  If
>>>> there are sinks that can handle MediaStreams containing multiple
>>>> audio or video tracks, then they should be able to use whatever
>>>> heuristics they want to select the Track(s) to play out.
>> I'm concerned with the non-deterministic behaviour that could happen
>> if we don't make something explicit here.
> I agree. We need to do something about this since we removed the order
> of the tracks and there's no logical "first" track of each kind anymore.
> But if the sink has some mechanism for enumerating tracks and selecting
> which one to render, then that should just work for MediaStreams as
> well. However, selecting a track is no guarantee that there's data on
> it; it might just be muted on track-level anyhow.
> /Adam
Received on Thursday, 21 March 2013 10:24:09 UTC

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