W3C home > Mailing lists > Public > public-media-capture@w3.org > April 2013

RE: Explicit track cloning

From: Jim Barnett <Jim.Barnett@genesyslab.com>
Date: Tue, 9 Apr 2013 15:20:33 +0000
To: Adam Bergkvist <adam.bergkvist@ericsson.com>, "public-media-capture@w3.org" <public-media-capture@w3.org>
Message-ID: <57A15FAF9E58F841B2B1651FFE16D2810373F7@GENSJZMBX02.msg.int.genesyslab.com>
I vote for *1*.  We just have to make it clear to developers what it means to put the same track in multiple MediaStreams.  As I understand it, a Track represents a set of settings/operations applied to the output of a device.  (These settings may or may not affect the state of the device.)  If a Track is in multiple MediaStreams, then any modification applied in one Stream is reflected immediately in the other.  I can't think of a use case for this off the top of my head[1], but I don't see any reason to disallow it. 

- Jim
1.  Suppose I am talking to someone over a PeerConnection and want my screen to reflect exactly what I am sending to the other side.  Would this be a case where I would have two MediaStreams, one added to a PeerConnection and the other connected to a <video> element?  If so, this is a case where I would want any change applied to a Track to show up in both MediaStreams simultaneously.  (I would want the audio muted in my local copy, but would want the video to be exactly the same.  So I would clone the audio track, or omit it altogether, and use the same video Track.)

-----Original Message-----
From: Adam Bergkvist [mailto:adam.bergkvist@ericsson.com] 
Sent: Tuesday, April 09, 2013 8:57 AM
To: public-media-capture@w3.org
Subject: Explicit track cloning


I got the task from the editor's team to sum up the discussion around explicit track cloning from [1] and propose some changes.

- we want explicit cloning.
- we already have a one to many relationship between sources and track instances
- we already have a one to many relationship between track instances and consumers (via MediaStream)

What wasn't entirely resolved:
- when addTrack() and the MediaStream() constructor doesn't auto-clone anymore, is it ok to add a track instance to more then one MediaStream?

API impact:
If we think it's ok to not put any limitations on adding the same track to several streams the API could behave as following (*1*):

Adopts the track instance (i.e. no cloning):
* MediaStream() constructor
* MediaStream.addTrack()

Clones the track instance:
* MediaStreamTrack.clone()

The current MediaStream() constructor is optimized for cloning an entire MediaStream since we identified that as a common use-case. Therefore, we could also have a MediaStream.clone() if we still want to have that operation as a one-liner.

If we think it's not ok for two track instances to live in different MediaStreams then we need to patch the above API a bit.

We've had two proposals to do this. One is to still have the addTrack() method clone the added track, but rename it to make this more clear (*2*). Unless that method would return the cloned track, we would still have the problem of finding a reference to the added track. It's however not that obvious how to patch the constructor in a similar way.

The second proposal was to use the
addTrack()/MediaStreamTrack.clone()-API mentioned above but, e.g., throw an exception when a track is added to a second stream (*3*). An readonly attribute on a track telling which MediaStream that it's currently belong to could help in this case.

So what should we pursue?

My preference would be (*1*) or (*3*).

Received on Tuesday, 9 April 2013 15:21:04 UTC

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