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RE: Track cloning behavior of MediaStream() constructor and addTrack()

From: Jim Barnett <Jim.Barnett@genesyslab.com>
Date: Tue, 19 Mar 2013 12:31:11 +0000
To: Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com>, "public-media-capture@w3.org" <public-media-capture@w3.org>
Message-ID: <57A15FAF9E58F841B2B1651FFE16D281031A97@GENSJZMBX02.msg.int.genesyslab.com>
If we do this, we'll have to specify what happens if different settings are applied to the same (uncloned) track in different MediaStreams.  I would think that the behavior would be the same as if the settings had been applied to the track in a single MediaStream.  (i.e., the new settings override the old ones, and both MediaSteams see the changes at the same time.)  Furthermore,  if Track1 is synchronized with Track2 in MediaStream1 and with Track3 in MediaStream2, then Track1, Track2, Track3 are synchronized everwhere (though their clones may not be).  

I agree that having the API implicitly clone tracks is a bad idea, and that having an explicit cloning operation would be better, but then we have to make sure that the uncloned track really is identical no matter what MediaStream it is in.
- Jim

-----Original Message-----
From: Stefan Håkansson LK [mailto:stefan.lk.hakansson@ericsson.com] 
Sent: Tuesday, March 19, 2013 4:31 AM
To: public-media-capture@w3.org
Subject: Re: Track cloning behavior of MediaStream() constructor and addTrack()

I like this proposal,

it makes it possible to select whether you clone a track, or just include the existing one in another MediaStream.

Also, if you clone using track.clone, than you'd automatically know what tracks are related (i.e. use the same source) and not need to detour via SourceId to find out the relations.

Personally I think we could even consider to simplify the MediaStream constructor so that it would not take any arguments but just create an empty MediaStream. You'd then have to use "addTrack" to add tracks to it.

Stefan

On 2013-03-18 15:47, Adam Bergkvist wrote:
> Hi
>
> As the spec is currently written, the MediaStream() constructor and 
> the
> addTrack() method create a new MediaStreamTrack instance to represent 
> a given track argument. Some of the changes we done lately, like 
> removed the ordered track list attributes, have made this API harder to work with.
>
> Independent track instances (or clones) are useful to control the 
> output to different consumers. In the case below, the self-view will 
> show video but the video to the other peer is disabled.
>
> var peerStream = new MediaStream(selfViewStream); // clone the stream 
> pc.addStream(peerStream); peerStream.getVideoTracks()[0].enabled = 
> false; selfViewStream.getVideoTracks()[0].enabled = true;
>
> However, the cloning behavior of MediaStream() and addTrack() causes 
> some unintuitive API behavior. For example, if we do:
>
> stream.addTrack(track);
>
> Then the following calls won't behave as one might expect.
>
> stream.getTrackbyId(track.id); // is null stream.removeTrack(track); 
> // is a no-op and won't remove track from
>                             // stream
>
> The instance track is not a part of stream and the new track instance 
> has a different id. This may also cause confustion if one tries to use 
> the track reference passed to addTrack() to influence the output of 
> stream (example below).
>
> stream.addTrack(track); // as above
> pc.addStream(stream);
> track.enabled = false; // won't disable the output of any track in
>                         // stream
>
> If we take the proposed approach for sync getUserMedia() as an example.
>
> var video1 = new VideoStreamTrack(constraints1); var video2 = new 
> VideoStreamTrack(constraints2); var stream = new MediaStream([video1, 
> video2]);
>
> The instances video1 and video2 are pretty much useless since new 
> track instances are created when the stream is constructed. It's also 
> not obvious which video tracks in stream that correspond to video1 and 
> video2, since the track order is undefined. The proposed sourceId 
> would make the pairing easier, as long as we keep the restriction that 
> only one track can represent a source in a MediaStream.
>
> I think the API would be more intuitive if MediaStream() and 
> addTrack() "adopted" the track arguments instead of cloned them. 
> Cloning would then be an explicit operation.
>
> To still support cloning, we could either support cloning of both 
> tracks and streams or simply only tracks.
>
> var clonedStream = stream.clone();
>
> A stream clone can be created by adding cloned tracks to a new stream.
>
> var tracks = stream.getAudioTracks().concat(stream.getVideoTracks());
> var trackClones = tracks.map(function (track) {
>      return track.clone();
> });
> var streamClone = new MediaStream(trackClones);
>
> What do people think about this?
>
> BR
> Adam
>
Received on Tuesday, 19 March 2013 12:31:41 UTC

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