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

From: Harald Alvestrand <harald@alvestrand.no>
Date: Tue, 19 Mar 2013 07:27:49 +0100
Message-ID: <51480565.4050303@alvestrand.no>
To: public-media-capture@w3.org
An alternate method of straightening out the API would be to rename the 
addTrack operator to addCopyOfTrack  or addTrackWithSameSourceAsTheArgument.

I think the ability to control tracks-in-streams independently is more 
important than the linkages you mention, but agree that the API name 
doesn't fit with what the operation does.

On 03/18/2013 03:47 PM, 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 06:30:38 UTC

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