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

Re: Constructors for MediaStreamTrack subtypes - remove?

From: Harald Alvestrand <harald@alvestrand.no>
Date: Sun, 11 Aug 2013 17:13:26 +0200
Message-ID: <5207AA16.3020307@alvestrand.no>
To: public-media-capture@w3.org
On 08/08/2013 03:35 PM, Stefan Håkansson LK wrote:
> On 2013-08-05 10:46, Harald Alvestrand wrote:
>> Hi,
>> at the moment, the spec contains constructors for VideoMediaStreamTrack
>> and AudioMediaStreamTrack - each taking a TrackConstraints set as parameter.
>>
>> These were part of an idea to revisit the way one acquires media
>> (different from getUserMedia, which returns tracks) - but the chairs are
>> unable to tell whether any of the brief proposals floated in Lyon and
>> Boston have gathered consensus as "THE proposal", or indeed whether we
>> have consensus to go ahead with this API addition.
>>
>> In the absence of such consensus, I suggest that we remove the
>> constructors from the spec at this time, so that it's clear that
>> implementations don't have to support these constructors that don't
>> produce useful objects at the moment.
>>
>> If we converge on a specific proposal that makes these constructors
>> useful, I think it is easy to add them back at that time.
>>
>> People who have a different opinion should reply to this message....
> Personally I think the possibility to create AudioMediaStreamTrack's and
> VideoMediaStreamTrack's (and out of them MediaStream's) is quite
> valuable in situations where the destination is a remote peer.
>
> It allows the application to set up the connection and media codecs etc.
> before (or in parallel to) asking for consent to use microphone and
> camera. This can enhance the user experience since it reduces the risk
> of audio clipping (as everything is already set up when the user accepts
> the use of camera and microphone). It also allows the app to not bother
> the user by asking for consent if for some reason the connection can't
> be established (or if the codecs supported do not match - we still don't
> have an MTI video codec).

That's where I get confused .... what if the properties of the stream 
depend on the device chosen at GetUserMedia time? Such as Cullen's 
standard example of a VP8-hardware camera and an H.264-hardware camera, 
where you can choose one or the other? (Of course, if you can tell the 
capabilities of the camera without opening it, we're back into 
fingerprinting land, for which user prompts are supposed to be our 
shield.... but "new" happens before the user prompt.)

So if one does this:

videoTrack = new VideoMediaStreamTrack(..)
videoStream = new MediaStream(videoTrack)

peerConnection.addStream(videoStream)
offer = peerConnection.createOffer(...)

will the offer contain all the codec configurations available to the 
browser, only those codec configurations that the browser knows it can 
support across all devices, some "browser default", or something else?

And if one later does (my preferred naming - I see no reason to reuse 
the name "getUserMedia):

navigator.attachUserMedia(videoStream, ....)

will a negotationNeeded event be fired if the capabilities of the source 
are different from the capabilities previously negotiated? If it's more 
capable than announced, less capable than announced, or only with 
different preferences?

It's this kind of complexity that leads me to think that we don't have a 
complete proposal for what I'm calling "attachUserMedia" yet.




>
> I also think it would be quite a minor addition to spec up how a
> MediaStream that contains only constructed audio and video tracks can be
> an argument to getUserMedia (but I don't know how big the implementation
> effort would be though).
>
>
>>          Harald
>>
>>
>>
>
Received on Sunday, 11 August 2013 15:13:16 UTC

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