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Assigning MediaStream objects to HTML media elements

From: Rich Tibbett <richt@opera.com>
Date: Tue, 13 Dec 2011 00:58:07 +0100
Message-ID: <4EE6950F.7010603@opera.com>
To: "public-media-capture@w3.org" <public-media-capture@w3.org>
Nine months ago we (Opera) were in the middle of some early 
experimentation work implementing an early proposal of the getUserMedia 
proposal. We found that we had to patch the then-current WHATWG <device> 
proposal with a number of additional API behaviors for when HTML media 
elements are hosting Media Stream objects.

In our early specification we defined the term 'stream mode' for when 
such DOM elements are assigned Stream objects for the purposes of local 
media playback. We documented the behavior of the DOM APIs for HTML 
media elements operating in stream mode in [1].

One of the key concepts we established is that MediaStream objects, when 
applied to audio/video elements, should act within a 'linear media 
timeline' model (also a defined term in [1]). Such a timeline is, for 
example, not seekable and has well-defined behavior in the case that the 
video element is paused and resumed. When a developer interfaces with 
the DOM APIs representing HTML Media elements, we define specific setter 
and getter behaviors to switch off things that don't make sense in a 
'linear media timeline' model.

For example, a developer cannot adjust the playbackRate of a live 
stream, captured from a user's webcam. I think we need to review and add 
these behaviors to the current specification to ensure we end up with 
consistent behavior across UAs implementing our future recommendation.

In the course of our early prototyping and also as part of the provided 
spec [1] we also introduced the ability to assign Stream objects 
directly to the .src of a video or audio HTML element (e.g. video.src = 
stream). This was not included in the official WHATWG draft at the time 
(since we didn't have window.URL.createObjectURL at that time and no-one 
really knew how else we could do it).

Right now in the W3C proposal developers are expected to indirectly mint 
a new temporary URL (via window.URL.createObjectURL) to assign a 
MediaStream to a video/audio element. To coincide with direct assignment 
of a Stream object to a video element, our work also defined how Stream 
objects that have been assigned to HTML media elements should be labeled 
if or when a developer attempts to resolve the src URL/URI. We settled 
on 'about:streamurl' to be a reserved, though unresolvable, about: URI, 
to indicate that the media element is displaying/playing an object that 
implements the MediaStream interface (and hence the media element is in 
stream mode and has all the behaviors of that mode as defined in [1]).

This direct object assignment requires less code to assign Stream 
objects to video/audio elements. It works really well to date in all 
Opera implementations and we'd like to apply this behavior to the W3C 
spec pending further discussion and feedback.

There are notably a number of things we might want to also discuss, such 
as what we should do if particular attributes (i.e. autoplay, loop) are 
included on any HTML media element declaration (but I'd assume they mean 
nothing in stream mode). The documentation in [1] should be a reasonable 
starting point for that discussion also.

I notice that roc's MediaStream API Processing proposal [2] touches on a 
few of these aspects to some degree also, albeit in less formal prose 
right now. I guess we're all thinking along the same thing and we just 
need to define the exact terms and algorithms in our specification.

Any feedback would be appreciated. The main question is: should we add 
such behaviors as these to our specification or not?

br/ Rich

[1] 
http://people.opera.com/richt/release/specs/device/o-device.html#stream-mode-behavior

[2] 
https://dvcs.w3.org/hg/audio/raw-file/tip/streams/StreamProcessing.html#mediastreams
Received on Tuesday, 13 December 2011 00:01:08 GMT

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