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Re: Rationalizing new/start/end/mute/unmute/enabled/disabled

From: Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com>
Date: Tue, 26 Mar 2013 10:38:11 +0100
Message-ID: <51516C83.70109@ericsson.com>
To: public-media-capture@w3.org
On 3/25/13 10:55 PM, Martin Thomson wrote:
> I think that it's fair to say that the current state of the
> MediaStream[Track] states is pretty dire.  At least from a usability
> perspective.
>
> Let's take a quick inventory:
>
> MediaStreamTrack:
>                  attribute boolean               enabled;
>      readonly    attribute MediaStreamTrackState readyState;
>                  attribute EventHandler          onstarted;
>                  attribute EventHandler          onmute;
>                  attribute EventHandler          onunmute;
>                  attribute EventHandler          onended;
>
> MediaStream:
>                  attribute boolean      ended;
>                  attribute EventHandler onended;
>
> On the track we have two state variables, one of them writeable, and
> four events.  At a bare minimum, this could be a single onstatechange
> event.
>
> For the stream, I really don't know why I would want to set ended =
> true on a stream.  The explanation doesn't cast any light on the
> matter either.  Let's pretend that this is read-only for the moment.
>
>
> Rendering
>
> I believe that the gUM document needs to be very clear about the
> rendering logic for MediaStream instances.  This is a little too
> amorphous in the current document.  I propose a section or sub-section
> entitled "Rendering MediaStreams" that has an explanation along the
> lines of the following.
>
> When a particular MediaStream instance is attached to a sink that
> consumes only one source (not a mixing sink, like <audio> probably
> will be), the output will be selected from the set of tracks in the
> stream that:
>
>   - are "live"; that is, in a readyState of "muted" or "unmuted" (not
> "new" or "ended")
>   - have enabled = true
>
> ...of course, media will only render (other than silence/black) if the
> selected track's readyState is "unmuted".  I'm assuming here that
> track selection does not examine the muted/unmuted state.
>
> ...but only if the stream itself is not ended.  Note that the stream
> can be un-ended by adding an un-ended track.
>
> A mixing sink (such as <audio>) renders multiple tracks
> simultaneously, combining all candidate tracks.

(Commenting on the entire "Rendering" section)

I agree to that we need to describe in more detail how MediaStream's 
interact with media (audio and video) elements. Jim took a stab at it 
last year, but it is time for an update. Audio and video are also 
different in that you can render only one video track in a media 
element, but mix all audio tracks.

However, I want to point out that the "resource fetch algorithm" of the 
html 5 Candidate rec 
(http://www.w3.org/TR/html5/embedded-content-0.html#concept-media-load-resource) 
already describes this quite detailed. I think we might get by with 
referring to that, but clarifying how certain things apply to MediaStreams.
Received on Tuesday, 26 March 2013 09:38:37 UTC

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