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Re: Proposal for Audio and Video Track Selection and Synchronisation for Media Elements

From: Mark Watson <watsonm@netflix.com>
Date: Mon, 21 Mar 2011 12:59:39 -0700
To: Philip Jägenstedt <philipj@opera.com>
CC: "public-html@w3.org" <public-html@w3.org>
Message-ID: <240A4C96-32E9-466E-B36E-7F92B4E66964@netflix.com>

On Mar 21, 2011, at 7:38 AM, Philip Jägenstedt wrote:

> I think that this proposal looks mostly good, some nitpicking:
> 
> On Mon, 21 Mar 2011 12:00:46 +0100, Ian Hickson <ian@hixie.ch> wrote:
> 
>> * Allowing authors to use CSS for presentation control, e.g. to
>>   control where multiple video channels are to be placed relative to
>>   each other.
> 
> It's clear how to do this declaratively, but with scripts there seems to  
> be a few steps involved:
> 
> 1. Create/find a second video element and position it with CSS.
> 
> 2. Binds the two video tracks together with the same MediaController using  
> .mediaGroup or creating a new controller.
> 
> 3. Set the src of the second video element to the same as the first.
> 
> 4. Wait until loadedmetadata and then enable the alternative video track.
> 
> This seems like it would work, but what if steps 2 and 3 happen in the  
> other order? Then the media framework has no hint that the two resources  
> will be correlated and will have to set up a completely separate decoding  
> pipeline for it. This is not very nice, because in some situations (same  
> playback rate and offset) it's possible to have a single decoding pipeline  
> and just plug in more decoders after the demuxer.
> 
> Is this a bug, or do we expect browsers to become clever enough to figure  
> out that the same decoding pipeline can be reused without any kind of  
> hint? Admittedly, this would help for games creating 10 Audio() elements  
> for the same URL.

For the case where the multiple video tracks are inband tracks in the same resource, would it make sense for the video elements in the same group to more explicitly share a source ? e.g. if no source is specified then a video element is assumed to have the same source as the other elements in the same group (which must then all have the same source) ?

Then you would not have step 3 above - it would be implicit in step 2 - and the ordering issue would not arise.

I agree that it is useful to know at the time that the source is specified for the second video element that it is linked to the first one, so that the media pipelines and other state can be established appropriately. For example if the source is an adaptive streaming one, then the adaptive streaming algorithms need to be aware of the association between streams.


> 
>>    <video src="movie.vid#track=Video&amp;track=English" autoplay  
>> controls mediagroup=movie></video>
>>    <video src="movie.vid#track=sign" autoplay mediagroup=movie></video>
> 
> This is using Media Fragment URI:  
> http://www.w3.org/2008/WebVideo/Fragments/WD-media-fragments-spec/#naming-track
> 
> It appears that the proposal assumes that track dimension can only be  
> specified one in a valid Media Fragment, but this is unfortunately not the  
> case. The MF spec states that "Multiple track specification is allowed,  
> but requires the specification of multiple track parameters."
> 
> Perhaps this is not a problem, #track=Alternative&track=Commentary would  
> just result in a resource with two tracks, and the first (Alternative)  
> would be used. However, how should this be reflected in audioTracks and  
> videoTracks? Should only the selected tracks be exposed there, or should  
> all tracks be exposed but the targetted ones enabled? I think the latter  
> makes more sense, but the former is more in line with MF.
> 
>> RISKS
>> * It's possible that it is still too early for us to be adding any
>>   kind of multi-track feature given the current implementation
>>   priorities of user agents.
> 
> Indeed, the complexity of implementing this is significant. It requires a  
> very capable media framework to do things like gapless looping of one  
> track synchronized with another and to determine when decoding pipelines  
> can be shared and not.
> 
> -- 
> Philip Jägenstedt
> Core Developer
> Opera Software
> 
> 
Received on Monday, 21 March 2011 20:00:16 UTC

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