webm with multiple concurrent tracks

[bryand...@gmail.com]

Can I create a WebM file with multiple audio/video tracks?

My use case is to record a WebRTC call where two people are speaking.

I want to have both A/V streams in the same file, then play that file back on a web page in 2 separate video elements, each of which will enable a different track from the source file.

I know that the Video element itself supports multi-track resources [1] and I believe that Matroska also allows this.

Is this possible / supported for WebM?  

Thanks

[1] http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#media-resources-with-multiple-media-tracks

[Frank Galligan]

Hi,

You could create a file with multiple audio and video tracks. At this point WebM still lists at most one video and audio track per file.

But before you go to far down this path, I would checkout if browsers actually support multiple tracks in the same video. Just because it is speced doesn't mean browsers will actually support it.

Also why do you want all of the tracks in the same file? Because of sync between all the tracks?

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[bryand...@gmail.com]

You make a good point about spec vs actual support.

One file is more convenient than multiple segments recorded separately, mainly for sync that is implicit in the file metadata instead of stored externally.  Also, I'd rather not transcode the video to mix down to one track.  Currently I keep multiple segments separate, and will probably keep doing this.

[Frank Galligan]

Yeah I would try and find out the actual support. But even if all of your target browsers support multiple tracks, I'm not sure they will support your use case... optimally. I would think that the 2 video tags would parse your video each separately and you would still have as many (if not more) sync issues between the tracks as if the 2 tracks were in separate video files. ***Usual disclaimer, I'm not an expert in multiple track support/implementation, so please investigate yourself.***

I still think it will be easiest for you to transcode the 2 video tracks side by side into a video file with one track, like how 3d stereo is handled.

[Silvia Pfeiffer]

I'm with Frank here: you need to put a mediagroup attribute on those two video elements anyway to get them in sync, which is incidentally not very well implemented in any browser yet. Might be worth opening bugs and chasing it up actually!

Silvia.