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Re: What does a track contain?

From: Harald Alvestrand <harald@alvestrand.no>
Date: Mon, 29 Feb 2016 12:31:43 +0100
To: public-media-capture@w3.org
Message-ID: <56D42C1F.6060602@alvestrand.no>
I think we should expect the sequence

<alpha-supporting canvas> -> mediaStreamTrack -> <video>

to deliver the alpha channel. Anything else seems like a bug.

But I think we have to admit that

<alpha-supporting canvas> -> mediaStreamTrack -> PeerConnection -> H.264
baseline encoding -> PeerConnection -> MediaStreamTrack -> <video>

is going to lose the alpha channel. We can play tricks with VP8, and I
think there exist H.264 extensions with alpha support too, but there are
times we're going to lose it.

I think that just has to be documented in webrtc-pc: "Encode what you can".



Den 24. feb. 2016 19:34, skrev Peter Thatcher:
> FYI, someone answered my question with this:
> 
> http://simpl.info/videoalpha/
> 
> Works in Chrome, but not Firefox.
> 
> On Wed, Feb 24, 2016 at 8:28 AM, Peter Thatcher <pthatcher@google.com
> <mailto:pthatcher@google.com>> wrote:
> 
>     I don't know enough about <video> to know the answer to this:  if it
>     has an alpha channel in it, and it's rendered on top of other web
>     elements, is it supposed to render such that you potentially see
>     through the video to the other web elements below?  In other words,
>     could I use a <video> to make a video of a translucent ghost fly
>     around a web page?  Is that the kind of thing we're going for here?
>       canvas -> track -> <video> -> ghost?
> 
>     On Wed, Feb 24, 2016 at 8:20 AM, Martin Thomson
>     <martin.thomson@gmail.com <mailto:martin.thomson@gmail.com>> wrote:
> 
>         On 24 February 2016 at 08:13, Peter Thatcher
>         <pthatcher@google.com <mailto:pthatcher@google.com>> wrote:
>         > What do you mean "throughout the media pipeline"?  Do you expect a track
>         > sent through a PeerConnection to have the alpha channel intact when it comes
>         > out the remote side?
> 
>         Well, we know that RTP video is generally incapable of handling
>         alpha
>         channels, so no.  Unless and until we find a codec that supports an
>         alpha channel (APNG perhaps).
> 
>         But the simple example of <canvas> piped to <video> presents
>         something
>         of a challenge.
> 
> 
> 
Received on Monday, 29 February 2016 11:32:18 UTC

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