W3C home > Mailing lists > Public > public-media-capture@w3.org > April 2013

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

From: Robert O'Callahan <robert@ocallahan.org>
Date: Tue, 9 Apr 2013 22:26:22 +1200
Message-ID: <CAOp6jLY363A1oZOV=rvKrCahmUv9VhoCUivXG1SPi9_2GiSAOA@mail.gmail.com>
To: Harald Alvestrand <harald@alvestrand.no>
Cc: "public-media-capture@w3.org" <public-media-capture@w3.org>
On Tue, Apr 9, 2013 at 8:02 PM, Harald Alvestrand <harald@alvestrand.no>wrote:

>  On 04/09/2013 12:16 AM, Robert O'Callahan wrote:
> On Tue, Apr 9, 2013 at 12:43 AM, Stefan Håkansson LK <
> stefan.lk.hakansson@ericsson.com> wrote:
>>  All tracks that we can decode. So e.g. if you play a resource with a
>>> video track in an <audio> element and capture that to a MediaStream, the
>>> MediaStream contains the video track.
>>  What if there are two video tracks? Only one of them is selected/played
>> naturally, but in principle both could be decoded. (What I am saying is
>> that we need to spec this up).
>  Definitely. Yes, I think we should decode them both.
> Not sure I get where this is coming from....
> I see absolutely no reason to decode a video stream until we know where
> it's going.
> The destination might be another PeerConnection with a compatibly
> negotiated codec, or a hardware device with special codec support, or a
> Recorder willing to store the bytes as previoiusly encoded.
> I think we should carefully *avoid* specifying exactly where and when
> decoding takes place. Only the observable result should be in the standard.

Sorry, yes, I totally agree. I didn't mean to suggest that data should be
decompressed. I just meant to say that both video tracks should be obtained
from the resource and be present in the MediaStream.

I don't want to go there at this time.
> We seriously run the risk of defining a toolkit that is able to do a set
> of tasks that nobody wants done (because they're not satisfied with the
> result), placing a heavy burden on browser implementors for very marginal
> benefit.

> We already support putting a video on a Canvas (in a somewhat cumbersome
> way, true). Should we focus on getting the video back out of the Canvas,
> and let people play with that toolkit before we start in on defining
> smaller toolkits that might not be able to do what people really want?

I'm not sure what you mean here. I prototyped ProcessedMediaStream (as in
MSP) a while back (minus the canvas support) and it worked well. Trying to
manipulate video by drawImage'ing to a canvas and re-encoding the result to
a MediaStream is quite likely to mean "they're not satisfied with the
result" (since it's subject to main-thread latency, among other problems).
Also, at least for us the simple ProcessedMediaStream API I just suggested
is not a heavy burden, it's a very small feature compared to almost
everything else being proposed in this group.

Anyway, we don't have to specify ProcessedMediaStream or anything like it
right now, but I think it's helpful to have some shared idea of where APIs
might go because that informs the decisions we make now. In particular
people have been talking about whether and how the source of a track might
change, and features like ProcessedMediaStream are relevant to that.

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Received on Tuesday, 9 April 2013 10:26:50 UTC

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