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Re: Blobs and MediaStreams (Re: getUserMedia API: How to use the LocalMediaStream object ?)

From: Rich Tibbett <richt@opera.com>
Date: Wed, 07 Mar 2012 18:00:14 +0100
Message-ID: <4F57941E.9000907@opera.com>
To: Adam Bergkvist <adam.bergkvist@ericsson.com>
CC: Randell Jesup <randell-ietf@jesup.org>, "robert@ocallahan.org" <robert@ocallahan.org>, "public-webrtc@w3.org" <public-webrtc@w3.org>, "Robert O'Callahan" <roc@ocallahan.org>, Chris Pearce <cpearce@mozilla.com>
Rich Tibbett wrote:
> Adam Bergkvist wrote:
>> On 03/07/2012 05:59 AM, Randell Jesup wrote:
>>> On 3/6/2012 6:21 PM, Robert O'Callahan wrote:
>>>>
>>>> I do want to get away from URL.createObjectURL(MediaStream) for
>>>> setting the src for a <video> element, and I think there's a
>>>> bigger opportunity here to unify media handling in a way that will
>>>> make future work easier.
>>>>
>>>> The MediaStreams Processing spec extends media elements' "src"
>>>> attribute so you can set it directly to a MediaStream: "video.src =
>>>> stream;". I've implemented that in my patches. It also adds an API to
>>>> capture a MediaStream from a media element, e.g. "stream =
>>>> video.captureStreamUntilEnded()".
>>>
>>> Right - I was forgetting the details of the Processing API proposal;
>>> that's what I was really suggesting here - we should use that portion of
>>> the MediaStream Processing API. To quote myself in the message that
>>> kicked off this discussion:
>>>
>>> URLs don't help us solve other problems (and add new ones);
>>> MediaStreams are a mechanism that lets us solve a whole class of
>>> problems (and encapsulate utility/semantics such as audio/video
>>> synchronization and multiple/alternate tracks).
>>
>> My objections to the "video.src = stream" approach pretty much goes away
>> if it would be possible to let the src-property be of type "any" (as
>> Robert suggests in the MediaStream Processing API). I'm not really sure
>> what the consequences are though.
>
> The following consequences were brought up in other recent discussions
> elsewhere:
>
> http://lists.w3.org/Archives/Public/public-webapps/2011OctDec/1522.html
>
> http://lists.w3.org/Archives/Public/public-webapps/2011OctDec/1516.html
>
> These problems are solvable and some good follow up emails follow the
> links highlighted above. I think this group or the MediaStream
> Processing API account for the concerns raised in these threads
> (particularly in the case of the latter since it already extends media
> elements to accept a type of 'any').

s/account for the concerns/should account for the concerns/

>
>>
>> It's pretty much the src attribute as a DOMString that I see issues
>> with. E.g.
>>
>> video1.src = stream;
>> log(video1.src); // "about:MediaStream"
>> video2.src = video1.src; // ??
>
> We actually produced a snapshot spec of our first implementation here
> describing how this worked:
>
> http://people.opera.com/richt/release/specs/device/o-device.html
>
> To summarize, a MediaStream object that was applied to a video.src
> simply stringified to 'about:streamurl'. So in the example above
> video2.src = video1.src would simply copy the actual object assignment
> from video1 to video2 and then log(video2.src) would also simply return
> 'about:streamurl'.
>
>>>>
>>>> It would also make sense to extend the "src" attribute to directly
>>>> accept Blob objects as well. The behavior is just like assigning to
>>>> the result of createObjectURL, except it has better GC behavior since
>>>> you don't have to manually revoke the URL.
>
> At the time of our (Opera's) initial implementation, given the challenge
> of assigning media streams to video elements, we went with the simplest
> approach possible: to assign MediaStream objects directly to video.src.
> That was before URL.createObjectURL existed.
>
> We will be implementing URL.createObjectURL and users will be able to
> assign Stream URLs to video.src as per the current proposal. The
> question really is whether we will then remove direct stream assignment
> or keep it as a developer-friendly optimisation. That probably depends
> on working out the issues referenced in the email links I provided above
> but it's an optimization that seems within reach for us.
>
> A number of folks from Opera have been arguing to keep this optimization
> since it seems relatively clean and straight-forward.
>
> - Rich
Received on Wednesday, 7 March 2012 17:00:56 GMT

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