W3C home > Mailing lists > Public > public-webrtc@w3.org > March 2012

Re: getUserMedia API: How to use the LocalMediaStream object ?

From: Adam Bergkvist <adam.bergkvist@ericsson.com>
Date: Mon, 5 Mar 2012 13:42:32 +0100
Message-ID: <4F54B4B8.2030506@ericsson.com>
To: Harald Alvestrand <harald@alvestrand.no>
CC: Randell Jesup <randell-ietf@jesup.org>, "public-webrtc@w3.org" <public-webrtc@w3.org>, "ludovic.vue@gmail.com" <ludovic.vue@gmail.com>
On 03/05/2012 11:03 AM, Harald Alvestrand wrote:
> On 03/05/2012 06:33 AM, Randell Jesup wrote:
>> Paste from the discuss-webrtc mailing list:
>> On 3/4/2012 12:58 PM, Ludo wrote:
>>> I am trying to use the getUserMedia API, but i get a problem at the
>>> moment to insert the stream object into the balise video.
>>> Here us the main part:
>>> <video autoplay id="test"></video>
>>>    navigator.getUserMedia('video', function(localMediaStream)
>>>    {
>>>      $('#test').attr('src',
>>> window.URL.createObjectURL(localMediaStream));
>>>     });
>>> I use opera Labs Camera whose support the API, but i get this error:
>>> "Uncaught exception: TypeError: Cannot convert 'window.URL' to object".
>>> I also tried without this method:
>>> <video autoplay id="test"></video>
>>>    navigator.getUserMedia('video', function(localMediaStream)
>>>    {
>>>      $('#test').attr('src', localMediaStream);
>>>     });
>>> The stream object is into the balise video but nothing happens...
>> I'm not sure why it doesn't work in Opera, but I will note that
>> media_element.src = MediaStream is the API we at Mozilla would prefer
>> to see adopted as a standard.  I'll let others here make the real
>> argument (roc in particular is on vacation, but I know this is his
>> preference), but I'll say that it's a much more obvious usage to the
>> application programmer (and the above is evidence to that for me), and
>> there may be some other advantages as well - perhaps easier tracking
>> of possible consumers of the output of a MediaStream object, while
>> createObjectURL() produces a string and thus you can't really know
>> when it might be used.
> I would also prefer the media_element.src = MediaStream.
> My second preference is to make createObjectURL() a function on the
> mediastream, rather than a free-floating function; I was writing some
> tests over the weekend, and realized that the fact that
> createObjectURL() is a free-floating function is *horrible* if you want
> to do testing with mocks.

I agree that createObjectURL()/revokeObjectURL() isn't the simplest API 
to use. However, it solves a problem in the web platform that isn't 
unique to WebRTC and MediaStream. The API is adopted from the File API 
where it's used to create URLs to Blobs. It can, e.g., be used together 
with a <input type="file"> element to select a image file and show it 
(without uploading the image to the server first).

var file = filePicker.files[0];
// profilePic is an <img>
profilePic.src = URL.createObjectURL(file);

Combine a <input type="file"> with a video element and you have a media 

I think we may need to support createObjectURL() to be consistent with 
the rest of the Web platform. The question is rather if we think it's 
necessary to have a second approach more in line with "vidio.src = stream".

Received on Monday, 5 March 2012 12:48:15 UTC

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