How to Implement WebRTC Screen Sharing

UPDATE: THE CONTENT OF THIS SPECIFIC ARTICLE ABOUT WEBRTC SCREEN SHARING IS OLD AND SLIGHTLY INACCURATE. PLEASE LOOK FOR ANOTHER RESOURCE AS AN EXAMPLE

After voice, video, instant messaging, screencasting, presence and sending files; the only missing piece for collaboration would be screen sharing. Here's how you do it in WebRTC.

[Konstantin Goncharuk (@kkooler) is Lead Developer at Videolink2.me – a free online video calls service with instant messages and… screen sharing. He was kind enough to provide the details on how to implement screen sharing in WebRTC on Chrome.]

Last couple of years web browsers extremely extended their functions from being "viewers for web pages" to all-in-one web application frameworks. Modern web browser provides local storage, database, multi-threading, peer to peer connection and dozens of other possibilities, that give developers a chance to bring user experience from native apps to web.

WebRTC for Screen sharing

Definitely the hero of last year was WebRTC — framework that provides direct data and media stream transfer between two browsers without external server involved. It’s breakthrough for online video communication apps, since developers don’t need to support hundreds of servers that act as a bridge for user media streams.

Now they only need to exchange participants “webrtc addresses” and let them use p2p connection for data transfer.

And WebRTC is production ready! We use it as one of connection channels in Videolink2.me to provide our users with high quality video calls service in web browsers.

But what will we see with WebRTC? Maybe screen sharing?

The current version of Chrome already supports plugin-free screen sharing. While there are thoughts on moving that into a mandatory Chrome extension, the implementation will probably stay the same. Why not prepare for it by implementing screen sharing?

Let’s take a look at how screen sharing works and how difficult is it to implement it in a web app.

WebRTC to share your screen

As I mentioned above, WebRTC enables video calling between two browsers. The next step for true collaboration is to enable screen sharing as well – which is exactly what Chrome's developers decided to do.

Chrome screen sharing doesn’t allow you to manipulate the shared screen, but only view it as a video stream. And as a developer, you work with the screen's stream the same way as you work with the web camera or microphone. And use WebRTC to transmit screen sharing the same way as usual media streams.

Interested? Let’s take a closer look.

Before developing you need to enable screen sharing: open chrome://flags/#enable-usermedia-screen-capture and enable "Enable screen capture support in getUserMedia()”.

Access to media streams

To get access to user media streams we use getUserMedia function, that takes object with description of desired stream and returns it back into callback.

That’s how it looks if you want to get user web camera with audio:

navigator.getUserMedia({audio: true, video: true}, function(stream) {
  //We’ve got media stream
  }, function() {
    //Error, no stream provided
  }
)

We use the same way to get user screen but with different stream description:

navigator.getUserMedia({
	audio: false,
	video: {
   	mandatory: {
       	chromeMediaSource: 'screen',
       	maxWidth: 1280,
       	maxHeight: 720
   	},
   	optional: []
	}
}, function(stream) {
   //We've got media stream
 }, function() {
   //Error, no stream provided
 }
)

We asked browser to provide screen media stream with maximum size 1280x720 without audio. After executing the code above, Chrome will ask our permission to share the screen and if we agree — pass our screen capture as a media stream to the callback.

Adding Video Stream to WebRTC connection

As soon as we got it we can add to WebRTC connection as any usual video stream:

navigator.getUserMedia({
	audio: false,
	video: {
   	mandatory: {
       	chromeMediaSource: 'screen',
       	maxWidth: 1280,
       	maxHeight: 720
   	},
   	optional: []
	}
}, function(stream) {
   pc.addStream(stream);
 }, function() {
   //Error, no stream provided
 }
)

So the received stream can be transmitted via WebRTC the same way as web camera and easily played in <video> component on the other side.

One restriction that should be taken into consideration — screen sharing works only with SSL enabled (https://) domains.

To see it in action, let’s make a simple example that will capture she creen and play it in the video player.

Our html page will contain button to start screen sharing and video to show shared screen:

<html>
<head>
  <title>Screen sharing example</title>
  <script src="https://code.jquery.com/jquery-1.10.1.min.js"></script>
</head>
<body>
  <p align="center"><input type="button" id="share_screen" value="Share screen"/></p>
  <p align="center"><video id="video"/></p>
</body>
</html>

Now we can add handler for click on share button, that will request screen media stream (using the code above) and play it in our video element:

<script language="javascript”>
    navigator.getUserMedia = navigator.webkitGetUserMedia || navigator.getUserMedia;
	$('#share_screen').click(function() {
     	navigator.getUserMedia({
              	audio: false,
              	video: {
                  	mandatory: {
                      	chromeMediaSource: 'screen',
                      	maxWidth: 1280,
                      	maxHeight: 720
                  	},
                  	optional: []
              	}
          	}, function(stream) {
              	document.getElementById(‘video’).src = window.URL.createObjectURL(stream);
              	$('#share_screen').hide();
          	}, function() {
              	alert('Error, my friend. Screen stream is not available. Try in latest Chrome with Screen sharing enabled in about:flags.');
            	}
     	)
	})
</script>

So instead of adding a stream to peer connection, we’re setting it as video surce.

And…That’s it! Here’s a page that we’ve made:

<html>
<head>
	<title>Screen sharing example</title>
	<script src="https://code.jquery.com/jquery-1.10.1.min.js "></script>
</head>
<body>
	<p align="center"><input type="button" id="share_screen" value="Share screen"/></p>
	<p align="center"><video id="video" autoplay/></p>
	<script language="javascript">
     	navigator.getUserMedia = navigator.webkitGetUserMedia || navigator.getUserMedia;
     	$('#share_screen').click(function() {
          	navigator.getUserMedia({
                   	audio: false,
                   	video: {
                       	mandatory: {
                           	chromeMediaSource: 'screen',
                           	maxWidth: 1280,
                           	maxHeight: 720
                       	},
                       	optional: []
                   	}
               	}, function(stream) {
                   	document.getElementById('video').src = window.URL.createObjectURL(stream);;
                   	$('#share_screen').hide();
               	}, function() {
                   	alert('Error, my friend. Screen stream is not available. Try in latest Chrome with Screen sharing enabled in about:flags.');
                 	}
          	)
     	})
	</script>
</body>
</html>

When the user clicks on the “Share screen” button, click handler will request media stream and display it in video player if the permission is granted.