Jan Grulich

Screen sharing support in WebRTC for Wayland sessions

Last time I wrote about possibility to share a screen of Plasma wayland session, using xdg-desktop-portal and our xdg-desktop-portal-kde backend implementation. Problem was that during that time, there was no application which would implement support for this, leaving my previous effort useless so far. Luckily, this should change pretty soon. I, together with my Red Hat collegues Tomáš Popela and Eike Rathke, have been working for past few weeks on bringing support for screen sharing on Wayland to web browsers. All modern browsers use WebRTC for all audio-video communication, including screen sharing, meaning that in a perfect world, just one implementation would be needed, which is not that exactly this case. Let’s go a bit into the details first.

Each system (Windows, Mac and X11) in WebRTC reimplements an abstract class called DesktopCapturer, which defines API used by applications to support screen sharing. For our wayland support, we started with a new implementation using Pipewire as the core technology used for screen content delivery and for the communication part, to request which screen to share and to obtain Pipewire stream information, we use xdg-desktop-portal, providing simple API to do so. Advantage of using xdg-desktop-portal is that it will work also in sandbox (Flatpak and Snap) and that there is support in Plasma (using xdg-desktop-portal-kde backend) and support for Gnome (using xdg-desktop-portal-gtk backend), both using same API. Using our desktop capturer implementation, WebRTC starts to communicate with xdg-desktop-portal, we set up a session associated with our request, we tell xdg-desktop-portal that we have interest in screen sharing so xdg-desktop-portals asks your backend implementation to provide a dialog to select a screen to share and once this is settled, we request to start screen sharing and backend implementation will set up Pipewire stream, sending us back file descriptor of a Pipewire remote which we can open and connect to it. Once we are connected, we finally start receiving buffers from Pipewire with screen data and providing them to applications. This so far sounds simple and that the work is basically done, but this is unfortunately not an ideal world.

We found out that e.g. Firefox uses some older copy of WebRTC, so while working on WebRTC trunk, to have our work ready for upstream, we had to modify slightly our changes for Firefox older copy in order to be able to test our changes. There is also one thing we haven’t figured out yet, the thing is that Firefox has its own dialog to select a screen (for other capturer implementations) and we were not able to avoid displaying it when our capturer is used. Another thing is Chromium, which seems to be using WebRTC in a different way when compared to Firefox as Chromium is using plugins so this is also something we have to figure out. There are probably still plenty of other things to do before all of this can be upstream, but we have made great progress on this so far. We even had couple of Bluejeans calls where both sides could share their screens, one running Gnome Wayland session and me running Plasma Wayland session.

And for those who like adventures, I have set up a Fedora COPR repository (currently building), with Firefox containing our changes so you can test it yourself, we will need testers soon or later anyway. You just need to make sure that Pipewire is running, otherwise this won’t work, portals (both xdg-desktop-portal and backend implementation) should be started automatically.

Screenshot of Firefox in action:

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