Jan Grulich

How to enable and use screen sharing on Wayland

Two days ago I wrote about our work on screen sharing in web browsers. While there was a lot of work done recently on this area, it’s not still in the state where everything would just work out of the box. There are few steps necessary to make this work for you and here is a brief summary what you need. This is not a distro specific how to, but given I use Fedora 28 and I know that everything you need is there, it’s most likely you will need to figure out the differences for your distribution or build it yourself.

PipeWire

PipeWire is the core technology used behind all of this. In Fedora you just need to install it, it’s available for Fedora 27 and newer. Once PipeWire is installed, you can just start it using “pipewire” command. If you want to see what’s going on, you can use “PIPEWIRE_DEBUG=4 pipewire”  to start PipeWire with debug information. For Fedora 29, there is a feature planned for PipeWire which should make it to start automatically.

Xdg-desktop-portal and xdg-desktop-portal-[kde,gtk]

We use xdg-desktop-portal (+ backend implementation) for communication between the app requesting to share a screen and between desktop (Plasma or Gnome). You need xdg-desktop-portal, which is the middle man between the app and backend implementation, compiled with screencast portal. This portal will be build automatically when PipeWire is present during the build. In Fedora you should be already covered when you install it. For backend implementation, if you are using Plasma, you need xdg-desktop-portal-kde from Plasma 5.13.x, again compiled with screencast portal, which is build when PipeWire is present. For Fedora 28+, you can use this COPR repository and you are ready to go. I highly recommend using Plasma 5.13.2, where I have some minor fixes and if you have a chance, try to compile upcoming 5.13.3 version from git (Plasma/5.13 branch), as I rewrote how we connect to PipeWire. Previously our portal implementation worked only when PipeWire was started first, now it shouldn’t matter. If you use Gnome, you can just install xdg-desktop-portal-gtk from Fedora repository or build it yourself. You again need to build screencast portal.

Enabling screen sharing in your desktop

Both Plasma and Gnome need some adjustments to enable screen sharing, as in both cases it’s an experimental feature. For Gnome you can follow this guide, just enable screen-cast feature using gsettings. For Plasma, you need to get KWin from Plasma 5.13.x, which is available for Fedora in the COPR repository mentioned above. Then you need to set and export KWIN_REMOTE=1 env variable before KWin starts. There is also one more thing needed for Gnome at this moment, you need to backport this patch to Mutter, otherwise it won’t be able to match PipeWire stream configuration with the app using different framerate, e.g. when using Firefox.

Edit: It seems that exporting KWIN_REMOTE=1 is not necessary, it probably was only during the time when this feature was not merged yet. Now it should work without it. You still need KWin from Plasma 5.13.

Start with screen sharing

Now you should be all set and ready to share a screen on Gnome/Plasma Wayland session. You can now try Firefox for Fedora 28 or Rawhide from this COPR repository. For Firefox there is a WebRTC test page, where you can test this screen share functionality. Another option is to use my  test application for Flatpak portals or use gnome-remote-desktop app.

Edit: I didn’t realize that not everyone knows about xdg-desktop-portal or PipeWire, below are some links where you can get an idea what is everything about. I should also mention that while xdg-desktop-portals is primarily designed for flatpak, its usage has been expanded over time as it perfectly makes sense to use it for e.g. Wayland, where like in sandbox, where apps don’t have access to your system, on Wayland apps don’t know about other apps or windows and communication can by done only through compositor.

 

Screen sharing in Plasma wayland session

One of the important missing features in Plasma wayland session is without a doubt possibility to share your screen or record you screen. To support this you need help of the compositor and somehow deliver all needed information to the client (application), in ideal way something what can be used by all DEs, such as Gnome. Luckily, this has been one of the primary goals of Pipewire, together with support for Flatpak. If you haven’t heard about Pipewire, it’s a new project that wants to improve audio and video handling in Linux, supporting all the usecases handled by PulseAudio and providing same level of handling for video input and output. With Pipewire supporting this, there was recently a new API added to xdg-desktop-portal for screen cast support and also for remote desktop. Using this API, applications can now have access to your screen content on Wayland sessions or in case they are running in sandbox. With various backend implementation, like xdg-desktop-portal-kde or xdg-desktop-portal-gtk, they just need to support one API to target all desktops. Screen cast portal works the way, that the client first needs to create a session between him and xdp (xdg-desktop-portal) backend implementation, user then gets a dialog with a screen he would like to share and starts screen sharing. Once he does that, xdp backend implementation creates a Pipewire stream, sends back response to the client with stream id and then client can connect to that stream and get its content. Once he no longer requests content of the selected stream, xdp backend implementation gets information that nobody is longer connected to the created Pipewire stream and can stop sharing screen information and xdp backend implementation is again ready to accept next requests for screen sharing. This is all happening in the background so there is really no cool picture I can show, at least this dialog which you get when you request to share a screen.

I finished support for screen cast portal in xdg-desktop-portal-kde last week and currently waiting for it to pass review and be merged to master. This is also currently blocked by two not merged reviews, one adding support for sending GBM buffers from KWin and one with new Remote Access Manager interface in KWayland, both authored by Oleg Chernovskiy, for which I’m really greatful. This all will hopefully land soon enough for Plasma 5.13. Testing this is currently a bit complicated as you need everything compiled yourself and besides my testing application there is really no app using this, except maybe Gnome remote desktop, but there should be support in future for this in Krfb, Chrome or in Firefox. Hopefully soon enough.

Last thing I would like to mention is for GSoC students. We also need remote desktop portal support to have full remote desktop experience so I decided to propose this as a GSoC idea so students can choose this interesting stuff as their GSoC work.

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