I've found some guides which say that this used to be complicated, but is now as simple as
sudo apt install obs-studio && sudo apt install v4l2loopback-dkms
This does indeed give me a "Start Virtual Camera" button on OBS Studio. When I click it, I am prompted for a password. And that's it. Nothing else happens. The button still says "Start Virtual Camera" (it should change to "Stop Virtual Camera"). And no virtual camera is detected by Firefox, Zoom, or Cheese.
Am I doing something wrong?
In my case (Ubuntu 21.04), this is related to the Secure Boot.
Ange1ique
's answer here worked for me. I will copy it to here, and add a clarification in case it helps others:uninstall
v4l2loopback
with:sudo apt-get remove --auto-remove v4l2loopback-dkms
install
v4l2loopback
again with:sudo apt-get install v4l2loopback-dkms
You will get a message about 'Configuring Secure Boot', which offers you the opportunity to create a key for this module. Read carefully and continue with tab(s) and an enter. Set the key/password and reboot.
On rebooting enter the key/password in the
Enroll MOK
section, and continue the restart.enter this in the terminal:
sudo modprobe v4l2loopback
(don't know if this is necessary, but this time I didn't get an error)now install OBS or just run it if already installed.
It turned out that disabling secure boot allowed modprobe to get the virtual camera to work. There may be another way to work this without disabling secure boot, but that worked for me. https://askubuntu.com/a/1198351/652 is the guide to disabling secure boot.
I found a solution months ago on an OBS forum for the same problem which involved reinstalling OBS from ppa.
I also added some packages at the end to be sure no v4l2loopback package is left behind. In the case you don't have OBS installed with flatpak, just make sure these packages are installed and up to date:
Sometimes applications like Firefox or OBS Studio, after exit, keep the kernel module v4l2loopback open.