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This module allows you to create "virtual video devices". Normal (v4l2) applications will read these devices as if they were ordinary video devices, but the video will not be read from e.g. a capture card but instead it is generated by another application. This allows you for instance to apply some nifty video effects on your Skype video... It also allows some more serious things (e.g. I've been using it to add streaming capabilities to an application by the means of hooking GStreamer into the loopback devices).
To get the main features of each new release, see the NEWS file. You could also have a look at the ChangeLog (which gets automatically generated and might only be of limited use...
For current issues, checkout https://github.com/umlaeute/v4l2loopback/issues Please use the issue-tracker for reporting any problems.
Before you create a new ticket in our issue tracker, please make sure that you have read this document and followed any instructions found within.
Also, please search the issue-tracker before reporting any problems: it's much better to add your information to an existing ticket than to create a new ticket with essentially the same information.
The issue tracker is meant to track specific bugs in the code (and new features). However, it is ill-suited as a user support forum.
If you have general questions or problems, please use the v4l2loopback tag
on Unix & Linux instead:
https://unix.stackexchange.com/questions/tagged/v4l2loopback
In order to build (compile,...) anything, you must have a working build-environment (compiler, GNU make,...). The kernel can be somewhat picky if you try to load a module that was compiled with a different compiler as was used to compile the kernel itself. So make sure to have the right compiler in place.
The v4l2loopback module is a kernel module.
In order to build it, you must have the kernel headers installed that match
the linux kernel with which you want to use the module (in most cases this will
be the kernel that you are currently running).
Please note, that kernel headers and kernel image must have exactly the same version.
For example, 3.18.0-trunk-rpi is a different version than 3.18.7-v7+, even though
the first few numbers are the same.
(Modules will be incompatible if the versions don't match. If you are lucky, the module will
simply refuse to load. If you are unlucky, your computer will spit in your eye or do worse.)
There are distribution-specific differences on how to get the correct kernel headers
(or to install a compilation toolchain).
Documenting all those possibilities would go far beyond the scope of v4l2loopback.
Please understand that we cannot provide support for questions regarding dependencies.
To build the kernel module, run:
$ make
This should give you a file named "v4l2loopback.ko", which is the kernel module
You cannot load a module built for a specific version of the kernel into another version of the kernel. So, if you have successfully built the module previously and have updated your kernel (and the matching headers) In the meantime, you really must clean the build before re-compiling the module. So run this before starting the build again:
$ make clean
To install the module, run "make install" (you might have to be 'root' to have all necessary permissions to install the module).
If your system has "sudo", do:
$ make && sudo make install
$ sudo depmod -a
If your system lacks "sudo", do:
$ make
$ su
(enter root password)
# make install
# depmod -a
# exit
(The depmod -a call will re-calculate module dependencies, in order to
automatically load additional kernel modules required by v4l2loopback.
The call may not be necessary on modern systems.)
See below for distribution-specific build instructions
or when using frameworks like DKMS.
Load the v4l2loopback module as root :
# modprobe v4l2loopback
Using sudo use:
$ sudo modprobe v4l2loopback
This will create an additional video-device, e.g. /dev/video0 (the number depends on whether you already had video devices on your system), which can be fed by various programs. Tested feeders:
In theory most programs capable of writing to a v4l2 device should work.
The data sent to the v4l2loopback device can then be read by any v4l2-capable application.
You can find a number of scenarios on the wiki at http://github.com/umlaeute/v4l2loopback/wiki
If you have a secure-boot enabled kernel, you might not be able to simply build a kernel module and insert it. (You will get SSL errors when building the module.) This is actually a security feature (as it prevents malicious code to be inserted into kernel-space).
If you are not allowed to insert the kernel module (running modprobe, or insmod), you have a few options
(consult your distribution's documentation on how to perform any of these steps):
You could also just try building the module via DKMS, and hope that it does all the magic for you.
If you need several independent loopback devices, you can pass the "devices" option, when loading the module; e.g.
# modprobe v4l2loopback devices=4
Will give you 4 loopback devices (e.g. /dev/video1 ... /dev/video5)
You can also specify the device IDs manually; e.g.
# modprobe v4l2loopback video_nr=3,4,7
Will create 3 devices (/dev/video3, /dev/video4 & /dev/video7)
# modprobe v4l2loopback video_nr=3,4,7 card_label="device number 3","the number four","the last one"
Will create 3 devices with the card names passed as the second parameter:
/dev/video3 -> device number 3
/dev/video4 -> the number four
/dev/video7 -> the last one
If you encounter problems detecting your device with Chrome/WebRTC you can try 'exclusive_caps' mode:
# modprobe v4l2loopback exclusive_caps=1
This will enable 'exclusive_caps' mode that only reports CAPTURE/OUTPUT capabilities exclusively. The newly created device will announce OUTPUT capabilities only (so ordinary webcam applications (including Chrome) won't see it). As soon as you have attached a producer to the device, it will start announcing CAPTURE capabilities only (so applications that refuse to open devices that have other capabilities apart from capturing can open it too.)
Options that you provided when loading the module (e.g. via modprobe) cannot be easily changed
on the fly.
In order to change these options, you must first unload the module with rmmod
(which will only work if no application is any longer accessing one of the loopback devices)
and then load it again (with the new options).
See also the section about DYNAMIC DEVICE MANAGEMENT.
you can set and/or query some per-device attributes via sysfs, in a human
readable format. See /sys/devices/virtual/video4linux/video*/
also there are some V4L2 controls that you can list with
$ v4l2-ctl -d /dev/video0 -l
keep_format(0/1): while set to 1, once negotiated format will be fixed forever,
until the setting is set back to 0sustain_framerate(0/1): if set to 1, nominal device fps will be ensured by means
of frame duplication when neededtimeout(integer): if >0, will cause a timeout picture (a null frame, by default)
to be displayed after (value) msecs of missing inputtimeout_image_io(0/1): if set to 1, the next opener will write to timeout frame
buffer$ v4l2loopback-ctl set-fps /dev/video0 25
or
$ echo '@100' | sudo tee /sys/devices/virtual/video4linux/video0/format
$ v4l2loopback-ctl set-caps /dev/video0 "UYVY:640x480"
Please note that GStreamer-style caps (e.g. video/x-raw,format=UYVY,width=640,height=480) or no longer supported!
You can define a timeout (in milliseconds), after which the loopback device will start outputting NULL frames, if the producer suddenly stopped.
$ v4l2-ctl -d /dev/video0 -c timeout=3000
Alternatively, you can calso provide a timeout-image, which will be displayed (instead of the NULL frames), if the producer doesn't send any new frames for a given period:
$ v4l2loopback-ctl set-timeout-image -t 3000 /dev/video0 service-unavailable.png
(this currently requires GStreamer 1.0 installed)
You can create (and delete) loopback devices on the fly, using the add (resp. delete) commands of the v4l2loopback-ctl utility.
When creating a new device, module options might be ignored. So you must specify them explicitly.
To create a new device /dev/video7 that has a label "loopy doopy", use:
$ sudo v4l2loopback-ctl add -n "loopy doopy" /dev/video7
Deleting devices is as simple as:
$ sudo v4l2loopback-ctl delete /dev/video7
The original module has been developed for linux-2.6.28; I don't have a system with such an old kernel anymore, so I don't know whether it still works. Further development has been done mainly on linux-2.6.32 and linux-2.6.35, with newer kernels being continually tested as they enter Debian.
Support:
v4l2loopack is now (since 2010-10-13) available as a Debian-package. https://packages.debian.org/source/stable/v4l2loopback
This means, that it is also part of Debian-derived distributions, including Ubuntu (starting with natty). The most convenient way is to install the package "v4l2loopback-dkms":
# aptitude install v4l2loopback-dkms
This should automatically build and install the module for your current kernel (provided you have the matching kernel-headers installed). Another option is to install the "v4l2loopback-source" package. In this case you should be able to simply do (as root):
# aptitude install v4l2loopback-source module-assistant
# module-assistant auto-install v4l2loopback-source
The Dynamic Kernel Module Support framework (DKMS) is designed to allow individual kernel modules to be upgraded without changing the whole kernel. It is also very easy to rebuild modules as you upgrade kernels.
If your distribution doesn't provide v4l2loopback-packages (or they are too old)
and you are experiencing troubles with code-signing, you probably should try this.
E.g. to build the v4l2loopback-v0.12.5 (but check the webpage for newer releases first!),
use something like the following (you might need to run the dkms commands as superuser/root):
version=0.12.5
# download and extract the tarball (tar requires superuser privileges)
curl -L https://github.com/umlaeute/v4l2loopback/archive/v${version}.tar.gz | tar xvz -C /usr/src
# build and install the DKMS-module (requires superuser privileges)
dkms add -m v4l2loopback -v ${version}
dkms build -m v4l2loopback -v ${version}
dkms install -m v4l2loopback -v ${version}
| distribution | dependencies |
|---|---|
| Fedora,... | gcc kernel-devel dkms |
| Debian, Ubuntu,... | dkms |
One can avoid manually loading the module by letting systemd load the module
at boot, by creating a file /etc/modules-load.d/v4l2loopback.conf with just
the name of the module:
v4l2loopback
This is especially convenient when v4l2loopback is installed with DKMS or with
a package provided by your Linux distribution.
If needed, one can specify default module options by creating
/etc/modprobe.d/v4l2loopback.conf in the following form:
options v4l2loopback video_nr=3,4,7
options v4l2loopback card_label="device number 3,the number four,the last one"
One can only add one option per line. These options also become the defaults when
manually calling modprobe v4l2loopback. Note that the double quotes can only
be used at the beginning and the end of the option's value, as opposed to when
they are specified on the command line.
If your system boots with an initial ramdisk, which is the case for most
modern distributions, you need to update this ramdisk with the settings above,
before they take effect at boot time. In Ubuntu, this image is updated with
sudo update-initramfs. The equivalent on Fedora is sudo dracut -f.
The most up-to-date version of this module can be found at http://github.com/umlaeute/v4l2loopback/.
Copyright (c) 2010-2021 IOhannes m zmoelnig
Copyright (c) 2016 Gavin Qiu
Copyright (c) 2016 George Chriss
Copyright (c) 2014-2015 Tasos Sahanidis
Copyright (c) 2012-2015 Yusuke Ohshima
Copyright (c) 2015 Kurt Kiefer
Copyright (c) 2015 Michel Promonet
Copyright (c) 2015 Paul Brook
Copyright (c) 2015 Tom Zerucha
Copyright (c) 2013 Aidan Thornton
Copyright (c) 2013 Anatolij Gustschin
Copyright (c) 2012 Ted Mielczarek
Copyright (c) 2012 Anton Novikov
Copyright (c) 2011 Stefan Diewald
Copyright (c) 2010 Scott Maines
Copyright (c) 2009 Gorinich Zmey
Copyright (c) 2005-2009 Vasily Levin
This package is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This package is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.
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