v4l2loopback - a kernel module to create V4L2 loopback devices

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).

NEWS

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...

ISSUES

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.

SEEKING HELP

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 Stack Overflow instead: https://stackoverflow.com/questions/tagged/v4l2loopback

DEPENDENCIES

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 than 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 that 3.18.7-v7+, even though the first few number 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 understnd that we cannot provide support for questions regarding dependencies.

BUILD

to build the kernel module run:

$ make

this should give you a file named "v4l2loopback.ko", which is the kernel module

INSTALL

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.)

RUN

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:

GStreamer-1.0: using the "v4l2sink" element
Gem(>=0.93) using the "recordV4L2" plugin 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

OPTIONS

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

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.)

ATTRIBUTES

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/video1 -l

keep_format(0/1): while set to 1, once negotiated format will be fixed forever, until the setting is set back to 0
sustain_framerate(0/1): if set to 1, nominal device fps will be ensured by means of frame duplication when needed
timeout(integer): if >0, will cause a timeout picture (a null frame, by default) to be displayed after (value) msecs of missing input
timeout_image_io(0/1): if set to 1, the next opener will write to timeout frame buffer

FORCING FPS

$ v4l2loopback-ctl set-fps 25 /dev/video0

$ echo '@100' | sudo tee /sys/devices/virtual/video4linux/video0/format

FORCING A GSTREAMER (1.0) CAPS

$ v4l2loopback-ctl set-caps "video/x-raw,format=UYVY,width=640,height=480" /dev/video0

SETTING STREAM TIMEOUT

$ v4l2-ctl -d /dev/video0 -c timeout=3000
(will output null frames by default)
$ v4l2loopback-ctl set-timeout-image service-unavailable.png /dev/video0
this currently requires GStreamer 1.0 installed

KERNELs

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:

= 5.0.0 should work
= 4.0.0 should work
= 3.0.0 might work
<< 3.0.0 may work (has not been tested in ages)
<= 2.6.27 will definitely NOT work

DISTRIBUTIONS

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

DOWNLOAD

the most up-to-date version of this module can be found at http://github.com/umlaeute/v4l2loopback/.

LICENSE/COPYING

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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