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104 lines
3.6 KiB
Markdown
104 lines
3.6 KiB
Markdown
# V4L2
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V4L2 is the second version of
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[Video4Linux](https://www.linuxtv.org/wiki/index.php/Main_Page).
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It functions as a driver for [webcams](/wiki/webcams.md) and [tv tuner cards](/wiki/video.md) and
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other devices.
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## v4l2loopback
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The following section is based on a
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[gist by ioquatix](https://gist.github.com/ioquatix/18720c80a7f7eb997c19eef8afd6901e).
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[v4l2loopback](https://github.com/umlaeute/v4l2loopback) is a utility to create
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a virtual video device.
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One use of it is in [OBS Virtual Camera](/wiki/obs.md#virtual-camera).
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### Setup
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To use v4l2loopback an according
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package needs to be installed.
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Some distributions have a package named `v4l2loopback-dkms` in the package
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manager, making the manual installation process unnecessary.
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Make sure that both `dkms` and `linux-headers` packages are installed aswell.
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### Usage
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This section focusses on various use-cases for the v4l2loopback software.
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### Create Temporary V4L2 Devices
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v4l2loopback can be manually started by running:
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```sh
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modprobe v4l2loopback card_label='V4L2 Loopback' video_nr=7 exclusive_caps=1
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```
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`video_nr=7` states the device path to use with v4l2loopback - in this case
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`/dev/video7`.
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This can be changed according to the needs.
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It is important that the device is not already in use by another application.
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To create more than one device more labels or numbers can be appended.
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An example command with 2 V4L2 video devices will look like the following command.
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```sh
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modprobe v4l2loopback card_label='OBS Virtual Camera','IP Webcam' video_nr=7,8 exclusive_caps=1
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```
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### Create Permanent V4L2 Devices
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For an automatic start at boot via [SystemD](./systemd.md) a service can be
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created - as described in [the SystemD entry](./systemd.md#run-command-on-boot).
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For v4l2loopback create the file `/etc/systemd/system/v4l2loopback.service`
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containing the following lines:
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```txt
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[Unit]
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Description=V4L2 Loopback
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[Service]
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Type=simple
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RemainAfterExit=yes
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ExecStart=/bin/modprobe v4l2loopback card_label='OBS Virtual Camera' video_nr=7 exclusive_caps=1
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ExecStop=/bin/rmmod v4l2loopback
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[Install]
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WantedBy=default.target
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```
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Note the line starting with `ExecStart=/bin/` is the same line described in the
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[above section](#create-temporary-v4l2-devices).
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Make sure to change it if needed.
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Afterwards enable the service to start at every boot process aswell as running it by using the
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following command:
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```sh
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sudo systemctl enable v4l2loopback
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sudo systemctl start v4l2loopback
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```
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## Troubleshooting
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This section focusses on various errors that can appear in the v4l2loopback software.
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### `Failed to start streaming on '/dev/video2' (Invalid argument)`
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This error does occur most of the time when trying to start
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[OBS' virtual camera](/wiki/obs.md#virtual-camera).
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This section is based on an
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[OBS Forum entry by rebelduck](https://obsproject.com/forum/threads/obs-virtual-camera-failed-to-start-streaming-on-dev-video2-invalid-argument.184717/)
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To fix it downgrade the packages `v4l2loopback-dkms` and `v4l2loopback-utils` to the last working
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version.
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For [Arch Linux](/wiki/linux/arch-linux.md) this can be achieved by using
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[Downgrade](/wiki/linux/package_manager.md#downgrading-packages).
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### `Error! Bad return status for module build on kernel`
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When updating the `v4l2loopback` package this error can occur after the creation of the
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[initramfs](/wiki/linux/mkinitcpio.md).
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This may point to an incompatibility of the package and kernel version which can only but easily
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and temporarily be fixed by downgrading the package as described in
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[the corresponding package manager section](/wiki/linux/package_manager.md#downgrading-packages).
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