OBS v4l2 plugin can't see the loopback camera device

[ali1234]

When selecting a v4l2 input device, only my real cameras are listed.

This is not a permissions problem - that would also prevent the gstreamer plugin from working. The virtual camera also has no problems working with mpv or web browsers, skype etc.

This can be worked around within OBS by using the gstreamer plugin instead of v4l2. Then use a pipeline like this:

v4l2src device=/dev/video10 ! video.

I realise you probably can't fix this but I think it would be helpful to add the workaround to the documentation.

[phlash]

Thanks! Looks like this is a result of how the OBS v4l2 plugin finds inputs:
https://github.com/obsproject/obs-studio/blob/7ae6f328a1784954b96abcb8d4247207251192df/plugins/linux-v4l2/v4l2-input.c#L337
[edit to correct poor assumption!]
in particular it enumerates /sys/class/video4linux which only contains physical devices. Worth reporting as a bug to OBS?
/sys/class/video4linux contains my virtual devices (once I have loaded the v4l2loopback module! Not sure why OBS doesn't offer these as inputs.. testing

In case you haven't found it, I have an experimental OBS plugin that uses backscrub, it works for me..
https://github.com/phlash/obs-backscrub

[phlash]

So, I've tried this myself, and OBS only lists inputs with a valid video stream at the time it is started. If I have backscrub running, then I can select my virtual camera and it all just works. If I don't have backscrub running, then I only get the option of physical camera.

[ali1234]

I tried starting backscrub before OBS and it did not make any difference. It is still not listed in the list of inputs. I am using 27.0.1.

[phlash]

OK, reopening this as something is different ❓

[phlash]

Righto - I'm running OBS 26.1.2 (stock Debian 11 / stable), here's what I have configured and see:

• v4l2 configuration:

 (/etc/modules-load.d/v4l2loopback)
v4l2loopback

(/etc/modprobe.d/v4l2loopback-options)
options v4l2loopback video_nr=10,11 exclusive_caps=1 card_label="Virtual Camera,Test Loopback"

this gives me four devices at boot, two from my camera (/dev/video0, /dev/video1) and two virtual (/dev/video10, /dev/video11)

• the following sym-links appear in /sys/class/video4linux:

rwxrwxrwx 1 root root 0 Aug 29 18:43 video0 -> ../../devices/pci0000:00/0000:00:14.0/usb1/1-6/1-6:1.0/video4linux/video0
lrwxrwxrwx 1 root root 0 Aug 29 18:43 video1 -> ../../devices/pci0000:00/0000:00:14.0/usb1/1-6/1-6:1.0/video4linux/video1
lrwxrwxrwx 1 root root 0 Aug 29 18:43 video10 -> ../../devices/virtual/video4linux/video10
lrwxrwxrwx 1 root root 0 Aug 29 18:43 video11 -> ../../devices/virtual/video4linux/video11

• I start backscrub as follows, input from first camera device, output to first virtual device:

% backscrub -c /dev/video0 -v /dev/video10

• Running OBS from a terminal (so I can see the logs):

% obs
...(add a V4L2 source)
info: v4l2-input: Start capture from 
error: v4l2-input: Unable to open device
error: v4l2-input: Initialization failed
info: User added source 'Video Capture Device (V4L2)' (v4l2_input) to scene 'Scene'
info: v4l2-input: /dev/video1 seems to not support video capture
libv4l2: error getting pixformat: Invalid argument
info: v4l2-input: Unable to open /dev/video11
info: v4l2-input: Found device 'Integrated Camera: Integrated C' at /dev/video0
info: v4l2-input: Found device 'Virtual Camera' at /dev/video10
info: v4l2-input: Found input 'Camera 1' (Index 0)
info: v4l2-controls: setting default for Power Line Frequency to 1
info: v4l2-controls: setting default for Exposure, Auto to 3
info: v4l2-input: Pixelformat: Motion-JPEG (unavailable)
info: v4l2-input: Pixelformat: YUYV 4:2:2 (available)
info: v4l2-input: Pixelformat: RGB3 (Emulated) (unavailable)
info: v4l2-input: Pixelformat: BGR3 (Emulated) (available)
info: v4l2-input: Pixelformat: YU12 (Emulated) (available)
info: v4l2-input: Pixelformat: YV12 (Emulated) (available)
info: v4l2-input: Stepwise and Continuous framesizes are currently hardcoded
info: v4l2-input: Stepwise and Continuous framerates are currently hardcoded
info: v4l2-input: Pixelformat: Motion-JPEG (unavailable)
info: v4l2-input: Pixelformat: YUYV 4:2:2 (available)
info: v4l2-input: Pixelformat: RGB3 (Emulated) (unavailable)
info: v4l2-input: Pixelformat: BGR3 (Emulated) (available)
info: v4l2-input: Pixelformat: YU12 (Emulated) (available)
info: v4l2-input: Pixelformat: YV12 (Emulated) (available)
info: v4l2-input: Stepwise and Continuous framesizes are currently hardcoded
info: v4l2-input: Stepwise and Continuous framerates are currently hardcoded
info: v4l2-input: Stepwise and Continuous framerates are currently hardcoded
info: v4l2-input: Start capture from /dev/video0
error: v4l2-input: Unable to set input 0
error: v4l2-input: Initialization failed
info: v4l2-input: Found input 'loopback' (Index 0)
info: v4l2-input: Pixelformat: YUYV 4:2:2 (available)
info: v4l2-input: Pixelformat: RGB3 (Emulated) (unavailable)
info: v4l2-input: Pixelformat: BGR3 (Emulated) (available)
info: v4l2-input: Pixelformat: YU12 (Emulated) (available)
info: v4l2-input: Pixelformat: YV12 (Emulated) (available)
info: v4l2-input: Stepwise and Continuous framerates are currently hardcoded
info: v4l2-input: Start capture from /dev/video10
info: v4l2-input: Input: 0
info: v4l2-input: Resolution: 640x480
info: v4l2-input: Pixelformat: YUYV
info: v4l2-input: Linesize: 1280 Bytes
info: v4l2-input: Framerate: 30.00 fps

..which seems to indicate that it cannot open /dev/video1 (some weird alias for my camera) nor /dev/video11 as there is no source feeding it. I can select (see screenshots below) either /dev/video0 or /dev/video10.

• adding a 'Chroma Key' filter then converts the green screen from backscrub into an alpha channel to blend my video over whatever other sources are feeding OBS.

[phlash]

Test repeated using OBS Studio 27.0.1-dfsg1-1 (from Debian bookworm / testing), and everything still works as above.

[ali1234]

This is likely to be caused by the snap container then.

[phlash]

I think so, thanks for raising the issue with snapcrafters, are we ok to close this here (possibly with a note in the docs and link to this ticket?)

[BenBE]

@phlash Once somebody™ added the note in the docs I'm fine with closing this.

[phlash]

docs updated in README.md