Allow virtual camera te be used by several applications

[juantinazzo]

Operating System Info

Other

Other OS

Manjaro

OBS Studio Version

32.0.4

OBS Studio Version (Other)

No response

OBS Studio Log URL

https://obsproject.com/logs/fEl1LwGUhO556fPj

OBS Studio Crash Log URL

No response

Expected Behavior

The virtual camera should be able to be used by several applications at the same time in Linux.

Current Behavior

The virtual camera can only be used with one application at the time, due to exclusive_caps:
pkexec modprobe v4l2loopback exclusive_caps=1 card_label='OBS Virtual Camera' && sleep 0.5

Steps to Reproduce

1. Start virtual camera
2. Try to use it in several browser tabs

...

Anything else we should know?

Reading the docs of v4l2loopback i can understand the reason behind 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.)

However, at least on Manjaro, both Firefox and Chromium seem to work well with exclusive_caps=0.

Right now my solution is to manually modprobe v4l2loopback at system startup, but it would better to have a configuration option in the settings for this.

[stephematician]

I agree this would be useful. However, there are reasons that v4l2loopback doesn't allow this at all (at least in more recent versions).

Apologies for super long reply.

First, a clarification. exclusive_caps doesn't mean exclusive capture. It means that the device presents exclusive capabilities.

This is a bit technical, but what it means is that when exclusive_caps=1 on /dev/video0 (or whatever loopback node we asked for), then the node will only announce one capability at a time - "output" or "capture" - when asked by an application (e.g. the browser).

When exclusive_caps=0, the node will announce that it can do both to any application that asks. This trips up a lot of browsers for whatever reason (hence why exclusive_caps=1 was introduced). =0 behaviour is also in violation of the V4L2 API, essentially, =0 is a legacy mode that the module allows but its not really helpful these days.

Secondly.

Another thing that happens to be in violation of the V4L2 API? Multiple applications reading the same node: https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/open.html#shared-data-streams

Basically: You can't open an ordinary webcam in multiple applications at the same time (with V4L2), and hence you can't open a v4l2loopback virtual camera, either.

Although v4l2loopback used to allow this; that has been deprecated, and I don't see it coming back any time soon. The loopback module is testing the limits of the V4L2 rules as it is (it allows one writer and one reader).

There's an argument to be made that because the loopback module is not "in tree" that it could do whatever it wants - why care about strict V4L2 API compliance? Lately (and this is partly my fault) it has been nudged in the direction of being more compliant with the API - and this seems to be necessary for quite a few use cases (e.g. gstreamer and pipewire are much more likely to play nicely with v4l2loopback as a result).

I'm just a contributor, though, so anyone else is free to add non-compliant features to module (if the maintainer agrees to it) as long as it doesn't break the most common use cases.†

Finally ... I 100% agree that multiple applications being able to view the camera would be great! How might that happen, you ask?

Well - at first glance, if more applications supported PipeWire and if OBS had a PipeWire virtual camera, we'd be set! But... there are serious blockages that OBS devs aren't in control of: #5377

Hope that helps explain why the loopback module won't allow this type of usage (regardless of exclusive_caps).

† Ironic side note: By making the module more compliant, I broke one of the more common use cases: OBS! (see #11891)

[stephematician]

If you want to duplicate the stream, btw, you can try using gstreamer to duplicate it to pipewire (virtual) sinks or duplicate it to multiple v4l2loopback nodes. Probably will chew up a bit of CPU, however.