design RFM: Virtual Camera Placeholder Image Configuration

(RFM: "Request for Mockup")

End goal

The placeholder image (the picture shown by the virtual camera when the virtual camera is inactive) should be configurable by the user. Not everyone wants the crossed out OBS logo (the current placeholder image) to show when their virtual camera is inactive.

Current alternatives

On Windows, it is possible to change the default placeholder image by overwriting its file in the virtualcam installation, which gets reset on every OBS update. There is an open pull request to define a location for the user to be able to put a placeholder image file at %APPDATA%\obs-studio\plugin_config\win-dshow\placeholder.png (see #4446).
On macOS 11 and 12, it's currently possible to replace the placeholder image by adding an image to ~/Library/Application Support/obs-studio/plugin_config/mac-virtualcam/placeholder.png. This workflow is far from ideal, as
On macOS 13 and newer, it's not possible to replace the placeholder file other than by recompiling OBS and the CMIO camera extension.
Unfortunately I'm not aware of the situation on Linux, but at least with V4L2 there might not even be a placeholder? Other people know better than me.

Other considerations

The actual implementation will differ wildly on the different operating systems. It should be expected that different implementations will happen at different times (or potentially never? See the note on Linux above)

Current ideas

We have a virtual camera settings dialog, it might make sense to put the placeholder selection there.

Current dialog:

Jan 16 '24 11:01 gxalpha

Can confirm that on Linux (at least, ubuntu 22.04, but I'm pretty sure it's the same for all), there is no need for a placeholder, as the virtual camera does not exist as a device until it is started in obs. As such it's not possible for it to be in a state where it's selectable as input by other programs, but does not receive video from obs.