Images are read as 8bits luminance values (0 = black, 255 = white)

Set the layer mode to Overlay on a webcam plugin, all black pixels become transparent, so you can see the previous plugin’s result in the black zones of the webcam !

In case your webcam is not recognized, you can use a V4L2 loopback device.

Install the kernel module:

$ sudo apt install v4l2loopback-dkms

Redirecting a webcam on /dev/video2 to /dev/video0:

$ sudo modprobe v4l2loopback
$ ffmpeg -i /dev/video2 -video_size 640x480 -pix_fmt yuyv422 -f v4l2 -vf hflip -s 640x480 /dev/video0