Redirect two webcam to a virtual camera using ffmpeg

Question

I've used

sudo modprobe v4l2loopback

to create a virtual camera #2. And I used

ffmpeg -i /dev/video0 -s 1280x720 -vcodec rawvideo -pix_fmt yuv420p -threads 0 -f v4l2 /dev/video2

to redirect camera #0 to the virtual camera #2. I would like to redirect both cameras #0 and #1 to the virtual camera #2, side by side, or may be one on the top of the other. How can I do that?

Answer 1

I think I've just found a solution:

ffmpeg -i /dev/video0 -i /dev/video1 -s 1280x720 -vcodec rawvideo -pix_fmt yuv420p -threads 0 -filter_complex hstack -f v4l2 /dev/video2

It works. Now I can show two cameras in a video conference.

But camera #0 show a delay (almost 1s) related to the second camera. How could I solve this delay problem on the first camera, without adding a delay to the second camera.