camera-fanout

Share one Linux webcam across multiple programs at once, using ffmpeg and v4l2loopback.

What it does

Linux only lets one process open /dev/video0 at a time. camera-fanout works around that by reading the real webcam once and copying ("fanning out") its frames into three v4l2loopback virtual devices (/dev/video10, /dev/video11, /dev/video12), so several tools can each open their own virtual camera simultaneously.

Two scripts:

• fanout.sh — loads the v4l2loopback module if needed, then runs a background ffmpeg process that reads /dev/video0 (MJPEG, 640x480@30) and writes raw YUYV422 frames to the three virtual devices. Manages a PID file at /tmp/camera-fanout.pid and logs to /tmp/camera-fanout.log.
• env.sh — meant to be sourced by other camera tools. If the fanout is running, it claims the next free virtual device (via a /tmp/vcam-claim-* lockfile) and exports VCAM_DEVICE / VCAM_INDEX. If the fanout isn't running, it falls back to /dev/video0.

Status

Working personal utility, single machine. Device numbers, resolution (640x480), and framerate (30) are hardcoded near the top of fanout.sh. The three virtual-device paths are likewise hardcoded and assume v4l2loopback is configured to create /dev/video{10,11,12}. Not packaged or generalized beyond that setup.

Requirements

• Linux with the v4l2loopback kernel module installed
• ffmpeg
• v4l2-ctl (from v4l-utils) for the status output
• sudo access to run modprobe v4l2loopback
• A real camera at /dev/video0

v4l2loopback should be configured to create three devices at /dev/video10, /dev/video11, and /dev/video12 (typically via /etc/modprobe.d/v4l2loopback.conf). If the devices come up at different numbers, edit the VCAMS array in fanout.sh.

Usage

./fanout.sh            # start the fanout (loads the module if needed)
./fanout.sh status     # show fanout / module / device status
./fanout.sh stop       # stop the fanout and clean up claim locks
./fanout.sh restart    # stop then start

In another camera tool, pick a virtual device automatically:

source /path/to/camera-fanout/env.sh
# now use "$VCAM_DEVICE" (e.g. /dev/video11) or "$VCAM_INDEX"

Notes / limitations

• Resolution, framerate, real-camera path, and the three virtual-device paths are all hardcoded in fanout.sh.
• The fanout decodes MJPEG once and writes raw YUYV422 to each device (one decode, three copies); there is no per-device transcoding.
• env.sh claims at most three devices; once all three are taken it falls back to sharing /dev/video10.
• Uses notify-send for a desktop notification on start if available (optional).