Shoulder Guard

Stop people reading your screen over your shoulder, and see who is standing behind you.
One keyboard shortcut. No system settings changed.

Side mirrors	Side mirrors + rear-view strip
Bigger mirrors	Bigger side mirrors, no strip

What it does

• Dim overlay: covers every monitor with a dark layer. You can still read your screen up close, but it is hard to read from a distance. Clicks and typing pass through the overlay, so you keep working as normal.
• Side mirrors: show the left and right edges of your webcam image in the bottom corners of the screen, like the side mirrors on a car.
• Rear-view strip: shows the top of the webcam image at the top of the screen.

Each shortcut toggles its feature: press it once to turn on, press again to turn off.

Shortcut	Action
Super+Shift+H	Dim overlay on/off
Super+Shift+J	Mirrors snapshot (fades out after a few seconds)
Super+Shift+K	Live side mirrors on/off
Super+Shift+L	Live side mirrors + rear-view strip on/off

Requirements

• Linux with GNOME (X11 or Wayland with XWayland)
• Python 3 with GTK 3 bindings (python3-gi, python3-gi-cairo)
• ffmpeg for the webcam mirrors

On Debian or Ubuntu:

sudo apt install python3-gi python3-gi-cairo gir1.2-gtk-3.0 ffmpeg

Install

git clone https://github.com/911RS/shoulder-guard.git
cd shoulder-guard
./install-shortcut.sh

You can pass your own key bindings:

./install-shortcut.sh '<Super><Shift>h' '<Super><Shift>j' '<Super><Shift>k' '<Super><Shift>l'

To remove the shortcuts:

./uninstall-shortcut.sh

Run without shortcuts

python3 guard.py              # dim overlay toggle
python3 mirrors.py            # one snapshot
python3 mirrors.py live       # live side mirrors toggle
python3 mirrors.py live top   # live side mirrors + rear-view strip toggle

Settings

Edit shoulder-guard.ini. After a change, press the shortcut twice (off, then on) to load the new values.

Setting	Default	Meaning
[guard] opacity	0.78	Overlay darkness, 0 = invisible, 1 = black
[mirrors] device	/dev/video0	Webcam device
[mirrors] hold	3	Seconds a snapshot stays visible
[mirrors] opacity	0.8	How solid the mirrors look
[mirrors] fps	10	Live mode frame rate
[mirrors] height	0.17	Side mirror height as a fraction of the screen
[mirrors] top_enabled	no	Always show the rear-view strip

The ini file lists every setting with a comment. Each setting can also be set with an environment variable, for example GUARD_OPACITY=0.9 or MIRROR_DEVICE=/dev/video2. To use an ini file in another location, set SHOULDER_GUARD_CONFIG=/path/to/file.ini.

Privacy

Webcam frames stay on your computer. Nothing is recorded or sent anywhere. Snapshot mode writes one temporary frame to $XDG_RUNTIME_DIR, which is cleared when you log out.