A tiny macOS LaunchAgent that watches your built-in display's brightness and mirrors it to every external monitor you have plugged in.
When macOS dims your MacBook (manually, or automatically via the ambient light sensor), ddc-mirror syncs the same brightness to all your externals. It picks the best mechanism per display automatically: Apple's native API for Studio/Pro Display XDR, DDC/CI for monitors on a direct cable, and profile-aware software dimming for monitors behind docks or hubs that strip DDC.
brew install emin93/tap/ddc-mirror
brew services start ddc-mirrorThat's it. There is no step two.
If an external monitor looks consistently darker or brighter than the MacBook display, add a small offset for external displays:
defaults write ch.emin.ddc-mirror externalBrightnessOffset -float 0.10
brew services restart ddc-mirror0.10 means +10 brightness points, so MacBook 50% maps to external 60%.
Negative values work too, and the final brightness is always clamped between 0%
and 100%.
- 🔌 Plug-and-sync. Connect a new monitor and it gets picked up on the next tick. Unplug one and it's gone. No restart, no config.
- 🖥️ Every monitor, every time. Multiple externals? They all sync. No "primary display" weirdness.
- 🤫 Invisible. No menu bar icon. No preferences pane. No hotkeys to remember. The only UI is the one Apple already ships.
- 🧘 Lightweight. ~55 KB binary, ~10 MB resident memory. You will not notice it is running.
- 🕵️ No telemetry. No analytics, no update pings, no crash reports phoned home. It physically cannot tell anyone you installed it.
- 💸 Free forever. MIT licensed. No pro tier, no paywall, no "upgrade for more features." There are no more features — that's the whole point.
Subscribes to Apple's private DisplayServices brightness-change push notification — the same signal that drives the macOS brightness HUD. Zero polling, zero wakeups when nothing's changing.
On startup, each external monitor is probed once and routed to the best available mechanism:
- 🍎 Apple-native API — for Studio Display & Pro Display XDR.
- 🔌 DDC/CI —
IOAVServiceWriteI2CVCP0x10, for monitors on a direct cable (Apple Silicon). - 🌗 Profile-aware software dimming — captures each display's existing ColorSync transfer table, then scales that table with
CGSetDisplayTransferByTablewhen hardware brightness is unavailable.
Each brightness change is debounced (~80 ms) and fanned out to every display.
Re-enumerates displays on hot-plug (CGDisplayRegisterReconfigurationCallback) and on wake (IORegisterForSystemPower). Restores ColorSync settings on exit. Survives reboots via brew services.
make
./ddc-mirrorThe whole thing is one .m file and a Makefile. No SwiftPM, no dependencies. Builds on Apple Silicon and Intel; on Intel, the DDC code path compiles out and every external falls back to profile-aware software dimming.