What a $23 paywall actually buys

A monitor utility wanted $23 for one feature, so I spent an evening finding out what that $23 actually buys. The answer turned out to be a genuinely interesting engineering problem, and the app I built to answer the question is now free and MIT licensed.

The feature

Your MacBook display is quietly clever: macOS reads the ambient light sensor and adjusts the built-in screen all day without you noticing. Your external monitor is not clever at all. It sits at whatever brightness you left it, so the display you actually stare at is the one that never adapts. When the sun goes down, one screen follows and the other burns a hole in your face.

Lunar solves this with a Pro feature called Sync Mode. That is the $23.

Monitors have had an API since 1998

The protocol is DDC/CI, and it has been in the specification since 1998. Your Mac can send I2C commands straight down the display cable to set hardware brightness, volume, even the input source. Most people have no idea the channel exists, which is why monitor control feels like it should be impossible and is actually just obscure.

On Apple Silicon it is obscure and undocumented. Getting at that channel means private IOKit APIs, and there is a whole underground of apps mapping this territory the hard way, Lunar and MonitorControl and BetterDisplay among them. Nothing in this category is sandboxed or App Store material, mine included.

The part I did not expect

The sync logic is small. Read the built-in display's ambient-adjusted brightness, run it through a per-display curve with a min, max and gamma, push it out over DDC. It is push driven, so it idles at zero percent CPU with about four wakeups per interval and no traffic at steady state.

Shipping it was the bigger job. A Mac app distributed outside the App Store needs a Developer ID signature, notarization by Apple, stapling, and an update mechanism. Lonar updates through Sparkle with an EdDSA-signed appcast, plus a Homebrew tap so installing is one line:

brew install --cask eimen2018/tap/lonar

The distribution pipeline took roughly as long as the feature. That is the part nobody puts in the demo video, and it is most of what separates a script from something another person can actually run.

What shipped

• Sync Mode, externals following the built-in display's ambient-adjusted brightness
• Sub-zero dimming, software dimming past the hardware minimum for pitch-dark rooms
• Volume control and input switching over DDC
• Scroll on the menu bar icon to dim every external at once

It works on any Apple Silicon Mac and any DDC-capable monitor, including the Studio Display. If you have ever wondered what is behind a Pro paywall, building it is a very effective way to find out.

Code and downloads on GitHub