OpenDisplay

OpenDisplay is a free and open source application that turns a spare iPhone or iPad into a genuine second monitor for a Mac, positioning itself as an alternative to Apple Sidecar, Duet Display, and Luna Display. Unlike Sidecar, it works across different Apple IDs and supports iPhones, not just iPads. Unlike Duet, there is no subscription, and unlike Luna, no hardware dongle is required — just two small apps and a cable or a WiFi connection.

The system pairs a Mac app that captures a virtual display created with CGVirtualDisplay and encodes frames using hardware H.264 via VideoToolbox, with an iOS app that listens for the stream and renders it through AVSampleBufferDisplayLayer. The phone listens while the Mac connects, which lets the same code run identically over USB through macOS's built-in usbmuxd daemon or over WiFi via Bonjour discovery, with touch and scroll events sent back as control messages.

Key features include:

• True display extension — macOS treats the device as a real monitor rather than a mirrored screen
• Retina/HiDPI rendering — the virtual display matches the device panel pixel-for-pixel
• Touch input — tap to click, drag, and two-finger scroll behave like a trackpad
• Portrait and landscape support — rotating the device rebuilds the display at native resolution
• Low-latency pipeline — TCP_NODELAY, frame-drop backpressure, and keyframe recovery keep streaming smooth

OpenDisplay is licensed under GPL-3.0, meaning modified versions must remain open source with attribution intact, though releases before v0.4.x remain available under MIT. The codebase is intentionally small, using Sparkle as its only runtime dependency for macOS auto-updates, and releases are automated with release-please using Conventional Commits. The project is self-hosted and private by design, with no servers, accounts, or analytics involved, and welcomes contributions toward its public roadmap of features like Apple Pencil support and encrypted WiFi transport.