opentrack is a cross-platform head-tracking application primarily used in flight simulators and other games to translate physical head movements into in-game camera or character movements. It features a modular plugin architecture with over 20 tracker backends (e.g., IR point tracking, neural network pose estimation via ONNX, ArUco markers, Wiimote, SteamVR, Tobii eye tracker) and numerous output protocols (FreeTrack, SimConnect, FSUIPC, UDP, vJoy, FlightGear, X-Plane, etc.). The core pipeline applies configurable filters (Accela, Hamilton, EWMA, Alpha Spectrum) and spline-based mapping curves before sending pose data to the target application. The application is built with Qt for the GUI, OpenCV for computer vision, and ONNX Runtime for neural network inference. It supports Windows, Linux, and macOS (though macOS is noted as unmaintained). The codebase is large and well-structured, with a clear separation between the plugin API, core logic, and platform compatibility layers. While some tracker modules are legacy or hardware-dependent, the core architecture and many utility components are reusable and demonstrate solid engineering practices.