ARToolKit is an open source tracking library for AR projects. It is supported on Android, iOS, Linux, and macOS. ARToolKit has the functionality to utilize the following:
- Single or stereo camera for position/orientation tracking
- Tracking of simple black squares
- Tracking of planar images
- Camera calibration
- Optical stereo calibration
- Optical head-mounted display support
It is fast enough for real-time AR applications. It is also free and open source software, with plugins for Unity and OpenSceneGraph. The downside to this software is that it has a huge variety of functions, so it is difficult to integrate the library, and it takes more time to explore all the available options and settings.