ARToolKit

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.