Working with AR technology starts rather mundane with getting some sort of video from a video source, usually an USB webcam or built in camera in a phone. One would believe that is the easiest part. But trust me it is not. There are more APIs and approaches out there then you care to learn about. Everybody trying to do things different or easier. And for what is worth it, the underlying device drivers are doing the "right thing" sometimes, but not always. One has to deal with threads that are spawned by the underlying APIs and whatever format the frames come up.
osgART is a cross platform toolkit for developing AR applications with the OpenSceneGraph API. osgART was invented by Julian Looser and then made by Raphael Grasset and Hartmut Seichter into a robust software framework with hundreds of useful features. The current working version is 2.x and is available from the osgART Website. osgART is been used in hundreds if not thousands of AR applications in research, education and commercial applications. It is most likely the most used open source AR application framework.