As part of our research in an HCI project, we need to track what users are drawing on pieces of paper in real time. Due to some requirements, we cannot replace the paper with anything digital. What we can do is tracking the user's pen movement using cameras, or use some sort of digital pen which works on paper (like Anoto or Livescribe), but with real time capabilities.
Using a camera might be more challenging due to occlusion, small size of pen and large size of the room, and so on. It seems that using a digital pen is a more viable option (I don't know of any product which offers this capability though), or maybe a completely different solution that I haven't thought of.
Any idea how can I tackle this problem? I appreciate if you can give me some pointers on technologies/tools you know which can help m
EDIT: I rephrased the question to fit the website's requirements better.