augmented reality long distance

I want to developp an application which track people (or people with marker in her hands), with HD camera.
The distance is above 5 meters.
I would like to use unity3d. My application must work on PC or Mac.

Which AR framework must I use (free or not) ?

Have you already experienced long distance tracking and unity 3D ?
Thank you

Thank you for your answer.
I’m currently use Vuforia but it works only on mobile device, not on PC or MAC…
I search another solution

Consider using the Kinect, which works fine enough for detecting people at distances 4 ft+. has a decent Unity framework

ARToolkit would be a great option to work with. Works well with both PC and Mac. The package is available in asset store.