What you can see in the video down here is the fast marker detection that I’ve developed for iOS (iPhone and iPad). The detection of 3D position and orientation of one or more markers can be used as a controller for a whole bunch of applications.

In the video (poor quality, sorry!), you can see the detection going on by the white text just superimposed over the hand.

This technique is very fast (@25fps on an iPad2!), ensuring extremely low latency on the   controls that follow, and robust (very high SNR), especially in odd lightining conditions (low lights or sharp contrasts); it can be applied to many other use cases.

What about using marker detector + an accelerometer? Check these two videos out; I’m using an iRing from IK Multimedia; the detection of the marker is plotted with red and green circles: it is quite fast (30FPS, negligible CPU load) and looks robust I think; accelerometer is working at 100hz, you can see top left bars for X,Y,Z accelerations (cables you see here are used for the accelerometer only). Hey there, I do not play guitar in life, so do not comment on it ok? =)

I’d like to make it, but wirless… anyone interested out there? 

This is another video, taken during the prototype development, where you can see the plot of the distance of the hand from the camera.