find out where a user is pointing on a wall

[gianni pinotto]

Hi, let's say I have a kinect placed on a wall (3 meters up from the floor) and a user in front of it.I want to identify where the left arm of the user is pointing on the wall, which area (suppose to divide the wall in 4 squares, I wanna find out which square the user is pointing to with his arm). How can I do that?

For now I tried to draw a straight line that starts at the elbow position and goes through the hand. But that's my problem. I can't draw a 3d line in processing without setting the P3D render in the size() and if I do that, I will get this error: SimpleOpenNI Error: createDepth: can't set videoMode!

What I simply need is to extend the skeleton forearm (from elbow till hand joints) with a straight line. I will know the distance between the wall and the hand then I will be able to find out where he is pointing. Ho can I extend the forearm line?

Thanks

GP