Dual virtual joysticks not anchored to the bottom of the screen

Hi,

I found plenty of docummentation about creating virtual controls for mobile versions of a game. What I am not able to find is any guide for creating a virtual joystick which is not anchored to a specific part of the screen, so the movenet is based on the delta of the touch slide.

A good example of this control is Geometry Wars 3. Any slide on the left part of the screen makes the player move in the same direction of the slide, and any slide on the right part of the screen makes the player shoot in the same direction of the gesture.

I don't even know if this control have a specific name. Any documentation or idea where I can start looking.

Regards