Using the relative mouse position
The relative mouse position is often used when you need unconstrained mouse movement. A typical example of such a use is a first person shooter game in a 3D environment. The relative mouse position represents how much the mouse pointer position changed in comparison with the previous state in all the axes.
Getting ready
The biggest problem with the relative mouse position is that the mouse pointer is constrained to the application window or the screen. You can solve this by centering the mouse cursor in the center of the application window after computing the relative cursor position or by using direct values from the mouse driver.
The relative mouse position has the big advantage of versatility because you can apply the mouse cursor speed modifier simply by multiplying the relative mouse position with a number. If that number is greater than 1, the mouse cursor will move faster. Multiplying by a number lesser than 1, will slow down the mouse cursor.
