glutTabletMotionFunc sets the tablet motion callback for the current window. The tablet motion callback for a window is called when the window has tablet input focus (normally, when the mouse is in the window) and the user generates tablet motion. The x and y callback parameters indicate the absolute position of the tablet "puck" on the tablet. The callback parameters are normalized to be within the range of 0 to 2000 inclusive.
Registering a tablet motion callback when a tablet device is not available is ineffectual and not an error. In this case, no tablet motion callbacks will be generated.
Passing NULL to glutTabletMotionFunc disables the generation of tablet motion callbacks. When a new window is created, no tablet motion callback is initially registered.