Anchun:
So much simplified description of this would be : ball thrown -> hit the surface -> camera activated to see where the ball hit-> sendsx,y position of the ball so that computer can say this is ball or safe.
In that case, a sensible approach would be to use the Arduino as an interface between the ball/ground contact sensor and the PC, and connect the camera directly to the PC, and do all your analysis and processing on the PC. If the outcome of that analysis needs to be output via any devices that can't interface directly to a PC (displays, lamps, sounders etc) then you may want to use the Arduino to interface to those too.