Figure 3. Tracker analysis: the reference frame is defined by two crossing white lines in (a) and (b), ( x , y ) . The mark er, indicated by the white arrow in (a), is used by T racker as a tracking point. The tennis ball, on the other hand, plays the role of a marker in (c). The running numbers, in (b) and (d), display the data points recorded by Tracker. The red bull's eyes identify the latest location recorded.