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Gathering training data

Recall that when the shooter started in the center of its range, it would score using the simple shooting policy: it began moving when the Ball Distance was 110 units or less. However, to get a diverse training sample, we replaced this shooting policy with a random shooting policy of the form ``at each opportunity, begin moving with probability tex2html_wrap_inline902 .'' To help choose x, we determined that the shooter had about 25 decision opportunities before the ball moved within 110 units of the Contact Point. Since we wanted the shooter to start moving before or after these 25 decision cycles with roughly equal probability so as to get a balanced training sample, we solved the equation tex2html_wrap_inline906 . Hence, when using the random shooting policy, the shooter started moving with probability 1/37 at each decision point.

Using this shooting policy, we then collected training data. Each instance consisted of four numbers: the three inputs (Ball Distance, Agent Distance, and Heading Offset) at the time that the shooter began accelerating and a 1 or 0 to indicate whether the shot was successful or not. A shot was successful only if it went directly from the front of the shooter into the goal as illustrated in Figure 3(b): a trial was halted unsuccessfully if the ball hit any corner or side of the shooter, or if the ball hit any wall other than the goal.

Running 2990 trials in this manner gave us sufficient training data to learn to shoot a moving ball into the goal. The success rate using the random shooting policy was 19.7%. In particular, only 590 of the training examples were positive instances.

Peter Stone
Thu Aug 22 12:51:13 EDT 1996