The decision of the Brain comes in the form of an action array passed to the `AgentAction()` function. The number of elements in this array is determined by the `Vector Action Space Type` and `Vector Action Space Size` settings of the agent's Brain. The RollerAgent uses the continuous vector action space and needs two continuous control signals from the brain. Thus, we will set the Brain `Vector Action Size` to 2. The first element,`action[0]` determines the force applied along the x axis; `action[1]` determines the force applied along the z axis. (If we allowed the agent to move in three dimensions, then we would need to set `Vector Action Size` to 3. Note the Brain really has no idea what the values in the action array mean. The training process just adjusts the action values in response to the observation input and then sees what kind of rewards it gets as a result.
Before we can add a force to the agent, we need a reference to its Rigidbody component. A [Rigidbody](https://docs.unity3d.com/ScriptReference/Rigidbody.html) is Unity's primary element for physics simulation. (See [Physics](https://docs.unity3d.com/Manual/PhysicsSection.html) for full documentation of Unity physics.) A good place to set references to other components of the same GameObject is in the standard Unity `Start()` method:
With the reference to the Rigidbody, the agent can apply the values from the action[] array using the `Rigidbody.AddForce` function:
The RollerAgent applies the values from the action[] array to its Rigidbody component, `rBody`, using the `Rigidbody.AddForce` function:
Joe Ward