Pursuing and evading

Pursuing and evading are great behaviors to start with because they rely on the most basic behaviors and extend their functionality by predicting the target's next step.

Getting ready

We need a couple of basic behaviors called Seek and Flee; place them right after the Agent class in the scripts' execution order.

The following is the code for the Seek behaviour:

using UnityEngine;
using System.Collections;
public class Seek : AgentBehaviour
{
    public override Steering GetSteering()
    {
        Steering steering = new Steering();
        steering.linear = target.transform.position - transform.position;
        steering.linear.Normalize();
        steering.linear = steering.linear * agent.maxAccel;
        return steering;
    }
}

Also, we need to implement the Flee behavior:

using UnityEngine;
using System.Collections;
public class Flee : AgentBehaviour
{
    public override Steering GetSteering()
    {
        Steering steering = new Steering();
        steering.linear = transform.position - target.transform.position;
        steering.linear.Normalize();
        steering.linear = steering.linear * agent.maxAccel;
        return steering;
    }
}

How to do it...

Pursue and Evade are essentially the same algorithm but differ in terms of the base class they derive from:

  1. Create the Pursue class, derived from Seek, and add the attributes for the prediction:
    using UnityEngine;
using System.Collections;

public class Pursue : Seek
{
    public float maxPrediction;
    private GameObject targetAux;
    private Agent targetAgent;
}
  2. Implement the Awake function in order to set up everything according to the real target:
    public override void Awake()
{
    base.Awake();
    targetAgent = target.GetComponent<Agent>();
    targetAux = target;
    target = new GameObject();
}
  3. As well as implement the OnDestroy function, to properly handle the internal object:
    void OnDestroy ()
{
    Destroy(targetAux);
}
  4. Finally, implement the GetSteering function:
    public override Steering GetSteering()
{
    Vector3 direction = targetAux.transform.position - transform.position;
    float distance = direction.magnitude;
    float speed = agent.velocity.magnitude;
    float prediction;
    if (speed <= distance / maxPrediction)
        prediction = maxPrediction;
    else
        prediction = distance / speed;
    target.transform.position = targetAux.transform.position;
    target.transform.position += targetAgent.velocity * prediction;
    return base.GetSteering();
}
  5. To create the Evade behavior, the procedure is just the same, but it takes into account that Flee is the parent class:
    public class Evade : Flee
{
    // everything stays the same
}

How it works...

These behaviors rely on Seek and Flee and take into consideration the target's velocity in order to predict where it will go next; they aim at that position using an internal extra object.

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