Artificial Intelligence  
Lecture 04 – Steering Behaviors  
Edirlei Soares de Lima  
<edirlei.lima@universidadeeuropeia.pt>  
Game AI – Model  
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Pathfinding  
Steering behaviors  
Finite state machines  
Automated planning  
Behaviour trees  
Randomness  
Sensor systems  
Machine learning  
Basics of Movement Algorithms  
Steering Behaviors  
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Steering Behaviors are designed to give  
autonomous characters the ability to  
navigate around their world in a life-like  
and improvisational manner.  
They are not based on complex strategies  
involving path planning or global  
calculations, but instead use local  
information.  
Combinations of steering behaviors can  
be used to achieve higher level goals.  
Steering Behaviors Data  
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A character can be defined by the following kinematic data structure:  
struct Kinematic:  
position # a 2 or 3D vector  
orientation # a single floating point value  
velocity # another 2 or 3D vector  
Steering behaviors operate with these kinematic data. Their output is a set  
of accelerations that will change the velocities of characters in order to  
move them around the level:  
struct SteeringOutput:  
linear  
# a 2 or 3D vector  
angular # a single floating point value  
Steering Behaviors – Overview  
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Seek and Flee:  
Steering Behaviors – Overview  
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Arrival:  
Steering Behaviors – Overview  
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Pursue and Evade:  
Steering Behaviors – Overview  
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Wander:  
Steering Behaviors – Overview  
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Path Following:  
Steering Behaviors – Overview  
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Collision Avoidance:  
Steering Behaviors – Overview  
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Obstacle Avoidance:  
Steering Behaviors – Overview  
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Separation:  
Steering Behaviors – Overview  
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Cohesion:  
Steering Behaviors – Overview  
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Alignment:  
Steering Behaviors – Overview  
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Flocking (Leader Following):  
Unity Implementation – Class Diagram  
Steering Data  
Steering  
Steering Behavior Base  
+
+
linear: Vector3;  
angular: float;  
-weight: float;  
+maxAcceleration: float;  
+maxAngularAcceleration: float;  
+drag: float;  
+abstract GetSteering(base): SteeringData;  
-
Steerings: Steering[];  
-
-
Start();  
FixedUpdate();  
Seek Behavior  
Arrive Behavior  
...  
-
target: GameObject;  
-target: GameObject;  
-
targetRadius: float;  
+GetSteering(base): SteeringData;  
-slowRadius: float  
+GetSteering(base): SteeringData;  
Base Classes  
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Steering Class:  
public abstract class Steering : MonoBehaviour  
{
public abstract SteeringData GetSteering(SteeringBehaviorBase  
steeringbase);  
}
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SteeringData Class:  
public class SteeringData{  
public Vector3 linear;  
public float angular;  
public SteeringData(){  
linear = Vector3.zero;  
angular = 0f;  
}
}
Base Classes  
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SteeringBehaviorBase Class:  
public class SteeringBehaviorBase : MonoBehaviour  
{
private Rigidbody rb;  
private Steering[] steerings;  
public float maxAcceleration = 10f;  
public float maxAngularAcceleration = 3f;  
public float drag = 1f;  
void Start()  
{
rb = GetComponent<Rigidbody>();  
steerings = GetComponents<Steering>();  
rb.drag = drag;  
}
.
..  
Base Classes  
void FixedUpdate ()  
{
Vector3 accelaration = Vector3.zero;  
float rotation = 0f;  
foreach (Steering behavior in steerings)  
{
SteeringData steering = behavior.GetSteering(this);  
accelaration += steering.linear;  
rotation += steering.angular;  
}
if (accelaration.magnitude > maxAcceleration){  
accelaration.Normalize();  
accelaration *= maxAcceleration;  
}
rb.AddForce(accelaration);  
if (rotation != 0)  
{
rb.rotation = Quaternion.Euler(0, rotation, 0);  
}
}
}
Seek Behavior  
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Seek behavior tries to match the position of  
the character with the position of the target.  
public class SeekBehavior : Steering  
{
[SerializeField] private GameObject target;  
public override SteeringData GetSteering(SteeringBehaviorBase  
steeringbase){  
SteeringData steering = new SteeringData();  
steering.linear = target.transform.position – transform.position;  
steering.linear.Normalize();  
steering.linear *= steeringbase.maxAcceleration;  
steering.angular = 0;  
return steering;  
}
}
Exercise 1  
1
) Flee is the opposite of seek. It tries to get as far from the  
target as possible. Implement and test the flee behavior  
class.  
Arrive Behavior  
p
{
o;  
Arrive Behavior  
float targetSpeed;  
if (distance > slowRadius)  
targetSpeed = steeringbase.maxAcceleration;  
else  
targetSpeed = steeringbase.maxAcceleration *  
(distance / slowRadius);  
Vector3 targetVelocity = direction;  
targetVelocity.Normalize();  
targetVelocity *= targetSpeed;  
steering.linear = targetVelocity –  
steeringbase.GetComponent<Rigidbody>().velocity;  
if (steering.linear.magnitude > steeringbase.maxAcceleration){  
steering.linear.Normalize();  
steering.linear *= steeringbase.maxAcceleration;  
}
steering.angular = 0;  
return steering;  
}
}
Pursue Behavior  
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Pursue behavior tries to predict where the target will be at  
some time in the future and aim toward that point.  
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How to predict the future position?  
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We assume the target will continue moving with the same velocity it  
currently has.  
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This is a reasonable assumption over short distances.  
Pursue Behavior  
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Pursue behavior tries to predict where the  
target will be at some time in the future and  
aim toward that point.  
public class PursueBehavior : Steering{  
[
[
SerializeField] private float maxPrediction = 2f;  
SerializeField] private GameObject target;  
public override SteeringData GetSteering(SteeringBehaviorBase  
steeringbase){  
SteeringData steering = new SteeringData();  
Vector3 direction = target.transform.position - transform.position;  
float distance = direction.magnitude;  
float speed = GetComponent<Rigidbody>().velocity.magnitude;  
.
..  
Pursue Behavior  
float prediction;  
if (speed <= (distance / maxPrediction))  
prediction = maxPrediction;  
else  
prediction = distance / speed;  
Vector3 predictedTarget = target.transform.position +  
(target.GetComponent<Rigidbody>().velocity * prediction);  
steering.linear = predictedTarget - transform.position;  
steering.linear.Normalize();  
steering.linear *= steeringbase.maxAcceleration;  
steering.angular = 0;  
return steering;  
}
Seek behavior code.  
}
Exercise 2  
2
) The opposite behavior of pursuit is evade. Once again, we  
calculate the predicted position of the target, but rather than  
delegating to seek, we delegate to flee. Implement and test  
the evade behavior class.  
Face Behavior  
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The face behavior makes a character look at its target.  
public class FaceBehavior : Steering{  
[SerializeField] private Transform target;  
public override SteeringData GetSteering(SteeringBehaviorBase  
steeringbase){  
SteeringData steering = new SteeringData();  
Vector3 direction = target.position - transform.position;  
float angle = Mathf.Atan2(direction.x, direction.z) *  
Mathf.Rad2Deg;  
steering.angular = Mathf.LerpAngle(transform.rotation.eulerAngles.y,  
angle, steeringbase.maxAngularAcceleration *  
Time.deltaTime);  
steering.linear = Vector3.zero;  
return steering;  
}
}
Look Where You’re Going Behavior  
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The “look where you’re going” behavior calculates the  
orientation using the current velocity of the character.  
public class LookWhereYoureGoingBehavior : Steering{  
public override SteeringData GetSteering(SteeringBehaviorBase  
steeringbase){  
SteeringData steering = new SteeringData();  
if (GetComponent<Rigidbody>().velocity.magnitude == 0)  
return steering;  
float angle = Mathf.Atan2(GetComponent<Rigidbody>().velocity.x,  
GetComponent<Rigidbody>().velocity.z) * Mathf.Rad2Deg;  
steering.angular = Mathf.LerpAngle(transform.rotation.eulerAngles.y,  
angle, steeringbase.maxAngularAcceleration *  
Time.deltaTime);  
steering.linear = Vector3.zero;  
return steering;  
}
}
Wander Behavior  
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The wander behavior moves a character aimlessly. The  
character must retain steering direction and make small  
random displacements to it each frame.  
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This behavior can be implemented in several ways, but one that  
produces good results is to constrain the steering force to the surface  
of a circle located slightly ahead of the character.  
Wander Behavior  
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The wander behavior moves a character  
aimlessly.  
public class WanderBehavior : Steering  
{
[SerializeField] private float wanderRate = 0.4f;  
[SerializeField] private float wanderOffset = 1.5f;  
[SerializeField] private float wanderRadius = 4f;  
private float wanderOrientation = 0f;  
private float RandomBinomial(){  
return Random.value - Random.value;  
}
private Vector3 OrientationToVector(float orientation){  
return new Vector3(Mathf.Cos(orientation), 0,  
Mathf.Sin(orientation));  
}
Wander Behavior  
public override SteeringData GetSteering(SteeringBehaviorBase  
steeringbase){  
SteeringData steering = new SteeringData();  
wanderOrientation += RandomBinomial() * wanderRate;  
float characterOrientation = transform.rotation.eulerAngles.y *  
Mathf.Deg2Rad;  
float targetOrientation = wanderOrientation +  
characterOrientation;  
Vector3 targetPosition = transform.position + (wanderOffset *  
OrientationToVector(characterOrientation));  
targetPosition += wanderRadius *  
OrientationToVector(targetOrientation);  
steering.linear = targetPosition - transform.position;  
steering.linear.Normalize();  
steering.linear *= steeringbase.maxAcceleration;  
return steering;  
}
}
Alignment Behavior  
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Alignment behavior gives a character the ability to align itself  
with (that is, head in the same direction and/or speed as)  
other nearby characters.  
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Can be implemented by finding all characters in the local  
neighborhood and averaging together the velocity of the nearby  
characters.  
Alignment Behavior  
public class AlignmentBehavior : Steering  
{
private Transform[] targets;  
[SerializeField] private float alignDistance = 8f;  
void Start(){  
SteeringBehaviorBase[] agents =  
FindObjectsOfType<SteeringBehaviorBase>();  
targets = new Transform[agents.Length - 1];  
int count = 0;  
foreach (SteeringBehaviorBase agent in agents){  
if (agent.gameObject != gameObject){  
targets[count] = agent.transform;  
count++;  
}
}
}
.
..  
Alignment Behavior  
public override SteeringData GetSteering(SteeringBehaviorBase  
steeringbase){  
SteeringData steering = new SteeringData();  
steering.linear = Vector3.zero;  
int count = 0;  
foreach (Transform target in targets){  
Vector3 targetDir = target.position - transform.position;  
if (targetDir.magnitude < alignDistance){  
steering.linear += target.GetComponent<Rigidbody>().velocity;  
count++;  
}
}
if (count > 0){  
steering.linear = steering.linear / count;  
if (steering.linear.magnitude > steeringbase.maxAcceleration){  
steering.linear = steering.linear.normalized *  
steeringbase.maxAcceleration;  
}
}
return steering;  
}
Path Following Behavior  
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Path following behavior takes a whole path as a target. The  
character moves along the path in one direction.  
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The target position is calculated in two stages:  
1. The current character position is mapped to the nearest point along the path;  
2. A target is selected which is further along the path than the mapped point by a  
fixed distance.  
Path Following Behavior  
public class FollowPathBehavior : Steering{  
[SerializeField] private PathLine path;  
[SerializeField] private float pathOffset = 0.71f;  
private float currentParam = 0;  
public override SteeringData GetSteering(SteeringBehaviorBase  
steeringbase){  
SteeringData steering = new SteeringData();  
Vector3 targetPosition;  
if (path.nodes.Length == 1)  
targetPosition = path.nodes[0];  
else{  
currentParam = path.GetParam(transform.position);  
float targetParam = currentParam + pathOffset;  
targetPosition = path.GetPosition(targetParam);  
steering.linear = targetPosition - transform.position;  
steering.linear.Normalize();  
steering.linear *= steeringbase.maxAcceleration;  
}
return steering;  
}
}
Path Following Behavior  
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In a alternative implementation, we first predict where the  
character will be in a short time and then map this to the  
nearest point on the path.  
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Smoother solution for complex paths with sudden changes of  
direction.  
Exercise 3  
3
) Based on the first implementation of the Path Following  
behavior, implement the alternative version of the behavior.  
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First predict where the character will be in a short time and then map  
this to the nearest point on the path.  
Collision Avoidance Behavior  
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Collision avoidance behavior tries to keep characters (which  
are moving in arbitrary directions) from running into each  
other.  
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The character must consider all other characters to determine (based  
on current velocities) when and where the two will make their nearest  
approach.  
Separation Behavior  
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Separation behavior gives a character the ability to maintain a  
certain separation distance from others nearby.  
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This can be used to prevent characters from crowding together.  
Separation Behavior  
public class SeparationBehavior : Steering{  
private Transform[] targets;  
[SerializeField] private float threshold = 2f;  
[SerializeField] private float decayCoefficient = -25f;  
void Start(){  
SteeringBehaviorBase[] agents =  
FindObjectsOfType<SteeringBehaviorBase>();  
targets = new Transform[agents.Length - 1];  
int count = 0;  
foreach (SteeringBehaviorBase agent in agents){  
if (agent.gameObject != gameObject){  
targets[count] = agent.transform;  
count++;  
}
}
}
.
..  
Separation Behavior  
public override SteeringData GetSteering(SteeringBehaviorBase  
steeringbase){  
SteeringData steering = new SteeringData();  
foreach (Transform target in targets){  
Vector3 direction = target.transform.position –  
transform.position;  
float distance = direction.magnitude;  
if (distance < threshold){  
float strength = Mathf.Min(decayCoefficient / (distance *  
distance), steeringbase.maxAcceleration);  
direction.Normalize();  
steering.linear += strength * direction;  
}
}
return steering;  
}
}
Cohesion Behavior  
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Cohesion behavior gives a character the ability to form a group  
with other nearby characters.  
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Its implemented by computing the “average position” (or “center of  
gravity”) of the nearby characters and then setting it as the target for  
seek behavior.  
Cohesion Behavior  
public class CohesionBehavior : Steering{  
private Transform[] targets;  
[SerializeField] private float viewAngle = 60f;  
void Start(){  
SteeringBehaviorBase[] agents =  
FindObjectsOfType<SteeringBehaviorBase>();  
targets = new Transform[agents.Length - 1];  
int count = 0;  
foreach (SteeringBehaviorBase agent in agents){  
if (agent.gameObject != gameObject){  
targets[count] = agent.transform;  
count++;  
}
}
}
.
..  
Cohesion Behavior  
public override SteeringData GetSteering(SteeringBehaviorBase  
steeringbase){  
SteeringData steering = new SteeringData();  
Vector3 centerOfMass = Vector3.zero;  
int count = 0;  
foreach (Transform target in targets){  
Vector3 targetDir = target.position - transform.position;  
if (Vector3.Angle(targetDir, transform.forward) < viewAngle){  
centerOfMass += target.position;  
count++;  
}
}
if (count > 0){  
centerOfMass = centerOfMass / count;  
steering.linear = centerOfMass - transform.position;  
steering.linear.Normalize();  
steering.linear *= steeringbase.maxAcceleration;  
}
return steering;  
}
Seek behavior code.  
}
Obstacle Avoidance Behavior  
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Obstacle avoidance behavior gives a character the ability to  
maneuver in a cluttered environment by dodging around  
obstacles.  
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Implementation:  
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The moving character casts one or more rays out in the direction of its motion.  
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If these rays collide with an obstacle, then a target is created that will avoid the  
collision, and the character does a basic seek on this target.  
Obstacle Avoidance Behavior  
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Number of rays:  
Obstacle Avoidance Behavior  
public class ObstacleAvoidanceBehavior : Steering{  
[SerializeField] private float avoidDistance = 1f;  
[SerializeField] private float lookahead = 2f;  
[SerializeField] private float sideViewAngle = 45f;  
public override SteeringData GetSteering(SteeringBehaviorBase  
steeringbase){  
SteeringData steering = new SteeringData();  
Vector3[] rayVector = new Vector3[3];  
rayVector[0] = GetComponent<Rigidbody>().velocity;  
rayVector[0].Normalize();  
rayVector[0] *= lookahead;  
float rayOrientation = Mathf.Atan2(  
GetComponent<Rigidbody>().velocity.x,  
GetComponent<Rigidbody>().velocity.z);  
float rightRayOrientation = rayOrientation + (sideViewAngle *  
Mathf.Deg2Rad);  
float leftRayOrientation = rayOrientation - (sideViewAngle *  
Mathf.Deg2Rad);  
.
..  
Obstacle Avoidance Behavior  
rayVector[1] = new Vector3(Mathf.Cos(rightRayOrientation), 0,  
Mathf.Sin(rightRayOrientation));  
rayVector[1].Normalize();  
rayVector[1] *= lookahead;  
rayVector[2] = new Vector3(Mathf.Cos(leftRayOrientation), 0,  
Mathf.Sin(leftRayOrientation));  
rayVector[2].Normalize();  
rayVector[2] *= lookahead;  
for (int i = 0; i < rayVector.Length; i++){  
RaycastHit hit;  
if (Physics.Raycast(transform.position, rayVector[i], out hit,  
lookahead)){  
Vector3 target = hit.point + (hit.normal * avoidDistance);  
steering.linear = target - transform.position;  
steering.linear.Normalize();  
steering.linear *= steeringbase.maxAcceleration;  
break;  
}
}
return steering;  
}
}
Combining Steering Behaviors  
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Individually, steering behaviors can achieve a good degree of  
movement sophistication. However, by combining steering  
behaviors together, even more complex movement can be  
achieved.  
There are two methods of combining steering behaviors:  
blending and arbitration.  
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Blending combines behaviors by executing all the steering behaviors  
and combining their results using a set of weights;  
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Arbitration selects one or more steering behaviors to have complete  
control over the character.  
Blending Behaviors  
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We can modify the base Steering class in order to add a  
weight to each behavior:  
public abstract class Steering : MonoBehaviour  
{
[SerializeField] private float weight = 1f;  
public abstract SteeringData GetSteering(SteeringBehaviorBase  
steeringbase);  
public float GetWeight(){  
return weight;  
}
}
Blending Behaviors  
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The weights are used in the SteeringBehaviorBase class to  
compute how much each behavior affects the movement:  
void FixedUpdate(){  
Vector3 accelaration = Vector3.zero;  
float rotation = 0f;  
foreach (Steering behavior in steerings){  
SteeringData steering = behavior.GetSteering(this);  
accelaration += steering.linear * behavior.GetWeight();  
rotation += steering.angular * behavior.GetWeight();  
}
if (accelaration.magnitude > maxAcceleration){  
accelaration.Normalize();  
accelaration *= maxAcceleration;  
}
rb.AddForce(accelaration);  
.
..  
Group Behavior: Flocking  
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The flocking behavior relies on blending three simple steering  
behaviors:  
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Separation: move away from characters that are too close;  
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Alignment: move in the same direction and at the same velocity as the  
group;  
–
Cohesion: move toward the center of mass of the flock.  
Group Behavior: Flocking  
•
•
In flocking behavior, separation is  
more important than cohesion,  
which is more important than  
alignment.  
The importance of each behavior  
can be established by setting the  
behaviors’ weights.  
Exercise 4  
4
) Create a scene with several characters and test the flocking  
behavior.  
–
–
Test different parameters to create a realistic flocking simulation.  
Evaluate what happens when other steering behaviors are combined  
with the flocking behavior, such as Collision Avoidance and Wander.  
Further Reading  
•
•
Buckland, M. (2004). Programming Game AI by Example. Jones & Bartlett  
Learning. ISBN: 978-1-55622-078-4.  
–
Chapter 3: How to Create Autonomously Moving  
Game Agents  
Millington, I., Funge, J. (2009). Artificial Intelligence for Games (2nd ed.).  
CRC Press. ISBN: 978-0123747310.  
–
–
Chapter 3.3: Steering Behaviors  
Chapter 3.4: Combining Steering Behaviors