Artificial Intelligence  
Lecture 08 Sensor Systems  
Edirlei Soares de Lima  
<edirlei.lima@universidadeeuropeia.pt>  
Game AI Model  
Pathfinding  
Steering behaviours  
Finite state machines  
Automated planning  
Behaviour trees  
Randomness  
Sensor systems  
Machine learning  
Decision Making  
In game AI, decision making is the ability  
of a character/agent to decide what to  
do.  
The agent processes a set of information  
that it uses to generate an action that it  
wants to carry out.  
Input: agent’s knowledge about the world;  
Output: an action request;  
Decision Making  
The knowledge can be broken down into external and  
internal knowledge.  
External knowledge: information about the game environment (e.g.  
characters’ positions, level layout, noise direction).  
Internal knowledge: information about the character’s internal state  
(e.g. health, goals, last actions).  
Sensing Information  
Game environments simulate the  
physical world, at least to some degree.  
In the real world, a person gains information  
about its environment by using its senses.  
If a loud noise is generated in the game,  
all characters that are close to the event  
must “hear the sound”.  
However, a character across the other end  
of the level may not heard it, and neither  
would a character behind a soundproof  
window.  
Sensing Information  
Is not possible to make a real simulation of how biological  
sensors work. Simplifications are indispensable.  
There is no point in simulating the way that sound travels from the  
headphones on a character’s head down its ear canal.  
Example:  
A character might be constantly looking at the position of the player.  
When the player gets near enough, the character suddenly engages its  
chase” action.  
It appears to the player as if the character couldn’t see the player until  
he got close enough.  
Sensory Modalities  
Four natural human senses are suitable for use in a game (in  
roughly decreasing order of use):  
Sight: the ability of character to see. Players can easily tell if it is being  
simulated badly.  
Touch: the ability of character to sense direct physical contact. Easily  
implemented using collision detection.  
Hearing: the ability of character to hear sounds. The real world sound  
propagation process is complex and usually is simplified in games.  
Smell: the ability of character to smell gases. Is a relatively unexplored  
sense in games.  
Sight  
There are several factors that can affect a character’s ability to  
see something:  
Sight Cone: cone shape that limits the view angle.  
Sight  
There are several factors that can affect a character’s ability to  
see something:  
Line of Sight: to see an object, the character needs a direct line of sight  
with the object.  
Sight  
There are several factors that can affect a character’s ability to  
see something:  
Distance: although humans have no distance limitation to their sight,  
most games add a distance limit.  
Atmospheric effects (such as fog) and  
the curve of the earth also limit the  
ability to see very long distances.  
Sight  
There are several factors that can affect a character’s ability to  
see something:  
Brightness: its notoriously difficult to see in dim light or dark rooms.  
Sight  
There are several factors that can affect a character’s ability to  
see something:  
Differentiation: its difficult see objects that do not contrast with their  
backgrounds. Camouflage works based on this principle.  
Hearing  
The real world sound propagation process is complex:  
Sound travels as compression waves through any physical medium.  
The wave takes time to move, and as it moves it spreads out and is  
subject to friction. Both factors serve to diminish the intensity  
(volume) of the sound with distance.  
The physical process is  
too complex to include  
in the AI of a game.  
Hearing  
Games usually simplify the sound propagation process: the  
sound uniformly reduce its volume over distance until it pass  
below some threshold.  
Sound travels through air around  
corners without a problem.  
Typically all materials are divided  
into two categories: those that do  
not transmit sound and those that  
do.  
The speed of sound is often fast  
enough not to be noticed.  
Sense Management Algorithm  
Algorithm:  
1
. Potential sensors (e.g. characters) are found and registered;  
2. The sense manager accepts signals: messages that indicate that something  
has occurred in the game level.  
Signal information: signal modality, intensity at its source, and the position of the source.  
3
4
5
6
. When a signal is sent to the sense manager, it finds all sensors within the  
maximum radius of the corresponding modality.  
. For each sensors, it calculates the intensity of the signal when it reaches the  
sensors and the time when that will happen.  
. If the intensity test passes, then the algorithm may perform additional tests  
(
e.g. line of sight).  
. If all tests pass, then a request to notify the character is posted in a queue.  
Sense Management Algorithm  
Attenuation in action:  
= ꢁꢂꢃꢄꢅꢆꢂℎ ꢇꢂꢄꢅꢅꢈꢇꢂꢀꢉꢅꢑ  
ꢂ = ꢂꢀꢒꢄ + (ꢓꢀꢁꢂꢇꢅꢔꢄ ꢂꢃꢇꢅꢁꢒꢀꢁꢁꢀꢉꢅꢁꢕꢄꢄꢓꢖꢇꢔꢂꢉꢃ)  
Class Diagram  
Sense Manager  
Notification  
Player  
0.*  
-
-
sensors:Sensor[];  
notificationQueue:Notification[];  
+time:float;  
+sensor:Sensor;  
+signal:Signal;  
+
+
senseManager:Sense Manager;  
signals:Signal[];  
1
1
1.* 1  
+
AddSignal(signal, emitter);  
SendSignals();  
Start();  
0
.*  
-
-
-
0.*  
-
-
Start();  
Update();  
Update();  
1
.*  
.*  
0
.*  
1
1
Modality  
Sensor  
Signal  
+
+
+
maximumRange:float;  
attenuation:float;  
inverseTransmissionSpeed:float;  
+threshold:float;  
+detectedModalities:Modality[];  
1
1.*  
+
+
strength:float;  
modality:Modality;  
1.*  
1
+
DetectsModality(modality):bool;  
+
ExtraTests(signal, emitter, sensor);  
+Notify(signal);  
Hearing Modality  
Sight Modality  
Smelling Modality  
+
ExtraTests(signal, emitter, sensor);  
+viewAngle:float;  
+ExtraTests(signal, emitter, sensor);  
+
-
-
ExtraTests(signal, emitter, sensor);  
TestSightCone(signal, emitter, sensor);  
TestLineOfSight(signal, emitter, sensor);  
Modality Classes  
Modality Class:  
public abstract class Modality : ScriptableObject{  
public float maximumRange;  
public float attenuation;  
public float inverseTransmissionSpeed;  
public abstract bool ExtraTests(Signal signal, Transform emitter,  
Sensor sensor);  
}
Modality  
+
+
+
maximumRange:float;  
attenuation:float;  
inverseTransmissionSpeed:float;  
+
ExtraTests(signal, emitter, sensor);  
Hearing Modality  
Sight Modality  
Smelling Modality  
Modality Classes  
Hearing Modality Class:  
[CreateAssetMenu(menuName = "Sense System/Modality/Hearing")]  
public class HearingModality : Modality{  
public override bool ExtraTests(Signal signal, Transform emitter,  
Sensor sensor){  
return true;  
}
}
Smelling Modality Class:  
[CreateAssetMenu(menuName = "Sense System/Modality/Smelling")]  
public class SmellingModality : Modality{  
public override bool ExtraTests(Signal signal, Transform emitter,  
Sensor sensor){  
return true;  
}
}
Modality Classes  
Sight Modality Class:  
[CreateAssetMenu(menuName = "Sense System/Modality/Sight")]  
public class SightModality : Modality  
{
public float viewAngle;  
public override bool ExtraTests(Signal signal, Transform emitter,  
Sensor sensor){  
if (!TestSightCone(signal, emitter, sensor))  
return false;  
if (!TestLineOfSight(signal, emitter, sensor))  
return false;  
return true;  
}
...  
Modality Classes  
private bool TestSightCone(Signal signal, Transform emitter,  
Sensor sensor){  
Vector3 targetDir = emitter.position - sensor.transform.position;  
float angle = Vector3.Angle(targetDir, sensor.transform.forward);  
if (angle < viewAngle)  
return true;  
return false;  
}
private bool TestLineOfSight(Signal signal, Transform emitter,  
Sensor sensor){  
Vector3 targetDir = emitter.position - sensor.transform.position;  
RaycastHit hit;  
if (Physics.Raycast(sensor.transform.position, targetDir,  
out hit, 10)){  
if (hit.transform == emitter)  
return true;  
}
return false;  
}
}
Signal and Notification Classes  
Signal Class:  
[
CreateAssetMenu(menuName = "Sense System/Signal")]  
Signal  
public class Signal : ScriptableObject{  
public float strength;  
public Modality modality;  
}
+
+
strength:float;  
modality:Modality;  
Notification Class:  
public class Notification {  
public float time;  
public Sensor sensor;  
Notification  
public Signal signal;  
public Notification(float t, Sensor sen, Signal sig){  
time = t;  
sensor = sen;  
+
+
+
time:float;  
sensor:Sensor;  
signal:Signal;  
signal = sig;  
}
}
Sensor Class  
Sensor Class:  
public class Sensor : MonoBehaviour {  
public float threshold;  
public Modality[] detectedModalities;  
public bool DetectsModality(Modality modality){  
foreach (Modality mod in detectedModalities){  
if (mod == modality)  
return true;  
}
Sensor  
return false;  
+threshold:float;  
+detectedModalities:Modality[];  
}
+
+
DetectsModality(mod):bool;  
Notify(signal);  
public void Notify(Signal signal){  
if (signal.modality is HearingModality)  
Debug.Log(name + " heard a sound!");  
if (signal.modality is SightModality)  
Debug.Log(name + " saw someone!");  
}
}
Sense Manager Class  
Sense Manager Class:  
public class SenseManager : MonoBehaviour  
{
private List<Sensor> sensors = new List<Sensor>();  
private SimplePriorityQueue<Notification> notificationQueue = new  
SimplePriorityQueue<Notification>();  
void Start()  
{
Sensor[] allSensors = Object.FindObjectsOfType<Sensor>();  
foreach (Sensor sensor in allSensors)  
sensors.Add(sensor);  
Sense Manager  
}
-
-
sensors:Sensor[];  
notificationQueue:Notification[];  
void Update()  
{
SendSignals();  
}
+
AddSignal(signal, emitter);  
-
-
-
SendSignals();  
Start();  
Update();  
...  
Sense Manager Class  
public void AddSignal(Signal signal, Transform emitter){  
foreach(Sensor sensor in sensors){  
if (!sensor.DetectsModality(signal.modality))  
continue;  
float distance = Vector3.Distance(emitter.position,  
sensor.transform.position);  
if (signal.modality.maximumRange < distance)  
continue;  
float intensity = signal.strength *  
Mathf.Pow(signal.modality.attenuation, distance);  
if (intensity < sensor.threshold)  
continue;  
if (!signal.modality.ExtraTests(signal, emitter, sensor))  
continue;  
float time = Time.time + (distance *  
signal.modality.inverseTransmissionSpeed);  
Notification notification = new Notification(time, sensor, signal);  
notificationQueue.Enqueue(notification, notification.time);  
}
SendSignals();  
}
...  
Sense Manager Class  
private void SendSignals()  
{
while (notificationQueue.Count > 0)  
{
Notification notification = notificationQueue.First;  
if (notification.time < Time.time)  
{
notification.sensor.Notify(notification.signal);  
notificationQueue.Dequeue();  
}
else  
{
break;  
}
}
}
}
Sense Manager - Objects  
Modalities:  
Signals:  
Exercise 1  
1
) Create a level geometry based on the  
illustrated map. Then:  
a) Add 10 NPCs to the level and implement a  
simple Finite State Machine (FSM) to control  
them (patrol, chase, kill);  
b) Add a controllable player character;  
c) Using the Sense Management system, allow  
the NPCs to see the player (modify the FSM  
to use the Sense Management system);  
Level map  
d) Add 10 hidden bombs in the level. When the  
player touches a bomb, it explodes  
(
explosion, will go to the explosion place to  
see what happened.  
producing sound). The NPCs that hear the  
Exercise 2  
2
) An alternative approach to implement sense management in  
a game consists of using the physics engine to detect and  
trigger sense functions. For this task, Unity has a great  
function to detect objects (sensors) within a sphere range:  
Collider[] Physics.OverlapSphere(Vector3 position, float radius,  
int layerMask = AllLayers);  
Implement and test the classes of a sense management  
system using the physics engine.  
Tips:  
Some possible classes: Sensor (objects that receive senses), Emitter (objects that  
generate senses to all sensors that are within a sphere range of the emitter);  
An explosion generates a single OverlapSphere call when the explosion occurs;  
An object that can be seen by sensors need to call the OverlapSphere function  
every frame.  
Sense Management  
Weakness 1: the system doesn’t take level geometry into  
consideration, other than for line-of-sight tests.  
Sense Management  
Weakness 2: timing discrepancy for moving characters.  
Sense Graph  
When the weaknesses of the Sense Management system are  
really a problem, there is a solution: Sense Graph.  
The game level geometry is transformed into a directed graph for  
sense management (similar to the pathfinding graph).  
Each node in the graph represents a region of the game level where  
signals can pass around.  
For all modalities, the node contains an attenuation value that  
indicates how a signal decays for each unit of distance it travels.  
Sense Graph  
The Sense Graph takes into consideration the level geometry:  
Sense Graph  
Sense graph for one-way glass:  
Sense Graph  
Air-conditioning in a sense graph:  
Further Reading  
Buckland, M. (2004). Programming Game AI by Example. Jones & Bartlett  
Learning. ISBN: 978-1-55622-078-4.  
Chapter 8: Practical Path Planning  
Millington, I., Funge, J. (2009). Artificial Intelligence for Games (2nd ed.).  
CRC Press. ISBN: 978-0123747310.  
Chapter 10.5: Sense Management