Introduction to AI in computer games

• How do you think about AI?
What is AI
2Fanafzar Game Studio

• The field of AI research was founded at a conference on
the campus of Dartmouth College in the summer of 1956
• “The science and engineering of making intelligent
machines” -- John McCarthy 1956
• The study and design of intelligent agents -- Russell &
Norvig
What is AI

• The Imitation Game (1950)
• A man (A), a woman (B), and an interrogator (C) who may
be of either sex.
Turing Test

• The Imitation Game
• We now ask the question, What will happen when a
machine takes the part of A in this game?
Turing Test

• Turing conjectured that, by the year 2000, a computer
with a storage of 10^9 units could be programmed well
enough to pass the test.
• The Turing test does not directly test whether the
computer behaves intelligently
– Some human behavior is unintelligent
– Some intelligent behavior is inhuman
• Real intelligence vs. simulated intelligence
Turing Test

• The Chinese room is a thought experiment by John Searle
which first appeared in his paper "Minds, Brains, and
Programs", published in Behavioral and Brain Sciences in
1980.
Chinese room

Chinese room

• Searle argued that software could pass the Turing Test
simply by manipulating symbols of which they had no
understanding.
• Searle concludes—the Turing Test cannot prove that a
machine can think.
Chinese room

• According to Strong AI, the correct simulation really is a
mind.
• In 1931, Kurt Gödel proved that it is always possible to
create statements that a formal system (such as an AI
program) could not prove.
Strong AI

• According to Weak AI, the correct simulation is a model
of the mind.
• Can machines think?
– boats and submarines do move through the water but
we do not call that swimming.
• Stuart Russell and Peter Norvig write: "AI researchers
have devoted little attention to passing the Turing test."
Weak AI

• The way computers "think" is vastly different from the
way a human thinks. --James Martin
• AI is faster and has a larger capacity for storage and
memory than any human.
• The largest nerves in the brain can transmit impulses at
around 90 meters per second, whereas a fiber optics
connection can transmit impulses at 300 million meters
per second, more than 3 million times faster.
Alien intelligence

• “A computer program is said to learn from experience E
with respect to some class of tasks T and performance
measure P, if its performance at tasks in T, as measured
by P, improves with experience E.” --Tom M. Mitchell
• REcognition, classification
• Online and Offline learning
• Supervised, Unsupervised, Reinforcement
Machine Learning

• Classification
Machine Learning - Supervised

• Clustering
– We don’t know number of classes
Machine Learning - Unsupervised
X Y
12.37 15.64
22.8 7.8
34 17
91 50
11.9 17
44 19
80 45
21 9
33.31 16.5
79 39
… …

• Clustering
Machine Learning - Unsupervised

• The goal of a reinforcement learning agent is to collect as
much reward as possible.
• Highly related to dynamic programming techniques
• Most famous technique is Q-learning
• Reinforcement Learning in First Person Shooter Games
– IEEE TRANSACTIONS ON COMPUTATIONAL INTELLIGENCE AND AI IN GAMES,
VOL. 3, NO. 1, MARCH 2011
• High-level Reinforcement Learning in Strategy Games
– International Conference on Autonomous Agents and Multiagent Systems
Machine Learning - Reinforcement

• A simple idea: use the theory of evolution as an
algorithm.
• A Population of Individuals
• Swarm intelligence
– Ant colony optimization
– Particle swarm optimization
– Bees algorithm
– Cuckoo search
Evolutionary Computing

• Individual = Chromosomes
• Mutation, Selection, and Crossover.
• Operating on dynamic data sets is difficult
• Tendency to converge towards local optima
• Randomness
Genetic Algorithm

• Simplified models of neural processing in the brain
Neural Networks

• Multilayer Perceptrons
Neural Networks

• How to train
• Black box
• Over fitting
• Computationally expensive
• Evolving Neural Controllers using GA for Warcraft 3-Real
Time Strategy Game
– 2011 Sixth International Conference on Bio-Inspired Computing
Neural Networks

• A good book about
GA and NN in games
Neural Networks

Do we really need AI in computer games?
What we expect from game AI?

• Efficiency
• Ease of Debugging (randomness)
• We don’t need general problem solver
• Believability
– We don’t need human level intelligence
– It doesn't really matter how NPC intelligence is
achieved, as long as the creatures in the game appear
believable.(weak AI)

• Video games provide a rich test bed for artificial
intelligence methods
• Designers need to control the behavior of NPCs
– Explicit control
– Implicit control
• It is very genre specific
• Avoid artificial stupidity

• Agent cycle
Agents as NPCs
THINKSENSE ACT

• Agent cycle
Agents as NPCs
MAP
GEOMETRY
ENTITIES
. . .
THINK ACT

• Agent cycle
Agents as NPCs
REMEMBER
REASON
BEHAVE
SENSE ACT

• Agent cycle
Agents as NPCs
ANIMATE
NAVIGATE
. . .
THINKSENSE

• Rule : if (condition) then action
• Production Rule System comprised of a database of rules,
each rule consists of an arbitrarily complex conditional
statements.
• They are fairly uncommon approach.
Rule Based

• Search Methods, discovering a sequence of actions or
states within a search space that satisfy some goal
• Goal-oriented behavior is still fairly rare in games.
Goal Oriented

• Everything in game world is triangle
• In door / Out door
• Path finding still is a problem
– Some pathfinding bugs(Video)
Introduction to Path planning

• Graph Theory – shortest path
• Single Source shortest path
• All pairs shortest path – Floyd
– Store the result
• Heuristic F(n) = D(n) + H(n)
A*/Dijkstra

• Near-Optimal Hierarchical Pathfinding,
– A. Botea, M. Muller, and J. Schaeffer, Journal of Game
Development, Volume 1
Hierarchical Pathfinding

• Grid/Tile Base
– Fast
– Easy to develop
– Memory Inefficient
– 2D and strategy games
Reviewing some pathfinding methods

• Waypoint graphs
– Manual

• Waypoint graphs
– Automated : Point of visibility

• Worlds require a ridiculous number of waypoints
• Difficult dynamic obstacle avoidance, if not impossible
• Is not shortest path – not optimal
• Impossible to do path-smoothing
• Zig Zag path

Fanafzar Game Studio 40
Inefficient

• Convex Polygons
– Manual/Automated

• Shortest path - Optimal
• Smaller Search Space

OK, But how
to
implement?

• Mesh simplification
• Rendering techniques
• Flood filling with AABBs (UDK)
• Voxelization
• Check Mikko Monone’s work, RecastNavigation
– http://digestingduck.blogspot.com

• The general process is as follows:
1. Voxelization
2. Generate Regions
3. Generate Contours
4. Generate Polygon Mesh
5. Generate Detailed Mesh
Navigation mesh generation process

1. Voxelization

2. Generate Regions

Douglas-Peucker simplification

4. Generate Polygon Mesh
– Triangulation
– Merge to Convex Polygon
– Benefits of Convex Polygon

• Path find with A*
– Graph nodes are convex polygons
– Corridor map
– A* is not complex to implement
– There are many optimization techniques

• Pathfinding is Not A Star, AUTODESK® KYNAPSE ®
MIDDLEWARE WHITE PAPER
– Path smoothing
– Path following
– Deal with other NPCs
– Deal with dynamic evolutions of game world

• Velocity obstacle
• There are some variations
– RVO, NLVO, FVO, HRVO, NHRVO, PVO
• Mikko uses RVO(Reciprocal Velocity Obstacles)

• Voxelization also used in Cover selection, Jumps,
Camera movement
– Automatic annotations in Killzone 3 --Mikko Mononen,
Paris Game AI Conference 2011

Navigation mesh, Zorvan Integration

• Generation Types
– Solo
– Tiled Navmesh
• Dynamic loading
• Deal with dynamic obstacles
• Off-Mesh Connections
• Convex polygons
Navigation mesh, Zorvan Integration

• How to simulate movement of intelligent objects
like bird, animals, cars, etc. --Craig Reynols
• Stanley and Stella in: Breaking the Ice (1987)
Steering Behaviors

• Simple Vehicle Model
– Mass scalar
– Position vector
– Velocity vector
– max_force scalar
– max_speed scalar
Steering Behaviors

• Seek
• Flee
• Flocking
• Pursuit
• Arrival
• Obstacle avoidance
• Path follow
• Leader follow
• Hide
Steering Behaviors

• Seek Vs Flee
Steering Behaviors

• Seek
Vector2D SteeringBehaviors::Seek(Vector2D TargetPos)
{
Vector2D diff = TargetPos - m_pVehicle->Pos();
Vector2D DesiredVelocity =
Vec2DNormalize(diff) * m_pVehicle->MaxSpeed();
return (DesiredVelocity - m_pVehicle->Velocity());
}
Steering Behaviors

• Obstacle avoidance
– Only circles
Steering Behaviors

• Combining Steering Behaviors
– Weighted Truncated Sum
– Weighted Truncated Running Sum with Prioritization
– Prioritized Dithering
• Open Steer
Steering Behaviors

Advantages
– Simplicity
– Reliability
– Predictability
– Efficiency
Disadvantages
– Local traps
• Oscillation
– Realism
• Jagged paths
– Scalability
Steering Behaviors

• Pure scripting!
• Structured
– FSM
– HFSM
– Behavior Tree
Decision Making

• Theory (Simplified)
– A set states, S
– An input vocabulary, e
– Transition function, T(s, e)
• Map a state and an input to another state
FSM
Patrol
(idle)
Combat
Return to
post
Pursue
Player Seen
Near PlayerFar from post
Reach post

• Finite State Machine (FSM) is the Most Commonly
used Game AI Technology Today
– Simple
– Efficient
– Easily extensible
– Powerful enough to handle a wide variety of situations
• Decisions only depend on current state
FSM

• Hard Coded
– Switch Statement
– Function pointers
– Polymorphism (State Pattern)
• Interpreted
– Data Driven
– Scripted
• Compiled
– machine code
– Generating source code
FSM

• Function Pointer-Based, Embedded Finite-State Machines
– Chapter 3.1, Game programming gems 1
• A Finite-State Machine Class
– Chapter 3.3 Game Programming Gems 3
FSM

• Each state can be a complete state machine in its own
right
• Original Paper
– Statecharts: A Visual Formalism for Complex Systems D. Harel
Science of Computer Programming 8, 1987
• Clustering states (XOR)
• Concurrency (AND)
HFSM

• Example
HFSM
Patrol
(idle)
Return to
post
Pursue
Player Seen
Near PlayerFar from post
Reach post
Combat
Attack 1 Attack 2
Success

• Cluster states
• The semantic of D is exclusive-or (XOR) of A and C
HFSM
A
C
B
α
β
δ
γ(cond)
D

• Parallel (AND combination)
HFSM
A
C
α β
E
G
F
α
δ
γ μ
A DY

Iranvij AI Editor

• Halo 2 [Bungie Software, 2004] was one of the first high-
profile games for which the use of behavior trees.
• Instead of a state, the main building block of a behavior
tree is a Task
• Conditions, Actions, and Composites
Behavior tree

• Selector
Behavior tree
terminate
keep trying
?

• Sequence
Behavior tree
keep going
bail out

• Example
Behavior tree
?
Door
Open?
Move
(into room)
Move
(to door)
Open
door
Move
(into room)
Condition Action

• Refactored tree
Behavior tree
?
Door
Open?
Move
(to door)
Open
door
Move
(into room)

• Decorators (such as Invertor)
• Random Selector
• Random Sequence
• Parallel

• Procedural Animation
• Procedural level generation
• Dynamic game difficulty balancing
Next Generation AI for game

• Artificial Intelligence For Games, Second Edition, Ian
Millington, John Funge
• Programming Game AI by Example, Mat Buckland
• Game programming gems series
• AI Game Programming Wisdom series
• AI Game Development: Synthetic Creatures with Learning
and Reactive Behaviors, Alex J. Champandard
• http://aigamedev.com/
• http://www.ai-blog.net/archives/000183.html
• http://www.critterai.org/nmgen_study
References

• http://digestingduck.blogspot.com/
References

Thank You
hojjatjafary@yahoo.com

Introduction to AI in computer games

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