Artificial Intelligence in
Game Design
Complex Steering Behaviors and
Combining Behaviors
Path Following
• Examples:
– Move from one location to another along known route
– Patrol area cyclically
– Implementation of high-level path planning
• Go to door of bookstore
• Go down hall
• Go into PC room
Path following
moves character
along these steps
Path Following
• Note that will never
follow path perfectly
– Like arrive, can’t ever
reach an exact location
– More realistic to move in
smooth curves
– More realistic if don’t
follow exact path every
time (patrolling)
Actual
path
Desired
path
Path Following
• Break desired path down into set of points along path
• Seek each point in sequence
– When “close enough” to point, seek next point
Possibly defined as slowRadius
Path Following
• Waypoints at important
path points
– Generated automatically
or chosen manually by
level designer
– Most accurate to meet
goal of path
– Path planning algorithms
used to create path from
waypoints
Desired
path
Wall Avoidance
• Goal: keep character some avoidDistance from walls
– Usually based on “bounding sphere” of object
• Most graphical objects represented this way
avoidDistance
Wall Avoidance
• Project path of character to some lookahead away
– lookahead > minimum distance required for stop at
maxAccel when traveling at maxSpeed
– Projection can use same algorithms that graphics use for
ray casting
• If wall within lookahead, steer away
lookahead
Wall Avoidance
• Determine collision point
• Project to point perpendicular to wall safeDistance away
• Seek that location
Collision point
avoidDistance
Seek this point
Wall Avoidance
• May still collide with wall
• Algorithm will keep steering until parallel to wall
New collision point
Seek this
new point
Wall Avoidance
• Corners can be problem if not detected by ray trace
– Very common problem in robotics
No collision
detected
• One solution: “whiskers”
Project three lines from character
Steer away
collision detected
Collision Avoidance
• Goal: avoid collisions with other characters who are
also in motion
– Is collision going to occur?
– If so, how should we steer?
• Location of closest approach
– Not where paths intersect
Collision Avoidance
safeRadius
• If distance less than some
safeRadius at that point
Flee that point
Flee where
obstacle will be
at that point
• If already in collision with
object, just Flee its current
location
Collision
detected
safeRadius
In collision
Flee
Collision Avoidance
• Much more difficult if multiple obstacles
– Determine time to collision of all
– Flee from location of first collision
– No perfect solution to problem
Later collision
No collision
First collision
Collision Avoidance
• Expensive to compute if many obstacles
– C characters checking for collisions with each other  C2 checks
• One solution: only check characters within some search
radius
Jumping
• NPC jumping over obstacles key to many games
– Run towards obstacle to achieve needed speed
– Jump over obstacle
– Land on other side
• Must be convincing
– Speed, launch point must
look realistic
– Character should not jump
unless can succeed
(unless player interrupts)
Going too slow
for realistic jump
Jump fails
Jumping
• Jump points
– Target for “run up” by NPC
– Usually have corresponding “landing pad”
• Large enough so covers all landing points from all of jump point
– Created by level designer
• Ideally room designed so jump point looks “natural”
Jump
point
Landing
pad
Jumping
• “Starting radius” for run up based on maxSpeed, maxAccel
– Outside radius  Seek jump point
– Inside radius  Arrive point outside radius
Seek jump point
Jump
point
Landing
pad
Combining Steering Behaviors
• What if multiple steering rules fire?
– What should resulting steering be?
Arrive gold
$ $
$
$
$
Flee player
– Blending results of rules
– Prioritizing some rules over others
Avoid wall
Blended Steering Behaviors
• Weighted sum of steering from different rules
– Weights = relative importance of action
– Can depend on “strength of stimulus (distance, etc.)
• Example:
– Flee player with weight 0.5/distance from player
– Arrive gold with weight 1/distance from gold
– Steering = sum of these
$ $
$
$
$
Blended Steering Behaviors
• Can result in “intelligent” path choices
$ $
$
$
$
Blended Steering Behaviors
• Flocking
– Seek “center of gravity” of flock
– Align with average direction
– Avoid direct collisions
Avoid collision
Average direction
Center of gravity
Blended Steering Behaviors
• Blending conflicting behaviors can give results that meet
no goals
flee
Stimuli perfectly
balanced – no
motion at all!
pursue
seek
result
Wall avoid
Prioritized Steering Behaviors
• Choose most important goal and execute its steering
completely
• Often proximity based
– Within safeRadius of wall  execute WallAvoid behavior
– Within attackRadius of player  execute Seek behavior
Wall avoid
has priority
Pursue
not considered
Prioritized Steering Behaviors
• Can group similar behaviors
– Give each unique priority
• Often based on “level of damage”
to character if not handled
– If total steering from highpriority group < threshold
try next highest priority group
Avoid obstacles group
Avoid walls to side
Avoid wall on turns
Avoid collisions group
Avoid cars to side
Avoid car in front
• Stimulus for high-priority behavior
not strong enough to matter
Follow path group
Seek next point on path
around track
Prioritized Steering Behaviors
Avoid obstacles group
Avoid walls to side
Avoid wall on turns
Steering
Next path point
Avoid collisions group
Avoid cars to side
Avoid car in front
Steer away from wall has priority
Ignore possible collisions with cars
Ignore next point on path
Follow path group
Seek next point on path
around track
Prioritized Steering Behaviors
Avoid obstacles group
Avoid walls to side
Avoid wall on turns
Minimal steering
from walls
Collision detected
Next path point
Steering
Avoid collisions group
Avoid cars to side
Avoid car in front
Avoid collision has priority
Avoid obstacles steering below threshold
Ignore next point on path
Follow path group
Seek next point on path
around track
Prioritized Steering Behaviors
Avoid obstacles group
Avoid walls to side
Avoid wall on turns
Minimal steering
from walls
Steering
Next path point
Minimal steering
from other cars
Avoid collisions group
Avoid cars to side
Avoid car in front
Seek next path point has priority
Avoid obstacles steering below threshold
Avoid collisions steering below threshold
Follow path group
Seek next point on path
around track
Prioritized Steering Behaviors
• Can still get stuck in cycles
flee
Flee enemies
– Steer for one high-priority behavior
• Far enough away so no longer priority
– Other behavior becomes priority
Seek target
• Steers back toward first problem
flee
seek
Higher Level Control of Steering
• Conflicts best resolved by higher-level reasoning
– What is current state?
– What overall goal are we attempting to accomplish?
Same room as player
Seek player
Player exits room
Different room as player
Enter room
where player is
Arrive door
Seek player
Arrive door