Artificial Intelligence in
Game Design
Terrain Analysis
Terrain Analysis
• Features of terrain have good/bad influence over map areas
– Good locations  Possible goals to move to
– Bad locations  Places to avoid when moving
Cover = good
High ground = good
Open space = bad
Slow to travel
through = bad
Terrain Analysis
• Must be based on realistic estimates of
how terrain affects play
• Game engine based
– Example: Engine reduces hit probability by
50% when target > 100 feet above
– Factor into NPC deciding to move to high
ground
– Most likely for “overhead perspective”
combat games
• Visibility based
– Relevant for FPS games
– Example: NPCs in shadow not visible to
player
– NPC should move to such areas
Terrain Analysis
• “Dangerous” terrain elements
– Things that can damage
character
• Terrain that might expose
character to attack
– Areas that force character to
move slowly
– Areas where character has
chance of getting stuck
– Often related to edge costs, but
not always
• Some slow terrain offers cover
• Forrest, darkness, etc.
GASOLINE
Terrain Example
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-2
-1
GASOLINE
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-1
-1
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-1
-1
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-1
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-1
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Terrain and Influence
• Areas have influence over other areas
• Examples:
Influence over
– High ground
– Shadow
Influence
over
– Cover
Influence
over
Influence
over
Levels of Influence
• Amount of influence may depend on relative values between areas
High influence over
Low influence over
• Influence of area i over area j for some property P = f (P(i), P(j))
– May be simple difference k × (P(i) – P(j))
Example: height(i ) = 50
height(j ) = 10
k = 0.2
Influence_Strength (i, j ) = 8
– Must be based on how game engine defines advantage of area i over area j
Influence and Distance
• Usual assumption: Influence of terrain decreases with distance
– Linear dropoff out to some “range” R
– Influence_Strength (i, j ) treated like “strength”
– Given distance D (i, j )
–
Influence (i, j ) = Influence_Strength (i, j )
× (R – D(i, j ) )/R
Influence over other areas 40 feet
below this one at distances 1 - 4
5
4
3
2
1
Influence over area 3 units
away and 40 feet lower = 2.0
0
1
2
3
4
5
Influence Example
B
A
Height = 50
Height = 40
D
C
Height = 30
Height = 20
Influence Over
A
B
C
D
A
-
-
20 * 0.2 * 0.5 =
2.0
10 * 0.2 * 0.25 =
0.5
B
10 * 0.2 * 0.75 =
1.5
-
30 * 0.2 * 0.75 =
4.5
20 * 0.2 * 0.5 =
2.0
C
-
-
-
-
D
-
-
10 * 0.2 * 0.75 =
1.5
-
Total Influence
• Influence over others – influence others have over it
• Total influence of area i = Σ influcence (i, j) – Σ influcence (j, i)
j
j
B
A
Height = 50
Height = 40
Total
Influence
A
D
C
Height = 30
Height = 20
B
2.0 + 0.5 – 1.5 1.5 + 4.5 + 2.0
= 1.0
= 8.0
C
D
-2.0 - 4.5 - 1.5
= -8.0
1.5 - 0.5 - 2.0
= -1.0
Combined Influences
• Total influence may involve many factors
– A is lower ground than B
– A has cover while B does not
– Influence of cover = 4.0
• Influence of B over A from height = 1.5
• Influence of A over B from cover = 4
• A has total influence over B of 2.5
A
Height = 40
B
Height = 50
Combined Influences
• Must also factor in unit-based influence maps
– Good location controlled by enemy is bad place to move to!
• Example:
• Influence of B over D from height = 2.0
• Influence of D over B from cannon = 3.0
• Total influence of D over B = 1.0
– B no longer a good place to move to!
• May not know that until you move there!
B
Height = 50
D
C
Height = 30
Height = 20
Map Analysis
• Complex analysis of crucial map areas
– Crucial for randomly generated terrain
– Example: Leaders: Clash of Nations
• Goals:
– Identify chokepoints between different game zones
– Identify hotspots that have influence over chokepoints
Map Analysis
• Smoothing (image analysis) used to remove small areas
of impassible objects
– Area around impassible tile T passible  T removed from
analysis
Map Analysis
• Random path generation often used
– For large number of examples:
• Select two random waypoints
• Use Dijkstra/A* to find shortest path between them
• Keep track of waypoints on path
• Store waypoints, path for each
– Waypoints with large number of paths are
candidate chokepoints
Map Analysis
• Verify chokepoints
– For each candidate chokepoint
rerun tests that passed
through it
– If resulting paths
significantly longer
(or infinite!)
chokepoint confirmed
Tactical Waypoints in Dungeons
• More difficult for dungeon world
– Tile centers may not give all good tactical locations
Best tactical
location based
on shadow
Best tactical
location based
on cover
Tactical Waypoints in Dungeons
• Automatic creation
–
–
–
–
Subdivide area into smaller tiles
Find tiles with highest influences over other tiles
Create new waypoints there
Computationally intensive, but only need to do once
Influence over
other points, few
points with
influence over it