Ghost AI

Overview

Ghost AI refers to Pac-Man’s legendary artificial intelligence system where four ghosts with individual “personalities” create complex, emergent behaviour from simple rules. Each ghost targets differently, switching between chase, scatter, and frightened modes—a masterclass in designing engaging AI from minimal complexity.

Fast Facts

Aspect	Detail
Game	Pac-Man (1980)
Designer	Toru Iwatani
Ghosts	Blinky, Pinky, Inky, Clyde
Insight	Personality through targeting

The Four Ghosts

Ghost	Name	Japanese	Personality
Red	Blinky	Oikake (Chaser)	Directly chases Pac-Man
Pink	Pinky	Machibuse (Ambusher)	Targets ahead of Pac-Man
Cyan	Inky	Kimagure (Fickle)	Complex, unpredictable
Orange	Clyde	Otoboke (Stupid)	Alternates chase/retreat

Targeting Modes

Mode	Behaviour	Trigger
Chase	Target based on personality	Default
Scatter	Target home corner	Timer-based alternation
Frightened	Random movement	Power pellet

Individual Targeting Rules

Blinky (Red) - The Shadow

Target = Pac-Man's current tile

Simple, relentless pursuit. Gets faster as dots decrease (“Cruise Elroy” mode).

Pinky (Pink) - The Ambusher

Target = 4 tiles ahead of Pac-Man's facing direction

Tries to cut off Pac-Man. Bug: when Pac-Man faces up, targets 4 tiles up AND 4 tiles left.

Inky (Cyan) - The Fickle One

1. Draw vector from Blinky to 2 tiles ahead of Pac-Man
2. Double that vector
3. Target = endpoint

Uses Blinky’s position, creating unpredictable flanking.

Clyde (Orange) - The Feigner

If distance to Pac-Man > 8 tiles:
    Target = Pac-Man's current tile
Else:
    Target = Clyde's scatter corner

Approaches then retreats, creating a “shy” personality.

Scatter Corners

Ghost	Corner
Blinky	Top-right
Pinky	Top-left
Inky	Bottom-right
Clyde	Bottom-left

During scatter mode, ghosts retreat to their corners, giving players breathing room.

Mode Timing (Level 1)

Phase	Mode	Duration
1	Scatter	7 seconds
2	Chase	20 seconds
3	Scatter	7 seconds
4	Chase	20 seconds
5	Scatter	5 seconds
6	Chase	20 seconds
7	Scatter	5 seconds
8	Chase	Forever

Why It Works

Design Choice	Effect
Individual personalities	Distinct threat types
Mode alternation	Tension and release
Simple rules	Predictable when learned
Emergent complexity	Ghosts interact unpredictably

Implementation Considerations

Challenge	Solution
Pathfinding	Simple: pick direction closest to target
Reversals	Ghosts reverse on mode change
Tunnels	Ghosts slow down
No backtracking	Can’t reverse unless mode changes

Legacy

Ghost AI demonstrates that compelling game AI doesn’t require complexity—it requires personality. The same principles appear in countless games: distinct enemy types, behaviour modes, and emergent interaction from simple rules.

Modern Applications

Game	Ghost AI Influence
Spelunky	Distinct enemy behaviours
Pac-Man Championship Edition	Refined original system
Any stealth game	Patrol/alert/chase modes

See Also

• Pac-Man
• State Machine
• Enemy Design