The Project | About Us | Contribute | Donations | License
HOME My courses Completed
◄ Previous
◄

Behavior Trees

Next ►
►

Behavior Trees (BTs) are a powerful and flexible way to model the decision-making process of game characters. They are widely used in the game industry due to their modularity, reusability, and ease of understanding.

Key Concepts of Behavior Trees

  1. Nodes: The basic building blocks of a behavior tree. Each node represents a specific behavior or decision.
  2. Root Node: The starting point of the behavior tree.
  3. Composite Nodes: Nodes that can have one or more child nodes. They control the flow of execution.
  4. Decorator Nodes: Nodes that modify the behavior of their child nodes.
  5. Leaf Nodes: The end nodes that perform actions or checks.

Types of Composite Nodes

  1. Sequence: Executes child nodes in order until one fails.
  2. Selector: Executes child nodes in order until one succeeds.
  3. Parallel: Executes all child nodes simultaneously.

Types of Decorator Nodes

  1. Inverter: Inverts the result of its child node.
  2. Repeater: Repeats the execution of its child node.
  3. Succeeder: Always returns success regardless of the child node's result.

Types of Leaf Nodes

  1. Action: Performs a specific task.
  2. Condition: Checks a condition and returns success or failure.

Example of a Behavior Tree

Let's consider a simple example where an NPC (Non-Player Character) patrols an area and chases the player if they are detected.

Behavior Tree Structure

  • Root
    • Selector
      • Sequence (Patrol)
        • Action (Move to Waypoint 1)
        • Action (Move to Waypoint 2)
      • Sequence (Chase Player)
        • Condition (Player Detected)
        • Action (Move to Player)

Pseudocode Implementation

class Node:
    def run(self):
        pass

class Action(Node):
    def __init__(self, action):
        self.action = action

    def run(self):
        return self.action()

class Condition(Node):
    def __init__(self, condition):
        self.condition = condition

    def run(self):
        return self.condition()

class Sequence(Node):
    def __init__(self, children):
        self.children = children

    def run(self):
        for child in self.children:
            if not child.run():
                return False
        return True

class Selector(Node):
    def __init__(self, children):
        self.children = children

    def run(self):
        for child in self.children:
            if child.run():
                return True
        return False

# Actions
def move_to_waypoint_1():
    print("Moving to Waypoint 1")
    return True

def move_to_waypoint_2():
    print("Moving to Waypoint 2")
    return True

def move_to_player():
    print("Chasing Player")
    return True

# Conditions
def player_detected():
    print("Player Detected")
    return True

# Building the Behavior Tree
patrol_sequence = Sequence([Action(move_to_waypoint_1), Action(move_to_waypoint_2)])
chase_sequence = Sequence([Condition(player_detected), Action(move_to_player)])
root = Selector([patrol_sequence, chase_sequence])

# Running the Behavior Tree
root.run()

Explanation

  • Action Nodes: move_to_waypoint_1, move_to_waypoint_2, and move_to_player are actions that the NPC can perform.
  • Condition Node: player_detected checks if the player is detected.
  • Sequence Node: patrol_sequence and chase_sequence are sequences of actions and conditions.
  • Selector Node: root selects between patrolling and chasing based on the conditions.

Practical Exercise

Exercise 1: Extend the Behavior Tree

Extend the behavior tree to include a new action where the NPC returns to a safe zone if the player's health is low.

Steps:

  1. Add a new condition to check if the player's health is low.
  2. Add a new action to move to the safe zone.
  3. Integrate these into the existing behavior tree.

Solution

# New Actions
def move_to_safe_zone():
    print("Moving to Safe Zone")
    return True

# New Conditions
def player_health_low():
    print("Player Health Low")
    return True

# Building the Extended Behavior Tree
return_to_safe_zone_sequence = Sequence([Condition(player_health_low), Action(move_to_safe_zone)])
root = Selector([return_to_safe_zone_sequence, patrol_sequence, chase_sequence])

# Running the Extended Behavior Tree
root.run()

Explanation

  • New Action Node: move_to_safe_zone is the action to move to the safe zone.
  • New Condition Node: player_health_low checks if the player's health is low.
  • New Sequence Node: return_to_safe_zone_sequence is the sequence for returning to the safe zone.
  • Updated Selector Node: root now includes the new sequence for returning to the safe zone.

Summary

Behavior Trees provide a structured and modular way to implement complex behaviors in game characters. By understanding and utilizing nodes, sequences, selectors, and decorators, you can create flexible and reusable AI behaviors. The practical exercise helps reinforce the concepts by extending the behavior tree with new actions and conditions.

◄ Previous
◄
Next ►
►
The Project | About Us | Contribute | Donations | License
© Copyright 2024. All rights reserved