Godot Behavior Tree & Navigation Tutorial (P4 - Enemy Behavior Tree)

Introduction

In this tutorial, we will create an enemy behavior tree using the Godot engine to enhance game AI. This guide follows a video tutorial by NickCY, where we will download a behavior tree plugin, set up an enemy scene, and refactor player and enemy scripts. You will learn how to implement pathfinding and movement tasks for your enemies, making your game more interactive and engaging.

Step 1: Download Behavior Tree Plugin

1. Visit the behavior tree plugin repository at fian46's GitHub.
2. Download the plugin files and extract them.
3. Open your Godot project, then navigate to the "Addons" folder.
4. Move the extracted plugin files into the "Addons" folder of your project.
5. Enable the plugin in Godot by going to "Project" > "Project Settings" > "Plugins" and toggling the behavior tree plugin to "Active".

Step 2: Create Enemy Scene

1. In Godot, create a new scene and name it Enemy.
2. Add a Node2D as the root node and rename it to Enemy.
3. Add a Sprite node as a child of the Enemy node to represent the enemy visually.
4. Assign a texture to the sprite to give the enemy a unique appearance.

Step 3: Make New Testing Behavior Tree Task

1. Create a new behavior tree (BT) for the enemy by right-clicking in the FileSystem panel and selecting "New Resource".
2. Choose "BehaviorTree" as the resource type and name it EnemyBT.
3. Open the behavior tree and add a root node.
4. Add child nodes, such as Sequence or Selector, according to your AI logic.
5. Save the behavior tree.

Step 4: Test the Behavior Tree

1. Attach the behavior tree to the Enemy node by adding a new script.
2. In the script, load the behavior tree resource using:

   var behavior_tree = preload("res://path/to/EnemyBT.tres")
   

3. Call the tick() method on the behavior tree in the _process(delta) function to update its state.

Step 5: Understand Behavior Trees

• Behavior trees are hierarchical structures that define AI behavior in a modular way.
• Each node represents a task or condition, which can be combined to create complex behaviors.
• Use Sequence nodes for tasks that must be completed in order and Selector nodes for tasks where only one needs to succeed.

Step 6: Implement Pathfinding Task

1. Create a new task in the behavior tree for pathfinding.
2. Use Godot's built-in navigation system to calculate paths.
3. Implement the pathfinding logic in the task's script, making sure to handle obstacles and target destinations.

Step 7: Create Move Task

1. Add a movement task to the behavior tree.
2. In the movement task script, use the following code to move the enemy towards a target:

   func move_towards(target_position):
       position = position.move_toward(target_position, speed * delta)
   

3. Ensure that the movement respects the navigation mesh.

Step 8: Test Enemy Behavior

1. Run the game to check if the enemy behaves according to the defined behavior tree.
2. Adjust the parameters in the behavior tree if necessary to improve the enemy AI's responsiveness.

Step 9: Create Player Scene

1. Create a new scene named Player.
2. Add a Node2D as the root and attach a Sprite for the player representation.
3. Configure player controls in the script to allow movement.

Step 10: Refactor Player and Enemy Scripts

1. Create a new Human class to encapsulate shared functionality for both player and enemy.
2. Move common properties and methods into the Human class.
3. Update the player and enemy scripts to inherit from this new class.

Step 11: Fix Player and Enemy Position

1. Ensure that both player and enemy positions are correctly set in the game world.
2. Adjust the initialization code in both scripts to set starting positions based on the game's layout.

Conclusion

In this tutorial, we successfully set up a behavior tree for an enemy in the Godot engine, implementing pathfinding and movement tasks. We also created a player scene and refactored the scripts for better organization. With these steps, you now have a foundational understanding of behavior trees in Godot, paving the way for more complex AI behaviors in your games. For further enhancements, consider exploring additional tasks or conditions to enrich your enemy AI.