Programming

• Behavior trees and state machines with LimboAI
• Using Scene Builder
• Physics layers and navigation regions

Behavior trees and state machines with LimboAI

LimboAI addon overview

We'll be using LimboAI for behavior trees and state machines

LimboAI docs | LimboAI on GitHub

 

LimboAI includes a demo project. It is highly recommended that you download it and check it out. When you run it, it has a nice little tutorial that walks you through the basics: https://github.com/limbonaut/limboai/tree/master/demo 

In its classes and nodes, LimboAI uses the following prefixes:

• BT: Behavior Tree
  • Nodes/classes that start with this have to do with parts of behavior trees
  • As an example, the BTPlayer node "plays" a behavior tree every frame and inside of that tree you have BTSequence, BTSelector, etc
• BB: BlackBoard
  • The blackboard is a shared place for agents to write values to.
  • These values can be read and written by any task in the tree.
• LimboHSM: Hierarchical State Machine
  • HSMs have LimboStates inside of them.
  • LimboHSM is itself a LimboState, and BTState can put states inside of behavior trees

Behavior trees

Read the intro to behavior trees here

Accessing LimboAI behavior tree editor

When installed, LimboAI adds a button to the top of the Godot editor, alongside the 2D/3D/Script buttons

When you click on a BTPlayer node, it has a BehaviorTree attached to it. When clicking that, it will automatically switch over to the LimboAI tab.

Building blocks: sequences and selectors

The main building blocks of a behavior tree are:

• BTSequence: Think of these like an AND operator on the child tasks.
  • As long as a task return SUCCESS, the next one will be executed.
• BTSelector: Think of these like an OR operator on the child tasks.
  • On the first task that returns SUCCESS, it will stop executing.

Writing custom tasks

Read the documentation on writing custom tasks.

Custom tasks go inside of ai/tasks in the project.

At the time of writing, we have:

• follow_path: Follows a Path3D node (i.e. for preset patrol paths)
• generate_navigation_path / follow_navigation_path: Using a NavigationAgent3D, generate and follow a path to a target in the blackboard
  • follow_navigation_path will follow the path until it gets close enough to the target, it will return RUNNING until it gets there
• distance_based_cooldown: Takes in a Curve2D and uses that to do a cooldown based on how close the agent is to the target
  • Useful for generating navigation paths more frequently the closer you are to the target, and less frequently the further you are
• in_shape_and_visible: Returns SUCCESS if there's a target in a ShapeCast3D that is visible (hit by a raycast, in "front" of the agent)
  • This also sets the target when it is visible

State machines

https://limboai.readthedocs.io/en/stable/hierarchical-state-machines/create-hsm.html

 

Using Scene Builder

Scene Builder overview

The Scene Builder addon has been installed an initialized in the project. This is an open source addon written in GDScript.

Scene Builder can be used to easily place assets inside of a scene.

AssetPlacer is a popular addon for this functionality but it: a) Costs money, b) Is not open source, c) Requires the .NET (C#) build of Godot.

Using Scene Builder

On the right panel of the Godot editor, there is a Scene Builder tab:

Selecting that, you can see a series of "Palettes" and below that categories.

When selecting an item from the icon list, that item can then be placed into the scene.

To rotate an item along the Y axis when placing it, click and drag left/right.

Parent node

At the bottom of the scene builder tab, there's a "Parent" option that can be used to set which node items that are added as children to.

To set this, drag a node from the scene tree over to the "Parent" option.

If this doesn't work, click "Find world 3d" first.

Adding scenes to Scene Builder

To add scene(s) to the Scene Builder list,

1. Select the scene(s) you want to add from within the "FileSystem" tab
2. Open the command pallette
   • On all operating systems: ` (backtick, to the left of the "1" key)
   • On macOS: Command+Shift+P
3. Search for / select "Create scene builder items"
   • 
4. A window will pop up asking for info
   • 
   • Leave the "Randomize" toggles un-checked (unless you do want some randomness)
   • In "Collection", type in the name of the collection you want the scenes to show up under
     • 
     • At the time of writing these are Environments, Props, and Characters
5. Click "Create scene builder items with icons"
6. Click "Reload items" in the Scene Builder tab's options to show the new item
   • 

If the item doesn't show up, it could mean that there is a UID issue.
Open the .tscn file itself and then save it and try again - in source control you may notice that its UID gets updated.

Commit this change in source control and everybody should see your object show up in their scene builder list.

Managing Scene Builder

Scene builder has two resources inside the project that it uses to store data.

Data/scene_builder/collection_names.tres contains all of the names of the collections that are used. Double click on this in the FileSystem to view it in the Inspector.

---

Data/scene_builder/scene_builder_database.tres contains the database of scenes, stored in a few nested dictionaries. Double click on this and open it in the inspector to edit/remove items from the different collections.

Physics layers and navigation regions

Physics layers

Current physics layers are:

1. Environment: Anything in the environment that the player should collide with and get stopped by
   1. The player and the over-the-shoulder camera will collide with these
2. Player: The player is on their own layer
3. Walls: Anything that the overhead camera should interact with
   1. In general, all Walls should also be in the Environment group
   2. Props that the player might hide behind (i.e. pillars) should be in Envrionment but not Walls
      1. This way, when the player has the overhead camera active and they move behind something, the camera doesn't "snap" in front of the object
4. Pickups: Things that the player can pick up
   1. The player has a shapecast on them that checks for this

 

Navigation regions

Meshes that should be part of the navigation mesh must be added to the navigation_mesh_source_group group. This is a global group that should 

Navigation regions should be set to have their "Source Geometry Mode" set to "Group Explicit". This makes it so that only nodes in the navigation_mesh_source_group group get parsed as part of navigation mesh baking: