Behaviour Trees

This week, we’ll be taking a look at behaviour trees in games and how they can be applied.

Behaviour Tree Recap

Recall from the lecture:

  • A behaviour tree is made of nodes, representing tasks

  • Basic actions in the games form the leaf nodes of our tree

  • Notes are combined together using composite tasks

  • Infomation can be shared between nodes using Blackboards

Lets review the most common types of composite types:

Selector

Run each node in order, if a node succeeds return SUCCESS, else return FAILURE when no nodes are left
Sequence

Run each node in order, if a node fails return FAILURE, else return SUCCESS when no nodes are left
Parallel

Run nodes at the same time, rules regarding success and failure are determined by the node itself

Example

Examine the following behaviour tree, and results table.

flowchart TB
  Selector --> A
  Selector --> B
  Selector --> Sequence
  Sequence --> C
  Sequence --> D
some rows of the table might not be needed.
Node Example 1 Example 2 Example 3

A

FAILURE

FAILURE

SUCCESS

B

SUCCESS

FAILURE

SUCCESS

C

FAILURE

SUCCESS

SUCCESS

D

FAILURE

SUCCESS

SUCCESS
Task: Evaluating Behaviour Trees

For each of the columns, what nodes would have been executed and in what order?

Using Behaviour Trees in Unity

There are many behaviour tree assets in Unity. A few years ago, I implemented one for COMP280. This was a 'code only' solution that some of you may remember if you were on the Unity track for COMP280 with me.

The repository is available on the github server. We’ll be using the same repo, but we’ll use it in slightly different ways. Find the Assets/Scripts/bt folder.

The controller script is located in BtController.cs. Use a tool (such as mermaid JS or similar) to map out what the behaviours implemented in the Create Tree method.

Task: Evaluating Behaviour Trees

Use a suitable diagramming tool to document the current behaviour. The tool that was used above was mermaidjs

Implementing new node types

Task: Implement randomised select and sequence

There are two 'advanced' form of composite nodes which can be implemented. These are non-deterministic versions of the `sequence' and `select' nodes. How would you implement these?

Reading

Game Engine Spesific

Game AI Pro

Last updated 2022-12-09