Artificial Intelligence in Game Development: Understanding Behavior Trees

Marco Secchi

600 min read
2024-11-18 10:34:35

0 Likes
0 Comments

Introduction

In the wild world of videogames, you'll inevitably encounter a foe that needs to be both engaging and captivating. This opponent isn't just a bunch of nice-to-see polygons and textures; it needs to be a challenge that'll keep your players hooked to the screen.

Let's be honest, as a game developer, crafting a truly engaging opponent is often a challenge that rivals the one your players will face!

In video games, we often use the term Artificial Intelligence (AI) to describe characters that are not controlled by the player, whether they are enemies or friendly entities. There are countless ways to develop compelling characters in video games. In this article, we'll explore one specific solution offered by Unreal Engine: behavior trees.

Note

Citations come from my https://github.com/marcosecchi/unreal-shooting-gym and open it up with Unreal Engine 5.4.

Once opened, you should see a level showing a lab with a set of targets and a small robot armed with a gun (A.K.A. RoboGun), as shown in Figure 1:

artificial-intelligence-in-game-development-understanding-behavior-trees-img-3

Figure 3. The AI behavior tree

Although this is a pretty simple behavior logic, there’s a lot of things involved here. First of all, you will notice that there is a Root node; this is where the behavior logic starts from.

After that, you will see that there are three gray-colored nodes; these are defined composite nodes.

“Composite nodes define the root of a branch and set the rules for its execution.”

Each of them behaves differently, but it is sufficient to say that they control the subtree that will be executed; as an example, the Shoot Sequence node will execute all the subtrees one after the other.

The purple-colored nodes are called tasks and they are basically the leaves of the tree, whose aim is to execute actions. Unreal Engine comes with some predefined tasks, but you will be able to create your own through Blueprints or C++.

As an example, consider the Shoot task depicted in Figure 4:

artificial-intelligence-in-game-development-understanding-behavior-trees-img-6

Figure 6. The character controller code

As you can see, it’s just a matter of running a behavior tree asset. Easy, isn’t it?

If you open the BP_RoboGun asset, you will notice that, in the Details panel, I have set the AI Controller Class to the RoboGunController; this will make the RoboGun pawn be automatically possessed by the RoboGunController.

Conclusion

This concludes this brief overview of the behavior tree system; I encourage you to explore the possibilities and more advanced features – such as writing your code the C++ way – by reading my new book “Artificial Intelligence in Unreal Engine 5”; I promise you it will be an informative and, sometimes, funny journey!

Author Bio

Marco Secchi is a freelance game programmer who graduated in Computer Engineering at the Polytechnic University of Milan. He is currently lecturer of the BA in Creative Technologies and of the MA in Creative Media Production. He also mentors BA students in their final thesis projects. In his spare time, he reads (a lot), plays (less than he would like) and practices (to some extent) Crossfit.