IINinja / godot-steering-toolkit

An asset kit of steering behaviors for Artificial Intelligence agents in the Godot game engine.

Godot Steering AI Framework

Godot Steering gives you tools to create complex AI agents using steering behaviors in Godot.

Steering Behaviors

There are two main aspects to creating AI-driven agents in games:

1. Decison-making. Based on what the AI knows, we want it to decide how should it behave, what should it react to, when, etc.
2. Reacting. Based on the decision the AI took, it needs to figure out how to move to its target or how to act.

Steering Behaviors help with reacting. They are behaviors the AI undergoes to steer its agents through the game world.

For example, an enemy spaceship pursuing the player through an asteroid field needs to avoid colliding with the asteroids. Yet, it should still move towards the player overall. Steering behaviors are good at that, and at producing smooth, natural motions.

Steering behaviors are a flexible and low performance algorithms to move around immediately, in a second-to-second resolution. Unlike more complex algorithms like the A* pathfinding algorithm, the agent doesn't get a complete path to the target.

Steering behaviors may not have as fine a resolution as complex pathfinding, but they are easier to use and more than enough for many games. Also, they combine well with more complex pathfinding methods like A*.

About this framework

This framework is designed to work with the Godot game engine. It is entirely in GDScript and designed to keep scene trees from getting bloated. It helps game developers making complex behaviors more easily with tools to prioritize, combine, and sequence modular steering behaviors together.

Design and Features

This framework takes a lot of its inspiration from the AI module from LibGDX: GDX-AI. It's an exllencent base to work from.

Some of the information below is covered in greater details in the GDX-AI Wiki, although this framework is quite different in the way it works. The GDX-AI wiki makes for an excellent primer on steering-based AI and includes a few diagrams, although you can skip the Java-specific part and technical details when reading it.

Below are the main concepts of our framework.

Agent

The Agent type holds information on the actor's position, velocity of travel, current orientation, as well as speed limits, mass, drag, size and other similar information. This information is used by the behaviors. It is up to the programmer to keep this information up-to-date.

Proximity

A way of defining a number of Agents within an area around an owner. For instance, in a spaceship game, the squad leader's ship would be the owner of the Proximity, and the agents would be in the group and their group-based behaviors would use that information to know where the area is and where it is going.

TargetAcceleration

The result of each behavior results in a desired change in velocity, linear or angular. This can be added to the current velocity to go up and down until the target is reached. The TargetAcceleration type holds both a linear and an angular acceleration, though whether they are used depends on which behavior is used.

Behaviors

• Combination Behaviors - More often than not, one behavior will not be enough to do everything you want your agent to do. These special behaviors take other behaviors and works to combine them. Combination behaviors can combine other combination behaviors.

  • Blended - Takes any number of behaviors and associates each with a strength that determines how strong that behavior is going to be. The final acceleration will be a combination of all behaviors in the blend.
  • Priority - Takes any number of behaviors, but the order matters - only the first to return a non-zero acceleration will be used, and all others ignored. Great for prioritizing avoiding firey death by crashing into an asteroid over pursuing the player.

• Individual Behaviors - Behaviors that only feature the agent and some target information, often another agent.

  • Seek/Flee - Produces acceleration to take the agent to where the target is right now. Flee is the opposite and moves away from where the target is.
  • Arrive - Produces acceleration to take the agent to where the target is, but slow down to zero velocity by the time it is on top of it.
  • MatchOrientation - Produces angular acceleration to turn the agent to match the target's own facing, and slow down to zero velocity by the time it is matched.
  • Pursue/Evade - Produces acceleration to take the agent to where the target will be by the time it is reached. Evade is the opposite and moves away from where the target is going to be.
  • Face - Produces angular acceleration to turn the agent towards the target. A turret could use this to keep a bead on enemies.
  • LookWhereYouAreGoing - Produces angular acceleration to turn the agent towards the direction it is traveling. This creates an agent that moves more naturally, especially if they can then choose to move independent of their travel upon need, like humans.
  • FollowPath - Produces acceleration that will lead the agent into following a given series of points that form a path. This can be a path generated by a complex pathfinding algorithm like A*.
  • Intersect - Produces acceleration that will lead the agent to cut through an imaginary line between two other agents. For example, a bodyguard could try to bodyblock for a projectile to protect another agent.
  • MatchVelocity - Produces acceleration that will lead the agent to have the same velocity as its target. Mimics that pretend they're the players could use this.
  • Jump - Produces acceleration that will lead the agent to have enough velocity to jump against gravity from a target starting point with enough speed to land at the target landing point.

• Group Behaviors - Behaviors that use Proximity objects to keep track of other agents in a given group.

  • Separation - Produces acceleration that will keep the agent away from a distance from other neighbors in the Proximity group. Units in a RTS could be made to smartly keeping away from neighbors with an contagious infection, or units in spaceships keeping away from their target because their weapons are dangerous to themselves in short range (missiles).
  • Alignment - Produces angular acceleration that will rotate the agent to face along with the other agents in the Proximity group. Spaceships in a flight formation, for example.
  • Cohesion - Produces acceleration that will aim to keep the agents in the Proximity group in a balanced group within the Proximity. An example is a sheep catching up to the rest of its flock.
  • Hide - Produces acceleration that will keep the agent behind an object in the Proximity and a target agent. It could be a stalker in the forest staying out of sight but following the player.
  • AvoidCollisions - Produces acceleration that will keep the agent from hitting objects in the Proximity group. For example, moving through a minefield without hitting any of the mines.
  • RaycastAvoidCollisions - Produces acceleration that will prevent the agent from hitting something that it is usually facing - great for discovering that there is an obstacle in the agent's direction of travel, whereas AvoidCollisions works in a radius around the agent.

Resources, links, and information

• RedBlobGames - An excellent resources for complex pathfinding like A*, graph theory, and other algorithms that are game-development related. Steering behaviors are not covered, but for anyone looking to study and bulk up on their algorithms, this is a great place.
• Understanding Steering Behaviors - Breakdowns of various behaviors by Fernando Bevilacqua with graphics and in-depth explanations that go beyond the scope of this document or GDX-AI's wiki entries.
• GDX-AI Wiki - Steering Behaviors - Descriptions of how LibGDX's AI submodule uses steering behaviors with a few graphics.