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Fusion Engine

Fusion Engine is a 2D game engine built specifically for creating reinforcement learning (RL) training environments, scripted entirely in Python. Its component model is inspired by Unity and its nested-scene model by Godot, but unlike either of those, the engine's whole purpose is producing lightweight, fast-stepping environments to train RL agents in — not shipping games.

Platform support

Fusion Engine currently only runs on Windows. There is no macOS or Linux support at this time.

This site covers the Python scripting API — the surface you write scripts against. For the engine's internal architecture (the rendering pipeline, the three physics solvers, the ECS, the editor, and the export pipeline) see the design document (PDF).

Where to start

  • New to Fusion? Start with Getting Started — install the engine, create a project, and attach your first script.
  • Writing scripts? See the Scripting Guide for the full Python API: components, exports, physics, input, and RL agents.
  • Looking up a class or function? Jump into the API Reference.

How the engine fits together

System Responsibility
Rendering Engine OpenGL-based rasterization renderer
Physics Engine Rigid body (PGS), soft body (XPBD), and fluid (PBF) simulation, unified through a shared impulse-based coupling layer
Object and Component Manager Manages entities and their components (ECS)
Editor ImGui-based editor UI, windows, and gizmos
File and Project Manager Project/scene serialization, resource paths (res://), and export
Script Manager The C++ ↔ Python bridge (pybind11) and script lifecycle

Reinforcement learning support (the fusion.RL module, the AgentComponent, headless training, and the training monitor) is an optional package, not part of the base engine — you enable it from the project configuration screen, and it's what pulls in Gymnasium, Stable-Baselines3, NumPy, and PyTorch into your project's virtual environment.

Quick example

Every Fusion script starts with from fusion import *, so the engine's classes and functions (Script, Vector3, export, component types, and so on) are available unqualified:

from fusion import *


class Enemy(Script):
    speed = export(3.0)

    def OnStart(self):
        self.transform = self.get_component(TransformComponent)

    def Process(self, delta):
        self.transform.world_position += Vector3(self.speed * delta, 0, 0)

speed here is an exported property — export(3.0) makes it show up as an editable field in the inspector, defaulting to 3.0, while still working as a normal Python attribute in code. See Exporting properties for the full set of export widgets (ranges, drag fields, color pickers, file pickers, and collapsible sections).