Fio is a liminal brush-based CSG editor and game engine that revives the spirit of classic tools like Radiant and Hammer. Built with a custom OpenGL/PyGame engine, it targets lightweight deployment on devices like the Snapdragon 8CX while maintaining OpenGL 3.3 compatibility. The unified forward renderer enables real-time lighting with stencil shadows without pre-baked compilation.

Designed for immediate "drop-in and play" workflows similar to CryEngine Editor, Fio packs powerful features into a compact package. The engine includes volumetric fog, glass and water shaders with configurable surface parameters, and an Entity I/O system inspired by Half-Life 2 for logic, triggers, and interactive elements. Terrain generation and OBJ model support round out the 3D capabilities.

Fio uses a JSON level format with magic number fingerprinting and operates under an MIT License, making it fully modular and open source. The project requires PyQt5, numpy, Pillow, PyOpenGL, pygame, and PyGL libraries. This modern reinterpretation of classic level editing tools offers developers a lightweight alternative for creating 3D worlds without the overhead of larger engines.