Global illumination

Global illumination (GI) is the light that reaches a surface after bouncing off other surfaces, added to the light that arrives directly from a source. Godot's documentation sums up the goal: "Global illumination aims to mimic how light behaves in real life" (Godot Engine documentation: Introduction to global illumination). It covers indirect diffuse light, from bounces and from emissive materials, and indirect specular light, which you see as reflections (Godot Engine documentation: Introduction to global illumination).

#Why it matters

Without GI, anything a light cannot reach directly is black, or filled with a flat ambient colour. With GI, a sunlit floor lights the ceiling above it, a red carpet tints nearby walls, and rooms get the soft gradients that make spaces read as real. Much of what artists call "good lighting" is bounce light.

#Baked and real-time GI

For decades, GI in games was calculated offline. Epic's Lumen engineers describe how it "had to be solved in an offline process called lightmap baking" (Unreal Engine tech blog: Unreal Engine 5 goes all-in on dynamic global…). Today there are both baked and real-time options; the trade-offs are covered in baked vs. dynamic lighting.

Each real-time technique has its own weakness, and Godot's documentation lists them. VoxelGI gives "Good reflections and indirect lighting, but beware of leaks". With SDFGI, "Dynamic objects can receive GI, but not contribute to it". SSIL is a "Good secondary source of indirect lighting, but no reflections" (Godot Engine documentation: Introduction to global illumination).

#Common mistakes

• Thin walls. Real-time GI leaks light through geometry it cannot resolve. Epic's rule: walls no thinner than 10 cm (Unreal Engine Documentation: Lumen Technical Details).
• Using ambient light instead of bounces. A flat ambient colour brightens everything equally, including corners that should be dark. Use it only as a fallback, which is how Godot frames it (Godot Engine documentation: Introduction to global illumination).
• Forgetting dynamic objects. With lightmaps, characters need light probes to match the scene (Godot Engine documentation: Introduction to global illumination).
• Ignoring the colour of big surfaces. A saturated floor tints the whole room through GI. Check your base colour (albedo) map values; see physically based rendering.

Related: real-time lighting, ambient occlusion, emissive materials, light and shadow.

#Further reading and watching