The 3D Handbook
Part 8 — Building in the engine Lighting and renderingPage 184 of 269 on the reading route
Key term

Global illumination

Light that bounces between surfaces before reaching the eye; why it matters for believable scenes, and the baked and real-time techniques Unreal, Unity and Godot use for it.

lightingglobal-illuminationlumenlightmaps

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.

A cross-section of a room. Sunlight enters through a window and hits a red carpet. Dashed red lines bounce from that point up to the ceiling and the far wall, which are tinted red.
Direct light (orange) hits the carpet; GI adds the bounces (red), which carry the carpet's colour onto the ceiling.Diagram by the handbook author
A box-shaped room with a red left wall, a green right wall and a white back wall, lit by a small white emissive square in the ceiling. Two dark blocks stand on the floor. The white walls and ceiling are tinted red and green near the coloured walls.
Colour bleeding in a Cornell-box-style room. The only light source is the white emissive square at the top, yet the white wall and ceiling are tinted red and green near the coloured walls, because light bouncing off them is reflected onto the rest of the scene. Render from the Godot documentation.Godot Engine documentation (Juan Linietsky, Ariel Manzur and the Godot community), CC BY 3.0

#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.

Engine Baked GI Real-time GI
Unreal Engine 5 Lightmaps (hidden when Lumen is on) (Unreal Engine Documentation) Lumen, the default GI and reflections system
Unity 6 Progressive GPU lightmapper; Max Bounces default 2 (Unity Manual) Enlighten Realtime Global Illumination (Unity Manual)
Godot 4 LightmapGI: "Excellent indirect lighting" (Godot Engine documentation) VoxelGI, SDFGI and SSIL, Forward+ renderer only

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).

A pale blue sci-fi interior. Sunlight comes through an opening at the back, but the walls, floor and ceiling are all the same flat blue-grey. A dark cube and a glossy sphere sit in the foreground.
No global illumination, only constant environment lighting. The sunlit opening has no effect on the rest of the room. Screenshot from Godot.Godot Engine documentation (Juan Linietsky, Ariel Manzur and the Godot community), CC BY 3.0
The same sci-fi interior with VoxelGI. The area around the sunlit opening is now filled with warm yellow bounced light, the shaded walls are darker, and the glossy sphere reflects the yellow light.
The same view with VoxelGI. Bounced sunlight tints the walls and floor yellow, and the sphere picks up the bounce in its reflection. The documentation warns that VoxelGI leaks light through walls thinner than one voxel. Screenshot from Godot.Godot Engine documentation (Juan Linietsky, Ariel Manzur and the Godot community), CC BY 3.0
Epic's example of large-world GI using Lumen's Far Field traces (Unreal Engine Documentation: Lumen Technical Details). Look at the areas the sun does not reach directly: their light and colour come from bounces, not from a flat ambient fill.Epic Games, Unreal Engine documentation

#Common mistakes

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

#Further reading and watching

Epic's Unreal Engine 5 reveal demo, linked from Epic's own Lumen tech blog. Watch for soft bounce light in areas no light reaches directly. Watch on YouTube

References

  1. Godot Engine contributors · Introduction to global illumination · Godot Engine documentation (source on GitHub), 2026 · docs · accessed 2026-10-21
  2. Daniel Wright, Krzysztof Narkowicz · Unreal Engine 5 goes all-in on dynamic global illumination with Lumen · Unreal Engine tech blog, 2022 · article · accessed 2026-10-21
  3. Epic Games · Lumen Global Illumination and Reflections · Unreal Engine Documentation, 2026 · docs · accessed 2026-10-21
  4. Unity Technologies · Baked Lighting Settings (Lighting Settings Asset reference) · Unity Manual, 2026 · docs · accessed 2026-10-21
  5. Unity Technologies · Lighting Settings Asset · Unity Manual, 2026 · docs · accessed 2026-10-21
  6. Epic Games · Lumen Technical Details · Unreal Engine Documentation, 2026 · docs · accessed 2026-10-21
  7. Epic Games · Lumen brings real-time global illumination to Fortnite Battle Royale Chapter 4 · Unreal Engine tech blog, 2022 · article · accessed 2026-10-21
  8. NVIDIA · Metro Exodus PC Enhanced Edition Out Now With Improved Ray Tracing And DLSS 2.0 · NVIDIA GeForce News, 2021 · article · accessed 2026-10-21

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Last updated 2026-10-22