In this lesson, you will learn how to inspect the simplified "Lumen Scene" representation that Unreal Engine uses to perform high-speed global illumination calculations.
When you use Lumen, the engine doesn’t actually perform ray tracing against your full-resolution Nanite geometry. Instead, it generates a simplified, lightweight proxy of your scene—called the Lumen Scene—to keep performance viable for dynamic lighting. To debug lighting artifacts or explain why an object isn't casting light correctly, you must be able to view this data directly.
Accessing the Visualization Modes
To see how the engine perceives your world, navigate to the Viewport menu in the top-left corner, select View Mode, hover over Lumen, and choose Lumen Scene.
When this mode is active, the scene will look like a simplified version of your geometry. You are looking at the "Surface Cache"—the collection of cards and meshes that Lumen uses as a low-resolution representation.
Why Inspect the Lumen Scene?
If you notice that global illumination is bleeding through walls or an object is missing its bounce lighting, the culprit is often the Lumen Scene representation. You should look for:
- Missing Geometry: If a wall is missing in the Lumen Scene view, Lumen will effectively treat it as invisible for lighting calculations, causing light leakage.
- Overlapping Cards: Lumen uses "cards" to capture light. If these cards are overlapping or incorrectly oriented, you may see flickering or "splotchy" lighting.
- Mesh Distance Field Quality: Since Lumen relies on Distance Fields for its internal calculations, visualizing the Lumen Scene helps you identify meshes that have poor Distance Field resolution, which results in blocky or inaccurate shadows.
Console Command Shortcut
If you want to quickly toggle this view while iterating, you can use the following console command in the backtick (`) menu:
r.Lumen.Visualize.Scene 1
Setting it to 0 returns your viewport to the standard Lit mode. By toggling this, you can verify that your high-poly Nanite assets are being properly "cardified" for the global illumination pass, ensuring your dynamic lighting remains both stable and accurate.
Key Takeaway: The Lumen Scene view allows you to see the simplified geometric proxy Lumen uses for calculations, enabling you to troubleshoot light leakage and lighting inaccuracies caused by poor Surface Cache coverage.