Why is thin geometry noisy in Lumen?

Lumen uses screen-space + distance field traces. Thin geometry (fences, hair, leaves) is poorly represented in distance fields; screen traces miss between samples. Result: flickering reflections.

What's Detail Trace Mode?

Higher-precision Lumen trace using full mesh data. Slower but accurate for thin objects. Enable via r.Lumen.ScreenProbeGather.DetailTrace 1.

View Lumen visualization (Show → Visualize → Lumen Scene). Thin geometry should be visible — not missing or sparse. Adjust mesh distance field resolution per asset if needed.

Fix: Unreal Lumen Screen Traces Noisy on Thin Geometry

Quick answer: Enable Detail Trace Mode for hero shots, bump mesh distance field resolution on thin assets, and increase max screen trace steps.

An open-world scene with chain-link fences and tall grass shows shimmering reflections under Lumen. Movement amplifies the noise. Thin geometry is hard for Lumen’s tracing approximations.

Distance Field Resolution

Open the static mesh asset:

Build Settings → Distance Field Resolution Scale: bump to 2.0 or 4.0 for thin meshes.
Rebuild distance field.

More resolution = better representation in Lumen’s SDF traces. Costs memory; only do for thin/expensive assets.

Detail Trace Mode

r.Lumen.ScreenProbeGather.DetailTrace 1
r.Lumen.Reflections.DetailTrace 1

Activate via console or project ini. Slower but more accurate. Profile cost on target hardware before shipping.

Screen Trace Step Count

r.Lumen.ScreenProbeGather.ScreenTraces.HZBTraversalNumThicknessStepsToDetermineHit 8

Raise hit-detection steps. Catches thin objects more reliably. Default ~4; 8 is a good balance.

Per-Asset Distance Field Settings

Hair, fences, leaves benefit from custom DF. Aerial vegetation can use 2-sided distance field generation to avoid thin-edge misses.

Lumen Scene Visualization

Show → Visualize → Lumen Scene. Confirms thin assets exist in the Lumen scene. If invisible: their DF is missing or too coarse.

Verifying

Walk past thin geometry; reflections stable. Spectroscope shows reduced temporal flicker. Frame rate within budget on target.

“Lumen handles bulk geometry well. Thin assets need explicit DF investment or Detail Mode.”

For cinematics, switch r.Lumen.Reflections.DetailTrace 1 just for those shots — quality matters more than runtime cost for marketing.