FXAA causes severe shadow flickering when Lumen is enabled in UE 5.5.4

General Issues and Bug Reporting
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Summary

FXAA causes excessive shadow noise and persistent flickering when Lumen is enabled, especially on static backgrounds. Disabling VSM does not resolve the issue.

Please select what you are reporting on:

Unreal Editor for Fortnite

What Type of Bug are you experiencing?

Stability

Steps to Reproduce

  1. Open a new or existing project in UE 5.5.4

  2. Enable Lumen (Global Illumination: Lumen, Reflections: Lumen)

  3. Set r.AntiAliasingMethod to 1 (FXAA)

  4. Place static meshes in a scene with directional lighting and shadows

  5. Observe the shadow areas during camera movement or from static viewpoints

Expected Result

FXAA should provide aliasing reduction without introducing excessive noise or flickering in shadows.

Observed Result

When Lumen is enabled, shadows appear noisy and exhibit heavy flickering. This occurs even in static scenes. Disabling Virtual Shadow Maps (r.Shadow.Virtual.Enable=0) does not resolve the issue.

Platform(s)

Windows 11, Unreal Engine 5.5.4

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Additional Notes

This issue does not occur with TAA or TSR. It appears FXAA lacks the temporal accumulation needed to stabilize Lumen’s dynamic lighting output.