Flickering from Screen Probe Gather Temporal Filter

For objects with thin, strand-like mesh details, Lumen screen probe gather temporal filter is not enough to overcome shadow flicker. Grass, hair, other foliage are all affected. I can dramatically change the flickering by turning r.Lumen.ScreenProbeGather.Temporal on/off. The only variable I have been able to alter and improve flickering artifacts is r.Lumen.ScreenProbeGather.Temporal.DistanceThreshold=1, which sounds like it discards the previous frame’s lighting results. Does this have a performance impact? What other cvars can we adjust to reduce the flickering? It is more noticeable in some lighting environments compared to others.

Steps to Reproduce
Use the foliage tool to place grass with thin strands in the default open world map at a high density. Observe flickering.

Hello,

Thank you for reaching out.

I’ve been assigned this issue, and we will be looking into this flickering for you.

Hello,

These tickets, documentation, and blog posts deal with Lumen and foliage, and contain a lot of information about dealing with flickering on grass and other foliage.

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https://dev.epicgames.com/documentation/en\-us/unreal\-engine/lumen\-technical\-details\-in\-unreal\-engine?application\_version\=5\.5\#lumenraytracing

https://dev.epicgames.com/documentation/en\-us/unreal\-engine/lumen\-global\-illumination\-and\-reflections\-in\-unreal\-engine?application\_version\=5\.5\#lumentwo\-sidedfoliage

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https://www.unrealengine.com/en\-US/tech\-blog/lumen\-brings\-real\-time\-global\-illumination\-to\-fortnite\-battle\-royale\-chapter\-4

Please let us know if this helps.