Small, bright, random color flashes in Volumetric Fog with Lumen 5.6.1

Development Rendering
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This is a quote from the documentation linked above:

Real-time rendering with Lumen works well for still shots or scenes where the camera moves forward and there are no animated objects in the frame.

2026-02-11_14-05-59
2026-02-11_14-05-591656×1324 425 KB

This is kinda ridiculous, right? XD And while I do appreciate the honesty, sadly (but unsurprisingly) this doc has been written by a “partner”, not by Epic themselves.

Anyway. Sorry for hijacking the thread a bit, but you mentioned “smudging”, and thus googling for “unreal engine lumen volumetric fog smudging” leads here, so I thought I’d share my 2 cents.

I’ve stumbled upon the same (or at least similar) issue as mentioned in the doc:

A few observations:

  • In the doc, the solution is switching from Lumen to Ray Tracing, which - as has been already said - is not really a solution for gamedev.
  • While this kinda seems like a Volumetric Fog issue, it’s simultaneously a Lumen issue, as disabling it (in Post Process volume, or via r.Lumen.DiffuseIndirect.Allow 0, like I demonstrate in the video) fixes the problem.
  • Disabling Temporal Reprojection (r.VolumetricFog.TemporalReprojection 0) also fixes the problem (allowing to keep Lumen), although it did introduce slight background flickering right above the horizon (not really visible in the video, but it was there before I stripped the scene of all assets for the demonstration).
  • Decreasing History Weight (e.g. r.VolumetricFog.HistoryWeight 0.5) also seems to fix the problem (with both Lumen and Temporal Reprojection enabled).
  • Messing with GridSizeZ and GridPixelSize didn’t really help in my case.
  • As a side note, decreasing Volumetric Scattering Intensity in the Directional Light e.g. to 0.1 seems to worsen the ghosting, allowing for easier debugging (for the video, I set it to 0.5).

Not sure which solution I’ll go with in the end; might end up disabling Lumen after all, as I did for our previous project. In the meantime, perhaps this will help someone in the future. Thanks for the links, BTW.

Edit: a related thread: https://forums.unrealengine.com/t/ue5-sky-volumetric-lighting-ghosting-any-solution/1283444

Edit 2: there’s a possibility that increasing View Distance too much in Exponential Height Fog’s Volumetric Fog settings may have made the issue so bad in the first place (at least in my case).