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

**URL:** <https://forums.unrealengine.com/t/small-bright-random-color-flashes-in-volumetric-fog-with-lumen-5-6-1/2680882>\
**Category:** Rendering\
**Tags:** unreal-engine, question\
**Created:** [December 1, 2025, 4:55pm UTC](https://forums.unrealengine.com/t/small-bright-random-color-flashes-in-volumetric-fog-with-lumen-5-6-1/2680882 "2025-12-01T16:55:05Z")\
**Posts on this page:** 1\
**Showing post:** 4

<div class="post-metadata">

**Author:** ![rosenand](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/rosenand/32/1141324_2.png) [@rosenand](https://forums.unrealengine.com/u/rosenand)\
**Post date:** [February 11, 2026, 3:49pm UTC](https://forums.unrealengine.com/t/small-bright-random-color-flashes-in-volumetric-fog-with-lumen-5-6-1/2680882/4 "2026-02-11T15:49:23Z")

</div>

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](https://d3kjluh73b9h9o.cloudfront.net/original/4X/0/d/f/0df6d01eda325940e5dbc521529319f3fae86a2d.jpeg)

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](https://forums.unrealengine.com/t/ue5-sky-volumetric-lighting-ghosting-any-solution/1283444/2)

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).

---

_[View the full topic](https://forums.unrealengine.com/t/small-bright-random-color-flashes-in-volumetric-fog-with-lumen-5-6-1/2680882)._