I haven’t been able to find threads about this specific issue, but from time to time, bright colored flashes (often red, green, blue, or a mix) appear in the volumetric fog. GI doesn’t seem to be impacted, but I haven’t noticed this bug when disabling Lumen. It has unfortunately been really hard to reproduce consistently. These flashes appear both in editor and in shipping builds.
I’m using SWRT Lumen and reflections, no Mega Lights, no VSM, no Nanite.
I captured one instance and made a GIF. I’m only using spots and point lights, and there are no emissive materials contributing to Lumen near the artefact (I’m using the node “Ray Tracing Quality Switch Replace = 0” in most materials). If anyone has any idea what it is and how to fix it, it’d be awesome!
Thanks for the reply, but setting TemporalReprojection to 0 made the fog super stuttery (it’s very dense in my scene). The solution and tips suggested are aimed at virtual production and sacrifice performance for visual quality, while I’m working on a game, so I can’t afford to tank performance
Though I did have the smearing issue they’re talking about in this post. Setting r.VolumetricFog.HistoryWeight to a lower value helped a lot! I found this Steam discussion, which also helped.
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 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).
