Fog Artifacts in 5.8

Development Rendering
, ,
1

When using thick fog combined with high frequency detailed meshes, artifacts start to show in 5.8. These are not visible in 5.7.

The common case for us is megascan assets like the river rock ground pieces you can get on FAB for example. But any asset that has a lot of depth variation over a small surface area can repro the issue. It is a bit difficult to see in the video, but large areas of nanite detail can show up very well in thick fog and is quite distracting.

I believe this may be happening with Volumetric Clouds as well, but I have not confirmed that yet.

I have tried reverting HeightFogCommon.ush and HeightFogPixelShader.usf back to 5.7 and that does not fix the issue.

I also tried turning off Fog Screen Space Scattering using convars, but that also did not fix the issue.

Any help here would be appreciated.

FogBug.mp4(8.01 MB)

2

Steps to Reproduce

  1. Create a “Basic” level.
  2. Turn on “Volumetric Fog” in the ExponentialHeightFog actor.
  3. Add a backstop wall
  4. Add “Icon_Base” meshes in a clump
  5. Add a cube and map a volumetric material to it
  6. Make the volumetric material have Albedo=0 and Extinction=1
  7. Roll camera in and out and observe artifacts
3

I tracked down the problem. It is actually due to TSR. The issue came in on 2/11/26 in TSRDetectThinGeometry.usf.

Commenting out this line will fix the problem:

//#define CONFIG_VARYING_RANGE_DETECTION (DIM_VARYING_RANGE_DETECTION)

I have verified this affects both fog and volumetric clouds in cases where either is thick and there are high frequency meshes in or behind the medium that produce lots of small depth changes.

4

Hey Tiantian,

That fixes it up as well and avoids needing to change some source code. I do notice with either my original fix or the convar, foliage does stand out a bit in dense fog when moving as well. It’s not nearly as egregious as it was without the fixes, but just thought I’d mention it.

Thank you for the reply!

-Jared

5

Hi Jared,

You can try set r.TSR.ThinGeometryDetection.Coverage.ShadingRange to 1 so we will not relax rejection on regular materials that is newly added in 5.8 for TSR.

Thanks,

Tiantian

6

It would be a kind of trade-off when Nanite Foliage is used. You can read the release page of thin geometry detection for TSR (https://dev.epicgames.com/documentation/unreal\-engine/thin\-geometry\-detection\-with\-temporal\-super\-resolution) for more details and control. Like if your fog can be translucent material afterDOF, it should be of much less visible.