What causes the flickering on grass meshes while using Lumen?


I can’t for the life of me figure out why my grass is flickering when I swap lighting over to Lumen in both 5.0.3 and 5.1.0.
I’ve tried many different thread and video approaches but can’t seem to pin point where this issue is coming from.
Any grass that is in shadow will cause this flickering issue. In the gif it is on the right side.

Things I have tried:
*Setting the mesh to Nanite
*Messing with Distance Field options on the mesh
*Using a plain material with no instructions other than base color
*Same thing happens on grass types in landscape and placed actors and foliage actors
*Shadow Maps and Virtual maps
*Higher lightmap resolution
*Different grass meshes do similar things


Any help here would be greatly appreciated as I’d love to use Lumen in my project but can’t with this grass flickering issue.

The only way I can get this issue to stop in editor is turn off “Screen Traces” but I assume that isn’t a solution

You can get around that by lowering r.Lumen.ScreenProbeGather.DownsampleFactor to 16 or 8 via console command, but you’ll get a noticeable performance hit.

Ah, I assumed it was potentially something related to WPO before 5.1 but it looks to be just something with the way lumen lights/shadows those individual grass blades.

The materials MUST be set to two sided foliage in order to fix this flickering issue that I am seeing.


does not work

You can try to disable the option “effect distance field lighting” for the foliage asset. I’ve found a lot of this flickering is caused by the ambient occlusion being generated by the glass blades. Unfortunately, the shading will lose some depth/contrast, but I found it completely fixed this issue for me.