I’m experiencing noticeable shimmering/noise on fur (thin/alpha-based geometry) when using Lumen with physically accurate light intensities (60k–120k lux), especially in indirect lighting.
Disabling screen traces:
r.Lumen.ScreenProbeGather.ScreenTraces 0
completely removes the issue, which suggests it’s caused by Lumen screen-space tracing rather than the material or shading model.
Switching to Hair shading model or disabling ray tracing in materials does not help.
Is there a way to disable or control screen traces per material/mesh (e.g. only for fur), or any recommended approach to stabilize this?
There are a number of hair specific lumen CVars that you can adjust to control Lumens behavior on fur and hair and that’s how you typically control screen trace behavior for hair/fur specifically. Some of the common ones are listed below and you can try toggling the on/off ones to narrow down the aspect causing the swimming noise - it might be ShortRangeAO. If you provide a sample of the fur we can be more precise.
r.Lumen.ScreenProbeGather.HairStrands.ScreenTrace Whether to trace against hair depth for hair casting shadow onto opaques.
r.Lumen.ScreenProbeGather.ScreenTraces.HairBias Bias for screen space rays traced from hair pixels. Usually hair has pretty complex geometry and requires a special bias value.
r.Lumen.ScreenProbeGather.HairStrands.VoxelTrace Whether to trace against hair voxel structure for hair casting shadow onto opaques
r.Lumen.ScreenProbeGather.ScreenTraces.HZBTraversal.SkipHairHits Whether to allow screen traces to hit hair shading model. Can be used to work around aliasing from high frequency hair cards geometry.
r.Lumen.ScreenProbeGather.HardwareRayTracing.HairBias Bias for rays traced from hair pixels. Usually hair RT representation heavily mismatches raster and requires a larger bias value.
r.Lumen.ScreenProbeGather.ShortRangeAO.HairScreenTrace Whether to trace against hair depth for hair casting shadow onto opaques.
r.Lumen.ScreenProbeGather.ShortRangeAO.HairVoxelTrace Whether to trace against hair depth for hair casting shadow onto opaques
Also if you haven’t already seen it, the Meerkat sample project which has a furry creature might be a good reference.