Hardware Lumen Surface Cache Artifacts on Nanite Geometry

When using Hardware Lumen with Nanite-enabled meshes in UE 5.6+, black / dark surface cache artifacts appear that result in generally darker and unstable lighting. These artifacts are not immediately visible in the standard lit view mode (screen traces hide the issue) but become evident when inspecting the surface cache.

The issue appears to be caused by a mismatch between the Nanite ray tracing proxy geometry (used for HW ray tracing) and the surface world positions reconstructed from the surface cache. The surface cache recovers world positions by combining the lumen mesh card location with the rasterized (from original Nanite mesh) depth map of the card. If these reconstructed positions do not align with the proxy geometry used for ray tracing, shadow and radiosity rays can intersect the mesh surface itself, producing dark spots and unstable lighting.

Disabling Nanite resolves the problem entirely. Adjusting Nanite LOD settings (e.g., Max Edge Length Factor) can reduce the artifacts at certain settings. Adjusting ray biases (e.g., r.LumenScene.DirectLighting.HardwareRayTracing.ShadowRayBias, r.LumenScene.Radiosity.HardwareRayTracing.SurfaceBias, r.LumenScene.Radiosity.HardwareRayTracing.SlopeSurfaceBias) can eliminate the issue at the expense of high shadow / ray bias.

• Create a blank project at maximum quality settings.
• Import the attached FBX mesh (or use a nanite enabled sphere if this is not available).
• Place the mesh in the startup level.
• Apply a neutral grey material (e.g., /Engine/EditorMeshes/ColorCalibrator/M_GreyBall) to better expose the issue.
• Toggle Surface Cache Viewmode on → observe unexpected dark artifacts.
• Disable the directional light in the view (eye icon > light types > directional lights) → artifacts are still present but less sharp/intense.
• In Static Mesh > Nanite Settings, set Max Edge Length Factor to 3 → artifacts are reduced but not gone. (Changes the LOD selection used when rasterizing depth to the surface cache)
• Disable Nanite on the static mesh → artifacts are fully gone.
• Adjust ray biases (e.g., set r.LumenScene.DirectLighting.HardwareRayTracing.ShadowRayBias to 9 and r.LumenScene.Radiosity.HardwareRayTracing.SlopeSurfaceBias to 5) → artifacts are eliminated on the sample mesh.