Lumen Technical Details
- Screen Traces first, then a more reliable method. Two methods: Hardware RT and Software RT through signed distance fields; HWRT higher quality, enabled by default, needs support, more expensive.
- Primary target: large open worlds at 60 FPS on next-gen consoles (High scalability). Secondary: indoor at 30 FPS; Epic scalability ~8 ms on next-gen consoles for GI+reflections at 1080p internal, relying on TSR.
- Surface Cache: automatic parameterization of nearby scene surfaces for fast lighting lookup at ray hits; material properties captured from multiple angles by Cards generated offline per mesh (default 12; Max Lumen Mesh Cards). Uncovered areas pink; don't bounce light, black in reflections. Skeletal meshes: six cards around pre-skinned bounds. Nanite accelerates captures; foliage/ISM only supported with Nanite. Surface cache direct+indirect lighting "amortized over multiple frames". Only simple interiors: walls/floors/ceilings separate meshes.
- Screen tracing supports any geometry; covers mismatches between Lumen Scene and triangle scene; limits art direction.
- Software RT: Mesh Distance Fields merged into a Global Distance Field; Detail Tracing (per-mesh SDF for first two meters then global; extremely high cost with kitbashing) vs Global Tracing (engine default). Limits: only static meshes, ISM, HISM, landscape; WPO not supported; modular geometry; walls no thinner than 10 cm; no very thin features / one-sided seen from behind; DF resolution from imported scale.
- HWRT: supports skinned meshes; intersects actual triangles; optional hit lighting; highest setup cost with many overlapping meshes; skinned meshes BVH update cost proportional to skinned triangles; Nanite meshes traced via Fallback Mesh (Fallback Relative Error). Platforms: Win10+ DX12 with RTX-2000+/RX 6000+, PS5, Xbox Series, Switch 2.
- "Software Ray Tracing is the only performant option in scenes with many overlapping instances, while Hardware Ray Tracing is the only way to achieve high quality mirror reflections".
- Large worlds: Lumen Scene around camera; relies on Nanite LOD and Multi-View rasterization for fast captures; doesn't require Nanite but captures very slow with many non-Nanite high-poly meshes. Fast camera movement -> indirect lighting pop-in. SWRT Lumen Scene covers 200 m (up to 800 m). Far Field (HWRT) to 1 km, requires World Partition HLOD1.
- Limitations: Lumen GI can't be used with lightmaps; "Lumen is not compatible with Forward Shading"; no PS4/XB1; VR a poor fit. SWRT: DX12 SM6, GTX-1070+.