- Lumen only uses Radiosity for lighting the Surface Cache, which is only seen indirectly through reflection or final gather rays. Lumen uses radiance caching for the main final gather (solving the first diffuse bounce) which is resampled for rough specular, as well as additional reflection rays for smoother reflections when roughness is below .4.
Check my Siggraph talk on Lumen’s Final Gather for more details:
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Yes Radiosity requires finite elements, for Lumen these are the Surface Cache texels. We trace from Surface Cache texels against the Global SDF, then sample last frame’s lighting at the hit, that gives infinite diffuse bounces through feedback. However if you do that naively you get a bunch of noise artifacts, so the Radiosity gather has some techniques to make the best use of a small number of rays (caching, probe interpolation and occlusion, spatial filtering, temporal accumulation).
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Screen and world space radiance probes are tracing rays which hit the Lumen Scene and sample its lighting (direct + indirect from Radiosity). When you set Ray Lighting Mode to Hit Lighting, it instead recalculates direct lighting at the hit point (tracing new shadow rays to each light), but still uses the Surface Cache at the hit point for indirect.
Basically Lumen Scene is only used for multi-bounce (and GI seen in reflections) while the Final Gather (using screen + world space radiance caches) is solving the first diffuse bounce with way higher quality, and solving rough reflections.