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Software Ray Tracing in Lumen

Myunghoon Cha

SIGGRAPH Advances in real-time rendering

Daniel Wright

Krzysztof Narkowicz

Patrick Kelly

Lumen: Real-time Global Illumination in Unreal Engine 5

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Why Lumen?

Indoor lighting often solved with baked lighting

Dynamic indirect lighting can solve this problem.

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Hybrid Ray tracing

Trace against the depth buffer first (Screen Tracing)

Trace against Signed Distance fields in computer shaders

Apply lighting to ray hits with Surface Cache

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Why Software tracing?

Option to scale down

Some GPUs don’t support Hardware Ray Tracing (HWRT)

60 fps for consoles

Full control over tracing code

Can’t replace HWRT, but can pick different tradeoffs

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Screen Tracing

Screen tracing supplement mismatches from the surface cache

Handles mismatches between Full triangle meshes and distance field.

Hierarchical Z Buffer traversal

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Mesh Distance Fields

Sparse representation

Mipmaps

Streamed from disk based on distance

Doubled default voxel density + 4x max resolution per mesh

Net memory cost: about half of UE4

10x faster Mesh distance field building

Lumen is highly rely on Mesh distance fields

Mesh Distance field is not a new technology. on UE5. It was already used in UE4 but it developed in UE5.

Mesh distance field is 3D texture that storing the distance data from the mesh.

In UE5, Sparse representation that only storing the data where it need to near the surface

Generating Mipmaps, so load the lower version depend on the distance

Mip0 resolution is based on the mesh size and mesh import settings, and then mip1 halves the resolution and doubles max object space distance, and so on.

It Double the default resolution in UE5

X, Y, Z and it’s cube so data is 8 times and 4 times max resolution per mesh

It sounds like memory take more than before but actually Net memory cost is about the half of UE4.

10 times faster mesh distance field building from UE4

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Surface Cache

Capture mesh at low resolution into atlas

Speedup on Nanite meshes: ~10~100x

Compute direct and indirect lighting

Low quality GI gather for multi-bounce

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Hybrid Ray tracing

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Datas and Quality

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Datas and Quality

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Software Ray Tracing Benefits

GI and rough reflections

Supported on all platforms

Versatile utility

Runtime LOD for complex scenes.

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Question?