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Btw, Lumen has amount of limitations in the software ray tracing mode.
https://docs.unrealengine.com/5.0/en-US/lumen-technical-details-in-unreal-engine/
Also quality problems:
Therefore, Lumen should not be used as an example to claim that hardware ray tracing is useless.
https://docs.unrealengine.com/5.0/en-US/lumen-technical-details-in-unreal-engine/
Limitations of Software Ray Tracing
Software Ray Tracing has some limitations relating to how you should work with it in your projects and what types of geometry and materials it currently supports.
Geometry Limitations:
Material Limitations:
- Only Static Meshes, Instanced Static Meshes, Hierarchical Instanced Static Meshes, and Landscape terrain are represented in the Lumen Scene.
- Foliage must be enabled with the setting Affect Distance Field Lighting found in the Foliage Tool settings.
Workflow Limitations:
- World Position Offset (WPO) is not supported.
- Transparent materials are ignored by distance fields and Masked materials are treated as Opaque.
- Masked materials can cause significant over-shadowing on foliage where large areas of leaves are masked out.
- Distance fields are built off of properties of the material assigned to the Static Mesh Asset rather than the override component.
- Overriding with a material that has a different Blend Mode or that has Two-Sided property enabled will cause a mismatch between the triangle representation and the mesh's distance field representation.
- Software Ray Tracing requires that levels be composed of modular geometry. Things like walls, floors, and ceilings should be separate meshes. Large single meshes, such as a Mountain or multi-story building, will have a poor distance field representation that can cause self-occlusion artifacts to appear.
- Walls should be no thinner than 10 centimeters (cm) to avoid light leaking.
- Distance Fields cannot represent extremely thin features, or one-sided meshes seen from behind. Avoid these types of artifacts by ensuring the viewer doesn't see the triangle back faces of one-sided meshes or only use closed geometry.
- Mesh Distance Field resolution is assigned based on the imported scale of the Static Mesh.
- A mesh that is imported very small and then scaled up on the component will not have sufficient distance field resolution. Instead, set the distance field resolution from the Static Mesh Editor's Build Settings if you use scaling on placed instances in a Level.
Also quality problems:
Therefore, Lumen should not be used as an example to claim that hardware ray tracing is useless.