Mesh distance fields

A mesh distance field is a 3D texture baked for a mesh in which every cell stores how far it is from the mesh's nearest surface. Epic's documentation calls it a signed distance field (SDF): "It works by storing the distance to the nearest surface in a volume texture", with positive values outside the mesh and negative values inside (Unreal Engine Documentation: Mesh Distance Fields). Because a ray can safely jump forward by the stored distance without hitting anything, distance fields make tracing rays through a scene far cheaper than testing triangles.

#What they are used for

In Unreal, mesh distance fields power several systems (Unreal Engine Documentation: Mesh Distance Fields; Unreal Engine Documentation: Lumen Technical Details):

• Dynamic ambient occlusion and shadowing for static mesh actors.
• Lumen's software ray tracing, the default mode: Lumen traces "against each mesh's distance field for the first two meters for accuracy" and against a merged Global Distance Field for the rest of each ray (Unreal Engine Documentation: Lumen Technical Details).
• Materials and effects, such as GPU particle collision and dynamic flow maps (Unreal Engine Documentation: Mesh Distance Fields).

The Global Distance Field is "a low-resolution Distance Field" that follows the camera, built by compositing every mesh's field into "a few volume textures centered around the camera, called clipmaps" (Unreal Engine Documentation: Mesh Distance Fields). Lumen's scene built from it covers 200 m from the camera by default, extendable to 800 m (Unreal Engine Documentation: Lumen Technical Details).

#Resolution and memory

Quality depends on the resolution of each mesh's volume texture, set with Distance Field Resolution Scale in the Static Mesh Editor (Unreal Engine Documentation: Mesh Distance Fields). "The maximum size volume texture any single mesh can have is 8 megabytes with a resolution of 128x128x128" (Unreal Engine Documentation: Mesh Distance Fields), and the editor shows each mesh's size in the viewport corner.

#Limitations and artefacts

Epic lists the main ones (Unreal Engine Documentation: Mesh Distance Fields; Unreal Engine Documentation: Lumen Technical Details):

• Rigid meshes only. Skeletal meshes do not cast distance field shadows; Epic points to capsule shadows instead (Unreal Engine Documentation: Mesh Distance Fields).
• No vertex animation. World Position Offset or displacement "may cause self-shadowing artifacts", because the field was generated offline (Unreal Engine Documentation: Mesh Distance Fields). See wind and world position offset.
• Non-uniform scaling is not handled correctly; uniform scaling "by two times or less" is usually fine (Unreal Engine Documentation: Mesh Distance Fields).
• Rounded corners and thin surfaces. Corners round off at low resolution, and thin surfaces are only represented by a negative cell inside the mesh (Unreal Engine Documentation: Mesh Distance Fields).
• Foliage. Dense grass can overflow internal buffers, so enable Affect Distance Field Lighting only on foliage types that need it (Unreal Engine Documentation: Mesh Distance Fields). Two-Sided Distance Field Generation suits masked foliage but "does come at a higher ray marching cost" (Unreal Engine Documentation: Mesh Distance Fields).

#In other engines

Godot 4 has a related technique, SDFGI (signed distance field global illumination), described on global illumination. The idea is shared: approximate the scene with distances, then march rays through it.

Related: global illumination, real-time lighting, ambient occlusion.

#Further reading and watching