subsurface

Light that enters a surface at one point and leaves at another — the transport that softens and warms the terminator on skin.

local subsurface = require("@builtin::systems.subsurfaceScattering.subsurface")

subsurface.mark(headId, { color = { 1, 0.3, 0.16 }, radius = 0.02 })
subsurface.unmark(headId)      --> true when it had been marked
subsurface.marked(headId)      --> { color, radius }, or nil
subsurface.list()              --> every entity scattering, sorted
subsurface.groups()            --> the marked set gathered by what it scatters
subsurface.setStrength(0.7)    --> how much of the spread reaches the frame
subsurface.active()            --> is the pass running
subsurface.clear()             --> unmark everything, release the pass

mark on an entity that is already marked replaces what it scatters. A radius below zero is an error rather than a silent clamp, and an entity that does not exist is an error rather than a mark that never draws. The pass is created with the first mark and released with the last.

groups is what the render feature draws from: meshes sharing a tint and a radius are one draw, so per-entity scattering costs one pass per distinct setting rather than one per mesh. It also drops entities that have since been despawned, so a removed character does not keep its group alive.

For scattering a scene should simply have, author the SubsurfaceScattering component instead — same settings, persists with the scene, edits live in the inspector.

The full description of the knobs and how the passes work is in the package readme: guides { path = "systems/subsurfaceScattering" }.