Skin Material and Subsurface Scattering
Real skin is not opaque: light enters the surface, scatters some distance beneath it and exits elsewhere. That is why skin has a reddish transition across the terminator, and why thin parts (ear rims, nostrils) glow when lit from behind. The engine models this with a material plus a post effect working together.
How the three pieces divide the work
Three components make up this feature, and understanding their split is the key to using it:
| Component | Kind | Responsible for |
|---|---|---|
SkinMaterial | Material | The shape of direct lighting: diffuse ramp, specular, back-lit transmission |
SkinSSS | Camera post effect | The diffusion: blurring the scatterable part in screen space and compositing it back |
SubsurfaceProfile | Data | The ratio of per-channel scatter radii — what makes it read as skin versus wax |
The important part is that stylization is separate from diffusion: the material decides what the lighting ramp looks like, and the post effect only spreads that ramp out. A stylized ramp therefore stays stylized after scattering instead of being averaged away.
That leads to a rule which is easy to get wrong: scatter tint, strength and radius live on the post effect, not on the material. There is no scatterRadius on the material; it is on the camera.
Minimal usage
import { SkinMaterial } from '@zephyr3d/scene';
const material = new SkinMaterial();
material.albedoTexture = skinColorTexture;
material.normalTexture = skinNormalTexture;
// The post effect must be enabled, or you get the material ramp with no diffusion
camera.skinSSS = true;Setting up the material without enabling camera.skinSSS is the most common mistake: nothing errors, but you see no scattering, because the diffusion step never runs.
Conversely the effect is energy conserving: the post effect subtracts the scatterable term and adds back a diffused version, so turning camera.skinSSS off leaves the image unchanged rather than suddenly darker or brighter.
Material side: the shape of the lighting
Parameters on SkinMaterial shape direct lighting and have nothing to do with diffusion:
| Property | Default | Purpose |
|---|---|---|
diffuseWrap | 0.28 | Diffuse wrap amount, letting light reach past the geometric terminator |
diffuseSoftness | 0.45 | How soft the ramp is |
shininess | 72 | Specular sharpness |
specularStrength | 1 | Specular intensity |
scatterWrap | 0.65 | Wrap of the scatterable term, setting the width of the transition band |
scatterStrength | 1.5 | Strength of the scatterable term written out |
scatterColor | (1, 0.42, 0.28) | Tint of the scatterable term |
transmissionStrength | 0 | Back-lit transmission strength; off by default |
transmissionPower | 4 | Directionality of the transmission |
shadowTint | black | Tints the shadowed part of the ramp; black is the original behaviour |
brightening | 0 | Overall diffuse gain |
The mask texture
The optional subsurfaceTexture carries three different things in its channels:
- R = skin mask — which pixels participate in scattering. Clothing, hair and eyes must be masked out here, otherwise they get tinted with the skin scatter colour.
- G = local softness — per-pixel adjustment of ramp softness.
- B = thickness — used by back-lit transmission; thin areas (ear rims, nostrils) take high values.
Transmission is off by default; to use it, raise transmissionStrength and supply thickness in B.
Post-effect side: diffusion and compositing
This group of camera properties controls the diffusion:
| Property | Default | Purpose |
|---|---|---|
skinSSS | false | Enable |
skinSSSStrength | 1 | Final composite strength |
skinSSSScatterRadius | 0.02 | World-space scatter radius; blur width scales with distance to keep it constant |
skinSSSSampleStep | 2 | Maximum pixel spacing between blur taps, capping the projected radius for close-ups |
skinSSSOpacity | 0.18 | Bias subtracted from the blurred skin mask before compositing |
skinSSSDepthScale | 80 | Depth rejection scale, preventing bleeding across depth discontinuities |
skinSSSColorBoost | 1 | Extra multiplier on the blurred result |
skinSSSSmoothness | 0 | Skin smoothing ("beauty filter") amount |
skinSSSScatterTint | white | Tints the light being redistributed |
skinSSSGlow | 0 | Additive, deliberately non-conserving bleed |
skinSSSProfilePreset | 'skin' | Scatter profile preset |
A few worth calling out:
skinSSSScatterRadius is in world space, not pixels. That means scattering correctly shrinks as a character walks away, without you retuning by distance. skinSSSSampleStep then caps the kernel in pixels so it does not grow unbounded in close-ups.
skinSSSGlow breaks energy conservation on purpose. At the default 0, light added on the dark side of the terminator is light removed from the lit side. Raising it only adds, making skin read as lit from within; around 1 approximates how the effect looked before it conserved energy.
skinSSSScatterTint multiplies only the difference term, so a warm tint colours the terminator without washing out the whole surface.
skinSSSSmoothness depends on a correct mask: smoothing samples weighted by the skin mask, so the R channel must exclude eyes, brows and lips or they get smoothed away too.
Scatter profiles
skinSSSProfilePreset sets the ratio between the red, green and blue scatter radii — and that ratio is what gives a scattering surface its character. Red travels furthest in skin, which is exactly the red-to-yellow gradient at the terminator. Changing the preset changes that ratio, which is why wax and jade run the same code path as skin rather than being special cases.
Available presets: skin, skin_thin, skin_default, skin_heavy_makeup, wax, wax_backlit, wax_soft, jade, jade_backlit, jade_soft.
The absolute size is still controlled by skinSSSScatterRadius; the preset only sets proportions.
This preset applies to the whole pass
camera.skinSSSProfilePreset is a whole-pass setting — one per camera. Using different profiles for different materials in the same image goes through a separate profile-slot path, where SubsurfaceProfile instances can be shared between materials (similar to Unreal's skin profile assets).
See also
- Custom Materials — the general material system
- Post-processing — the camera post-processing chain
- Lighting — configuring lights