# Runtime clear coat

Progressive image jobs accept an independent dielectric GGX coat over an opaque
metallic/roughness substrate. A dark, moderately rough plastic base with a smooth
coat can show narrow softbox reflections while retaining a dark body. Chrome uses
the existing metallic surface; it does not need a coat.

```json
{"command":"render.start","params":{"progressive":true,"raytrace":{"clear_coats":[{"body_index":0,"body_name":"Housing","weight":1,"roughness":0.12,"ior":1.5}]}}}
```

`render.preflight` accepts the same override. Index and exact name must both match
the captured `render.scene` body. At most 64 distinct bodies can be overridden.
Weight is finite in [0,1], perceptual roughness in [0.045,1], and relative IOR in
[1,3]. Unknown fields and repeated indices are errors. Weight zero is a no-op.
Coats cannot be combined with runtime transmission on the same body. Overrides
apply to the body and its faces in the job snapshot, never to the saved document,
preview, undo history or appearance format. They require progressive rendering.

The implementation adds single-scattering GGX/Smith coat reflection with its own
Schlick Fresnel term. For cosine `c`, define the conservative directional bound
`B(c) = Fcoat(c / sqrt(2 * (1 + sqrt(1 - c*c))))`. The base BRDF is multiplied by
`(1-weight*B(N.V))*(1-weight*B(N.L))`. This is deliberately stronger attenuation
than macro-normal Fresnel, which can leak energy with rough coats over shiny bases.

For any reflected upper-hemisphere direction, the view/half-vector cosine is at
least the argument used in B. Schlick decreases with cosine, and a unit-Fresnel
single-scattering Smith GGX lobe reflects at most one. Thus the coat contributes
at most `weight*B(N.V)`; the attenuated unit-energy base contributes at most
`1-weight*B(N.V)`. The symmetric product retains reciprocity. This conservative
allocation can darken grazing views and omits internal inter-layer scattering and
refracted substrate directions; it is not a full layered transport solution.
Importance-sampled f64 furnace quadrature covers shiny bases, rough coats, partial
weights, IORs and grazing directions, alongside analytic and reciprocity checks.

Continuation selects the coat GGX lobe with probability `weight/2`, otherwise the
existing equal cosine/GGX base mixture. Evaluation uses the complete layered BRDF;
the PDF is the same combined mixture for continuation and direct-light MIS. GGX
samples below the surface are rejected without renormalizing the PDF. Zero weight
preserves the old BRDF and random-number consumption exactly. GPU BRDF/PDF tests
compare the production shader to the independent f64 numerical contract, including
roughness 0.045 and grazing directions.

The per-triangle resolved material occupies 64 bytes, including a 16-byte coat
record. Storage-limit checks and peak-memory estimates include this additional
record.

The model was designed from the public descriptions of
[layered materials in PBRT](https://pbr-book.org/4ed/Light_Transport_II_Volume_Rendering/Scattering_from_Layered_Materials)
and [Filament's clear-coat model](https://google.github.io/filament/main/filament.html#materialsystem/clearcoatmodel).
These motivate independent interfaces and substrate attenuation; this implementation
is original and deliberately simpler than a multilayer random walk. No reference
implementation source was copied.