ADR-0038 — Environment specular is split-sum, analytical until a cubemap exists
Status: Accepted Date: 2026-09-23
Context
JRV-0072 gives a metal no diffuse environment light. When the direct highlight leaves, the gold card goes flat. A reflection of the other triangle, a probe, and an imported HDR cubemap are later tickets. The sky that already exists is two hemispheres of linear radiance.
Decision
Environment specular is added after environment diffuse, in the same HDR target, before exposure.
The lookup is one function, jarvig_hemisphere(direction). Today that direction is the reflection vector, and roughness² blends it toward the hemisphere the normal faces. That blend is the stand-in for a prefiltered mip. It is not color * (1 - roughness).
The Fresnel integral is Brian Karis's analytical environment BRDF. F0 is the same value the direct term uses. Grazing view angles raise the reflectance. Ambient occlusion does not scale this term.
A later cubemap replaces the hemisphere lookup. It may take the reflection vector and a roughness mip. MaterialSurface does not grow an environment texture slot for that.
Alternatives Considered
- Multiply the sky by
(1 - roughness). Rejected. A rough mirror and a sharp one would only change brightness, not direction. - Sample a cubemap now. Rejected. There is no imported environment asset, and the asset database is not this ticket.
- A constant fill so the metal is not dark. Rejected.
- Darken the reflection with AO. Rejected until a specular-occlusion model exists. Direct specular stays free of AO.
Consequences
docs/rendering/environment-lighting.md records the formula. JRV-0074 can introduce a local probe without changing the surface contract. The bootstrap metal reflects the sky, not the checker triangle.
Supersedes
Nothing. ADR-0036 still owns the environment light. ADR-0037 still owns which side is shaded. This decision adds the specular lobe those feed.