# ADR 0036: Surface ABI v1 and Solari Evaluator Parity

## Status

Accepted

## Context

Arbitrary full-pipeline WGSL cannot be carried safely between Bevy's raster pipelines and Solari's
ray-hit shaders. A custom fragment shader may own bindings, entry points, derivatives, discard, or
other stage-specific behavior that has no direct ray-tracing equivalent. Treating such a shader as
a `StandardMaterial` in Solari would produce a different material, while tracing skinned source
vertices would produce bind-pose geometry instead of the rasterized deformation.

Blacksite needs one constrained material expression that can produce the same surface values in
forward/deferred raster rendering and at Solari ray hits, while leaving transforms, skinning,
lighting, fog, shadows, and render-pipeline bindings under engine ownership.

## Decision

- `blacksite_surface` owns versioned Surface ABI v1. A custom evaluator implements:

  ```wgsl
  fn evaluate(
      input: SurfaceInput,
      params: SurfaceParams,
      samples: SurfaceSamples,
  ) -> Surface
  ```

  `SurfaceInput` provides UV0, world position, and world normal. `Surface` returns base color,
  tangent-space normal, emissive, metallic, perceptual roughness, reflectance, occlusion, alpha,
  and lit/unlit model selection.
- The ABI has 16 `vec4` parameter lanes, eight sampled 2D texture slots with UV transforms, and a
  400-byte `SurfaceUniform`. Shader schemas map typed Bool/Float/Vec2/Vec3/Color/Enum properties and
  texture properties into that fixed layout. Exceeding either limit is a validation error.
- Evaluators cannot declare bindings or shader entry points and cannot use compute/storage,
  ray-query, derivative, discard, barrier/subgroup, texture-store, or atomic constructs. The engine
  supplies resources, entry points, vertex transforms/skinning, lighting, fog, and shadows.
- Raster rendering composes the evaluator between the engine-owned Surface header and footer and
  installs it as an `ExtendedMaterial<StandardMaterial, SurfaceExtension>` for forward, deferred,
  and prepass pipelines. Tangent-space normals are applied when the mesh supplies tangents.
- The maintained `third_party/bevy_solari` fork packs the same uniform and texture set, namespaces
  validated evaluator functions, and generates a shader-ID dispatch library for ray hits. Eligible
  non-deformed geometry uses the evaluator for hit material fields, tangent-space normal, emissive,
  unlit behavior, and cutout candidate acceptance.
- Opaque and cutout are the supported alpha modes. Solari treats geometry as potentially
  non-opaque and evaluates the exact Surface alpha and cutoff before accepting a cutout ray
  candidate.
- Plain materials without a custom evaluator keep Bevy's standard raster and Solari material path.
  Dependency revisions cover Material Instances, their direct base, shader schema, and evaluator.
  Naga syntax/semantic validation and the cross-pipeline restrictions run before installation;
  invalid updates produce diagnostics and retain the last successfully installed evaluator/library.
- Meshes with `SkinnedMesh`, runtime morph weights, or morph targets are excluded from Solari until
  an instance-keyed deformed-vertex and BLAS update path exists. They continue to render through
  Bevy's raster path; Solari diagnostics report the exclusion. No bind-pose ray-tracing fallback is
  allowed.

## Consequences

- Custom material code is portable because it describes a surface, not an entire render stage.
- Raster and Solari evaluate the same source for supported, Solari-eligible static or rigid
  geometry. Normal parity requires tangents; Solari compatibility diagnostics identify missing
  tangents and other unsupported mesh layouts.
- Dynamic skinned/morph Solari geometry remains deliberate future work. This is a geometry parity
  limit, not permission to substitute source/bind-pose vertices.
- ABI layout or semantic changes require a new version and migration. The fixed lanes and texture
  count trade arbitrary resource layouts for deterministic cross-renderer packing.
- Blend, transmission, refraction, custom vertex displacement, arbitrary entry points, and
  stage-specific shader behavior are unsupported by Surface ABI v1.
- Maintaining evaluator parity requires carrying the focused Solari fork until the necessary hooks
  exist upstream; its dispatch and cutout behavior are covered by local tests.
