OC-001 Minimal Optical Closure — Fixture

Status: Stage 0 · Design only · not yet implemented
Epistemic tier: Implemented engine behavior (once built) · Experimental interpretation (results)

Fixture concept

OC-001 is a three-object scene:

1. Reference sphere — a plain checkerboard sphere with no field interaction, used as a straight-transport baseline.
2. Probe sphere — a checkerboard sphere with a polar dent feature, placed inside or adjacent to the GRIN field volume.
3. GRIN field volume (north-offset) — a radial gradient index field centered above the probe sphere, curving rays away from the probe's dent region at the default observer distance.

The observer begins far enough away that the dent/checker region on the probe sphere is not reached by the curved transport. Moving the observer closer reopens the region. The fixture documents this observer-dependent behavior as a sweep table, not as a single binary classification.

Fixture design (probe metadata only — no intersection configuration)

{
  "name": "oc_001_minimal_optical_closure",
  "description": "Observer-dependent optical accessibility probe. Reference sphere, probe sphere with polar dent, and a north-offset GRIN field volume.",
  "probeMetadata": [
    {
      "colliderName": "reference_sphere",
      "role": "reference",
      "center": [-1.8, 0, 0],
      "radius": 0.6,
      "material": {
        "uvMode": "analytic_spherical",
        "checkerTilesU": 6,
        "checkerTilesV": 6
      }
    },
    {
      "colliderName": "probe_sphere",
      "role": "probe",
      "center": [0, 0, 0],
      "radius": 0.8,
      "material": {
        "uvMode": "analytic_spherical",
        "checkerTilesU": 8,
        "checkerTilesV": 8,
        "hasPolarDent": true,
        "polarDentLatitude": 0.85,
        "polarDentDepth": 0.15
      }
    }
  ]
}

probeMetadata is not intersection configuration

The probeMetadata block is used only for post-hit UV computation and diagnostic classification. It does not configure the hit system. The Godot scene's physics geometry determines what gets hit; probeMetadata tells the interpretive layer how to read that result.

Expected output set

Output	Channel	Description
Beauty render	checker_albedo (diagnostic)	Checker pattern as seen from the default observer position
Straight vs curved comparison	ray_hit_flag (existing)	Hit map with field off vs field on
UV coordinate overlay	surface_uv_coords (diagnostic)	False-color UV map at hit points
Checkerboard overlay	checker_albedo (diagnostic)	Binary checker state per pixel
Curvature / field overlay	bend_magnitude_metric (existing)	Field curvature at each sample
Hit classification map	collider ID per pixel	Which object was hit
Accessibility diagnostic map	DiagnosticState enum	5-class probe accessibility per pixel
Observer sweep table	all diagnostics per observer origin	How accessibility changes with distance

What this fixture does not demonstrate

• Not a Godot parity comparison.
• Not a Core-vs-Core comparison (Phase 1 is Godot-side only).
• Not proof of hidden geometry.
• Not proof of optical closure in physical space.
• Simulation-bounded claim only.