# D-239 (REVISED post-verification): Tile derivation contract — coarse→fine refinement chain (Q-101)

domain: architecture · type: confirmed · date: 2026-06-07 · resolves: Q-101

**Decision.** The walkable tile is materialised by a **three-carrier refinement chain**, each
stage a pure deterministic function of `(seed, atlas, body-params, position)` per D-227:
`RegionProfile` (~1 km) → `ChunkContext` (64 m) → `VoxelColumn` (1 m). Each scale boundary is its
own derivation with its own failure modes.

### Determinism — no authoring at the derivation layers, ever
L1–L5 derivation has **zero per-body override hooks**. All authorial control lives **upstream** at
the body-parameter / atlas / system-specialization layer; the gating params (`GlaciationGrade`,
`precipitation_class`, `RIVER_THRESHOLD`, `tectonic_class`) are **derived** from body params
(stellar type / orbit / hydrosphere / lithology), never authored. `RIVER_THRESHOLD` specifically is
a derived **per-body-class** value `(hydrosphere, tectonic_activity, precipitation_class) → threshold`,
not the global constant `200`. Lore-anchored bodies are honoured by **setting params** and stand as
**validation cases** — if a body reads wrong, fix its params, never patch the derivation.
**Validation caveat:** derived terrain can only honour a lore body if its params *permit* it — e.g.
tidal flats require a moon param (the D-228 tidal term); confirm each validation body's params before
treating its terrain as a contract (Velen's tidal-flat coast presumes Velen has a moon — verify).

### Anti-"squaring" (warp)
A global position-keyed **domain-warp field** suppresses grid/seam artifacts. The warp is a
**stateless, hash-based pure function of `(seed, body_id, position)`** — no lookup table, no
thread-local cache (so it's order-independent across threads/platforms). It keeps **f64 sub-metre
precision through to the final voxel coordinate, then quantises by truncation to the integer voxel
address** — a cast, not a comparison, so IEEE-754-deterministic across targets. The ±8 m warp range
makes FMA-contraction ULP variance (~1e-15 m) unable to shift the rounded voxel, so no platform
guards are needed. The warp is *position math*, not a structural decision — D-010 integer discipline
on all material/morphology decisions is preserved.

### Morphology — 8 families
Selected by a strict **decision tree over integer `RegionProfile` inputs**: LavaField · FjordWall ·
CliffCoast · BraidedDelta · DuneStrand · IncisedGorge · MeanderReach · AlluvialPlain (fallback).
Requires adding `tectonic_class` to `RegionProfile`. MountainPass is a zone label sharing IncisedGorge
geometry. **Hard gates (boolean, applied pre-selection in tree order):** fjord requires
`GlaciationGrade ≥ 2`; LavaField requires `tectonic_class = Volcanic`; lithology bounds slope/form
(see laws below).

### Frozen 17-zone `MorphologyZone` display vocabulary
Derived labels per D-228, **enum→label map frozen at authoring time**. The canonical freeze point is
the Rust `MorphologyZone` enum; **adding/renaming a zone requires a D-record amendment** (a code
reviewer enforces this). The 17: open ocean, lake, tidal flat, dune strand, cliff coast, fjord,
delta, estuarine, alluvial plain, river bank, meander reach, braided plain, valley floor, mountain
pass, alpine, volcanic, wetland. **Note — zones ≠ families:** four zones (tidal flat, estuarine,
alpine, wetland) have no distinct generator family; they are derived sub-classifications from
family + elevation/water-height conditions (e.g. tidal flat = BraidedDelta at the tidal margin;
alpine = AlluvialPlain/IncisedGorge above the treeline-elevation threshold).

### Zone seams — prevent incompatible, allow valid (don't patch)
The boolean gates are discontinuities on continuous inputs, so "continuous→continuous" is NOT the
mechanism. The mechanism is the **decision-tree gate ordering + a build-time compatibility matrix**:
*incompatible* family pairs (e.g. MeanderReach↔Volcanic — geomorphically incoherent) cannot be
adjacent classifier outputs; the matrix is a **build-time invariant test**, never a runtime override.
*Valid* geomorphic seams (cliff↔fjord at the glaciation threshold, lithology faults) are **permitted
sharp transitions** — kept sharp (real geology is sharp), made non-grid/organic by the warp; no
modulation feathering. The warp also guarantees seams don't produce degenerate slivers at chunk scale
(preserving the per-chunk tactical-content rule below).

### Believability laws (binding — inline, not just titles)
- **Drainage monotonicity:** channels descend; respect the D8 thalweg (D-208); tributaries join from
  upstream; river mouths sit at sea level.
- **Lithology → landform:** Rock → vertical faces/cliffs; Sand → ≤ angle of repose (~32°), dunes not
  cliffs; Gravel → braided channels/fans, not single-thread meander; Soil → rolling/floodplain;
  Wetland → ≤5° flats; Lava → sheets/shield slopes + lava tubes, immature drainage.
- **Glaciation → form:** fjord requires GlaciationGrade ≥ 2; U-valleys ≥ 1; moraine ridges ≥ 1;
  cirques ≥ 2. GlaciationGrade 0 high-relief is V-ridges (fluvial), never glacial U-profiles.
- **Climate → vegetation:** treeline transitions Forest → Scrub → Barren (no Forest→Barren skip);
  riparian Thicket/Scrub band 1–3 tiles along every perennial waterway; boundaries follow
  contours/moisture, not chunk edges.

### Game-feel constraints
Every chunk carries ≥1 tactical decision point; cover ≠ concealment (mechanically distinct);
high-ground asymmetric-but-not-dominant; chokepoints narrow enough to matter (river crossings
3–15 m, gorge floors 2–8 m); **seasonal/tidal state must produce real passability changes, not
cosmetic ones** — BraidedDelta/MeanderReach `ElevationDelta` calibrated so channels fall below the
Q-105 high-water threshold and levees stay above it (flood = genuinely impassable, not recoloured).

### Mechanics
- No per-tile `flow_direction[64×64]` in `ChunkContext` (D8 ≈ 152 m/cell — coarser than a chunk) →
  one basin-direction + global meander-curve params. Features with wavelength > 64 m (meanders,
  dunes, cliff continuations) seed from **Region-or-higher**, not the chunk seed.
- **Caching:** `RegionProfile` is stored in `BodyWorldState` (D-203) (~6k/body, populated in the
  D-206 background pass) so the Atlas reads zone labels without triggering voxel derivation; voxel
  derivation is on-demand + cached, never persisted (D-227). Budget ~2.2–4.2 ms/chunk **(validate
  per-family in Phase 4 — FjordWall/IncisedGorge are far costlier than the AlluvialPlain fallback).**

**Resolves:** Q-101. **Touches:** Q-102 (cohesion = the warp). **Leaves open:** Q-103 (mutator op
schema), Q-105 (region seasonal/clock state — the ElevationDelta calibration above is a forward
contract to it).
**Cross-refs:** D-227, D-228, D-203, D-206, D-208/209/210, D-010, D-234, D-142, D-217.
**Raised by:** tile-derivation-contract workshop (Tyre, Gestalt, Troblum, Miri) + lead-interviewed
decisions, 2026-06-07; verified by an adversarial pass from the same four.
**Dissent:** Tyre's initial cross-family elevation-blend was resolved against (prevent-at-source).
Early-integer-truncation of the warp (raised against Gestalt's `ElevationDelta` ranges and by Tyre)
was resolved against in favour of f64-to-voxel.