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Docs/archive/CODEX-TASK-003-extrareach-perlin-bound.md

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+# Codex task 003 — three `ExtraReach` formulas use an FBM bound this file already proved wrong
+
+**Owner:** Codex (Model Luna, xHigh) · **Orchestrator:** Claude · **Branch:** `experimental`
+**Status:** specified, not started
+**Kind:** ⚠️ **correctness of a box verdict** — the class of bug that deletes collision. Not a perf task.
+
+---
+
+## Why this exists
+
+`VoxelDensityOpStack.cpp` contains a rigorous, written derivation that `|Perlin3D| ≤ 1.5`, exposes it
+as `PerlinAbsBound` (line ~2336), and uses it correctly for the tunnel warp dilation (~2452). The
+comment there is explicit that the loose "~[-1,1]" figure from the noise header is **not** to be
+relied on, and `OPSTACK-HANDOFF.md` records the standard: *a bound in a box verdict must be PROVED,
+not observed — over-estimating costs CPU, under-estimating deletes collision.*
+
+**Three `ExtraReach` formulas in the same file silently assume `sup|FBM| ≤ 1.0`.** Each carries the
+comment "FBM ∈ [-1,1]", which is exactly the claim the file disproves 1800 lines earlier.
+
+And `VoxelNoise::FBM` **is normalised** — it returns `Total / MaxValue` where `MaxValue = Σ Amp`
+(`VoxelNoise.h` ~272). So `sup|FBM| = sup|Perlin3D|` exactly: **1.5, not 1.0.** The octave sum
+neither amplifies nor attenuates the bound.
+
+### What that costs, per archetype, at the shipped defaults
+
+`Identity` from these sources means "no primitive within `ExtraReach` of the box", i.e. `Sdf ≥
+ExtraReach` throughout. Roughness then does `Sdf += FBM · VOXEL_NOISE_SCALE · Strength`
+(`FSdfRoughnessMod::Eval`), so worst case `Sdf' ≥ ExtraReach − B·1.25·|Roughness|`. Soundness
+requires `Sdf'` to stay at or above the downstream carve/fill threshold.
+
+| archetype | `ExtraReach` at defaults | downstream threshold | needs `B ≤` | verdict at `B = 1.5` |
+|---|---|---|---|---|
+| **VerticalShafts** (`Rough 3.0`) | `1.25·3 + 2 + 1` = **6.75** | carve blend **2.0** | **1.27** | ⛔ **UNSOUND** (margin −0.875) |
+| **Maze** (`Rough 2.0`) | `1.25·2 + 2 + 1` = **5.5** | carve blend **2.0** | **1.40** | ⛔ **UNSOUND** (margin −0.25) |
+| **FloatingIslands** (`Rough 4.0`, `K 5.0`) | `1.25·4 + 2·5 + 1` = **16.0** | fill `K` **5.0** (+ `K/6` SmoothMin dip) | **2.03** | ✅ sound — but only because `K` is large. Sound by parameter luck, not by construction. |
+
+Break-even roughness for the two carve archetypes is `|Rough| ≤ 1.6`; they ship at 3.0 and 2.0.
+
+**How alarmed to be, stated honestly.** No strate has `bUseOperatorStack` ticked, so nothing in the
+running game is affected today. The brute-force tile scans report 0 violations — but they *sample*,
+and they were sampling against a shaft source that proved **zero** tiles until `e002bd4`, so the
+shaft path has never been exercised at all. The empirical sup of this Perlin is estimated at
+~1.0–1.1, which is *below* the 1.27 the shafts need — which is why nothing has been seen yet, and
+also why the margin is uncomfortably thin. The bug is that the verdict rests on an unproved bound,
+which is the thing this codebase has already decided it does not do.
+
+## The fix
+
+1. **Hoist `PerlinAbsBound` to file scope and rename it `VF_PerlinAbsBound`**, so there is **one**
+   definition rather than a class-static plus three implicit `1.0`s. Keep the existing derivation
+   comment with it — it is the justification, not decoration.
+
+   **Naming, resolved:** the file-scope helpers in this file are all `VF_`-prefixed
+   (`VF_NearCaveSurface`, `VF_DistPointSegment`, `VF_NoCaveOverBox`), so a file-scope constant takes
+   the same prefix. That means this is a **rename**, not just a move:
+   - delete the `static constexpr float PerlinAbsBound = 1.5f;` class-static inside `FRoomGraphSource`
+     (~2336), moving its whole derivation comment with it;
+   - **update `FRoomGraphSource`'s own use at ~2452** (`P.CaveWarpStrength * VOXEL_NOISE_SCALE *
+     PerlinAbsBound`) to the new name. This is the one place where the warp dilation is computed and
+     it must keep computing the identical value — the rename must not change its arithmetic.
+   - after the edit, `grep -n "PerlinAbsBound" ` must show **only** `VF_PerlinAbsBound` occurrences.
+2. **Multiply the roughness term by it in all three `ExtraReach` formulas** (~4106 VerticalShafts,
+   ~4215 FloatingIslands, ~4247 Maze):
+
+   ```cpp
+   // before
+   FMath::Abs(P.SurfaceRoughness) * VOXEL_NOISE_SCALE
+   // after
+   FMath::Abs(P.SurfaceRoughness) * VOXEL_NOISE_SCALE * VF_PerlinAbsBound
+   ```
+3. **Fix the three comments.** Each says "FBM ∈ [-1,1]". Replace with the real statement: `FBM` is
+   normalised (`Total / MaxValue`), so `sup|FBM| = sup|Perlin3D| =` the proved `PerlinAbsBound`.
+   A comment that states a refuted bound is how this happened in the first place.
+
+## ⚠️ Invariants
+
+1. **This must not change density by one bit.** `ExtraReach` is read **only** inside
+   `EffectOverBox` (verified: every other occurrence is a comment or the `float ExtraReach;` member
+   declaration — no `Eval`, no `GetCells`). The eight equivalence tests compare `Eval` bit for bit
+   and must stay green. **If you find yourself editing an `Eval`, stop — you have the wrong site.**
+2. **The change direction is strictly conservative**: larger `ExtraReach` ⇒ more `CarveOnly`, fewer
+   `Identity` ⇒ *fewer* tiles proved uniform. It can only cost CPU, never open a hole. Do not
+   "balance" it by tightening something else in the same edit.
+3. **Anonymous-namespace placement.** Put the hoisted constant **above the labelled end of the
+   anonymous namespace**, not anchored on the FACTORIES banner — anchoring there puts it outside and
+   the brace added with it closes nothing. This mistake has been made twice in this file and the
+   file says so.
+4. **`FRoomGraphSource`'s warp dilation changes NAME ONLY.** It already uses the bound correctly and
+   is the reference implementation for this fix; the value it computes must be bit-identical after
+   the rename. Do not alter its formula, its `√2` factor, or anything else in that function.
+5. Comments are French + English; match the surrounding file.
+
+## Acceptance
+
+- `git diff --stat` shows **one** file: `Source/VoxelForge/Private/VoxelDensityOpStack.cpp`.
+- All three `ExtraReach` definitions include the bound; no fourth site exists (`BuildTunnelNetworkStack`
+  has no `ExtraReach` — it uses `PerlinAbsBound` directly for the warp).
+- No `Eval` body changed.
+- After the build: the eight equivalence tests stay green (density unchanged), and the box-verdict
+  lines for **Maze** and **VerticalShafts** may report *fewer* proved tiles than before. **A drop
+  there is the expected, correct outcome, not a regression** — it is the cost of a sound bound.
+  Record the before/after in `OPSTACK-PROGRESS.md`.
+
+## Notes for the reviewer (Claude)
+
+- Confirm the constant is genuinely at file scope inside the anonymous namespace and that the
+  class-static is gone, not shadowed — two definitions that can drift is the failure this fixes.
+- Confirm all three call sites got it. Two out of three is worse than none, because it looks done.
+- Confirm no `Eval`, `GetCells`, or `GetCellsAt` body appears in the diff.