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