VoxelForge/OPSTACK-HANDOFF.md

Handoff — VoxelForge operator stack, 2026-07-29 (T1.d delivered and measured)

Paste the block below into a fresh session. Everything it refers to is on disk and in git.

State: 8 of 8 archetypes ported and green. Tile-skipping now actually works and is measured: 11 of 40 tiles proved AllSolid at production defaults, 14641 voxels brute-forced, 0 violations. AUDIT §C2 is fixed. One commit is written but not yet built — see "First action".

---

You're picking up the VoxelForge UE5 voxel plugin on branch experimental (already checked out — do not create another). I'm Jahni. The design and the history are written down so you don't re-derive them.

Read first, in this order

1. CLAUDE.md — project rules. Rule #1 is absolute: never build, compile, or run the editor. I build everything myself. When code is done, stop, say "ready to build", list the likely compile-error spots, and wait.
2. OPSTACK-PROGRESS.md — THE LAST ENTRY FIRST. Append-only log; the resume point.
3. OPSTACK-PLAN.md — the plan. §2.6.1 is the acceptance bar and supersedes §2.6.
4. OPSTACK-DECOMPOSITION.md — per-archetype breakdown. §0.2 (the amplitude bound) is now implemented, not pending; §2 TunnelNetwork and §8 Underwater are history, not instructions.
5. AUDIT-2026-07.md — §C2's SDF-cache half is FIXED (2026-07-28), its live-edit half (OC_Chunk / BM_Chunk / FChunkBiomeCache) is still open; §C10 is SOLVED, don't reopen; §C9's library half is the top open theoretical risk with 0 measured exposure.
6. CODEMAP.md — navigation. Trust symbol names over line numbers.

Where things stand

All 8 archetypes have an operator-stack twin, per-strate opt-in, each equivalence-tested bit for bit against its original density function. The switch and the stack are two complete, interchangeable implementations.

Everything sits behind UVoxelStrateDefinition::bUseOperatorStack; the ported list lives only in UVoxelStrateManager::UsesOperatorStackForChunk (all 8). No strate asset has the box ticked — that is my call and I still haven't made it. GetDensityAt and ClassifyTile build the stack through the same factory, VF_BuildOpStackForChunk — a second copy would be a hole, not a bug.

✅ T1.d — the tile-skipping prize — is real, and it is measured

FRoomGraphSource::EffectOverBox answers spatially. The result, brute-forced voxel by voxel:

[production defaults]  11 of 40 tiles proved AllSolid — 14641 voxels checked, 0 violations
[dense fixture]         0 of 40                       — correct, and structurally inevitable

One function's verdict is inherited by FSdfConvertOp, the twelve detail modifiers (via VF_NoCaveOverBox) and FWormFieldSource — fourteen operators from one place. That is what the C1 wiring was built for.

⚠️⚠️ THE ONE INVARIANT THAT CAN DELETE COLLISION — read before touching any op

FRoomGraphSource::EffectOverBox returning Identity now means Sdf ≥ T, NOT Sdf == FLT_MAX, where T = max(3·SDFBlendRadius, WormNetworkRange). That is sound only because all three consumers of the SDF channel were read one by one:

consumer	threshold
FSdfConvertOp::Eval	Sdf >= Blend, and the tunnel stack passes MakeSdfCarve(P.SDFBlendRadius, …) ⇒ K
the twelve modifiers	VF_NearCaveSurface ⇒ 3K
FWormFieldSource::Eval	CaveSDF >= WormNetworkRange ⇒ WormNetworkRange

Any new consumer of InOut.Sdf must have a threshold ≤ T, or be added to that max. An op reading Sdf < 100 would see false Identity verdicts and produce tiles with no geometry and no collision. The warning is written at the site you land on when you add one.

(The −K slack covers any number of primitives because SmoothMin's penalty is exactly zero once |A−B| ≥ K, so the running minimum saturates at K below the smallest term. Without that observation the slack would scale with the ~88 tunnels in a cache and the criterion would be dead.)

First action: build, then read ONE line

The last commit (e002bd4, VerticalShafts) is written and NOT built. Everything before it is built and green.

Build, run the VoxelForge filter, and read Box verdicts over 60 VerticalShafts tiles.

0 was the number for the whole project's life. Its EffectOverBox used to return CarveOnly because a shaft merely existed within a Spacing*1.6 halo — true almost everywhere at ShaftSpacing 55 / ShaftDensity 0.6. It now rebuilds the connectors the way GetCells does and tests the real capsules, with Z exact and XY conservative.

• Non-zero, and violations still 0 ⇒ it worked; record it and move on.
• Still 0 ⇒ the warning in that test names what to check first: ExtraReach inflates both remaining tests, so compare it against ShaftMaxRadius before touching either test. Do not re-derive from scratch — that is exactly what cost three rounds on TunnelNetwork.

Then, in order

1. PERF — still unparked, and now the biggest open item. The op path is measurably slower. One cause found and fixed (the column memo discarded itself every chunk). Remaining suspects in order: the hashed column lookup vs GSurfColCache's direct-indexed box, then per-voxel virtual dispatch. Also measured and stated: the gate is tested twelve times per voxel instead of once (stage B5's deliberate trade). Measure before optimising — that is the §C10 lesson, and this session re-learned it the hard way.
2. The warp squeeze — PARKED with its ceiling measured, my recommendation is leave it. The WARP SHARE line says over half the remaining blocking is the query-box dilation, not geometry (production: rooms 0.9 → 0.4, tunnels 2.4 → 1.1 with the dilation zeroed). The only remaining route is proving sup|Perlin3D| down from the proved 1.5 toward its apparent ~1.0–1.1, worth ~27 % of the dilation. Spot-checking a grid is not a proof and a wrong sup is a hole. A negative result is already recorded so nobody repeats it: bounding the warp locally (evaluate at the box centre, shift, dilate by the variation) is worse — a rigorous per-axis Lipschitz bound is 4·1.875 + 1 = 8.5 per unit cell, and 8.5 × 0.206 (the half-box in noise units) = 1.75 exceeds the global range bound of 1.5.
3. AUDIT §C9 library half — sinf/cosf are not IEEE-754 specified, so MSVC's CRT and glibc's libm can differ. Currently 0 samples within 1e-6 of the isosurface, i.e. no measured risk. Run CrossPlatformDigest on Linux, compare the SHAPE digest, pin it. The real fix if ever needed is a deterministic in-house sin/cos.
4. AUDIT §C2's remaining half — OC_Chunk, BM_Chunk, FChunkBiomeCache are still keyed without the layout version. That is the live-edit staleness class ("I tweaked the asset and one patch kept the old shape"), not the determinism class, which is fixed.
5. Phase 3 — ops as data assets. A design conversation, not a transcription. Don't start it unprompted. What makes it possible is already in place: ops depend on capabilities (IVoxelBiomeField), never on UVoxelGenerator.

Debts — status changed, read this before acting on the old text

1. "Box bounds read STRATE params but a per-room op can raise them" — DORMANT, not urgent. Checked rather than paid, and the check reversed the premise: when the source proves Identity the twelve modifiers are Identity soundly (their bNearCaveSurface gate never opens, so no room op can enable anything), and when it answers Both it supplies no MaxCarveOverBox, so the default FLT_MAX kills every hypothesis regardless of what the modifiers claim. It goes live the day FRoomGraphSource gains a MaxCarveOverBox — bounding the converter's 2·BaseDensity would make the modifiers' own numbers matter for the first time. Written at the site.
2. AUDIT §C2 — FIXED on the switch path. GetDensityWithParams now takes required ParamsFingerprint + LayoutVersion. Required, not defaulted, so a caller that forgets fails to compile. The CRC is taken once per chunk where the params memo already lives (CP_TunnelFP) — a MemCrc32 per voxel on the hottest path would have been a real regression. Note the audit's own suggested alternative ("add chunk Z to the key") is both insufficient (Interleaved makes Alpha depend on chunk XY too) and destructive (chunk XY is deliberately absent so WorldX ± 1 gradient probes don't thrash the box — ARCHITECTURE §8.10).

Hard rules that prevent real bugs

• Density sign: negative = solid at the mesher. Inside the op stack the convention is INTERNAL (positive = solid), negated once by the caller. The SDF channel uses standard SDF convention.
• Identity from the room source means Sdf ≥ T. See the boxed invariant above. This is the single most dangerous thing in the current code.
• Never run both density paths in one world. Comparing them is legitimate — §C10 is closed since FPSemantics = Precise, and all eight equivalence tests compare bit for bit. They are port-correctness oracles, not fidelity checks: §2.6.1 requires same seed ⇒ same world on every peer, not resemblance to the pre-refactor world.
• Every cache key includes LayoutVersion AND the params. See §C2 and the overhang regression of 2026-07-27, where omitting the params silently deleted the overhang and only 1 sample in 20 000 crossed the isosurface.
• A bound in a box verdict must be PROVED, not observed. |Perlin3D| ≤ 1.5 is derived from GradDot's two-distinct-axes form and the per-axis weighted bound of 0.5 — not from the header's "~[-1,1]". Over-estimating costs CPU; under-estimating deletes collision.
• ProcessQueue stays EQueueMode::Mpsc; Epoch carries through every async path; don't "optimize" the ARCHITECTURE §8.10 invariants.
• Commit per coherent unit with a real message. experimental is pushed and tracked (origin/experimental, since 2026-07-29) — keep it in sync. NEVER push main, which stays the known-good fallback at the commit it has always been. ⚠️ A pushed commit here is not a "verified green" marker: the branch carries unbuilt work by design, and only OPSTACK-PROGRESS.md says what was actually built.
• Update CODEMAP §3, ARCHITECTURE §8, tick OPSTACK-PLAN, append to OPSTACK-PROGRESS.md.
• When inserting a class into VoxelDensityOpStack.cpp / VoxelHeightOpStack.cpp, put it ABOVE the labelled end of the anonymous namespace. Anchoring on the FACTORIES banner puts it outside, and the brace added with it closes nothing. Made that mistake twice; both files say so.
• Match the codebase's spelling of engine macros. KINDA_SMALL_NUMBER, not UE_KINDA_SMALL_NUMBER — the plugin uses the unprefixed form everywhere.

Method lessons this refactor actually paid for

Ordered by how much they cost.

• ⭐ Instrument what you ASSUMED, not just what you changed. This is the expensive one, learned over four rounds in one session. The warp dilation — CaveWarpStrength · VOXEL_NOISE_SCALE · PerlinAbsBound, a constant chosen in the first commit — inflated a 10-voxel tile into a 50-voxel query box, 125× the volume. Four separate tightenings (the worm, the columns, the sampler, the tunnel disjunction) were each individually correct and each landed around that untouched term. The tunnel fix, predicted "an order of magnitude", delivered 25 % — and the instrument said so, and I credited the tunnels. When a fix under-delivers against its predicted size, suspect the constant you never measured.
• Instrument before hypothesising. §C10 cost six builds and five refuted hypotheses. In this session the attribution line (AllSolid killed by: …) was written after two wrong guesses and immediately named a third operator nobody had looked at. A diagnostic that lists candidate causes without measuring them is still a guess wearing rigour — my "either the tiles straddle cave or the source isn't reaching Identity" warning offered two causes and both were wrong.
• Verify the premise before reasoning from it. Six times now a confident chain rested on an unchecked assumption and the check reversed it. Latest three: RoomSpacing was 42 (the fixture overrides it) while I did three rounds of arithmetic with the header default of 80 — the number was printing in the report I kept quoting; "the plugin bets on |Perlin3D| ≤ 0.8" was wrong (the cache rebuilds when the warped query leaves the box, so that expansion is a perf heuristic); and the per-room-op debt "must be paid first" was wrong (it is dormant). Include the premises you are confident enough about not to look up — especially a default, when a fixture exists whose whole job is overriding defaults.
• A sampler must cover at least one period of what it samples. The tunnel test drew tile XY from ±32 voxels with RoomSpacing 80 and a guaranteed origin room at (0,0) — it measured the spine hub and called it the world. The shaft test had the identical bug (±48 against ShaftSpacing 55). Both now print their own extent in units of the pattern's period.
• A test fixture tuned for coverage can be antagonistic to the thing you are measuring. EnableTunnelFeatures densifies (RoomSpacing 80→42, RoomDensity 0.35→0.85) so the equivalence check isn't comparing solid rock to solid rock — and at that density the room cull radius equals the lattice spacing, so no box can ever be proved. 0 proved there is the correct answer. The box verdict is therefore measured on both densities, and the dense run must stay at 0.
• Diagnostics report THIS run; history goes in the log. The test output had accumulated hardcoded numbers from previous runs beside live ones ("32 of 34 tiles" printed while the live figure was 21 of 28). Unreadable, and self-inflicted.
• Read the code, not the comment. The cliff modifier's comment promises a sampled Z±1 gradient; the code samples nothing and uses a Z-stretched Perlin it calls VertGrad. Ported as written — and written down, so nobody "fixes" it from the comment.
• A perf change can be a correctness change. The column-memo optimisation silently deleted the overhang; the tests caught it the same day. Invisible to inspection, and it produced plausible terrain.
• Coverage is a number, not a boolean. Four related traps, each producing a green run that proved almost nothing:
  • A test that prints nothing on success is indistinguishable from one that never ran.
  • A guard that only trips at zero notices absence, it does not measure coverage. Use fractions.
  • A success message that asserts coverage instead of reporting it reads as evidence while measuring nothing.
  • A check can be vacuous as well as a counter. "Nothing leaked" is worthless unless something happened.
• Enabling a feature is not evidence it fired — ask the structure, not the output. Setting PitDensity did nothing (wrong struct). Prefer the check that can fail for exactly one reason — and when a zero has several possible causes, give each one its own number.
• An oracle that shares the defect under test proves nothing. The stale-cache check compares each stack against itself evaluated alone. (Since §C2 was fixed, the test call sites now pass a real params fingerprint, so the original no longer shares the defect either.)
• One definition, not two kept in sync. VF_BuildOpStackForChunk exists because a tile skipped on the verdict of a stack that is not the one producing its density is a hole. The same reasoning is why GetLastRoomBoxDiagnostic reads back what the operator computed instead of letting the test re-derive the criterion, and why the two-density tile scan is one lambda called twice.
• Don't assert a number you want to improve. Check 4 asserted 0 proved — honest when written, and it would have forbidden the entire T1.d gain. What it asserts now is that no proved tile is wrong (brute force, every voxel); the proved count is reported.