VoxelForge/Standalone/Research/WorldAssemblerM3/Astra2/REVIEW.md

[INFERENCE] **Build a tighter passage cache first.** Ring stamping is promising, but the code already pushes segment contributions into a tile lattice. The main opportunity is reducing coverage, overlap, and off-grid fallback work—not reversing a purely pull-based architecture.

References below: `H` = `Standalone/Core/src/ShapeWorldInternal.h`; `C` = `Standalone/Core/src/VoxelForgeCore.cpp`; `M` = `Standalone/Source/VoxelForge/Private/VoxelMarchingCubesMesher.cpp`. Report references mean `Standalone/Research/WorldAssemblerM3/LinuxNNN/REPORT.md`.

**1. Cost model**

[CODE] `prepareSweptTubeBase` allocates per-passage `(cells+3)³` sample records, including a one-step halo. It walks candidate bounding boxes, lattice samples, then fine segments; individual segment bounds reject some pairs. Accepted pairs compute projection, deterministic square root, radius, floor profile, and potentially shell distance. A final pass flattens the selected tube and caches relief (`H:11436`, `H:11489`, `H:11521`, `H:11659`).

[CODE] During density evaluation, aligned samples retrieve that cache, then still perform relief weighting, floor shaping, mouth fades, shell composition, and host-field composition. Misses invoke two hierarchy searches: nearest body/floor and nearby shells—not necessarily a linear whole-route scan (`H:1247`, `H:1346`, `H:1427`, `H:1435`).

[MEASURED] L119 reports 29.40M preparation pairs and 24.46M segment visits across its four LOD0–2 profiles. Preparation fell 59.8% versus L118, while fallbacks increased 55–58%. L117 attributes about 95% of the tile-worker regression to passage-feature tiles. Thus the diagnosis is sound, but those counters do not establish equivalent costs per operation.

[INFERENCE] For an illustrative straight crossing, assume exposed surface perimeter \(P=16\) m, tile crossing length \(L=32\Delta\), and exact band ±one lattice step. Unique band samples are approximately \(2PL/\Delta^2\):

| [INFERENCE] Normal tile | LOD0 | LOD1 | LOD2 |
|---|---:|---:|---:|
| Spacing Δ | 0.25 m | 0.5 m | 1 m |
| Band samples | 4,096 | 2,048 | 1,024 |
| Fraction of 35³ prepared grid | 9.6% | 4.8% | 2.4% |

[INFERENCE] These are geometric estimates, not measured “typical tiles”; clipping, floor/outer-shell surfaces, bends, and crossings change them. If today fraction \(f\) of the grid receives \(k\) segment evaluations, and stamping averages \(m\) contributions per band sample, the expensive-work ratio is approximately `(table fraction × m)/(f × k)`, multiplied by relative operation cost. For `f=0.5, k=2, m=2`, that is 19%, 9.6%, 4.8%. Allocation, classification, host evaluation, and composition remain. Dense rings can make \(m\) large and erase this advantage.

**2. Concrete difficulties**

- [CODE] **Skipping:** passage-bound overlap currently vetoes a uniform proof and returns `Mixed` (`H:5684`). [INFERENCE] Retain conservative bounds, expanded for approximation and reconstruction support. “No band samples written” proves neither solid nor air: a thin shell can lie between samples.

- [CODE] **Seams:** cache coordinates derive from integer world coordinates; nearest ties use segment index (`H:11448`, `H:11564`, `H:11640`). [INFERENCE] Generate rings from route-global parameters, never tile-local stepping. Include every contributor affecting shared samples and halos; use stable ordering and arithmetic. Independently clipping ring generation at tile boundaries risks disagreement.

- [INFERENCE] **LOD:** distinguish 25 cm curve error from lattice spacing. A fixed 25 cm band is insufficient around 1 m cells; cover interpolation corners and gradient support. A 0.75 m shell can disappear at coarse resolution. Independently reconstructed LOD fields also disagree off-grid; define one canonical point-query field.

- [CODE] **Off-grid:** `sweptSample` rejects non-lattice coordinates; public point queries call `DensityAtUnclamped` without requiring a tile cache (`H:11721`; `C:1875`). Gradients normally use density-grid values, with unclamped fallback (`M:1972`). [INFERENCE] Interpolation needs all eight valid corners, including outside loaded tiles. It changes gradients and zero crossings; interpolating only passage channels also differs from interpolating the fully composed field.

- [CODE] **Composition:** passage bodies and shells use separate hard minima; final composition includes host/native-floor exceptions (`H:1496–1519`). The floor corner uses smooth-max, implemented through smooth-min (`H:3493`, `H:6940`). [INFERENCE] Do not assume the measured smooth-min cost is tunnel-to-tunnel joining. Preserve separate void/shell channels and composition order. Arbitrary “rock/air” magnitudes can move interpolated crossings and smooth blends.

- [CODE] **Shape:** floor selection is independent of minimum round distance; the code explicitly warns that minimizing flattened segment distances makes downhill floors jump (`H:1328`). Floor evaluation uses horizontal projection, authored `h(s)`, and XY mouth blending (`H:3448`). Shells have rounded crests and five-metre mouth fades (`H:1461`, `H:3503`). [INFERENCE] A conventional spline-normal D-section changes this geometry. Stamped isolated rings can scallop; rasterize bounded slabs between frames while preserving gravity-relative floors, shell unions, and interior priority.

- [MEASURED] **Walkability:** L119’s maximum sampled floor-root offset was 7.884 mm; L113 documented voxel-aligned floor ribs. [INFERENCE] A 25 cm curve tolerance does not authorize 25 cm floor error or guarantee ≤15° grade. Audit reconstructed density and meshes, including mouths, overlaps, support, and headroom—not only planner `h(s)`.

- [CODE] **Threads:** copied worker scopes share prepared data through a shared pointer; active scope is thread-local (`H:353`, `H:11411`; `C:2854`). [INFERENCE] Finish stamping before publication. Parallel ring writes require deterministic reductions; mutable shared lazy caches introduce synchronization and lifetime problems.

**3. Smallest useful change**

[CODE] Preparation already precedes `GenerateMesh`, whose fill calls lattice evaluators (`C:2747`, `C:2892`; `M:1488`). Route segments are already prepared globally with 10 cm chord tolerance and an 8 m spacing cap (`H:4601`).

[INFERENCE] Replace preparation’s broad pair enumeration with conservative slab-to-grid spans or small spatial bins. Keep indexed lookups during pull-based fill. Store body/floor ownership and shell contributions separately; use explicit outside/interior/band states. A dense index plus compact records may beat a sparse hash map. Additional frames can be route-global, but this does not eliminate existing per-tile rasterization.

**4. Alternatives and recommendation**

[MEASURED] L115 already reduced coarse candidates 74.1% without reducing fine pairs. L118’s restriction of fallbacks to prepared spans changed output; L116’s 20.8% pair reduction did not recover readiness.

[INFERENCE] An analytic local frame still needs a parameter lookup, especially near folds. Prefer globally indexed conservative segment bins, shared by preparation and exact point queries.

[INFERENCE] Build in three steps: instrument unique samples, overlap, misses by caller, memory, and per-LOD time; implement tighter bins/slabs while preserving geometry; then separately trial 25 cm curve tolerance and canonical interpolation. Accept only reproducible near-readiness gains against L119/L109 plus unchanged walk/seam requirements and cross-compiler/worker determinism. Kill the approach if saved distance work becomes rasterization, memory, or reconstruction cost.