# VoxelForge — Architecture & Design Deep-Dive > The 2026 redesign in detail: archetypes, the (0,0) spine, disturbances, content/atmosphere, > biomes, and the **performance invariants** (the `§8.10` "don't regress" list). Read this > before touching generation / strates / passages. > Navigation, file index & conventions live in [CODEMAP.md](CODEMAP.md). Section numbers (`§8.x`) > are preserved so existing cross-references keep resolving. ## 8. Archetypes, spine, disturbances, content & carving (2026 redesign) A large A-to-Z expansion. The world is a stack of strates the player descends through; each strate can be a fundamentally different *archetype*, connected at (0,0). ### 8.1 Archetypes (`ECaveGeneratorType`, VoxelStrateTypes.h) Each archetype has its own param `USTRUCT` (on `UVoxelStrateDefinition`, EditCondition-gated by `GeneratorType`) and its own density function in `VoxelGenerator.cpp`, dispatched by the `switch` in `GetDensityAt`. | Archetype | Params struct | Density fn | Idea | |-----------|---------------|------------|------| | TunnelNetwork | `FStrateGenerationParams` | `GetDensityWithParams` | rooms+tunnels (original) | | FlatPlain / CrystalChamber | `FSlabGenerationParams` | `GetSlabDensity` | floor/ceiling void (original) | | Maze | `FMazeGenerationParams` | `GetMazeDensity` | tight corridors on a 3D lattice (per-voxel, no cache; edge = lower node + axis hash) | | SurfaceWorld | `FSurfaceGenerationParams` | `GetSurfaceDensity` | heightfield terrain: domain-warped continents+ridged mtns+detail, a low-freq **relief map** (`M`) that scales mountains/elevation for plains↔highland variety, **F20 heightfield terrain ops** (`Surface|Ops` — all default off ⇒ byte-identical): **Cliff** (slope-gated steepening — where the analytic structural slope > `CliffSlopeThreshold`, push the height away from the local mean by `CliffSharpness` ⇒ gentle slopes become sheer walls/canyon faces that hug real steep ground, gentle areas untouched; 4 structural resamples only when enabled), **Terrace** (relief-gated plateau quantize + `TerraceHardness` soft-round↔crisp-mesa), **LayerLines** (sedimentary sine shelves, slope-expressed) — pure per-column height remaps in the single oracle `ComputeSurfaceTerrainZ` (`SampleSurfaceStructuralZ` = pre-op raw height, re-sampled at an XY offset for Cliff's slope), biome-selected + border-blended for free via each biome's `SurfaceParams` + the height output-lerp; plus **phase-2 Overhang** (the first VOLUMETRIC op — real jutting shelves like a cliff lip): in `SurfaceDensityFromColumn`, for AIR voxels in the window `(TerrainZ, TerrainZ+OverhangHeight]` above a steep slope, the heightfield is re-sampled UPHILL (toward the cliff) by a reach that GROWS with height and unioned in. Low in the window the shift is ~0 (borrows nearby low rock ⇒ stays air over the void); high up it reaches the far cliff (solid) ⇒ a shelf attached to the cliff, tapering out over the void with air underneath. Per-column `OverhangAmp`(=strength·slope-gate) + unit uphill dir `(DirX,DirY)` are resolved once in `ComputeSurfaceColumn` — the gradient sampled at the REACH scale (`OverhangReach`) so a point out over the void can "see" the cliff to know which way is uphill — and cached on `FSurfaceColumn`. It's genuine 3D (per-voxel structural re-eval), so it's gated hard to steep overhang columns; the union only ADDS rock (never removes), capped at `TerrainZ+OverhangHeight`, so `ClassifyTile` forces Mixed only in `(TerrainZ, TerrainZ+OverhangMargin]` (upward-only; margin = max `OverhangHeight`) — a shelf never holes a trivially-skipped tile, and only the thin cliff-edge band of air tiles is woken (NOT far-field like phase-3 spikes/holes). Overhang undersides classify as ground rock by the F17 rule (down-facing but below `TerrainZ`). Known v1 limit: applies at all LODs (Step-agnostic) — may alias far; gate to fine tiles later. Beaches at water line, high sky-cap ceiling. The cap is shapeable terrain in its own right (`ComputeSurfaceCeiling`, `Surface|Sky` params: `CeilingUndulation` broad inverted hills/valleys, `CeilingRidgeStrength` hanging ridgelines, `CeilingRoughness`+freq fine bumps, `CeilingWarp*`) — defaults (strengths 0, freq 0.04) = old flat-ish cap. (`Surface|Macro` params = the cheap precursor to biomes; `ReliefStrength=0` ⇒ old uniform terrain.) **Sky-cap vs ground is a PER-TRIANGLE surface class (F17), not a per-tile verdict**: the mesher classifies each unique vertex on the **worker** — only down-facing verts (`N.Z<-0.1`) pay a memoized `GetSurfaceHeightAt` column query; nearer `CeilSurf` ⇒ sky-cap, nearer `TerrainZ` ⇒ terrain overhang stays ground (a future SurfaceWorld cave roof — down-facing but below `TerrainZ` — also lands ground by the same rule; non-surface strates always ground). Triangles take the majority class of their 3 verts and are packed as **two contiguous index runs** (ground then cap, `FVoxelMeshData::NumCeilingTriangles`; skirts inherit their source triangle's run) → RMC **polygroups 0/1** → one **section per non-empty group** (`ApplyMeshToTile`, material slot = group index): slot 0 = strate `OverrideMaterial`/default (min-corner chunk), slot 1 = `CeilingMaterial` resolved at the tile's **TOP chunk** (mid as gap fallback, then min; fallback: ground material) — a coarse tile is 2^level chunks tall, so its min corner can sit in a lower strate/gap while the cap belongs to the strate above (this was the "far cap = ground material" residue), so the shadowless overhead rock is tinted separately instead of reading flat/bright. Shadow is **per section** via `FRealtimeMeshSectionConfig::bCastsShadow` (NOT the component `SetCastShadow` — RMC's proxy ignores the component flag; this is also why level≥2 far tiles only stopped casting once the section flag was wired): ground casts at level≤1, the cap section never casts, so the rock ceiling never shadows the terrain below it. History: v1 was a game-thread centre height-oracle (misclassified coarse far tiles → terrain material on the cap underside); v2 a whole-tile worker normal VOTE — which painted **mixed coarse tiles** (one far tile spanning terrain AND cap) entirely with the winner's material, and put sky material under terrain overhangs. The per-triangle class fixes both and is the identity channel caves/F8 will reuse. | | VerticalShafts | `FVerticalShaftParams` | `GetVerticalShaftDensity` | full-height shafts + horizontal connectors + partial ledges | | FloatingIslands | `FFloatingIslandParams` | `GetFloatingIslandDensity` | asymmetric islands: flat land top + underside tapering to a point, lobed (domain-warped) outline, in an open void | | Underwater | `FStrateGenerationParams` + water | (reuses `GetDensityWithParams`) | tunnel rock + high water table | All density fns share the convention: internal **positive=solid**, apply origin spine → vertical boundary seal → inter-strate passages → XY edge seal, then `return -Density` (MC: negative=solid). The final MC-facing edge pass in `GetDensityAt` is repeated after disturbances so the global rim cannot be reopened by a post-process; the diff layer remains the explicit player override. StrateManager provides params per chunk via `GetMaze/Surface/VerticalShaft/FloatingIslandParamsForChunk` (macro `VF_ARCHETYPE_PARAMS_GETTER`) — no cross-boundary blend (Hard transitions between archetypes). On top of the archetype, an optional **biome** layer (§8.14) modulates terrain & content WITHIN a strate via a window-invariant XY field — currently wired into SurfaceWorld. ⚠️ **The `switch` above is no longer the only density path.** 6 of the 8 archetypes (all but TunnelNetwork and Underwater) also exist as **operator stacks**, selected per strate by `bUseOperatorStack` and evaluated instead of the `switch`; each is bit-identical to the function in its row. The design lives in `OPSTACK-PLAN.md` / `OPSTACK-DECOMPOSITION.md`, the symbol index in `CODEMAP §3.2d` — not repeated here. What matters for *this* document: the archetype table describes what the world IS, and both paths compute it. ### 8.2 (0,0) spine & hybrid connections - `ApplyOriginSpine` (VoxelGenerator.cpp, static helper) carves a guaranteed open vertical column at XY (0,0) in every strate's **interior** (seals untouched). Radius = `UVoxelGenerator::OriginSpineRadius` ← `VoxelSettings::OriginSpineRadius`. Called before every `ApplyBoundarySeal`. - `VerticalShafts` treats that structural column as a **connector endpoint**, not as a second density primitive. A thread-local rebuild collects a direct-indexed geometric halo once (with the stock settings: 9×9 tree-emission cells and a 15×15 roll), emits tree links only for the inner 3×3 cells, and resolves each parent from its complete ±2-cell candidate window. Each shaft chooses the nearest shaft whose jittered distance to (0,0) is strictly smaller; ties use the lowest cell index. If that window has no parent, the deterministic ±3 lower-origin neighbour fallback fires; a window touching the origin uses the spine directly, with a final direct-spine fallback for an unusually empty finite halo. The wider collection is therefore fully contained for every emitted shaft and the parent is independent of chunk or evaluation-cell boundaries. Every parent edge strictly decreases origin distance or terminates at the spine, which proves that no shaft is orphaned or cyclic. The existing probabilistic `VoxelHash::Pair` links and `Spacing*1.6` cutoff remain as texture and loops; they are additive, not the connectivity law. Tree links use a radius strictly above the proven `sup|FBM|=1.5` roughness envelope (`max(ConnectorRadius, SurfaceRoughness*VOXEL_NOISE_SCALE*1.5 + 1)`) and choose a shared ledge-free Z interval. `BuildVerticalShaftStack` mirrors the same tree, random links, windows, hashes, radius, and connector-Z rule; `VF_ApplyOriginSpine` remains the sole owner of the vertical column itself. - Descent is **player-dug** through the thin seals at (0,0). The single auto-opened connection is the **surface entry shaft** at (0,0) through the top of strate 0 (`GeneratePassages`, `bOpenSurfaceEntry`). - **Hybrid extras:** auto-carved *shortcut* passages per boundary, placed away from (0,0). Now fully **per-strate** — see §8.8 (the upper strate's `PassageConfig` drives count/style/shape). The tree is a construction guarantee for the shaft field: every seeded shaft has one inward edge, and every chain strictly decreases distance to the origin until it reaches a local minimum that links to the spine. The fallback resolver preserves that proof, so no shaft is an orphan or part of a cycle. It does not alter passage placement. The acceptance measurement remains the actual arrival→departure flood-fill at the independently placed passage endpoints. ### 8.2a XY edge seal (bounded-world invariant) `UVoxelSettings::WorldRadiusVoxels` and `EdgeSealThickness` define the bounded world in actor-space XY around `(0,0)`. The default radius is 8192 voxels (~2.05 km at 25 cm/voxel) and the default solid ramp is 64 voxels. A radius of **0 is a true no-op**: the hot path returns before doing any arithmetic, so the legacy unbounded field is unchanged. From `R - Thickness` to `R`, `VF_ApplyXYEdgeSeal` raises the internal positive-solid density with the same `SmoothStep01` ramp idiom as the vertical seal; at and beyond `R` it forces positive `BaseDensity`. The generator negates once for marching cubes, so the externally visible sealed density is negative (solid). This is the fourth structural post and is appended automatically by `FVoxelOpStack::AppendStructuralPost`, after the spine, vertical seal, and passage carve. Passage carving deliberately remains before the edge seal: passages may pierce a vertical strate seal, but a passage whose mouth or body reaches the XY rim is overwritten by the edge force and cannot open a hole through the world boundary. The legacy density functions apply the same post in the same order. `GetDensityAt` reapplies the MC-facing form after disturbances as a backstop for out-of-layout air and any disturbance carve; player diff edits still run last by design and remain authoritative. `FXYEdgeSealOp` is `IsXYPure() == true`, uses only a squared radial early-out in the common interior case, and returns `AllSolid` from `ClassifyBox` only when the closest point of the whole box is at least one voxel into the ramp (or beyond `R`). Its `ForcedMarginOverBox` reports the actual positive lower bound, conservatively reduced after the one-voxel proof margin; it never treats the mathematical inner edge as solid. The same proof is used by the global `ClassifyTile` shell guard, so tiles wholly in the proven outer band can skip T1.d generation/collision work. A box touching the band but not meeting that proof remains `Mixed`. ### 8.3 Disturbance layer (the "wow" post-process) `FStrateDisturbanceParams` (on the definition, all archetypes). `ApplyDisturbances` (VoxelGenerator.cpp static, **MC convention**) runs in `GetDensityAt` after dispatch: chasms (carve air), bridges (solid spans), ridges (solid blades). The XY edge seal is reapplied after this layer, so disturbances cannot reopen the outer shell; the layer otherwise stays inside the vertical seal bands. Provided per chunk by `StrateManager::GetDisturbanceParamsForChunk`. ### 8.4 Cross-chunk determinism (the seam-prevention invariant) `BuildChunkCache` (VoxelCaveMorphology.cpp) uses **two regions**: a wide *COLLECT* region (`2*MaxTunnelLength + MaxInfluence`) over which connectivity is decided (NN filtered to `<= MaxTunnelLength`, origin cap = deterministic top-N by hash), and a tight *STORE* region (`+MaxInfluence`) kept for per-voxel eval. This makes the room/tunnel graph window-invariant. **If you add a connectivity rule with longer edges, the COLLECT region must still cover the max edge reach, and decisions must not depend on the stored window.** ### 8.5 Content scatter & water — `VoxelContentManager.h/.cpp` (NEW) `UVoxelContentManager` (owned by `AVoxelWorld`, game-thread). TWO INDEPENDENT subsystems: **(A) DECORATIONS — distance-based WORLD GRID (the no-pop system, 2026-06-17).** Decorations are placed on a fixed world XY cell grid (**1 cell = 1 chunk footprint**, `DECO_CELL_VOXELS = CHUNK_SIZE`) and streamed by DISTANCE from the player, **fully decoupled from clipmap tiles / LOD**. **THE MARCH RUNS ASYNC ON WORKER THREADS** (mirrors mesh gen — `GetDensityAt` is thread-safe; the synchronous-on-game-thread first cut was a perf disaster + starved streaming → seams, so it was moved off-thread). Driven by `AVoxelWorld::Tick → UpdateDecorations(playerWorldPos)`, three phases: **(1)** recompute the desired cell set (`RebuildDesiredCells`) only when the player crosses a cell boundary OR changes strate — clears out-of-range loaded cells, queues cells that are NOT loaded and NOT in flight (`PendingLaunch`, nearest-first). A loaded region is NEVER re-streamed in place while it stays in range (only cleared when it leaves), so decorations don't FLICKER as the player moves. **TWO STREAMING GRIDS (`FStrateDecoration::StreamTier`, 2026-06-26):** the stream RADIUS is a property of the GRID, never of an entry — mixing radii inside one grid would force an in-place re-stream when the player crosses an entry's radius (the original tier system's flicker bug). So there are exactly two self-contained grids (`FDecoGrid`, each with its own region map + builds + queues + HISMs): **NearGrid** (`DecorationNearRadiusChunks`, FINE `DecorationSpacingVoxels` column grid — dense groundcover near the player) and **FarGrid** (`DecorationRadiusChunks`, COARSE `DecorationFarSpacingVoxels` column grid — trees/landmarks/RARE props visible everywhere; the coarse grid is what makes a rare prop cheap, since the worker march cost scales with column count). An entry picks a grid via `StreamTier` (default Far); the palette is partitioned by tier so each grid marches only its own subset. A given world XY is covered by a Far region always, plus a Near region when close (separate HISMs → crossing the near boundary never touches the far region). **(2)** `LaunchDecoTasks` (per grid, sharing ONE `MaxConcurrentDecorationTasks` budget — each throttles against the other's in-flight count): snapshot the update's decoration palette (built once on the GAME thread — biome, see below) then fire an async `UE::Tasks` march (`BuildCellSpawns`, `BackgroundNormal`, capped at `MaxConcurrentDecorationTasks` in flight via `InFlightCells`). **(3)** `ProcessDecoResults`: drain finished tasks' results (`Mpsc` queue → `ReadyResults`), epoch-guarded (`DecoEpoch`, bumped on clear/strate-change so stale in-flight results are discarded) + range-checked, and **apply (spawn) budgeted** (`MaxDecorationCellsPerFrame` — the only game-thread cost, SpawnActor/AddInstance). `BuildCellSpawns` (worker) finds each column's surface point(s) and rolls the entries there (shared `PlaceAtCrossing`). Candidate columns are **snapped to INTEGER voxel XY** (integer jitter) so the generator's surface-column cache (T1.a, §8.10) applies — FRACTIONAL XY bypasses it and recomputes the noise-heavy heightfield+biome on every sample. **TWO column strategies by archetype:** **(a) SurfaceWorld → HEIGHT ORACLE (`Ctx.bSurfaceWorld`), NO marching.** `Generator::GetSurfaceHeightAt(x,y, chunkZ → TerrainZ, CeilSurf)` returns the heightfield surface + sky-cap ceiling in O(1) (it shares the density path's `ResolveSurfaceChunkParams`/`ComputeSurfaceColumn` via its own thread_local per-chunk cache, so it's bit-identical to the rendered ground). Per column: query centre + 4 neighbours (gradient → floor/ceiling normals), place a Floor crossing at `TerrainZ` and a Ceiling crossing at `CeilSurf` (if open space below). A single `GetDensityAt` at the surface verifies the column isn't CARVED (passage/spine/diff make it air → skip; the oracle is the raw heightfield and doesn't know carving). ~5 height evals + 1-2 density samples/column vs hundreds marched. **(b) other archetypes (caves/shafts/islands) → ray-march** the strate Z-band (`GetStrateUnrealZRange`, voxel coords) via `GetDensityAt` at a COARSE step (`DecorationMarchStepVoxels`), each air↔solid sign change **bisection-refined** (4 iters → accuracy independent of step). Either way **a prop sits at the SAME world position at every LOD → no pop**. (march) The top cap/seal + the open air are always marched first; the scan only stops after `DecorationColumnDepthVoxels` of CONTIGUOUS solid once it has ENTERED the open space (trims dead bedrock below the ground without ever stopping short of it — a "below the first crossing" cap was wrong: on a surface world the first crossing is the high CEILING, so it stopped mid-air before reaching the ground = no floor props). Outward normal = normalized density gradient (solid→air, matches the mesher), classified Floor/Wall/Ceiling by `normal.Z`. Each crossing rolls every `FStrateDecoration` independently: surface-type, density gate (`DecoHash(cell,column,crossing,entry,seed)`), water-relative, align/yaw/scale, per-cell `MaxPerChunk` + global actor cap → a `FDecoSpawn{EntryIdx, bInstanced, Xf}`. The game thread spawns from the result's `Entries` snapshot. `DecorationMaxCrossingsPerColumn` caps cave columns (surface worlds have 1). **Shutdown:** `NotifyShutdown()` (called from `AVoxelWorld::EndPlay`) flags + spin-waits on the in-flight task count before UObject teardown (tasks read the Generator); `BeginDestroy` is the backstop. **Determinism:** pure hash of (cell, column, crossing, entry, seed) + the density surface snap. **Decorations exist ONLY in the player's current strate** (march is strate-bounded) → a strate change wipes + rebuilds them, and there is **no cross-strate light bleed to cull** (the old `SetActiveStrate` light-culling pass is SUBSUMED — gone). **Render paths:** `ActorClass` → real actors (lights/logic, pricey game-thread spawn); `InstancedMesh` → HISM (no tick/actor/collision, emissive glows far), per-cell-per-entry. **Per-entry HISM tuning for dense groundcover** (`FStrateDecoration`, only the InstancedMesh path): `CullDistance` (cm; 0 = no cull — the lever that makes dense grass affordable: placed thickly, drawn only near → GPU cost bounded by area-within-cull, NOT the stream radius), `bCastShadow` (default true; turn OFF for grass — dense instanced shadows are the dominant foliage cost), `MaxSlopeAngle` (deg from flat = acos(|N.Z|); 90 = no filter, ~35 keeps grass off cliffs — applied in the worker's `PlaceAtCrossing`). **Placement-constraint gates (all in `PlaceAtCrossing`, deterministic, zero-cost at defaults):** `MinSlopeAngle` (lower companion to Max — band a prop onto a tilt range, e.g. 30..70 = slopes only), `bWallExcludeOverhangs` (wall-only-upright: drop normals with N.Z < 0 so downward overhangs don't take wall props). **Shared vocabulary (2026-07-06):** these gates + spawn/transform/render fields now live on `FPlacementProfile` (embedded as `Profile` on `FStrateDecoration`, `FStrateLandmark`, and the coming `FStrateSetPiece`), so all three scatter primitives are authored identically. Rotation unified to `RotationOffset` (fixed) + `RandomRotation` (per-axis hash-random) — decoration's ctor defaults `RandomRotation.Yaw = 360` (full random heading, replacing the old `bRandomYaw`/`MinYaw`/`MaxYaw`; banded yaw = offset + a smaller random range). Distribution is unchanged; the exact per-instance yaw values reshuffle once (different hash mix). **F7 AWARE PLACEMENT (2026-07-06):** `FPlacementProfile::Conditions` = a list of `FTerrainCondition` DERIVED PREDICATES (relief / moisture / biome-border weight, each an inclusive [Min,Max] band, optional invert), AND-ed and evaluated by `Generator::EvaluateTerrainConditions` at the candidate XY — "conditions, not annotations": nothing is stored, the phenomenon is queried from the analytic fields (SampleRelief / SampleMoisture / SampleBiomeAt) on demand, so it's deterministic + worker-safe (the caller hands over the strate's already-resolved `FBiomeContext`, so no re-resolve). Opt-in per entry (empty list = zero cost); wired into both the deco worker (`BuildCellSpawns`) and landmark placement (`SpawnLandmarkInstance`). This is the shared core of the coming quest `FindFeature` locator (same predicate, run as an outward search) and the anchor gate for `FStrateSetPiece`. The BP bridge `AVoxelWorld::GetVoxelSurfaceHeightAt` exposes the trace-free deterministic ground/ceiling height so authored ruin/set-piece Blueprints self-arrange on the real surface before it meshes. Next types (water-edge band, relief-peak local-max, slope) are additive. **F7 COMPANIONS (relational decoration, `FDecoCompanion`):** each `FStrateDecoration` may list `Companions` (satellites: rocks/mushrooms around a tree). When `PlaceAtCrossing` emits a parent spawn, it immediately rolls each companion (probability → count in `[CountMin,CountMax]` → disk offset `RadiusMin/MaxVox`), all hashed off the PARENT's hash `H` → a pure function of the parent, so NO "did a tree spawn here?" search. Each satellite offsets from the parent in the XY plane (voxel space) and then, by default (`bSnapToSurface`), **re-snaps to the real surface at its OWN XY** via `FindLandmarkColumn` (cheap `GetSurfaceHeightAt` oracle on SurfaceWorld, a short ray-march in caves; worker-safe — reads only `Gen`) — this kills floaters on uneven ground; a satellite that finds no surface at its spot is skipped. Turn `bSnapToSurface` off to inherit the parent's exact height+normal (cheapest, flat ground only). Satellites can also be **gated by their own `Profile.Conditions`** evaluated at the satellite XY (e.g. a unique mushroom only at a biome border). They carry their own `Profile` transform/render. `FDecoSpawn` gained `CompanionIdx` (−1 = the entry; else index into `Companions`) so `MergeCellResult`/`ApplyRegion` resolve the satellite's mesh/actor + render tuning from `Companions[ci].Profile` (the region mesh bucket now stores an `FPlacementProfile`, not a whole entry). **Two levels:** a companion may carry `SubCompanions` (`FDecoSubCompanion` — a distinct type, since UHT can't reflect a self-recursive `FDecoCompanion`) that spawn ON each level-1 satellite (moss on a rock); level-2 **inherits the L1 satellite's snapped point** (no re-snap — the cost lever that keeps nesting cheap) and can still gate on its own `Conditions`. `FDecoSpawn::SubIdx` routes L2 back to `Companions[ci].SubCompanions[sj]`. A hard **per-parent budget** (`GMaxCompanionsPerParent`, 256) caps the total L1+L2 count so a misconfiguration can't blow up regardless of authored counts. Bounded (count × parents, ≤ budget), deterministic, streams inside the existing two-grid deco system untouched. Depth beyond 2 (and per-entry references for arbitrary nesting) is the remaining flagged follow-up. **F7 SET-PIECES — FOLDED INTO LANDMARKS (2026-07-06).** Set-pieces (ruins/shrines/monuments) and landmarks (mini-suns) had no real reason to be separate once both shared the spawn core and gained `Conditions`, so `FStrateSetPiece`/`UpdateSetPieces` were MERGED into `FStrateLandmark`/`UpdateLandmarks` — one primitive, one `UVoxelStrateDefinition::Landmarks` list. Each entry has an `AnchorMode`: **HashLattice** (scatter on a coarse lattice, `SpacingChunks`) or **PassageMouth** (enumerate `StrateManager->GetPassages()`, keep endpoints whose Upper/LowerStrateIndex == the current strate — `bAtDescentMouths` = the hole going down, `bAtArrivalMouths` = where you land; endpoints are global VOXEL coords). Feature-conditioning is just `Profile.Conditions` on either mode. **Exclusion (self-awareness, optional):** `ExclusionRadiusChunks` (0 = OFF, the default — mini-suns don't exclude) + `Priority`; a candidate is suppressed if a HIGHER-RANKED one's disk covers it (rank = Priority, then hash — deterministic). `UpdateLandmarks` now gathers all candidates → resolves exclusion (SKIPPED entirely when no entry opts in, so pure scatter keeps its old O(n) cost; HashLattice keeps the original hash salt 0x1A2D5u so mini-sun positions are unchanged) → spawns survivors via `SpawnFromProfile` (+ the orb wrapper). **Exclusion is now POP-FREE:** each entry is gathered in `StreamRadius + MaxExcl` (MaxExcl = the strate's largest `ExclusionRadiusChunks`); the extra "ring" candidates carry `bSpawnable = false` and only SUPPRESS (never spawn), so every conflictor of an in-range candidate is always present regardless of player position → a candidate's fate is a pure function of (seed, layout), no edge-of-radius flicker. **Decoration footprint (`bSuppressDecorationsUnder` + `SuppressRadiusChunks`):** a landmark mesh doesn't change density, so the deco placer can't see it — instead, on spawn it calls `RemoveDecorationsInSphere` to clear grass in its footprint, stores the footprint on `FLandmarkInstance`, and `ApplyRegion` re-clears under any loaded suppressing landmark when a deco region streams in fresh (so temple floors stay clear as you leave/return). **PLAYER DIG → GRASS REMOVAL:** `AVoxelWorld::ApplyModification` (the single funnel for every carve/fill brush) calls `RemoveDecorationsInSphere(Center·VOXEL_SIZE, Radius·VOXEL_SIZE)` after the diff+remesh — instant, flicker-free (only the affected HISM instances go, via `GetInstancesOverlappingSphere`+`RemoveInstances`; nothing is cleared+rebuilt), so grass never floats over a dug hole. This only patches the LIVE instances; the placer's existing density check (`D(VX,VY,hC) <= 0.5f`) already keeps any future natural rebuild correct. (Editing a Static-mobility HISM re-caches its proxy — fine for player-paced digging.) Not handled by immediate removal: new grass on a freshly-exposed ledge / regrowth after fill-back — both self-correct on the next natural re-stream. **Freeze note:** a huge set-piece mesh hitches on register (game-thread proxy/distance-field build — NOT async-fixable; the spawn is game-thread by engine rule; the asset is a hard ref so already resident) → mitigate ASSET-side (Nanite on the mesh, bake distance fields), optionally budget spawns across frames. `ApplyDecoResult` buckets spawns per entry and builds each HISM with ONE batched `AddInstances` (single cluster-tree build, set cull/shadow BEFORE `RegisterComponent`) — the game-thread hitch-killer for dense cells. **Per-entry tier (`StreamTier`, default Far)** picks NearGrid or FarGrid; radius + column spacing are PER-GRID settings, never per-entry (a per-entry radius would re-introduce the in-place re-stream flicker — see the two-grid rationale above). This REPLACES the vestigial `MaxLODLevel`; the dead `DecorationActorRadiusChunks` setting is repurposed as `DecorationNearRadiusChunks`. `CullDistance` still bounds GPU draw on top (orthogonal to which grid streams the entry). **No LOD area-density compensation** (placement is per real surface point, density-stable with distance). **SpawnDensity semantics CHANGED** vs the old vertex scatter: it rolls per column surface-point (not per mesh vertex) → expect a one-time density re-tune. **Settings (`Voxel|Content`):** `DecorationRadiusChunks` (6 — FAR reach in cells), `DecorationNearRadiusChunks` (3 — NEAR reach), `DecorationSpacingVoxels` (4 → 8×8 cols/cell — NEAR/fine grid), `DecorationFarSpacingVoxels` (4 by default → raise to 8–16 for a cheap coarse FAR grid; the rare-prop lever), `DecorationRegionSizeCells` (4 — RxR cells per region/HISM), `DecorationMarchStepVoxels` (2 — coarse, bisection-refined; cave march only), `DecorationMaxCrossingsPerColumn` (4 — cave march only), `DecorationColumnDepthVoxels` (160 — bedrock march cap; cave march only), `MaxDecorationCellsPerFrame` (2 — apply/spawn budget), `MaxConcurrentDecorationTasks` (4 — in-flight task cap; 0 disables decorations). **COST:** surface worlds now use the O(1) oracle (cheap); caves ray-march. The work is OFF the frame (worker threads) — game thread only pays the budgeted spawn. If streaming slows, lower `MaxConcurrentDecorationTasks` / raise `DecorationMarchStepVoxels` / shrink radii/spacing. Default `DecorationRadiusChunks=6` ≈ props ~48 m out — raise for far flora (cost ~r²). **(A2) LANDMARKS — rare large objects on a COARSE HASH LATTICE (`UpdateLandmarks`, 2026-06-26).** The right primitive for sparse, far-visible objects like the underground "mini-suns" — where the per-chunk decoration grid fails: at a 2048-chunk radius that grid enumerates ~13M cells per cell-crossing on the game thread and FREEZES. Landmarks instead live on a per-entry hash lattice (cell = `FStrateLandmark::SpacingChunks` chunks), so a radius-R disk holds only ~(R/Spacing)² candidates (≈16 at R=2048, Spacing=512). Listed strate-wide on `UVoxelStrateDefinition::Landmarks`. Each Tick, for each entry, walk the small lattice box around the player within `StreamRadiusChunks`: `DecoHash(cell,entry,seed)` rolls existence (`SpawnProbability`), a jittered XY (`JitterFraction` — effective min spacing ≈ Spacing·(1−Jitter)), then a SINGLE-column surface find (`FindLandmarkColumn`: SurfaceWorld height oracle, else one density ray-march) snaps to the chosen `SurfacePlacement` (default Ceiling = sky-cap). Gates mirror decorations (biome via `GetDominantBiomeAt`, slope band, water-relative). Foliage-style transform tweaks: `LocationOffset` (world XYZ), `RotationOffset` + per-axis `RandomRotation`, `Min/MaxScale`, `bAlignToSurface`. Spawned as a real actor (`ActorClass`, for a sun's light) or one Static `UStaticMeshComponent` (`InstancedMesh`, `CullDistance`=0 → never cull). All SYNCHRONOUS on the game thread (so few candidates it never hitches); a cell's surface-find runs only the first frame it enters range, then is cached in `LandmarkInstances` (keyed `FIntVector(cellX,cellY,entry)`, null entry = "evaluated, nothing placed" so it isn't retried). Deterministic (hash → no pop, infinite reach). Strate-bounded (wiped on strate change, like decorations). This is the real home for "rare prop at all distances" — the job the decoration FarGrid could approximate at moderate range but not at extreme radius. **(B) WATER — tile-driven, level-0 only (continuous plane, never pops).** `PopulateTileWater(tile)` in `ApplyMeshToTile` (level-0 tiles), `ClearTileWater(tile)` in `UnloadTile`. One scaled engine plane (`/Engine/BasicShapes/Plane`) per water-surface chunk (per-chunk-Z plane logic assumes a single chunk's vertical span — hence level-0 only), keyed `TMap` (reflected UPROPERTY). Water Z: `bHasWater` + `WaterLevelRelative` → `StrateManager::GetWaterLevelWorldZForChunk`. Biome `WaterMaterial` overrides `UVoxelStrateDefinition::WaterMaterial` (level stays strate-global). `ClearAll`/`SetSeed` on `ChangeSeed`/regenerate clears both subsystems (decorations re-stream on the next Tick via the INT_MIN sentinels). **Per-biome content (§8.14):** decorations resolve the dominant biome **PER COLUMN** on the worker (`ResolveBiomeSampleAt` via the strate's `FBiomeContext`, box-cached → one rebuild per chunk footprint). The update builds ONE flat decoration palette (every biome's list concatenated; `CurrentEntryBiome[i]` tags entry `i` with its context-biome index, -1 = strate fallback), and `PlaceAtCrossing` rolls only the entries the column's biome owns. This replaced the old per-CELL `GetDominantBiomeAt` (one biome for a whole 8 m cell → axis-aligned border snapping); borders now follow the warped-Voronoi field at column resolution, no straight lines. Water still uses the chunk-centre `GetDominantBiomeAt` for its material. `Initialize` now also takes `UVoxelSettings*` (for the grid tunables). `ContentMaxLevel` is now legacy/dead for decorations. ### 8.6 Atmosphere — `VoxelAtmosphereManager.h/.cpp` (NEW) `UVoxelAtmosphereManager` (owned by `AVoxelWorld`, gated by `bManageAtmosphere`). `UpdateForPlayer(pos)` each Tick, reacts only on strate change. Drives a managed `UExponentialHeightFogComponent` + movable `USkyLightComponent` from the player's strate (`FogColor/FogDensity/bVolumetricFog/AmbientLightColor/AmbientLightIntensity`), and spawns PERSISTENT ceiling/floor "layer" actors (`Def->CeilingLayerActor`/`FloorLayerActor` + ZOffsets + rotations) that follow the player in XY — the sky-island sea-of-clouds / two-sided fog. `Def->AtmosphereActor` (a full BP with your own fog/sky/postprocess) OVERRIDES the managed fog+sky for that strate. `Reset()` on ChangeSeed/EndPlay. (Skylight ambient underground is weak — captures a dark scene; fog is the strong visual.) **Per-biome atmosphere (§8.14):** `UpdateForPlayer` also resolves the player's dominant biome and, when the biome has `bOverrideAtmosphere`, its fog/sky beats the strate's (reacts on biome change, not just strate change). `ApplyFogSky(Def, Biome)` is the shared path; layer actors + the full `AtmosphereActor` BP stay strate-level. Needs the generator injected (`Initialize(..., Generator)`). ### 8.7 Inter-strate bedrock gap `VoxelSettings::InterStrateGapChunks` (N) inserts N chunks of SOLID bedrock between consecutive strates (`StrateManager::Initialize` leaves the gap in the layout). `IsGapChunk` detects it; `GetDensityAt` renders gap chunks as solid + passages only (no caves/spine/seal) so the player digs (0,0) through the gap to descend. `GetStrateUnrealZRange` gives a strate's cm Z range. ### 8.8 Inter-strate passages — PER-STRATE (`FStratePassageConfig` on the definition) Each strate's `PassageConfig` (VoxelStrateTypes.h) controls its descent tunnels to the layer below: `Connections`, `Style` (`EVoxelPassageStyle`: Straight/Worm/Spiral/Cascading), `MouthRadius`/`MidRadius` (tapered width → `FVoxelPassage::ControlRadii` + `VoxelSDF::TaperedCapsule`), `ReachMin/Max` (depth into each strate), `DistanceMin/Max` (from the (0,0) spine), `Wander`, `Segments`, `VerticalWobble`, Spiral/Cascade params. Built in `StrateManager::GeneratePassages` as control-point chains. **Worm = independent fBM per horizontal axis** (`PassageFBM` static) with a flat-top envelope → organic squirm (NOT a 1D zigzag, NOT a same-freq 2-channel spiral). `EvaluateModifierSDF` (per voxel) first builds a `thread_local` **per-chunk shortlist** of passages whose bounds reach this chunk (rebuilt on chunk change / `PassagesVersion` bump) — chunks with no passage near return `FLT_MAX` immediately — then **bounding-sphere-culls** each shortlisted passage (`FVoxelPassage::BoundCenter/BoundRadiusSq`). Both are perf-critical (§8.10). The (0,0) surface entry is a simple straight tube. Placement, reach, and shape draws for inter-strate passages are keyed by `(seed, upper-strate index, connection index, per-value salt)` through `VoxelHash`, so one passage's connection count cannot shift any later passage. Global passage settings were removed from `VoxelSettings`. After a passage's XY is selected, `GeneratePassages` asks the **destination** strate for a full lower-mouth landing point through the pure free function `VF_SuggestLandingPoint` (`VoxelCaveMorphology`). TunnelNetwork and Underwater keep the nearest hash-room vertical placement at the requested XY, with `MakeStrateSeed(world-seed, strate-index)` matching the room graph's existing identity; FlatPlain and CrystalChamber recompute their slab void band and reject column-overlap points. Maze snaps to the nearest roughness-safe horizontal lattice corridor within one cell, VerticalShafts to the exact axis of a roughness-safe shaft on the drainage tree within one shaft spacing, and FloatingIslands to a validated blob top within one island spacing. Those three return false when no footing exists inside that explicit lateral budget. GeneratePassages interpolates the snapped XY through the control-point chain and recomputes its conservative bound; SurfaceWorld remains deliberately unanswerable because production terrain can be selected through manager-owned biome/per-column context. Declined answers (unsupported or no-footing) keep the historical random reach and emit a warning. This query is source-level only — it does not construct an operator stack, touch a cache, or call back into `UVoxelStrateManager` while `Initialize` is building the layout. The resulting lower endpoint is clamped to the destination's seal-free interior. ### 8.9 Carving — brush shapes + editor controls `FVoxelModification` has `EVoxelBrushShape {Sphere,Box,Capsule}` + `BoxExtent`/`CapsuleEnd`/ `Falloff` + `GetWorldBounds`. `UVoxelDiffLayer::GetDensityOffset` switches per shape; chunk overlap uses the shape AABB. `AVoxelWorld`: `CarveBox/FillBox/CarveCapsule/FillCapsule/ ApplyModification` (BlueprintCallable) + `EditorCarveSphere/EditorFillSphere` (CallInEditor) driven by `EditorBrush*` props. ### 8.10 Performance invariants (DON'T regress) - **Streaming** (`UpdateChunksAroundPosition`): rebuild/cull the desired set ONLY when the player crosses a chunk boundary (`LastUpdateCenter`); use the `DesiredSet` TSet for the cull; idle via `bAllChunksLoaded`. Stationary player ≈ free. (Old per-frame O(loaded×desired) scan = 22ms.) - **LOD** changes HOT-SWAP (`LoadChunk` only, never unload-first) → no holes. LOD reconciliation lives in the PERSISTENT per-frame submit loop (same loop as new-chunk loads), NOT as a one-shot on the boundary-cross frame — a one-shot drops every chunk past the task budget and strands it at a stale LOD. Idle (`bAllChunksLoaded`) only when a full scan finds no loads AND no LOD mismatches outstanding. - **SDF cache** (`GetDensityWithParams`): search-BOX validity, not chunk-key — gradient ±1 sampling must not thrash the (expensive) rebuild. - **Operator channel metadata** (`FVoxelOpStack`): `ChannelReads`, `ChannelWrites`, and `IsAdditive` are called only when an operator is added to a stack and their values are cached in the stack entry. `ValidateChannelOrder` is assembly/diagnostic-only; no declaration virtual may enter `Eval`'s per-voxel loop. If `FVoxelOpSample` gains a field, add its channel bit and update every operator declaration and the validator together. - **VerticalShafts field cache**: shaft rolls and connector decisions, including the structural tree and structural-spine connector endpoint, are rebuilt only when the thread-local centre cell or a geometry-affecting parameter changes. Each rebuild rolls a wider direct-indexed geometric halo (stock settings: 9×9 tree-emission cells / 15×15 total roll); it stores/emits only the inner 3×3 shafts and their capsules for the per-voxel loop, while spatial culling selects cached connectors. The complete ±2 parent and ±3 fallback windows plus connector reach are contained by that halo, so no parent decision depends on the evaluation cell. No cell or pair hash, widened neighbourhood scan, or tree construction may move into the per-voxel loop; the operator-stack source follows the same cache contract. - **Per-chunk param cache** in `GetDensityAt`: GenType + param struct + disturbance cached thread-locally by `(DensityCacheOwnerId, ChunkCoord, LayoutVersion)`; the process-unique owner ID prevents cross-world reuse while adding only one `uint64` compare per voxel. Don't remove the owner or layout key, and don't move the fetch/blend back to per-voxel. - **XY edge seal hot path** (§8.2a): `WorldRadiusVoxels == 0` returns immediately; otherwise the common interior case is a squared-distance test with no square root. The forcing op receives the two global settings through `FVoxelOpContext` once per chunk, while `ClassifyBox`/`ClassifyTile` share the radial proof. This adds no cache, state, RNG, or clipmap stream, and does not touch the density grid's two-pass MC loop or any existing `thread_local` cache. - **Biome cache** (`ResolveBiomeSampleAt`/`FChunkBiomeCache`, §8.14): validity is a world-XY BOX + ChunkZ + Seed, NOT a chunk key — same reason as the SDF cache. The cell classification is noise-heavy; a chunk-key would thrash it on gradient-normal / +X/+Y boundary samples. Keep the box halo (≥ CHUNK_SIZE) + cell margin (warp + CellSize) so the 3x3 lookup never misses. - **Passage cull** (§8.8) + **morphology two-region** (§8.4): both are per-voxel-cost critical. - **Per-chunk passage shortlist** (`EvaluateModifierSDF`): runs per voxel and is called from every archetype's `ApplyPassageCarving`. Keeps a `thread_local` shortlist (passage INDICES) of passages whose bounds reach the current chunk, rebuilt only on chunk change or `PassagesVersion` bump (incremented in `GeneratePassages`). Most chunks have NO passage near → instant `FLT_MAX` return instead of walking the whole `Passages` array per voxel. Conservative superset (chunk bounding sphere vs passage bound) ⇒ bit-identical carve. Store indices + version, never pointers (the array is rebuilt on `RebuildStrates`). - **Gen tasks run at `UE::Tasks::ETaskPriority::BackgroundNormal`** (`LoadTile`): worker gen yields to foreground game/render tasks. Without it, raising `MaxConcurrentTasks` past the spare-core count saturates the scheduler and starves the frame (the "concurrency > ~12 = stutter" symptom). Keep gen at background priority so the frame keeps its cores. - **Mesher density grid + margin ring** (`GenerateMesh`): sample each grid point ONCE into a flat `(CHUNK_SIZE/Step + 1 + 2)³` array (the `+2` is a 1-point MARGIN ring, indices −1..GridDim, for T1.b normals). The cell loop reads 8 corners from it; per-cell sampling would call `GetDensityAt` ~8× too often. Geometry is bit-identical (edge positions unchanged). Don't refactor back to per-corner `GetDensity` and don't drop the margin ring (normals + seamless borders need it). The cell loop is **two-pass**: pass 1 reads the 8 corner densities + builds the MC case index and `continue`s on no-surface cells (≈70% of cells); pass 2 computes the 8 positions + grid-gradients ONLY for surface cells. Don't hoist position/gradient back above the case-index test. The `DensityGrid` and vertex-dedup `TMap` are `thread_local` and reused per worker (Reset / keep capacity) — don't make them per-call locals (re-allocates ~170 KB + a hash map every tile). **CAPTURE-DURING-MESHING (sanctioned reuse, doesn't regress the above):** `GenerateMesh`'s optional `OutCaptureGrid` copies the already-filled `DensityGrid` interior (`CHUNK_SIZE³`, quantized via `VF_QuantizeDensity`) out for the density clipmap (mini-sun shadows) — a PURE READ added after the grid loop. It does NOT touch the grid shape, the two-pass loop, the margin ring, or the thread_local reuse. Only level-0 full-res tiles request it (`Step==1<