fix(physics): skip mesh-AABB-fallback cylinder for landblock stabs
ISSUES #83 Phase A1. Landblock stabs (entity.Id 0xC0XXYY00+n per LandblockLoader.cs:55) were being registered with TWO collision shadows: the correct per-part BSP at `entity.Id*256 + partIdx`, AND a redundant mesh-AABB-fallback cylinder at `entity.Id`. The fallback clamped to 1.5m radius, centered at the building's mesh origin, producing user-reported "thin air" collisions inside cottages and within 2m of building exteriors. The fallback was originally designed for canopy-only-BSP procedural scenery (0x80XXYY00+n) — trees whose BSP covers the canopy but not the trunk. Landblock stabs have full BSP coverage and don't need it. Probe evidence (launch-thinair capture): - 0xC0A9B479 cylinder fallback (Holtburg cottage): 104 hits in a short capture session, all inside the cottage main room (cell=0xA9B4013F), ~2m from the building's mesh origin. - 0xA9B47900 BSP (the actual cottage walls): 52 legitimate hits. Fix: one new bool _isLandblockStab + one clause in the existing mesh-AABB-fallback gate. Spec: docs/superpowers/specs/2026-05-21-cylinder-fallback-dedup-design.md. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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# Cylinder fallback dedupe for landblock stabs (Phase A1)
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**Date:** 2026-05-21
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**Status:** Spec — awaiting user approval before plan-writing.
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**Phase:** ISSUES #83 fix sequence — Phase A1 of A1→A2→A3→A4.
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**Author:** Claude Opus 4.7.
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## Summary
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Landblock stabs (entity.Id `0xC0XXYY00+n` — buildings + structures
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from `LandBlockInfo.Objects` and `LandBlockInfo.Buildings`) are
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registered with TWO collision shadows in
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`GameWindow.RegisterShadowsForLandblock`:
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1. **BSP shadow per part** (`id = entity.Id * 256 + partIdx`, truncates
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to e.g. `0xA9B47900` for the Holtburg cottage) — the correct
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wall-by-wall collision.
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2. **Mesh-AABB-fallback cylinder** (`id = entity.Id` = `0xC0A9B479`)
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— a 1.5 m-clamped invisible disc at the mesh origin. **This is the
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bug**: it blocks the player in "thin air" inside cottages and near
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building exteriors.
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The fallback path was originally designed to cover canopy-only-BSP
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trees (`0x80XXYY00+n` procedural scenery) where the visible trunk
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isn't covered by the BSP. For landblock stabs the BSP covers the
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whole structure; the fallback is redundant and incorrect.
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Fix: gate the mesh-AABB-fallback registration path on "NOT a landblock
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stab." One extra clause in the existing `if` condition; ~5 lines of
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code; no math changes.
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## Evidence
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From the 2026-05-21 thin-air-collision capture
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(`launch-thinair.utf8.log`):
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```
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[entity-source] id=0xA9B47900 entityId=0xC0A9B479 type=BSP note=partIdx=0 hasPhys=true
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[entity-source] id=0xC0A9B479 entityId=0xC0A9B479 type=Cylinder note=mesh-aabb-fallback
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```
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Same `entityId`, same `gfxObj=0x01000A2B`, same landblock
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`0xA9B4FFFF`. The 1.5 m cylinder is the user-reported "thin air"
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collider.
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Hit attribution counts (this capture):
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| obj id | type | hits | role |
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|---|---|---:|---|
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| `0xC0A9B479` | Cylinder (fallback) | **104** | invisible disc — the bug |
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| `0xA9B47900` | BSP (correct walls) | 52 | real cottage walls |
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| `0xB5001600` | BSP (interior static) | 97 | inn fireplace / furniture |
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The 104 fallback hits happen inside cell `0xA9B4013F` (cottage main
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room) at ~2 m from the building's mesh origin
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`entOrigin_lb=(130.5, 11.5, 94.0)`.
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## Root cause
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`src/AcDream.App/Rendering/GameWindow.cs:5826-5828`:
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```csharp
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bool _isOutdoorMesh = ((entity.Id & 0x80000000u) != 0) // scenery
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|| ((entity.Id < 0x40000000u) // stab
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&& (_srcPrefix == 0x01000000u || _srcPrefix == 0x02000000u));
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```
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`(entity.Id & 0x80000000u) != 0` matches BOTH `0x80...` (procedural
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scenery) AND `0xC0...` (landblock stabs). The mesh-AABB-fallback gate
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at line 6050-6052:
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```csharp
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if (!isPhantomSetup
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&& (_isOutdoorMesh || (entityBsp == 0 && entityCyl == 0))
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&& entity.MeshRefs.Count > 0)
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```
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triggers the fallback for both classes. For stabs (which have full BSP
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coverage) this is wrong.
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## Fix
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Add `_isLandblockStab` flag near the existing `_isOutdoorMesh`
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definition, and AND it into the fallback gate:
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```csharp
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bool _isLandblockStab = (entity.Id & 0xFF000000u) == 0xC0000000u;
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```
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Then in the gate:
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```csharp
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if (!isPhantomSetup
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&& !_isLandblockStab // ← NEW: stabs have BSP; fallback is redundant
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&& (_isOutdoorMesh || (entityBsp == 0 && entityCyl == 0))
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&& entity.MeshRefs.Count > 0)
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```
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That's the entire code change. ~3 lines added.
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## Acceptance criteria
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1. `dotnet build` green.
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2. Re-launch with `ACDREAM_PROBE_BUILDING=1 ACDREAM_DEVTOOLS=1`. Open
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the log and grep `[entity-source].*note=mesh-aabb-fallback`. For
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every match, the `entityId` must NOT start with `0xC0`.
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3. **Visual verification (the acceptance test):** walk inside a
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Holtburg cottage; the player should be able to walk anywhere in
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the room without being blocked by invisible walls in "thin air."
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Walking near the outside walls of buildings should not show
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invisible barriers 2 m before the visible wall.
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4. The procedural scenery cylinder coverage stays — walking up to a
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tree trunk should still stop at the trunk (not pass through),
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confirming we didn't regress the original purpose of the fallback.
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## Out of scope
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- **PHSP inversion fix** (Phase A2) — separate spec.
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- **Synthesis removal** (Phase A3) — separate spec; needs A1+A2 first.
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- **Multi-cell iteration** (Phase A4) — separate spec.
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- **Distinguish CScenery vs CBuildingObj at retail level** — retail
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has two different classes; we use one path here. Refactoring to
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match retail's class layout is post-M2 polish.
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- **`0xB5001600` interior static collisions** (97 hits) — these
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attribute to a 0x40-prefix interior entity (probably inn fireplace
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or furniture). Whether they're legitimate or bugged is a separate
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investigation. Out of this phase's scope.
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## Risks
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- **R1: Trees with canopy-only BSP regress** (player walks through
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trunk). The fallback was added for this case. *Mitigation:*
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unaffected — trees are `0x80xxxxxx` scenery, not `0xC0xxxxxx`
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stabs. The fix is precisely scoped to stabs. Verified by the
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entity-ID layout in `src/AcDream.Core/World/LandblockLoader.cs:55`.
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- **R2: Some buildings might have INCOMPLETE BSP** (e.g., a roof
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with no floor BSP). After fix, those buildings lose the fallback
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cylinder coverage. *Mitigation:* none in this phase. If a user
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reports a building with no collision after this fix, we add per-
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building inspection. Retail design is "BSP only for stabs"; this
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is the correct path.
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- **R3: The 0xB5xx interior statics (entity.Id 0x40xxxxxx) may have
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the same doubled-collision bug** (BSP + cylinder fallback). They
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don't have the 0xC0 prefix so our fix wouldn't apply. *Mitigation:*
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out of scope. Investigated as a follow-up if visual regression
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surfaces.
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## References
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- Capture log: `launch-thinair.utf8.log` (2026-05-21, uncommitted).
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- Entity ID layout: [`src/AcDream.Core/World/LandblockLoader.cs:40-55`](src/AcDream.Core/World/LandblockLoader.cs:55).
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- Affected code: [`src/AcDream.App/Rendering/GameWindow.cs:5791-6206`](src/AcDream.App/Rendering/GameWindow.cs:5791) (the per-entity registration loop in `RegisterShadowsForLandblock`).
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- Prior phase: spec
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[`2026-05-21-indoor-walk-miss-probe-design.md`](2026-05-21-indoor-walk-miss-probe-design.md).
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- Findings: [`docs/research/2026-05-21-walk-miss-capture-findings.md`](../../research/2026-05-21-walk-miss-capture-findings.md).
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