# Indoor Walking Phase 1 — BSP cluster (#84 / #85 / #86)

**Status:** Brainstormed 2026-05-19. Awaiting user spec review before plan.
**Scope:** Diagnostic-first investigation pass across the three "indoor walking is broken" bugs that share a cell-BSP / picker root-cause cluster. Surface evidence with a single probe + one capture session, then ship surgical fixes (one commit per issue).
**Predecessors:**
- Indoor cell rendering Phase 1 (`docs/superpowers/specs/2026-05-19-indoor-cell-rendering-fix-design.md`) — the five `[indoor-*]` render-side probes.
- Indoor cell rendering Phase 2 (`docs/superpowers/specs/2026-05-19-phase2-indoor-cell-rendering-fix-design.md`) — silent-failure surfacing + WB Setup-prefix guard. Made floors render.
- Handoff: `docs/research/2026-05-19-indoor-followup-handoff.md`.

The indoor cell rendering Phase 1+2 pair made floors render. The moment floors rendered, nine pre-existing indoor bugs (`docs/ISSUES.md` #78-#86) became user-observable. This phase tackles the **BSP cluster** subset: #84, #85, #86.

`#78` (outdoor stabs visible through floor) is in the same handoff cluster but a fundamentally different code path (render-side visibility / stencil), so it's deferred to a separate phase. `#79-#83` (lighting / terrain / stairs) are in different clusters.

---

## 1. What we know from the code

Pre-investigation reads (2026-05-19) of the three issue surfaces:

### `#84` (blocked by air indoors) — cell BSP IS consulted

The handoff hypothesized "cell BSP isn't being used". Code reading says otherwise:

- **Cell BSP IS cached.** `PhysicsDataCache.CacheCellStruct` ([src/AcDream.Core/Physics/PhysicsDataCache.cs:131](src/AcDream.Core/Physics/PhysicsDataCache.cs:131)) stores `BSP`, `PhysicsPolygons`, `Vertices`, `WorldTransform`, `InverseWorldTransform`, and pre-resolved polygons (planes computed at cache time).
- **Cell BSP IS consulted in collision.** `Transition.FindEnvCollisions` ([src/AcDream.Core/Physics/TransitionTypes.cs:1188-1241](src/AcDream.Core/Physics/TransitionTypes.cs:1188)) has an explicit indoor branch gated on `cellLow >= 0x0100` that:
  1. Looks up `cellPhysics` via `engine.DataCache.GetCellStruct(sp.CheckCellId)`,
  2. Transforms the player's sphere to cell-local space via `InverseWorldTransform`,
  3. Calls `BSPQuery.FindCollisions` with the cell's pre-resolved polys,
  4. Returns `cellState` if `!= OK`.

So #84's root cause is not "wiring missing". It's one of: (a) extra physics-only polys with no visible counterpart, (b) `+0.02f` Z-bump misalignment between cellTransform (applied to physics) and player Z (computed from terrain), (c) `BSPQuery` returning false positives at certain poly side-types, (d) `cellTransform` quaternion error on rotated cells. Capture data will pin which.

### `#85` (pass through walls outside→in) — likely asymmetric path

Walking outside-in keeps `CheckCellId` as the outdoor land cell (low byte `0x00xx-0x00FF`), so the indoor cell-BSP branch at TransitionTypes.cs:1192 is **gated out by design** (`cellLow >= 0x0100` is false). The only collision tested on the outside-in approach is:

- **Terrain** (always tested),
- **Outdoor stab BSPs** ([`PhysicsDataCache.GetGfxObj`](src/AcDream.Core/Physics/PhysicsDataCache.cs) for `LandBlockInfo.Objects`) — building stab is hit via `FindObjCollisions`.

L.2d slice 1+1.5 ported `CBuildingObj` collision (per CLAUDE.md), so the outer building shell SHOULD be hit. If #85 reproduces, hypotheses:

1. The outdoor stab BSP for the Inn covers floor+roof but is missing wall polys (authoring shape — retail's interior cells own the walls, outdoor shell is a partial envelope).
2. The outdoor stab BSP has wall polys but with one-sided normals; outside approach hits the back face which BSP treats as "behind plane" → no collision (`feedback_no_patching_collision` memory's faithful-port rule means we'd need to follow retail's handling).
3. The L.2g dynamic-physics-state flag work doesn't include outdoor building shells in the collision sweep for the player's CheckCellId.
4. **Retail's actual behavior** may be that outside-in BSP probing queries the EnvCell's BSP across the cell boundary — retail's `CCellStructure::find_env_collisions` may walk neighbor-cell BSPs.

### `#86` (click selection penetrates walls) — root cause definitively pinned by code reading

`WorldPicker.Pick` ([src/AcDream.Core/Selection/WorldPicker.cs:88-160](src/AcDream.Core/Selection/WorldPicker.cs:88), and the screen-rect overload at line 202) is **pure ray-sphere against entity AABBs**. There is no cell BSP test, no scenery BSP test, no terrain test. Any entity along the ray within `maxDistance` is a candidate; nothing occludes.

No probe needed for #86. Fix is structural: add a cell-BSP ray-poly occlusion test that runs once per `Pick` call and culls entities whose ray-distance exceeds the nearest wall hit.

---

## 2. The three issues

| # | Title | Code path | Fix shape |
|---|---|---|---|
| #84 | Blocked by air indoors | `Transition.FindEnvCollisions` cell branch | TBD — pinned by probe capture |
| #85 | Pass through walls outside→in | `FindObjCollisions` outdoor-stab path or cross-cell BSP probing | TBD — pinned by probe capture |
| #86 | Click selection penetrates walls | `WorldPicker.Pick` (both overloads) | Add cell-BSP ray-poly occlusion test |

---

## 3. Architecture

```
                    [indoor-bsp] probe
                            ↓
        ┌───────────────────┴────────────────────┐
        ▼                                        ▼
  Movement path                          Picker path
  (FindEnvCollisions cell branch)        (WorldPicker.Pick)
        │                                        │
        ├─→ #84: blocked by air                  └─→ #86: click through walls
        └─→ #85: pass through walls              (cause already pinned by code reading)
            (cause TBD — needs capture)
```

The probe spans only the movement path. #86's diagnosis is already known; its fix is independent of the capture and can land in parallel.

---

## 4. Components

### Component 1 — `PhysicsDiagnostics.IndoorBspEnabled`

New static toggle on `AcDream.Core.Physics.PhysicsDiagnostics`. Mirrors the existing `ResolveProbeEnabled` / `CellProbeEnabled` pattern:

- Backed by `ACDREAM_PROBE_INDOOR_BSP` env var read once at startup.
- Mutable at runtime via the DebugPanel checkbox.
- Zero-cost when off — checked before any string formatting.

Also extends `PhysicsDiagnostics.IndoorAllEnabled` cascading the way Phase 1 cascaded the render-side `ACDREAM_PROBE_INDOOR_ALL`.

### Component 2 — `[indoor-bsp]` log site

One `Console.WriteLine` block in `Transition.FindEnvCollisions` ([TransitionTypes.cs:1222](src/AcDream.Core/Physics/TransitionTypes.cs:1222)), wrapping the existing `BSPQuery.FindCollisions` call. Captured fields per call:

| Field | Source | Why |
|---|---|---|
| `cellId` | `sp.CheckCellId` | Which cell's BSP was queried (hex, full 32-bit) |
| `localPos` | `localCenter` | Sphere foot center in cell-local space (3 floats) |
| `localPrevPos` | `localCurrCenter` | Sphere previous-frame foot center in cell-local space |
| `worldPos` | `footCenter` | Sphere foot center in world space (for cross-ref with user-reported spot) |
| `result` | `cellState` | `TransitionState` enum (`OK` / `Collided` / etc.) |
| `polyId` | `ci.LastHitCellPolyId` (NEW field if needed) | Which cell poly was hit, if any |
| `polyNormal` | `cellPhysics.Resolved[polyId].Plane.Normal` | Local-space normal (3 floats) — diagnoses one-sided / orientation bugs |
| `sidesType` | `cellPhysics.Resolved[polyId].SidesType` | `Front` / `Back` / `Both` — diagnoses #85 candidate |
| `walkable` | `ci.LastKnownContactPlaneValid` | Walkable surface tracking state |

Log line format (one line, pipe-separated, machine-greppable):

```
[indoor-bsp] cell=0xA9B40100 wpos=(82.45,71.23,1.04) lpos=(0.45,2.10,1.02) result=Collided poly=0x0042 n=(0.00,1.00,0.00) sides=Front walkable=true
```

If `BSPQuery.FindCollisions` doesn't already expose the hit poly id, the log fields shrink to what's available without expanding the BSPQuery API. A separate small change to surface `lastHitPolyId` from `BSPQuery` would be in-scope for this phase if needed.

### Component 3 — DebugPanel checkbox

Adds a checkbox row in the DebugPanel's Diagnostics section (already hosts the L.2a `Resolve` and `Cell-transit` toggles, plus the Phase 1 `Indoor walk/cull/upload/lookup/xform` toggles). Surface area: ~3 lines. No new file.

### Component 4 — `WorldPicker` cell-BSP occluder

Two implementation options:

**Option C1 — Inline in `WorldPicker.Pick`.** Add a `cellOccluder` callback parameter `Func<Vector3, Vector3, float>?` that returns the nearest wall-hit `t` along the ray (or `float.PositiveInfinity` if no hit). Inside `Pick`, after computing the entity hit `t`, gate by `entityHit < cellOccluder(origin, direction)`.

**Option C2 — Separate `CellBspRayOccluder` static class.** New file `src/AcDream.Core/Selection/CellBspRayOccluder.cs`. Function `NearestWallT(Vector3 origin, Vector3 direction, IEnumerable<CellPhysics> loadedCells)` — Möller-Trumbore ray-triangle against each cell's resolved polys, returns nearest `t`. WorldPicker calls it once per `Pick` invocation.

**Recommend C2.** Reasons: testable in isolation (synthetic cell + ray); two `WorldPicker.Pick` overloads share one implementation; future picker improvements (entity body refine, scenery BSP refine) get a parallel structure to copy.

The caller (`GameWindow` Use/Select handlers) must supply the loaded `CellPhysics` set. `PhysicsDataCache` already has `GetCellStruct(id)` so the caller iterates currently-loaded `LoadedCell` ids from `CellVisibility._cellLookup` (Holtburg radius 4 keeps maybe 80 cells loaded — fast Möller-Trumbore).

### Component 5 — Fix patches (TBD)

Concrete commits drafted only after capture data lands. Candidates by issue:

**#84**:
- Remove `+0.02f` Z bump from the physics-side `cellTransform` while keeping it for render's `cellMeshRef` (separate transforms). Or apply the bump symmetrically (also bump player Z by `+0.02f` when entering an indoor cell).
- Filter out physics-only polys with no visible counterpart, IF capture data shows phantom polys are the issue.
- Patch `BSPQuery.FindCollisions` side-type handling, IF capture data shows specific side-types misbehaving.

**#85**:
- Port retail's outside-in BSP cross-cell probing — query an EnvCell's BSP from an outdoor cell when the sphere overlaps the EnvCell's world AABB. Reference: PDB-named `CCellStructure::find_env_collisions` and neighbors.
- OR ensure outdoor building-shell stab BSPs include wall polys with two-sided handling.
- Path picked from capture evidence + decomp grep.

---

## 5. Data flow

### Capture session

User runs the canonical Holtburg launch (`ACDREAM_LIVE=1`, `+Acdream` char) with `ACDREAM_PROBE_INDOOR_BSP=1` + `ACDREAM_PROBE_RESOLVE=1` (latter already shipped from L.2a). Three scripted scenarios:

1. **Inside Inn walkaround (~30 s)** — walk slowly around the common room, attempt to reproduce #84. Note world-position when an invisible block happens.
2. **Outside-in approach (~30 s)** — stand 5+ m west of the Inn, sprint at the west wall. Reproduce #85.
3. **Inside-out sanity (~30 s)** — stand inside, walk into east wall from interior. This SHOULD block (per issue text); confirms inside-out path works.

Total launch: one. Captures all three.

### Offline analysis

```
grep "\[indoor-bsp\]" launch.log | head -200    # see what fired during scenario 1
grep "\[resolve\]" launch.log | grep "obj=0x"    # see which objects were hit during scenario 2
grep "\[cell-transit\]" launch.log               # confirm cell ids during transitions
```

Diagnosis per issue:

- **#84**: in scenario-1 lines, find `result=Collided` events where world-pos is in open space (no visible wall). Cross-ref `polyId` with the cell's `cellStruct.PhysicsPolygons` to identify what the offending poly is. Compare its local-Z with player's local-Z to test the Z-bump hypothesis.
- **#85**: in scenario-2 lines, expect zero `[indoor-bsp]` events (gated out). Check `[resolve]` lines for the moment the player crosses the wall plane — did `FindObjCollisions` fire for any building stab? If yes, what poly? If no, the outdoor stab path is missing wall geometry → fix shape is the cross-cell BSP probing.
- **#86**: no capture needed. Code reading already pinned the cause; fix is structural.

### Fix application

Per CLAUDE.md "no workarounds" rule:
- The probe data must point at one specific code site before any fix lands.
- Each fix commit cites the evidence in its message ("`[indoor-bsp] cell=0x... wpos=... poly=... n=...` — the poly at local-Z=0.0 is the floor poly; player local-Z=-0.02 from the +0.02f bump puts foot below floor → spurious floor-up push at cell boundary").
- No try/catch swallow, no early-return guard at the symptom site.

---

## 6. Commit shape

```
1. feat(physics): Cluster A — indoor BSP collision probe
   - PhysicsDiagnostics.IndoorBspEnabled toggle + env var + DebugPanel checkbox
   - [indoor-bsp] log site in TransitionTypes.FindEnvCollisions cell branch
   - (if needed) BSPQuery.LastHitPolyId surfacing

   [CAPTURE SESSION — user-driven, no commit]

2. fix(physics): Cluster A #84 — <root cause from probe>
   - One surgical change to TransitionTypes / GameWindow / BSPQuery
   - Commit message cites probe evidence line
   - Closes ISSUES.md #84

3. fix(physics): Cluster A #85 — <root cause from probe + decomp>
   - One surgical change to TransitionTypes or PhysicsDataCache
   - Commit message cites probe evidence + retail decomp anchor
   - Closes ISSUES.md #85

4. fix(picker): Cluster A #86 — cell-BSP ray occlusion in WorldPicker
   - New CellBspRayOccluder static class (Option C2)
   - WorldPicker.Pick (both overloads) consults occluder before returning hit
   - Unit test covering synthetic wall-between-camera-and-entity case
   - Closes ISSUES.md #86

5. docs(roadmap+issues): Cluster A shipped — close #84/#85/#86, update roadmap
   - ISSUES.md moves three issues to Recently closed
   - docs/plans/2026-04-11-roadmap.md shipped table updated
   - CLAUDE.md "Currently in Phase L.2..." line advanced if appropriate
```

Visual verification gate sits between commits 4 and 5. User confirms each acceptance criterion in the live client before closing.

---

## 7. Files touched

**Definite:**

- `src/AcDream.Core/Physics/PhysicsDiagnostics.cs` — new `IndoorBspEnabled` toggle.
- `src/AcDream.Core/Physics/TransitionTypes.cs` — `[indoor-bsp]` log site at the cell branch.
- `src/AcDream.App/UI/Panels/DebugPanel.cs` (or wherever the diagnostics checkboxes live) — UI toggle.
- `src/AcDream.Core/Selection/WorldPicker.cs` — call the new occluder.
- `src/AcDream.Core/Selection/CellBspRayOccluder.cs` — new file.
- `src/AcDream.App/Rendering/GameWindow.cs` — wire `LoadedCell` set / `CellPhysics` enumeration into the Use/Select handlers' picker calls.
- `tests/AcDream.Core.Tests/Selection/WorldPickerCellOcclusionTests.cs` — new unit test for #86 fix.
- `docs/ISSUES.md` — close #84/#85/#86.
- `docs/plans/2026-04-11-roadmap.md` — shipped table entry.

**TBD (depends on capture):**

- `src/AcDream.App/Rendering/GameWindow.cs:5362` (+0.02f Z bump site).
- `src/AcDream.Core/Physics/BSPQuery.cs`.
- `src/AcDream.Core/Physics/PhysicsDataCache.cs`.

---

## 8. Error handling

- Probe always behind `PhysicsDiagnostics.IndoorBspEnabled`. Zero-cost when off.
- Probe writes to `Console.WriteLine`, captured by the launch.log `Tee-Object` pipe (matches existing probe convention).
- `CellBspRayOccluder` returns `float.PositiveInfinity` when no cells are loaded (outdoor camera). Picker behaves exactly as today in that case.
- No try/catch around fix sites. If a fix doesn't behave, the user reports the residual symptom and the probe re-fires to identify the new cause.

---

## 9. Testing

### Unit tests

- **`WorldPickerCellOcclusionTests`** (new): synthetic `CellPhysics` with one wall poly between origin and an entity at 5 m. `Pick` returns null. Remove the wall — `Pick` returns the entity. Verifies the occluder is wired and triangulates correctly.
- **`CellBspRayOccluderTests`** (new): direct unit tests for the Möller-Trumbore intersection — ray hits poly front, back, edge, miss, parallel-to-poly. Standard ray-triangle coverage.
- **Existing tests**: `dotnet test` green. `WorldPickerTests` + `WorldPickerRectOverloadTests` + all `BSPQuery` tests must remain green.

### Visual verification (user-driven)

Three checks, one per issue:

1. **#84 acceptance** — User walks the common-room loop in Holtburg Inn. No invisible blocks. Probe shows no `TransitionState != OK` events at positions away from visible walls/furniture.
2. **#85 acceptance** — User stands 5+ m west of the Inn, runs at the west wall. Player blocks at the wall plane (within ~0.05 m of the visible wall surface). User cannot enter the building except via a door portal.
3. **#86 acceptance** — Mouse over a wall pixel from outside the Inn → cursor shows no selection. Mouse over an NPC through an open door portal → cursor shows the NPC selection ring (selection still works through real apertures).

---

## 10. Acceptance criteria

- All three issues meet their respective acceptance gates above (visual confirmation by user).
- `dotnet build` green.
- `dotnet test` green (new tests + all existing).
- Roadmap "shipped" table updated.
- `docs/ISSUES.md` #84/#85/#86 moved to "Recently closed" with commit SHAs.
- A short post-phase handoff doc (`docs/research/<ship-date>-indoor-walking-phase1-shipped-handoff.md`) records the probe evidence + the three root causes, parallel to the existing Phase 1+2 docs.

---

## 11. Phase name + roadmap placement

**Proposed name:** "Indoor walking Phase 1 — BSP cluster (#84/#85/#86)".

Reasons:
- Continues the "Indoor X Phase N" naming established by Phase 1 (probes) + Phase 2 (rendering fix).
- Distinguishes from indoor RENDERING work (which is done) — the focus has shifted to indoor WALKING.
- "Phase 1" implies more phases follow (Phase 2 likely = #78 outdoor-stab visibility cluster).

**Roadmap placement:** Add to `docs/plans/2026-04-11-roadmap.md` ahead-table as the next item in the indoor track. Insert after the Indoor cell rendering Phase 2 entry. Cross-link to ISSUES.md #84/#85/#86.

**Milestone:** This is parallel to the M2 critical path (which is F.2 / F.3 / F.5a / L.1c / L.1b). M1 already landed and is frozen. Indoor walking work is a quality-of-life parallel track — the user's recent commits put it ahead of M2 work because the rendering Phase 2 ship made it actionable.

---

## 12. Out of scope

- **#78** — outdoor stabs/buildings visible through rendered floor. Different code path (visibility / stencil). Filed for Indoor walking Phase 2.
- **#79-#82** — lighting / terrain shading. Cluster B in the handoff. Separate phase.
- **#83** — walking up stairs broken. Standalone issue. May share code with this phase if the cell BSP fix touches step-up; address opportunistically only if so.
- **Refactoring `WorldPicker`** beyond adding the occluder. The existing two-overload structure stays.
- **Stage B picker refine** (Möller-Trumbore against entity body polygons) — Issue #71, deferred per existing roadmap.

---

## 13. Risks

1. **Capture is inconclusive.** If the probe fires zero unexpected events during scenario 1 (i.e., #84 cannot be reproduced live during the capture), we extend the probe to also log `BSPQuery` internals or capture a longer session. Probably one more launch.
2. **#85 fix requires significant retail-decomp port.** Cross-cell BSP probing (querying an EnvCell's BSP from an outdoor cell) is not in the current code. The retail decomp at `named-retail/acclient_2013_pseudo_c.txt` has `CCellStructure::find_env_collisions` and neighbors that handle this. If the port is non-trivial (more than ~100 lines), promote #85 to its own dedicated phase rather than including it here. Decision point: after the capture, before commit 3.
3. **`CellBspRayOccluder` performance.** Möller-Trumbore against ~80 cells × ~50 polys each = ~4K triangle tests per `Pick` call. Picker fires once per click — acceptable. If we ever move to hover-pick (every frame), this needs an acceleration structure; not in scope here.
4. **Probe gets noisy.** If `FindEnvCollisions` fires at 30 Hz × N cells, the log can grow fast. Add a per-call rate limit only if the capture log is unreadable; default to unlimited (Phase 1+2 didn't need limiting).