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+# Phase A8.F — camera-collision root cause + handoff (2026-05-29, session 2)
+
+## TL;DR
+
+The A8.F "flap" (building walls / ground blinking in and out) and the
+missing/transparent-wall symptoms are **not** a builder or enforcement bug.
+Their root cause is the **3rd-person camera EYE passing through walls**: the
+A8.F renderer computes "am I inside a building?" and "which side of each
+doorway am I on?" from the camera eye position, so when the eye clips a wall
+those decisions flip frame-to-frame.
+
+This session: (1) **fixed + committed** the worst symptom — the cellar terrain
+flood (commit `9417d3c`); (2) established the recursive-clip **builder actually
+works** for most cells (the prior "Bug B" framing was wrong); (3) **reframed the
+root cause** to the camera, with the user's help; (4) researched the camera
+system (Opus agent + my verification).
+
+**The decision that blocks the next session** (brainstorm with the user FIRST):
+retail's camera does **NOT** collide with walls and pull the eye in — it lets
+the eye clip and **fades the player to translucent** instead (a mechanism
+acdream already ports). So "make the camera move closer on a wall hit" is a
+**deliberate divergence from retail**, not a faithful port. The next session
+must choose an approach (spring arm vs. decouple visibility from the eye) and
+get user sign-off before coding, per the CLAUDE.md no-redesign rule.
+
+---
+
+## What this session shipped / established
+
+- **Bug A FIXED + committed (`9417d3c`):** an empty `OutsideView` while inside a
+  building now draws **no** outdoor terrain/scenery (empty mask = "no outdoors
+  visible," not "all outdoors"). Previously the `else` branch disabled the
+  stencil and flooded ungated terrain over the cell interior. **Visual-confirmed
+  by the user: cottage cellar walls are solid now, no terrain bleed-through.**
+  App tests 108/108. All behind `ACDREAM_A8_INDOOR_BRANCH=1`; default play
+  unaffected.
+- **"Bug B" (builder under-produces) is substantially NOT real.** The pv-dump
+  census showed `PortalVisibilityBuilder` produces correctly *narrowed*
+  `OutsideView` regions for most cells (`0172/0173/0162/015E/0165/016F` →
+  `polys=1`). The empty cases are mostly legitimate (a windowless cellar can't
+  see daylight) or driven by the camera being in an invalid position (see root
+  cause). The handoff predecessor's Finding 2 ("never narrows") does not hold.
+- **Root cause reframed → the camera.** Confirmed below.
+- **Camera research done** (Opus agent, findings verified against the decomp +
+  the code by this session).
+
+### Diagnostics added (committed in `9417d3c`, all opt-in)
+
+- `PortalVisibilityBuilder.Build` now emits a **`CAMPORTAL[i]` census** under
+  `ACDREAM_A8_DUMP_PV=1`: per camera-cell portal, before the BFS guards, it logs
+  `other=`, `polyLen=`, `hasPlane=`, `interiorSide=`, `planeN=`. This is what let
+  us see that the cottage front-door exit portal was *culled by the side test*
+  (`interiorSide=False`) rather than missing.
+- `[opaque]` probe (`ACDREAM_PROBE_ENVCELL=1`) — opaque cell-render stats
+  (`cells`/`tris`) **before** the transparent loop overwrites them (the existing
+  `[envcells]` line reads post-loop and is misleading — ignore its `cells=1
+  tris=0`).
+- `tools/A8CellAudit` `portals <cellId>` now **replicates `BuildLoadedCell`'s
+  polygon-vertex resolution** and prints `BUILDER_SEES=OK/EMPTY` per portal, so
+  exit-portal validity is checkable offline without a launch.
+
+---
+
+## The visual symptoms (user-observed, 2026-05-29, with `ACDREAM_A8_INDOOR_BRANCH=1`)
+
+1. Cellar walls **solid** (Bug A fix confirmed). ✓
+2. **Flap**: buildings/ground disappear when passing from inside to outside.
+3. Cellar entrance no longer covered by terrain (Bug A fix). ✓
+4. Looking into certain windows from **outside**: back walls missing.
+5. Inside **other** buildings: external + internal walls transparent (showing
+   sky / NPCs / particles).
+
+**The user's key correlation:** items 2/4/5 happen **when the camera passes
+through a door, or when standing inside and panning the camera through a wall**.
+They're intermittent — not always reproducible — which fits a camera-position
+trigger, not a static rendering bug.
+
+---
+
+## Root cause: the camera eye drives A8.F visibility, and it clips through walls
+
+`camPos` is the **camera eye**, extracted from the inverse of the active
+camera's View matrix:
+
+```
+GameWindow.cs:7270-7271
+  Matrix4x4.Invert(camera.View, out var invView);
+  var camPos = new Vector3(invView.M41, invView.M42, invView.M43);  // = the eye
+```
+
+(`camera.View` is `CreateLookAt(_dampedEye, …)`, so the translation is the
+chase-camera eye — **not** the player avatar position, which is tracked
+separately for interior lighting at `GameWindow.cs:7415-7417`.)
+
+That eye then drives all three A8.F visibility decisions:
+
+1. **Camera-cell + portal BFS** — `_cellVisibility.ComputeVisibility(camPos)`
+   (`GameWindow.cs:7323`) → `FindCameraCell` via `PointInCell(camPos, …)`. The
+   BFS portal side-test (`CellVisibility.cs:466-481`) culls portals the eye is on
+   the wrong side of.
+2. **Strict inside-building gate** (`GameWindow.cs:7343-7346`):
+   `cameraInsideBuilding = a8IndoorBranchEnabled && PointInCell(camPos,
+   CameraCell) && CameraCell.BuildingId != null`. Picks the inside-out vs
+   outside-in render branch.
+3. **Per-portal interior-side cull in the recursive-clip builder** —
+   `PortalVisibilityBuilder.CameraOnInteriorSide(cell, i, cameraPos)`
+   (`PortalVisibilityBuilder.cs:196-203`; cull site ~`:124`):
+   transforms `cameraPos` to cell-local and dot-tests against the portal plane.
+
+**The flap mechanism:** when the eye damps to a position outside the room (or in
+the next room), `PointInCell(eye)` flips and `CameraOnInteriorSide` inverts.
+The camera-cell ping-pongs, the inside/outside branch switches, and the exit
+portal through a doorway is culled-then-uncovered frame-to-frame → walls/ground
+blink. The 3-frame `CellSwitchGraceFrameCount` hysteresis (`CellVisibility.cs:167`)
+only masks single-frame blips; a sustained multi-frame clip defeats it.
+
+**Hard evidence (this session's capture):** at cottage cell `0xA9B40170` the
+front-door exit portal was valid (`polyLen=4`) but the census showed
+`interiorSide=False` — i.e. the eye was on the *outdoor* side of the door plane
+`(0,-0.995,0.105)`. The plane math puts the cull threshold at eye Y < −8.50 with
+the door at Y≈−8.5: **the eye had poked out through the front wall while the
+player stood inside.** The same portal projected fine when the BFS reached
+`0170` from a deeper room (`0173`) where the eye was well inside.
+
+---
+
+## KEY FINDING: retail does NOT collide the camera — it fades the player
+
+This **reframes the intended fix** and needs user attention.
+
+- **Retail's camera does no eye-vs-wall collision.** `CameraManager::UpdateCamera`
+  (0x00456660, decomp `:95505-95953`) computes the eye as
+  `pivot + viewer_offset`, damps it (`Frame::interpolate_origin`, `:95922`), and
+  writes it straight to the render viewer via
+  `SmartBox::PlayerPhysicsUpdatedCallback` (0x00452d60, decomp `:91842`) — no
+  raycast, no sphere sweep, no BSP query in between. The `CameraManager` struct
+  (`acclient.h:35238-35263`) has **no** collision-radius / clipped-distance /
+  obstruction field.
+- **Retail's anti-clip mechanism is to fade the *player***.
+  `CameraSet::UpdateCamera` (0x00458ae0) calls
+  `CPhysicsObj::SetTranslucencyHierarchical(player, …)` (decomp `:97679`, `:97698`,
+  `:97725`, `:97737`) — opaque at ≥0.45 m camera-to-pivot, fully transparent at
+  ≤0.20 m. **acdream already ports this** as `RetailChaseCamera.ComputeTranslucency`
+  (`RetailChaseCamera.cs:367-376`, far=0.45 / near=0.20).
+
+**Verification (mine, this session):** I confirmed in the decomp that
+`SetTranslucencyHierarchical` is called from `CameraSet::UpdateCamera` (the four
+lines above), that `CameraManager::UpdateCamera`'s sole caller writes the eye
+with no intervening clip, that `RetailChaseCamera.cs` has zero
+collision/physics references (only `PlayerTranslucency`), and that `camPos` is
+the eye. (I relied on the Opus agent's full read of `CameraManager::UpdateCamera`
+for "zero collision calls in the whole function body"; I spot-checked the entry,
+the damping tail, the caller, and the struct — all consistent.)
+
+**Implication:** "make the camera move closer when it hits a wall" is **not what
+retail does**. A spring arm would be a *modern divergence* from the retail
+behavior acdream faithfully ported. Per CLAUDE.md ("do not replace working
+retail-faithful logic with a modern redesign without explicit user approval"),
+the next session must get the user's sign-off on the approach before coding —
+*but note* that here "retail-faithful" literally means "no collision," which is
+the behavior that produces the A8.F flap. So some divergence or decoupling is
+unavoidable to fix the flap.
+
+---
+
+## acdream's current camera (file:line)
+
+All under `src/AcDream.App/Rendering/` unless noted.
+
+- **Two chase cameras, toggled per-frame:** legacy `ChaseCamera.cs` (rigid) and
+  default `RetailChaseCamera.cs` (damped). Selected by
+  `CameraController.Active` (`CameraController.cs:20-33`) via
+  `CameraDiagnostics.UseRetailChaseCamera` (default ON,
+  `src/AcDream.Core/Rendering/CameraDiagnostics.cs`).
+- **Eye computation (`RetailChaseCamera.Update`, `:86-142`):**
+  `pivotWorld = playerPos + (0,0,1.5)`; `targetEye = pivotWorld +
+  forward*(-Distance·cosPitch) + up*(Distance·sinPitch)` (`:113-117`); then
+  exponential damping `_dampedEye = Lerp(_dampedEye, targetEye, alpha)` (`:121-133`);
+  published as `Position` + `View = CreateLookAt(_dampedEye, …)` (`:136-137`).
+  **No geometry test anywhere between target and publish.**
+- **The "input-lag for turning/jumping" port** = the RetailChaseCamera's three
+  smoothing mechanisms, all verified faithful to the decomp:
+  - Exponential damping (follow-lag): `:121-133` + `ComputeDampingAlpha:323-329`
+    ↔ retail `:95866-95923` (`stiffness*dt*10` clamped).
+  - 5-frame velocity-averaged, slope-aligned heading (the jump/slope feel):
+    `:94-107`, `:290-314`, `ComputeHeading:211-257` ↔ retail `old_velocities[5]`
+    `:95644-95677`.
+  - Mouse low-pass filter (input lag on flicks): `FilterMouseDelta:164-177` +
+    `FilterMouseAxis:340-358` ↔ retail `CameraSet::FilterMouseInput` (0x00457530,
+    `:96250-96279`); wired at `GameWindow.cs:1171-1199`.
+- **Constants** (from retail `CameraSet::SetDefaultOffsets` 0x00458F80,
+  `:97916-97967`): `PivotHeight=1.5`, `Distance=2.61` (=|(0,−2.5,0.75)|),
+  `Pitch=0.291 rad`, distance clamp `[2,40]`, pitch clamp `[−0.7,1.4]`. **No
+  collision-radius constant exists** (no collision is done).
+- **No 1st-person mode** (retail `SetInHead` 0x00458CE0 unported).
+- Spec: `docs/superpowers/specs/2026-05-18-retail-chase-camera-design.md`
+  explicitly scopes collision OUT (`:454-457`: "we don't attempt 'camera
+  collides with wall' — same as retail").
+
+---
+
+## Integration: acdream already has the collision machinery
+
+A camera "spring-arm" sweep can reuse the player's existing swept-sphere engine
+(`src/AcDream.Core/Physics/`):
+
+- **`PhysicsEngine.ResolveWithTransition(curPos, targetPos, cellId, radius,
+  height, stepUp, stepDown, isOnGround, body, moverFlags, entityId)`**
+  (`PhysicsEngine.cs:589`) → `Transition.FindTransitionalPosition`
+  (`TransitionTypes.cs:653`) → per sub-step `FindEnvCollisions`
+  (`:1933`, indoor branch fetches `GetCellStruct(cellId)` → `cellPhysics.BSP.Root`
+  → `BSPQuery.FindCollisions`).
+- **`BSPQuery`** primitives (`BSPQuery.cs`): `PointInsideCellBsp` (`:1034`),
+  `SphereIntersectsCellBsp` (`:1077`), `FindCollisions` (`:1637`),
+  `SphereIntersectsPoly` (`:2085`). A purpose-built pivot→eye cast off these is
+  likely a **better fit** than the full `Transition` (which carries unwanted
+  step-up / walkable / gravity semantics).
+- **`CellPhysics`** (`PhysicsDataCache.cs:511-564`): per-cell `BSP` (collision),
+  `CellBSP` (point-in-cell), polys/planes, transforms. Fetched via
+  `GetCellStruct(cellId)`.
+- Player sweep params for reference: `radius 0.48`, `height 1.2`
+  (`PlayerMovementController.cs:1107-1108`). A camera probe wants a **small**
+  radius (e.g. `PhysicsGlobals.DummySphereRadius = 0.1`), `height 0` (single
+  sphere), `isOnGround:false`, `body:null` (no contact-plane persistence).
+- **Slot-in point:** clamp `targetEye` toward `pivotWorld` **between
+  `RetailChaseCamera.cs:117` (target) and `:131` (damp)** so the damping eases
+  toward the clamped point. This requires injecting a physics/collision probe
+  into `RetailChaseCamera` (currently GL-free, no engine ref); App→Core is the
+  allowed dependency direction, so inject `PhysicsEngine` or a narrow
+  `ICameraCollisionProbe` interface.
+
+---
+
+## The design decision (MUST brainstorm with the user before coding)
+
+Fixing the flap requires the eye (or the point feeding visibility) to stop
+leaving the room. Three shapes, none purely retail-faithful:
+
+- **Option A — modern spring arm (leading candidate; matches user's instinct).**
+  Sweep pivot→targetEye, stop the eye at the first wall hit. Fixes *both* the
+  render (no see-through) *and* the A8.F visibility (eye stays inside → stable
+  camera-cell + side-tests). **Diverges from retail** (retail lets the eye clip +
+  fades the player). The already-ported translucency fade still applies and will
+  fade *less* (eye stays farther from the player when not clipping). Needs user
+  sign-off for the divergence.
+- **Option B — decouple visibility from the rendered eye.** Leave the rendered
+  eye retail-faithful (clips + fades), but feed `ComputeVisibility` /
+  `PointInCell` / `CameraOnInteriorSide` a *clamped* point. Fixes the flap, but
+  the render still shows through walls when the eye clips (you'd see exterior
+  through the wall). Less satisfying visually.
+- **Option C — make A8.F robust to an outside eye.** When the eye is outside but
+  the player is inside, use the *player's* cell for the camera-cell + side tests.
+  Similar outcome to B (flap fixed, render still clips).
+
+**Recommendation to put to the user:** Option A. It's the only one that fixes
+both the render and the visibility, it matches the user's stated expectation,
+and acdream already owns the collision machinery to do it. The cost is an
+explicit, documented divergence from retail's no-collision design. Confirm with
+the user, then (per roadmap discipline) file it as a phase before coding.
+
+---
+
+## Open design questions for the implementation session
+
+1. **Approval for the retail divergence** (Option A spring arm) — the gating item.
+2. **Sweep radius:** small (≈0.1–0.2 m). Too small → eye hugs walls / near-plane
+   clips; too large → eye yanks in aggressively in tight rooms.
+3. **Which primitive:** purpose-built `BSPQuery.FindCollisions` ray/sphere cast
+   (recommended) vs. full `ResolveWithTransition` (wrong semantics for a camera).
+4. **Indoor vs outdoor geometry:** indoor walls are in `CellPhysics.BSP` (per
+   cell). **Cottage exterior shells live in landblock-baked GfxObjs**, not cells
+   (cf. issue #98/#101 — cottage floors/walls in GfxObj `0x01000A2B` etc.). A
+   cell-BSP-only sweep fixes the indoor case but misses outdoor shells (the
+   `FindObjCollisions` / ShadowEntry path would be needed for those). Decide scope.
+5. **1st-person fallback:** none today; a spring arm must no-op at distance 0 if
+   1st-person is added.
+6. **Interaction with the ported translucency fade** (`ComputeTranslucency`) —
+   verify the fade still behaves once the eye stops clipping.
+7. **Pivot reference:** sweep from `pivotWorld` (head, `:113`), not the feet.
+
+---
+
+## Apparatus / diagnostics (committed `9417d3c`; opt-in)
+
+Launch (PowerShell), then walk `+Acdream` into a Holtburg cottage:
+
+```powershell
+$env:ACDREAM_DAT_DIR="$env:USERPROFILE\Documents\Asheron's Call"; $env:ACDREAM_LIVE="1"
+$env:ACDREAM_TEST_HOST="127.0.0.1"; $env:ACDREAM_TEST_PORT="9000"
+$env:ACDREAM_TEST_USER="testaccount"; $env:ACDREAM_TEST_PASS="testpassword"
+$env:ACDREAM_A8_INDOOR_BRANCH="1"; $env:ACDREAM_A8_DUMP_PV="1"; $env:ACDREAM_PROBE_ENVCELL="1"
+dotnet run --project src\AcDream.App\AcDream.App.csproj --no-build -c Debug 2>&1 | Tee-Object -FilePath "a8f.log"
+```
+
+- `ACDREAM_A8_DUMP_PV=1` — `[pv-dump]` per camera cell incl. the **`CAMPORTAL[i]`
+  census** (polyLen + interiorSide per portal, before the guards) + the
+  EXIT-CULLED/PROJ/CLIP trace + `OUTSIDEVIEW polys=N`.
+- `ACDREAM_PROBE_ENVCELL=1` — `[opaque]` cell-render stats (reliable; pre-loop).
+- `ACDREAM_PROBE_VIS=1` — `[buildings]`/`[draworder]`/`[stencil]`/`[envcells]`
+  (heavy: ~17 GL queries × several calls per frame; keep captures short).
+- `tools/A8CellAudit portals <cellId>` — offline portal dump with
+  `BUILDER_SEES=OK/EMPTY` per portal.
+
+**A future "camera-cell ≠ player-cell" probe** would directly confirm the
+flap-by-frame: log when `PointInCell(eye)` disagrees with the player's cell.
+
+---
+
+## Current state / safety
+
+- **Default game safe** — everything gated behind `ACDREAM_A8_INDOOR_BRANCH=1`.
+- Bug A fix + diagnostics committed (`9417d3c`); App tests 108/108.
+- Builder works (not the bug); camera-collision work **not started** (awaiting the
+  design decision).
+- Tree clean as of `9417d3c` plus untracked launch logs (`a8f-*.log`).
+
+---
+
+## Pickup prompt
+
+> Read `docs/research/2026-05-29-a8f-camera-collision-handoff.md`. The A8.F flap /
+> missing-walls are caused by the 3rd-person camera EYE clipping through walls and
+> destabilizing the camera-cell + portal side-tests (the eye drives
+> `PointInCell` / `CameraOnInteriorSide` via `camPos` at GameWindow.cs:7271).
+> Bug A (cellar terrain flood) is already fixed + committed (`9417d3c`); the
+> recursive-clip builder works. **Before coding, brainstorm the fix with the user**
+> (`superpowers:brainstorming`): retail does NOT collide the camera — it fades the
+> player (`SetTranslucencyHierarchical`, already ported as
+> `RetailChaseCamera.ComputeTranslucency`), so a "spring arm that pulls the eye in"
+> is a deliberate divergence needing sign-off. Leading option: a modern spring-arm
+> camera collision (Option A) sweeping pivot→targetEye via a small-radius
+> `BSPQuery.FindCollisions` against `CellPhysics.BSP`, clamping `targetEye` between
+> `RetailChaseCamera.cs:117` and `:131`. Watch the open questions (sweep radius,
+> outdoor shells live in GfxObjs not cells, 1st-person, the existing fade). File a
+> roadmap phase once the approach is agreed.
+
+---
+
+## Reference index
+
+**acdream code** (`src/…`):
+- `AcDream.App/Rendering/RetailChaseCamera.cs` — eye `:113-137`, damping
+  `:121-133`/`:323-329`, heading `:211-257`, mouse filter `:340-358`,
+  translucency `:367-376`.
+- `AcDream.App/Rendering/ChaseCamera.cs` (legacy), `CameraController.cs:20-33`,
+  `ICamera.cs`.
+- `AcDream.Core/Rendering/CameraDiagnostics.cs` — `UseRetailChaseCamera`, tunables.
+- `AcDream.App/Rendering/GameWindow.cs` — eye `:7270-7271`, visibility `:7323`,
+  `cameraInsideBuilding` `:7343-7346`, camera updates `:6851`/`:6862`, mouse-filter
+  `:1171-1199`.
+- `AcDream.App/Rendering/PortalVisibilityBuilder.cs` — `CameraOnInteriorSide`
+  `:196-203`, cull `:124`, CAMPORTAL census in `Build`.
+- `AcDream.App/Rendering/CellVisibility.cs` — `ComputeVisibility` `:272`,
+  `PointInCell` `:367`, BFS side-test `:466-481`, grace `:167`.
+- `AcDream.Core/Physics/` — `PhysicsEngine.cs:589`, `TransitionTypes.cs:653/1933`,
+  `BSPQuery.cs:1034/1077/1637`, `PhysicsDataCache.cs:511-564`,
+  `PlayerMovementController.cs:1107-1108`.
+- Spec: `docs/superpowers/specs/2026-05-18-retail-chase-camera-design.md` (collision
+  out-of-scope `:454-457`).
+
+**Retail decomp** (`docs/research/named-retail/acclient_2013_pseudo_c.txt`):
+- `CameraManager::UpdateCamera` @ 0x00456660 (`:95505-95953`) — NO collision;
+  damping `:95866-95923`; velocity ring `:95644-95677`.
+- `CameraSet::UpdateCamera` @ 0x00458AE0 (`:97643-97742`) — player-fade
+  `SetTranslucencyHierarchical` `:97679/97698/97725/97737`.
+- `CameraSet::FilterMouseInput` @ 0x00457530 (`:96250-96279`).
+- `CameraSet::SetDefaultOffsets` @ 0x00458F80 (`:97916-97967`) — pivot (0,0,1.5),
+  viewer (0,−2.5,0.75).
+- `SmartBox::PlayerPhysicsUpdatedCallback` @ 0x00452d60 (`:91842`) — sole
+  UpdateCamera caller; eye → render with no clip.
+- `CameraManager` struct `acclient.h:35238-35263` (no collision fields).