# U.4c flap characterization — real-dat evidence (Task U.4c-1)

**Date:** 2026-05-31. **Tool:** `tools/A8CellAudit/Program.cs` `portals` subcommand (extended this
session to dump per-portal plane + centroid-`InsideSide` + dat `PortalSide`). DAT-only, no live session.

## TL;DR

The doorway flap is a **direct `0xA9B40171 → 0xA9B40170` portal** side-test flip — **not** a multi-hop
PVS-reachability problem. The window/exit cell `0xA9B40170` is a *direct portal neighbour* of the
camera cell `0xA9B40171`. The root cause is **H2 (confirmed): our per-frame side test
(`CameraOnInteriorSide`) derives the sidedness *sense* from the cell centroid
(`GameWindow.BuildLoadedCell` ~5648), independently of — and anti-correlated with — the dat's
authored per-portal `PortalSide` flag, which retail's `PView::InitCell` (decomp:432896) uses.**
This supersedes the spec's leading hypothesis (H1, stab_list set-grounding): the stab_list/PVS
grounding (Layer 1 data, already plumbed) remains a correct retail-faithful improvement and the
`seen_outside` anchor is real, but the **flap fix is the side-test port**, not set-grounding.

## Real cell topology (Holtburg cottage, landblock 0xA9B40000)

`A8CellAudit portals 0xA9B40171 0xA9B40170 0xA9B40174 0xA9B40175`:

```
0xA9B40171 (camera/root, seenOutside=Y) — 3 portals, all flags=ExactMatch, all BUILDER_SEES=OK:
  portal[0] -> 0xA9B40170   polyId=54   N=(0,-1,0)  d=-7.65  centroidDot=-4.400  ourInsideSide=1  datPortalSide=0
  portal[1] -> 0xA9B40173   polyId=55   N=(-1,0,0)  d= 4.10  centroidDot=-2.450  ourInsideSide=1  datPortalSide=0
  portal[2] -> 0xA9B40175   polyId=56   N=(0,0,-1)  d=-0.00  centroidDot=-4.600  ourInsideSide=1  datPortalSide=0

0xA9B40170 (window cell, seenOutside=Y) — 2 portals, BUILDER_SEES=OK:
  portal[0] -> 0xFFFF (EXIT/outdoor)  flags=5            polyId=4  N=(0,-0.99,0.10) d=-8.56 centroidDot=-0.346 ourInsideSide=1 datPortalSide=0
  portal[1] -> 0xA9B40171             flags=ExactMatch,PortalSide  polyId=5  N=(0,-1,0)       d=-7.65 centroidDot=+0.480 ourInsideSide=0 datPortalSide=1

0xA9B40174 (seenOutside=Y) -> 0xA9B40175
0xA9B40175 (seenOutside=Y) -> 0xA9B40171, -> 0xA9B40174
```

## Findings

1. **Direct portal, not multi-hop.** `0170` is `0171`'s portal[0]. The [vis] flap (`cells=4 …0170`
   vs `cells=3` without it, at stable `root=0xA9B40171`) is the `0171→0170` side test flipping as the
   camera moves within `0171`. Multi-hop PVS grounding (H1) would not fix a direct-portal flip.

2. **`PortalSide` is an authored, direction-asymmetric dat flag.** The same opening carries
   `PortalSide` UNSET on `0171→0170` and SET on the reverse `0170→0171`. It is in
   `portal.Flags` (`DatReaderWriter.Enums.PortalFlags.PortalSide`), already surfaced into
   `LoadedCell.Portals[i].Flags` as `(ushort)portal.Flags` (GameWindow:5614-5618). **Our builder
   never reads it** — it uses `ClipPlanes[i].InsideSide`, which `BuildLoadedCell` derives from the
   cell centroid (`centroidDot >= 0 ? 0 : 1`, ~5648).

3. **Our `InsideSide` is anti-correlated with the dat `PortalSide`.** For `0171→0170`:
   `InsideSide=1`, `PortalSide=0`. For `0170→0171`: `InsideSide=0`, `PortalSide=1`. We synthesize a
   sidedness sense (with our own cross-product normal winding) that is opposite to the authoritative
   dat value. A centroid guess is also structurally fragile for non-convex / oddly-placed portals
   and for the 3rd-person camera **eye** (which can sit on the far side of an interior portal plane
   while still rooted in the cell).

4. **Camera cell `0171` has `seenOutside=Y`.** Retail keeps the landscape engaged while the camera
   is in `0171` (`RenderNormalMode` 92649; `grab_visible_cells` 311878). Terrain should not vanish
   from `0171`. (Retail still gates the *per-frame draw* on `outside_view` non-empty in `DrawCells`
   432715 — so the real fix is keeping `0170`'s exit in the per-frame view, i.e. fixing the side
   test, not floating terrain off `outside_view`.)

5. **All portal polygons resolve (`BUILDER_SEES=OK`).** Not a degenerate-polygon bug.

## The fix (evidence-selected: H2)

Port retail `PView::InitCell` (decomp:432896, sidedness at 432928-432968) sidedness into the
builder's portal-traversal test: use the dat's authored `PortalSide` (from `portal.Flags`) for the
traversal *sense*, replacing the centroid-derived `InsideSide`. The plane (Normal, D) still comes
from the portal polygon geometry; only the *sense* source changes (centroid guess → authored bit).

**Open detail — the exact sign-correct mapping (do NOT guess; port + validate).** InitCell's
condition is convoluted in the BN decomp (a "clearly behind ⇒ cull" branch, an on-plane branch, and
`if (computed_sidedness == portal_side) cull else traverse`), and it interacts with the portal
polygon's normal orientation/winding. The `A8CellAudit` `retailTraversesAtCentroid` column was a
first-cut formula and is UNRELIABLE (it reported "no-traverse" for all of `0171`'s portals, which is
absurd — proof the naive sense was wrong). Resolve the exact sense by reading InitCell carefully,
porting it, and validating that (a) all of `0171`'s neighbours stay reachable from interior poses
and (b) `0170` (and its exit) stays reachable across a camera sweep near the `0171→0170` plane —
then the live visual gate. Keep `Builder_BackFacingPortal_NotTraversed` / sealed-cellar /
window-narrowing tests faithful (a fix that just "always traverses" is a band-aid and must fail
them).

## Apparatus

`A8CellAudit portals <cellIds...>` now prints, per portal: plane `N`/`d`, `centroidDot`, our
`InsideSide`, the dat `PortalSide`. Use it on any flap cell to see the centroid-vs-authored
divergence. The committed Task-1 synthetic flap test (`Build_NearBoundaryIntermediatePortal…`) models
the *mechanism* (a side-test flip drops a downstream exit cell) but via a multi-hop `InsideSide`
cull; U.4c-3 should re-author it (or add a sibling) to faithfully model the **direct-portal
`PortalSide`-vs-centroid** divergence, so the test is a clean gate for the actual fix.