research(render): Phase U.4c-1 — characterize the flap on real dat evidence
A8CellAudit portals dump extended to print per-portal plane + centroid-derived InsideSide vs the dat's authored PortalSide. Real Holtburg cottage cells show: the flap is a DIRECT 0xA9B40171->0xA9B40170 portal side-test flip (0170 is a direct neighbour, not multi-hop), and our centroid-derived InsideSide is anti-correlated with the dat PortalSide that retail InitCell (432896) uses. Evidence selects H2 (port the side test) over H1 (PVS set-grounding). Camera cell 0171 seenOutside=Y. Full reading + fix direction + open sign question in the note. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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docs/research/2026-05-31-u4c-flap-characterization.md
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# U.4c flap characterization — real-dat evidence (Task U.4c-1)
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**Date:** 2026-05-31. **Tool:** `tools/A8CellAudit/Program.cs` `portals` subcommand (extended this
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session to dump per-portal plane + centroid-`InsideSide` + dat `PortalSide`). DAT-only, no live session.
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## TL;DR
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The doorway flap is a **direct `0xA9B40171 → 0xA9B40170` portal** side-test flip — **not** a multi-hop
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PVS-reachability problem. The window/exit cell `0xA9B40170` is a *direct portal neighbour* of the
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camera cell `0xA9B40171`. The root cause is **H2 (confirmed): our per-frame side test
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(`CameraOnInteriorSide`) derives the sidedness *sense* from the cell centroid
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(`GameWindow.BuildLoadedCell` ~5648), independently of — and anti-correlated with — the dat's
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authored per-portal `PortalSide` flag, which retail's `PView::InitCell` (decomp:432896) uses.**
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This supersedes the spec's leading hypothesis (H1, stab_list set-grounding): the stab_list/PVS
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grounding (Layer 1 data, already plumbed) remains a correct retail-faithful improvement and the
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`seen_outside` anchor is real, but the **flap fix is the side-test port**, not set-grounding.
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## Real cell topology (Holtburg cottage, landblock 0xA9B40000)
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`A8CellAudit portals 0xA9B40171 0xA9B40170 0xA9B40174 0xA9B40175`:
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```
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0xA9B40171 (camera/root, seenOutside=Y) — 3 portals, all flags=ExactMatch, all BUILDER_SEES=OK:
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portal[0] -> 0xA9B40170 polyId=54 N=(0,-1,0) d=-7.65 centroidDot=-4.400 ourInsideSide=1 datPortalSide=0
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portal[1] -> 0xA9B40173 polyId=55 N=(-1,0,0) d= 4.10 centroidDot=-2.450 ourInsideSide=1 datPortalSide=0
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portal[2] -> 0xA9B40175 polyId=56 N=(0,0,-1) d=-0.00 centroidDot=-4.600 ourInsideSide=1 datPortalSide=0
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0xA9B40170 (window cell, seenOutside=Y) — 2 portals, BUILDER_SEES=OK:
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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
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portal[1] -> 0xA9B40171 flags=ExactMatch,PortalSide polyId=5 N=(0,-1,0) d=-7.65 centroidDot=+0.480 ourInsideSide=0 datPortalSide=1
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0xA9B40174 (seenOutside=Y) -> 0xA9B40175
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0xA9B40175 (seenOutside=Y) -> 0xA9B40171, -> 0xA9B40174
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```
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## Findings
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1. **Direct portal, not multi-hop.** `0170` is `0171`'s portal[0]. The [vis] flap (`cells=4 …0170`
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vs `cells=3` without it, at stable `root=0xA9B40171`) is the `0171→0170` side test flipping as the
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camera moves within `0171`. Multi-hop PVS grounding (H1) would not fix a direct-portal flip.
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2. **`PortalSide` is an authored, direction-asymmetric dat flag.** The same opening carries
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`PortalSide` UNSET on `0171→0170` and SET on the reverse `0170→0171`. It is in
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`portal.Flags` (`DatReaderWriter.Enums.PortalFlags.PortalSide`), already surfaced into
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`LoadedCell.Portals[i].Flags` as `(ushort)portal.Flags` (GameWindow:5614-5618). **Our builder
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never reads it** — it uses `ClipPlanes[i].InsideSide`, which `BuildLoadedCell` derives from the
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cell centroid (`centroidDot >= 0 ? 0 : 1`, ~5648).
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3. **Our `InsideSide` is anti-correlated with the dat `PortalSide`.** For `0171→0170`:
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`InsideSide=1`, `PortalSide=0`. For `0170→0171`: `InsideSide=0`, `PortalSide=1`. We synthesize a
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sidedness sense (with our own cross-product normal winding) that is opposite to the authoritative
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dat value. A centroid guess is also structurally fragile for non-convex / oddly-placed portals
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and for the 3rd-person camera **eye** (which can sit on the far side of an interior portal plane
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while still rooted in the cell).
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4. **Camera cell `0171` has `seenOutside=Y`.** Retail keeps the landscape engaged while the camera
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is in `0171` (`RenderNormalMode` 92649; `grab_visible_cells` 311878). Terrain should not vanish
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from `0171`. (Retail still gates the *per-frame draw* on `outside_view` non-empty in `DrawCells`
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432715 — so the real fix is keeping `0170`'s exit in the per-frame view, i.e. fixing the side
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test, not floating terrain off `outside_view`.)
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5. **All portal polygons resolve (`BUILDER_SEES=OK`).** Not a degenerate-polygon bug.
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## The fix (evidence-selected: H2)
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Port retail `PView::InitCell` (decomp:432896, sidedness at 432928-432968) sidedness into the
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builder's portal-traversal test: use the dat's authored `PortalSide` (from `portal.Flags`) for the
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traversal *sense*, replacing the centroid-derived `InsideSide`. The plane (Normal, D) still comes
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from the portal polygon geometry; only the *sense* source changes (centroid guess → authored bit).
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**Open detail — the exact sign-correct mapping (do NOT guess; port + validate).** InitCell's
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condition is convoluted in the BN decomp (a "clearly behind ⇒ cull" branch, an on-plane branch, and
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`if (computed_sidedness == portal_side) cull else traverse`), and it interacts with the portal
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polygon's normal orientation/winding. The `A8CellAudit` `retailTraversesAtCentroid` column was a
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first-cut formula and is UNRELIABLE (it reported "no-traverse" for all of `0171`'s portals, which is
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absurd — proof the naive sense was wrong). Resolve the exact sense by reading InitCell carefully,
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porting it, and validating that (a) all of `0171`'s neighbours stay reachable from interior poses
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and (b) `0170` (and its exit) stays reachable across a camera sweep near the `0171→0170` plane —
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then the live visual gate. Keep `Builder_BackFacingPortal_NotTraversed` / sealed-cellar /
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window-narrowing tests faithful (a fix that just "always traverses" is a band-aid and must fail
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them).
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## Apparatus
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`A8CellAudit portals <cellIds...>` now prints, per portal: plane `N`/`d`, `centroidDot`, our
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`InsideSide`, the dat `PortalSide`. Use it on any flap cell to see the centroid-vs-authored
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divergence. The committed Task-1 synthetic flap test (`Build_NearBoundaryIntermediatePortal…`) models
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the *mechanism* (a side-test flip drops a downstream exit cell) but via a multi-hop `InsideSide`
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cull; U.4c-3 should re-author it (or add a sibling) to faithfully model the **direct-portal
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`PortalSide`-vs-centroid** divergence, so the test is a clean gate for the actual fix.
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