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+# AR Tracking Overhaul: Worker Offload, Camera Model, and Coplanar Pose Ambiguity
+
+**Date:** 2026-09-23
+**Project:** newbury-exhibit-v3
+**Status:** ongoing
+**Tags:** ar, opencv, solvepnp, aruco, web-worker, camera-calibration, performance, caching, test-rig
+
+## Context
+
+v3 replaced v2's per-marker POS-IT tracking with a joint multi-marker OpenCV
+board solve. On-device testing surfaced a cascade of distinct faults that all
+presented as "the ghost is in the wrong place or missing": a frozen page, ghosts
+rendering off-screen, ghosts vanishing entirely, and stale builds masking fixes.
+This session diagnosed each from HUD telemetry rather than guesswork, fixed them,
+and built a printable A3 test rig — including a foldable marker tower — to make
+tracking evaluation repeatable.
+
+## Key Decisions
+
+- **Run OpenCV.js in a Web Worker** (`cv-worker.js`), not on the main thread.
+  The `@techstark/opencv-js` build embeds ~8 MB of WASM as a base64 data URI
+  inside a 10.8 MB script, so parsing + base64 decode + WASM compile all happen
+  synchronously — seconds of frozen UI and a "page unresponsive" prompt.
+  *Rejected:* loading via Blob URL with a progress bar (made memory worse: three
+  copies of 10.8 MB); extracting the WASM to a sibling file at Docker build time
+  to enable streaming compile (mechanical string surgery on emscripten glue, too
+  fragile).
+
+- **Solves are fire-and-forget, one in flight at a time.** If a solve is still
+  running the frame is skipped rather than queued, so the main thread never waits
+  and latency can't accumulate. The 1€ filter absorbs the ~1 frame delay.
+
+- **FOV applies to the image's LONG axis, not `videoWidth`.** iOS hands back
+  portrait streams (720×1280); assuming the angle spanned `videoWidth` made the
+  focal length 1280/720 = 1.78× too short. This was measured on device twice —
+  the first "fix" (deriving vertical FOV from the video + `object-fit: cover`
+  crop) was correct in principle but still keyed off `videoWidth`, so the same
+  1.78× error survived it.
+
+- **Throttle detection to 20 Hz while rendering at full rate.** Detection
+  (`getImageData` + ArUco decode) dominates frame cost, and the crests don't
+  move — only the phone does.
+
+- **Cap the detection canvas LONG side, not its width.** Fixing width at 960
+  tripled pixel count on portrait streams (960×1707 vs 960×540).
+
+- **Kept detection resolution at 960 after checking the arithmetic.** Mid-change
+  I was raising it to 1280 on the theory that missed crests were a decode
+  problem — then computed that 960 already yields ~23 px per marker cell, far
+  above ArUco's needs. Raising it would have cost +78% CPU for probably nothing.
+  Exposed as a knob instead so the theory can be tested rather than assumed.
+
+- **Deduplicate marker ids within a frame, keeping the largest quad.** Two quads
+  decoding to the same id hand solvePnP two contradictory image points for one
+  set of world points, collapsing the pose (observed: `seen [0,2,0,4]`,
+  reproj 5.4 px).
+
+- **Gate poses on physics, not reprojection error.** A coplanar point set has two
+  poses that reproject almost equally well, so error can't separate them (a
+  flipped pose was accepted at 1.3 px). Gates: camera must be above the sheet;
+  camera can't teleport 20 cm between frames. Rejections keep the *previous*
+  pose rather than applying a bad one. Escape hatch after 12 consecutive
+  rejections so a bad reference can't lock tracking out.
+
+- **The real cure for ambiguity is geometry: a marker tower.** Gates suppress a
+  symptom; non-coplanar anchors make the mirrored pose mathematically impossible.
+  *Rejected:* wrapping a crest sheet around the 85 mm cylinder — ArUco needs
+  planar facets. Chose a four-sided prism + top face so at least one crest is
+  readable from any angle (worst case 45° incidence).
+
+- **Tower face orientations derived, not guessed.** Unrolling a prism advances
+  arc position and the marker's printed-right axis together, so left-to-right on
+  the flat net is yaw *increasing*: front(180), left(270), back(0), right(90).
+  Verified against the real `anchorWorldMatrix` before shipping, because a wrong
+  yaw yields plausible-but-wrong ghost placement — the hardest kind to debug.
+
+- **Motion amounts are server-authoritative.** `playlist.js` sends explicit
+  `bobAmp`/`radius`/`speed` with every spawn, so client defaults never applied.
+  A client-side "3 mm bob" fix was dead code until the server default changed.
+
+- **Test rig is a single source of truth.** The page that draws the printed
+  sheets also builds the scene + playlist JSON from the same constants, so print
+  and config cannot drift apart.
+
+## Technical Details
+
+**World frame:** origin at front-left of the sheet; +X right, +Z away from
+viewer, +Y up; centimetres. Marker print sizes stay in mm (`sizeMM`), converted
+once at the solver boundary.
+
+**Camera model** (`exhibit.js`):
+```js
+const focalPxFor = (w, h) => (Math.max(w, h) / 2) / Math.tan(degToRad(HFOV_DEG) / 2);
+// updateCameraIntrinsics(): scale = max(innerWidth/vw, innerHeight/vh)  // object-fit: cover
+// visH = min(vh, innerHeight/scale);  camera3.fov = 2*atan((visH/2)/focal)
+// _crop.{x,y} = visible fraction, used to put the aim-check px into viewport NDC
+```
+Verified rendered frustum matches true visible frame to <0.05° in portrait,
+landscape, and landscape-screen cases.
+
+**Worker protocol** (`public/js/ar/cv-worker.js`):
+- `{type:'init'}` → `{type:'ready'}` | `{type:'failed', msg}`
+- `{type:'solve', id, obj, img, n, W, H, f}` → `{type:'pose', id, ok, C, m3, reproj, n, why}`
+- `{type:'reset'}` — clears warm start + gate state
+- `obj`/`img` are transferred `Float64Array`s (no copy). Worker returns camera
+  position `C` and the 3×3 world-from-camera rotation `m3`; the client builds the
+  quaternion (worker never touches three.js).
+
+**Frame conversion** (validated against synthetic projections, error ~0):
+solvePnP gives world→cvCamera `(R,t)`; cv cam is +X right, +Y down, +Z forward.
+`C = -Rᵀt`; three.js world-from-camera = `Rᵀ · diag(1,-1,-1)`.
+
+**Worker constants:** `MAX_REPROJ_PX = 4` (was 8), `MIN_CAM_Y_CM = 1`,
+`MAX_JUMP_CM = 20` (allowance scales with elapsed time), `JUMP_GRACE_MS = 500`,
+`MAX_REJECT_STREAK = 12`, `PREV_TTL = 1500`.
+
+**localStorage knobs:** `nbx.engine` (opencv|posit), `nbx.hfovDeg` (default 60),
+`nbx.detectHz` (20), `nbx.detectLong` (960).
+
+**Server motion defaults** (`server/playlist.js`, these override the client):
+`bobAmp 0.3`, `bobHz 0.4`, `wanderRadius 2.5`, `wanderVertical 0.4`,
+`pathSpeed 1.5` — previously 5–10, 60, 10, 8, sized for a large floor.
+
+**A3 test rig** (`/admin/testsheet.html`), A3 landscape = 42.0 × 29.7 cm:
+- Page 1: crests 0–5 at (5,4) (21,4) (37,4) (5,25.7) (21,25.7) (37,25.7) cm,
+  60 mm, `mount:'flat'`, `yawDeg:180` (prints upright). 100 mm scale bar.
+- Page 2: tower net — 85 mm square × 160 mm tall, four 85 mm panels + 15 mm glue
+  tab + 85 mm top flap; net 355 × 245 mm. Crests 6–9 on the sides at 100 mm
+  height (`mount:'wall'`, yaw 180/270/0/90, position pushed ±4.25 cm along the
+  face normal), crest 10 on top (`mount:'flat'`, `yawDeg:180`).
+- Anchor heights now 0 / 10 / 16 cm — genuinely non-coplanar.
+- Walker loop: 12–30 cm × 8.85–20.85 cm, 60 cm perimeter, ~40 s/lap at 1.5 cm/s.
+- Static ghost at (21, 22) cm, behind the tower from the front (occlusion test).
+
+**Cache policy** (`server/server.js`): `no-cache` for `.js`/`.html`,
+`immutable, max-age=31536000` for `/vendor/opencv.js`. A cache-busting query on
+the entry module would NOT have worked — ES module imports fetch under their own
+URLs, so `detect.js` etc. would stay stale.
+
+**Deploy:** `cd ~/newbury-exhibit-v3 && bash deploy.sh`. Rollback to v2:
+`cd ~/newbury-exhibit && docker compose up -d --build`.
+
+## Code Changes
+
+- `public/js/ar/cv-worker.js` — **new.** OpenCV board solve off-thread; reusable
+  Mats; warm-start + SQPNP/IPPE cold start; reprojection, camera-height and
+  teleport gates with reported rejection reason.
+- `public/js/ar/solve-cv.js` — rewritten as a worker client; keeps anchor
+  geometry and the 1€ filter on the main thread; retains the previous pose on
+  rejection; exposes `lastReject`.
+- `public/js/ar/detect.js` — report raw vs scene-filtered detections; dedupe
+  repeated ids keeping the largest quad; expose `dupes`.
+- `public/js/exhibit.js` — worker-backed solver; long-axis focal model; camera
+  intrinsics from video + cover-crop; 20 Hz detection throttle; long-side canvas
+  cap; scene fetched over HTTP so tracking survives a dead WebSocket; cylinder
+  occluders; dev Motion button; aim-check and rejection telemetry in the HUD.
+- `public/index.html` — moved `#dbg` below the HUD row (it was covering the
+  tracking chip and swallowing taps); added the `#motionBtn` control.
+- `public/js/ghosts/behavior.js` — idle float scaled to the display: 3 mm bob
+  (was 6 cm), matching shimmer for wander/path; table-scale fallbacks.
+- `server/playlist.js` — server-side motion defaults rescaled and made tunable
+  via `behaviors.bobAmp`/`bobHz`.
+- `server/server.js` — explicit cache policy for app code vs the WASM blob.
+- `public/admin/testsheet.html` — **new.** Two-page printable A3 test rig plus
+  one-click scene + playlist load, all from shared constants.
+- `docs/PAGES.md` — **new.** Page index, API endpoints, HUD reading guide, and
+  the gotchas worth not rediscovering.
+
+## Open Questions / Next Steps
+
+- **Why do crests fail to decode?** Observed 5 clearly visible, only 2 decoded.
+  Resolution is NOT the explanation (~23 px/cell at 960). Candidates: strict
+  `maxHammingDistance: 0` rejecting marginal reads under blur/glare; adaptive
+  thresholding on uneven lighting. Relaxing to hamming 1 is plausible now that a
+  known-id whitelist, dedupe and pose gates exist, but it risks a misread landing
+  on a whitelisted id. Test `nbx.detectLong = 1280` first to settle it empirically.
+- **Validate the tower on-device.** Success criteria: `markers` count rises,
+  `REJ:` stays quiet, and `proj` tracks `px` while circling the tower.
+- **Tune `nbx.hfovDeg`.** 60° is a guess; if a constant `proj`/`px` ratio remains,
+  adjust and watch `reproj` minimise.
+- **WebSocket flapping.** HUD showed `ws conn` persisting with ghosts clearing on
+  reconnect. Tracking no longer depends on it, but ghosts do. Likely the
+  openresty vhost missing `Upgrade`/`Connection` headers on `/ws`.
+- **Upstream proxy overrides cache headers.** openresty replaces the app's
+  `no-cache` with a daily `expires`, so deploys don't reach devices until it
+  lapses. Needs a vhost change; private-tab testing is the workaround.
+- **Layout editor doesn't understand cylinder occluders or wall anchors** — it
+  may mangle the tower layout on save.
+- **Delete the posit engine** once the opencv path is confirmed on the real town.
+
+## Artifacts
+
+- Repo: `https://gitea.hideawaygaming.com.au/jessikitty/newbury-exhibit-v3`
+- v2 (rollback path): `https://gitea.hideawaygaming.com.au/jessikitty/newbury-exhibit-v2`
+- `docs/PAGES.md` — page index, HUD guide, gotchas
+- `public/admin/testsheet.html` — the printable test rig
+- OpenCV.js build pinned at `@techstark/opencv-js@4.12.0-release.1`
+  (5.0.0 does not initialise; `solvePnPGeneric` and `cornerSubPix` are absent
+  from this build, which is why ambiguity is handled by gating rather than by
+  comparing both IPPE solutions)