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+# ARKit-only body position (cut Mac MediaPipe pose/face)
+
+**Date**: 2026-06-30
+**Status**: design approved (decisions made), pending spec review
+**Scope**: `shared/AVLiveWire` (tag), `iphone-arbody` (send 2D skeleton + rebuild),
+`data_only_viz/{iphone_usb_source.py, scripts/iphone_usb_bridge.py, multi.py, arkit_joint_map.py}`,
+`launcher/concert/launch_concert.sh` (POSE_FILTER). Hands already cut (iPhone Vision).
+
+## Context
+
+Under `--iphone-usb` the matrix's body modulation (`/pose/center`, `/pose/kin`,
+`/pose/sho_span`, `/pose/limb_span`, `/pose/head`) is computed from the **Mac
+MediaPipe pose** (2D image landmarks), with ARKit only *fused* into the 3D
+world body (`arkit_fuse`, 14 joints). The 2D body position the matrix uses is
+therefore MediaPipe (CPU, jittery) — the user wants it 100% ARKit.
+
+**The wall:** the iPhone sends the ARKit skeleton in **3D world** coords
+(`root * jointModelTransforms`, tag `.skeleton`). The matrix `/pose/center`
+needs **2D image-normalized** coords. The iPhone app already computes a 2D
+projection (`ARCamera.projectPoint` → `skeleton2D`) for its on-device overlay
+but does NOT send it. So ARKit-only 2D position requires the iPhone to send the
+2D-projected skeleton.
+
+## Decisions (from brainstorming)
+
+- **Full ARKit body**: under `--iphone-usb`, build the body (2D + 3D) from the
+  ARKit skeleton; **cut Mac MediaPipe pose AND face** (hands already cut). Lose
+  `/pose/mouth` + the face mesh (accepted — max CPU, position 100% ARKit).
+- The iPhone sends the **2D-projected skeleton** (viewport-normalized) over USB,
+  in addition to the existing 3D world skeleton.
+- `arkit_fuse` is dropped from `POSE_FILTER` (ARKit becomes the source, not a fuse).
+
+## Design
+
+### Wire format (`shared/AVLiveWire`)
+- Add a `skeleton2D` tag (e.g. value 5). Payload: 91 × (x, y) Float32
+  (viewport-normalized 0..1) + 91 × valid (UInt8). Mirrors `SkeletonPayload`
+  but 2 floats/joint instead of 3.
+
+### iPhone app (`iphone-arbody`)
+- In `publishUSB` (or alongside `updateSkeleton2D`), build a `Skeleton2DPayload`
+  from the already-computed `skeleton2D` points (`projectPoint` viewport coords)
+  divided by `viewportSize` → normalized 0..1, with the tracked flags; send via
+  `usb.send(tag: .skeleton2D, ...)`. Keep the existing 3D `.skeleton` send (the
+  matrix's `/pose/kin` 3D body still uses it).
+- **Rebuild + redeploy** on the iPhone (`xcodegen generate` → Xcode → ⌘R). USER
+  STEP — cannot be done remotely.
+
+### Mac decode (`data_only_viz`)
+- `iphone_usb_source` + `scripts/iphone_usb_bridge.decode_skeleton2D`: decode
+  `TAG_SKELETON2D` → `state.persons_arkit_2d[pid]` (91 × (x,y) normalized) +
+  `persons_arkit_2d_t`.
+
+### Mac pose loop (`multi.py`, under `--iphone-usb`)
+- **Cut MediaPipe pose + face** inference (like hands): `pose_res`/`face_res`
+  skipped; `faces=[]`.
+- **Build the 2D body** (`bodies`, MP33 image-normalized) from `persons_arkit_2d`
+  via the `ARKIT91_TO_MP33` map (extended with head/neck → MP33 slot 0). Missing
+  MP33 slots (face/fingers) → low-confidence/clamped defaults.
+- **Build `bodies3d`** (MP33 world) from `persons_arkit_joints` (3D world) via the
+  same map (this replaces the `arkit_fuse` splice — ARKit is now the source).
+- Emit `/pose/*` from the ARKit body. The id/tracking can use a single pid (the
+  ARKit body) — the matrix is single-person.
+
+### Mapping (`arkit_joint_map.py`)
+- Extend `ARKIT91_TO_MP33` with head/neck → MP33 0 (nose) so `/pose/center` +
+  `/pose/head` have a head anchor. (ARKit `head_joint` index — confirm against the
+  Apple enum during implementation.)
+
+### Launcher
+- Drop `arkit_fuse` from `POSE_FILTER` (no longer fusing): set
+  `POSE_FILTER=median+kalman+lookahead+ik` for the body (ARKit is already smooth;
+  keep the temporal filters). Verify the matrix mod still feels right.
+
+## Testing
+
+- **AVLiveWire**: `swift test` (the wire round-trip for the new tag, in the
+  shared package's loopback tests).
+- **Mac decode + body-build**: unit-test `decode_skeleton2D` + the ARKit→MP33
+  body builder in isolation (no torch / no mediapipe needed for those pure
+  functions) — feed a synthetic 91-joint 2D array, assert the MP33 body + a
+  sane `/pose/center` (mean of mapped joints).
+- **Live (on the rig, after the iPhone rebuild)**: body position tracks from
+  ARKit (stable, no MediaPipe jitter); `/pose/center`/`kin`/`sho_span` respond;
+  CPU drops (no MediaPipe inference); the matrix's pose-driven modulation works;
+  the face mesh + `/pose/mouth` are gone (accepted).
+
+## Risks / dependencies
+
+- **iPhone rebuild is a manual user step** (Swift, on-device) — each iteration of
+  the Swift change needs a rebuild+redeploy. Get the wire format right first time.
+- Untestable end-to-end locally (needs the device + ARKit). Mac-side pure
+  functions are unit-tested; the integration is verified live.
+- Coordinate orientation: `projectPoint` viewport coords depend on the interface
+  orientation — normalize consistently (the app already handles orientation for
+  its overlay; reuse that).
+
+## Out of scope
+
+- Face/mouth modulation (cut with MediaPipe face).
+- Multi-person (ARKit body tracking is single-body; the matrix is single-person).
+- Hand position (already iPhone Vision).
+
+## Decided defaults
+
+- Full ARKit body, cut MediaPipe pose+face, send 2D skeleton from the iPhone,
+  drop `arkit_fuse`, head/neck added to the mapping.