fix(pose): swap chirality only, default on

User-confirmed live: ARBodyTracker Vision chirality arrives inverted
vs the mirrored view. HAND_SWAP_LR now inverts the chirality mapping
inside route_hands (cx fallback untouched, it is already screen
relative) and defaults to 1. Also: HandPersistenceGate greedy-match
limitation documented + malformed-entry guard (review follow-ups).

data_only_viz/action_head_pub.py

@@ -200,9 +200,10 @@ class ActionHeadPublisher(threading.Thread):
         # "iphone" / "mediapipe".
         self._finger_source = os.environ.get("FINGER_SOURCE", "auto").lower()
         self._fdbg_n = 0
-        # Safety knob: swap the two routed slots if the iPhone app's camera
-        # config inverts Vision chirality (fix live without redeploy).
-        self._hand_swap_lr = os.environ.get("HAND_SWAP_LR", "0") not in (
+        # ARBodyTracker's Vision chirality arrives inverted vs the user's
+        # mirrored view (observed live 2026-07-02); default-swap corrects it.
+        # Set HAND_SWAP_LR=0 if a future app build fixes chirality at source.
+        self._hand_swap_lr = os.environ.get("HAND_SWAP_LR", "1") not in (
             "0", "", "false", "False",
         )
         self._hand_near_min = float(os.environ.get("HAND_NEAR_MIN", "0.10"))

data_only_viz/hand_display.py

@@ -263,6 +263,11 @@ class HandPersistenceGate:
 
     min_frames=1 disables gating: every incoming hand is drawable on the
     first call (useful for the single-person MediaPipe fallback path).
+
+    Known limit: matching is greedy nearest-track — two hands crossing
+    within `radius` can inherit each other's track (and a ghost appearing
+    exactly where a real hand just left inherits its count). Acceptable
+    for a display gate; do not reuse for gesture-state tracking.
     """
 
     def __init__(self, min_frames: int = 3, radius: float = 0.15) -> None:
@@ -288,8 +293,11 @@ class HandPersistenceGate:
         used: set[int] = set()
 
         for h_idx, hand in enumerate(hands):
-            wx = hand[0].x
-            wy = hand[0].y
+            try:
+                wx = hand[0].x
+                wy = hand[0].y
+            except (TypeError, IndexError, AttributeError):
+                continue   # malformed entry: never drawable, no track
 
             # Find the nearest existing track within radius (greedy).
             best_dist = float("inf")

data_only_viz/hand_slots.py

@@ -7,9 +7,11 @@ First match per slot wins; extras dropped.
 Fallback (chirality absent/misaligned): sort valid hands by screen cx ascending
 (flipped when mirror=True) and assign leftmost to slot 0, next to slot 1.
 
-`swap=True`: swaps the two slots at the very end (safety knob: if the iPhone
-app's camera config inverts Vision chirality, HAND_SWAP_LR=1 fixes the
-direction live without redeploy).
+`swap=True`: inverts the chirality INTERPRETATION only (chir 0 -> right slot,
+1 -> left slot). ARBodyTracker's Vision chirality arrives inverted vs the
+user's mirrored view (observed live 2026-07-02), so HAND_SWAP_LR defaults to
+1 in the consumers; set 0 if a future app build fixes chirality at the
+source. The cx fallback is screen-relative and is never swapped.
 
 `near_min > 0`: after routing, replace a slot with None when the hand's
 wrist-to-middle-MCP distance (normalised) is below `near_min`.
@@ -72,8 +74,9 @@ def route_hands(
     chirality: int list aligned 1-to-1 with `hands` (0=left, 1=right).
                None / wrong length -> cx fallback.
     mirror:    flip cx for the fallback path so screen-left stays slot 0.
-    swap:      swap the two slots at the very end (both paths). Safety knob
-               for a source whose Vision chirality arrives inverted.
+    swap:      invert the chirality mapping (0 -> right, 1 -> left). Safety
+               knob for a source whose Vision chirality arrives inverted.
+               Never touches the cx fallback (already screen-relative).
     near_min:  replace a routed slot with None when hand_size < near_min.
     """
     chir_aligned = (
@@ -94,9 +97,12 @@ def route_hands(
     slots: list = [None, None]
 
     if chir_aligned and any(c is not None for c, _ in valid_pairs):
-        # ---- Chirality path: chir 0 -> slot 0 (left), 1 -> slot 1 (right) ----
+        # ---- Chirality path: chir 0 -> slot 0 (left), 1 -> slot 1 (right);
+        # swap inverts the mapping when the source chirality is flipped ----
         for c, hand in valid_pairs:
             target = c if c in (0, 1) else None
+            if target is not None and swap:
+                target = 1 - target
             if target is not None and slots[target] is None:
                 slots[target] = hand
     else:
@@ -118,8 +124,4 @@ def route_hands(
             if slots[i] is not None and _hand_size(slots[i]) < near_min:
                 slots[i] = None
 
-    # ---- swap: exchange the two slots at the very end (safety knob) ----
-    if swap:
-        slots[0], slots[1] = slots[1], slots[0]
-
     return slots

data_only_viz/renderer.py

@@ -165,9 +165,10 @@ class MetalRenderer(NSObject):
         self._hand_gate = HandPersistenceGate(
             min_frames=int(os.environ.get("HAND_PERSIST_FRAMES", "3"))
         )
-        # Safety knob: swap the two routed hand slots if the iPhone app's
-        # camera config inverts Vision chirality (fix live without redeploy).
-        self._hand_swap_lr = os.environ.get("HAND_SWAP_LR", "0") not in (
+        # ARBodyTracker's Vision chirality arrives inverted vs the user's
+        # mirrored view (observed live 2026-07-02); default-swap corrects it.
+        # Set HAND_SWAP_LR=0 if a future app build fixes chirality at source.
+        self._hand_swap_lr = os.environ.get("HAND_SWAP_LR", "1") not in (
             "0", "", "false", "False",
         )
         self._init_skel_cpu_buffer()

data_only_viz/tests/test_hand_slots.py

@@ -89,24 +89,25 @@ def test_mirror_flips_cx_fallback_order():
 
 
 # ---------------------------------------------------------------------------
-# swap: exchanges the two slots at the very end (both paths)
+# swap: inverts the CHIRALITY interpretation only (source chirality flipped);
+# the cx fallback is screen-relative and must never be affected.
 # ---------------------------------------------------------------------------
 
-def test_swap_swaps_chirality_slots():
+def test_swap_inverts_chirality_mapping():
     h_l = _hand(cx=0.8)  # Vision says left (chir=0) but source is inverted
     h_r = _hand(cx=0.2)  # Vision says right (chir=1)
     result = route_hands([h_l, h_r], [0, 1], swap=True)
-    assert result[0] is h_r   # slots exchanged at the end
-    assert result[1] is h_l
+    assert result[0] is h_r   # chir 1 -> left slot under swap
+    assert result[1] is h_l   # chir 0 -> right slot under swap
 
 
-def test_swap_swaps_cx_fallback_slots():
-    """swap=True also exchanges the slots on the cx fallback path."""
+def test_swap_does_not_alter_cx_fallback():
+    """swap only concerns chirality; the cx fallback stays screen-relative."""
     h_a = _hand(cx=0.2)
     h_b = _hand(cx=0.8)
     result = route_hands([h_a, h_b], None, swap=True)
-    assert result[0] is h_b
-    assert result[1] is h_a
+    assert result[0] is h_a   # unchanged: leftmost keeps slot 0
+    assert result[1] is h_b
 
 
 def test_swap_single_hand_moves_slot():