fix(viz): mirror-conjugate point rotation

Live report: video straight but skeleton rotated wrong. Flip and
90-degree rotation do not commute — the video is flipped THEN rotated
while points are rotated THEN display-flipped. Points now use the
mirror-conjugated mode (cw/ccw swapped under mirror), proven by a
composition test.

data_only_viz/iphone_usb_source.py

@@ -51,6 +51,19 @@ def rotate_norm_xy(x: float, y: float, mode: str) -> "tuple[float, float]":
     return f(x, y) if f else (x, y)
 
 
+def effective_point_rotate(mode: str, mirror: bool) -> str:
+    """Point-rotation mode equivalent to the video pipeline's transform.
+
+    The VIDEO is horizontally flipped at decode (mirror) THEN rotated by
+    multi.py; the POINTS are rotated here THEN flipped at display time by
+    the renderer. Flip and 90-degree rotations do not commute: under the
+    mirror the point rotation must be the mirror-conjugate of the video
+    rotation (cw <-> ccw swapped; 180 commutes)."""
+    if mirror and mode in ("cw", "ccw"):
+        return "cw" if mode == "ccw" else "ccw"
+    return mode
+
+
 def _make_point(x: float, y: float, z: float, conf: float) -> PoseKp:
     """Wrap raw floats from the wire into a PoseKp landmark.
 
@@ -150,8 +163,9 @@ class IphoneUSBSource:
         self._mirror = mirror and _cfg.concert_mirror
         # VIDEO_ROTATE also applies to the iPhone 2D landmarks (skeleton2D +
         # hands) so they stay aligned with the rotated video (multi.py only
-        # rotates the frame).
-        self._video_rotate = _cfg.video_rotate or "none"
+        # rotates the frame). Mirror-conjugated: see effective_point_rotate.
+        self._video_rotate = effective_point_rotate(
+            _cfg.video_rotate or "none", self._mirror)
         self._codec = av.codec.CodecContext.create("hevc", "r") if av is not None else None
         self._lock = threading.Lock()
         self._frame = None            # latest BGR np.ndarray

data_only_viz/tests/test_iphone_usb_source.py

@@ -301,3 +301,23 @@ def test_rotate_norm_xy_matches_rot90():
     assert rotate_norm_xy(0.2, 0.7, "180") == pytest.approx((0.8, 0.3))
     assert rotate_norm_xy(0.2, 0.7, "none") == (0.2, 0.7)
     assert rotate_norm_xy(0.2, 0.7, "garbage") == (0.2, 0.7)
+
+
+def test_effective_point_rotate_mirror_conjugation():
+    """Video = flip THEN rotate; points = rotate THEN flip (renderer).
+    Prove the conjugated mode lands points where the video pixel lands:
+    flip_x(T(x,y)) must equal rot(flip_x(x,y)) for T = conjugate(rot)."""
+    from data_only_viz.iphone_usb_source import (
+        effective_point_rotate, rotate_norm_xy,
+    )
+    flip = lambda x, y: (1.0 - x, y)
+    for video_mode in ("ccw", "cw", "180", "none"):
+        pt_mode = effective_point_rotate(video_mode, mirror=True)
+        for (x, y) in [(0.2, 0.7), (0.9, 0.1), (0.5, 0.5)]:
+            video_dest = rotate_norm_xy(*flip(x, y), video_mode)
+            point_dest = flip(*rotate_norm_xy(x, y, pt_mode))
+            assert video_dest == pytest.approx(point_dest), (
+                video_mode, x, y)
+    # No mirror: modes pass through untouched
+    assert effective_point_rotate("ccw", mirror=False) == "ccw"
+    assert effective_point_rotate("none", mirror=True) == "none"