added preflight check

tests/test_masked_hhd.py

@@ -70,17 +70,22 @@ def test_sensor_detect_motion_ignores_invalid_region():
         backend="cpu",
     )
     try:
+        # get_img() resizes the frame to the config's camera.resolution_stitch
+        # and stores valid_mask at that same resolution, so the flow buffer must
+        # be sized from sensor.valid_mask rather than from the raw frame.
         frame = np.zeros((728, 672, 3), dtype=np.uint8)
         mask = np.zeros((728, 672), dtype=bool)
         mask[180:540, 160:512] = True
         sensor.get_img(frame=frame, already_processed=True, valid_mask=mask)
 
-        sensor.flow = np.zeros((728, 672, 2), dtype=np.float32)
-        sensor.flow[~mask] = 10.0
+        flow_shape = sensor.valid_mask.shape
+        assert sensor.valid_mask.any() and not sensor.valid_mask.all()
+        sensor.flow = np.zeros((*flow_shape, 2), dtype=np.float32)
+        sensor.flow[~sensor.valid_mask] = 10.0
         assert not sensor.detect_motion(threshold=1.0)
 
         sensor.flow[:] = 0.0
-        sensor.flow[mask] = 10.0
+        sensor.flow[sensor.valid_mask] = 10.0
         assert sensor.detect_motion(threshold=1.0)
     finally:
         sensor.disconnect()