(DiamondLightSource/hyperion#1283) Automatically apply the snapshot m…

…icrons_per_pixel when the snapshot is triggered instead of requiring the plan to apply it manually.

src/dodal/devices/oav/grid_overlay.py

@@ -133,6 +133,7 @@ class SnapshotWithGrid(MJPG):
     last_path_outer = Component(Signal)
     last_path_full_overlay = Component(Signal)
 
+    # scaling factors for the snapshot at the time it was triggered
     microns_per_pixel_x = Component(Signal)
     microns_per_pixel_y = Component(Signal)
 

src/dodal/devices/oav/oav_detector.py

@@ -201,6 +201,7 @@ def __init__(self, *args, params: OAVConfigParams, **kwargs):
         super().__init__(*args, **kwargs)
         self.parameters = params
         self.subscription_id = None
+        self._snapshot_trigger_subscription_id = None
 
     def wait_for_connection(self, all_signals=False, timeout=2):
         connected = super().wait_for_connection(all_signals, timeout)
@@ -213,4 +214,20 @@ def wait_for_connection(self, all_signals=False, timeout=2):
             self.zoom_controller.level.unsubscribe(self.subscription_id)
         self.subscription_id = self.zoom_controller.level.subscribe(cb)
 
+        self.add_snapshot_trigger_hook()
+
         return connected
+
+    def add_snapshot_trigger_hook(self):
+        def apply_current_microns_per_pixel_to_snapshot(*args, **kwargs):
+            """Persist the current value of the mpp to the snapshot so that it is retained if zoom changes"""
+            microns_per_x_pixel = self.parameters.micronsPerXPixel
+            microns_per_y_pixel = self.parameters.micronsPerYPixel
+            self.snapshot.microns_per_pixel_x.put(microns_per_x_pixel)
+            self.snapshot.microns_per_pixel_y.put(microns_per_y_pixel)
+
+        self._snapshot_trigger_subscription_id = (
+            self.snapshot.last_saved_path.subscribe(
+                apply_current_microns_per_pixel_to_snapshot
+            )
+        )

tests/devices/unit_tests/test_oav.py

@@ -29,11 +29,14 @@ def fake_oav() -> OAV:
     fake_oav.snapshot.box_width.put(50)
     fake_oav.snapshot.num_boxes_x.put(15)
     fake_oav.snapshot.num_boxes_y.put(10)
+    fake_oav.snapshot.x_size.sim_put(1024)  # type: ignore
+    fake_oav.snapshot.y_size.sim_put(768)  # type: ignore
 
     fake_oav.cam.port_name.sim_put("CAM")  # type: ignore
     fake_oav.proc.port_name.sim_put("PROC")  # type: ignore
 
     fake_oav.wait_for_connection()
+    fake_oav.zoom_controller.set("1.0x").wait()
 
     return fake_oav
 
@@ -79,6 +82,23 @@ def test_snapshot_trigger_saves_to_correct_file(
         assert calls_to_save == expected_calls_to_save
 
 
+@patch("requests.get")
+@patch("dodal.devices.areadetector.plugins.MJPG.Image.open")
+def test_snapshot_trigger_applies_current_microns_per_pixel_to_snapshot(
+    mock_open: MagicMock, mock_get, fake_oav
+):
+    image = PIL.Image.open("test")  # type: ignore
+    mock_open.return_value.__enter__.return_value = image
+
+    expected_mpp_x = fake_oav.parameters.micronsPerXPixel
+    expected_mpp_y = fake_oav.parameters.micronsPerYPixel
+    with patch.object(image, "save"):
+        st = fake_oav.snapshot.trigger()
+        st.wait()
+        assert fake_oav.snapshot.microns_per_pixel_x.get() == expected_mpp_x
+        assert fake_oav.snapshot.microns_per_pixel_y.get() == expected_mpp_y
+
+
 @patch("requests.get")
 @patch("dodal.devices.areadetector.plugins.MJPG.Image.open")
 @patch("dodal.devices.oav.grid_overlay.add_grid_overlay_to_image")