Better fix for exposure drift with more complete testing. Also fix return value for capture_to_memory

src/openflexure_microscope_server/things/camera/__init__.py

@@ -342,7 +342,7 @@ class BaseCamera(lt.Thing):
         logger: lt.deps.InvocationLogger,
         metadata_getter: lt.deps.GetThingStates,
         buffer_max: int = 1,
-    ) -> None:
+    ) -> int:
         """Capture an image to memory. This can be saved later with ``save_from_memory``.
 
         Note that only one image is held in memory so this will overwrite any image

src/openflexure_microscope_server/things/camera/picamera.py

@@ -226,7 +226,7 @@ class StreamingPiCamera2(BaseCamera):
         if not self._setting_save_in_progress and self.streaming:
             with self._streaming_picamera() as cam:
                 cam_value = cam.capture_metadata()["ExposureTime"]
-            if abs(cam_value - self._exposure_time) > 30:
+            if cam_value != self._exposure_time:
                 self._exposure_time = cam_value
                 self.save_settings()
         return self._exposure_time
@@ -250,7 +250,8 @@ class StreamingPiCamera2(BaseCamera):
             "Brightness": 0,
             "ColourGains": self.colour_gains,
             "Contrast": 1,
-            "ExposureTime": self.exposure_time,
+            # Must also set plus 1 or the exposure drifts with start and stop stream.
+            "ExposureTime": self.exposure_time + 1,
             "Saturation": 1,
             "Sharpness": 1,
         }