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+Metrics and video recording
+===========================
+
+Observers are attached to pipeline steps to be notified when a method is called.
+
+## observers.py
+
+For this use case we need to record gaze analysis metrics on *ArUcoCamera.on_look* call and to record sector screen image on *ArUcoCamera.on_copy_background_into_scenes_frames* signal.
+
+```python
+import logging
+
+from argaze.utils import UtilsFeatures
+
+import cv2
+
+class ScanPathAnalysisRecorder(UtilsFeatures.FileWriter):
+
+	def __init__(self, **kwargs):
+
+		super().__init__(**kwargs)
+
+		self.header = "Timestamp (ms)", "Path duration (ms)", "Steps number", "Fixation durations average (ms)", "Explore/Exploit ratio", "K coefficient"
+
+	def on_look(self, timestamp, frame, exception):
+		"""Log scan path metrics."""
+
+		if frame.is_analysis_available():
+
+			analysis = frame.analysis()
+
+			data = (
+				int(timestamp), 
+				analysis['argaze.GazeAnalysis.Basic.ScanPathAnalyzer'].path_duration, 
+				analysis['argaze.GazeAnalysis.Basic.ScanPathAnalyzer'].steps_number,
+				analysis['argaze.GazeAnalysis.Basic.ScanPathAnalyzer'].step_fixation_durations_average,
+				analysis['argaze.GazeAnalysis.ExploreExploitRatio.ScanPathAnalyzer'].explore_exploit_ratio,
+				analysis['argaze.GazeAnalysis.KCoefficient.ScanPathAnalyzer'].K
+			)
+
+			self.write(data)
+
+class AOIScanPathAnalysisRecorder(UtilsFeatures.FileWriter):
+
+	def __init__(self, **kwargs):
+
+		super().__init__(**kwargs)
+
+		self.header = "Timestamp (ms)", "Path duration (ms)", "Steps number", "Fixation durations average (ms)", "Transition matrix probabilities", "Transition matrix density", "N-Grams count", "Stationary entropy", "Transition entropy"
+
+	def on_look(self, timestamp, layer, exception):
+		"""Log aoi scan path metrics"""
+
+		if layer.is_analysis_available():
+
+			analysis = layer.analysis()
+
+			data = (
+				int(timestamp),
+				analysis['argaze.GazeAnalysis.Basic.AOIScanPathAnalyzer'].path_duration, 
+				analysis['argaze.GazeAnalysis.Basic.AOIScanPathAnalyzer'].steps_number,
+				analysis['argaze.GazeAnalysis.Basic.AOIScanPathAnalyzer'].step_fixation_durations_average,
+				analysis['argaze.GazeAnalysis.TransitionMatrix.AOIScanPathAnalyzer'].transition_matrix_probabilities,
+				analysis['argaze.GazeAnalysis.TransitionMatrix.AOIScanPathAnalyzer'].transition_matrix_density, 
+				analysis['argaze.GazeAnalysis.NGram.AOIScanPathAnalyzer'].ngrams_count,
+				analysis['argaze.GazeAnalysis.Entropy.AOIScanPathAnalyzer'].stationary_entropy,
+				analysis['argaze.GazeAnalysis.Entropy.AOIScanPathAnalyzer'].transition_entropy
+			)
+
+			self.write(data)
+
+class VideoRecorder(UtilsFeatures.VideoWriter):
+
+	def __init__(self, **kwargs):
+
+		super().__init__(**kwargs)
+
+	def on_copy_background_into_scenes_frames(self, timestamp, frame, exception):
+		"""Write frame image."""
+
+		logging.debug('VideoRecorder.on_map')
+
+		image = frame.image()
+
+		# Write video timing
+		cv2.rectangle(image, (0, 0), (550, 50), (63, 63, 63), -1)
+		cv2.putText(image, f'Time: {int(timestamp)} ms', (20, 40), cv2.FONT_HERSHEY_SIMPLEX, 1, (255, 255, 255), 1, cv2.LINE_AA)
+
+		self.write(image)
+```