feat: modify VAFE to keep only n expanded Features

The problem that I had overlooked until now (!!) due to only using small
exemplar videos for testing was that VideoActivityFeatureExtractor keeps
a fraction of small (0.5s) Features. This is not so problematic for a short
exemplar video, but ends up witha  lot of jumpy Features in an actual source
video.

Fix approach:

Like LoudnessFE, keep a specific number and expand the duration (which I think I
will do for LoudnessFE too), dropping any that would be consumed in that range

pipeline/feature_extractors.py

@@ -296,12 +296,18 @@ class VideoActivityFeatureExtractor(FeatureExtractor):
 
     #TODO: minimum duration -- consider whether to do here, or expand duration post-consolidation
     """
-    def __init__(self, input_files=None, config=None):
+    _CONFIG_DEFAULT_NUM_FEATURES = 5 # keep the top 5 activity moments
+    _CONFIG_DEFAULT_MIN_DURATION = 5.00 # seconds
+    def __init__(self, input_files=None, config=None,
+                 num_features=_CONFIG_DEFAULT_NUM_FEATURES,
+                 min_duration=_CONFIG_DEFAULT_MIN_DURATION):
         if not input_files:
             raise ValueError("No input files provided!")
         self.input_files = input_files
         self.config = config
         self.features = []
+        self._num_features = num_features
+        self._min_duration = min_duration
 
     def _scdet(self, video_file):
         """Run scdet filter on the video file"""
@@ -347,6 +353,29 @@ class VideoActivityFeatureExtractor(FeatureExtractor):
         scores = sorted(scores, key=lambda x: x[1], reverse=True)
         return scores[:int(len(scores) * (percent / 100))]
 
+    def _keep_num(self, features, num=_CONFIG_DEFAULT_NUM_FEATURES) -> list:
+        """Keep the top n activity features (default: 5)
+
+        Approach:
+         - for range in 0-n
+           + expand the nth top feature to min duration
+            (move start back by 0.5*min_duration, end forward by 0.5*min_duration)
+           + drop any features that are now in that feature's range
+         - return the top n features
+
+        Each feature is a Feature object, with an Interval object
+        """
+        for i in range(num):
+            # expand the feature to min_duration
+            features[i].interval.move_start(-0.5*self._min_duration, relative=True)
+            features[i].interval.move_end(0.5*self._min_duration, relative=True)
+            # drop any features that are now in that feature's range
+            features = [f for f in features if
+                        f.interval.start < features[i].interval.start or
+                        f.interval.end > features[i].interval.end]
+
+        return features[:num]
+
     def setup(self):
         pass
 
@@ -354,11 +383,16 @@ class VideoActivityFeatureExtractor(FeatureExtractor):
         for file in self.input_files:
             scores = self._scdet(file.path)
             means = sorted(self._nonoverlap_mean(scores), key=lambda x: x[1], reverse=True)
+
+            features = []
             for time, score in self._drop_lowest(means, 66):
-                self.features.append(Feature(interval=Interval(start=time, duration=0.500),
+                features.append(Feature(interval=Interval(start=time, duration=0.500),
                                              source=file, feature_extractor="videoactivity",
                                              score=score))
 
+            # prune features list to keep self.num_features
+            self.features = self._keep_num(features, self._num_features)
+
     def teardown(self):
         pass