feat(preprocessor): add frame selector

gnes/preprocessor/video/shotdetect.py

@@ -14,6 +14,7 @@
 #  limitations under the License.
 
 import numpy as np
+import math
 from typing import List
 
 from gnes.preprocessor.base import BaseVideoPreprocessor
@@ -99,12 +100,13 @@ def apply(self, doc: 'gnes_pb2.Document') -> None:
                     shot_len = len(frames)
                     c.weight = shot_len / num_frames
                     if self.sframes > 0 and shot_len > self.sframes:
-                        begin = 0
-                        if self.sframes < 3:
-                            begin = (shot_len - self.sframes) // 2
-                        step = (shot_len) // self.sframes
-                        frames = [frames[_] for _ in range(begin, shot_len, step)]
-
+                        if shot_len >= 2 * self.sframes:
+                            step = math.ceil(shot_len / self.sframes)
+                            frames = frames[::step]
+                        else:
+                            idx = np.sort(np.random.choice(shot_len, self.sframes, replace=False))
+                            frames = [frames[idx_] for idx_ in idx]
+
                     chunk_data = np.array(frames)
                     c.blob.CopyFrom(array2blob(chunk_data))
             else: