Add a warning if any clip can't be obtained from a video in VideoClips. (#2513)

* Add a warning if a clip can't be get from a video in VideoClips

* Update torchvision/datasets/video_utils.py
Co-authored-by: Philip Meier <github.pmeier@posteo.de>

* Add a test
Co-authored-by: Philip Meier <github.pmeier@posteo.de>

test/test_datasets_video_utils.py

@@ -119,6 +119,16 @@ class Tester(unittest.TestCase):
         self.assertTrue(clips.equal(idxs))
         self.assertTrue(idxs.flatten().equal(resampled_idxs))
 
+        # case 3: frames aren't enough for a clip
+        num_frames = 32
+        orig_fps = 30
+        new_fps = 13
+        with self.assertWarns(UserWarning):
+            clips, idxs = VideoClips.compute_clips_for_video(video_pts, num_frames, num_frames,
+                                                             orig_fps, new_fps)
+        self.assertEqual(len(clips), 0)
+        self.assertEqual(len(idxs), 0)
+
 
 if __name__ == '__main__':
     unittest.main()

torchvision/datasets/video_utils.py

 import bisect
 import math
+import warnings
 from fractions import Fraction
 from typing import List
 
@@ -204,6 +205,9 @@ class VideoClips(object):
         )
         video_pts = video_pts[idxs]
         clips = unfold(video_pts, num_frames, step)
+        if not clips.numel():
+            warnings.warn("There aren't enough frames in the current video to get a clip for the given clip length and "
+                          "frames between clips. The video (and potentially others) will be skipped.")
         if isinstance(idxs, slice):
             idxs = [idxs] * len(clips)
         else: