Support reading video from url (#531)

* add url support for VideoReader

* add a comment

* add unittest

* use a connectable url for ci

mmcv/video/io.py

@@ -62,6 +62,8 @@ class VideoReader:
     """
 
     def __init__(self, filename, cache_capacity=10):
+        # Check whether the video path is a url
+        if not filename.startswith(('https://', 'http://')):
             check_file_exist(filename, 'Video file not found: ' + filename)
         self._vcap = cv2.VideoCapture(filename)
         assert cache_capacity > 0

tests/test_video/test_reader.py

@@ -45,8 +45,10 @@ class TestVideoReader:
     def setup_class(cls):
         cls.video_path = osp.join(osp.dirname(__file__), '../data/test.mp4')
         cls.num_frames = 168
+        cls.video_url = 'https://www.learningcontainer.com/wp-content/uploads/2020/05/sample-mp4-file.mp4'  # noqa: E501
 
     def test_load(self):
+        # read from video file
         v = mmcv.VideoReader(self.video_path)
         assert v.width == 294
         assert v.height == 240
@@ -57,6 +59,16 @@ class TestVideoReader:
         import cv2
         assert isinstance(v.vcap, type(cv2.VideoCapture()))
 
+        # read from video url
+        v = mmcv.VideoReader(self.video_url)
+        assert v.width == 320
+        assert v.height == 240
+        assert v.fps == 15
+        assert v.frame_cnt == 1889
+        assert len(v) == 1889
+        assert v.opened
+        assert isinstance(v.vcap, type(cv2.VideoCapture()))
+
     def test_read(self):
         v = mmcv.VideoReader(self.video_path)
         img = v.read()