Avoid relying on PyAV-provided video frame count

They are not reliable.

In particular, MP4 has a feature called "edit lists" that allows you to set
a custom playback order for the media data. With edit lists, you could only
specify that a particular range of frames should be played, or that a range
should be played multiple times, etc. See the following for technical
details:

https://developer.apple.com/documentation/quicktime-file-format/edit_list_atom

FFmpeg follows edit lists when decoding videos. However, the frame count
returned by PyAV's `Stream.frames` property is the number of frames in the
raw media data and does not reflect the modifications applied by an edit
list.

When we build a video manifest, we use `Stream.frames` if it's non-zero.
Therefore, in the presence of an edit list we will obtain a frame count that
does not match the actual number of frames that we can get out of the video.

FWIW, edit lists are probably not the only way that `Stream.frames` could be
inaccurate, it's just the reason behind a specific problem I encountered.

Since we already have to handle the situation where `Stream.frames` is not
available, just pretend it doesn't exist and always count frames by
traversing the entire video. I don't think it even matters much, since we
have to do it anyway to build the rest of the manifest.

We also have to stop validating the frame count in a user-provided manifest,
which is unfortunate, but it doesn't seem worthwhile to decode the entire
video just for that.

CHANGELOG.md

@@ -26,7 +26,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
-- TDB
+- Incorrectly determined video frame count when the video contains an MP4 edit list
+  (<https://github.com/opencv/cvat/pull/6929>)
 
 ### Security
 

cvat/apps/engine/task.py

@@ -932,7 +932,6 @@ def _update_status(msg):
                                                               manifest_path=db_data.get_manifest_path())
                             manifest.init_index()
                             manifest.validate_seek_key_frames()
-                            manifest.validate_frame_numbers()
                             assert len(manifest) > 0, 'No key frames.'
 
                             all_frames = manifest.video_length

utils/dataset_manifest/core.py

@@ -46,10 +46,6 @@ def __init__(self, source_path, chunk_size, force):
                         )
                     self.height, self.width = (frame.height, frame.width)
 
-                    # not all videos contain information about numbers of frames
-                    if video_stream.frames:
-                        self._frames_number = video_stream.frames
-
                     return
 
     @property
@@ -63,6 +59,9 @@ def _get_video_stream(container):
         return video_stream
 
     def __len__(self):
+        assert self._frames_number is not None, \
+            "The length will not be available until the reader is iterated all the way through at least once"
+
         return self._frames_number
 
     @property
@@ -498,7 +497,7 @@ def _write_base_information(self, file):
 
     def _write_core_part(self, file, _tqdm):
         iterable_obj = self._reader if _tqdm is None else \
-            _tqdm(self._reader, desc="Manifest creating", total=len(self._reader))
+            _tqdm(self._reader, desc="Manifest creating", total=float("inf"))
         for item in iterable_obj:
             if isinstance(item, tuple):
                 json_item = json.dumps({
@@ -510,17 +509,12 @@ def _write_core_part(self, file, _tqdm):
 
     def create(self, *, _tqdm=None): # pylint: disable=arguments-differ
         """ Creating and saving a manifest file """
-        if not len(self._reader):
-            tmp_file = StringIO()
-            self._write_core_part(tmp_file, _tqdm)
+        tmp_file = StringIO()
+        self._write_core_part(tmp_file, _tqdm)
 
-            with open(self._manifest.path, 'w') as manifest_file:
-                self._write_base_information(manifest_file)
-                manifest_file.write(tmp_file.getvalue())
-        else:
-            with open(self._manifest.path, 'w') as manifest_file:
-                self._write_base_information(manifest_file)
-                self._write_core_part(manifest_file, _tqdm)
+        with open(self._manifest.path, 'w') as manifest_file:
+            self._write_base_information(manifest_file)
+            manifest_file.write(tmp_file.getvalue())
 
         self.set_index()
 
@@ -576,15 +570,6 @@ def validate_seek_key_frames(self):
                 self.validate_key_frame(container, video_stream, key_frame)
                 last_key_frame = key_frame
 
-    def validate_frame_numbers(self):
-        with closing(av.open(self._source_path, mode='r')) as container:
-            video_stream = self._get_video_stream(container)
-            # not all videos contain information about numbers of frames
-            frames = video_stream.frames
-            if frames:
-                assert frames == self.video_length, "The uploaded manifest does not match the video"
-                return
-
 class ImageProperties(dict):
     @property
     def full_name(self):