[backends] Add workaround for #575 to VideoStreamCv2

Author: Breakthrough

scenedetect/backends/opencv.py

@@ -54,6 +54,22 @@
 )
 
 
+def _get_capture_api(path: str) -> int:
+    """API preference to use when opening `path` with `cv2.VideoCapture`. This exists as a
+    workaround for https://scenedetect.com/issue/575, where certain versions of the opencv-python
+    package cause intermittent segfaults on exit. The issue only happens on Windows when opening
+    PNG image sequences.
+
+    By default OpenCV resolves any capture backend by probing for compatibility, and tries FFmpeg
+    first. Usually this fails, and it falls back gracefully to CAP_IMAGES. However, this also seems
+    to cause issues on interpreter exit, as the stack trace implicates something in the ffmpeg DLL.
+    """
+    is_local = not any(identifier in path for identifier in ("://", " ! "))
+    if os.name == "nt" and is_local and path.lower().endswith(".png"):
+        return cv2.CAP_IMAGES
+    return cv2.CAP_ANY
+
+
 def _get_aspect_ratio(cap: cv2.VideoCapture, epsilon: float = 0.0001) -> float:
     """Display/pixel aspect ratio of the VideoCapture as a float (1.0 represents square pixels)."""
     # Versions of OpenCV < 3.4.1 do not support this, so we fall back to 1.0.
@@ -326,8 +342,10 @@ def _open_capture(self, frame_rate: FrameRate | None = None):
             if self._path_or_device < 0:
                 raise ValueError("Invalid/negative device ID specified.")
             input_is_video_file = False
+            capture_api = cv2.CAP_ANY
         else:
             assert isinstance(self._path_or_device, str)
+            capture_api = _get_capture_api(self._path_or_device)
             input_is_video_file = not any(
                 identifier in self._path_or_device
                 for identifier in NON_VIDEO_FILE_INPUT_IDENTIFIERS
@@ -337,7 +355,7 @@ def _open_capture(self, frame_rate: FrameRate | None = None):
             if input_is_video_file and not os.path.exists(self._path_or_device):
                 raise OSError("Video file not found.")
 
-        cap = cv2.VideoCapture(self._path_or_device)
+        cap = cv2.VideoCapture(self._path_or_device, capture_api)
         if not cap.isOpened():
             raise VideoOpenFailure(
                 "Ensure file is valid video and system dependencies are up to date.\n"

tests/test_backend_opencv.py

@@ -17,6 +17,8 @@
 For VideoStream tests that validate conformance, see test_video_stream.py.
 """
 
+import os
+
 import cv2
 import pytest
 
@@ -39,6 +41,27 @@ def test_open_image_sequence(test_image_sequence: str):
     assert sequence.position == 29
 
 
+@pytest.mark.skipif(os.name != "nt", reason="Workaround only applies on Windows.")
+def test_open_png_sequence_skips_ffmpeg_probe(test_image_sequence: str, monkeypatch):
+    """PNG inputs must be opened with `cv2.CAP_IMAGES` on Windows because certain versions of the
+    opencv-python 5.x package cause intermittent segfaults on exit.
+
+    See https://scenedetect.com/issue/575 for details."""
+    calls = []
+    real_capture = cv2.VideoCapture
+
+    def spy(*args):
+        calls.append(args)
+        return real_capture(*args)
+
+    monkeypatch.setattr(cv2, "VideoCapture", spy)
+    sequence = VideoStreamCv2(test_image_sequence, frame_rate=25.0)
+    sequence.reset()
+    assert calls == [(test_image_sequence, cv2.CAP_IMAGES)] * 2
+    assert sequence.capture.getBackendName() == "CV_IMAGES"
+    assert sequence.read() is not False
+
+
 def test_capture_adapter(test_movie_clip: str):
     """Test that the VideoCaptureAdapter works with SceneManager."""
     cap = cv2.VideoCapture(test_movie_clip)

website/pages/changelog.md

@@ -794,7 +794,5 @@ Development
  - [api] Scene-list output functions for EDL, FCPXML, FCP7 XML, and OTIO now accept an open text file handle, a string path, or a `pathlib.Path`; paths are opened and closed automatically [#567](https://github.com/Breakthrough/PySceneDetect/issues/567)
  - [feature] Add `--min-out-length`/`min_out_length` to `detect-threshold`/`ThresholdDetector`, which ignores fades that stay below the threshold for less than the given duration [#278](https://github.com/Breakthrough/PySceneDetect/issues/278)
  - [improvement] `detect-threshold` now ignores fade-outs shorter than `0.1s` by default. Use `--min-out-length 0` to restore the previous behavior. The API default remains `0` [#278](https://github.com/Breakthrough/PySceneDetect/issues/278)
-
-## PySceneDetect 0.8 (TBD)
-
 - [feature] Added `save-keyframes` command to export detected cuts using `# keyframe format v1` for Aegisub-compatible tools [#534](https://github.com/Breakthrough/PySceneDetect/issues/534). Frame numbers are currently approximate for VFR input [#569](https://github.com/Breakthrough/PySceneDetect/issues/569)
+ - [bugfix] Fix intermittent segfault on exit. Isolated to Windows builds with OpenCV 5.x when opening PNG image sequences with the `VideoStreamCv2` backend. [#575](https://github.com/Breakthrough/PySceneDetect/issues/575)