[docs] Update backend docs for MoviePy backend.

.github/workflows/build.yml

@@ -45,6 +45,7 @@ jobs:
           git fetch --depth=1 https://github.com/Breakthrough/PySceneDetect.git refs/heads/resources:refs/remotes/origin/resources
           git checkout refs/remotes/origin/resources -- tests/resources/
 
+      # TODO: Cache this: https://github.com/actions/cache
       # TODO: Install ffmpeg/mkvtoolnix on all runners.
       - name: Download FFMPEG
         if: ${{ matrix.os == 'windows-latest' }}

docs/cli/backends.rst

@@ -7,6 +7,8 @@ Backends
 
 PySceneDetect supports multiple backends for video input. Some can be configured by using :ref:`a config file <scenedetect_cli-config_file>`. Installed backends can be verified by running ``scenedetect version --all``.
 
+Note that the `scenedetect` command output is generated as a post-processing step, after scene detection completes. Most commands require the ability for the input to be replayed, and preferably it should also support seeking. Network streams and other input types are supported with certain backends, however integration with live streams requires use of the Python API.
+
 
 =======================================================================
 OpenCV
@@ -27,3 +29,16 @@ PyAV
 The `PyAV <https://github.com/PyAV-Org/PyAV>`_ backend (`av package <https://pypi.org/project/av/>`_) is a more robust backend that handles multiple audio tracks and frame decode errors gracefully.
 
 This backend can be used by specifying ``-b pyav`` via command line, or setting ``backend = pyav`` under the ``[global]`` section of your :ref:`config file <scenedetect_cli-config_file>`.
+
+
+=======================================================================
+MoviePy
+=======================================================================
+
+MoviePy launches ffmpeg as a subprocess, and can be used with various types of inputs. If the input supports seeking it should work fine with most operations, for example, image sequences or AviSynth scripts.
+
+.. warning::
+
+    The MoviePy backend is still under development and is not included with current Windows distribution. To enable MoviePy support, you must install PySceneDetect using `python` and `pip`.
+
+This backend can be used by specifying ``-b moviepy`` via command line, or setting ``backend = moviepy`` under the ``[global]`` section of your :ref:`config file <scenedetect_cli-config_file>`.

scenedetect/backends/moviepy.py

@@ -12,11 +12,9 @@
 #
 """:class:`VideoStreamMoviePy` provides an adapter for MoviePy's `FFMPEG_VideoReader`.
 
-Uses string identifier ``'moviepy'``.
-
-.. warning::
-
-    The MoviePy backend is still under development. Some features are not yet supported.
+MoviePy launches ffmpeg as a subprocess, and can be used with various types of inputs. Generally,
+the input should support seeking, but does not necessarily have to be a video. For example,
+image sequences or AviSynth scripts are supported as inputs.
 """
 
 from logging import getLogger
@@ -72,13 +70,7 @@ def __init__(self, path: AnyStr, framerate: Optional[float] = None, print_infos:
         self._frame_number = 0
         # We need to manually keep track of EOF as duration may not be accurate.
         self._eof = False
-        # MoviePy doesn't support extracting the aspect ratio yet, so for now we just fall
-        # back to using OpenCV to determine it.
-        try:
-            self._aspect_ratio = VideoStreamCv2(self._path).aspect_ratio
-        except VideoOpenFailure as ex:
-            logger.warning("Unable to determine aspect ratio: %s", str(ex))
-            self._aspect_ratio = 1.0
+        self._aspect_ratio: float = None
 
     #
     # VideoStream Methods/Properties
@@ -121,6 +113,15 @@ def duration(self) -> Optional[FrameTimecode]:
     @property
     def aspect_ratio(self) -> float:
         """Display/pixel aspect ratio as a float (1.0 represents square pixels)."""
+        # TODO: Use cached_property once Python 3.7 support is deprecated.
+        if self._aspect_ratio is None:
+            # MoviePy doesn't support extracting the aspect ratio yet, so for now we just fall
+            # back to using OpenCV to determine it.
+            try:
+                self._aspect_ratio = VideoStreamCv2(self._path).aspect_ratio
+            except VideoOpenFailure as ex:
+                logger.warning("Unable to determine aspect ratio: %s", str(ex))
+                self._aspect_ratio = 1.0
         return self._aspect_ratio
 
     @property

scenedetect/backends/pyav.py

@@ -10,10 +10,7 @@
 # PySceneDetect is licensed under the BSD 3-Clause License; see the
 # included LICENSE file, or visit one of the above pages for details.
 #
-""":class:`VideoStreamAv` provides an adapter for the PyAV av.InputContainer object.
-
-Uses string identifier ``'pyav'``.
-"""
+""":class:`VideoStreamAv` provides an adapter for the PyAV av.InputContainer object."""
 
 from logging import getLogger
 from typing import AnyStr, BinaryIO, Optional, Tuple, Union