Add option for the user to pass in ffmpeg thread count (#291)

Co-authored-by: Nicolas Hug <[email protected]>

src/torchcodec/decoders/_video_decoder.py

@@ -36,6 +36,11 @@ class VideoDecoder:
             This can be either "NCHW" (default) or "NHWC", where N is the batch
             size, C is the number of channels, H is the height, and W is the
             width of the frames.
+        num_ffmpeg_threads (int, optional): The number of threads to use for decoding.
+            Use 1 for single-threaded decoding which may be best if you are running multiple
+            instances of ``VideoDecoder`` in parallel. Use a higher number for multi-threaded
+            decoding which is best if you are running a single instance of ``VideoDecoder``.
+            Default: 1.
 
             .. note::
 
@@ -58,6 +63,7 @@ def __init__(
         *,
         stream_index: Optional[int] = None,
         dimension_order: Literal["NCHW", "NHWC"] = "NCHW",
+        num_ffmpeg_threads: int = 1,
     ):
         if isinstance(source, str):
             self._decoder = core.create_from_file(source)
@@ -82,7 +88,10 @@ def __init__(
 
         core.scan_all_streams_to_update_metadata(self._decoder)
         core.add_video_stream(
-            self._decoder, stream_index=stream_index, dimension_order=dimension_order
+            self._decoder,
+            stream_index=stream_index,
+            dimension_order=dimension_order,
+            num_threads=num_ffmpeg_threads,
         )
 
         self.metadata, self.stream_index = _get_and_validate_stream_metadata(

test/decoders/test_video_decoder.py

@@ -55,8 +55,9 @@ def test_create_fails(self):
         with pytest.raises(ValueError, match="No valid stream found"):
             decoder = VideoDecoder(NASA_VIDEO.path, stream_index=1)  # noqa
 
-    def test_getitem_int(self):
-        decoder = VideoDecoder(NASA_VIDEO.path)
+    @pytest.mark.parametrize("num_ffmpeg_threads", (1, 4))
+    def test_getitem_int(self, num_ffmpeg_threads):
+        decoder = VideoDecoder(NASA_VIDEO.path, num_ffmpeg_threads=num_ffmpeg_threads)
 
         ref_frame0 = NASA_VIDEO.get_frame_data_by_index(0)
         ref_frame1 = NASA_VIDEO.get_frame_data_by_index(1)