Implement probe_video_metadata_from_header

src/torchcodec/decoders/_core/__init__.py

@@ -2,4 +2,9 @@
 
 from .video_decoder_ops import *  # noqa
 
-from ._metadata import get_video_metadata, StreamMetadata, VideoMetadata
+from ._metadata import (
+    get_video_metadata,
+    get_video_metadata_from_header,
+    StreamMetadata,
+    VideoMetadata,
+)

src/torchcodec/decoders/_core/_metadata.py

@@ -1,13 +1,15 @@
 import json
+import pathlib
 
 from dataclasses import dataclass
-from typing import List, Optional
+from typing import List, Optional, Union
 
 import torch
 
 from torchcodec.decoders._core.video_decoder_ops import (
     _get_container_json_metadata,
     _get_stream_json_metadata,
+    create_from_file,
 )
 
 
@@ -83,6 +85,11 @@ def best_video_stream(self) -> StreamMetadata:
 
 
 def get_video_metadata(decoder: torch.Tensor) -> VideoMetadata:
+    """Return video metadata from a video decoder.
+
+    The accuracy of the metadata and the availability of some returned fields
+    depends on whether a full scan was performed by the decoder.
+    """
 
     container_dict = json.loads(_get_container_json_metadata(decoder))
     streams_metadata = []
@@ -92,7 +99,8 @@ def get_video_metadata(decoder: torch.Tensor) -> VideoMetadata:
             StreamMetadata(
                 duration_seconds=stream_dict.get("durationSeconds"),
                 bit_rate=stream_dict.get("bitRate"),
-                # TODO_OPEN_ISSUE: We should align the C++ names and the json keys with the Python names
+                # TODO_OPEN_ISSUE: We should align the C++ names and the json
+                # keys with the Python names
                 num_frames_retrieved=stream_dict.get("numFrames"),
                 num_frames_computed=stream_dict.get("numFramesFromScan"),
                 min_pts_seconds=stream_dict.get("minPtsSecondsFromScan"),
@@ -112,3 +120,7 @@ def get_video_metadata(decoder: torch.Tensor) -> VideoMetadata:
         best_audio_stream_index=container_dict.get("bestAudioStreamIndex"),
         streams=streams_metadata,
     )
+
+
+def get_video_metadata_from_header(filename: Union[str, pathlib.Path]) -> VideoMetadata:
+    return get_video_metadata(create_from_file(str(filename)))

test/decoders/test_metadata.py

@@ -1,20 +1,43 @@
+import functools
+
 import pytest
 
 from torchcodec.decoders._core import (
     create_from_file,
     get_ffmpeg_library_versions,
     get_video_metadata,
+    get_video_metadata_from_header,
     scan_all_streams_to_update_metadata,
     StreamMetadata,
 )
 
 from ..utils import NASA_VIDEO
 
 
-def test_get_video_metadata():
-    decoder = create_from_file(str(NASA_VIDEO.path))
-    scan_all_streams_to_update_metadata(decoder)
-    metadata = get_video_metadata(decoder)
+def _get_video_metadata(path, with_scan: bool):
+    decoder = create_from_file(str(path))
+    if with_scan:
+        scan_all_streams_to_update_metadata(decoder)
+    return get_video_metadata(decoder)
+
+
+@pytest.mark.parametrize(
+    "metadata_getter",
+    (
+        get_video_metadata_from_header,
+        functools.partial(_get_video_metadata, with_scan=False),
+        functools.partial(_get_video_metadata, with_scan=True),
+    ),
+)
+def test_get_metadata(metadata_getter):
+    with_scan = (
+        metadata_getter.keywords["with_scan"]
+        if isinstance(metadata_getter, functools.partial)
+        else False
+    )
+
+    metadata = metadata_getter(NASA_VIDEO.path)
+
     assert len(metadata.streams) == 6
     assert metadata.best_video_stream_index == 3
     assert metadata.best_audio_stream_index == 4
@@ -43,8 +66,9 @@ def test_get_video_metadata():
     assert best_stream_metadata.bit_rate == 128783
     assert best_stream_metadata.average_fps == pytest.approx(29.97, abs=0.001)
     assert best_stream_metadata.codec == "h264"
-    assert best_stream_metadata.num_frames_computed == 390
+    assert best_stream_metadata.num_frames_computed == (390 if with_scan else None)
     assert best_stream_metadata.num_frames_retrieved == 390
+    assert best_stream_metadata.num_frames == 390
 
 
 @pytest.mark.parametrize(

test/decoders/test_video_decoder_ops.py

@@ -249,11 +249,6 @@ def test_create_decoder(self, create_from):
 
     # Keeping the metadata tests below for now, but we should remove them
     # once we remove get_json_metadata().
-    # Note that the distinction made between test_video_get_json_metadata and
-    # test_video_get_json_metadata_with_stream is misleading: all of the stream
-    # metadata are available even without adding a video stream, because we
-    # always call scanFileAndUpdateMetadataAndIndex() when creating a decoder
-    # from the core API.
     def test_video_get_json_metadata(self):
         decoder = create_from_file(str(NASA_VIDEO.path))
         metadata = get_json_metadata(decoder)