[Bugfix] Align OpenCV video metadata timeline

tests/multimodal/media/test_video.py

@@ -10,7 +10,11 @@
 from PIL import Image
 
 from vllm.assets.base import get_vllm_public_assets
-from vllm.assets.video import video_to_ndarrays, video_to_pil_images_list
+from vllm.assets.video import (
+    video_get_metadata,
+    video_to_ndarrays,
+    video_to_pil_images_list,
+)
 from vllm.multimodal.media import ImageMediaIO, VideoMediaIO
 from vllm.multimodal.video import VIDEO_LOADER_REGISTRY, VideoLoader
 
@@ -112,6 +116,20 @@ def test_opencv_video_io_colorspace(tmp_path, is_color: bool, fourcc: str, ext:
         assert np.nanmean(sim) > 0.99
 
 
+def test_opencv_video_metadata_matches_sampled_frame_timeline(tmp_path):
+    image_path = f"{tmp_path}/test_metadata_image.png"
+    Image.new("RGB", (8, 8), color=(255, 0, 0)).save(image_path)
+    video_path = f"{tmp_path}/test_metadata_video.mp4"
+    create_video_from_image(image_path, video_path, num_frames=10, fps=5.0)
+
+    metadata = video_get_metadata(video_path, num_frames=4)
+
+    assert metadata["fps"] == pytest.approx(5.0)
+    assert metadata["duration"] == pytest.approx(2.0)
+    assert metadata["frames_indices"] == [0, 3, 6, 9]
+    assert metadata["total_num_frames"] == 4
+
+
 NUM_FRAMES = 10
 FAKE_OUTPUT_1 = np.random.rand(NUM_FRAMES, 1280, 720, 3)
 FAKE_OUTPUT_2 = np.random.rand(NUM_FRAMES, 1280, 720, 3)

vllm/assets/video.py

@@ -15,6 +15,10 @@
 from .base import get_cache_dir
 
 
+def _sample_frame_indices(total_frames: int, num_frames: int) -> npt.NDArray:
+    return np.linspace(0, total_frames - 1, num_frames, dtype=int)
+
+
 @lru_cache
 def download_video_asset(filename: str) -> str:
     """
@@ -47,7 +51,7 @@ def video_to_ndarrays(path: str, num_frames: int = -1) -> npt.NDArray:
     frames = []
 
     num_frames = num_frames if num_frames > 0 else total_frames
-    frame_indices = np.linspace(0, total_frames - 1, num_frames, dtype=int)
+    frame_indices = _sample_frame_indices(total_frames, num_frames)
     for idx in range(total_frames):
         ok = cap.grab()  # next img
         if not ok:
@@ -86,13 +90,14 @@ def video_get_metadata(path: str, num_frames: int = -1) -> dict[str, Any]:
 
     if num_frames == -1 or num_frames > total_frames:
         num_frames = total_frames
+    frame_indices = _sample_frame_indices(total_frames, num_frames)
 
     metadata = {
         "total_num_frames": num_frames,
-        "fps": duration / num_frames,
+        "fps": fps,
         "duration": duration,
         "video_backend": "opencv",
-        "frames_indices": list(range(num_frames)),
+        "frames_indices": frame_indices.tolist(),
         # extra field used to control hf processor's video
         # sampling behavior
         "do_sample_frames": num_frames == total_frames,