[Nikhi00718]

Description

Without OpenCV, videos whose first video stream follows an audio stream cannot be decoded. The PyAV fallback passes the video's global container index to decode(video=...), but that selector indexes only video streams. In an audio-first MP4, the first video has container index 1 and video index 0: read() silently returns False, and reading the dimensions raises IndexError: tuple index out of range.

Select video stream 0, consistently with streams.video[0], for initial decoding, frame-count fallback, display-rotation metadata and decoder reset during backward seeking.

Motivation and Context

Stream order is valid container metadata and should not prevent VideoInfo.from_video_path, get_video_frames_generator or the fallback capture from reading the file. This affects the default installation without OpenCV. No external model or download is needed to reproduce it: write an AAC stream before an MPEG-4 stream in a local MP4, then read it with the fallback.

Changes Made

• Correct the four PyAV decode selectors.
• Add real MP4 and Matroska fixtures with audio-first and video-first stream orders, checking dimensions, frame count, reads and backward seeking.
• Cover the public video APIs with synchronous and prefetched reads.
• Add an Unreleased changelog entry.

Testing

Windows, Python 3.13.15, PyAV 18.0.0, NumPy fallback backend (OpenCV absent). Commands use the existing development environment's Python with this worktree's src on PYTHONPATH.

• Clean develop at 6ecb27963: python -m pytest tests/cv2/test_video.py -k audio_before_video -q --tb=short — 4 failed with the stream-index error, 2 passed.

• Fixed: python -m pytest tests/cv2/test_video.py tests/utils/test_video.py -q --tb=short — 112 passed, 5 skipped.

• Clean baseline: python -m pytest src tests --cov=supervision -q --tb=short — 5694 passed, 105 skipped.

• Fixed full suite: python -m pytest src tests --cov=supervision -q --tb=short — 5700 passed, 105 skipped.

• python -m pre_commit run --all-files — all hooks passed, including mypy.

• Added/updated tests, and the full suite passes locally

• Updated docstrings for the corrected behavior

• Added a changelog entry in docs/changelog.md under Unreleased

Additional Notes

The capture continues to select the first video stream. Codec selection, audio remuxing and the OpenCV backend are unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94%. Comparing base (ffd0ac0) to head (eddb4b0).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2746   +/-   ##
=======================================
  Coverage       94%     94%           
=======================================
  Files           83      83           
  Lines        13612   13612           
=======================================
+ Hits         12796   12807   +11     
+ Misses         816     805   -11     

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[Copilot]

🟢 Approval recommended

The narrowly scoped correction consistently selects the intended stream and includes targeted regression coverage, with no unresolved findings.

0 open findings

What changed in this PR

Fixes PyAV fallback video reading when audio precedes the first video stream.

Changes:

• Selects video stream 0 consistently for decoding, metadata, frame counting and decoder resets.
• Adds regression coverage for stream ordering, backward seeking and public video readers.
• Documents the fix in the changelog.

File	Description
tests/​cv2/​test_video.py	Adds MP4, Matroska and public-reader regression tests.
src/​supervision/​_cv2/​_video.py	Corrects four PyAV video-stream selectors.
docs/​changelog.md	Records the audio-first video fix.

🧠 Review effort: Balanced

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