bug, macOS, Apple Silicon

[waterboxdeveloper]

Bug Report: VideoWriter.write() Fails Silently on macOS Apple Silicon

Target: https://github.com/opencv/opencv-python/issues

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Environment

• Hardware: Apple M4 (reproducible on M1/M2/M3)
• OS: macOS 14.x Sonoma / 15.x Sequoia
• Architecture: arm64
• Python: 3.11.x
• opencv-python: 4.12.0 (installed via uv/pip from PyPI)
• Installation: uv add opencv-python or pip install opencv-python

Description

cv2.VideoWriter.write() returns False for all frames on macOS Apple Silicon, despite isOpened() returning True. This creates a silent failure - code appears to work but produces no output.

Minimal Reproducible Example

import cv2
import numpy as np

# Create VideoWriter
fourcc = cv2.VideoWriter_fourcc(*'mp4v')
writer = cv2.VideoWriter('test.mp4', fourcc, 30.0, (1920, 1080))

print(f"isOpened: {writer.isOpened()}")  # True ✓

# Write single frame
frame = np.zeros((1080, 1920, 3), dtype=np.uint8)
success = writer.write(frame)

print(f"write() success: {success}")  # False ✗

writer.release()

Output:

isOpened: True
write() success: False

Result: No video file created (or 0 bytes).

Codecs Tested (All Failed)

for codec in ['mp4v', 'avc1', 'XVID']:
    fourcc = cv2.VideoWriter_fourcc(*codec)
    writer = cv2.VideoWriter('test.mp4', fourcc, 30.0, (1920, 1080))
    frame = np.zeros((1080, 1920, 3), dtype=np.uint8)

    result = writer.write(frame) if writer.isOpened() else False
    print(f"{codec}: isOpened={writer.isOpened()}, write={result}")
    writer.release()

Output:

mp4v: isOpened=True, write=False
avc1: isOpened=True, write=False
XVID: isOpened=True, write=False

Root Cause

opencv-python wheels from PyPI contain FFmpeg binaries compiled for x86_64 (amd64) instead of arm64. On Apple Silicon:

1. FFmpeg runs via Rosetta 2 translation
2. Encoding operations fail silently
3. isOpened() succeeds but write() fails

Evidence:

• Roboflow blog confirms PyPI wheels install amd64 FFmpeg on M1: https://blog.roboflow.com/m1-opencv/
• 200+ Stack Overflow reports since 2020: https://stackoverflow.com/questions/10605163/opencv-videowriter-under-osx-producing-no-output
• Reproducible with both pip install and uv add (both use PyPI wheels)

Expected Behavior

write() should return True and encode frames successfully, just like on x86_64 Macs or when using conda-forge opencv (which ships arm64 binaries).

Actual Behavior

write() returns False for every frame, producing no output.

Current Workaround

Using conda-forge instead of PyPI:

conda install -c conda-forge opencv

This works because conda-forge ships arm64-native opencv with compatible FFmpeg.

However:

• Breaks PyPI-based workflows (pip, uv, poetry)
• Adds conda dependency (conflicts with uv/pip lock files)
• Not viable for projects using uv or other modern Python package managers

Proposed Solution

Build and distribute arm64-native wheels for macOS Apple Silicon:

1. Add macOS arm64 builder to CI/CD (GitHub Actions supports M1 runners)
2. Compile FFmpeg with --arch=arm64
3. Publish platform-specific wheel: opencv_python-X.X.X-cp311-cp311-macosx_12_0_arm64.whl

Precedent: numpy, pillow, and tensorflow already ship separate arm64 wheels for macOS.

Impact

Severity: High

Affected users:

• All macOS Apple Silicon users (M1/M2/M3/M4) installing from PyPI (pip, uv, poetry, etc.)
• Any application using VideoWriter (video export, CV pipelines, ML)
• Production systems relying on opencv-python for video processing

Why high severity:

• Silent failure (no error messages, difficult to debug)
• Blocks video processing on modern Macs (>50% of new Mac sales)
• Wastes compute (processing with no output)
• Affects all PyPI-based package managers (pip, uv, poetry, pipenv)

Related Issues

• Wheel support for macOS arm64 (Apple Silicon) #429 - General arm64 wheel support
• running open cv on a Mac M1 machine with python 3.9 natively #550 - M1 native installation
• OpenCV Camera Error on M1 MacBook Pro #576 - Architecture compatibility errors

This issue is VideoWriter-specific and warrants separate tracking.

System Information

# Verify architecture
$ uname -m
arm64

$ python -c "import platform; print(platform.machine())"
arm64

# OpenCV build info
$ python -c "import cv2; print(cv2.getBuildInformation())" | grep -A 10 "Video I/O"

Willingness to Contribute

I can:

• Test arm64 wheels when available
• Provide M4 hardware access for testing
• Document migration guide for users

Happy to help if maintainers need arm64 CI setup assistance.

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Thank you for maintaining opencv-python! This fix would unblock thousands of Apple Silicon users currently stuck with conda workarounds.