Python中OpenCV读取MATLAB生成的未压缩AVI文件时cap.read()崩溃问题求助

I recently ran into a frustrating issue where using cap.read() in OpenCV to load an uncompressed AVI file generated by MATLAB's VideoWriter would immediately crash my Python program with a segmentation fault (zsh: segmentation fault python3 t.py). Here's a breakdown of the problem, my debugging steps, and the solutions I found:

Problem Details #

• The video file plays normally in IINA, and ffmpeg can extract frames successfully, but converting it to other formats fails.
• OpenCV's cap.isOpened() returns True, and I can retrieve all metadata (FPS, frame count, width/height) correctly. However, cap.get(cv2.CAP_PROP_FOURCC) returns 0.0, which is a red flag.
• MATLAB-generated grayscale AVIs work fine with OpenCV, but this BGR24 color version does not.

Test Code #

import cv2
video_path = '/Users/aravind/Downloads/test.avi'
cap = cv2.VideoCapture(video_path)
print(cap.get(cv2.CAP_PROP_FOURCC))
print("FPS:", cap.get(cv2.CAP_PROP_FPS))
print("Frame count:", cap.get(cv2.CAP_PROP_FRAME_COUNT))
print("Width:", cap.get(cv2.CAP_PROP_FRAME_WIDTH))
print("Height:", cap.get(cv2.CAP_PROP_FRAME_HEIGHT))
while cap.isOpened():
    print("Working here")
    ret, frame = cap.read() # Crash occurs here

FFprobe Output #

{
  "streams": [
    {
      "index": 0,
      "codec_name": "rawvideo",
      "codec_long_name": "raw video",
      "codec_type": "video",
      "codec_tag_string": "[0][0][0][0]",
      "codec_tag": "0x0000",
      "width": 192,
      "height": 192,
      "coded_width": 192,
      "coded_height": 192,
      "has_b_frames": 0,
      "pix_fmt": "bgr24",
      "level": -99,
      "r_frame_rate": "30/1",
      "avg_frame_rate": "30/1",
      "time_base": "1/30",
      "start_pts": 0,
      "start_time": "0.000000",
      "duration_ts": 60,
      "duration": "2.000000",
      "bit_rate": "26991946",
      "nb_frames": "60",
      "extradata_size": 9,
      "disposition": {
        "default": 0,
        "dub": 0,
        "original": 0,
        "comment": 0,
        "lyrics": 0,
        "karaoke": 0,
        "forced": 0,
        "hearing_impaired": 0,
        "visual_impaired": 0,
        "clean_effects": 0,
        "attached_pic": 0,
        "timed_thumbnails": 0,
        "non_diegetic": 0,
        "captions": 0,
        "descriptions": 0,
        "metadata": 0,
        "dependent": 0,
        "still_image": 0,
        "multilayer": 0
      }
    }
  ]
}

FFmpeg Frame Extraction Success #

ffmpeg version 8.1 Copyright (c) 2000-2026 the FFmpeg developers
built with Apple clang version 21.0.0 (clang-2100.0.123.102)
configuration: --prefix=/opt/homebrew/Cellar/ffmpeg/8.1 --enable-shared --enable-pthreads --enable-version3 --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gpl --enable-libsvtav1 --enable-libopus --enable-libx264 --enable-libmp3lame --enable-libdav1d --enable-libvmaf --enable-libvpx --enable-libx265 --enable-openssl --enable-videotoolbox --enable-audiotoolbox --enable-neon
libavutil 60. 26.100 / 60. 26.100
libavcodec 62. 28.100 / 62. 28.100
libavformat 62. 12.100 / 62. 12.100
libavdevice 62. 3.100 / 62. 3.100
libavfilter 11. 14.100 / 11. 14.100
libswscale 9. 5.100 / 9. 5.100
libswresample 6. 3.100 / 6. 3.100
Input #0, avi, from '/Users/aravind/Downloads/test.avi':
  Duration: 00:00:02.00, start: 0.000000, bitrate: 26813 kb/s
  Stream #0:0: Video: rawvideo, bgr24, 192x192, 26991 kb/s, 30 fps, 30 tbr, 30 tbn
File '/tmp/test_frame.png' already exists. Overwrite? [y/N] y
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> png (native))
Press [q] to stop, [?] for help
Output #0, image2, to '/tmp/test_frame.png':
  Metadata:
    encoder         : Lavf62.12.100
  Stream #0:0: Video: png, rgb24(pc, gbr/unknown/unknown, progressive), 192x192, q=2-31, 200 kb/s, 30 fps, 30 tbn
  Metadata:
    encoder         : Lavc62.28.100 png
[image2 @ 0xbe7018280] The specified filename '/tmp/test_frame.png' does not contain an image sequence pattern or a pattern is invalid.
[image2 @ 0xbe7018280] Use a pattern such as %03d for an image sequence or use the -update option (with -frames:v 1 if needed) to write a single image.
[out#0/image2 @ 0xbe6c54180] video:2KiB audio:0KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: unknown
frame=    1 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.03 bitrate=N/A speed=7.15x
elapsed=0:00:00.00

Root Cause #

Looking at the ffprobe output, the key issue is the codec_tag: "0x0000" — MATLAB's VideoWriter didn't set a valid FOURCC identifier for the uncompressed BGR24 video. OpenCV relies on FOURCC to determine how to decode frame data; without a valid tag, it can't correctly parse the raw pixel data, leading to a memory access violation (segmentation fault). Grayscale AVIs work because they likely use a FOURCC that OpenCV recognizes by default.

Solutions #

1. Transcode the Video with FFmpeg #

Add a valid FOURCC tag that OpenCV understands using ffmpeg:

ffmpeg -i test.avi -c:v rawvideo -pix_fmt bgr24 -fourcc CVFP test_fixed.avi

CVFP corresponds to OpenCV's CV_FOURCC('C','V','F','P'), which is a supported FOURCC for uncompressed BGR24 video. After transcoding, OpenCV will read the file without crashing.

2. Use PyAV (FFmpeg-Based Library) to Read the Video #

PyAV is a Python wrapper for FFmpeg that handles rawvideo formats correctly. Install it first:

pip install av

Then use this code to read the video and convert frames to OpenCV format:

import av
import cv2
import numpy as np

video_path = '/Users/aravind/Downloads/test.avi'
container = av.open(video_path)
fps = container.streams.video[0].average_rate

for frame in container.decode(video=0):
    # Convert PyAV frame to OpenCV-compatible BGR array
    img = frame.to_ndarray(format='bgr24')
    cv2.imshow('Frame', img)
    # Control playback speed
    if cv2.waitKey(int(1000 / fps)) & 0xFF == ord('q'):
        break

cv2.destroyAllWindows()

3. Modify MATLAB's Video Generation #

Generate the AVI with a valid FOURCC that OpenCV recognizes. In MATLAB, you can specify the compression method or set the FOURCC directly:

% Create VideoWriter object with uncompressed format
vidObj = VideoWriter('test_compatible.avi', 'Uncompressed AVI');
% Set FOURCC to CVFP (OpenCV's BGR24 format)
vidObj.FourCC = ['C' 'V' 'F' 'P'];
vidObj.FrameRate = 30;
open(vidObj);

% Write your frames here (example with a random frame)
frame = uint8(rand(192,192,3)*255);
writeVideo(vidObj, frame);

close(vidObj);

This ensures the output AVI has a FOURCC tag that OpenCV can interpret correctly.

内容的提问来源于stack exchange，提问作者RajaKrishnappa