How to hide/disable ffmpeg erros when using OpenCV (python)?

Question

I'm using OpenCV python to capture a video. This is my code

import cv2

cap = cv2.VideoCapture("vid.mp4")
while True:
    flag, frame =  cap.read()

    if not flag:
        cv2.imshow('video', frame)

    if cv2.waitKey(10) == 27:
        break

When a frame is not ready it produces an error like this

enter image description here

or

Truncating packet of size 2916 to 1536
[h264 @ 0x7ffa4180be00] AVC: nal size 2912
[h264 @ 0x7ffa4180be00] AVC: nal size 2912
[h264 @ 0x7ffa4180be00] no frame!

[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7ffa41803000] stream 0, offset 0x14565: partial file

I wanted to find a way to hide this error! I guess that this error is being produced by ffmpeg. Is there any way to hide or disable it?

This error is produced when I call cap.read(). And I also tried to wrap it with try ... except ... but it doesn't work because it doesn't throw any exceptions.

Thanks @mguijarr for your comment. Yes that's true when you are using ffmpeg in command line. But when I want to do it in python, what should I do? — Mehran
– Mehran, Commented Oct 1, 2013 at 1:48

Matias · Accepted Answer · 2019-12-20 13:50:16Z

2

One way to hide ffmpeg errors is by redirecting sterr elsewhere. I found this brilliant example on how to hide the errors.

import ctypes
import io
import os
import sys
import tempfile
from contextlib import contextmanager

import cv2

libc = ctypes.CDLL(None)
c_stderr = ctypes.c_void_p.in_dll(libc, 'stderr')


@contextmanager
def stderr_redirector(stream):
    original_stderr_fd = sys.stderr.fileno()

    def _redirect_stderr(to_fd):
        libc.fflush(c_stderr)
        sys.stderr.close()
        os.dup2(to_fd, original_stderr_fd)
        sys.stderr = io.TextIOWrapper(os.fdopen(original_stderr_fd, 'wb'))

    saved_stderr_fd = os.dup(original_stderr_fd)
    try:
        tfile = tempfile.TemporaryFile(mode='w+b')
        _redirect_stderr(tfile.fileno())
        yield
        _redirect_stderr(saved_stderr_fd)
        tfile.flush()
        tfile.seek(0, io.SEEK_SET)
        stream.write(tfile.read().decode())
    finally:
        tfile.close()
        os.close(saved_stderr_fd)

f = io.StringIO()
with stderr_redirector(f):
    # YOUR CODE HERE

f.close()

answered Dec 20, 2019 at 13:50

Matias

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1 Comment

user9876 Over a year ago

Since Python 3.5, there is standard library support for this as contextlib.redirect_stderr (and contextlib.redirect_stdout). You should probably use that rather than reinventing the wheel - especially such a complicated wheel!

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