使用Python子进程管理FFMPEG冻结 [英] Piping to FFMPEG with Python subprocess freezes

问题描述

使用以下代码，我可以使用Python，Numpy和FFMPEG二进制文件将视频框架管理到FFMPEG：

With the following code, I am able to pipe frames of a video to FFMPEG using Python, Numpy and the FFMPEG binaries:

from __future__ import print_function
import subprocess
import numpy as np
import sys

npshape = [480, 480]
cmd_out = ['ffmpeg',
           '-y', # (optional) overwrite output file if it exists
           '-f', 'rawvideo',
           '-vcodec','rawvideo',
           '-s', '%dx%d'%(npshape[1], npshape[0]), # size of one frame
           '-pix_fmt', 'rgb24',
           '-r', '24', # frames per second
           '-i', '-', # The input comes from a pipe
           '-an', # Tells FFMPEG not to expect any audio
           '-vcodec', 'mpeg4',
           'output.mp4']

fout = subprocess.Popen(cmd_out, stdin=subprocess.PIPE, stderr=subprocess.PIPE).stdin

for i in range(24*40):
    if i%(24)==0: 
        print('%d'%(i/24), end=' ')
        sys.stdout.flush()

    fout.write((np.random.random(npshape[0]*npshape[1]*3)*128).astype('uint8').tostring())

fout.close()

如果我写了不到37秒的框架，但如果我尝试写更多的东西，代码就挂起来了。这个行为的根本原因是什么？

This works fine if I write anything less than 37 seconds worth of frames, but if I try to write anything more, the code just hangs. What is the underlying cause for this behaviour? How can I fix it?

推荐答案

一个非常可能的罪魁祸首是令人厌恶的恶臭 subprocess.Popen 行。不仅你不理会它的返回值 - 你绝对不会这样做，以确保子流程完成某一点和/或检查其退出代码 - 你也使 stderr 一个管道，但从来没有读过 - 所以当缓冲区填满时，进程必须挂起。

A highly probable culprit is the disgustingly stinky subprocess.Popen line. Not only you ignore its return value - which you must never do, in order to ensure the subprocess' completion by certain point and/or check its exit code - you also make stderr a pipe but never read it - so the process must be hanging when its buffer fills.

这应该修复它：

p = subprocess.Popen(cmd_out, stdin=subprocess.PIPE)
fout = p.stdin

<...>

fout.close()
p.wait()
if p.returncode !=0: raise subprocess.CalledProcessError(p.returncode,cmd_out)

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