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• How to stop perl buffering ffmpeg output

  4 février 2017, par Sebastian King

  I am trying to have a Perl program process the output of an ffmpeg encode, however my test program only seems to receive the output of ffmpeg in periodic chunks, thus I am assuming there is some sort of buffering going on. How can I make it process it in real-time ?

  My test program (the tr command is there because I thought maybe ffmpeg’s carriage returns were causing perl to see one big long line or something) :

  #!/usr/bin/perl
  
  
  
  $i = "test.mkv"; # big file, long encode time
  
  $o = "test.mp4";
  
  
  
  open(F, "-|", "ffmpeg -y -i '$i' '$o' 2>&1 | tr '\r' '\n'")
  
          or die "oh no";
  
  
  
  while(<f>) {
  
          print "A12345: $_"; # some random text so i know the output was processed in perl
  
  }
  
  </f>

  Everything works fine when I replace the ffmpeg command with this script :

  #!/bin/bash
  
  
  
  echo "hello";
  
  
  
  for i in `seq 1 10`; do
  
          sleep 1;
  
          echo "hello $i";
  
  done
  
  
  
  echo "bye";

  When using the above script I see the output each second as it happens. With ffmpeg it is some 5-10 seconds or so until it outputs and will output sometimes 100 lines each output.

  I have tried using the program unbuffer ahead of ffmpeg in the command call but it seems to have no effect. Is it perhaps the 2>&1 that might be buffering ?
  Any help is much appreciated.

  If you are unfamiliar with ffmpeg’s output, it outputs a bunch of file information and stuff to STDOUT and then during encoding it outputs lines like

  frame=  332 fps= 93 q=28.0 size=     528kB time=00:00:13.33 bitrate= 324.2kbits/s speed=3.75x

  which begin with carriage returns instead of new lines (hence tr) on STDERR (hence 2>&1).

• Adding audio to an OpenCV video

  21 février 2019, par pcrunn

  I’m trying to make a script that automates the process of making a video and i’m stuck on the part that i add the audio, I’m not sure if i’ll need to use ffmpeg for that and if so, i’m not sure how to use it the proper way to work with OpenCV (Which I use for rendering the video).

  Thanks for your time, Here is my current code (that I found from a website)

  # Creating the video... The worst part. Literally
  
  
  
  width = 1920
  
  height = 1080
  
  FPS = 30
  
  length = 0
  
  
  
  for song in songs:
  
      length+=song.info.length
  
  
  
  length = int(length)
  
  
  
  fourcc = VideoWriter_fourcc(*'MP42')
  
  video = VideoWriter('result.avi', fourcc, float(FPS), (width, height))
  
  for _ in range(FPS*length):
  
      frame = np.random.randint(0, 256,
  
                                (height, width, 3),
  
                                dtype=np.uint8)
  
      video.write(frame)
  
  
  
  video.release()

• How to cut out first 5 seconds with youtube_dl and ffmpeg in python

  23 février 2021, par SvenXP

  i have a python script to download and save a MP3 and i would like to add code to cut out 5 seconds from the beginning of the MP3.

  


  def download():
    ydl_opts = {
        'format': 'bestaudio/best',
        'outtmpl': 'c:/MP3/%(title)s.%(ext)s',
        'cookiefile': 'cookies.txt',
        'postprocessors': [{
            'key': 'FFmpegExtractAudio',
            'preferredcodec': 'mp3',
            'preferredquality': '192',
        }],
    }
    with youtube_dl.YoutubeDL(ydl_opts) as ydl:
        ydl.download([inpYTLinkSong.get()])


  


  I found some code for command line to cut x seconds :

  


  ffmpeg -ss 00:00:15.00 -i "OUTPUT-OF-FIRST URL" -t 00:00:10.00 -c copy out.mp4

  


  So i think i have to get the -ss part into the postprocessor part in my script, something like :

  


  'postprocessors': [{
        'key': 'FFmpegExtractAudio',
        'preferredcodec': 'mp3',
        'preferredquality': '192',
        'ss':'00:00:05.00'
    }],


  


  But of course its not working with 'ss' or 'duration' (found in ffmpeg docu).

  


  So any ideas what i have to put there instead of 'ss' ?