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• failed to set frame number in opencv in cpp

  28 février 2018, par Madhu Nadendla

  I am trying to set frame position of opened videofile using OpenCV in C++ but it returns 0.

  solution 1

  bool success = capture.set(CV_CAP_PROP_POS_FRAMES, noFrame);
  
  
  
  double frameRate = capture.get(CV_CAP_PROP_FPS);

• Ffmpeg output stream closed

  29 novembre 2023, par excalibur

  There is one problem in my code that uses ffmpeg to convert webm videos to mp4 and set duration to 10 seconds, it works well but, sometimes returns "output Stream closed" error. What can be a reason of this problem, and how can i solve it ?

  


  My code :

  


  async convertWebmVideoToMp4(buffer: any) {
    try {
        return new Promise((resolve, reject) => {
            const videoMp4Buffer = Buffer.from(buffer);

            const readable = Readable.from(videoMp4Buffer);

            const outputBuffer = [];

            const outputStream = new Writable({
                write(chunk, encoding, callback) {
                    outputBuffer.push(chunk);
                    callback();
                },
            });

            const ffmpegProcess = ffmpeg()
                .input(readable)
                .outputFormat('mp4')
                .videoCodec('libx264')
                .audioCodec('aac')
                .duration(10)
                .outputOptions([
                    '-movflags frag_keyframe+empty_moov',
                    '-preset ultrafast',
                    '-c:a aac',
                    '-r 30',
                    '-tune fastdecode',
                ])
                .on('end', () => {
                    outputStream.end();
                    const finalOutputBuffer = Buffer.concat(outputBuffer);
                    resolve(finalOutputBuffer);
                })
                .on('error', (err, stdout, stderr) => {
                    console.error(err);
                    reject(err);
                });

            outputStream.on('error', err => {
                this.logger.error(err);
            });

            ffmpegProcess.pipe(outputStream, { end: true });
        });
    } catch (error) {
        throw error;
    }
}


  


  My code should return the buffer of video that type is mp4 and length is 10 seconds, but it returns "output stream closed" error.

  


• 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).