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• FFmpeg - avcodec_receive_frame returns 0 but frames are invalid

  9 juillet 2019, par Jinx

  I’ve been trying to extract images from videos, but not those with PNG codec. My code works fine with those with JPEG. avcodec_receive_frame worked successfully but the data of the frames was like trash ? Do I have to do something special to demuxing when dealing with PNG ?

  Exception thrown at 0x00007FFF7DF34B9A (msvcrt.dll) in Program.exe : 0xC0000005 : Access violation reading location 0x00000000000003F0 when calling avcodec_send_frame in my saveImage function, which means I was accessing invalid or unalloacted memory I guess. How this happened ?

  

  Just suppose all the function calls returned 0 until exception thrown.

  Decoding :

  bool ImageExtractor::decode() {
  
      // some other code here
  
      ret = avcodec_send_packet(codec_ctx, packet); // returned 0
  
      ret = avcodec_receive_frame(codec_ctx, frame); // returned 0
  
      if (ret == 0) {
  
          if (count >= target_frame) {
  
              snprintf(buf, sizeof(buf), "%s/%d.png", destination.toLocal8Bit().data(), count);
  
              saveImage(frame, buf); // a function that writes images on disk 
  
         }
  
      // some other code here
  
  }
  
  
  
  
  
  bool ImageExtractor::saveImage(AVFrame *frame, char *destination) {
  
       AVFormatContext *imgFmtCtx = avformat_alloc_context();
  
       pFormatCtx->oformat = av_guess_format("mjpeg", NULL, NULL);
  
  
  
       // some other code here
  
  
  
      if (!frame)
  
          std::cout << "invalid frame \n";
  
  
  
      if (!imgCodecCtx) // AV_CODEC_ID_MJPEG(7)
  
          std::cout << "invalid codec ctx \n";
  
  
  
      ret = avcodec_send_frame(imgCodecCtx, frame); // everything stopped here
  
  }

  Demuxing :

  avformat_open_input(&format_ctx, source.toLocal8Bit(), nullptr, nullptr);
  
  vsi = av_find_best_stream(format_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, nullptr, 0);
  
  codec_par = avcodec_parameters_alloc();
  
  avcodec_parameters_copy(codec_par, format_ctx->streams[vsi]->codecpar);
  
  
  
  AVCodec* codec = avcodec_find_decoder(codec_par->codec_id); // AV_CODEC_ID_PNG(61)
  
  codec_ctx = avcodec_alloc_context3(codec);
  
  avcodec_parameters_to_context(codec_ctx, codec_par);
  
  avcodec_parameters_free(&codec_par);
  
  avcodec_open2(codec_ctx, codec, 0);

• subprocess.py returns a File Not Found error

  7 avril 2021, par wasneeplus

  As part of a video analysis script I wanted to find the duration of a video file. For this I found the script offered in the first answer to this question : How to get the duration of a video in Python ?

  


  import subprocess

def get_length(filename):
    result = subprocess.run(["ffprobe", "-v", "error", "-show_entries",
                             "format=duration", "-of",
                             "default=noprint_wrappers=1:nokey=1", filename],
        stdout=subprocess.PIPE,
        stderr=subprocess.STDOUT)
    return float(result.stdout)


  


  This code works fine when my friend runs it in a Jupyter server environment, but when I try it on my laptop the trouble starts.

  


  When I imput the following filename into the function :

  


  filename = "C:\\Users\\benja\\OneDrive - De Haagse Hogeschool\\Onderzoeken 3\\8V.mp4"


  


  I get the following error :

  


      Traceback (most recent call last):&#xA;  File "c:/Users/benja/OneDrive - De Haagse Hogeschool/Onderzoeken 3/python_script.py", line 9, in <module>&#xA;    num_of_frames = math.floor((pf.get_length(filename) - 1)) * 30&#xA;  File "c:\Users\benja\OneDrive - De Haagse Hogeschool\Onderzoeken 3\python_funcs.py", line 21, in get_length&#xA;    stderr=subprocess.STDOUT)&#xA;  File "C:\Users\benja\Anaconda3\lib\subprocess.py", line 466, in run&#xA;    with Popen(*popenargs, **kwargs) as process:&#xA;  File "C:\Users\benja\Anaconda3\lib\subprocess.py", line 769, in __init__&#xA;    restore_signals, start_new_session)&#xA;  File "C:\Users\benja\Anaconda3\lib\subprocess.py", line 1172, in _execute_child&#xA;    startupinfo)&#xA;FileNotFoundError: [WinError 2] Het systeem kan het opgegeven bestand niet vinden&#xA;</module>

  


  I do realise that my problem is almost identical to that of several other questions on here. However, their solutions don't seem to work for me. I have tried to :

  


  

  • Add the location of ffmpeg-win64-4.2.2.exe to the Path system variable.

  


  • Add the location of python.exe to the ComSpec system variable.

  


  • Put the videofile in the same directory as the script.

  


  


  I would be most grateful if someone could point me in the right direction. Thank you in advance.

  


• Why process output reader returns null data ? When same code in other process works correct

  26 août 2019, par Владимир Водов

  I try to get output from ffmpeg process but cant get output.
  In another processes and commands it works correctly but output returns immideately when start !

          using (var process = new Process())
  
          {
  
              process.StartInfo = new ProcessStartInfo()
  
              {
  
                  FileName = LinkHelper.IPFS_PATH,
  
                  Arguments = cmd,
  
                  UseShellExecute = false,
  
                  CreateNoWindow = true,
  
                  RedirectStandardOutput = true
  
              };
  
  
  
              process.ErrorDataReceived += FfmpegErrorRecieved;
  
              process.Start();
  
  
  
              using (StreamReader reader = process.StandardOutput)
  
              {
  
                  string output = await reader.ReadToEndAsync();
  
                  Console.WriteLine(output);
  
              }               
  
              process.WaitForExit();
  
          }

  Before output handle !

  Update :
  As szatmary said ffmpeg use output error instead standard output, so when you initialize process.StandartInfo don’t forget initialize property "RedirectStandardError" to TRUE !

  Here is correct code :

  private async Task DetectFFmpegCamerasAsync()
  
      {
  
          var cmd = "-list_devices true -f dshow -i dummy";
  
          using (var process = new Process())
  
          {
  
              process.StartInfo = new ProcessStartInfo()
  
              {
  
                  FileName = LinkHelper.FFMPEG_PATH,
  
                  Arguments = cmd,
  
                  UseShellExecute = false,
  
                  CreateNoWindow = true,
  
                  RedirectStandardError = true
  
              };
  
  
  
              process.Start();
  
  
  
              using (StreamReader reader = process.StandardError)
  
              {
  
                  string output = await reader.ReadToEndAsync();
  
                  Console.WriteLine($"Camera detection output: \n {output}");
  
              }               
  
              process.WaitForExit();
  
          }
  
      }