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• Thread Safety of LibAv/FFMpeg ?

  14 septembre 2022, par user71512

  Is LibAV/FFMpeg thread safe ? For example. Could I read from a file using AVFormatContext* in one thread and decode the read packet it in another with simple additions of mutexes or is the thread safetyness of the library a "don't know don't care" type deal ? I know that libav has basic support for encoder threads but I'm trying more of a blackbox type approach where I break it up into multiple threads (Source -> Decoder -> Filter -> Encoder -> Sink) and trying to understand the complications of such.

  


  Anyone with any experience with ffmpeg and threads and would like to chime in with any other info pertinent to this would also be greatly appreciated.

  


• Cannot suppress ffmpeg output from ruby

  3 avril 2014, par Danny

  I have ruby on rails app that allows users to upload videos. When a video is added, I have a before_save filter that uses ffmpeg to generate a series of thumbnails. The problem is that ffmpeg is producing tons of console output when I'm saving a video item in the rails console, and when I run my tests.

  My environment :

  • Host Machine : OS X 10.9.2
  • Vagrant Box : Ubuntu 10.04.4
  • ffmpeg version : SVN-r0.5.9-4:0.5.9-0ubuntu0.10.04.3
  • ruby version : 1.9.3-p194

  Command I'm running :

  `ffmpeg -v 0 -ss #{timestamp} -i #{video_file.path} -y -f image2 -vcodec mjpeg -vframes 1 -s 640*360 #{thumbnail_path}/thumbnail#{i}.jpg`

  This version of ffmpeg on my VM doesn't seem to care about the "-v 0" option. I've also tried "-loglevel quiet" which causes ffmpeg to error, indicating that the option isn't recognized (both loglevel and v work on my host machine's ffmpeg).

  Tried using both exec() and system(), which both caused execution to hang. Tried to redirecting output to a file by doing :

  `ffmpeg -v 0 -ss #{timestamp} -i #{video_file.path} -y -f image2 -vcodec mjpeg -vframes 1 -s 640*360 #{thumbnail_path}/thumbnail#{i}.jpg > #{thumbnail_path}/output.txt`

  Still see output. Next I tried :

  `ffmpeg -v 0 -ss #{timestamp} -i #{video_file.path} -y -f image2 -vcodec mjpeg -vframes 1 -s 640*360 #{thumbnail_path}/thumbnail#{i}.jpg &> dev/null`

  Still seeing output ! Finally I tried :

  $stdout.reopen("#{thumbnail_path}/output.txt", "w")
  
  $stderr.reopen("#{thumbnail_path}/error.txt", "w")
  
  `ffmpeg -v 0 -ss #{timestamp} -i #{video_file.path} -y -f image2 -vcodec mjpeg -vframes 1 -s 640*360 #{thumbnail_path}/thumbnail#{i}.jpg`
  
  $stdout = STDOUT
  
  $stderr = STDERR

  Holy cow, that worked ! Well, sort of. No more verbose output when running tests, BUT somehow anytime this runs I get kicked out of the rails console.

  Does anyone have a more elegant solution ?

• Convert individual pixel values from RGB to YUV420 and save the frame - C++

  24 mars 2014, par learner

  I have been working with RGB->YUV420 conversion for sometime using the FFmpeg library. Already tried the sws_scale functionality but its not working well. Now, I have decided to convert each pixel individually, using colorspace conversion formulae. So, following is the code that gets me few frames and allows me to access individual R,G,B values of each pixel :

  // Read frames and save first five frames to disk
  
      i=0;
  
      while((av_read_frame(pFormatCtx, &packet)>=0) && (i<5)) 
  
      {
  
          // Is this a packet from the video stream?
  
          if(packet.stream_index==videoStreamIdx) 
  
          {   
  
              /// Decode video frame            
  
              avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
  
  
  
              // Did we get a video frame?
  
              if(frameFinished) 
  
              {
  
                  i++;
  
                  sws_scale(img_convert_ctx, (const uint8_t * const *)pFrame->data,
  
                            pFrame->linesize, 0, pCodecCtx->height,
  
                            pFrameRGB->data, pFrameRGB->linesize);
  
  
  
                  int x, y, R, G, B;
  
                  uint8_t *p = pFrameRGB->data[0];
  
                  for(y = 0; y < h; y++)
  
                  {  
  
                      for(x = 0; x < w; x++) 
  
                      {
  
                          R = *p++;
  
                          G = *p++;
  
                          B = *p++;
  
                          printf(" %d-%d-%d ",R,G,B);
  
                      }
  
                  }
  
  
  
                  SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, i);
  
              }
  
          }
  
  
  
          // Free the packet that was allocated by av_read_frame
  
          av_free_packet(&packet);
  
      }

  I read online that to convert RGB->YUV420 or vice-versa, one should first convert to YUV444 format. So, its like : RGB->YUV444->YUV420. How do I implement this in C++ ?

  Also, here is the SaveFrame() function used above. I guess this will also have to change a little since YUV420 stores data differently. How to take care of that ?

  void SaveFrame(AVFrame *pFrame, int width, int height, int iFrame)
  
  {
  
      FILE *pFile;
  
      char szFilename[32];
  
      int  y;
  
  
  
      // Open file
  
      sprintf(szFilename, "frame%d.ppm", iFrame);
  
      pFile=fopen(szFilename, "wb");
  
      if(pFile==NULL)
  
          return;
  
  
  
      // Write header
  
      fprintf(pFile, "P6\n%d %d\n255\n", width, height);
  
  
  
      // Write pixel data
  
      for(y=0; ydata[0]+y*pFrame->linesize[0], 1, width*3, pFile);
  
  
  
      // Close file
  
      fclose(pFile);
  
  }

  Can somebody please suggest ? Many thanks !!!