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• ffmpeg encoded h.264 frame rate is slightly wrong

  26 juillet 2021, par Timmmm

  I have some code to encode video to an MP4/H.264 file. It's basically the same as this example but encoding to MP4/H.264 with no audio.

  


  I set the time_base/framerate like this :

  


      codecContext->time_base = AVRational{1, 30};
    codecContext->framerate = AVRational{30, 1};
...
    stream->time_base = codecContext->time_base;


  


  And it all works fine (the quality is terrible but that's another issue). However the weird thing is that the frame rate comes out at 30.303030, not 30. VLC and ffmpeg itself both confirmed this.

  


  30.303030 is 30 * 100/99 but I can't see why that would ever happen.

  


  I did find a few other people reporting this odd frame rate here, here and here but nobody seems to have worked out why.

  


  What's up with that frame rate ?

  


  Edit : One of those questions led me to MediaInfo, which actually does report the frame rate as 30.000 FPS, whereas FFMPEG reports 30.30, which is presumably rounded from VLC's value of 30.303030.

  


  Edit 2 : Aha ! I think it's because I encode exactly 100 frames. I wonder if VLC and ffmpeg calculate the frame rate by dividing the total number of frames by the difference between the presentation times of the first and last frames. That would definitely yield 30.303030 :

  


  100 / (99/30 - 0/30) = 30.303030

  


  I confirmed this by encoding 200 frames and the frame rate changes to 30.15. So this is possibly just a bug in the frame rate calculation of VLC and ffmpeg.

  


• FFmpeg overwrite an AVPacket

  26 janvier 2019, par Alejandro Ramírez

  Hello guys I am working with video. I wan to encrypt the I-frames of one video compressed in H.264. So I get the AVPacket from the video and compare if (AVPacket.flags & AV_PKT_FLAG_KEY) to know if the packet has an I frame, but when I try to print the AVPacket.data I don’t have any information to encrypt.
  Where can I get the information regarding to I-frame. ahead a put my code, thank you.

  #include <iostream>
  
  extern "C" {
  
  #include <libavcodec></libavcodec>avcodec.h>
  
  #include <libavformat></libavformat>avformat.h>
  
  #include <libswscale></libswscale>swscale.h>
  
  #include <libavutil></libavutil>avutil.h>
  
  }
  
  #define INBUF_SIZE 4096
  
  //#define AV_INPUT_BUFFER_PADDING_SIZE 32
  
  int main (int argc, char * argv[])
  
  {
  
      //av_register_all();//omit
  
      //revisar el número de argumentos en el video
  
      AVFormatContext *pFormatCtx = NULL;
  
      AVCodec *dec = NULL;
  
      AVCodecContext *pCodecCtx = NULL;
  
      AVStream *st = NULL;
  
      AVDictionary *opts = NULL;
  
      AVFrame *frame;
  
      AVPacket avpkt;
  
      uint8_t inbuf [INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
  
      FILE *f;
  
      int frame_count;
  
      int video_stream_index = -1;
  
  
  
      //av_init_packet (&amp;avpkt);
  
  
  
      memset (inbuf + INBUF_SIZE, 0, AV_INPUT_BUFFER_PADDING_SIZE);
  
      if (avformat_open_input (&amp;pFormatCtx, argv[1], NULL, NULL) != 0)
  
          return -1;
  
      if (avformat_find_stream_info (pFormatCtx, NULL) &lt; 0)
  
          return -1;
  
      //video_stream_index = av_find_best_stream (pFormatCtx,AVMEDIA_TYPE_VIDEO, -1, -1, &amp;dec,0);
  
      video_stream_index = av_find_best_stream (pFormatCtx,AVMEDIA_TYPE_VIDEO, -1, -1, &amp;dec,0);
  
      if ( video_stream_index &lt; 0)
  
          return -1;
  
      std::cout &lt;&lt; "video_stream; " &lt;&lt; video_stream_index &lt;&lt; "\n";
  
      st = pFormatCtx -> streams [video_stream_index];
  
      std::cout &lt;&lt; "number of frames " &lt;&lt; st -> nb_frames &lt;&lt; "\n";
  
      std::cout &lt;&lt; "event_flags " &lt;&lt; st -> event_flags &lt;&lt; "\n";
  
      //pCodecCtx = st -> codec;//deprecated
  
  
  
      dec = avcodec_find_decoder (st -> codecpar -> codec_id);
  
      std::cout &lt;&lt; "codec_id: " &lt;&lt; st -> codecpar -> codec_id &lt;&lt; "\n";
  
      std::cout &lt;&lt; "AV_CODEC_ID_H264: " &lt;&lt; AV_CODEC_ID_H264 &lt;&lt; "\n";
  
      if (!dec)
  
          return -1;
  
      pCodecCtx = avcodec_alloc_context3 (dec);
  
      if (!pCodecCtx)
  
          return -1;
  
      //av_dict_set (&amp;opts, "refcounted_frames", "0", 0);
  
      avcodec_parameters_to_context (pCodecCtx, st -> codecpar);
  
  
  
  
  
      std::cout &lt;&lt; "todo bien \n";    
  
      if (avcodec_open2 (pCodecCtx, dec, &amp;opts) &lt; 0)
  
          return -1;
  
      /*************hasta aqui buen codigo*********************************************/ 
  
      frame = av_frame_alloc ();
  
      if (!frame)
  
          return -1;
  
      av_init_packet (&amp;avpkt);
  
      avpkt.data = NULL;
  
      avpkt.size = 0;
  
      f = fopen (argv[1], "r");
  
      int times = 1;
  
      while (av_read_frame (pFormatCtx, &amp;avpkt) >= 0){
  
          AVPacket oripkt = avpkt;
  
          if (oripkt.stream_index == video_stream_index){
  
              if (oripkt.flags &amp; AV_PKT_FLAG_KEY){
  
                  std::cout &lt;&lt; "times: " &lt;&lt; times ++ &lt;&lt; "\n";
  
                  std::cout &lt;&lt; "avpkt.flags: " &lt;&lt; oripkt.flags &lt;&lt; "\n";
  
                  std::cout &lt;&lt; "tam avpkt.data: " &lt;&lt; sizeof(oripkt.data) &lt;&lt; "\n";
  
                  std::cout &lt;&lt; "tam avpkt.data: " &lt;&lt; oripkt.data &lt;&lt; "\n";
  
                  std::cout &lt;&lt; "oripkt.size: " &lt;&lt; oripkt.size &lt;&lt; "\n";
  
                  std::cout &lt;&lt; "oripkt.side_date_eme: " &lt;&lt; oripkt.side_data_elems &lt;&lt; "\n";
  
                  if (!oripkt.data)
  
                      std::cout &lt;&lt; "no tengo dinero \n";
  
              }
  
          }
  
      }
  
  
  
      std::cout &lt;&lt; "Fin del programa " &lt;&lt; "\n";
  
  }
  
  </iostream>

• Android video conversion [on hold]

  28 octobre 2013, par Jeffrey Grima

  I am a student who is doing his bachelors degree thesis. I need to implement a mobile application to convert video files from one format to another and also using a mobile service to do the same thing and compare how long did it take to perform the same tests but with different approaches.

  After doing some research on how to implement the conversion on the mobile application I have only found FFMpeg. I would like to know if any of you guys have in mind like other video converting API`s for android or something that i should consider before submitting my proposal.