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• FFMPEG Audio/Video out of sync after transcoding video in segments

  25 mai 2017, par Idan

  My system transcodes videos in a very specific way.

  1. separating the video and the audio
  2. transcoding the audio stream
  3. segmenting the video
  4. transcoding each of the segments of the video
  5. concat all segments back to 1 video
  6. merging the new transcoded video and audio back together

  While at 99% of the times the process works as it should and the result is a valid video+audio file. in 1% of the times, I get the video and audio out of sync.

  When investigated the issue I noticed that the sync issue appears in a specific segment/s. Meaning, if the video was sliced into 100 segments, the sync can be ok for the first 50 segments, then something happens and the audio or video gets an offset and goes out of sync. It can occur in any number of segments in one video.

  I guess it’s something to do with timestamps getting lost in the process and segments changing their length in the process but I have no idea how I can overcome it. Open for suggestions.

  The commands I use for each step (paths were shorten and may not match, not real issue there) :

  Segmenting the video :

  ffmpeg -fflags +genpts -i $INPUT_FILE -c copy -map 0:0 -flags -global_header -segment_time 10 -break_non_keyframes 0 -reset_timestamps 1 -segment_list segments.list -segment_list_type ffconcat -write_empty_segments 0 -segment_format mp4 -f segment seg-%d.mp4

  Transcoding audio :

  ffmpeg -i $INPUT_FILE  -vn -c:a aac -threads 1 -ac 2 -b:a 125588 audio.mp4

  Transcoding each of the segments :

  ffmpeg -y -i $f -an -vcodec libx264 -threads 4 -r 30 -pix_fmt yuv420p -crf 20 -preset:v fast -profile:v main -level:v 4.1 transcoded/$f

  Concat segments :

  ffmpeg -y -f concat -i segments.list  -c copy -movflags +faststart file_video.mp4

  Combine video and audio :

  ffmpeg -y -i file_video.mp4 -i file_audio.mp4 -c copy -shortest file_out.mp4

• combine three videos between specific time using ffmpeg

  9 décembre 2014, par Krisi

  I have a situation where I want to combine 3 videos

  The first video (guide.mp4) is the guide and the other 2 are small pieces that need to go inside the "guide"

  In a graphical manner, I have the following

  |         .---1.mp4---.                            .-----2.mp4-----.    |
  
  |---------+-----------+------------guide.mp4-------+---------------+----|
  
  0s        4s          9s                          18s             25s   28s

  the 1.mp4 and 2.mp4 will cover the guide between these times.
  the videos are of same size and all are .mp4 files

  I am pretty sure that this is covered, but I looked a lot, but since I myself am not good in english, I dont know how I could possibly look it up with success...

  Thanks in advance.

• 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>