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• how av_freep in ffmpeg works ?

  11 août 2016, par Billy Bob

  I have some questions regarding void pointer in C. I found out the following code, part of ffmpeg that I couldn’t understand... Could someone explain to me how it works ?

  void av_freep(void *arg)
  
  {
  
          void *val;
  
  
  
          memcpy(&val, arg, sizeof(val));
  
          memcpy(arg, &(void *){ NULL }, sizeof(val));
  
          av_free(val);
  
  }

  and later it’s called like this :

  char *str;
  
  av_freep(&str);

  my questions :

  1. how passing address of str (&str) doesn’t trigger "incompatible type" warning when it’s compiled ? Shouldn’t &str type is become char** ?
  2. what’s the meaning of &(void *) NULL in one of memcpy parameter ?
  3. why it’s trying to free "val" which is not allocated ?

  Thanks !

• FFMPEG API real-time encoding, skipped frames

  24 octobre 2015, par Paul Knopf

  I am using FFMPEG to encode live video. If I can’t keep up with the live encoding, I need to drop frames. Currently, I just skip the encoding of a raw frame, and the PTS for the next frame is the same. Meaning, even if there is a skipped frame, the pts is always 1,2,3,4,5,6....

  This causes the playback of the file to skip ahead in time. Instead, I would like to add blank frames to the muxed file. I tried making PTS indicate a skipped frame by setting it to 1,2,4,5,8,...., but the FFMPEG h264 encoders depend on PTS being consecutive.

  How do I pass empty data to my mp4/container, indicating a blank space in time, so that decoders won’t appear to jump ahead in time, but instead just see a delay/freeze.

• Syncronize RTSP with computer time

  17 décembre 2012, par Dídac Pérez

  I am successfully using libav to receive the video stream from an RTSP network source. The point is that I need to syncronize my computer's time with the video capturing, meaning that I need to know which datetime of my computer corresponds to the first frame (pts = 0). My API calls are the following ones :

  av_register_all()
  
  avcodec_register_all()
  
  avformat_network_init()
  
  avformat_open_input()
  
  avformat_find_stream_info()
  
  av_read_play()
  
  loop
  
    av_init_packet()
  
    av_read_frame()
  
    [...]
  
    av_free_packet
  
  end loop

  With the calls above, I successfully read frames, but I do need to know how can I know the exact absolute datetime that corresponds to the first frame, since it has a pts of 0. Maybe I can use a time() function or GetSystemTime (I am using Windows) between two calls of the above, but do not really know how libav works. I will appreciate your help,

  Kind regards,

  Dídac Pérez