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• the same audio have different length using different tools (librosa,ffprobe)

  29 octobre 2019, par Fathy Eltanany

  I want to measure an audio file’s duration.
  I’m using two different tools and got different values.

  1. ffprobe :
     I’m using this line to get duration using ffprobe

   ffprobe -i audio.m4a -show_entries format=duration -v quiet -of csv="p=0"

  result :780.320000 seconds
  2. Librosa (python library)
  and using this line to get duartion using librosa

  y1, sr1 = librosa.load(audio_path, sr=44100)
  
  librosa.get_duration(y1, sr1) * 1000

  result 780329.7959183673 milliseconds

  Does anyone know what’s causing the difference ?

• x264 encoding with libav

  25 mars 2014, par user3453729

  I try to encode raw image data to x264 with libav :

  AVPacket vpkt = { 0 };
  
  av_init_packet(&vpkt);
  
  
  
  int got;
  
  int ret = avcodec_encode_video2(vcodec, &vpkt, frameyuv.get(), &got);
  
  
  
  if (!ret && got && vpkt.size) {
  
      if (vpkt.pts != AV_NOPTS_VALUE) {
  
          vpkt.pts = av_rescale_q(vpkt.pts, vcodec->time_base, videost->time_base);
  
      }
  
      if (vpkt.dts != AV_NOPTS_VALUE) {
  
          vpkt.dts = av_rescale_q(vpkt.dts, vcodec->time_base, videost->time_base);
  
      }
  
  
  
      vpkt.stream_index = videost->index;
  
  
  
      if(vcodec->coded_frame->key_frame) {
  
          vpkt.flags |= AV_PKT_FLAG_KEY;
  
      }
  
      /* -> will return -22 if max_b_frames > 0 */
  
      ret = av_interleaved_write_frame(oc, &vpkt);
  
  }

  Runs fine when vcodec->max_b_frames is set to 0, but on any other value av_interleaved_write_frame returns -22 (invalid argument).

  /* will fail */
  
  c->max_b_frames = 3;
  
  /* -> ok*/
  
  c->max_b_frames = 0;

  Why ? Am i missing something ?

  Codec options are

  AVDictionary *opts = NULL;
  
  av_dict_set(&opts, "vprofile", "baseline", 0);
  
  
  
  /* ... */
  
  c->codec_type = AVMEDIA_TYPE_VIDEO;
  
  c->bit_rate = 500 * 1000;
  
  c->width = VideoWidth;
  
  c->height = VideoHeight;
  
  c->time_base.den = fps;
  
  c->time_base.num = 1;
  
  c->pix_fmt = AV_PIX_FMT_YUV420P;

  Container format is mp4.

• Using ffmpeg with Imagick

  19 mars 2014, par user3240613

  I am trying to generate thumbnails from videos in imagick, by extracting a single frame from them, using the ffmpeg application.

  I use this code currently :

  $image->newPseudoImage( null, null, 'ffmpeg:video.mp4[50]');

  It works. But it is not an ideal solution.
  I want to generate the thumbnail from a 50% position in the video, but i do not know how long the video is, so I can't do something like ffmpeg:video.mp4[500001]. And even if I knew the length, I still couldn't do it because running this ffmpeg:video.mp4[1000] takes almost 20 seconds to execute (ffmpeg:video.mp4[50] takes one or two seconds only).

  When i try to add some extra parameters like "ffmpeg:video.mp4[50] -ss 50" it returns error.

  The only other option I can think of, is using the exec to directly execute the ffmpeg command in the shell like "ffmpeg -i video.mp4 -vframes 1 -o screenshot.jpg" or something like that. Would that actually be more efficient solution than using the newpseudoimage method ?