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• Treating a video stream as playback with pausing

  21 janvier 2020, par kealist

  I am working on an application that streams multiple h264 video streams to a video wall. I am using libav/ffmpeg libs to stream multiple video files at once from inside the application. The application will control playback speed, seeking, pausing, resuming, stopping, and the video wall will only be receiving udp streams.

  I want to implement streaming such that if the videos are paused, the same frame is sent continually so that it looks as if it is a video window in a paused state.

  How can i insert copies of the same h264 frame into the stream so that it does not mess up sending of later frames ?

  My code is almost an exact port of transcoding.c from ffmpeg.exe. Planning on retaining a copy of the last frame/packet, and when paused to send this. Is this likely to function properly, or should I approach this a different way.

  while (true)
  
  {
  
      if (paused) {
  
          // USE LAST PACKET
  
      }
  
      else 
  
      {
  
          if ((ret = ffmpeg.av_read_frame(ifmt_ctx, &packet)) < 0)
  
              break;
  
      }
  
      stream_index = packet.stream_index;
  
  
  
      type = ifmt_ctx->streams[packet.stream_index]->codec->codec_type;
  
      Console.WriteLine("Demuxer gave frame of stream_index " + stream_index);
  
      if (filter_ctx[stream_index].filter_graph != null)
  
      {
  
          Console.WriteLine("Going to reencode&filter the frame\n");
  
          frame = ffmpeg.av_frame_alloc();
  
          if (frame == null)
  
          {
  
              ret = ffmpeg.AVERROR(ffmpeg.ENOMEM);
  
              break;
  
          }
  
  
  
          packet.dts = ffmpeg.av_rescale_q_rnd(packet.dts,
  
                  ifmt_ctx->streams[stream_index]->time_base,
  
                  ifmt_ctx->streams[stream_index]->codec->time_base,
  
                  AVRounding.AV_ROUND_NEAR_INF | AVRounding.AV_ROUND_PASS_MINMAX);
  
          packet.pts = ffmpeg.av_rescale_q_rnd(packet.pts,
  
                  ifmt_ctx->streams[stream_index]->time_base,
  
                  ifmt_ctx->streams[stream_index]->codec->time_base,
  
                  AVRounding.AV_ROUND_NEAR_INF | AVRounding.AV_ROUND_PASS_MINMAX);
  
  
  
  
  
  
  
          if (type == AVMediaType.AVMEDIA_TYPE_VIDEO)
  
          {
  
  
  
              ret = ffmpeg.avcodec_decode_video2(stream_ctx[packet.stream_index].dec_ctx, frame,
  
                  &got_frame, &packet); 
  
  
  
          }
  
          else
  
          {
  
              ret = ffmpeg.avcodec_decode_audio4(stream_ctx[packet.stream_index].dec_ctx, frame,
  
                  &got_frame, &packet);
  
          }
  
          if (ret < 0)
  
          {
  
              ffmpeg.av_frame_free(&frame);
  
              Console.WriteLine("Decoding failed\n");
  
              break;
  
          }
  
          if (got_frame != 0)
  
          {
  
              frame->pts = ffmpeg.av_frame_get_best_effort_timestamp(frame);
  
              ret = filter_encode_write_frame(frame, (uint)stream_index);
  
              // SAVE LAST FRAME/PACKET HERE
  
              ffmpeg.av_frame_free(&frame);
  
              if (ret < 0)
  
                  goto end;
  
          }
  
          else
  
          {
  
              ffmpeg.av_frame_free(&frame);
  
          }
  
      }
  
      else
  
      {
  
          /* remux this frame without reencoding */
  
          packet.dts = ffmpeg.av_rescale_q_rnd(packet.dts,
  
                  ifmt_ctx->streams[stream_index]->time_base,
  
                  ofmt_ctx->streams[stream_index]->time_base,
  
                  AVRounding.AV_ROUND_NEAR_INF | AVRounding.AV_ROUND_PASS_MINMAX);
  
          packet.pts = ffmpeg.av_rescale_q_rnd(packet.pts,
  
                  ifmt_ctx->streams[stream_index]->time_base,
  
                  ofmt_ctx->streams[stream_index]->time_base,
  
                  AVRounding.AV_ROUND_NEAR_INF | AVRounding.AV_ROUND_PASS_MINMAX);
  
          ret = ffmpeg.av_interleaved_write_frame(ofmt_ctx, &packet);
  
          if (ret < 0)
  
              goto end;
  
      }
  
      ffmpeg.av_free_packet(&packet);
  
  }

• Decoder return of av_find_best_stream vs. avcodec_find_decoder

  7 octobre 2016, par Jason C

  The docs for libav’s av_find_best_stream function (libav 11.7, Windows, i686, GPL) specify a parameter that can be used to receive a pointer to an appropriate AVCodec :

  decoder_ret - if non-NULL, returns the decoder for the selected stream

  There is also the avcodec_find_decoder function which can find an AVCodec given an ID.

  However, the official demuxing + decoding example uses av_find_best_stream to find a stream, but chooses to use avcodec_find_decoder to find the codec in lieu of av_find_best_stream’s codec return parameter :

  ret = av_find_best_stream(fmt_ctx, type, -1, -1, NULL, 0);
  
  ...
  
  stream_index = ret;
  
  st = fmt_ctx->streams[stream_index];
  
  ...
  
  /* find decoder for the stream */
  
  dec = avcodec_find_decoder(st->codecpar->codec_id);

  As opposed to something like :

  ret = av_find_best_stream(fmt_ctx, type, -1, -1, &dec, 0);

  My question is pretty straightforward : Is there a difference between using av_find_best_stream’s return parameter vs. using avcodec_find_decoder to find the AVCodec ?

  The reason I ask is because the example chose to use avcodec_find_decoder rather than the seemingly more convenient return parameter, and I can’t tell if the example did that for a specific reason or not. The documentation itself is a little spotty and disjoint, so it’s hard to tell if things like this are done for a specific important reason or not. I can’t tell if the example is implying that it "should" be done that way, or if the example author did it for some more arbitrary personal reason.

• Révision 22576 : Suppression d’un code mort, mort depuis longtemps.

  23 novembre 2015, par marcimat@rezo.net

  Déjà signalé par rénato en 2006 (http://spip-dev.rezo.narkive.com/mHG6Qlt6/small-bug-in-phraser-champs),
  ce if () vaut toujours true. On l’enlève du coup.

  Précisément, cela vient tout à l’origine d’une erreur lors d’un passage de ereg à strpos en r3964, strpos ne comprennant
  pas l’expression [0-9]. Redmine n’affichant plus les révisions / diff avant la révision 9600, je mais des liens git :
  https://git.spip.net/rCSPIP57683758d1ebe3a6ffa5095035e88768b14d86f2 ou https://github.com/spip/SPIP/commit/57683758d1ebe3a6ffa5095035e88768b14d86f2

  C’était à une époque ou la constante NOM_DE_CHAMP coupait les balises nommées en héxadécimal en 2 : `#FF3300` par exemple avait `#FF` en balise, et
  le reste était dans la suite, et il fallait potentiellement recorriger ce problème.
  Cette constante a été améliorée ensuite, pas longtemps après en r4407, pour attraper tout le contenu de ces balises :
  https://github.com/spip/SPIP/commit/fef3ff59fad8896d1c8946346eac1b47befc588f

  Donc, en théorie, depuis cette date, le test if() n’a plus lieu d’être.