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• ffmpeg C API - creating queue of frames

  25 mai 2016, par lupod

  I have created using the C API of ffmpeg a C++ application that reads frames from a file and writes them to a new file. Everything works fine, as long as I write immediately the frames to the output. In other words, the following structure of the program outputs the correct result (I put only the pseudocode for now, if needed I can also post some real snippets but the classes that I have created for handling the ffmpeg functionalities are quite large) :

  AVFrame* frame = av_frame_alloc();
  
  int got_frame;
  
  
  
  // readFrame returns 0 if file is ended, got frame = 1 if
  
  // a complete frame has been extracted
  
  while(readFrame(inputfile,frame, &got_frame)) {
  
    if (got_frame) {
  
      // I actually do some processing here
  
      writeFrame(outputfile,frame);
  
    }
  
  }
  
  av_frame_free(&frame);

  The next step has been to parallelize the application and, as a consequence, frames are not written immediately after they are read (I do not want to go into the details of the parallelization). In this case problems arise : there is some flickering in the output, as if some frames get repeated randomly. However, the number of frames and the duration of the output video remains correct.

  What I am trying to do now is to separate completely the reading from writing in the serial implementation in order to understand what is going on. I am creating a queue of pointers to frames :

  std::queue queue;
  
  int ret = 1, got_frame;
  
  while (ret) {
  
    AVFrame* frame = av_frame_alloc();
  
    ret = readFrame(inputfile,frame,&got_frame);
  
    if (got_frame) 
  
      queue.push(frame);
  
  }

  To write frames to the output file I do :

  while (!queue.empty()) {
  
    frame = queue.front();
  
    queue.pop();
  
    writeFrame(outputFile,frame);
  
    av_frame_free(&frame);
  
  }

  The result in this case is an output video with the correct duration and number of frames that is only a repetition of the last 3 (I think) frames of the video.

  My guess is that something might go wrong because of the fact that in the first case I use always the same memory location for reading frames, while in the second case I allocate many different frames.

  Any suggestions on what could be the problem ?

• How to explain the given ffplay C code snippet ?

  20 juillet 2015, par Jerikc XIONG

  The following code snippet is from ffplay :

  static int decoder_decode_frame(Decoder *d, AVFrame *frame, AVSubtitle *sub) {
  
      int got_frame = 0;
  
  
  
      do {
  
          int ret = -1;
  
  
  
          if (d->queue->abort_request)
  
              return -1;
  
  
  
          if (!d->packet_pending || d->queue->serial != d->pkt_serial) {
  
              AVPacket pkt;
  
              do {
  
                  if (d->queue->nb_packets == 0)
  
                      SDL_CondSignal(d->empty_queue_cond);
  
                  if (packet_queue_get(d->queue, &pkt, 1, &d->pkt_serial) < 0)
  
                      return -1;
  
                  if (pkt.data == flush_pkt.data) {
  
                      avcodec_flush_buffers(d->avctx);
  
                      d->finished = 0;
  
                      d->next_pts = d->start_pts;
  
                      d->next_pts_tb = d->start_pts_tb;
  
                  }
  
              } while (pkt.data == flush_pkt.data || d->queue->serial != d->pkt_serial);
  
              av_free_packet(&d->pkt);
  
              d->pkt_temp = d->pkt = pkt;
  
              d->packet_pending = 1;
  
          }
  
  
  
          switch (d->avctx->codec_type) {
  
              case AVMEDIA_TYPE_VIDEO:
  
                  ret = avcodec_decode_video2(d->avctx, frame, &got_frame, &d->pkt_temp);
  
                  if (got_frame) {
  
                      if (decoder_reorder_pts == -1) {
  
                          frame->pts = av_frame_get_best_effort_timestamp(frame);
  
                      } else if (decoder_reorder_pts) {
  
                          frame->pts = frame->pkt_pts;
  
                      } else {
  
                          frame->pts = frame->pkt_dts;
  
                      }
  
                  }
  
                  break;
  
              case AVMEDIA_TYPE_AUDIO:
  
                  ret = avcodec_decode_audio4(d->avctx, frame, &got_frame, &d->pkt_temp);
  
                  if (got_frame) {
  
                      AVRational tb = (AVRational){1, frame->sample_rate};
  
                      if (frame->pts != AV_NOPTS_VALUE)
  
                          frame->pts = av_rescale_q(frame->pts, d->avctx->time_base, tb);
  
                      else if (frame->pkt_pts != AV_NOPTS_VALUE)
  
                          frame->pts = av_rescale_q(frame->pkt_pts, av_codec_get_pkt_timebase(d->avctx), tb);
  
                      else if (d->next_pts != AV_NOPTS_VALUE)
  
                          frame->pts = av_rescale_q(d->next_pts, d->next_pts_tb, tb);
  
                      if (frame->pts != AV_NOPTS_VALUE) {
  
                          d->next_pts = frame->pts + frame->nb_samples;
  
                          d->next_pts_tb = tb;
  
                      }
  
                  }
  
                  break;
  
              case AVMEDIA_TYPE_SUBTITLE:
  
                  ret = avcodec_decode_subtitle2(d->avctx, sub, &got_frame, &d->pkt_temp);
  
                  break;
  
          }
  
  
  
          if (ret < 0) {
  
              d->packet_pending = 0;
  
          } else {
  
              d->pkt_temp.dts =
  
              d->pkt_temp.pts = AV_NOPTS_VALUE;
  
              if (d->pkt_temp.data) {
  
                  if (d->avctx->codec_type != AVMEDIA_TYPE_AUDIO)
  
                      ret = d->pkt_temp.size;
  
                  d->pkt_temp.data += ret;
  
                  d->pkt_temp.size -= ret;
  
                  if (d->pkt_temp.size <= 0)
  
                      d->packet_pending = 0;
  
              } else {
  
                  if (!got_frame) {
  
                      d->packet_pending = 0;
  
                      d->finished = d->pkt_serial; // FLAG
  
                  }
  
              }
  
          }
  
      } while (!got_frame && !d->finished);
  
  
  
      return got_frame;
  
  }

  It’s difficult for me to understand the following code :

  d->finished = d->pkt_serial; // FLAG

  Can anyone help me ?

  Thanks.

• Set H264 high quality encoding in FFMPEG

  25 décembre 2015, par hiitiger

  I’m working on a function, using ffmpeg to encode a serial images to a MP4 file.
  I use H264 codec, and want to have a high quality output.

  Other things now goes well, but the video quality looks obvious loss.

  After some Google, I set codec context like this

  video_st = avformat_new_stream(oc, 0); 
  
  ....
  
  pVideoCodec  = avcodec_find_encoder(fmt->video_codec); //AV_CODEC_ID_H264
  
  ....
  
  avcodec_get_context_defaults3(video_st->codec, pVideoCodec);
  
  video_st->codec->codec_id =  fmt->video_codec;
  
  video_st->codec->width = m_outWidth;
  
  video_st->codec->height = m_outHeight;
  
  video_st->codec->time_base.den = m_fps;
  
  video_st->codec->time_base.num = 1;
  
  video_st->codec->pix_fmt = AV_PIX_FMT_YUV420P;
  
  
  
  AVDictionary *param = nullptr;
  
  av_dict_set(&param, "qp", "0", 0);
  
  av_dict_set(&param, "preset", "medium", 0);

  I also try to set bit_rate. But all turned out to be bad.

  I want a visually loss less output video.

  So what parameters should I set or anything else did I missed ?