MKV Formatting

[dishmael]

When running the remuxing code against an MP4 as input and MKV as output, the media plays in VLC but the progress bar does not update. If I run ffmpeg -i input.mp4 -map 0 -c copy output.mkv, the file works fine in VLC. I've been going nuts trying to figure out what's missing to get VLC to play the output correctly. Any pointers?

[leandromoreira]

are you adding any extra opts like

    av_dict_set(&opts, "movflags", "frag_keyframe+empty_moov+default_base_moof", 0);

?

[dishmael]

No, I set the dictionary to NULL. Here's the code base w/o a lot of the error checking for brevity.

// remux.c
#include <stdlib.h>
#include <stdio.h>
#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>

int main(int argc, char **argv)
{
  AVFormatContext *input_format_context = NULL, *output_format_context = NULL;
  AVDictionary *opts = NULL;
  AVPacket packet;
  const char *in_filename, *out_filename;
  int ret, i;

  if (argc < 3)
  {
    fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]);
    return 1;
  }

  in_filename = argv[1];
  out_filename = argv[2];

  if ((ret = avformat_open_input(&input_format_context, in_filename, NULL, NULL)) < 0)
  {
    fprintf(stderr, "Could not open input file '%s'", in_filename);
    goto end;
  }
  if ((ret = avformat_find_stream_info(input_format_context, NULL)) < 0)
  {
    fprintf(stderr, "Failed to retrieve input stream information");
    goto end;
  }

  avformat_alloc_output_context2(&output_format_context, NULL, NULL, out_filename);
  if (!output_format_context)
  {
    fprintf(stderr, "Could not create output context\n");
    ret = AVERROR_UNKNOWN;
    goto end;
  }

  for (i = 0; i < input_format_context->nb_streams; i++)
  {
    AVStream *out_stream;
    AVStream *in_stream = input_format_context->streams[i];
    AVCodecParameters *in_codecpar = in_stream->codecpar;

    out_stream = avformat_new_stream(output_format_context, NULL);
    if (!out_stream)
    {
      fprintf(stderr, "Failed allocating output stream\n");
      ret = AVERROR_UNKNOWN;
      goto end;
    }

    ret = avcodec_parameters_copy(out_stream->codecpar, in_codecpar);
    if (ret < 0)
    {
      fprintf(stderr, "Failed to copy codec parameters\n");
      goto end;
    }
  }
  // https://ffmpeg.org/doxygen/trunk/group__lavf__misc.html#gae2645941f2dc779c307eb6314fd39f10
  av_dump_format(output_format_context, 0, out_filename, 1);

  ret = avio_open(&output_format_context->pb, out_filename, AVIO_FLAG_WRITE);
  if (ret < 0)
  {
    fprintf(stderr, "Could not open output file '%s'", out_filename);
    goto end;
  }

  // https://ffmpeg.org/doxygen/trunk/group__lavf__encoding.html#ga18b7b10bb5b94c4842de18166bc677cb
  ret = avformat_write_header(output_format_context, &opts);
  if (ret < 0)
  {
    fprintf(stderr, "Error occurred when opening output file\n");
    goto end;
  }

  while (1)
  {
    AVStream *in_stream, *out_stream;
    ret = av_read_frame(input_format_context, &packet);
    if (ret < 0)
      break;
    in_stream = input_format_context->streams[packet.stream_index];
    out_stream = output_format_context->streams[packet.stream_index];

    // Adjust PTS/DTS
    packet.pts = av_rescale_q_rnd(packet.pts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX);
    packet.dts = av_rescale_q_rnd(packet.dts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX);
    packet.duration = av_rescale_q(packet.duration, in_stream->time_base, out_stream->time_base);
    // https://ffmpeg.org/doxygen/trunk/structAVPacket.html#ab5793d8195cf4789dfb3913b7a693903
    packet.pos = -1;

    // https://ffmpeg.org/doxygen/trunk/group__lavf__encoding.html#ga37352ed2c63493c38219d935e71db6c1
    ret = av_interleaved_write_frame(output_format_context, &packet);
    if (ret < 0)
    {
      fprintf(stderr, "Error muxing packet\n");
      break;
    }
    av_packet_unref(&packet);
  }

  // https://ffmpeg.org/doxygen/trunk/group__lavf__encoding.html#ga7f14007e7dc8f481f054b21614dfec13
  av_write_trailer(output_format_context);

end:
  avformat_close_input(&input_format_context);
  /* close output */
  if (output_format_context && !(output_format_context->oformat->flags & AVFMT_NOFILE))
    avio_closep(&output_format_context->pb);
  avformat_free_context(output_format_context);
  if (ret < 0 && ret != AVERROR_EOF)
  {
    fprintf(stderr, "Error occurred: %s\n", av_err2str(ret));
    return 1;
  }
  return 0;
}

[dishmael]

I finally figured out the issue. I needed to set packet.flags |= AV_PKT_FLAG_KEY; - once I set that, the progress bar worked correctly.

while (1)
{
  AVStream *in_stream, *out_stream;
  ret = av_read_frame(input_format_context, &packet);
  if (ret < 0)
    break;
  in_stream = input_format_context->streams[packet.stream_index];
  out_stream = (*output_format_context)->streams[packet.stream_index];

  // Rescale PTS/DTS
  av_packet_rescale_ts(&packet, in_stream->time_base, out_stream->time_base);
  packet.flags |= AV_PKT_FLAG_KEY;
  packet.pos = -1;

  // https://ffmpeg.org/doxygen/trunk/group__lavf__encoding.html#ga37352ed2c63493c38219d935e71db6c1
  ret = av_interleaved_write_frame(*output_format_context, &packet);
  if (ret < 0)
  {
    fprintf(stderr, "Error muxing packet\n");
    break;
  }
  av_packet_unref(&packet);
}