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• Interact with ffmpeg from a .NET program ?

  18 septembre 2011, par Shimmy

  I'm trying to create a .NET wrapper for media-file conversion using ffmepg, here is what I've tried :

  static void Main(string[] args)
  
  {
  
    if (File.Exists("sample.mp3")) File.Delete("sample.mp3");
  
  
  
    string result;
  
  
  
    using (Process p = new Process())
  
    {
  
      p.StartInfo.FileName = "ffmpeg";
  
      p.StartInfo.Arguments = "-i sample.wma sample.mp3";
  
  
  
      p.StartInfo.UseShellExecute = false;
  
      p.StartInfo.RedirectStandardOutput = true;
  
  
  
      p.Start();
  
  
  
      //result is assigned with an empty string!
  
      result = p.StandardOutput.ReadToEnd();
  
  
  
      p.WaitForExit();
  
    }
  
  }

  What actually happens is the content of the ffmpeg program is printed out to the Console app, but the result variable is an empty string. I want to control the conversion progress interactively, so the user doesn't even have to know I'm using ffmpeg, but he still knows the conversion progress' details and what percentage etc. the app is up to.

  Basically I would also be happy with a .NET wrapper for a P/Invoke to conversion function ONLY (I am not interested in a whole external library, unless I can extract the PI function from it).

  Anyone with experience in ffmpeg & .NET ?

  Update
  Please view my further question, how to write input to a running ffmpeg process.

• Video encoding and keyframes

  24 février 2013, par Tishu

  I am transcoding a video frame by frame and using x264+ffmpeg to encode. The original video plays fine, but the first few frames of my transcoded vide show grey artefacts. I understand this is because of time compression and these artefacts disappear after a few frames.

  See these two pictures which are the first and second frames. The third frame is normal (i.e. no grey artefact and not blurry like the second one)
  
  

  How can I force the first frame to be a key frame (ie fully encoded in my output video) so that these artefacts do not show ?

  Edit - more details

  Here is what I am doing more in details. I used bit form differents tutorials to read a video frame by frame and reencode each frame to a new video. My encoding parameters are the following :

  avcodec_get_context_defaults3(c, *codec);
  
  c->codec_id = codec_id;
  
  c->bit_rate = output_bitrate;
  
  /* Resolution must be a multiple of two. */
  
  c->width    = output_width;
  
  c->height   = output_height;
  
  /* timebase: This is the fundamental unit of time (in seconds) in terms
  
   * of which frame timestamps are represented. For fixed-fps content,
  
   * timebase should be 1/framerate and timestamp increments should be
  
   * identical to 1. */
  
  st->r_frame_rate.num = output_framerate_num;
  
  st->r_frame_rate.den = output_framerate_den;
  
  c->time_base.den = output_timebase_den;
  
  c->time_base.num = output_timebase_num;
  
  c->gop_size      = 3; /* emit one intra frame every twelve frames at most */
  
  c->pix_fmt       = STREAM_PIX_FMT;
  
  if (c->codec_id == AV_CODEC_ID_MPEG2VIDEO) {
  
      /* just for testing, we also add B frames */
  
      c->max_b_frames = 2;
  
  }
  
  if (c->codec_id == AV_CODEC_ID_MPEG1VIDEO) {
  
      /* Needed to avoid using macroblocks in which some coeffs overflow.
  
       * This does not happen with normal video, it just happens here as
  
       * the motion of the chroma plane does not match the luma plane. */
  
      c->mb_decision = 2;
  
  }
  
  c->max_b_frames = 2;
  
  c->scenechange_threshold = 0;
  
  c->rc_buffer_size = 0;
  
  c->me_method = ME_ZERO;

  Then I process each frame, probably doing something wrong there. The decoding bit :

  while(av_read_frame(gFormatCtx, &packet)>=0) {
  
      // Is this a packet from the video stream?
  
      if(packet.stream_index==gVideoStreamIndex) {
  
          // Decode video frame
  
          avcodec_decode_video2(gVideoCodecCtx, pCurrentFrame, &frameFinished, &packet);
  
          // Did we get a video frame?
  
          if(frameFinished) {
  
              [...]
  
              if(firstPts == -999) /*Initial value*/
  
                  firstPts = packet.pts;
  
              deltaPts = packet.pts - firstPts;
  
              double seconds = deltaPts*av_q2d(gFormatCtx->streams[gVideoStreamIndex]->time_base);
  
              [...]
  
              muxing_writeVideoFrame(pCurrentFrame, packet.pts);
  
          }
  
      }
  
  }

  The actual writing :

  int muxing_writeVideoFrame(AVFrame *frame, int64_t pts)
  
  {
  
  frameCount = frameCount +1;
  
  if(frameCount > 0)
  
  {
  
      if (video_st)
  
          video_pts = (double)video_st->pts.val * video_st->time_base.num /
  
                      video_st->time_base.den;
  
      else
  
          video_pts = 0.0;
  
  
  
      if (video_st && !(video_st && audio_st && audio_pts < video_pts))
  
      {
  
          frame->pts = pts;//av_rescale_q(frame_count, video_st->codec->time_base, video_st->time_base);
  
          write_video_frame(oc, video_st, frame);
  
      }
  
  }
  
  
  
  return 0;
  
  }
  
  
  
  static int write_video_frame(AVFormatContext *oc, AVStream *st, AVFrame *frame)
  
  {
  
      int ret;
  
      static struct SwsContext *sws_ctx;
  
      //LOGI(10, frame_count);
  
      AVCodecContext *c = st->codec;
  
  
  
      /* encode the image */
  
      AVPacket pkt;
  
      int got_output;
  
      av_init_packet(&pkt);
  
      pkt.data = NULL;    // packet data will be allocated by the encoder
  
      pkt.size = 0;
  
  ret = avcodec_encode_video2(c, &pkt, frame, &got_output);
  
  if (ret < 0) {
  
      fprintf(stderr, "Error encoding video frame: %s\n", av_err2str(ret));
  
      exit(1);
  
  }
  
  /* If size is zero, it means the image was buffered. */
  
  if (got_output) {
  
      if (c->coded_frame->key_frame)
  
          pkt.flags |= AV_PKT_FLAG_KEY;
  
      pkt.stream_index = st->index;
  
      /* Write the compressed frame to the media file. */
  
      ret = av_interleaved_write_frame(oc, &pkt);
  
  } else {
  
      ret = 0;
  
  }
  
  
  
      if (ret != 0) {
  
          LOGI(10, av_err2str(ret));
  
          exit(1);
  
      }
  
      frame_count++;
  
      return got_output;
  
  }

• Video encoding and keyframes

  24 février 2013, par Tishu

  I am transcoding a video frame by frame and using x264+ffmpeg to encode. The original video plays fine, but the first few frames of my transcoded vide show grey artefacts. I understand this is because of time compression and these artefacts disappear after a few frames.

  See these two pictures which are the first and second frames. The third frame is normal (i.e. no grey artefact and not blurry like the second one)
  
  

  How can I force the first frame to be a key frame (ie fully encoded in my output video) so that these artefacts do not show ?

  Edit - more details

  Here is what I am doing more in details. I used bit form differents tutorials to read a video frame by frame and reencode each frame to a new video. My encoding parameters are the following :

  avcodec_get_context_defaults3(c, *codec);
  
  c->codec_id = codec_id;
  
  c->bit_rate = output_bitrate;
  
  /* Resolution must be a multiple of two. */
  
  c->width    = output_width;
  
  c->height   = output_height;
  
  /* timebase: This is the fundamental unit of time (in seconds) in terms
  
   * of which frame timestamps are represented. For fixed-fps content,
  
   * timebase should be 1/framerate and timestamp increments should be
  
   * identical to 1. */
  
  st->r_frame_rate.num = output_framerate_num;
  
  st->r_frame_rate.den = output_framerate_den;
  
  c->time_base.den = output_timebase_den;
  
  c->time_base.num = output_timebase_num;
  
  c->gop_size      = 3; /* emit one intra frame every twelve frames at most */
  
  c->pix_fmt       = STREAM_PIX_FMT;
  
  if (c->codec_id == AV_CODEC_ID_MPEG2VIDEO) {
  
      /* just for testing, we also add B frames */
  
      c->max_b_frames = 2;
  
  }
  
  if (c->codec_id == AV_CODEC_ID_MPEG1VIDEO) {
  
      /* Needed to avoid using macroblocks in which some coeffs overflow.
  
       * This does not happen with normal video, it just happens here as
  
       * the motion of the chroma plane does not match the luma plane. */
  
      c->mb_decision = 2;
  
  }
  
  c->max_b_frames = 2;
  
  c->scenechange_threshold = 0;
  
  c->rc_buffer_size = 0;
  
  c->me_method = ME_ZERO;

  Then I process each frame, probably doing something wrong there. The decoding bit :

  while(av_read_frame(gFormatCtx, &packet)>=0) {
  
      // Is this a packet from the video stream?
  
      if(packet.stream_index==gVideoStreamIndex) {
  
          // Decode video frame
  
          avcodec_decode_video2(gVideoCodecCtx, pCurrentFrame, &frameFinished, &packet);
  
          // Did we get a video frame?
  
          if(frameFinished) {
  
              [...]
  
              if(firstPts == -999) /*Initial value*/
  
                  firstPts = packet.pts;
  
              deltaPts = packet.pts - firstPts;
  
              double seconds = deltaPts*av_q2d(gFormatCtx->streams[gVideoStreamIndex]->time_base);
  
              [...]
  
              muxing_writeVideoFrame(pCurrentFrame, packet.pts);
  
          }
  
      }
  
  }

  The actual writing :

  int muxing_writeVideoFrame(AVFrame *frame, int64_t pts)
  
  {
  
  frameCount = frameCount +1;
  
  if(frameCount > 0)
  
  {
  
      if (video_st)
  
          video_pts = (double)video_st->pts.val * video_st->time_base.num /
  
                      video_st->time_base.den;
  
      else
  
          video_pts = 0.0;
  
  
  
      if (video_st && !(video_st && audio_st && audio_pts < video_pts))
  
      {
  
          frame->pts = pts;//av_rescale_q(frame_count, video_st->codec->time_base, video_st->time_base);
  
          write_video_frame(oc, video_st, frame);
  
      }
  
  }
  
  
  
  return 0;
  
  }
  
  
  
  static int write_video_frame(AVFormatContext *oc, AVStream *st, AVFrame *frame)
  
  {
  
      int ret;
  
      static struct SwsContext *sws_ctx;
  
      //LOGI(10, frame_count);
  
      AVCodecContext *c = st->codec;
  
  
  
      /* encode the image */
  
      AVPacket pkt;
  
      int got_output;
  
      av_init_packet(&pkt);
  
      pkt.data = NULL;    // packet data will be allocated by the encoder
  
      pkt.size = 0;
  
  ret = avcodec_encode_video2(c, &pkt, frame, &got_output);
  
  if (ret < 0) {
  
      fprintf(stderr, "Error encoding video frame: %s\n", av_err2str(ret));
  
      exit(1);
  
  }
  
  /* If size is zero, it means the image was buffered. */
  
  if (got_output) {
  
      if (c->coded_frame->key_frame)
  
          pkt.flags |= AV_PKT_FLAG_KEY;
  
      pkt.stream_index = st->index;
  
      /* Write the compressed frame to the media file. */
  
      ret = av_interleaved_write_frame(oc, &pkt);
  
  } else {
  
      ret = 0;
  
  }
  
  
  
      if (ret != 0) {
  
          LOGI(10, av_err2str(ret));
  
          exit(1);
  
      }
  
      frame_count++;
  
      return got_output;
  
  }