Recherche avancée

Sur d’autres sites (15267)

• How can I extract PGS/SUP subtitles from mkv container via libav

  24 août 2024, par Elia

  Trying to write a C code that does the equivalent of the following ffmpeg command :

  


  ffmpeg -i video.mkv -map 0:"$STREAM_INDEX" -c:s copy subtitles.sup


  


  I have managed to read the media container, locate the subtitle streams, and filter the relevant one based on stream metadata. However I only succeeded in getting the bitmap data, without the full PGS header and sections.

  


  Tried dumping packet->data to a file hoping it includes full sections, but the data doesn't start with the expected PGS magic header 0x5047.
  
Also using avcodec_decode_subtitle2 doesn't seem to be useful as it only gives bitmap data without the full PGS header and sections.

  


  What steps are supposed to happen after finding the relevant subtitle stream that allows extracting it as raw data that exactly matches the output of the ffmpeg command above ?

  


  Some sample code :

  


      while(av_read_frame(container_ctx, packet) == 0) {
      if(packet->stream_index != i) {
        continue;
      }

      // Try 1 - dump packet data to file
      fwrite(packet->data, 1, packet->size, fout);

      // Try 2 - decode subtitles
      int bytes_read = 0, success = 0;
      AVSubtitle sub;

      if ((bytes_read = avcodec_decode_subtitle2(codec_context, &sub, &success, packet)) < 0) {
        fprintf(stderr, "Failed to decode subtitles %s", av_err2str(bytes_read));
        goto end;
      }
      fprintf(stdout, "Success! Status:%d - BytesRead:%d NumRects:%d\n", success, bytes_read, sub.num_rects);

      if (success) {
        // Only managed to extract bitmap data here via sub.rects
      }

      av_packet_unref(packet);
    }


  


• c++, ffmpeg tanscoding : time_base differs depending on the container

  1er mai 2022, par prostraction

  I transcode video (mkv and mp4). When mkv transcoded to mkv, output is fine (output video fps and duration are same as input), but if mkv transcoded to mp4, output fps is less than input 2 times and duration of output video is more than input 2 times.

  


  I transcode only video, audio writing as decoded packet from input file.

  


  Video stream and context created like this :

  


  out_stream = avformat_new_stream(ofmt_ctx, NULL);
avcodec_parameters_copy(out_stream->codecpar, in_codecpar);
out_stream->codecpar->codec_tag = 0;
codec_encode  = avcodec_find_encoder(out_stream->codecpar->codec_id);
context_encode = avcodec_alloc_context3(codec_encode);
context_encode->width       = width;
context_encode->height      = height;
context_encode->pix_fmt     = codec_encode->pix_fmts[0];
context_encode->time_base   = av_inv_q(in_stream->r_frame_rate);
out_stream->time_base       = context_encode->time_base;
out_stream->r_frame_rate    = in_stream->r_frame_rate;


  


  Transcoding (simplified) :

  


   int64_t i = 0; 
 while (true) {
        av_read_frame(ifmt_ctx, pkt);
        in_stream = ifmt_ctx->streams[pkt->stream_index];
        pkt->stream_index = stream_mapping[pkt->stream_index];
        pCodecCtx = ifmt_ctx->streams[pkt->stream_index]->codec;
        pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
        error = avcodec_open2(pCodecCtx, pCodec, nullptr);
        if (pkt->stream_index == AVMEDIA_TYPE_VIDEO) {
             ....
             avcodec_decode_video2(pCodecCtx, frame, &frameFinished, pkt);
             ....
             // manipulate with frame
             ....
             frame->pts = i;
             avcodec_send_frame(context_encode, frame);
             while ((ret = avcodec_receive_packet(context_encode, pkt_encode)) >= 0) {
                  if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
                       break;
                  av_packet_rescale_ts(pkt_encode, context_encode->time_base, out_stream->time_base);
                  av_interleaved_write_frame(ofmt_ctx, pkt_encode);
                  av_packet_unref(pkt_encode);
             }
             i++;
        }
         else {
            av_packet_rescale_ts(pkt, in_stream->time_base, out_stream->time_base);
            av_interleaved_write_frame(ofmt_ctx, pkt);
        }
        av_packet_unref(pkt);
   }


  


  Mediainfo of output mkv transcoded video (mkv -> mkv) :

  


  

  • Frame rate : 23.976 (24000/1001) FPS

  


  


  Mediainfo of output mp4 transcoded video (mkv -> mp4) :

  


  

  • Frame rate : 11.988 (12000/1001) FPS

  


  • Original frame rate : 23.976 (24000/1001) FPS

  


  


  When video context created, time_base values are (mkv -> mp4 and mkv -> mkv) :

  


  FPS input: (24000/1001)
FPS output: (24000/1001)
context_decode->time_base (1001 / 48000)
context_encode->time_base (1001 / 24000)
in_stream->time_base (1 / 1000)
in_stream->codec->time_base (1001 / 48000)
out_stream->time_base (1001 / 24000)
out_stream->codec->time_base (0 / 1)


  


  When video frame is writing, time_base values are (mkv -> mp4) :

  


  context_encode->time_base (1001 / 24000)
out_stream->time_base (1 / 48000)


  


  But if mkv->mkv :

  


  context_encode->time_base (1001 / 24000) 
out_stream->time_base (1 / 1000)


  


  ffmpeg av_dump :

  


  Input #0, matroska,webm, from '24fps2.mkv':
 - Stream #0:0: Video: h264 (High), yuv420p(tv, bt709, progressive), 1920x1080 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
 - Stream #0:1: Audio: aac (LC), 48000 Hz, stereo, fltp (default)

Output #0, mp4, to 'temp_read.mp4':
 - Stream #0:0: Video: h264 (High), yuv420p(tv, bt709, progressive), 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 23.98 tbr, 23.98 tbn
 - Stream #0:1: Audio: aac (LC), 48000 Hz, stereo, fltp


  


  But if I manually set time_base to be equal to FPS/2 of input video :

  


  AVRational temp;
temp.num = 500;
temp.den = 24001;
context_encode->time_base   = temp;
out_stream->time_base       = context_encode->time_base;
out_stream->r_frame_rate    = in_stream->r_frame_rate;


  


  When video stream and context created, time_base values are (mkv -> mp4) :

  


  context_encode->time_base (500 / 24001)
out_stream->time_base (500 / 24001)


  


  When video frame is writing, time_base values are (mkv -> mp4) :

  


  context_encode->time_base (500 / 24001)
out_stream->time_base (1 / 48000)


  


  And video FPS and duration is correct :

  


  

  • Frame rate : 23.976 (24000/1001) FPS

  


  


  What is wrong with time_base and av_packet_rescale in this case and how it could be fixed ?

  


• What is different between h.265/HEVC in a container(mp4, mkv....) and h.265/HEVC Annex B raw file ?

  22 juillet 2018, par Adam Estel

  What is different between h.264/HEVC in a container (mp4, mkv....) and h.265/HEVC Annex B raw file ? I mean about codec, pixel formats and why can’t hevc Annex B contain audio stream ?

  I researched google for a while but I couldn’t understand it fully yet.