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• FFmpeg screencast recording : which codecs to use ?

  24 avril 2013, par mkaito

  I've been experimenting with recording screencasts using FFmpeg's X11grab module, which has worked more or less fine so far. I understand that a/v encoding is a complex process with many fine details, but I'm doing my best to learn.

  I'd like to do "lightweight" recording of a video stream, that puts as little strain as possible on the system while the stream is being recorded. I record two audio streams separately with pacat and sox. Later, the whole thing is filtered, normalized, encoded, and combined into a Matroska container.

  Right now, I'm having ffmpeg record a rawvideo stream to be fed to x264's yuv4 demuxer. I experimented with ffv1 and straight x264 recording before. My system can't handle real time encoding with x264 on the settings I want for the final stream, so I have to recompress separately once the recording is done. I've found that ffv1 gives me terrible frame dropping, and yuv4 too, but less so. I suspect this is due to hard drive speed, even if I'm sitting in a SATA3 Caviar Black that's being used exclusively to hold the recorded data.

  The question is, which combination of video codecs should I look at ? Record straight in x264 and recompress to "better" x264 later ? Raw video, then compress ? How would I go about pinpointing issues such as the frame drops I've been experiencing ?

  EDIT : This is the ffmpeg line I currently use.

  ffmpeg -v warning -f x11grab -s 1920x1080 -r 30000/1001 -i :0.0\
  
  -vcodec rawvideo -pix_fmt yuv420p -s 1280x720\
  
  -threads 0\
  
  recvideo.y4m

• HTML5 video player stutters even after video is loaded [migrated]

  24 octobre 2013, par Adi Ulici

  I am using jwplayer to display videos on my websites but cannot manage to make them running smoothly. They stutter, especially in full screen, although I wait for the video to fully load.

  I tried decreasing the video quality from by decreasing the bitrate from 3000k to 1800k (I use ffmpeg to convert the videos). The resolution of the videos is 1280x720 (HD).

  Any tips what should I try next ? Thanks

• libavcodec/libx264 do not produce B-frames

  6 novembre 2013, par Rob Schmidt

  I am writing an application in C++ that uses libavcodec with libx264 to encode video. However, the encoded data ended up being much larger than I expected. I analyzed the results and discovered that my encoding never produced B-frames, only I- and P-frames.

  I created a standalone utility based on the ffmpeg source code and examples to test my encoder setup. It reads in an H.264 file, re-encodes the decoded frames, and outputs the result to a file using the ITU H.264 Annex B format. I also used ffmpeg to perform the same operation so I could compare against a known good implementation. My utility never outputs B-frames whereas ffmpeg does.

  I have since tried to figure out what ffmpeg does that my code doesn't. I first tried manually specifying encoder settings related to B-frames. This had no effect.

  I then tried running both ffmpeg and my utility under gdb and comparing the contents of the AVStream, AVCodecContext, and X264Context prior to opening the encoder and manually setting any fields that appeared different. Even with identical settings, I still only produce I- and P-frames.

  Finally, I thought that perhaps the problem was with my timestamp handling. I reworked my test utility to mimic the pipeline used by ffmpeg and to output timestamp debugging output like ffmpeg does. Even with my timestamps identical to ffmpeg's I still get no B-frames.

  At this point I don't know what else to try. When I run ffmpeg, I run it with the command line below. Note that aside from the "superfast" preset, I pretty much use the default values.

  ffmpeg -v debug -i ~/annexb.264 -codec:v libx264 -preset superfast -g 30 -f h264 ./out.264

  The code that configures the encoder is listed below. It specifies the "superfast" preset too.

  static AVStream *add_video_stream(AVFormatContext *output_ctx, AVCodec **output_codec, enum AVCodecID codec_id)
  
  {
  
      *output_codec = avcodec_find_encoder(codec_id);
  
      if (*output_codec == NULL) {
  
          printf("Could not find encoder for '%s' (%d)\n", avcodec_get_name(codec_id), codec_id);
  
          return NULL;
  
      }
  
  
  
      AVStream *output_stream = avformat_new_stream(output_ctx, *output_codec);
  
      if (output_stream == NULL) {
  
          printf("Could not create video stream.\n");
  
          return NULL;
  
      }
  
      output_stream->id = output_ctx->nb_streams - 1;
  
      AVCodecContext *codec_ctx = output_stream->codec;
  
  
  
      avcodec_get_context_defaults3(codec_ctx, *output_codec);
  
  
  
      codec_ctx->width = 1280;
  
      codec_ctx->height = 720;
  
  
  
      codec_ctx->time_base.den = 15000;
  
      codec_ctx->time_base.num = 1001;
  
  
  
  /*    codec_ctx->gop_size = 30;*/
  
      codec_ctx->pix_fmt = AV_PIX_FMT_YUVJ420P;
  
  
  
      // try to force B-frame output
  
  /*    codec_ctx->max_b_frames = 3;*/
  
  /*    codec_ctx->b_frame_strategy = 2;*/
  
  
  
      output_stream->sample_aspect_ratio.num = 1;
  
      output_stream->sample_aspect_ratio.den = 1;
  
  
  
      codec_ctx->sample_aspect_ratio.num = 1;
  
      codec_ctx->sample_aspect_ratio.den = 1;
  
  
  
      codec_ctx->chroma_sample_location = AVCHROMA_LOC_LEFT;
  
  
  
      codec_ctx->bits_per_raw_sample = 8;
  
  
  
      if ((output_ctx->oformat->flags & AVFMT_GLOBALHEADER) != 0) {
  
          codec_ctx->flags |= CODEC_FLAG_GLOBAL_HEADER;
  
      }
  
  
  
      return output_stream;
  
  }
  
  
  
  
  
  int main(int argc, char **argv)
  
  {
  
      // ... open input file 
  
  
  
      avformat_alloc_output_context2(&output_ctx, NULL, "h264", output_path);
  
      if (output_ctx == NULL) {
  
          fprintf(stderr, "Unable to allocate output context.\n");
  
          return 1;
  
      } 
  
  
  
      AVCodec *output_codec = NULL;
  
      output_stream = add_video_stream(output_ctx, &output_codec, output_ctx->oformat->video_codec);
  
      if (output_stream == NULL) {
  
          fprintf(stderr, "Error adding video stream to output context.\n");
  
          return 1;
  
      }
  
      encode_ctx = output_stream->codec;
  
  
  
      // seems to have no effect
  
  #if 0
  
      if (decode_ctx->extradata_size != 0) {
  
          size_t extradata_size = decode_ctx->extradata_size;
  
          printf("extradata_size: %zu\n", extradata_size);
  
          encode_ctx->extradata = av_mallocz(extradata_size + FF_INPUT_BUFFER_PADDING_SIZE);
  
          memcpy(encode_ctx->extradata, decode_ctx->extradata, extradata_size);
  
          encode_ctx->extradata_size = extradata_size;
  
      }
  
  #endif // 0
  
  
  
      AVDictionary *opts = NULL;
  
      av_dict_set(&opts, "preset", "superfast", 0);
  
      // av_dict_set(&opts, "threads", "auto", 0); // seems to have no effect
  
  
  
      ret = avcodec_open2(encode_ctx, output_codec, &opts);
  
      if (ret < 0) {
  
          fprintf(stderr, "Unable to open output video cocec: %s\n", av_err2str(ret));
  
          return 1;
  
      }
  
  
  
      // ... decoding/encoding loop, clean up, etc.
  
  
  
      return 0;
  
  }

  My test utility produces the following debug output in which you can see there are no B-frames produced :

  [libx264 @ 0x1b8c9c0] using mv_range_thread = 56
  
  [libx264 @ 0x1b8c9c0] using SAR=1/1
  
  [libx264 @ 0x1b8c9c0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
  
  [libx264 @ 0x1b8c9c0] profile High, level 3.1
  
  Output #0, h264, to './out.264':
  
      Stream #0:0, 0, 1/90000: Video: h264, yuvj420p, 1280x720 [SAR 1:1 DAR 16:9], 1001/15000, q=-1--1, 90k tbn, 14.99 tbc
  
  
  
  <snip>
  
  
  
  [libx264 @ 0x1b8c9c0] frame=   0 QP=17.22 NAL=3 Slice:I Poc:0   I:3600 P:0    SKIP:0    size=122837 bytes
  
  [libx264 @ 0x1b8c9c0] frame=   1 QP=18.03 NAL=2 Slice:P Poc:2   I:411  P:1825 SKIP:1364 size=25863 bytes
  
  [libx264 @ 0x1b8c9c0] frame=   2 QP=17.03 NAL=2 Slice:P Poc:4   I:369  P:2159 SKIP:1072 size=37880 bytes
  
  [libx264 @ 0x1b8c9c0] frame=   3 QP=16.90 NAL=2 Slice:P Poc:6   I:498  P:2330 SKIP:772  size=50509 bytes
  
  [libx264 @ 0x1b8c9c0] frame=   4 QP=16.68 NAL=2 Slice:P Poc:8   I:504  P:2233 SKIP:863  size=50791 bytes
  
  [libx264 @ 0x1b8c9c0] frame=   5 QP=16.52 NAL=2 Slice:P Poc:10  I:513  P:2286 SKIP:801  size=51820 bytes
  
  [libx264 @ 0x1b8c9c0] frame=   6 QP=16.49 NAL=2 Slice:P Poc:12  I:461  P:2293 SKIP:846  size=51311 bytes
  
  [libx264 @ 0x1b8c9c0] frame=   7 QP=16.65 NAL=2 Slice:P Poc:14  I:476  P:2287 SKIP:837  size=51196 bytes
  
  [libx264 @ 0x1b8c9c0] frame=   8 QP=16.66 NAL=2 Slice:P Poc:16  I:508  P:2240 SKIP:852  size=51577 bytes
  
  [libx264 @ 0x1b8c9c0] frame=   9 QP=16.55 NAL=2 Slice:P Poc:18  I:477  P:2278 SKIP:845  size=51531 bytes
  
  [libx264 @ 0x1b8c9c0] frame=  10 QP=16.67 NAL=2 Slice:P Poc:20  I:517  P:2233 SKIP:850  size=51946 bytes
  
  
  
  <snip>
  
  
  
  [libx264 @ 0x1b8c9c0] frame I:7     Avg QP:13.71  size:152207
  
  [libx264 @ 0x1b8c9c0] frame P:190   Avg QP:16.66  size: 50949
  
  [libx264 @ 0x1b8c9c0] mb I  I16..4: 27.1% 30.8% 42.1%
  
  [libx264 @ 0x1b8c9c0] mb P  I16..4:  6.8%  6.0%  0.8%  P16..4: 61.8%  0.0%  0.0%  0.0%  0.0%    skip:24.7%
  
  [libx264 @ 0x1b8c9c0] 8x8 transform intra:41.2% inter:86.9%
  
  [libx264 @ 0x1b8c9c0] coded y,uvDC,uvAC intra: 92.2% 28.3% 5.4% inter: 50.3% 1.9% 0.0%
  
  [libx264 @ 0x1b8c9c0] i16 v,h,dc,p:  7%  7% 77%  8%
  
  [libx264 @ 0x1b8c9c0] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu:  7% 15% 49%  6%  4%  3%  5%  3%  8%
  
  [libx264 @ 0x1b8c9c0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 19% 25% 24%  6%  7%  4%  6%  3%  6%
  
  [libx264 @ 0x1b8c9c0] i8c dc,h,v,p: 72% 14% 10%  4%
  
  [libx264 @ 0x1b8c9c0] Weighted P-Frames: Y:0.0% UV:0.0%
  
  [libx264 @ 0x1b8c9c0] kb/s:6539.11
  
  </snip></snip>

  ffmpeg, on the other hand, produces the following output that is almost identical but includes B-frames :

  [libx264 @ 0x20b9c40] using mv_range_thread = 56
  
  [libx264 @ 0x20b9c40] using SAR=1/1
  
  [libx264 @ 0x20b9c40] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
  
  [libx264 @ 0x20b9c40] profile High, level 3.1
  
  [h264 @ 0x20b8160] detected 4 logical cores
  
  Output #0, h264, to './out.264':
  
    Metadata:
  
      encoder         : Lavf54.63.104
  
      Stream #0:0, 0, 1/90000: Video: h264, yuvj420p, 1280x720 [SAR 1:1 DAR 16:9], 1001/15000, q=-1--1, 90k tbn, 14.99 tbc
  
  Stream mapping:
  
    Stream #0:0 -> #0:0 (h264 -> libx264)
  
  
  
  <snip>
  
  
  
  [libx264 @ 0x20b9c40] frame=   0 QP=17.22 NAL=3 Slice:I Poc:0   I:3600 P:0    SKIP:0    size=122835 bytes
  
  [libx264 @ 0x20b9c40] frame=   1 QP=18.75 NAL=2 Slice:P Poc:8   I:984  P:2045 SKIP:571  size=54208 bytes
  
  [libx264 @ 0x20b9c40] frame=   2 QP=19.40 NAL=2 Slice:B Poc:4   I:447  P:1581 SKIP:1572 size=24930 bytes
  
  [libx264 @ 0x20b9c40] frame=   3 QP=19.78 NAL=0 Slice:B Poc:2   I:199  P:1002 SKIP:2399 size=10717 bytes
  
  [libx264 @ 0x20b9c40] frame=   4 QP=20.19 NAL=0 Slice:B Poc:6   I:204  P:1155 SKIP:2241 size=15937 bytes
  
  [libx264 @ 0x20b9c40] frame=   5 QP=18.11 NAL=2 Slice:P Poc:16  I:990  P:2221 SKIP:389  size=64240 bytes
  
  [libx264 @ 0x20b9c40] frame=   6 QP=19.35 NAL=2 Slice:B Poc:12  I:439  P:1784 SKIP:1377 size=34048 bytes
  
  [libx264 @ 0x20b9c40] frame=   7 QP=19.88 NAL=0 Slice:B Poc:10  I:275  P:1035 SKIP:2290 size=16911 bytes
  
  [libx264 @ 0x20b9c40] frame=   8 QP=19.91 NAL=0 Slice:B Poc:14  I:257  P:1270 SKIP:2073 size=19172 bytes
  
  [libx264 @ 0x20b9c40] frame=   9 QP=17.90 NAL=2 Slice:P Poc:24  I:962  P:2204 SKIP:434  size=67439 bytes
  
  [libx264 @ 0x20b9c40] frame=  10 QP=18.84 NAL=2 Slice:B Poc:20  I:474  P:1911 SKIP:1215 size=37742 bytes
  
  
  
  <snip>
  
  
  
  [libx264 @ 0x20b9c40] frame I:7     Avg QP:15.95  size:130124
  
  [libx264 @ 0x20b9c40] frame P:52    Avg QP:17.78  size: 64787
  
  [libx264 @ 0x20b9c40] frame B:138   Avg QP:19.32  size: 26231
  
  [libx264 @ 0x20b9c40] consecutive B-frames:  6.6%  0.0%  0.0% 93.4%
  
  [libx264 @ 0x20b9c40] mb I  I16..4: 30.2% 35.2% 34.6%
  
  [libx264 @ 0x20b9c40] mb P  I16..4: 13.9% 11.4%  0.3%  P16..4: 60.4%  0.0%  0.0%  0.0%  0.0%    skip:13.9%
  
  [libx264 @ 0x20b9c40] mb B  I16..4:  5.7%  3.3%  0.0%  B16..8: 15.8%  0.0%  0.0%  direct:25.7%  skip:49.5%  L0:43.2% L1:37.3% BI:19.5%
  
  [libx264 @ 0x20b9c40] 8x8 transform intra:39.4% inter:77.2%
  
  [libx264 @ 0x20b9c40] coded y,uvDC,uvAC intra: 90.7% 26.6% 3.0% inter: 34.0% 4.1% 0.0%
  
  [libx264 @ 0x20b9c40] i16 v,h,dc,p:  7%  7% 77%  9%
  
  [libx264 @ 0x20b9c40] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu:  7% 16% 51%  5%  4%  3%  5%  3%  7%
  
  [libx264 @ 0x20b9c40] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 22% 27% 20%  6%  6%  3%  6%  3%  6%
  
  [libx264 @ 0x20b9c40] i8c dc,h,v,p: 71% 15% 11%  3%
  
  [libx264 @ 0x20b9c40] Weighted P-Frames: Y:0.0% UV:0.0%
  
  [libx264 @ 0x20b9c40] kb/s:4807.16
  
  </snip></snip>

  I'm sure I'm missing something simple, but I can't for the life of me see what it is. Any assistance would be greatly appreciated.