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• My crazy linux.conf.au week

  9 février 2012, par silvia

  In January I attended the annual Australian Linux and Open Source conference (LCA). But since I was sick all of January and had a lot to catch up on, I never got around to sharing all the talks that I gave during that time.

  Drupal Down Under

  It started with a talk at Drupal Down Under, which happened the weekend before LCA. I gave a talk titled “HTML5 video specifications” (video, slides).

  I spoke about the video and audio element in HTML5, how to provide fallback content, how to encode content, how to control them from JavaScript, and briefly about Drupal video modules, though the next presentation provided much more insight into those. I explained how to make the HTML5 media elements accessible, including accessible controls, captions, audio descriptions, and the new WebVTT file format. I ran out of time to introduce the last section of my slides which are on WebRTC.

  Linux.conf.au

  On the first day of LCA I gave a talk both in the Multimedia Miniconf and the Browser Miniconf.

  Browser Miniconf

  In the Browser Miniconf I talked about “Web Standardisation – how browser vendors collaborate, or not” (slides). Maybe the most interesting part about this was that I tried out a new slide “deck” tool called impress.js. I’m not yet sure if I like it but it worked well for this talk, in which I explained how the HTML5 spec is authored and who has input.

  I also sat on a panel of browser developers in the Browser Miniconf (more as a standards than as a browser developer, but that’s close enough). We were asked about all kinds of latest developments in HTML5, CSS3, and media standards in the browser.

  Multimedia Miniconf

  In the Multimedia Miniconf I gave a “HTML5 media accessibility update” (slides). I talked about the accessibility problems of Flash, how native HTML5 video players will be better, about accessible video controls, captions, navigation chapters, audio descriptions, and WebVTT. I also provided a demo of how to synchronize multiple video elements using a polyfill for the multitrack API.

  I also provided an update on HTTP adaptive streaming APIs as a lightning talk in the Multimedia Miniconf. I used an extract of the Drupal conference slides for it.

  Main conference

  Finally, and most importantly, Alice Boxhall and myself gave a talk in the main linux.conf.au titled “Developing Accessible Web Apps – how hard can it be ?” (video, slides). I spoke about a process that you can follow to make your Web applications accessible. I’m writing a separate blog post to explain this in more detail. In her part, Alice dug below the surface of browsers to explain how the accessibility markup that Web developers provide is transformed into data structures that are handed to accessibility technologies.

• ffmpeg libx264 AVCodecContext settings

  21 mai 2014, par integra753

  I am using a recent windows (Jan 2011) ffmpeg build and trying to record video in H264. It is recording fine in MPEG4 using the following settings :

  c->codec_id = CODEC_ID_MPEG4;
  
  c->codec_type = AVMEDIA_TYPE_VIDEO;
  
  c->width = VIDEO_WIDTH;
  
  c->height = VIDEO_HEIGHT;
  
  c->bit_rate = c->width * c->height * 4;
  
  c->time_base.den = FRAME_RATE;
  
  c->time_base.num = 1;
  
  c->gop_size = 12;
  
  c->pix_fmt = PIX_FMT_YUV420P;

  Simply changing CODEC Id to H264 causes avcodec_open() to fail (-1). I found a list of possible settings Encoding h.264 with libavcodec/x264. I have tried these, without setting pix_fmt, avcodec_open() still fails but if I additionally set c->pix_fmt = PIX_FMT_YUV420P ; then I get a divide by zero exception.

  I then came across a few posts on here that say I should set nothing (with exception of code_id, codec_type, width, height and perhaps bit_rate and pix_fmt) as the library now chooses the best settings itself. I have tried various combinations, still avcode_open() fails.

  Does anyone have some advice on what to do or some settings that are current ?

  Thanks.

  Here are one set of H264 settings which give the issue I describe :

  static AVStream* AddVideoStream(AVFormatContext *pOutputFmtCtx, 
  
  int frameWidth, int frameHeight, int fps)
  
  {
  
  AVCodecContext* ctx;
  
  AVStream* stream;
  
  
  
  stream = av_new_stream(pOutputFmtCtx, 0);
  
  if (!stream) 
  
  {
  
      return NULL;
  
  }
  
  
  
  ctx = stream->codec;
  
  
  
  ctx->codec_id = pOutputFmtCtx->oformat->video_codec; //CODEC_ID_H264
  
  ctx->codec_type = AVMEDIA_TYPE_VIDEO;
  
  ctx->width = frameWidth;             //704
  
  ctx->height = frameHeight;           //576
  
  ctx->bit_rate = frameWidth * frameHeight * 4;
  
  
  
  ctx->coder_type = 1;  // coder = 1
  
  ctx->flags|=CODEC_FLAG_LOOP_FILTER;   // flags=+loop
  
  ctx->me_cmp|= 1;  // cmp=+chroma, where CHROMA = 1
  
  ctx->partitions|=X264_PART_I8X8+X264_PART_I4X4+X264_PART_P8X8+X264_PART_B8X8; // partitions=+parti8x8+parti4x4+partp8x8+partb8x8
  
  ctx->me_method=ME_HEX;    // me_method=hex
  
  ctx->me_subpel_quality = 7;   // subq=7
  
  ctx->me_range = 16;   // me_range=16
  
  ctx->gop_size = 250;  // g=250
  
  ctx->keyint_min = 25; // keyint_min=25
  
  ctx->scenechange_threshold = 40;  // sc_threshold=40
  
  ctx->i_quant_factor = 0.71; // i_qfactor=0.71
  
  ctx->b_frame_strategy = 1;  // b_strategy=1
  
  ctx->qcompress = 0.6; // qcomp=0.6
  
  ctx->qmin = 10;   // qmin=10
  
  ctx->qmax = 51;   // qmax=51
  
  ctx->max_qdiff = 4;   // qdiff=4
  
  ctx->max_b_frames = 3;    // bf=3
  
  ctx->refs = 3;    // refs=3
  
  ctx->directpred = 1;  // directpred=1
  
  ctx->trellis = 1; // trellis=1
  
         ctx->flags2|=CODEC_FLAG2_BPYRAMID+CODEC_FLAG2_MIXED_REFS+CODEC_FLAG2_WPRED+CODEC_FLAG2_8X8DCT+CODEC_FLAG2_FASTPSKIP;  // flags2=+bpyramid+mixed_refs+wpred+dct8x8+fastpskip
  
  ctx->weighted_p_pred = 2; // wpredp=2
  
  // libx264-main.ffpreset preset
  
  ctx->flags2|=CODEC_FLAG2_8X8DCT;
  
  ctx->flags2^=CODEC_FLAG2_8X8DCT;    // flags2=-dct8x8
  
  
  
  // if set this get divide by 0 error on avcodec_open()
  
  // if don't set it get -1 error on avcodec_open()
  
  //ctx->pix_fmt = PIX_FMT_YUV420P; 
  
  
  
  return stream;

  }

• ffmpeg libx264 AVCodecContext settings

  21 mai 2014, par integra753

  I am using a recent windows (Jan 2011) ffmpeg build and trying to record video in H264. It is recording fine in MPEG4 using the following settings :

  c->codec_id = CODEC_ID_MPEG4;
  
  c->codec_type = AVMEDIA_TYPE_VIDEO;
  
  c->width = VIDEO_WIDTH;
  
  c->height = VIDEO_HEIGHT;
  
  c->bit_rate = c->width * c->height * 4;
  
  c->time_base.den = FRAME_RATE;
  
  c->time_base.num = 1;
  
  c->gop_size = 12;
  
  c->pix_fmt = PIX_FMT_YUV420P;

  Simply changing CODEC Id to H264 causes avcodec_open() to fail (-1). I found a list of possible settings Encoding h.264 with libavcodec/x264. I have tried these, without setting pix_fmt, avcodec_open() still fails but if I additionally set c->pix_fmt = PIX_FMT_YUV420P ; then I get a divide by zero exception.

  I then came across a few posts on here that say I should set nothing (with exception of code_id, codec_type, width, height and perhaps bit_rate and pix_fmt) as the library now chooses the best settings itself. I have tried various combinations, still avcode_open() fails.

  Does anyone have some advice on what to do or some settings that are current ?

  Thanks.

  Here are one set of H264 settings which give the issue I describe :

  static AVStream* AddVideoStream(AVFormatContext *pOutputFmtCtx, 
  
  int frameWidth, int frameHeight, int fps)
  
  {
  
  AVCodecContext* ctx;
  
  AVStream* stream;
  
  
  
  stream = av_new_stream(pOutputFmtCtx, 0);
  
  if (!stream) 
  
  {
  
      return NULL;
  
  }
  
  
  
  ctx = stream->codec;
  
  
  
  ctx->codec_id = pOutputFmtCtx->oformat->video_codec; //CODEC_ID_H264
  
  ctx->codec_type = AVMEDIA_TYPE_VIDEO;
  
  ctx->width = frameWidth;             //704
  
  ctx->height = frameHeight;           //576
  
  ctx->bit_rate = frameWidth * frameHeight * 4;
  
  
  
  ctx->coder_type = 1;  // coder = 1
  
  ctx->flags|=CODEC_FLAG_LOOP_FILTER;   // flags=+loop
  
  ctx->me_cmp|= 1;  // cmp=+chroma, where CHROMA = 1
  
  ctx->partitions|=X264_PART_I8X8+X264_PART_I4X4+X264_PART_P8X8+X264_PART_B8X8; // partitions=+parti8x8+parti4x4+partp8x8+partb8x8
  
  ctx->me_method=ME_HEX;    // me_method=hex
  
  ctx->me_subpel_quality = 7;   // subq=7
  
  ctx->me_range = 16;   // me_range=16
  
  ctx->gop_size = 250;  // g=250
  
  ctx->keyint_min = 25; // keyint_min=25
  
  ctx->scenechange_threshold = 40;  // sc_threshold=40
  
  ctx->i_quant_factor = 0.71; // i_qfactor=0.71
  
  ctx->b_frame_strategy = 1;  // b_strategy=1
  
  ctx->qcompress = 0.6; // qcomp=0.6
  
  ctx->qmin = 10;   // qmin=10
  
  ctx->qmax = 51;   // qmax=51
  
  ctx->max_qdiff = 4;   // qdiff=4
  
  ctx->max_b_frames = 3;    // bf=3
  
  ctx->refs = 3;    // refs=3
  
  ctx->directpred = 1;  // directpred=1
  
  ctx->trellis = 1; // trellis=1
  
         ctx->flags2|=CODEC_FLAG2_BPYRAMID+CODEC_FLAG2_MIXED_REFS+CODEC_FLAG2_WPRED+CODEC_FLAG2_8X8DCT+CODEC_FLAG2_FASTPSKIP;  // flags2=+bpyramid+mixed_refs+wpred+dct8x8+fastpskip
  
  ctx->weighted_p_pred = 2; // wpredp=2
  
  // libx264-main.ffpreset preset
  
  ctx->flags2|=CODEC_FLAG2_8X8DCT;
  
  ctx->flags2^=CODEC_FLAG2_8X8DCT;    // flags2=-dct8x8
  
  
  
  // if set this get divide by 0 error on avcodec_open()
  
  // if don't set it get -1 error on avcodec_open()
  
  //ctx->pix_fmt = PIX_FMT_YUV420P; 
  
  
  
  return stream;

  }