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• ffmpeg VP9 - What do the quality settings change ?

  27 mai 2015, par Yay295

  ffmpeg has a -quality setting when encoding VP9 with options best, good, and realtime. How do these options affect the other available encoding options (ex. -speed, -slices, -frame-parallel, etc...) ? I read somewhere that -best and -good -speed 0 will give about the same quality, with the later being much faster. To me this makes it sound like the quality settings just change the other options (such as -speed) as if they are merely presets and one could achieve the same results manually. Is this true, or do the quality settings affect things I couldn’t change with other options ?

• ffmpeg invalid argument for vp9 encoding

  17 juin 2015, par user3083672

  I’m following the recommended settings for vp9 encoding here : http://wiki.webmproject.org/ffmpeg/vp9-encoding-guide

  Using this command :

  ffmpeg -i test.mp4 -c:v libvpx-vp9 -pass 1 -b:v 1000K -threads 8 -speed 4 \
  
  -tile-columns 6 -frame-parallel 1 -auto-alt-ref 1 -lag-in-frames 25 \
  
  -an -f webm /dev/null

  produces the following error :

  [NULL @ 0000000005a209c0] Unable to find a suitable output format for '\'
  
  \: Invalid argument

  Any ideas ?

• Summary Video Accessibility Talk

  1er janvier 2014, par silvia

  I’ve just got off a call to the UK Digital TV Group, for which I gave a talk on HTML5 video accessibility (slides best viewed in Google Chrome).

  The slide provide a high-level summary of the accessibility features that we’ve developed in the W3C for HTML5, including :

  • Subtitles & Captions with WebVTT and the track element
  • Video Descriptions with WebVTT, the track element and speech synthesis
  • Chapters with WebVTT for semantic navigation
  • Audio Descriptions through synchronising an audio track with a video
  • Sign Language video synchronized with a main video

  I received some excellent questions.

  The obvious one was about why WebVTT and not TTML. While for anyone who has tried to implement TTML support, the advantages of WebVTT should be clear, for some the decision of the browsers to go with WebVTT still seems to be bothersome. The advantages of CSS over XSL-FO in a browser-context are obvious, but not as much outside browsers. So, the simplicity of WebVTT and the clear integration with HTML have to speak for themselves. Conversion between TTML and WebVTT was a feature that was being asked for.

  I received a question about how to support ducking (reduce the volume of the main audio track) when using video descriptions. My reply was to either use video descriptions with WebVTT and do ducking during the times that a cue is active, or when using audio descriptions (i.e. actual audio tracks) to add an additional WebVTT file of kind=metadata to mark the intervals in which to do ducking. In both cases some JavaScript will be necessary.

  I received another question about how to do clean audio, which I had almost forgotten was a requirement from our earlier media accessibility document. “Clean audio” consists of isolating the audio channel containing the spoken dialog and important non-speech information that can then be amplified or otherwise modified, while other channels containing music or ambient sounds are attenuated. I suggested using the mediagroup attribute to provide a main video element (without an audio track) and then the other channels as parallel audio tracks that can be turned on and off and attenuated individually. There is some JavaScript coding involved on top of the APIs that we have defined in HTML, but it can be implemented in browsers that support the mediagroup attribute.

  Another question was about the possibilities to extend the list of @kind attribute values. I explained that right now we have a proposal for a new text track kind=”forced” so as to provide forced subtitles for sections of video with foreign language. These would be on when no other subtitle or caption tracks are activated. I also explained that if there is a need for application-specific text tracks, the kind=”metadata” would be the correct choice.

  I received some further questions, in particular about how to apply styling to captions (e.g. color changes to text) and about how closely the browser are able to keep synchronization across multiple media elements. The earlier was easily answered with the ::cue pseudo-element, but the latter is a quality of implementation feature, so I had to defer to individual browsers.

  Overall it was a good exercise to summarize the current state of HTML5 video accessibility and I was excited to show off support in Chrome for all the features that we designed into the standard.