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• DIY video file streaming from linux/osx to iOS devices

  27 septembre 2016, par sfactor

  This is for a hobby project. I want to learn about video streaming and also create something that’s useful for me as well.

  The project should be able to run a server on my macbook and a client on my iPad that will allow me to watch the videos I’ve got stored on my laptop without having to copy them in my iPad over my wifi.

  I know there are solutions like Plex, Air Video etc. that allow me to do it. But since my goal is to practice writing some client/server code, I want to create something myself with the basic functionality of these apps.

  I figured I’d probably need something like ffmpeg and Apple’s HTTP Live Streaming (HLS), but I don’t have a Apple developer account to be able to use it. I do have Xcode in my mac. So, some free 3rd party library for HLS or something equivalent. I also fond this tool called https://www.bento4.com.

  How would I go about getting started with such an application and what are the libraries I could use to accomplish this ?

• FFmpeg CRF control using x264 vs libvpx-vp9

  19 octobre 2016, par igon

  I have some experience using ffmpeg with x264 and I wanted to do a comparison with libvpx-vp9. I tested a simple single pass encoding of a raw video, varying the crf settings and presets both with x264 and libvpx-vp9. I am new to libvpx and I followed this and this carefully but I might have still specified wrong combination of parameters since the results I get do not make much sense to me.

  For x264 I did :

  ffmpeg -i test_video.y4m -c:v libx264 -threads 1 -crf <crf> -preset <preset> -y output.mkv 
  
  </preset></crf>

  and obtained the following results :

  codec  , settings                        , time        , PSNR      ,bitrate
  
  libx264,['-crf', '20', '-preset', 'fast'],13.1897280216, 42.938337 ,15728
  
  libx264,['-crf', '20', '-preset', 'medium'],16.80494689, 42.879753 ,15287
  
  libx264,['-crf', '20', '-preset', 'slow'],25.1142120361, 42.919206 ,15400
  
  libx264,['-crf', '30', '-preset', 'fast'],8.79047083855, 37.975141 ,4106
  
  libx264,['-crf', '30', '-preset', 'medium'],9.936599016, 37.713778 ,3749
  
  libx264,['-crf', '30', '-preset', 'slow'],13.0959510803, 37.569511 ,3555

  This makes sense to me, given a crf value you get a value of PSNR and changing the preset can decrease the bitrate but increase the time to encode.

  For libvpx-vp9 I did :

  ffmpeg -i test_video.y4m -c:v libvpx-vp9 -threads 1 -crf <crf> -cpu-used <effort> -y output.mkv 
  
  </effort></crf>

  First of all I thought from tutorials online that the -cpu-used option is equivalent to -preset in x264. Is that correct ? If so what is the difference with -quality ? Furthermore since the range goes from -8 to 8 I assumed that negative values where the fast options while positive values the slowest. Results I get are very confusing though :

  codec     , settings                      , time        , PSNR     ,bitrate
  
  libvpx-vp9,['-crf', '20', '-cpu-used', '-2'],19.6644911766,32.54317,571
  
  libvpx-vp9,['-crf', '20', '-cpu-used', '0'],176.670887947,32.69899,564
  
  libvpx-vp9,['-crf', '20', '-cpu-used', '2'],20.0206270218,32.54317,571
  
  libvpx-vp9,['-crf', '30', '-cpu-used', '-2'],19.7931578159,32.54317,571
  
  libvpx-vp9,['-crf', '30', '-cpu-used', '0'],176.587754965,32.69899,564
  
  libvpx-vp9,['-crf', '30', '-cpu-used', '2'],19.8394429684,32.54317,571

  Bitrate is very low and PSNR seems unaffected by the crf setting (and very low compared to x264). The -cpu-used setting has very minimal impact and also seems that -2 and 2 are the same option.. What am I missing ? I expected libvpx to take more time to encode (which is definitely true) but at the same time higher quality transcodes. What parameters should I use to
  have a fair comparison with x264 ?

  Edit : Thanks to @mulvya and this doc I figured that to work in crf mode with libvpx I have to add -b:v 0. I re-ran my tests and I get :

      codec     , settings                                 , time        , PSNR     ,bitrate
  
  libvpx-vp9,['-crf', '20', '-b:v', '0', '-cpu-used', '-2'],57.6835780144,45.111158,17908
  
  libvpx-vp9,['-crf', '20', '-b:v', '0', '-cpu-used', '0'] ,401.360313892,45.285367,17431
  
  libvpx-vp9,['-crf', '20', '-b:v', '0', '-cpu-used', '2'] ,57.4941239357,45.111158,17908
  
  libvpx-vp9,['-crf', '30', '-b:v', '0', '-cpu-used', '-2'],49.175855875,42.588178,11085
  
  libvpx-vp9,['-crf', '30', '-b:v', '0', '-cpu-used', '0'] ,347.158324957,42.782194,10935
  
  libvpx-vp9,['-crf', '30', '-b:v', '0', '-cpu-used', '2'] ,49.1892938614,42.588178,11085

  PSNR and bitrate went up significantly by adding -b:v 0

• Anomalie #4650 : ?var_mode=inclure fait une grosse bouillie de contenus avec le squelette-dist

  7 février 2021, par Franck D

  Hello, je pense que ce ticket est l’équivalent avec sans doute une meilleurs solution de https://core.spip.net/issues/3531
  Si bien, qu’il faudra faire la fermeture des deux quand cela sera ok :)
  Franck