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• aacenc_ltp : adjust and speed up autocorrelation calculations

  17 octobre 2015, par Rostislav Pehlivanov

  aacenc_ltp : adjust and speed up autocorrelation calculations
  There were some errors in the calculation as well as an entire
  
  unnecessary loop to find the gain coefficient. Merge the
  
  two loops.
  
  Thanks to @ubitux for the suggestions and testing.
  

  • [DH] libavcodec/aacenc_ltp.c
  • [DH] tests/fate/aac.mak

• Animation speed adjustment using ffmpeg in Python

  5 novembre 2015, par neither-nor

  I’m been for years using stock ffmpeg script to sequentially snitch together temporal plots in Python. However, I cannot figure out the trivial issue of how to, for instance, slow down the animation speed so that the resultant video file has a longer duration.

  Example :

  import matplotlib.pyplot as plt
  
  import os, sys
  
  
  
  for t in range(100):
  
      plt.cla()
  
      plt.text(0.5, 0.5, 'time %02d'%(t+1))
  
      plt.draw()
  
  
  
      fname = '_tmp%02d.png'%(t+1)
  
      plt.savefig(fname)
  
  
  
  os.system("ffmpeg -i _tmp%02d.png -pix_fmt yuv420p -r 20 -b:v 20M flipbook.mp4")
  
  os.system("rm _tmp*.png")    

  The resulting "flip book" takes 4s and the time stamp increases steadily. However, I tried to make the animation last twice as long by testing the following :

  1. Change 20 after -r to 1 : this still lasts 4s but now the time stamp "leaps" nonlinearly

  2. Change 20M to 1M : no discernible effect

  I can’t find much information about this line of code, either the usage of each flag or how to modify aspects of it (e.g.,speed).

• swresample/resample : speed up build_filter for Blackman-Nuttall filter

  5 novembre 2015, par Ganesh Ajjanagadde

  swresample/resample : speed up build_filter for Blackman-Nuttall filter
  This uses the trigonometric double and triple angle formulae to avoid
  
  repeated (expensive) evaluation of libc’s cos().
  Sample benchmark (x86-64, Haswell, GNU/Linux)
  
  test : fate-swr-resample-dblp-44100-2626
  
  old :
  
  1104466600 decicycles in build_filter(loop 1000),     256 runs,      0 skips
  
  1096765286 decicycles in build_filter(loop 1000),     512 runs,      0 skips
  
  1070479590 decicycles in build_filter(loop 1000),    1024 runs,      0 skips
  new :
  
  588861423 decicycles in build_filter(loop 1000),     256 runs,      0 skips
  
  591262754 decicycles in build_filter(loop 1000),     512 runs,      0 skips
  
  577355145 decicycles in build_filter(loop 1000),    1024 runs,      0 skips
  This results in small differences with the old expression :
  
  difference (worst case on [0, 2*M_PI]), argmax 0.008 :
  
  max diff (relative) : 0.000000000000157289807188
  
  blackman_old(0.008) : 0.000363951585488813192382
  
  blackman_new(0.008) : 0.000363951585488755946507
  These are judged to be insignificant for the performance gain. PSNR to
  
  reference file is unchanged up to second decimal point for instance.
  Reviewed-by : Michael Niedermayer <michael@niedermayer.cc>
  
  Signed-off-by : Ganesh Ajjanagadde <gajjanagadde@gmail.com>
  

  • [DH] libswresample/resample.c