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• matplotlib funcanimation save issue

  20 avril 2015, par Richie Abraham

  I have been trying some animation recently using matplotlib animations. It has been going great, i create an ffmpeg writer and save it as a video file. However i face an issue whenever the function that FuncAnimation calls returns more than one object.

  Below is a small snippet of my code base. When I return both im0 and im1, the video file created only has im1 , although the plt.show command works as expected ( showing both the videos ). If i return just a single im0, then it works as expected. IT also works as expected if i return both im0 and im1 with alpha=0.5.

  Can anyone shed some light on what is happening underneath the hood ?

  fig, ax = plt.subplots(1)
  
  def animate(i):
  
      im0=ax.imshow(np.ma.masked_array(imgl[i][:,:,0], mask=get_blob(i)),cmap='cubehelix')
  
  
  
      im1=ax.imshow(imgl[(i-100)%len(imgl)][:,:,0],cmap='cubehelix')
  
  
  
      return [im1,im0]
  
  
  
  
  
  
  
  ani = animation.FuncAnimation(fig, animate, frames=200,
  
                                interval=10, blit=True,repeat=False)
  
  ani.save('ps.mp4', writer=writer)
  
  plt.show()

• what FFMPEG performance settings to use for processing videos for the web

  19 juin 2015, par eAbi

  I have a few questions regarding usage of ffmpeg for processing videos for the web. I’m a beginner so please bear with me (although I read some docs on the internet)

  Performance

  • First of all, given the fact that FFMPEG utilizes all cores at 100%, what is the actual parallelism efficiency ?

  Let’s assume the following scenario. I have a video (fullHD, doesn’t matter what encoders / compression format was used to obtain the video) and I want to resize (downscale) to various sizes (e.g. 240px, 480px and 720px height) using mp4 format (thus using libx264 with aac codecs).

  Using ffmpeg, I see that all of my laptop’s cores (8) are used at 100% and I was wondering what scenarios can improve the overall performance of the whole processing task. So this leads us to basically 2 scenarios : Assuming the video mentioned above as input, for obtaining the 3 output videos (@ 240px, 480px and 720px height sizes), we :

  1. Process input video and obtain 1 output video at a time, and let all the cores work at the same time at 100% ;
  2. Process the video to obtain all output videos in parallel, by bounding each output video to a single processor core which’ll work at 100% ;

  So the question is actually reduced to the parallelism efficiency of the ffmpeg program.

  This means that letting ffmpeg process the task procVideo - which takes 1 input video to produce 1 single output video (transcoding/downscaling and so on) - on N processor cores doesn’t mean it finish the task N times faster than letting it run the same task bound to a single core. So if the efficiency is smaller than 100%, it’s better to have N procVideo tasks in parallel, each bound to a single core, rather than doing the task sequentially for each output video.

  ---

  Codecs

  Other than the above performance problem, the usage of codecs bugs me. I am trying to obtain mp4 videos because of the wide implementation of the format in html5 browsers.

  So having a video as input in any format, I want to convert it to mp4. So I’m using libx264 codec with aac.

  • Use libx264, x264 or h264 for video encoding/decoding ?
  • Use libfdk_aac, libaacplus or aac for audio encoding/decoding to aac ?

  Also, I would like to know what are the licesing fees for each of the above codec, as the online resources on these are quite limited / hard to understand.

  If anyone could shed some light on those questions, I would really be grateful ! Thanks for your time !

• Using FFMPEG library with iPhone SDK for video encoding

  21 juin 2015, par user203349

  I need to encode several pictures shot by the iphone camera into a mp4 video file and I know FFMPEG can do this (the application TimeLapser and ReelMoments do it already). I plan to use this in my app iMotion (available in the appstore).

  I successfully install and compile the ffmpeg for the iphone SDK with this link :
  http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/2009-October/076618.html

  But now I’m stuck here in my XCode project. What should I do next to use the FFMPEG library for video encoding ? The Apple documentation about external library using is very light and I just can find any tutorial on the web wich explain how to do this.

  If anybody can help me, I would be very grateful..`

  Thanks

  Aurelien Potier