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• Matplotlib : Live animation works fine but displays a blank plot when being saved

  18 juillet 2017, par Loïc Poncin

  I made a little forest fire animation. My code is at the end of the question.

  Here is some information before I ask my question :

  • No tree : forest[i,j] = 0
  • A tree : forest[i,j] = 1
  • A tree on fire : forest[i,j] = 2

  Basically what happens is that constructforest creates a 2 dimensional array called forest of size n by m with a probability of tree occupancy called p. After that setonfire sets on fire the forest and while the forest can burn spreadfire spread the fire.

  When I run forestfire with the Python prompt or the IPython prompt I get a nice animation but when I go check the video file that I saved I only see a blank plot.

  I did some research, I found many questions about this issue but none of the advice I read was helpful :

  • matplotlib animation produces a blank
  • Matplotlib animation not working in IPython Notebook (blank plot)
  • Animation from matplotlib not working in spyder
  • Spyder Python Animation not working

  Can someone tell me what is going on please ?

  forestfire.py

  from random import random
  
  
  
  import numpy as np
  
  
  
  import matplotlib.pylab as plt
  
  import matplotlib.colors as mcolors
  
  import matplotlib.animation as animation
  
  
  
  
  
  def hazard(p):
  
      r=random()
  
      assert p>=0 and p<=1
  
      return r <= p
  
  
  
  
  
  def constructforest(n,m,p):
  
      forest = np.zeros((n,n))
  
      for i in xrange(n):
  
          for j in xrange(m):
  
              if hazard(p):
  
                  forest[i,j] = 1
  
      return forest
  
  
  
  
  
  def setfire(forest,i,j):
  
      forest[i,j] = 2
  
      return forest
  
  
  
  
  
  def spreadfire(forest):    
  
  
  
      n,m=forest.shape
  
      c = np.copy(forest)
  
  
  
      for i in xrange(n):
  
          for j in xrange(m):
  
  
  
              if c[i,j] == 1:
  
  
  
                  Y, X = xrange(max(0,i-1),min(n,i+2)), xrange(max(0,j-1),min(m,j+2))
  
  
  
                  for y in Y:
  
                      for x in X:
  
  
  
                          if c[y,x] == 2:
  
                              forest[i,j] = 2                        
  
      return forest
  
  
  
  
  
  def canburn(forest):    
  
  
  
      n,m=forest.shape
  
      c = np.copy(forest)
  
  
  
      for i in xrange(n):
  
          for j in xrange(m):
  
  
  
              if c[i,j] == 1:
  
  
  
                  Y, X = xrange(max(0,i-1),min(n,i+2)), xrange(max(0,j-1),min(m,j+2))
  
  
  
                  for y in Y:
  
                      for x in X:
  
  
  
                          if c[y,x] == 2:
  
                              return True                      
  
      return False
  
  
  
  
  
  def forestfire(forest):
  
  
  
      fig, ax = plt.subplots()
  
  
  
      movie = []    
  
  
  
      # Colormap
  
      red, green, blue = [(1,0,0,1)], [(0,1,0,1)], [(0,0,1,1)]   
  
  
  
      colors = np.vstack((blue, green, red))
  
      mycmap = mcolors.LinearSegmentedColormap.from_list('my_colormap', colors)
  
  
  
      # Initialization
  
      k = 0
  
  
  
      forest = spreadfire(forest)
  
  
  
      im = plt.imshow(forest, animated=True, cmap = mycmap, interpolation="none", origin='lower')
  
      movie.append([im])
  
  
  
      # Fire propagation
  
      while canburn(forest):
  
          k += 1
  
          print k
  
  
  
          forest = spreadfire(forest)
  
  
  
          im = plt.imshow(forest, animated=True, cmap = mycmap, interpolation="none", origin='lower')
  
          movie.append([im])
  
  
  
      return animation.ArtistAnimation(fig, movie, blit=True, repeat_delay=100)
  
  
  
  
  
  
  
  ani = forestfire(setfire(constructforest(101,101,0.4),50,50))
  
  
  
  ani.save("forestfire_test.mp4", writer = 'ffmpeg', fps=5, dpi=500)

  EDIT

  As requested by @Y.Luo by @ImportanceOfBeingErnest in the comments I downgraded matplotlib to 2.0.0 and I changed the framerate of the animation but forestfire_test.mp4 still displays a blank plot.

  Here are my settings :
  
  

• FFmpeg and Jupyter Notebooks

  12 mai 2022, par Clerni

  I'm getting the error RuntimeError: Requested MovieWriter (ffmpeg) not available when trying to run this simple example of creating and displaying an animation in a Jupyter Notebook.

  


  ---------------------------------------------------------------------------&#xA;RuntimeError                              Traceback (most recent call last)&#xA; in <module>&#xA;      8 &#xA;      9 # Define the meta data for the movie&#xA;---> 10 FFMpegWriter = manimation.writers[&#x27;ffmpeg&#x27;]&#xA;     11 metadata = dict(title=&#x27;Movie Test&#x27;, artist=&#x27;Matplotlib&#x27;,&#xA;     12                 comment=&#x27;a red circle following a blue sine wave&#x27;)&#xA;&#xA;/usr/local/lib/python3.8/dist-packages/matplotlib/animation.py in __getitem__(self, name)&#xA;    164         if self.is_available(name):&#xA;    165             return self._registered[name]&#xA;--> 166         raise RuntimeError(f"Requested MovieWriter ({name}) not available")&#xA;    167 &#xA;    168 &#xA;&#xA;RuntimeError: Requested MovieWriter (ffmpeg) not available&#xA;&#xA;</module>

  


  Running !pip install ffmpeg didn't help, as ffmpeg is already installed, apparently :

  


  Requirement already satisfied: ffmpeg in /home/username/.local/lib/python3.8/site-packages


  


  How can I make this work ?

  


• Does the incoming codec or audio-video sync matter when encoding from Xvfb using FFMPEG ?

  15 décembre 2022, par Nav

  How I'm capturing :
  
I'm using Xvfb on Linux, to capture a video and audio from an app that is also running on Linux. I'm using -f x11grab when capturing that video headlessly from Xvfb's frambuffer using FFMPEG, and encoding it using a few other parameters like -c:v libx264 -pix_fmt yuv420p.

  


  The questions :
  
Since I'm reading from Xvfb framebuffer :

  


  

  1. Would it matter that the app uses SVC to encode the video, and I'm using H.264 ?

  


  2. Would it matter if I use a different framerate than the app is using ?

  


  3. Would it matter if I use a different pix_fmt than what the app is using ?

  


  


  Reason for asking :

  


  
  
As shown in the image (blue is the app, red is FFMPEG, green is Xvfb) ;

  


  

  • The CPU percentage consumed by FFMPEG shoots up when the app's CPU percentage shoots up. I've also encountered audio being ahead of video for some participants, but I'm unable to figure out whether it's due to network lag or due to FFMPEG's encoding.

  


  • Also, the aim is to reduce CPU consumption, so I was looking to see if I could do FFMPEG encoding in the same format as the app.