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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 :
  
  

• How to get video file length uploaded to Amazon S3 ?

  25 mai 2015, par TSP

  I am using plupload to upload video files to Amazon S3 and am playing it using JWPlayer. Before video file is played, I display list of video files uploaded to S3. In this list I would like to display the duration of the video.

  I have read the ffmpeg approach used with PHP. Is there a better approach to get the duration ?

  Regards

• How to lossless rotate video from Sony A7Rii in FFMPEG avoiding codec error ?

  12 janvier 2020, par Jeffs

  I have a bunch of videos filmed on a Sony A7Rii that are upside down and need to be flipped 180. I’d like to do this lossless and been trying to use the following command :

  ffmpeg -i input.mp4 -c copy -metadata:s:v:0 rotate=180 output.mp4

  I understand this updates orientation information in the metadata of the video file, avoiding the need to reencode. However, it is generating an error relating to the audio codec :

  [mp4 @ 000001db6a69cd80] Could not find tag for codec pcm_s16be in stream #1, codec not currently supported in container
  Could not write header for output file #0 (incorrect codec parameters ?) : Invalid argument
  Stream mapping :
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)

  After some searching, I understand this is expected behaviour because the MP4 container isn’t supposed to have audio encoded with that codec. Unfortunately for me, that is how the Sony A7Rii produces files.

  What FFMPEG command will offer the best work-around ? Ideally I’d not re-encode the audio, but that would be tolerable if there is no other way.

  Thanks !