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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 :
  
  

• avformat/mov : Free temp buffer upon negative sample_size error.

  19 mai 2020, par Dale Curtis

  avformat/mov : Free temp buffer upon negative sample_size error.
  2d8d554f15a7a27cfeca81467cc9341a86f784e2 added a new error condition
  
  to mov_read_stsz() but forgot to free a temporary buffer when it
  
  occurs.
  Signed-off-by : Dale Curtis <dalecurtis@chromium.org>
  
  Signed-off-by : Michael Niedermayer <michael@niedermayer.cc>
  

  • [DH] libavformat/mov.c

• My webcam works fine, while using cv2.Videocapture() but while trying to read from hard drive it shows error cap.isOpened() returns false, why ?

  27 septembre 2017, par Hu Yang

  import cv2
  video_capture = cv2.VideoCapture("rtsp ://admin:Trust1986_@10.58.94.47:554/h264/ch1/main/av_stream")
  video_capture.isOpened()

  False

  Other information below here, and ffmpeg I had installed.

    Video I/O:
  
      DC1394 1.x:                  NO
  
      DC1394 2.x:                  NO
  
      FFMPEG:                      NO
  
        avcodec:                   NO
  
        avformat:                  NO
  
        avutil:                    NO
  
        swscale:                   NO
  
        avresample:                NO
  
      GStreamer:                   NO
  
      OpenNI:                      NO
  
      OpenNI PrimeSensor Modules:  NO
  
      OpenNI2:                     NO
  
      PvAPI:                       NO
  
      GigEVisionSDK:               NO
  
      Aravis SDK:                  NO
  
      UniCap:                      NO
  
      UniCap ucil:                 NO
  
      V4L/V4L2:                    NO/NO
  
      XIMEA:                       NO
  
      Xine:                        NO
  
      Intel Media SDK:             NO
  
      gPhoto2:                     NO
  
  
  
  root@ubuntu:/home/face_recognition/opencv/ffmpeg/ffmpeg-3.3.4# ffmpeg -version
  
  ffmpeg version 3.3.4 Copyright (c) 2000-2017 the FFmpeg developers
  
  built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04.3)
  
  configuration: 
  
  libavutil      55. 58.100 / 55. 58.100
  
  libavcodec     57. 89.100 / 57. 89.100
  
  libavformat    57. 71.100 / 57. 71.100
  
  libavdevice    57.  6.100 / 57.  6.100
  
  libavfilter     6. 82.100 /  6. 82.100
  
  libswscale      4.  6.100 /  4.  6.100
  
  libswresample   2.  7.100 /  2.  7.100
  
  root@ubuntu:/home/face_recognition/opencv/ffmpeg/ffmpeg-3.3.4# 
  
  root@ubuntu:/home/face_recognition/opencv/ffmpeg/ffmpeg-3.3.4# python3.6m -V
  
  Python 3.6.2
  
  root@ubuntu:/home/face_recognition/opencv/ffmpeg/ffmpeg-3.3.4# pkg-config --modversion opencv  
  
  3.3.0

  what shall I do next step ? Thanks.