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• Exception in Tkinter callback while saving animation with matplotlib

  24 août 2017, par paul

  I’m a little hesitant about asking this, since there seem to be many "Exception in Tkinter callback" questions, but I cannot find one that fits the problem I have here.

  I am trying to save an MP4 animation (of a percolation simulation) using matplotlib with ffmpeg. The code works fine on my home laptop, but not on my work PC. It also works fine if I replace the anim.save line with plt.show(), but I do want to save the animation. I’m using Python 3.5.2 on Ubuntu 17.04 (and I have ffmpeg installed).

  Here is the error :

  >>> Exception in Tkinter callback
  
  Traceback (most recent call last):
  
    File "/usr/lib/python3.5/tkinter/__init__.py", line 1558, in __call__
  
      return self.func(*args)
  
    File "/usr/lib/python3.5/tkinter/__init__.py", line 604, in callit
  
      func(*args)
  
    File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_tkagg.py", line 373, in idle_draw
  
      self.draw()
  
    File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_tkagg.py", line 354, in draw
  
      FigureCanvasAgg.draw(self)
  
    File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_agg.py", line 474, in draw
  
      self.figure.draw(self.renderer)
  
    File "/usr/lib/python3/dist-packages/matplotlib/artist.py", line 62, in draw_wrapper
  
      draw(artist, renderer, *args, **kwargs)
  
    File "/usr/lib/python3/dist-packages/matplotlib/figure.py", line 1165, in draw
  
      self.canvas.draw_event(renderer)
  
    File "/usr/lib/python3/dist-packages/matplotlib/backend_bases.py", line 1809, in draw_event
  
      self.callbacks.process(s, event)
  
    File "/usr/lib/python3/dist-packages/matplotlib/cbook.py", line 563, in process
  
      proxy(*args, **kwargs)
  
    File "/usr/lib/python3/dist-packages/matplotlib/cbook.py", line 430, in __call__
  
      return mtd(*args, **kwargs)
  
    File "/usr/lib/python3/dist-packages/matplotlib/animation.py", line 661, in _start
  
      self._init_draw()
  
    File "/usr/lib/python3/dist-packages/matplotlib/animation.py", line 1221, in _init_draw
  
      self._draw_frame(next(self.new_frame_seq()))
  
  StopIteration

  The code producing the error is :

  def percolate():
  
  
  
      # initialize an instance of the Percolator class
  
      perc = Percolator(1, 100, 0.1)
  
  
  
      # initialize the image
  
      fig, ax = plt.subplots()
  
      im = plt.imshow(perc.states)
  
  
  
      anim = animation.FuncAnimation(fig, perc.update, perc.update_iter, repeat=False, fargs=(im, ), save_count=perc.n**2)
  
      anim.save("perc.mp4")

  I can reproduce the code for the Percolator class if necessary, but that part is working fine. It has two functions : update_iter, a generator function that yields True as long as the animation should continue, and update, which takes (the result of the iterator and) im as inputs, and its last two lines are

  im.set_array(self.states)
  
  return im,

  UPDATE :

  Here’s an MWE.

  import numpy as np
  
  import matplotlib.pyplot as plt
  
  from matplotlib import animation
  
  
  
  
  
  class Percolator:
  
  
  
      def __init__(self):
  
          self.i = 0
  
          self.states = np.zeros((10, 10))
  
          self.end = False
  
  
  
      def update(self, garbage=None, im=None):
  
          self.i += 1
  
          if self.i == 10:
  
              self.end = True
  
          im.set_array(self.states)
  
          return im,
  
  
  
      def update_iter(self):
  
          while self.end == False:
  
              yield True
  
  
  
  def percolate():
  
  
  
      perc = Percolator()
  
      fig, ax = plt.subplots()
  
      im = plt.imshow(perc.states)
  
  
  
      anim = animation.FuncAnimation(fig, perc.update, perc.update_iter, repeat=False, \
  
                                     fargs=(im, ), save_count=100)
  
  
  
      anim.save("perc.gif", writer="imagemagick")

  In this example, the Percolator class doesn’t do anything interesting - it sets up a 10x10 grid of 0s, and each call to its update function sets the image to the same 10x10 grid.

  If the frames attribute of FuncAnimation is set to 50 (say), rather than to perc.update_iter, then there is no error and the image is saved correctly. So the problem seems to be with my generator function. I want to use the generator function because I want to keep creating new frames until some condition on perc.states is met - here, boringly, I’ve just asked it to keep going for 10 iterations.

  System details : Python 3.5.3, matplotlib 2.0.0, Ubuntu 17.04.

  UPDATE 2 :

  Same problem after upgrading to matplotlib 2.0.2. Also, printing some output along the way reveals that the error occurs at the end of the iterations. In fact, if update_iter is changed to :

  def update_iter(self):
  
      print(self.end)
  
      while self.end == False:
  
          yield True

  ... then the output is :

  False
  
  False
  
  False
  
  False
  
  False
  
  False
  
  False
  
  False
  
  False
  
  False
  
  False
  
  True
  
  >>> True
  
  Exception in Tkinter callback
  
  Traceback (most recent call last):
  
  etc.

• How to extract frames from a video at browser using javascript ? [on hold]

  13 septembre 2018, par Hrisheekesh R

  Problem Statement :

  1. User picks a video to upload
  2. 20 screenshots at random timestamps at the video is captured inside the browser
  3. The screenshots are sent to a server.

  Is there a way to capture the frames inside a browser ?

• What fourcc should I use when encoding mp4 videos with VP9 ?

  15 juin 2020, par Jamie

  My goal is to write an mp4 video using the cv2 Python library, which itself depends on ffmpeg. What code should I use as the fourcc parameter, i.e., :

  



  fourcc = cv2.VideoWriter_fourcc(*'????')