title: matplotlib FuncAnimation frame interval
body: <p>I am currently experimenting with the matplotbib FuncAnimation and try some exmaples. Everthing runs fine, however, I produce videos via ffmpeg with anim.save(...) and I don't get my animation to play faster/slower. Neither changing </p>
  
  <pre><code>FuncAnimation(...,interval=x,...)
  </code></pre>
  
  <p>nor</p>
  
  <pre><code>anim.save(...,fps=x.)
  </code></pre>
  
  <p>had any effect on the video output. What is the difference between the two ('frames'/'fps' should be 'interval', no?)? Here my reduced code:</p>
  
  <pre><code>import numpy as np
  import matplotlib.animation as animation
  import matplotlib.pyplot as plt
  
  class Ball:
  
      def __init__(self,initpos,initvel,radius,M):
          self.pos=initpos
          self.vel=initvel
          self.radius=radius
          self.M=M
  
  
      def step(self,dt):
  
          self.pos += self.vel*dt
          self.vel[2] += -9.81*dt*self.M
  
  initpos=np.array([5.0,5.0,10.0])
  initvel=np.array([0.0,0.0,0.0])
  radius=0.25
  M=1
  
  
  test_ball = Ball(initpos,initvel,radius,M)  
  dt=1./10
  
  fig = plt.figure()
  ax = fig.add_axes([0, 0, 1, 1], projection='3d')
  
  pts = []
  pts += ax.plot([], [], [], 'bo', c="blue")
  
  def init():
      for pt in pts:
      pt.set_data([], [])
      pt.set_3d_properties([])
      return pts
  
  def animate(i):
  test_ball.step(dt) 
  for pt in pts:
      pt.set_data(test_ball.pos[0],test_ball.pos[1])
      pt.set_3d_properties(test_ball.pos[2])
      pt.set_markersize(10)
      return pts 
  
  anim = animation.FuncAnimation(fig, animate,init_func=init,frames=50,interval=1)
  
  
  mywriter = animation.FFMpegWriter()
  anim.save('mymovie.mp4',writer=mywriter,fps=10)
  </code></pre>
  
  <p>Hope, someone can help me. Thanks a lot.</p>
  
  <p>PS: By the way, I was wondering as well </p>
  
  <ol>
  <li>what the init funcion does because the code also runs perfectly without it but it is in most of the inet-examples. </li>
  <li>what markersize is for an ax.plot(...,'o',...) point. Radius in which unit?  </li>
  </ol>