text: #!/usr/bin/env python3
  # -*- coding: utf-8 -*-
  """
  Created on Mon Apr 30 15:28:09 2018
  
  @author: dataquanty
  """
  
  
  import numpy as np
  from math import sqrt, pi, acos,cos
  from matplotlib import pyplot as plt
  from scipy.misc import imsave
  from bisect import bisect_left
  
  h , w = 1000, 1000
  img = np.ones((h,w))
  
  
  center = (500,500)
  r = [20,80,200,300,400,500,600]
  r = np.exp(range(1,8)).astype(int)
  lagval = [0,pi,0,pi,0,pi,0]
  maxd = 810
  r = range(10,maxd,20)
  lagval = [0,pi]*int(len(r)/2)
  
  lagval = np.random.rand(len(r))*pi
  lagval = [-pi/4,pi/3]*int(len(r)/2)
  lagval = [0,0.05]*int(len(r)/2)
  
  def dist(a,b):
      return sqrt((a[0]-b[0])**2+(a[1]-b[1])**2)
  
  
  for i in range(h):
      for j in range(w):
          if (i,j) == center:
              img[i][j]=0
          
          else: 
              d = dist((i,j),center)
              k = bisect_left(list(r),d) 
              
              
              #dist((i,j),center)<= r1:
              val = (j-center[1])/d
              img[i][j] = cos(acos(val)-lagval[k])
              """
              angle = acos((j-center[1])/dist((i,j),center))
              if i > center[0]:
                  angle = 2*pi - angle
              val = ((angle - lagrad)%(2*pi))/2*pi
              img[i][j] = val
              """
  
  
  #imsave('figLag_pi_s2.png',img)
  plt.figure(figsize=(10,10))       
  plt.imshow(img,cmap='gray')
  
  #interpolation='nearest'
  plt.show()