text: #!/usr/bin/env python
  
  '''
  Simple "Square Detector" program.
  
  Loads several images sequentially and tries to find squares in each image.
  '''
  
  # Python 2/3 compatibility
  from __future__ import print_function
  import sys
  PY3 = sys.version_info[0] == 3
  
  if PY3:
      xrange = range
  
  import numpy as np
  import cv2 as cv
  
  
  def angle_cos(p0, p1, p2):
      d1, d2 = (p0-p1).astype('float'), (p2-p1).astype('float')
      return abs( np.dot(d1, d2) / np.sqrt( np.dot(d1, d1)*np.dot(d2, d2) ) )
  
  def find_squares(img):
      img = cv.GaussianBlur(img, (5, 5), 0)
      squares = []
      for gray in cv.split(img):
          for thrs in xrange(0, 255, 26):
              if thrs == 0:
                  bin = cv.Canny(gray, 0, 50, apertureSize=5)
                  bin = cv.dilate(bin, None)
              else:
                  _retval, bin = cv.threshold(gray, thrs, 255, cv.THRESH_BINARY)
              contours, _hierarchy = cv.findContours(bin, cv.RETR_LIST, cv.CHAIN_APPROX_SIMPLE)
              for cnt in contours:
                  cnt_len = cv.arcLength(cnt, True)
                  cnt = cv.approxPolyDP(cnt, 0.02*cnt_len, True)
                  if len(cnt) == 4 and cv.contourArea(cnt) > 1000 and cv.isContourConvex(cnt):
                      cnt = cnt.reshape(-1, 2)
                      max_cos = np.max([angle_cos( cnt[i], cnt[(i+1) % 4], cnt[(i+2) % 4] ) for i in xrange(4)])
                      if max_cos < 0.1:
                          squares.append(cnt)
      return squares
  
  def main():
      from glob import glob
      for fn in glob('../data/pic*.png'):
          img = cv.imread(fn)
          squares = find_squares(img)
          cv.drawContours( img, squares, -1, (0, 255, 0), 3 )
          cv.imshow('squares', img)
          ch = cv.waitKey()
          if ch == 27:
              break
  
      print('Done')
  
  
  if __name__ == '__main__':
      print(__doc__)
      main()
      cv.destroyAllWindows()
  