code: def _get_3rd_point(a, b):
      """To calculate the affine matrix, three pairs of points are required. This
      function is used to get the 3rd point, given 2D points a & b.
  
      The 3rd point is defined by rotating vector `a - b` by 90 degrees
      anticlockwise, using b as the rotation center.
  
      Args:
          a (np.ndarray): point(x,y)
          b (np.ndarray): point(x,y)
  
      Returns:
          np.ndarray: The 3rd point.
      """
      assert len(a) == 2
      assert len(b) == 2
      direction = a - b
      third_pt = b + np.array(
          [-direction[1], direction[0]], dtype=np.float32
      )
  
      return third_pt
  
imports: import numpy
  import cv2
  import copy
label_desc: Use image node_id 3 for calling a global function with example usage: _get_3rd_point(a, b) and returns: third_pt