class_docstr: Only calculate the unknown region if trimap provided.
code: def __init__(self):
      self.sad_diffs = 0
      self.count = 0
  
imports: import cv2
  import numpy
  from scipy.ndimage.filters import convolve
  from scipy.special import gamma
  from skimage.measure import label
label_desc: Use image node_id 1 to create a new SAD object  with example: obj = SAD()