class_docstr: Only calculate the unknown region if trimap provided.
  Refer to: Refer to: https://github.com/open-mlab/mmediting/blob/master/mmedit/core/evaluation/metrics.py
code: def evaluate(self):
      conn = self.conn_diffs / self.count if self.count > 0 else 0
      return conn
  
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 3 for calling the Conn obj's underlying member method code with example usage: obj.evaluate() and returns: conn