class_docstr: Making binary mask from detection data with ICDAR format.
  Typically following the process of class `MakeICDARData`.
code: def polygon_area(self, polygon):
      return cv2.contourArea(polygon)
  
file_path: PaddleOCR/benchmark/PaddleOCR_DBNet/data_loader/modules/make_shrink_map.py
imports: import numpy
  import cv2
label_desc: Use image node_id 4 for calling the MakeShrinkMap obj's underlying member method code with example usage: obj.polygon_area(polygon) and returns: cv2