code: def _water_detection(img_data):
      data = np.asarray(img_data).copy()
      data[data == 150] = 31
      data[data == 199] = 18
      data[data >= 200] = data[data >= 200] - 100
  
      return data
  
imports: import numpy
  from trollimage.colormap import Colormap
  from satpy.enhancements import exclude_alpha, using_map_blocks
label_desc: Use image node_id 2 for calling a global function with example usage: _water_detection(img_data) and returns: data