code: def obj_box_coord_upleft_butright_to_centroid(coord):
      
      if len(coord) != 4:
          raise AssertionError("coordinate should be 4 values : [x1, y1, x2, y2]")
      x1, y1, x2, y2 = coord
      w = x2 - x1
      h = y2 - y1
      x_c = x1 + w / 2.
      y_c = y1 + h / 2.
      return [x_c, y_c, w, h]
docs: Convert one coordinate [x1, y1, x2, y2] to [x_center, y_center, w, h].
      It is the reverse process of ``obj_box_coord_centroid_to_upleft_butright``.
  
      Parameters
      ------------
      coord : list of 4 int/float
          One coordinate.
  
      Returns
      -------
      list of 4 numbers
          New bounding box.