code: def gradient_to_center(x, ctr=0.5):
      """return normalised vector pointing to center"""
      v = ctr - x
      v /= (v**2).sum() ** 0.5
      return v
  
imports: import numpy
  from math import gamma, pi, exp
label_desc: Use image node_id 4 for calling a global function with example usage: gradient_to_center(x, ctr) and returns: v