code: def PSNR(y_true, y_pred, MAXp=1):
      """
      Evaluates the PSNR value:
          PSNR = 20 * log10(MAXp) - 10 * log10(MSE).
  
      Args:
          y_true: ground truth.
          y_pred: predicted value.
          MAXp: maximum value of the pixel range (default=1).
      """
      return (
          -10.0 * K.log(K.mean(K.square(y_pred - y_true))) / K.log(10.0)
      )
  
label_desc: Use image node_id 1 for calling a global function with example usage: PSNR(y_true, y_pred, MAXp) and returns: unknown