import tensorflow as tf


    @private_method
    def _PS(self, I, r):
        X = tf.transpose(I, [2, 1, 0])  # (r, w, b)
        X = tf.batch_to_space_nd(X, [r], [[0, 0]])  # (1, r*w, b)
        X = tf.transpose(X, [2, 1, 0])
        return X
