The calibration set is usually a subset of the training/validation dataset or actual application images (at least 100 images for performance). The input function is a python importable function to load the calibration dataset and perform data preprocessing. The vai_q_tensorflow quantizer can accept an input_fn to do the preprocessing which is not saved in the graph. If the preprocessing subgraph is saved into the frozen graph, the input_fn only needs to read the images from dataset and return a feed_dict.