1. INTRODUCTION
Nowadays, face recognition has become a common authentication method in various scientific applications. However, there is an important issue that face recognition system must face in practice. That is, the captured faces are usually of low resolution (LR) because of the influence of distance, lighting, poses, weather, and the limitation of cameras. This leads to the poor performance of traditional face recognition algorithms. To solve this issue, many face hallucination (FH, also called face super-resolution) techniques, have been proposed over the past decade; see, for example, [1], [2], [3], [4].