... Nowadays, with the rapid development of mobile devices, a huge amount of effort has been devoted to dealing with various tasks in computer vision and machine learning tasks, such as activity recognition [34,41], motion tracking [6,61,64], behavior prediction [35,36,47], and political ideology prediction [2,44,45,62]. Among them, classification based on image sets, as a promising technique, has received significant attention and has been increasingly applied for many practical applications, such as video surveillance [58], action recognition [50], and face recognition [12,19]. ...