“…From the view of the representation, spatial graphs and spatio-temporal graphs are dominant. Specifically, handcrafted features (e.g., LBP [81,83], Gabor [22,115], HOG [36,87,104]) or deep-based features (e.g., CNN [74,115], VGG [91], ResNet [39]) are employed to enhance the node representation similar to many non-graph FER methods [20,31]. For reasoning approaches, early studies prefer to capture the relations of an individual node from predefined graph structures using tracking strategies (e.g., displacement projection [76] and DNG [90]) or general machine learning models (e.g., RF [40], RNN [22], CNN [36]).…”