“…Facial expressions can be divided into six basic emotions, namely, anger (An); disgust (Di); fear (Fe); happiness (Ha); sadness (Sa); surprise (Su); and one neutral (Ne) emotion [ 9 ], contempt (Co), was subsequently added as one of the basic emotions [ 10 ]. Recognition of these emotions can be categorized into image-based [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ] and video-based [ 38 , 39 , 40 , 41 , 42 , 43 ] approaches. Image-based approaches only use information about the static input image to determine the category of facial expression; on the other hand, except when the spatial features extracted from a static image are available, video-based approaches can also use temporal information of a dynamic image sequence to capture the temporal changes of facial appearance when some facial expression occurs.…”