“…In this domain features can further be distinguished as (1) frame-level features vs. features that integrate information over time (time-window or video level), (2) geometric vs. appearance features, and (3) local vs. global features. A variety of frame-level features have been used for recognizing facial pain expression: (1) generic shape features (most often plain landmark coordinates) [1], [2], [5], [8], [12], [16], [17], [20]- [22], [25], [31], [33], [34], [38], [40], [42], [63], [89]; (2) generic appearance features, which include plain pixel representations ("SAPP", "CAPP", and similar) [1], [2], [8], [20], [21], [38], [43], [66], [73], Local Binary Pattern (LBP) [3], [12]- [14], [26], [30], [35], [39], [41], [42], [63], Histogram of Oriented Gradients (HOG) [4], [5], [14], [26], [60], [62], Gabor [18],…”