“…To overcome the difficulties mentioned above, low‐level features such as pixel‐level motion and appearance have been widely applied [1, 9, 13, 15, 19, 20, 23, 26, 29, 33, 35–37, 43–47, 52]. Optical flow is one of the most popular features for motion pattern, which can be used to describe the position and motion direction of each local patch and generate bag‐of‐words representation [15, 33, 35–37, 46]. Histograms of local optical flow [13], a histogram of gradient and optical flow [20], and a multi‐scale histogram of optical flow [23, 47] can encode the flow into a histogram and generate rotation and translation invariant representation.…”