Frame difference is obtained by the absolute difference
value of two frames. It can reflect the movements in the
frames. Supposing the background of video is static, if the
difference is below some value, it means there is no
movement and it may be caused by noise. Otherwise, it
caused by movement. This value is called threshold. So
moving pixels and static background pixels can be
distinguished by thresholding T in (2). And we call this
image mask. The value of T is determined in part B ofⅡ.