Simple question about HS algorithm's formul(Optical flow)

In the below pic, I can not understand what U vector is? It says flow field but I can not imagie what really is the flow field?

• I think it is derivative of I, dI / dt (frame to frame). And you aim to solve for it – MimSaad Jul 3 '18 at 19:51
• u is the displacement field which deforms your initial frame. each intensity value at pixel x moved to x+u in the next frame. – Mohammad M Jul 4 '18 at 13:16