1. A method for processing an image sequence, the method comprising: providing a fully constrained nonlinear equation set including: a forward displaced frame difference equation, a backward displaced frame difference equation, and a plurality of displacement vector invariant equations; receiving an input image sequence comprising a pair of image frames individually including multidimensional image data corresponding to a plurality of pixel locations at different times; and using at least one processor, solving the equation set using an iteration equation and the image data of the pair of image frames to determine a displacement field describing displacement vectors at pixel locations at one time by solving displacement field values at another time from the plurality of displacement vector invariant equations.