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, wherein the multidimensional image data of the pair of frames is two-dimensional; wherein the forward displaced frame difference equation is l(i+Δx ij (t 1 ), j+Δy ij (t 1 ), t 2 )−l ij (t 1 )=0; wherein the backward displaced frame difference equation is l ij (t 2 )−l(i+Δx ij (t 2 ), j+Δy ij (t 2 ), t 1 ); and wherein the plurality of displacement vector invariant equations consists of: a first forward displacement vector invariant equation, being Δx ij (t 2 )+Δx(i+Δx ij (t 2 ), j+Δy ij (t 2 ), t 1 )=0, and a second forward displacement vector invariant equation, being Δy ij (t 2 )+Δy(i+Δx ij (t 2 ), j+Δy ij (t 2 ), t 1 )=0; where l is an intensity of the image data; i is a pixel index in a horizontal direction x; j is a pixel index in a vertical direction y orthogonal to the horizontal direction x; t 1 is the one frame time; t 2 is the other frame time; Δx ij (t 1 ) is a displacement vector at pixel location i and j at the one time in the horizontal direction; Δx ij (t 2 ) is a displacement vector at pixel location i and j at the other time in the horizontal direction; Δy ij (t 1 ) is a displacement vector at pixel location i and j at the one time in the vertical direction; and Δy ij (t 2 ) is a displacement vector at pixel location i and j at the other time in the vertical direction.