1. A method for determining an optical flow field between a pair of images, comprising the steps of: (a) decomposing each of the pair of images into image pyramids; (b) transforming the pair of decomposed images at a first pyramid scale to second derivative representations under an assumption that a brightness gradient of pixels in the pair of decomposed images is constant; (c) estimating discrete-time derivatives of the second derivative image representations; (d) applying an optical flow estimation process to the discrete-time derivatives to produce a raw optical flow field; (e) scaling the raw optical flow field; and (f) repeating steps (b)-(e) for the pair of images at another pyramid scale until all pyramid scales have been visited to produce a final optical flow field, wherein spatiotemporal gradient estimations in step (c) are warped by a previous raw optical flow estimation.