1. A system for producing a motion vector field for a current frame, said system comprising: (a) an estimator that selects an initial motion vector from a set of candidate motion vectors; and (b) a refiner that refines said initial motion vector using gradient-based motion determination in a manner that minimizes a cost function that includes a first term and a second term, said first term based on at least one of an optical flow constraint and brightness constancy constraint and measuring how well a refinement fits with pixel data by using spatial and temporal image derivatives, said second term measuring spatial inconsistency in said motion vector field resulting from said refinement, and where said refiner operates in a manner free from searching regions of frames other than said current frame for content similar to objects associated with respective motion vectors in said motion vector field, and where said estimator selects a motion vector having a resolution at least as coarse as a pixel of said current frame, and said refiner refines the selected said motion vector to have a resolution finer than a pixel of said current frame.