1. A method of sub-pixel motion estimation, the method comprising: a) estimating a first motion vector to integer-pixel accuracy, wherein the first motion vector maps a reference image to a current image; b) computing image gradient information of the reference image; and c) determining a second motion vector to sub-pixel accuracy by adding a sub-pixel motion vector ΔP to the first motion vector, wherein the sub-pixel motion vector ΔP is estimated based on the image gradient information, and wherein the sub-pixel motion vector ΔP improves from integer-pixel accuracy to sub-pixel accuracy with no more than one iterative search step in applying Lucas-Kanade algorithm.