1. A method of estimating jitter between two video frames in a digital video sequence, comprising: comparing a pixel subset of one video frame to candidate pixel subsets within a search area in the other video frame to determine a preliminary matching pixel subset from the candidate pixel subsets; determining an actual matching pixel subset from the other video frame based on the location of the preliminary matching pixel subset relative to said search area; and calculating a global jitter vector based on the pixel subset and the actual matching pixel subset thereby to estimate the jitter; checking the reliability of the actual matching pixel subset; and only calculating the global jitter vector if the actual matching pixel subset is deemed to be reliable; wherein reliability of the actual matching pixel subset is based on at least one of frame contrast measure and steepness measure; and wherein said determining comprises: determining if the preliminary matching pixel subset is adjacent a border of the search area; if not, designating the preliminary matching pixel subset as the actual matching pixel subset; otherwise, altering the search area; and comparing the pixel subset to candidate pixel subsets within the altered search area to determine the actual matching pixel subset; wherein said altering comprises: shifting the search area in the direction of the preliminary matching pixel subset.