1. Apparatus for processing a matrix of picture element (pixel) data values representing an image, to define edges within the image comprising: means for defining a potential edge location in a predetermined direction between first and second ones of the pixel data values which first and second pixel data values are adjacent in the matrix of pixel data values; means for determining a first edge strength value in the predetermined direction said first edge strength value being a measure of a difference in magnitude between the first and second pixel data values; manes for determining a second edge strength value in the predetermined direction said second edge strength value being a measure of a difference in magnitude over a group of N successive pixel data values in the predetermined direction where N is an integer greater than 2, the N successive pixel values being defined on the matrix of pixel data and including the first and second pixel data values; means for analyzing the N pixel data values to determine if the N pixel data values define an edge at the potential edge location and for modifying the second edge strength value to indicate a null edge if it is determined that the N pixel values do not define an edge at the potential edge location; means for comparing the first edge strength value to the second edge strength value to assign one of the first and second edge strength values to the potential edge location.