1. A computer-implemented video quality assessment method comprising using a computer: generate respective edge maps for a reference video field/frame and a test video/frame; generate data relating to (i) edges contained within the respective edge maps and (ii) edges contained within corresponding sub-field/frame elements of the respective edge maps; count edge pixels within the sub-elements of the test and reference fields/frames; determine respective difference values between respective counts of corresponding sub-field/frame elements in the test and reference fields/frames; generate an edge parameter value in dependence on each of the difference values; and use the edge parameter values to produce a video quality measurement value.