1. A method of encoding a coding unit of a digital video sequence, the method comprising: computing a brightness measure for each region in a plurality of regions in the coding unit, wherein the brightness measure indicates a relative brightness between the region in the coding unit and a corresponding region in a reference coding unit; determining a number of regions in the plurality of regions with significant brightness change by comparing each brightness measure to a region brightness change threshold; detecting whether there is global brightness change in the coding unit by comparing the number of regions to a global brightness threshold; and when global brightness change is detected, computing weighted prediction factors to be used for weighted prediction in encoding the coding unit.