1. A method for determining a metric used in motion compensated video coding of a scene comprising frames, the metric measuring a difference between a first image block contained in a reference frame and a second image block contained in a current frame, the first and second image blocks comprising the same number of rows and columns of pixels, the method comprising: (a1) generating a pattern matrix having the same number of rows and columns as the first and second image blocks, for selecting a subset of pixels in the first and second image blocks; (b1) obtaining a characteristic of each pixel in the subset of pixels generated in the step (a1); and (c1) processing the characteristic of the subset of pixels for determining the metric.