Schematic view of the product of ω^Tω for a fixed value of x. The result is a squared and symmetric matrix of order (N_y + 1) × O + 1 where N_y is the height of the packed swath. The outer side of the ‘squares’ on the main diagonal is O + 1. The squares overlap by one row or column. The offset from the top left corner is determined by the central line: a y_c of zero implies v_min = 0. The in and out elements are the footprints of the first and the last subpixels that fall in a given detector pixel x, y. A central line offset of zero would set in = 1 and out = 0. All elements outside the main diagonal boxes are zero. The values inside a box are known explicitly as shown on the sketch.