Displacement vectors calculated in the CROSS-CORR step map each pixel in the raw science image to the geometric space of the ITF, where the final displacement coordinates are floating point coordinates. If the final coordinates for the science pixel are within 0.125 pixel (in either the line or sample direction) of the integer coordinates of an ITF pixel, the science image pixel is assumed to be coincident with the ITF pixel and the FN computed will be based upon data contributed only from the single ITF pixel. If the science image pixel is displaced by more than 0.125 pixel from the integer coordinates of any ITF pixel, the FN computed will be based upon data contributed from a 4 × 4 matrix of ITF pixels surrounding the final position in question. Both of these scenarios are described in further detail in the next section.