A method of selecting a homography model from point correspondences of an associated image pair; a method of filtering point correspondences from a pair of associated images; a method of calculating a homography transformation; and a method of image differencing; including implementing one or more sampling constraints, e.g. partition zones and an exclusion zone, that provide a more even spread of point selection over the image, and/or that provide for selected points to be spread further apart, than would be the case if the one or more sampling constraints were not implemented. Repeated selections of combinations of points may be selected in a deterministic manner, and the deterministic manner may be tailored to an escape condition.