Although fringe shifting and image processing are popular for extracting fractional fringe orders, few methods are well adapted for geometric moiré patterns. The main reasons are (1) the intensity distribution across geometric moiré fringes is a complicated function of fringe order, rather than the simple harmonic function of classical interference fringes; and (2) in practice, the function is not fixed but varies from region to region in the fringe pattern. In the idealized case of equal bar and space widths of moiré gratings, the (averaged) intensity distribution is triangular . In real systems, the finite aperture of the camera lens acts as a filter that distorts the triangular distribution by rounding its corners: the rounding of corners by filtering is much stronger for closely spaced fringes than widely spaced fringes and, therefore, the intensity distribution is not a fixed function. In addition, the function varies in practice because the ratio of bar and space widths of the moiré grating usually varies from region to region across the field.