1. A method of determining shading values in a graphics processing system using an approximation of a Beckmann Distribution Function (BDF), the BDF being defined as: 15 D = 1 4 &it; m 2 &it; cos 4 &it; β &it; e - ( tan &it; &it; β m ) 2 where, m&equals;Root mean square slope of microfacets &bgr;&equals;Angle between {overscore (N)} and {overscore (H)} the method comprising the steps of: (a) reformulating the BDF in terms of the scalar product of the surface normal {overscore (N)} and the viewpoint-light half-vector {overscore (H)} ({overscore (N)}·{overscore (H)}) to produce a modified BDF; (b) dividing a portion of the BDF into a plurality of intervals, n, each interval defined by first and second control points at its boundaries; and, (c) approximating each interval using a cubic approximation to produce coefficients for use in determining the shading values.