With bump mapping, there are actually two surface normals that should be considered for lighting. The unperturbed normal is based on the surface's large-scale geometry, while the perturbed normal is based on the small-scale structure. Either normal can create self-shadowing situations. Figure 8 shows a situation where the perturbed normal is subject to illumination. However, the point on the surface should not receive illumination from the light because the unperturbed normal indicates that the point is in shadow due to the large-scale geometry [10].
In order to account for self-shadowing due to the perturbed surface normal and the unperturbed normal the lighting equation 1 should be rewritten as in Equation 7.