The fast subsurface scattering shaders all work by creating a lightmap that stores the shaded object's front and back surfaces, their depths, and light intensity. During rendering, the lightmap is sampled to create several light layers, which incorporate color and depth information. These layers are then added together to produce the final subsurface scattering effect that is applied to the object's surface.
The light layers are added together in the following order: specular highlights, diffuse illumination, front scattering, and back scattering.
