The secret of velvety skin

Abstract.

Asperity scattering adds a 'surface lobe' to the usual diffuse, backscatter, and specular lobes of rough surfaces. Although rarely acknowledged, it is an important effect in many materials that are covered with a thin layer of sparse scatterers, such as dust or hairs. In common cases where single scattering predominates, asperity scattering adds important contributions to the structure of the occluding contour and the edge of the body shadow. This is because the bidirectional reflectance distribution function (BRDF) is inversely proportional to the cosines of both the illumination and viewing angles. The BRDF is generally low (and typically negligible) except when either the illuminating rays or visual directions graze the surface. Because asperity scattering selectively influences the edges in the image of an object, it has (as judged by photometric magnitudes) a disproportionally large effect on (human) visual appreciation. We identify it as a neglected but often decisive visual cue in the rendering of human skin. Its effect is to make smooth cheeks look 'velvety' or 'peachy', that is to say, soft (the appearances of both velvet and peachy skin are dominated by asperity scattering). This is a most important aesthetic and emotional factor that is lacking in Lambertian (looks merely dullish and paper-like), 'skin-type' BRDF (looks like glossy plastic), or even translucent (looks 'hard', vitreous) types of rendering.