Subsurface Scattering - The Magic of Computer Graphics - page 112

Figure 4.24 Albedo map for the human head model. (From [Donner and Jensen 05] c

2005 ACM,

Inc. Included here by permission.) (See Color Plate IV.)

Human skin is not homogeneous; its density and reflectance properties vary
across its surface. This is caused in part by variations in the thickness of the der-
mal layers, but also depends on the concentration of various pigments. The head
model shown in Figure 4.23 was obtained through a high-resolution digital scan.
An albedo map for the model was captured from a photograph of the person's
face taken under uniform illumination ( Figure 4.24 ) . The color variation in the
final image comes from the albedo map convolved with the reflectance function
as described above. 2
Figure ?? shows an image of the human head model rendered with the mul-
tipole model constructed for human skin combined with the albedo map derived
from Figure 4.24. The rendering includes specular highlights from the BRDF
model. Specular reflection is generally regarded as a surface-level effect and is
handled independently from subsurface scattering. Figure 4.24(i) and (j) contain
close-ups of the face rendered using a dipole approximation and the multilayer
model. The effect of scattering in the epidermis is clearly visible in the multilayer
model image. In contrast, the dipole model rendering looks overly translucent—
it looks more like a wax model of a human head. Translucency is an important
effect in the appearance of human skin, as the back-lit image of Figure 4.24(k)
shows.

2 The most significant pigment in human skin is melanin ; different amounts of this pigment is
the primary cause of differences in skin color. The particular individual who served as the model
for the 2005 paper apparently has a low concentration of melanin. Although this helps illustrate the
effectiveness of the multipole model, Figure 4.24 is certainly not representative of human skin color.
Donner has since developed a more general human skin model that better handles variation in melanin
concentration [Donner et al. 08].