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tiple scattering in a hair volume with simple formulas. So Moon and Marschner
took a particle-simulation approach. The method they employed is based on the
concept of a volume photon map (see Section 2.4), but the specific properties of
scattering in hair necessitated some modifications to the method. The first major
difference in Moon and Marschner's algorithm is that in the particle simulation
phase, photons are deposited uniformly along the particle paths instead of at sur-
face intersections. The other major difference is that because light scattering in
hair volume is highly directional, the photon map is represented in a 5D space
that includes direction instead of the usual 3D space. That is, in the nearest neigh-
bors search for particle lookup, particles are “near” each other if they are close in
spatial proximity and also have nearly the same direction. In the normal photon
map method, closeness is only measured in spatial position.
Figure 8.59 shows the results of the particle simulation method compared to
other methods of rendering a lock of hair. The image in part (a) was rendered
using direct lighting only; without any scattering it looks black. The image in
part (b) is a reference image rendered using path tracing. The image in part (c)
was rendered using the volumetric particle simulation. Although it lacks some
of the fine detail visible in the path-traced image, the overall appearance matches
very well. The image in part (d) was rendered with the volumetric method, but
under the assumption that the scattering is isotropic. Without the strong forward
(a) (b) (c) (d)
Figure 8.59
Renderings of light hair using volumetric multiple scattering. The model is rendered
(a) with direct lighting only (there is essentially no color), (b) using path tracing, for a
reference image, (c) using the volumetric particle simulation method, and (d) assuming the
scattering is isotropic. Note that the forward scattering assumption is necessary to repro-
duce the color. (From [Moon and Marschner 06] c
2006 ACM, Inc. Included here by
permission.) (See Color Plate XIV.)
