Graphics Reference
In-Depth Information
0.0 0.1 0.2 0.3 0.4 0.5
0.6 0.7 0.8
0.9 1.0
Figure 8.66
Effect of the density factor. Normal human hair ranges from 0
.
6to0
.
8. Lower density
means less scattering, which makes the hair appear darker. (From [Zinke et al. 08] c
2008
ACM, Inc. Included here by permission.) (See Color Plate XVI.)
source (shown in the red hatched region in the figure);
d
b
represents the density of
fibers in the local hair cluster from which backscattering is possible (in the blue
hatched region in the figure). Values of the density factors
d
f
and
d
b
for human
hair range from about 0
7 for most of the examples
in the paper. Figure 8.66 illustrates the effect of changing these values. At values
near zero, there is little multiple scattering, which causes the hair to appear much
darker as explained in the previous section.
To render a point in a hair volume using the method, the incident light, at-
tenuated by the global multiple scattering function, is integrated over the incident
directions. This includes unoccluded incident light, the reflection of which is
computed directly from the Marschner model. There are several ways this might
be done. A ray-“shooting” approach simply casts rays from the evaluation point
and counts intersection with other hair fibers. If there are none, the Marschner
model is applied. Otherwise the global forward-scattering function is computed
for the number of intersections as described above. While this can be done as
part of the rendering process, a more efficient method is to precompute this at-
tenuation along all shadow paths, and store the results in a volumetric data struc-
ture for subsequent lookup during rendering. This process employs
deep opacity
maps
[Yuksel and Keyser 08], which can be implemented on a GPU if the hair
volume is approximated by a collection of layers. Figure 8.67 contains a compar-
ison of results. Part (a) of the figure is rendered using brute-force path tracing,
Part (b) is rendered using the dual scattering approach with ray shooting, Part (c)
is a snapshot of the hardware implementation.
The conceptual separation of global and local multiple scattering certainly has
theoretical importance, as it creates the opportunity to use different approaches
for approximating the two effects independently. However, its primary impact
has been the method for fast rendering of hair volumes with accuracy comparable
to path tracing. This method came into practical use quickly after its publication.
The first use of this method in entertainment works was in Disney's film
Tangled
(2010) [Sadeghi et al. 10]. Hair rendering was once considered a very specific
rendering problem; however, the volumetric approaches definitely have a kind of
.
6to0
.
8; both were fixed at 0
.
