Graphics Reference
In-Depth Information
that Jensen and Buhler suggested in the “fast translucency” paper were a good
choice, as they correspond roughly to the surface color and the translucency, re-
spectively. A larger mean free path implies more transparency.
Hery extended this approach to control positionally dependent multiple scat-
tering using textures [Hery 05]. Representing the diffuse reflectance and mean
free path parameters in textures is straightforward to implement: at each sam-
ple point, the scattering computation proceeds with the parameter values obtained
from the textures. However, the fact that these parameters are more intuitive does
not mean that they correspond directly to the final appearance of the rendered
object. In movie production, the process of modeling subsurface scattering of-
ten starts with a photograph of the live actor or detailed model of the character
(known as a
maquette
) taken under uniform illumination. The goal of photo-
realistic rendering is to match the scattering characteristics of the actual object,
although there is also flexibility for artistic control. Adjusting the parameters to
match the photograph is a tedious process of trial and error. Hery showed how
the scattering parameters can be estimated directly from a photographic record as
an inverse process. The mean free path
l
d
=
1
/
σ
tr
is obtained from a measured
value of
σ
tr
, i.e., from Table 4.1. This parameter is set to be uniform over the
whole object. Next, texture maps from the photograph are constructed for the ob-
ject. The geometric model typically consists of micropolygons (polygons smaller
than screen pixels in the final image), so the image is sampled and interpolated
at the resolution appropriate for the micropolygons. The diffuse albedo parame-
ters are determined as described in Section 4.3.4. The result is an albedo map for
the object that can be artistically adjusted according to the particular production
needs. The object is then rendered with the finished albedo map. This combi-
nation of physical accuracy and artistic control is a compelling advantage of the
texture-based technique.
After the invention of the texture-based scattering parameter control, use of
the BSSRDF shader spread rapidly at ILM and it was soon utilized in various
other projects. Examples of its use include the skin of Yoda and the growing
plants in the movie
Star Wars Episode III: Revenge of the Sith
[Lucas 05], the
sea spray in
Poseidon
[Petersen 06], and the skin of Davy Jones in
Pirates of
the Caribbean: Dead Man's Chest
[Verbinski 06]. Various creative efforts were
employed to construct albedo maps and to represent the incident light according
to the particular needs of each situation.
The BSSRDF shader at ILM has been extended to more general multilayer
objects. Donner and Jensen's multipole model was not practical for production
work because the parameters cannot be determined from photographs or albedo
maps by direct inversion of the model formulas. One of the biggest problems was
how to create a multilayer model that could be controlled intuitively by the artists.
