Graphics Reference
In-Depth Information
(a) BRDF (b) BSSRDF
Figure 4.25 Test images for the BSSRDF shader developed at ILM. (a) An image rendered with the
BRDF shader. (b) The same image rendered with the dipole-based BSSRDF shader. (Cour-
tesy of Christophe Hery c
Industrial Light & Magic.) (See Color Plate V.)
4.3.6 BSSRDF Models in Motion Picture Production
The greatest demand for subsurface scattering in movie production comes from
the need for realistic skin rendering of CG characters. The computational cost of
physically based subsurface scattering had prevented it from being used in movies
until the early 2000s. The dipole-based BSSRDF model developed by Jensen and
his colleagues was really the first model simple enough yet sufficiently accurate to
meet the demands of the motion picture industry. This model was first considered
during production of Star Wars Episode II: Attack of the Clones [Lucas 02], in
a scene where a CG character and an actor hug each other. In this scene, the
actor was temporarily replaced by a CG digital double. The intention was to
use the dipole BSSRDF model to render the skin of the actor. Christophe Hery
implemented the model as a RenderMan shader at ILM. Test images with and
without the BSSRDF shader are shown in Figure 4.25. However, its use was
postponed because of stability concerns and time constraints. The implementation
was later used in the film Harry Potter and the Chamber of Secrets ; the BSSRDF
model was used to render a CG character named Dobby [Columbus 02].
Hery's RenderMan BSSRDF shader was implemented using a depth buffer,
as the ray-tracing capabilities of RenderMan were not well developed at the time.
As noted earlier, the dipole-based BSSRDF model does not require ray tracing.
To compute the exitance at a point p on an object, sample points are chosen by
taking points near p on the tangent plane at p (which is presumably a reasonable
approximation to the surface at p ) and projecting those onto the surface. If the
light source is visible from a sample point, the exitance at p for the sample point
is computed using the diffuse BSSRDF formula of Equation (4.13). If the light
is not visible from a sample point, the sample point is replaced by a point on the
opposite side of the object (perpendicular to the tangent plane). The computation
is repeated for each sample point, and the sum of these results gives the exitance
Search WWH ::




Custom Search