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at point p due to multiple scattering. The “fast translucency” algorithm, which
employs sampling as preprocessing, nearly doubled the computation speed. The
“fast translucency” paper by Jensen and Buhler also describes a way of using
a specular reflection to approximate single scattering, so the multiple scattering
model by itself sufficiently handles subsurface light transport.
The BSSRDF shader at ILM faced a great turning point during the production
of Lemony Snicket's A Series of Unfortunate Events [Silberling 04]. The biggest
challenge for ILM in this film was to replace the face of a real baby with CG.
The BSSRDF shader used to render the Dobby character in the Harry Potter film
used only direct lighting. While this worked well for that imaginary character,
it was found to be insufficient for the scene with the human baby's skin. The
problem was that the omission of global illumination (indirect lighting) resulted
in inadequate photorealism. However, the BSSRDF model was developed for a
single incident light direction. Including indirect illumination requires repeating
the scattering computation for a collection of indirect samples. Unfortunately,
the computation is expensive and the number of samples needed is large. To
reduce the number of samples, the researchers employed a scheme where the in-
cident flux (or irradiance) is stored at surface sample points and the actual incident
light is computed from interpolating these samples. This approach is similar to
the irradiance caching techniques described in Chapter 2.
Interpolation is well suited to multiple scattering computations, because the
models are based on the assumption that multiple scattering is a diffusion pro-
cess. This causes incident light to be blurred, so a certain amount of noise in
the representation of the incident light tends to be smoothed out by subsurface
scattering. This means that the incident irradiance at each sample point does not
need to be computed very accurately, so a simple interpolation method suffices.
Accounting for indirect illumination for subsurface scattering produced a greater
improvement in the photorealism of the rendered images than the staff at ILM
expected. It also showed the importance of accurate incident light for the dipole
multiple scattering model.
Another innovation that came about during the production of Lemony Snicket's
A Series of Unfortunate Events was to provide the artists more intuitive param-
eters to control multiple scattering. The dipole model depends on the diffuse
scattering and absorption coefficients
σ s and
σ a , which represent how much dif-
fused light is scattered or absorbed. As described earlier, these are not particu-
larly intuitive. Before the work on the baby's skin the parameters for scattering in
skin ( Table 4.1 ) were treated by the BSSRDF shader as constant across the entire
skin. This proved inadequate for the baby's face; the effect of multiple scattering
needed to vary with position, and the control of this needed to be based on both
physical and artistic considerations. The diffuse reflectance and mean free path
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