Graphics Reference
In-Depth Information
inherited the approach of the original PRT paper along with SH representation of
BRDFs. Precomputation of radiance transfer was performed, and SH approxima-
tions were stored as point clouds for the RenderMan software to read.
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Spherical harmonics expansions were applied to both environment lighting
and BRDFs. Combining the three components (environment lighting, radiance
transfer, and BRDFs) yielded the rendering result. Radiance transfer is a primary
realism component yet is costly to compute. Precomputing radiance transfer pro-
vides the ability to change both the lighting and surface materials interactively,
which greatly contributed to the ease of exploring the lighting and surface expres-
sion to get to the desired quality. PRT was also used for the final rendering, and it
contributed to the overall quality of all rendered scenes regardless of their type or
complexity.
The significant advantage of using PRT in
Avatar
was that there was essen-
tially no difference between final renderings and test iterations—the same PRT
was used for both to avoid recomputation. This enabled the artists to work on
lighting tests knowing the final renderings would be the same, except at higher
pixel resolution. This definitely differs from hardware-accelerated rendering
pipelines where the final images are generated by an entirely different software
system.
However there was one critical problem in the PRT approach. Spherical har-
monics do not work well for high-frequency lighting. In
Avatar
, the scenes in-
clude all kinds of lighting and reflection. Instead of SH basis functions, Gaus-
sian functions were employed to represent high-frequency lighting. Lighting was
expanded with Gaussian bases and stored, and then convolved with a radiance
transfer term derived from the SH and BRDF terms, which were also represented
with a Gaussian. This idea proved to be very useful for the representation of high-
frequency lighting details, such as sharp specular highlights and reflections. The
implementation in the
Avatar
project required lots of ideas different from those
presented in the original PRT papers, some of which remain trade secrets. For ex-
ample, high contrast in the environment lighting could create ringing artifacts in
an SH approximation; proprietary methods were developed for suppressing these
artifacts at render time.
Until recently, the bottleneck of PRT, in the sense of moving the implementa-
tion into movie production, was the costly precomputation process that involves
ray tracing. However, ray tracing has come to be more and more prevalent in
the motion picture industry since 2008 (as described in Section 2.5.5). The use
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A “point” in a RenderMan “point cloud” is a kind of little disc having a surface normal, radius,
and an area. Point clouds are used to estimate or cache information such as occlusion or one bounce
indirect diffuse illumination. The PRT used in
Avatar
mainly computed occlusion in the preprocess
and stored the results as a point cloud so that RenderMan can refer to it at render time.
