The latest Homing Beacon has arrived and today we get to talk about
how ILM rendered hundreds of Wookiees. With the common yet
inaccurate phrase "it's all done on computers" spreading
widely among moviegoers, people often wonder what makes
computer-generated imagery (CGI) difficult or time-consuming. The
computer is essentially creating an accurate simulation of the
complex behavior of light interacting against a modeled surface.
What makes CGI realistic is precise recreation of a light ray's
particular bounce, refraction or absorption when it hits a given
object. These calculations are done for every frame of the movie,
often taking many, many hours per frame.
Organic textures with rough shapes and odd curves add complexity
to the light paths, as do qualities such as translucency. When it
comes to creating a furry digital creature, the realistic movement
and sway of all those thousands of hair strands is taxing enough for
the computer to calculate -- now consider adding that each hair is
interacting with the light in a specific way, and reflecting and
refracting light into its environment and surrounding hairs.
"It takes an awfully long time to generate hair for a lot of
hero Wookiees," says Sequence Supervisor and Development Lead
Patrick Conran, "and even with our most severe pipeline
efficiencies turned all the way up, you've got a hundred
Wookiees. It takes a long time before you ever manage to start
rendering anything."
Before the Wookiee efficiency solution was cracked, ILM had to
finish one shot for the November 2004 teaser trailer of a vanguard
of Wookiee warriors charging from an embankment. "We had five
practical Wookiee suits spread over four different shoots composited
together, and we had to take that and add them into a miniature and
digital matte painting background, and then generate our CG Wookiees
behind them," explains Tim Fortenberry, Digital Effects Artist.
"We rendered it the old fashioned way. It took something like
5,000 processing hours initially and a terabyte and a half on disc.
So it was definitely not a good way to do the entire sequence."
The artists and technicians at ILM found a better way to create
crowds of Wookiees to fit the already tight schedule of Episode III
postproduction. "Looking at the concept art, we noticed that
[Kashyyyk] was pretty overcast and there is pretty low contrast. We
definitely used that to our advantage," says Fortenberry.
Rather than calculate the quality of light striking and bouncing
off all the fur on their digital Wookiees for each shot and angle,
ILM "baked in" a pre-set quality of light to all the
Wookiee models. "We were able to do this without using any
spotlights or deep shadows, which really made it much more
manageable," says Conran.
Also helping speed the process along was letting a Wookiee's
distance from the camera dictate the detail of the model. While the
practical Wookiee suits were used for the closest of shots, the hero
digital models had somewhere in the neighborhood of 800,000 hairs on
their surface. The ones further in the background would have less,
say 40,000 hairs. They'd be so small in "camera space"
though that it wouldn't be noticeable, and it would speed up the
computer's calculations. The ones furthest from camera had no hair
at all -- they instead had thickened bodies with flat, painted hair
texture on them.
Another time-saving technique had to do with the flexibility of
the Wookiee models. After composing a shot with hundreds of Wookiee
warriors, someone may want to change specific Wookiee characters.
Rather than having to re-render the scene to accommodate a
completely different Wookiee model, ILM built an "über-Wookiee."
"This model had all the variations built into it," says
Fortenberry "It essentially puts all the changes to the Wookiee
model at the very last step before rendering. So, if there are
changes, there's no going back to Creature Development or the
Animation Department. You'd turn on the features you'd want on the
Wookiee, and dial out the ones you don't."
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