Computer Graphics World

May 2009 22 n n n n Lights•Shaders familiar with so we tried to preserve some of what they have seen in previous films." To rig the machines, ILM used tech- niques developed originally for Transform- ers, which give animators flexibility in choosing which parts connect to what (see "Heavy Metal," July 2007), but for this film, the riggers sometimes put limitations on that flexibility. "We're working with heavy machinery and equipment," Chu says. "We wanted the models to move like they would in reality and to be robotic, so sometimes we put boundaries on what the rig could do, and sometimes we just put limitations on ourselves. We needed to get the right pose from the camera angle but still keep an eye on realism." e most difficult machine and most in- tricate rig was that for the harvester, a 60- foot biped that looks something like a head- less terminator, but with two main arms, two inner arms, and weapons that pop out. As they had done for the Moto-Termina- tors, ILM redesigned the harvester to cre- ate a beefier machine than in the original concept art. e Moto-Terminators, which function as a team to collect humans, slide out from plates in the harvester's legs. For the Hydrobots, ILM matched the Winston models in look and animation style, and intercut them with the practi- cal puppet in several shots. e studio also intercut with Winston's T-600 terminator, although the fighting shots were generally CG, and they matched the digital T-800's mechanical endoskeleton to the Winston puppet for close-up shots. e new shaders helped make those transitions seamless. The Importance of Shaders "We had a lot of metal," Conran says. "at's what we were thinking about going into this film." Before this movie, ILM's most recent advancement in creating metal surfaces were the techniques used in Iron Man to create brushed metal (see "Power Suits," May 2008). "We used [the brushed-metal tech- niques] where appropriate," Conran says, "but for this film, we needed cast iron and grungy metal, so we did a major push un- der the hood with our shaders." e new Pixar RenderMan shaders and image- based lighting helped the crew create the skin-covered terminators, as well. Developed by ILM's research and devel- opment department under the leadership of Christophe Hery, these new shaders use "importance sampling" for reflections and raytracing, and in doing so, combine specular highlights and reflection. Explains Conran, "Traditionally, when you're raytracing to sample the environ- ment, you cast one ray in a cone; you blur the environment to mimic the surface properties. With our new technique, you sample the environment and the BRDF (bidirectional reflectance distribution func- tion) of the surface. en, you cast rays and gather light from all parts of the image using multiple rays weighted according to the specular." Conran continues: "Usually, specular high- lights are CG cheats for doing reflections. We're trying to get away from that. If you sample reflections using the specular func- tion, then you marry specular and reflection, which are really the same thing. We get a more realistic look, and we get it quicker." e more realistic look comes in part because the surfaces self-reflect more accu- rately with the new shader. "e amount of light given out by a light source is the amount that gets reflect- ed back, other than the amount of light the surface absorbs," Snow explains. "at's the real light physics. Getting creatures like our terminators to self-reflect is vital. We've been cheating self-reflections for as long as we've been doing this stuff, and frankly, I think that's probably what leads to the 'Oh, it looks CG' comments. Being direct about the way a surface bounces light itself makes such a difference in the realism. I can't live without it anymore." e shader has another advantage, as well: It gives the lighters more consistent results. "Before, even though specular and reflection are the same, we computed them different- ly," says Philippe Rebours, digital produc- tion supervisor at ILM. e separate com- putations for reflections from the surface and from the environment often produced inconsistent looks through a sequence as, for example, lighters moved spotlights. "Now, specular and reflection are the same, and diffuse and indirect are the same, and the creature reacts the same way as it did on the turntable," Rebours says. "So that's a win." at meant the crew could move the creatures in this film from high-contrast, bleached-out desert envi- ronments into dark interiors more often and with less hassle. The beefy looking Moto-Terminators rode through three design cycles before director McG got the attack-dog look he wanted. To give the animated bikes intelligent moves, animators referenced footage of stunt riders on location.

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