Issue link: http://digital.copcomm.com/i/110536
n n n n CG I /Stereo 3D Layout artists kept the stereo neutral and used traditional techniques, like camera drift, to heighten the emotion in some dramatic scenes, such as those between Fiona and Shrek (top). For the intense scene between Shrek and Rumpelstiltskin (bottom), however, they applied stereo depth purposefully to drive a feeling of discomfort. for granted in legacy films, but Shrek has been in development 14-plus years. Nothing just loaded and ran.��� Alex Ongaro, head of effects, led the teams that created the magical poof and spun Shrek into and back out of alternate reality, with effects artist Andrew Kim contributing to both. The poof is a filigree pattern of gold dust that occurs when Shrek signs the magic contract and at other points in the film. The technique involved directing particles to follow modeled shapes yet still react to wind and other dynamics in a scene. ���Imagine bi-dimensional swirling particles,��� Ongaro says. ���We animated the particles procedurally, saved them on disk, and then instanced those geometries into a Maya scene that was dynamic, so the wind and turbulences affected them.��� (DreamWorks is submitting a SIGGRAPH sketch on this procedure, too.) Once Shrek signs the contract, the world he knows rips apart into a storm of papers that fly around and reassemble into an alternate version of the world that Shrek thinks, at first, is the same world. Ongaro again worked with Andrew Kim on this world-changing event. ���We started developing the concept while the movie was still in storyboards,��� Ongaro says. ���We decided that since everything centered on the contract, why not imagine that 14 May 2010 the world is made of paper that tears apart?��� To create the effect, the artists used a combination of tools, including Side Effects Software���s Houdini. They started by building a simplified version of the set using simple planes, which sounds easy enough. ���The problem was that this is a stereo movie,��� Ongaro says. ���We needed to go from a 3D movie to effects in paper.��� The answer was to render the final lighting version of the scene from both eyes and then project the images on to the set made of simple planes. ���Basically using projection mapping from the camera and the two different eyes gave us the 3D effect once it was on the planes,��� Ongaro says. ���We took this environment and tore it apart procedurally in Houdini. From Houdini, we knew where the edges were, so we could have them affected by fields. The paper needed to deform, but instead of doing that in Houdini, we did it at render time with a displacement map, a world-space noise map.��� In addition, they sent a spherical tornado of gold light into the scene as the world tears apart. ���It was very challenging,��� Ongaro says. ���The lighting department had to update the papers like they were tearing apart, with light coming from the outside.��� The tornado is made of millions of particles, which needed to look as if they were backlit, as if they had subsurface scattering. Custom tools handled global illumination and ambient occlusion within the tornado, but the self-shadowing didn���t look right. ���We decided to compute normals from the particles to do a quick and dirty ambient occlusion pass,��� Ongaro says. ���We���d take a group of particles, compute the difference between the vectors, and get a gradient. We���d point the normal in a less-dense area.��� In addition to these effects, the team grew trees with a previously developed system, poured martinis using Maya, kicked dust with Maya particles, filled the air with smoke using a volume renderer, and helped the characters create mud angels using particles and displacement maps. Perhaps the most beautiful effects, though, are the god rays filled with glittering dust. ���The beams of light come into the audience, and you can almost taste the dust,��� Mitchell says. ���You want to run your fingers through it. It���s bizarre.��� For this effect, the artists developed a particle shader within the studio���s proprietary rendering system. ���We couldn���t create the god rays in compositing because this is a stereo movie,��� Ongaro says. ���So we used particles as the source of the rays, and at render time, filled cones with particles that fade off. It turned out to be beautiful.��� Come Again Even though this is the last Shrek film, it is not the last reunion for at least some of the familiar characters. Already under way is a film titled Puss In Boots, which is scheduled for 2011. And with DreamWorks promising three films a year in the future, many of the crew on this film will reunite on another film. In the 10 years since DreamWorks Animation created the first Shrek, which won the first Oscar for Best Animated Feature, computer graphics technology and the artists using it have advanced at lightning speed. But, they aren���t out of school yet. ���We have a lot of experience and great artists who are masters of their craft, but they just keep getting better,��� Cooper says. ���Everyone���s skill set and ability and understanding of what they are building and making allows us to keep making richer pictures. We���re always coming up with new techniques. We take so much for granted, but in fact, no one is a total master of this medium because the medium hasn���t been around long enough to be mastered.��� n Barbara Robertson is an award-winning writer and a contributing editor for Computer Graphics World. She can be reached at BarbaraRR@comcast.net.