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Yeatman Gets His 'G-Force'

In his directorial debut, Hoyt Yeatman raised a G-Force of guinea pigs with the help of Sony Pictures Imageworks, and Bill Desowitz uncovers their CG and 3-D secrets.

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The G-Force guinea pigs needed to hold their tools, so Imageworks made a toe look more like a thumb All images © Disney Enterprises Inc..

Hoyt Yeatman, the VFX vet, has been nurturing G-Force for six years, ever since his then pre-school son first hatched the notion of guinea pig superspys. The concept took hold immediately when Yeatman did some research and discovered that there was more scientific validity to the concept than one would imagine. But realizing this rather unique hybrid took a lot longer than expected for Yeatman, who chose to direct as well. He even pitched to Michael Bay early on before enticing veteran producer Jerry Bruckheimer (Pirates of the Caribbean).

As if animating a guinea pig geek squad to combat a giant robot and matching the CG to the live-action plates wasn't challenging enough, Yeatman also decided to add stereoscopic 3-D to the mix as a post conversion. Thankfully, he was in great hands with Sony Pictures Imageworks, which not only excels at hybrid films with CG characters but also at 3-D. Scott Stokdyk (Spider-Man) handled vfx supervision, Troy Saliba (Monster House) served as animation director and Rob Engle (Beowulf) led the stereoscopic team. In fact, G-Force turned out to exceed even Spider-Man 3 in both shot count (1,287) and complexity (480 CG shots, 61 minutes of vfx screentime, 128 character rigs, including 15 robot rigs, and 1,861 total 3-D shots). (Engle serves on a G-Force 3-D panel next Thursday at SIGGRAPH 2009 in New Orleans, Room 260-262.)

"Because of the way we were shooting it, we pushed all the 3-D decision making into post, so it was like a DI suite," Yeatman says. "We called it the "depth grade suite," where we went in and customized how the depth was going to work from cut to cut."

Making this hybrid work smoothly offered its own set of challenges: "We developed sequences that were heavily animated, and trying to build background plates out of live action can be a real pain, because suddenly you're locked into something that needs to evolve," Yeatman adds.

"I'm very proud of the lighting and what I've found over the years is that when you have to put photoreal characters or environments in CG one of the problems is matching all the specular highlights and to make it look like it fits. The DP, Bojan Bozelli, overexposed and over-processed the negative and what that would do is increase the dynamic range of the Kodak Vision 3 to almost 11 stops."

According to Stokdyk, G-Force provided a great opportunity to showcase Imageworks' character animation and the whole film was built around letting the performance come out and letting the animators have some fun to tell the story. "We shot it in a way to facilitate that: we tried to get plates for everything we could. And we worked really hard to get close to the ground and work with depth of fields to have sweet spots for characters to live in. We tried to capture all of the areas and environments on film so we could track the characters in. And then we'd do the standard surveying and photography of a lot of the backgrounds.

"I think what gave Hoyt a real advantage being a first-time director was it was his idea and his baby. If there were any questions story or character related, it was all in his head. That is what I hope for out of any filmmaker, which is a clear direction of what the story is and who the characters are. In terms of doing the visual effects, of course, he's been where I was on a movie so many times, so in many ways he just let me do my job and trusted me and we had a great shared language. It was actually very easy for me to get into his head what he wanted look wise. We had a shared language of how to balance light and key and fill and specular highlights and subsurface. We just used our short-hand and went where we needed to go.

"In fact, we used his HDR cam to capture environments, so it was very important to him that we were able to work fast and efficient and get a real good level of realism. It is computer controlled and has four digital SLR cameras that are synched and he has a program where he takes a series of bracket exposure and combines them into an HDR single image and actually stitches together images from all four cameras together. And that pops out and goes into our system. And so we've done a lot of image-based rendering over the past couple of years and we basically used his system as input and upgraded all of our tools. So Hoyt's philosophy was to use the right tools to make everything look as real as possible. And he also wanted a lot of flexibility in terms of cameras. When we were shooting, he wanted to get the right height and down into the world of the guinea pigs, and then, in post, he wanted to be to have the performance of the guinea pigs drive the camera and have it seem like there was an A level camera operator there capturing rehearsed performances as if it was shot with real actors and very carefully choreographed."

Sony made advancements in its Katana in-house software, in which light placement around the CG characters was tweaked so the artists had the ability to move, vary and control them. "And then we used the rest of the image to bring in a low level ambient and fill contribution. So we basically upgraded our image-based lighting system to add more artistic input into the process. We had incremental improvements in hair (nice control of magnets) for where their gear made contact with the fur; we converted from more nurbs-based modeling to subdivision surfaces so our hair system had to be upgraded to work well without that."

There were animation challenges as well: "Guinea pigs have three toes and four fingers, but our characters needed to hold their tools, so we made one look more like a thumb," Saliba explains. "And they are able to stand on two legs, and we lightened up their irises to be able to read the subtlety of feelings in their eyes."

G-Force utilized multi-camera rigs for stereoscopic 3-D, allowing Imageworks to emphasize and de-emphasize the depth in different objects in a scene.

Stereoscopically, Imageworks was able to take advantage of everything it has developed throughout the past five years."We've refined the toolset to support all CG features, and we applied them to the live-action environment," Engle suggests. "What was particularly rewarding was working with Hoyt and Scott to come up with a unique and exciting 3-D experience. The main challenge is that the film was shot entirely using 2-D traditional techniques. Hoyt and the other filmmakers wanted to use the 2-D tools that have been refined over the past 100 years of filmmaking to provide as much flexibility on set. The scope of this film goes from housefly to 80-foot-tall killer robot, so what we did was convert the plate photography from 2-D to 3-D and then integrate our fully rendered, stereoscopic CG elements, whether it was the robot, the guinea pigs or the house fly. The other challenge was that Disney really wanted to make this an immersive experience and to come up with ways to put you in the guinea pigs' world. And the other unique challenge was to occasionally break that out into the audience. We set up this technique where we letterboxed the entire film and broke the mask and take advantage of bringing the world into the audience space.

"Because all of the stereoscopic is synthesized, we are mapping it to a 3-D virtual world. A perfect example is the technique we use call multi-camera rigs. You isolate individual elements in a scene and create separate stereo parameters and a separate stereo base for each of the objects in a scene that you want to break out. What it allows us to do is emphasize and de-emphasize the depth in different objects in the scene. Because we are synthesizing, we can use those techniques on anything, whether it's a guinea pig or live-action character. If a human needed to look more dimensional, we gave them a separate camera rig and isolate them and expand to their stereo. And a large part of the film is completely CG synthesized worlds that, from our standpoint, was just like Beowulf or Polar Express."

Yeatman, who is definitely interested in directing again, concludes that "it was an amazing experience, as you could imagine, managing a lot of very creative people -- you're working with titans. Sony really picked up the pace and was able to manage the hundreds of shots that went through and the changes that occurred with the 3-D process and did a really good job."

Bill Desowitz is senior editor of AWN and VFXWorld.