Imageworks visual effects supervisor Scott Stokdyk gives Ian Failes a run down of his work for G-Force, a Hoyt Yeatman-directed feature about an elite troupe of guinea pigs who must save the world from an evil billionaire.

fxg: What was the experience like working on a film by Hoyt Yeatman, given his visual effects experience?

Scott: It has been incredibly cool, from the start of the production right to the end. Even on set, Hoyt's probably forgotten more about camera gear than I will ever know. So on set he was into special camera rigs and setups and creative things like that. We were testing these tiny HD cameras for specialty shots and probe lenses and other incredible things.

In post, I've been able to really understand his vision very easily and understand his language. On a personal side, he's told me some great stories. I wish I had written down some of them, because you could write a book from his stories.

09Aug/gf/gf2fxg: When did you get involved in the production and what sort of concept work and previs did you contribute?

Scott: I've been on the show for about two years. I got in during the script breakdown sessions and initial previs, working with the previs editors and Hoyt. We didn't do a lot of heavy duty shoot-prep in terms of the previs. There was basically only one motion control shot on the whole movie that we had to prep curves for. Everything else pretty much involved setting up the visual feel of what it was going to be.

On second unit there was a lot of plate photography for a chase sequence in the guinea pigs' RDD. There are federal police chasing them in SUVs and everything. A lot of the previs came in really handy for the plate photography there, but on first unit there was really a lot of 'puppet theatre' as we liked to call it. Here, Hoyt and Troy Saliba, our animation director, would get the props people to move the stand-in stuffies around on sticks. The first unit camera operator, Patrick Loungway, would get down close to them and block out the shot. A lot of it was definitely off-the-cuff and working with the location and environment we had and trying to compose stuff on the day.

09Aug/gf/gf3fxg: Were there any special things done on set to facilitate what would be done later with visual effects?

Scott: One of the things we did to capture our HDRI on set was use Hoyt's HDR cam. Traditionally at Imageworks we've used a series of bracketed fish-eye photographs to capture our HDR images for environments and for image-based lighting on set. In this case, Hoyt's got a really nice self-contained system that captures everything and stitches it all together and delivers it in a nice package.

We also found that sometimes using real guinea pigs for interaction with the actors gave us much better and nuanced performances. In the past we've used stuffies or silicone bucks that the actors can grab onto. But what we found for G-Force was that the actor's best performance was holding a real guinea pig.

We shot the first part of the movie with very docile, easy to handle guinea pigs. It was really nice having the guinea pig fur to see how the interaction worked. But by the end when we came back for re-shoots and we'd had some experience with these shots, we knew what that interaction looked like. So we actually used what's called a 'skinny pig' for the hand interaction. It's basically a hairless guinea pig, like a hairless cat, which gave us a good actor interaction and made for an easier paint-out.

And for everything we shot, we put these hairy round spheres out there for filming for fur reference. If the guinea pigs in the scene were meant to be wet, we'd douse the reference spheres with water. If the wet guinea pig was falling down on some concrete or something, we'd throw a wet reference fur ball down and have that on film as reference.

fxg: How did you then go about building the digital guinea pigs?

Scott:: The production office that Hoyt worked out of had adopted a series of guinea pigs. The original concept for the whole movie came from this guinea pig that Hoyt's son had borrowed from school called 'Cuteguy'. I think by the time we had started shooting the movie, unfortunately, Cuteguy had passed away. I think guinea pigs don't tend to live that long - maybe four or five years.

So the production office got a bunch of different types of guinea pigs, including some that looked like the original Cuteguy. We basically brought our own photographers and videographers in and shot the guinea pigs for looks and action. We would film them indoors and outdoors. We'd do these pretty rigorous film shoots where we'd capture the environment, extra images and shoot grey spheres and the guinea pigs.

So basically we just got a bunch of really, really nice acquisition photography. Then we brought it back to Imageworks and started in the computer. The computer build of our characters then started with that photography in terms of hand modeling. There are also some pretty big anatomy books around for guinea pigs. So much is known about them - I mean, there's a reason for the term 'guinea pig' - they're very well understood in the medical and scientific communities. We had scientific and medical journals to look at, but really this show is about performance, so we had to depart a little from that.

09Aug/gf/g7During the design phase, Troy and I were trying to take things that were cute and real anatomically about the guinea pigs but add layers of performance onto them. If you look at the back of a guinea pig and its hind feet, its legs and skeleton will suck into the skin. It almost looks like a big potato sack. It looks cute, but if you need to have them run and jump, those legs had to extend out and you needed more definition. So we had to pay a lot of attention to what really happens with guinea pigs and what we needed to happen with them.

A rule of thumb was, 'We should have these guinea pigs be able to look photo-real if we pose them and hit key frames for what a real guinea pig could do.' The body should react and the skin should react like a real guinea pig. But our guinea pigs needed to do things like reach their hands above their heads. To be able to do that we needed to stretch out their arms longer than normal.

The next layer involved combing and grooming the hair. We have some great hair people here. We went in and paid a lot of attention to detail to the grooming. There was some really sophisticated feathering and borders between colour layers on the guinea pigs. Depending on the breed of the guinea pig, they can have very smooth, shiny hair or they can have lumpy and more wild hair. We drew inspiration from different breeds to bring some of that into our guinea pigs. I think it was the combing layer of texture that adds to the realism.

fxg: Do you use proprietary tools for your hair and fur?

Scott: We do. Over the years for different shows, we've built on Maya tools and Maya solvers and also made our own custom tools. There's a number of different ways to solve for dynamics and to comb the creature, but sometimes once you've run through the animation you might like one area and not like another area. Our team developed a way to do hair dynamics compositing. If you liked motion in the head but not the legs, you could pick from different simulations and composite different areas. That gave us a way to almost art-direct something that's very procedural.

09Aug/gf/gf4fxg: How did the gear and gadgets worn by the guinea pigs impact on fur interaction?

Scott: You've hit on one of our biggest challenges with the hair and fur. That interaction was pretty brutal. Basically the hair team had to go in and use a series of magnets, effectively. The magnets are attached to the geometry that's interacting with them. The magnets push away the hair in a very, very local area and try to take care of collisions between the gear and the body of the guinea pig. It's a very difficult problem that we had a great solution for maybe 95 per cent of the time. Five per cent of the time time there was hair penetration and we had to go back in and clean it up.

09Aug/gf/gf1fxg: What work was involved for the other animals and robots in the film?

Scott: The cool thing about this show was that we did get to animate and render creatures down to cockroach and fly size and up to giant robot size - and everything in between! The cool thing about the small scale stuff was the we got into a world you don't really see, like flying around as a fly. Or down at the floor level as a guinea pig or cockroach.

Then we got to our 80 foot robot, it was all about making something that was impressive and cool and classically 'big movie fx' cool, but we had to work the guinea pigs into that. Hoyt worked with a concept designer named Will Reeves and he came up with some fantastic concept drawings. We kept referring back to them for the whole movie because I think he really captured something in those designs. For the robots, Hoyt always wanted them put together in a non-prefabricated way and a way that's kind of half-designed. If we could get in the mind of the creator of these crazy robots, we'd see that they're made of 'found' materials sometimes oddly pieced together but always connected mechanically with cables.

So they're made of all these different appliances and pieces of a house and a basement. It becomes almost a giant walking junk pile, but it's got a structure underneath. It already looks really cool from very far but as we got into the requirements of the movie, we had to make the insides of this giant robot the environment for a guinea pig. We had to make the guts of this robot interesting and dynamic. This character animation actually became an environment for a smaller scale of character animation. So we had shots inside the robot where mechanically everything's swinging and there are sparks going, with characters scrambling around in that environment.

09Aug/gf/g5fxg:What sort of lighting and rendering issues did that bring up?

Scott: Well, the first thing is the incredible amount of geometry involved just to do that, because we had to fill the screen space over and over again and maintain realism. On top of that there was a whole cable system involved. The cables are props that the characters can use to animate with and also things that bind the moving parts together. So that was a whole set of curves we had to turn into cable geometry. We actually used a good combination of the Arnold ray tracer as a renderer and RenderMan. So in the same all-CG scene we might have very complex moving geometry with interactive sparks that have to light it. For that we'd use global illumination from the Arnold renderer to get a very nuanced feel to that background. Then we had to tie in that lighting to the foreground characters who were primarily RenderMan (all our CG characters were done in RenderMan). We almost treated it as if the background was a shot plate through the Arnold renderer and the foreground was a RenderMan render of CG characters into that other plate, but that plate was actually virtual and we had all the perfect lighting incarnations for that background.

09Aug/gf/g6fxg: How did the 3D component of the film impact on your visual effects shots?

Scott: We learned that it was going to be 3D about half way through shooting. I think in a way that was a benefit for us. With all the complex camera rigs and how we were shooting in a very organic, natural way, I don't think we could have survived using a stereo camera rig on set. So in post we treated the entire show as if it were a typical effects show and split an entirely different team led by Rob Engle to do the stereoscopic conversion.

We've done a lot of work here at Imageworks converting all-CG shows to stereo shows, and we've traditionally done that as a whole separate team. Of the 60 minutes of the film Imageworks delivered as effects work, about a third of it was completely virtual. So while we were working on that, there was always a push to try and get as much of the movie as virtual as possible because that conversion to stereo is easier for the team. For the rest of it, the other 40 minutes of effects work, we tried to get those shots done as early as possible for the stereo team and not do things that would break down their pipeline. We didn't do a lot of finished/completed shots in Flame because we wanted to use our in-house Katana lighting software to make it easier to convert into 3D.

As we worked on our shots, we also had to think about what worked in 3D and what didn't. We tried not to break some rules about what would make it jarring for the audience, like breaking the left-hand side of frame. You have to think about depth of field issues and how they're related to the stereo.

fxg: Have you got any other thoughts on G-Force to add?

Scott: I'm looking back on it now, and I don't think i could have asked for a better show to work on because I did get to work with great performance animation and your 'giant robot' all in the same movie! And the twist of having it go to stereo opened up a whole new world of thinking, because I think there is a big future in that. I think there's also a convergence between animated movies and visual effects movies. The fact that almost a third of our work was like a photo-real animated feature was incredibly cool. And then of course working with Hoyt was a whole other incredible experience, picking up on all his knowledge and history.

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