Building A Better Bunny
Behind The Scenes At Blue Sky Studios

By Audrey Doyle

When Chris Wedge and his team at Blue Sky Studios began working on "Bunny" eight years ago, they knew the short computer-animated film was special. But no one knew just how special.

Ever since it premiered last fall, "Bunny"—the first animated film whose environments were rendered entirely using radiosity—has won the Golden Nica Award in Computer Animation at Prix Ars Electronica '99, an award for Best Digital Animation Overall at the New York Film Festival, and First Prize at the International Children's Cinema 45^th International Short Film Festival Oberhausen. It was also honored earlier this year in Monaco at Imagina, with three Grand Prize awards: Grand Prize of Imagina, Grand Prize Fiction, and Grand Prize 3D Animation.

But perhaps the icing on the cake was the Academy Award it earned in March for Best Animated Short Film of 1998. "'Bunny' has gotten us more attention than anything we've done, I think because we didn't compromise in its creation," says Wedge, who is vice president of creative development and one of the founders of Harrison, New York-based Blue Sky Studios in 1987. "We took the time to do it right, and it was truly gratifying to have that commitment and hard work so well rewarded."

What is it about "Bunny" that has made this film such a success? For some, it's the charming yet poignant story it tells. For others, it's the pioneering techniques that gave the computer-generated film its warm, photorealistic style. Whatever it is, it seems to have worked: in addition to receiving the aforementioned awards, "Bunny" has been invited to screen at numerous festivals, including the Cannes Film Festival, the Newport Film Festival, the Toronto Worldwide Short Film Festival, and the Sydney Film Festival. In addition, it has been accepted to the SIGGRAPH '99 Electronic Theater.

Indeed, "Bunny" has enjoyed phenomenal success in the past six months, and the future appears equally as bright. Here is a behind-the-scenes look at what has made this computer animation such an extraordinary accomplishment.

It Starts With the Story

A 7-minute film written and directed by Wedge, "Bunny" tells the story of an old, widowed Bunny who receives a late-night visitor—a hairy moth—while baking a cake in her kitchen. It appears as though the moth is pursuing Bunny, despite her attempts to shoo it away. Why is the moth stalking her? Who does it represent? Only after Bunny undergoes a moving transformation do viewers comprehend that the moth is actually her dearly departed husband who is here to lead Bunny to the afterlife.

According to Wedge, he came up with the Bunny character because he wanted to create a film with elements of storybook fantasy. In fact, some of his visual inspirations came from the Uncle Wiggly books he loved as a child. But he wanted this story to be a little edgier than the Uncle Wiggly books. So he switched the gender of the lead character from male to female, and instead of a cane he gave the character a walker, or "hopper," as Wedge describes it.

Then he started to kick the story around, beginning with location and situation. "Some of the interiors, the designs of the environments, that were in those Uncle Wiggly stories got me going," he says. But he adds that he was mostly inspired by memories he had of the kitchens he knew of grandmotherly people he remembered from when he was growing up. "When I think of where all that stuff came from, they’re definitely deep memories. I think a lot of us have memories that are tinged with fantasy."

As for Bunny's personality, Wedge says he knew from the start that he wanted her to be a surly, bitter, and lonely old woman, not a sweet, grandmotherly type, and you can see these personality traits in the way Bunny moves as well as in her facial expressions. In fact, Wedge says the only thing he and his team didn't do was stick a burning cigarette in her mouth and make her wince every once in a while when the smoke got in her eyes. "I didn’t want to take it that far," he chuckles. But he did want to make it clear that she was "bitter and lonely, and that there was no way to touch her heart."

Although Wedge says that "Bunny" was initially going to be a story about an old, widowed rabbit who must bake her own birthday cake because she is all alone in life, he realized he wasn't going to be able to get that point across in a 7-minute film that had no dialog whatsoever. So he decided to turn "Bunny" into a suspense story and began to incorporate elements of suspense into his storyboards.

But developing the story line for "Bunny" was not an easy process. According to Wedge, he and his team had already animated the middle sequence of the film, in which Bunny smacks the moth into the cake batter and throws the cake pan into the oven, before they were sure how the film should begin or end. "It was scary," says Wedge. "The middle was all boarded and in production, and I had beginnings and endings that I just didn’t feel were strong. So I kept changing them."

After toying around with various beginning and ending sequences, Wedge put a halt to the project until he had the story line ironed out. Finally, he decided that the moth should represent something very close to Bunny, but that Bunny was pushing it away because she didn't understand it. That is when Wedge decided that the moth should represent Bunny's dead husband. From there, the story took shape, first with the moth attempting time and time again to get Bunny's attention, and then, once it did, drawing Bunny into the oven, which transports her to the afterlife.

One thing Wedge and his team never did was change the story line based on limitations in technology. "I always knew we had all the animation technology and talent that we needed to pull it off; so this was completely story-driven," he maintains. "I just said if we want to do it, we’ll do it. We’ll figure out how to get it done. So those changes we made didn’t have to do with what we could or couldn’t do with the software available at the time."

Innovative Technology

While its moving story line has certainly contributed to the popularity of "Bunny," in CG circles what makes this piece so special is the fact that it is the first computer-animated film whose environments were rendered entirely using radiosity. Radiosity is an advanced rendering technique that mimics the subtlest properties of natural light. For "Bunny," the use of radiosity created an unparalleled dimensionality and organic realism never before attempted in a computer-animated film. Helping the Blue Sky team accomplish this feat was CGI Studio, the company's proprietary lighting software.

According to Wedge, as he was writing the story line for "Bunny," the Blue Sky crew was developing CGI Studio with the notion that it would help Wedge bring his idea to fruition. "The inspiration for 'Bunny' and the CGI Studio technology are completely intertwined," he says. "When we first started the company, we had a lot of ideas about what our technology was going to be able to do. This is when I started looking for ideas. Back then, people were using the term photorealistic very loosely. When they said 'photorealistic,' it was a photorealistic image of a plastic ball casting a shadow. We had different ideas, and we talked about the technology quite a bit together. And as we were developing the software and getting very little business in the door, we had more time to think about how we’d like to apply it."

Based on the concept of ray tracing, CGI Studio simulates the way light behaves when it shines on real objects and surfaces in a natural environment. The software, which features an object-oriented graphics programming language, incorporates Monte Carlo Integration, a well-known algorithm in the computer graphics community that Blue Sky programmers included in CGI Studio to enable them to easily determine where the light was coming from in each scene in the film. The actual quality of the lighting itself was handled in CGI Studio. Combining Monte Carlo Integration with CGI Studio gave the Blue Sky animators a realistic-looking background without having to determine how much light and shadow would be required in each area of the environment.

"CGI Studio is a lighting product, and this Monte Carlo Integration algorithm is the radiosity end of it," explains Blue Sky Studios' Dave Walvoord, who was digital effects supervisor on "Bunny". "Combining them gives us this realistic-looking background that means we don’t have to go into each little area of the environment and decide how much light and shadow should be there. We get to spend less people time. What we’re concerned with more is just moving the light and letting it do its work, and then looking at it."

According to Walvoord, the Blue Sky team chose to use radiosity techniques to render only the environments—not the Bunny and moth characters. "We learned right away that radiosity didn't have to be everywhere. When we did apply radiosity to the Bunny and moth characters, the renders were going through the roof, but the quality wasn't improving any. So, we decided to use it only on the backgrounds, which is where it really contributes."

One of the reasons the animators didn't need radiosity rendering for Bunny and the moth had to do with the way the characters were styled. In terms of Bunny, it had to do with her fur. "About 2 years ago, we put together the final piece for me, which was the fur technology," says Wedge. "We had already begun animation, assuming that the fur would come. In fact, for a while we had a naked bunny jumping around, swinging her spoon. Then about 1.5 years ago, our first fur renders started to appear."

Bunny's fur has a great deal of color variation. Because her fur is so visually complex anyway, there was no need to animate the character with radiosity rendering. "If Bunny had white fur, then we would have needed the visual complexity that radiosity rendering would have given us," comments Walvoord. "But because her fur was multi-colored, and because we were putting in all the shadows anyway, we didn't need it. Radiosity rendering didn't add anything for us, and it was more expense in terms of render time and design cycle time than we wanted to pay." As it is, he says, it took the Blue Sky crew 14.75 hours to render each frame of the animation (rendering was done on 14 Compaq AlphaServer RenderPlex systems, with a total of 164 processors). "If we were going to use radiosity on Bunny, it would have more than doubled our rendering time," he says.

According to Walvoord, Bunny's fur was quite complex. "There were more than 14,000 clumps of hair on Bunny's body," he says. These clumps of hair—each clump consisting of approximately 50 hairs—were actually invisible cones that were texture-mapped with a mixture of short and long hairs. In total, there were 5640 long-hair cones, 8524 short-hair cones, and 269 shaggy-hair cones. Adobe Photoshop and Interactive Effects' Amazon 3D Paint were used for texture-map creation.

While Bunny didn't require radiosity rendering due to the complexity of her fur, the moth character didn't require it due to the simplicity of his design. "The moth really had a single-color fur, so it wasn't inherently as visually complex," states Walvoord. "But it was also a much simpler character—it had a very simple shape, not a lot of moving parts. So it was easier to get the look we wanted without going to the expense of radiosity." The fur on both Bunny and the moth was created in CGI Studio.

In addition to radiosity and fur, Blue Sky's research and development division, headed by vice president/director of R&D Carl Ludwig, also worked on applications to create flared lights and volumetric shadows, which allowed the characters to cast long shadows through shafts of light. "This is especially seen in the part of the film when the stove opens and you see that big, blue light shining out from it," says Walvoord. "Then, when Bunny sticks her face in the stove, it casts these long, deep shadows, really nice shadows." According to Walvoord, all lighting was done in CGI Studio. "The lighting was very crucial in this film, in terms of setting the mood and telling the story," he notes.

Modeling and Animation Challenges

Rendering and lighting the scenes as well as creating the fur in "Bunny" were quite challenging, to be sure. Also challenging was the method with which the Blue Sky team modeled and animated the objects and characters in the film.

Work on the models in "Bunny" began long before the NURBS modeling method was developed. In fact, says, Walvoord, one of the things that made this project difficult was the fact that nearly all of the models were built using an old modeling method called Constructive Solid Geometry. "This technique is a lot like working with Legos," he explains. "You're working mainly with rectangles, spheres/ellipsoids, and cones. With this technique, you can either stick these primitives together, or you can use one primitive to cut a hole in another one. Today’s modeling software doesn’t take that approach. CSG was probably abandoned when NURBS was developed, which is an easier way to generate models."

Although CSG made the project difficult, for Walvoord it also made the project fun. "When I started on this project two years ago, they showed me this stuff and said, here, try and make this sink render. I was looking at stuff that had been done when I was in high school, before I even understood that computer graphics existed! So it was like I had to relearn my job all over again. But it also made the job fun."

While most of the models were created using the CSG method, some of the models—such as the chain on the porch light—were built late in the production process, in Alias/Wavefront's PowerAnimator. Nearly all of the animation was done in Softimage. To make the CSG models animate in Softimage, Blue Sky wrote a program that converted the CSG models into polygonal representations that Softimage could understand. "These polygonal representations were very simple in their make-up, mainly to save memory," Walvoord notes. Because Amazon 3D Paint works with polygonal models, the team had to texture the CSG models using a combination of Photoshop and proprietary tools.

The only animation in "Bunny" that was done in CGI Studio is the scene at the end of the film in which 100 or so moths are flying around the afterlife environment. To animate the moths in that sequence, Blue Sky animator Rhett Collier wrote procedural animation code implemented through CGI Studio. "He figured out through math formulas how a moth would flap its wings. Then he transferred that over to his procedural program, which controlled how the moths came into the scene, making sure they didn't interpenetrate each other, and varying the way each one flapped," says Doug Dooley, one of the lead animators on the project. "It was a brilliant program, and very sophisticated procedural animation." All modeling and animation were done on Silicon Graphics workstations.

The Future

Although extremely pleased with the accolades they have received for the ground-breaking techniques they implemented while making "Bunny," Chris Wedge and the rest of the team at Blue Sky Studios feel that perhaps the most important thing they received from working on this film was the knowledge they gained. "What we took away from 'Bunny' was a huge learning experience," says Walvoord. "We learned that radiosity is hard, and that it costs a lot. But we do think it could be the future."

And, speaking of the future: "We'd really like to make a feature film, and we have several things in the works," hints Wedge. "One thing about the Oscar, and even the nomination, was that it opened a lot of doors for us. We've been talking to Fox, and I had meetings with Jeffrey Katzenberg and Disney's Tom Schumaker. Everyone knows us now.

"I don’t think there are a lot of people out there who are going to be making frames that look like A Bug’s Life, or Mighty Joe Young, or whatever it is that the latest special effect or film is going to offer. It’s all cutting edge," Wedge continues. But he is confident that in the near future, the technologies used in films such as Bugs, Mighty Joe, and "Bunny" will be more generally available. "We threw our cast out pretty far on this one," he concludes. "It took a while to reel it back in, but look at where it got us."

Click image to see a mpeg clip from Bunny

Audrey Doyle is a freelance writer based in Boston. Besides a writer for Digital Producer, she is the former Editor-in-Chief of Digital Magic Magazine, Managing Editor of Computer Graphics World, and a frequent contributor to Video Systems, Millimeter and CGW. She can be reached by email here.