Bouncing light rays, intricate shadows, and subsurface scattering all complicate the computer rendering of anything in nature and require the development of algorithms that provide a shortcut for modeling an object’s visual appearance—a technique called phenomenology.

Occasionally computer animators can seem more like virtual gardeners than illustrators. Trees, for instance, have long been a challenge to re-create convincingly. “It’s difficult to have models for veins and bark and for the interaction of light with the leaves, which are somewhat translucent,” says Przemyslaw Prusinkiewicz, a professor of computer science at the University of Calgary in Alberta. “And the overall shape is very complicated.” Too often, computer-rendered trees looked like topiaries, stripped of the free-form variation that real trees possess.

To create a believable tree, you need both true-to-life textures—Prusinkiewicz and his colleagues have recently created a tool for simulating the tiny hairs on the surface of a leaf—and you need a realistic branch structure. Branches, of course, are an iterated phenomenon: A branch develops, then sprouts new branches, which sprout even more. The exact size and position of each branch along the chain affects all the others, as gravity and available light shape its growth. If you try to simulate the end results, you may get trees that have an artificiality to them. In order to make convincing virtual trees, some animators now simulate the entire growth process. The trunks of the trees in Shrek 2’s lush forests were predefined by computer animators, but the branches were all grown organically from digital seeds.

Clouds and fire also pose significant hurdles for machine rendering. David Ebert, director of the Purdue University Rendering and Perceptualization Lab, has been exploring cloud simulations for more than a decade. “Clouds are a very amorphous phenomenon,” he says. “You have all these tiny particles of water, ice, and snow. Light enters the cloud and is scattered around, and some of it is directed to your eye. And while it’s being directed to your eye, it’s passing through the atmosphere, which has air particles scattering light along that direction. So you really have a very complex 3-D collection of small particles that you need to simulate.” Fire is even more chaotic. “There you have actual combustion occurring—so instead of just having light being reflected by all these particles, you actually have light being emitted by particles,” Ebert says. “Then you’ve got gas that gives off light that’s transparent but also dust and soot particles that are opaque—if you shine a bright light on a flame, you’ll actually see a shadow behind it. So you’ve really got a lot of complexity.”

Creating realistic leaf motion turns out to be an important element in modeling virtual trees. To get the right effect, Shrek 2’s team of animators found that simulating an invisible fluid sloshing around the tree created a more convincing motion than using a wind simulation.

Thanks to our visual acuity and our gift for pattern recognition, we humans have an extraordinary capacity to detect small aberrations in simulated nature. Ebert tells a story of working with storm experts from the University of Oklahoma. “We were creating renderings based on data they’d supplied us, and we had an atmospheric scientist come over and look at one of the renderings we’d done.” With one glance she could tell that there was something wrong with the image: The cloud tower at the very back of the storm was too smooth; it lacked the telltale cauliflower shape that you’d normally see in a supercell formation. “Oh, there’s no medium-scale turbulence in that model—something’s wrong,” she said. In fact, a software bug had corrupted the original data that Ebert had been sent, but it took another week for the researchers to discover the problem. Yet someone could see it in the clouds in a single glance.

Ebert has also discovered that looking at clouds from both sides—their real incarnations and their simulated doubles on the screen—can reveal new things about the complexity of the natural world. One of the scientists working with his animation team is an expert on cumulus cloud formations. “We’ll be looking at a cloud, and we’ll say: ‘How do we simulate that really hard edge there?’” Ebert says. “And our colleague will say, ‘Well, I’m not really sure.’ So to improve our models we’ve started to ask questions that even atmospheric scientists don’t know the answers to.”

As more of our entertainment comes from computer-rendered worlds, through movies or video games or other online social environments, replicating the complexity of nature will become an increasingly commonplace computer task. At this year’s Academy Awards, computer scientist Henrik Wann Jensen received a technical achievement Oscar for his pioneering research in subsurface scattering. The prize itself is a measure of the pace of technology. Special-effects awards used to be all about spaceships, explosions, and robots from the future. Now they’re giving prizes for capturing the subtleties of human skin and hair.

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