Got milk?

Making its digital version look realistic

September 17, 2001|By Karen Kaplan | Karen Kaplan,SPECIAL TO THE SUN

Animators working on the movie "Shrek" spent two months crafting a technically complex 3 1/2 -second shot. The star wasn't the rotund green ogre in the movie's title role, but a frothy glass of milk.

Considering that Hollywood special-effects wizards have managed to create dinosaurs that stalk real actors, computer-generated hair that appears to blow naturally in the wind and countless fiery explosions, an ordinary glass of pure white milk might not seem too difficult.

In reality, the task is among the most daunting in the field of computer graphics. Even with two months of effort, the milk in Shrek is a long way from the quality animators would like to see.

Stanford University computer scientist Henrik Wann Jensen has been searching for a way to create realistic digital milk, and after a year of work, he has come up with what computer graphics experts say is the first solution that is true to life.

"It seems like something that's so simple, but no one has rendered a convincing picture of a glass of milk," said Jensen, who presented his findings last month at the annual Siggraph computer-graphics conference at the Los Angeles Convention Center.

In his quest to breathe light - and therefore life - into purely digital objects, Jensen relied on a simple pocket laser pointer and a few bags of groceries from the local supermarket.

He also used the same advanced mathematics and physics that optics experts use to study the behavior of individual photons of light.

"Who'd have thought so much science would be needed just to make milk look good?" said Eugene Fiume, chairman of the computer science department at the University of Toronto and an expert on creating realistic computer-generated images.

Hollywood stands to be the biggest beneficiary of all this science. Jensen's technique is expected to go a long way toward making skin and faces look far more realistic than they did in Final Fantasy: The Spirits Within, the most ambitious attempt yet to digitally create a photo-realistic world. It could help usher in the long-awaited arrival of lifelike virtual actors who perform dangerous stunts or even fill starring roles.

A half-dozen digital entertainment and special-effects companies already have invited Jensen to brief them on his technique. Pixar Inc., the studio that produced the groundbreaking Toy Story films, has hired him as a consultant for its upcoming film Nemo and other projects.

"When I see that kind of thing happening, I realize they've been thirsting for this," said Mark Levoy, an associate professor at Stanford who worked on the technique with Jensen. "It's going to make an impact quickly."

Scientists have tried for years to infuse the artificial worlds created by computer graphics with the translucent qualities that bring a sense of life to organic materials such as milk.

"Pretty much everything about milk is hard," said Jonathan Gibbs, the film's lead effects animator at PDI/DreamWorks. "It's surprising. It's such an absolutely mundane thing."

Accurately rendering translucent materials is one of the last significant hurdles in computer graphics. Biologists define life based on an organism's ability to convert nutrients into energy so it can grow and reproduce.

In the world of computer graphics, life boils down to things such as color, fluidity of movement and, most of all, translucence.

Capturing the way light softens the tip of a nose, glints off a leafy tree or glows inside a glass of milk is key to convincing the human eye that it is witnessing something real.

The problem is the enormous complexity in the way light interacts with translucent objects.

When particles of light, called photons, hit a glass of milk, a small portion of them will bounce right off. But most will enter the glass and keep on going until they bump into fat globules and other molecules suspended in the liquid. Each photon could bounce around hundreds of times before exiting the glass. In the meantime, the mishmash of photons illuminates the glass of milk, producing its telltale glow.

The effect can be observed easily by shining a concentrated beam of red light from a laser pointer into the glass and seeing it turn pink.

The zigzagging paths of these millions of photons can be modeled on a computer. But it would take months for today's computers to process all the calculations. "It's not really an appropriate way to approach the problem," Fiume said.

So animators looked for shortcuts. They created computer models that assumed light bounces off translucent surfaces, or maybe knocks around a few times under the surface before exiting. Those techniques tend to make objects take on an artificial, plastic-looking appearance. Animators compensate with a few tricks to add light to their pictures.

At Sony Imageworks, artists working on the movie Stuart Little added a pinkish glow to the ears of the title character - a mouse - whenever he stood in front of a light source, said Scott Stokdyk, digital-effects supervisor there.

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