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Simulating Skin - Hollywood Comes Calling

The Computer Science and Engineering department's Graphics and Vision research group got a major boost in February, when a co-director received one of Hollywood's coveted Academy Awards. Assistant professor Henrik Wann Jensen shared the Technical Achievement Award with two former colleagues - Stanford's Pat Hanrahan, and Cornell's Stephen Marschner - for their pioneering research in mimicking the way that light disperses beneath the surface of skin and other translucent materials.

Put simply, Jensen was hailed for helping to pave the way for photo-realistic, computer-generated humans in the movies.

The Academy of Motion Picture Arts and Sciences, which rarely honors academics outside the film industry, specifically cited the impact of a 2001 SIGGRAPH paper on which Jensen was the principal author. In 'A Practical Model for Subsurface Light Transport,'he provided a mathematical model for rendering the effect of light on translucent surfaces, such as skin or marble.

Even before the paper was published, Jensen was invited to speak at major visual-effects companies such as Industrial Light & Magic (ILM) and Pixar, which subsequently incorporated the technology into their visual-effects software. They created characters such as Dobby in Harry Potter and the Chamber of Secrets, and Gollum in the second and third installments of the Lord of the Rings trilogy. "It has been heartening to see how quickly Hollywood adopted this technology," says Jensen. "Almost all big-budget films with extensive visual effects now incorporate at least some elements of our model, and with Gollum and Dobby, for the first time movie-goers had the sense that they were watching flesh-and-bone characters.'

Henrik Wann Jensen

Jensen has grappled with ways to replicate the appearance of natural phenomena and materials since the mid 1990s, when he developed (and wrote a book about) 'photon mapping'-a method to replicate the look of light on a scene from one or more sources. That process is widely used in the computergraphics industry, but the researcher recognized that it fell short when rendering translucent materials such as milk, marble, and snow, or skin, eyes and teeth.

Previous visual-effects technology assumed that light on any surface reflects from the same point where it hits, as it does on a metal surface. The result: images that appear hard-like rock or plastic. "That is why the early successes in digital animation, such as Jurassic Park and Toy Story, featured primarily nonhuman characters," explains the Danish academic. 'Now we are entering an era when skin can be rendered true to life, allowing audiences to forget that the character is synthetic rather than real."

Drawing insight from a medical text, Jensen realized that existing effects did not take into account a phenomenon known as 'subsurface scattering.'On translucent materials, light penetrates the surface and scatters, and the photons then reflect out from various points away from where they entered, at varying angles. Jensen's new model mathematically accounts for the way those photons scatter.

Meanwhile, Jensen's awardwinning work is contributing to growing interest in CSE's Graphics and Vision group, which now counts ten faculty members and over twenty graduate students. Jensen leads the graphics research, while the computer-vision side is led by professors David Kriegman and Serge Belongie. The founders have dubbed their group, The Pixel Lab. 'At the most basic level, the pixel is what we have in common,'explains Jensen. 'Our combined vision and graphics approach makes the UCSD program unique.'Adds Belongie: 'The vision people do analysis, while the graphics people do synthesis, creating images. But we work together.'

"the gloaming" Cyrus Jam's rendering
of a lost city in the mountains
Indeed, Jensen and Belongie recently introduced a new technique for more efficiently rendering scenes that feature distant natural illumination-a technique that could speed up the work of Hollywood special-effects wizards and videogame developers. The group also recently expanded to include a leading computer graphics designer from industry. Steve Rotenberg is a former director of research at Angel Studios, a top videogame design firm. And this quarter, Rotenberg is teaching a course on computer-animation programming-CSE 169-giving students a crash course in the tools that could one day land them great jobs making tomorrow's visual-effects movies. Already, the student who placed first in Jensen's competition in his Rendering Algorithms course last spring, Cyrus Jam, has gone on to a full-time career at ILM.