The Softening of Computer Graphics.

A new light-scattering model takes the edge off of computer-generated images

When it comes to translucent objects, computer graphics techniques tend to take a hard line. Most rendering systems pay little heed to the fact that light scatters inside translucent materials and leaves the surface at a different location from where it entered--a physics phenomenon called subsurface scattering. Rather, the majority of rendering algorithms are built using bidirectional reflectance distribution functions (BRDF), which assume that light entering at a point on a surface reflects at that same point. In reality, this is true only for metal objects. In all other materials, particularly translucent ones, light scatters.

This light-scattering effect can be seen when a laser pointer is shone on a translucent material. The area around the point of illumination starts glowing because of the light scattering inside the material. By ignoring this physical reality, many computer-generated representations of translucent objects, such as snow, milk, or skin, have an unnatural, hard appearance. Even for materials that don't seem very translucent, the BDRF methods create a hard, CG-looking effect, because they don't locally blend surface features such as color and geometry.

It's not that software developers are unaware of the physics of light, but they often choose to overlook certain aspects of it given the incumbent computational challenges. Existing techniques for simulating subsurface scattering are computationally impractical, says Henrik Wann Jensen, a researcher in the Computer Graphics Lab at Stanford University. "Typically, such techniques have involved costly Monte Carlo ray-tracing or photon-mapping approaches that simulate the scattering of individual photons," he says. "These methods are particularly costly for materials such as milk and skin, where the photons can scatter hundreds of times before leaving the material."

In an effort to soften things up without breaking the computational bank, Jensen and Graphics Lab researchers Stephen Marschner, Marc Levoy, and Pat Hanrahan have developed a Bidirectional Scattering Surface Reflectance Distribution Function (BSSRDF) that samples the surface of a translucent material and approximates the diffusion of light inside it. Instead of following the paths of individual protons, the system relies on a rapid, image-based measurement technique for determining the optical properties of translucent materials. Consequently, it is many orders of magnitude faster than Monte Carlo-based raytracing. In fact, says Jensen, the researchers are able to render translucent materials almost as fast as the traditional "hard" rendering techniques. And, he says, "our model is accurate enough that it allows us to measure the scattering properties of materials simply by fitting our theory to the scattering profile observed when ...