All non-metallic materials are translucent to some degree. This means that light scatters inside the material before being either absorbed or leaving the material at a different location. This phenomenon is called subsurface scattering.

In 1998, I started exploring techniques for simulating volumetric subsurface scattering at MIT. Previous methods had only considered 1D transport wrapped into a BRDF model and therefore ignored the transport of light through the material. At MIT, we developed techniques based on photon mapping and path tracing to simulate the darkening of many wet materials as well as the translucency of weathered stone.

One of the key innovations was the development of a new technique based on a diffusion approximation. This approach is much faster than tracing individual photons and is simple enough that a BSSRDF (Bidirectional Scattering-Surface Reflectance Distribution Function) can be formulated. The BSSRDF is particularly suited for highly scattering translucent materials where the assumptions in the ubiquitous BRDF approximation break down.

As an example, the BSSRDF was used to render the translucent marble bust shown here. Note how the BSSRDF captures the soft and smooth appearance as well as the light diffusing through the marble – something that a BRDF cannot simulate. The BSSRDF research has been rapidly adopted by the movie industry and won an Academy Award in 2004.

Rendering Translucent Materials

In recent years, we have worked on techniques for rapid evaluation of the BSSRDF model and extensions to simulate diffusion in multilayered translucent materials such as human skin. Recently, we developed a generalized Lorenz-Mie theory for computing the optical properties of translucent materials and scattering media.

A Translucent Marble Bust

This sequence of images shows a translucent marble bust (Diana the Huntress) illuminated from behind. The images illustrate some of the different techniques for simulating subsurface scattering. The image on the left was rendered using photon mapping with 200,000 photons used to simulate multiple scattering inside the material. The image in the center was rendered using a BRDF-based model for subsurface scattering. Notice how the BRDF fails to capture the light transport through the material, and even though the shape of the reflected light is correct, the material does not look translucent.

Multilayered Skin Model

This face model was rendered using a layered skin model described in this paper. The skin is composed of three layers: epidermis, dermis, and the bloody dermis. The scattering parameters for the individual layers are from measured data available in the medical physics literature. Note that the multilayered model gives the skin a less waxy look compared to the standard BSSRDF (dipole diffusion) based skin rendering shown below.

Rendering Milk

This image shows how milk can be rendered realistically using the BSSRDF model. From left to right, we see skim milk, whole milk, and diffuse milk. The diffuse milk has been rendered using a traditional BRDF model, which results in a hard appearance making the milk look more like white paint rather than milk.

Translucent Teapot

This translucent teapot was rendered in 7 seconds on a 800MHz dual-P3 PC using a new fast hierarchical rendering technique for translucent materials. This method was developed with Juan Buhler and allows for efficient rendering of complex translucent objects.

Comparing BRDF and BSSRDF

Note the translucency around the nose and the color bleeding in the shadows. These effects are achieved even though the setup is very simple – one light source, only one binary map is used to distinguish the lip from the skin region, and a bumpmap is used to add more detail to the skin.

Granite and Marble Bunny

Even though granite seems like a solid material, it still scatters light below the surface. This is due to micro cracks and small air bubbles in the material. For granite, the translucency is most noticeable in the quartz grains.

Stone subsurface scattering is an important aspect of capturing reality in computer-generated images. By using techniques such as photon mapping and path tracing, we can simulate the transport of light through materials and create more realistic renderings. The BSSRDF approach has been shown to be particularly effective for rendering translucent materials and has been adopted by the movie industry.