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.

True subsurface scattering cannot be simulated using the ubiquitous BRDF (Bidirectional Reflectance Distribution Function). Instead it requires a full simulation of the BSSRDF (Bidirectional Scattering Surface Reflectance Distribution Function). This is fairly costly, but it can be done quite efficiently using photon mapping as described in my book as well as my papers. Particularly relevant is is this SIGGRAPH'2001 paper presenting the first practical BSSRDF model for computer graphics - this paper uses a fast dipole diffusion approximation in combination with an accurate term for single scattering. A follow-up paper presented a hierarchical evaluation technique for the BSSRDF which is substantially faster (our test results showed a speed improvement of a factor 150).

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 from PDI and it is described in this paper.

BRDF rendering	BSSRDF rendering

This face model has been rendered using the BRDF and BSSRDF described in this paper. The BRDF model is a simplified version of the BSSRDF that does not account for subsurface light transport (it assumes that the light scatters at a single point on the surface). Note that the BRDF model does include subsurface scattering - the contributed is integrated into a single scattering term and a diffuse term. In contrast to the BRDF the BSSRDF does simulate subsurface light transport and gives the skin a more natural translucent appearance. Note in particular 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. The face was modeled by Steven Stahlberg.

This bust of Diana the Huntress is made of translucent marble. It has been rendered using subsurface scattering to capture the translucency including multiple scattering in the marble. Just 200,000 photons were used to simulate multiple scattering in the marble.

This is the same marble bust as shown above. The difference is that the entire bust is shown and that the light source is in front of the bust. This reduces the translucency effect significantly. This was one of the first images with subsurface scattering that I rendered.

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.

A granite and a marble version of the Stanford bunny.