Subsurface Scattering

When light neither "passes through", nor "bounces off".

Many real-world materials are neither transparent nor opaque, including milk, marble, and other stones. These materials admit some light beneather their surface, where it is bounced around for a bit, only to re-emerge elsewhere on the surface. The effect of this "subsurface scattering" is the difference between the appearance of chalk, or liquid paint, and milk.

left-to-right: skim milk, whole milk, and 'diffuse' milk (showing the way non-scattering algorithms would render it) click to enlarge