Rendering Translucent Materials
Subsurface Scattering, commonly abbreviated as SSS, is an essential lighting phenomenon and rendering technique that is crucial for creating realistic computer-generated imagery of any non-metallic, translucent material. Unlike a perfectly opaque object where light only bounces off the surface, a translucent object allows light to penetrate its surface, scatter around amongst the particles inside, and then exit at a different point. This is what gives such materials their characteristic soft, glowing appearance.
The Key to Realistic Skin
The most important and well-known use of SSS is for rendering believable human skin. Skin is not a simple, opaque surface. It is a complex layered material. When light hits skin, it enters the epidermis and dermis, bounces around off blood cells and other tissues, and exits, often with a reddish tint. SSS is the technique that simulates this effect. Without it, digital characters look hard, waxy, and plastic-like—a classic sign of the ‘uncanny valley.’ With SSS, they gain the subtle softness and warmth that is characteristic of real skin.
This effect is especially noticeable in strong backlighting, where the light will appear to glow through the thinner parts of the body, such as the ears or nostrils.
Subsurface Scattering: Other Materials
Beyond skin, SSS is vital for rendering a wide range of other common materials, including:
- Marble
- Wax (e.g., a candle)
- Milk and other dairy products
- Jade
- Grapes and other fruits
- Gummy candies