What is Subsurface Scattering
Subsurface scattering works by stimulating how light penetrates a translucent surface like a grape, and is absorbed and scattered. Subsurface scattering is critical for creating materials for all different kinds of surfaces like paper, marble, wax, and most importantly, skin. If there is no subsurface scattering on skin, then it won’t look realistic because there is a look that skin has- with a level of translucency. (Pluralsight.com, 2017).
When light hits a surface, multiple things happen at once; some of the light is reflected off the surface, giving specular light. However, with some materials that have a level of translucency some of the light rays are actually going to be absorbed into the surface. Once inside, the light rays will scatter all around and exit the surface at different locations, providing subsurface scattering. It may not be apparent at first, but human skin is actually very translucent. To see an example of this, hold a bright light behind someone’s ear. You’ll be able to see how the light is absorbed, and passes through the skin. It also illuminates the inner workings so you’ll likely be able to see blood vessels, skin pores, etc. (Pluralsight.com, 2017).
Lighting shining through the fingers. (Pluralsight.com, 2017).
Translucent object have two major components and subsurface scattering will allow you to create both of these components.
The first is forward scattering, this is when light enters the front of the object and is reflected back toward the viewer. This is what actually gives materials their soft appearance, like wax or skin.
The second component is called back scattering, and this occurs when a light is illuminating the backside of the object, and the light rays actually pass completely through to the other side. As mentioned previously, a great example of this would be if you set up a light behind someone’s ear or placed your hand directly in front of a light source.
Without subsurface scattering it would be very difficult to simulate these two distinct components of translucent materials like wax or skin.
The physics behind Subsurface Scattering
When a photon hits a surface, it has the change to be either reflect, absorb, or penetrate the surface. These two characteristics show reflectance of local illumination models: Where the final color at any point, need only consider that point with no influence from other points. This concept is implemented in a number of shading models (or BRDFs – bidirectional reflectance distribution functions) that you may be familiar with: Blinn, Phong, Lambert, etc. These models approximate local illumination well, but when a photon enters into a translucent surface and bounces around inside, it may very well exit the surface at a point where it did not enter. This is what happens in subsurface scattering, light enters a point P and exits at another point, thereby making local illumination models unfit for representing these non-local effects.
Tutorial on Arnold and SSS
Pluralsight.com. (2017). Understanding Subsurface Scattering – Capturing the Appearance of Translucent Materials. [online] Available at: https://www.pluralsight.com/blog/film-games/understanding-subsurface-scattering-capturing-appearance-translucent-materials [Accessed 21 Mar. 2017].