Improve your 3D lighting with sub-surface scattering

During the early days of CGI, lighting was truly reflective. A CGI light ray would hit a surface and bounce off, which was great if all you wanted to animate was chrome balls spinning. But CGI artists wanted to recreate or augment reality, and this means an understanding of what light does when it interacts with a surface.

In most cases, light actually enters a surface and is channelled through it to help create the look that we know as ‘real’. This property of light interaction is called sub-surface scattering (SSS), and gaining an understanding of how it works will enable you to really get a handle on the true subtleties of realistic rendering.

Classic examples of where you will see SSS working include wax, viscous liquids such as milk and all over your body. Your own face can demonstrate a range of light diffusion examples occurring within your skin depending on how it is lit. If your character is strongly back-lit, the thinner fleshy areas of the ears should let more light through than the other parts of the head. If your character is lit in profile, the terminator (the area where a form transitions from light into shadow) should display a red colour where light passes through the top layer of the skin. And if your character is more front-lit, facial shadows should still have a red-tinted edge, as light has travelled into the skin and is coming out at a slightly different direction.

As you can see, the type of light can change the SSS characteristics of an object, and the changes can be quite subtle. In rendering, this means that using SSS can be a time-consuming process. Renderers are starting to implement much more efficient SSS algorithms for both real-time and pre-rendered sequences.

As with all CGI, it’s a good idea to look at how these processes are dealt with in traditional art. Artists have long used techniques such as layering oil paints, with dark colours laid down first, lighter colours added on top, and highlights added last. These layering techniques can be used with texture maps along with other image-based techniques, depending on your renderer, for enhancing your shaders to really get the most of the creative opportunities that SSS can bring to your work.

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