Sub Surface Scattering (SSS), sometimes known as Subsurface Light Transport, is a method for simulating light passing through objects and generally affects skin. The name quite literally means light scattering from the point beneath a surface.
The focus of Subsurface Scattering is one what we’d typically deem partially translucent objects, which is why it most obviously affects the quality of characters’ skin and eyes. This feature isn’t exclusive to skin though, as it can also add substance to other partially translucent objects in games, including paper, wax, and hair. However, SSS is still used almost exclusively for skin in video games these days, particularly as we find alternate methods of lighting.
Below we have an example of human skin without subsurface scattering (left) and with subsurface scattering (right). As you’ll see, the inclusion of a complex bump map for the skin necessitates SSS in order to more authentically represent the appearance of real skin. Play a game from around 2007-2010 and you’ll see plenty of examples of characters looking like the left-hand picture.
As for how this manifests in-game, the prominence of the effect can vary. You’re going to most notice this effect during in-engine cut-scenes when the camera’s up close to a character. However, during general gameplay the effect will be fairly minor, although certainly more noticeable in first-person titles.
How demanding is Sub Surface Scattering?
Subsurface Scattering isn’t very resource intensive at all and typically only has a minor impact on the GPU. During typical benchmarks we found SSS affected frame rates in the region of 0-2%, meaning its performance impact is negligible. You will typically find Sub Surface Scattering just has an On and Off toggle rather than any more granular choices.
Is it worth enabling Sub Surface Scattering?
Due to Sub Surface Scattering having little to no impact on gaming frame rates we would recommend you enable this graphics setting at all times. In particular, first-person games can benefit a lot more from Subsurface Scattering. The effect will often apply to your character’s arms, as well as when you get up close to NPCs. Disabling SSS should be a last resort for performance-related issues.