similar to the recent texturing posts, how does subsurface scattering work and how is it implemented in games?
Subsurface scattering is what happens when light isn’t completely reflected off of a surface, but penetrates into the surface by some amount and reflects off of stuff beneath the surface and partially reflected and refracted through that depth. This is how it works:
Normally, when you look at an opaque object, what you see is determined by the light reflecting directly off of its surface. That can be shiny or matte, but you see only what is on the surface of the thing. There are also objects that are (semi-)transparent - they may refract or bend light as it passes through them, partially handling colors and such as well. Regardless, the light bounces off of something and enters your eye, allowing you to see it in a process like this:
However, sometimes the surface isn’t completely opaque but it also isn’t transparent. Light will actually penetrate some surfaces to some depth before it reflects back out. This is what the “sub” in sub-surface scattering refers to - it goes under the surface before it reflects back out. The surface isn’t transparent on the way out, so the light coming back out of that area beneath the surface gets scattered based on qualities of the material it’s traveling through. As a result, you get something like this:
The depth to which the light can penetrate beneath the surface and the effects of that material on the light’s color, intensity, direction, etc. are defined and controlled by the data within the subsurface scattering map. A subsurface scattering map is essentially another kind of texture map that defines the depth that light can penetrate this particular part of the model, and what kind of effects that penetration/reflection will have on the light that comes out. This allows technical artists can tweak the individual visual behaviors for different materials like skin, wood, blood, ooze, fur, fuzz, etc. as well as where these different individual locations are located on the textured model.
The result is that we can now define exact areas where light can penetrate into the model, others where it cannot, and our artists can tweak those values to say how far the light penetrates and what the material does to the light that gets reflected from beneath the surface. This is what lets tech artists do things like make certain parts of the dragon (tail, horns, ridges, etc.) react to light shined on it differently than other parts.
[Join us on Discord] and/or [Support us on Patreon]
The FANTa Project is being rebooted. [What is the FANTa project?]
Got a burning question you want answered?
Short questions: Ask a Game Dev on Twitter
Long questions: Ask a Game Dev on Tumblr
Frequent Questions: The FAQ