SubSurface Properties

The Subsurface Properties control the effect of translucency – or light that is scattered beneath the surface.

Subsurface Scattering (SSS) simulates the effect of light entering a translucent object and scattering inside it. Some of this light is absorbed and some is scattered back to the surface. SSS is a crucial component that allows you to accurately simulate many kinds of materials including plastics, marble, milk, skin etc.

Maxwell Render has a highly sophisticated set of parameters designed to simulate both surface and subsurface scattering. You will find Subsurface Properties for each BSDF as a collection of parameters under a collapsible rollout. These parameters are:

Scattering

Scattering color is the reflectance of inner particles causing subsurface scattering. This means that the incoming light will be reflected/ scattered in this color.

Coef

This coefficient defines the amount of particles inside the medium. Coef=0 (default) means there will be no subsurface scattering. In other words, the rays will pass through without hitting a particle. The higher the coefficient value, the more opaque/ less translucent the medium is. For example, lemonade is more translucent while marble is more opaque.

Asymmetry

Asymmetry defines the isotropy of scattering. Asym=0 (default) means that light rays will be scattered equally in all directions. A negative value will let the light rays go through while a positive value will send the rays backwards. Besides the volumetric subsurface scattering just explained here, Maxwell Render also has a Single Sided mode which helps you simulate thin translucent materials like paper, leaves, and lampshades. The remaining parameters under this rollout only control Single Sided scattering.

While the parameters under Subsurface Properties define the characteristics inside the medium, they are not enough to give the material its final look. We also have to specify other properties of the light ray that enters the material. Here, Transmittance, Attenuation, Nd and Roughness play key roles.

Transmittance

When a ray hits the surface, you need to allow it to enter the medium so it can create subsurface scattering. Set a transmittance color, which defines the color of the rays allowed to enter the medium. If the incoming light is white, the color of the rays beneath the surface will be equal to the transmittance color.

Attenuation

No transmittance color will create proper subsurface scattering without a sufficient amount of attenuation. Attenuation defines the decay of light travelling inside an object. The lower the value, the less transmissive and translucent the object will be. It is important to understand that you can control the amount of light allowed to enter an object using attenuation (together with transmittance color) and the amount of cloudiness/ translucency using scattering color and the coefficient. See the images below.

Nd

Nd is the index of refraction/reflection. It is responsible for the Fresnel effect on the surface of a material, and also controls the bending of rays when they pass through a surface. It is essential that you use values between 1.0 and 2.5 to avoid unrealistic results. Space or air begins at 1.01 (for vapor, smoke, etc.) and dielectrics do not go higher than 2.5. The recommended range for common translucent materials is between 1.2 and 1.7.

Roughness

Roughness defines the analytical perturbation on micro surfaces. Just like in any other material, it primarily controls the diffusion of specularity, and it is also responsible for the diffusion of rays on the surface before they go through the medium.

So even with zero reflectance, roughness has an effect on the diffusion of rays. The rest of the parameters including abbe, r2, anisotropy, angle, and bump are fully compatible with subsurface scattering, allowing you to simulate the corresponding effects at the same time.