Sub Surface ScatteringΒΆ

The Sub surface scattering BSDF simulates the light scattering inside a material like the skin, the marble, a vegetable etc..

The Library/Materials/Surface shader includes a simple, flexible and stable SSS BSDF.

To achieve a translucence through a single polygon (like a paper sheet or leaves) use the User Guide/Shading/Translucence color.

Quick setup

Setup a skin shader
  1. In the RenderGraph, add a RenderGraph/Preset/Skin node.
  2. Set the Surface > Diffuse > Color with a skin texture or leave the default color.
  3. Tweak the Surface > SSS > Width to achieve the desired waxiness.

The default surface shader and the skin preset.

Setup a single color scattering
  1. In the RenderGraph, add a Materials/Surface node.
  2. Set the Surface > SSS attribute to 1.0.
  3. Set the Surface > SSS > Layer 1 Color to white.
  4. Set the Surface > SSS > Layer 2 Weight to 0.0 to disable the second lobe.
  5. Set the Surface > SSS > Layer 3 Weight to 0.0 to disable the third lobe.
  6. Set the Surface > Diffuse > Color with your material main color.
  7. Tweak the Surface > SSS > Width to achieve the desired waxiness.

The SSS set to 0.0 and a white diffuse color, then with a single white lobe SSS and last with an orange diffuse color.

Waxiness

Unlike most BRDF, the SSS is dependent on the size of the objects. The Surface > SSS > Width attribute controls the global scale of the SSS BSDF. With a small width, the SSS looks sharp, with a large width, it looks waxy.

The SSS Width set to 0.05, 0.1 and 0.2.

Multi gaussian blurring

The SSS BRDF is achieve through a triple gaussian blur of the incoming light. Each gaussian lobe has its own color, width and weight. The Surface > SSS > Norm. Diffuse Color attribute normalizes the result of the three gaussian lobes to guarantee the integral SSS color is white. The overall visible color is simply the Surface > Diffuse > Color.

The default SSS gaussian setup.

The default SSS setup is composed of a large red lobe, a middle green lobe and a final sharp blue lobe.

Front and Back scattering

The SSS BSDF includes two controls to increase or decrease the front and back scattering. The front scattering controls the amount of light scattering on the front of the surface. The back scattering controls the amount of light scattering on the back of the surface.

The SSS front scattering set to 0.5, 1.0 and 2.0.

The SSS back scattering set to 0.5, 1.0 and 2.0.

To increase the SSS effect through the character ears, increase the Back Scattering. To stay energy conservative, keep the product Front Scattering*Back Scattering below 1.0. Not being energy conservative is ok though.

SSS Set

The Surface > SSS > Set attribute is the object User Guide/Scene Graph/Set in which the light can scatter. If this attribute is left empty, the light will scatter only through the object itself.

The SSS Set attribute is empty the light scatters inside each object only, and set to Diffuse, the light scatters through the both objects.

Albedo blurring

The light scattering through the object is multiplied by a mix between the input and output surface diffuse colors. The attribute Surface > SSS > Blur controls this mix and makes the diffuse color texture more or less blurry.

The SSS blur attribute set to 0.0, 0.5 and 1.0.

Approximation Depth

To save some render time, the SSS can be approximated after one or more bounces by a diffuse BRDF using the Surface > SSS > Approximation Depth.

The SSS approximation depth set to 0 (always approximated), 1 and 2.

Additional resources

The Library/Materials/Surface shader for details. The User Guide/Shading/Translucence section.