What are the two available SSS modes?


Starting from Corona Renderer 1.7, there are two subsurface scattering modes that can be enabled for the CoronaMtl under the "Volumetrics and SSS" rollout:

  1. "SSS"
  2. "Volumetric Scattering"

These two modes should be used in slightly different scenarios, and are characterized by different properties.




1. "SSS" Mode



The “SSS” mode is suitable for quickly setting up simple materials, which do not require such effects as translucency, refraction, or opacity. This means that the material usually has to have a defined, opaque surface, and that its volume is very thick. Examples of such materials include:




When using the “SSS” mode, translucency and refraction properties of the material are ignored and greyed-out:




Opacity can be still controlled, however if it’s lowered, no volume will be visible inside the material, meaning that it is only controlling the visibility of the object’s surface:




Below are the available controls, which define how the final SSS effect looks:


Amount

Controls how much the material’s color will be affected by the subsurface scattering as opposed to diffuse reflection. A value of 0 results in no subsurface scattering at all, while a value of 1 results in full subsurface scattering and no diffuse component.


Radius

Defines the subsurface scattering radius – how far a color will scatter from a place that was hit by a light ray.


Scatter color

Defines the scatter color of subsurface scattering – the color that can be observed in the shadowed parts of the material.




2. "Volumetric Scattering" Mode



Volumetric scattering is suitable for materials relying on additional effects such as translucency, refraction, or opacity. Materials, which require this mode are usually very thin or refractive, and this includes:



A good method for setting up a material using volumetric scattering is to make its surface completely transparent using Opacity set to 0. You can then adjust the volumetric scattering parameters to see their effect, without interference from other effects, such as reflectivity or translucency, and finalize the material settings once satisfied with the look of the volume.


The available properties defining the volume of the material include:


Absorption

This defines the density of the material. It is controlled using two parameters:


Absorption Distance
Controls how quickly the light passing through an object will become colored and darker. With a lower value, the light will be very quickly absorbed and so appear more strongly colored and darker, even with thin pieces of the material. With higher values, the light will travel farther through the material before these effects happen, so that even thicker pieces of the material will not show so much coloring and darkening of the light.


Absorption Color
The color that the white light will take on after passing through the material by the “Absorption Distance”. 



Scattering

The color of the scattering media. It affects the coloring of light bounced back toward the viewer by the volume of the material, in much the same way as Diffuse controls the coloring of light bounced back toward the viewer by the surface.


It is controlled by three parameters:

 

Scattering Color

This controls the color of the light after it is scattered.


Directionality

This controls the direction in which light is scattered. The default of 0 means that light gets scattered evenly in all directions. Positive values mean that light gets scattered forward, along the same path it was already traveling. Negative values mean that light gets scattered backward, back along the path it was traveling.


Single bounce only

Once light has been scattered, it is still passing through the volume and has a chance that it may be scattered again. By enabling this checkbox, light can only be scattered once, after which it will pass through the volume without further scattering (though it will continue to be affected by absorption) and this gives faster rendering with the trade-off being a darker result and potentially less realism depending on how much the scattering affects the result.



When using the Volumetric Scattering mode, to make the SSS effects visible, at least one of the following material properties has to be enabled:

  • Translucency
  • Refraction
  • Opacity

Lowering opacity when using the Volumetric Scattering mode will only make the object's surface transparent, and will reveal the object's volume underneath:



See also:

How to render skin?

How to use subsurface scattering?