You must use SSS Set Name in order to spread SSS across multiple objects. For example, from a face to an eyeball.
Specifies the weight of the SSS contribution.
The SSS Weight in this case is comparable to having the Diffuse Weight in an Ai Standard shader that only has a Diffuse component. Reducing the SSS Weight value to zero still shows visible shading because the Specular component is still on.
Overall multiplier of the SSS radius of the three layers.
Increasing the radius multplier value can radically change the appearance of the material, from looking like leather to marble.
SSS is very scale dependent. You will need to adjust the radius multiplier depending on the size of your model. If you were to render using the default Skin settings, you may get something that does not look correct. Alternatively, adjusting the scene scale can have similar results.
Radius Multiplier from 1 to 100
For each of the three layers, the following controls are provided:
This allows you to provide different effects near the surface of the skin, at a deeper layer of skin, and behind the skin (you will see from the default colors for example that a red color for the deep scattering picks up the red color from blood beneath the skin, with more typical flesh tones used for the other two layers), and control how much contribution there is from each.
In general you should keep the shallow radius low in order to emulate a diffuse shading effect. The Mid and Deep scatter layers can then be used to add more SSS.
You may also want to experiment with adding some extra textures to the scatter colors. These were made by desaturating and gaining the color to pull out the dark areas, like his eyebrows and stubble. This was multiplied over flat colors of the values from the shader. Obviously a lot more time could be spent here to add a lot more detail in the subsurface and the specular to make him him look more realistic. In order to represent the lower dermal skin layer, the Deep Scatter image has been gaussian blurred and has less frequency detail.
The Epidermal SSS texture is connected to the Shallow Scatter attribute.
The Upper dermal SSS texture is connected to the Mid Scatter attribute.
The Lower dermal SSS texture is connected to the Deep Scatter attribute.
Modified colors for Shallow, Mid and Deep scatter layers
Example of Ai Skin shader connections
Simplified skin shading workflow
Instead of using three unique textures to control the three skin layers (Shallow, Mid and Deep Scatter) you can simplify the workflow using just one texture. Instead of tinting the files with a painting program, you could try using one diffuse map and then use the Maya multiplyDivide node in combination with a flat ramp color texture to create three different sss layers with the skin shader. Once you are happy with the skin tone you can then replace the three inputs with three different maps and then add more unique details to the maps. This way enables you to work faster to balance the skin tone without having to keep switching to an external painting program.
Radius in which light travels through the surface of the skin before exiting. Lower values produce less scattering and the skin can appear more opaque. Higher values give a larger sense of translucency, similiar to wax.
The SSS radius is scale dependent and works best with real-world units. You can see the effect that 'scatter radius' has on a mesh with different scale sizes below:
SSS is not evaluated in indirect rays traced by the SSS. Indirect rays fired by the SSS engine from one object that ends up hitting another object will return black. This is in part legacy inherited from the point-cloud based SSS engine, and in part a fundamental limitation of diffusion based BSSRDFs (which their inventor, Henrik wann Jensen, always warned were only exact for infinite, semi-infinite media and not for curved/thin surfaces etc). We have investigated how to fix this limitation of BSSRDFs but so far our efforts have not been successful. A workaround is to connect a Standard material to the diffuse attribute of a Rayswitch shader.
In the example below, you can see that green light is bounced from the cube on the left onto the sphere. However, no light is bounced from the cube when a Skin shader is assigned to it.