The skin shader node is provided to give an easy to use generic skin shader. Further examples of customer work using the Skin shader can be found here.
The parameters are presented in several different groups:
As you can see, the shader is broken up into different SSS layers ('Shallow Scatter', 'Mid Scatter' and 'Deep Scatter'), each one with its own scattering radius, color, and weighting factor. There are also two separate specular/reflection layers.
Specifies the weight of the SSS contribution.
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
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:
'Sheen Layer' settings are the same. It is recommended that you use 'Sheen Layer' for tight specular reflections and 'Specular' for broader specular reflections.
The color the specular reflection will be modulated with.
Sheen Layer (yellow) and Specular Color (red)
Controls the glossiness of the specular reflections. The lower the value, the sharper the reflection. In the limit, a value of 0 will give you a perfectly sharp mirror reflection, whilst 1.0 will create reflections that are close to a diffuse reflection.
The specular weight.
The amount of reflection will be dependent on the viewing angle of the surface following the Fresnel equations (which depends on the IOR value).
The visual effect is that the reflection increases as the viewer's angle of incidence, with respect to the surface normal, approaches 0.
The white powdery look, apparent on the grapes in the images below, is achieved using a high roughness value. Notice the effect that increasing the IOR has on the specular roughness.
There is currently an issue with Specular Fresnel (IOR) which produces a darkening effect around the edges of the Skin shader. This issue is unrelated to bump mapping. However, the effect is more apparent when using bump mapping because it increases the areas where there are grazing angles. This issue is due to be fixed in a future release. A workaround is to disable 'Fresnel Affects SSS' found within the 'Advanced' options of the Skin shader.
Bump mapping increases darkening effect caused by IOR
By default 'Specular in Secondary Rays' is disabled, so for diffuse and glossy rays, the Skin shader evalutes the SSS layers only. This makes the shader faster to render. When 'Specular in Secondary Rays' is enabled, the Skin shader evalutes the Specular layers as well as the SSS layers for diffuse and glossy rays.
Determines whether fresnel computations affect SSS. It is recommended that this should be left on by default.
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
Enabling this option will take into account the effect that Displacement Autobump has on the ray traced BSSRDF's result. This helps capture the high frequency details of the surface more accurately when using Autobump.
Beware that enabling this option will triple shader evaluations with SSS, resulting in much longer render times.
Sets the opacity of the skin.
The color or texture used to affect the opacity of the skin. This is set to full white by default, which means fully opaque skin. If not set to full white, be sure to have the Opaque flag off in the Arnold Parameters property of the skin object, or else Arnold will bypass any transparency-related calculations to optimize render times.
Ramp texture connected to Opacity Color. Black areas of map are non-opaque.