Subsurface scattering is light passing through and diffusing within a thin translucent material. Human skin is an example of such a material as it is made up of many thin translucent layers. A Subsurface Scattering material is ideal for re-creating skin in 3D computer graphics and is why the Mental Ray Subsurface Scattering Fast Skin shader was specifically created.
To compare the benefits of using the SSS Fast Skin shader, a model of the human head was set-up with a three point lighting system and a Phong material. The result is a plastic like artificial look due to the harshness of the shadows.
The model has been setup to use three Mental Ray area lights, as they produce softer shadows. The key light is positioned above the face and to the right, there is then a soft low fill light to reveal some of the shadow detail in the checks, and then a strong back light to model the jaw and lift the face from the background.
The default SSS Fast Skin Shader
Before attempting to recreate any material it is always good to have a reference
image handy. While 3d graphics can improve upon nature, a reference image can be
used when things start to 'just not look right'.
Applying the SSS Fast Skin shader with the default settings produces a wax like material. Note though the difference in the softer gradation of shadows.
The SSS Fast Skin Shader is dependant on the scale of the model. It is best to use models that approximate real world sizes. The head model in this example is 190 mm wide, which, including the ears is the average width of the human face. Scale conversion can be used when the scale isn't quite right by adjusting the overall subsurface scattering effect. In 3ds max 7, Scale Conversion is found under the Advanced Options. Setting it to 100 divides each of the three Diffuse layer's radius: the Epidermal, Subdermal and Back surface layers. The result is a return to the harsher, darker shadows, and effectively disables the scattering.
Setting the Scale Conversion to 0.1 produces scattering that is too big and grainy. The result looks like a red balloon as too much of the Subdermal layer (the lower bloody layer) is showing through.
Back scattering controls how much light is allowed to pass through the object and can be used to show veins and arteries. To see the effect, turn everything off, except Back Scattering Weight. Applying the following settings produces a silhouette with partial transparency.
3-Layer Diffuse Subsurface Scattering
Unscattered diffuse weight = 0.0
Epidermal Scatter weight = 0.0
Subdermal scatter weight = 0.0
Back scatter weight = 1.0
-Layer Specularity and Reflections
Overall specular weight = 0.0
Scale Conversion = 1.0
Here the model has been rotated and lowered to expose more of the ear so as to highlight the back scattering effect.